SABP-A-022

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Copyright©Saudi Aramco 2008. All rights reserved.

Best Practice

SABP-A-022 25 August 2008 Stainless Steel Fabrication, Testing and Installation

Document Responsibility: Materials & Corrosion Control Standards Committee

Saudi Aramco DeskTop Standards Table of Contents 1 Scope and Purpose............................................ 2 2 Conflicts and Deviations..................................... 2 3 References......................................................... 2 4 Definitions and Abbreviations............................. 5 5 Introduction to Stainless Steels.......................... 6 6 Classes of Stainless Steel.................................. 6 7 Composition, Corrosion Resistance and Sensitization..................................... 9 8 Materials Selection........................................... 11 9 Castings .......................................................... 12 10 Fabrication....................................................... 13 10.1 Welding and Corrosion Resistance....... 13 10.2 Welding of Stainless Steels................... 14 10.3 Storage.................................................. 32 10.4 Shop Fabrication................................... 32 10.5 Pickling, Passivation & Iron Removal.... 33 10.6 Field Fabrication.................................... 34 11 Hydrostatic Testing.......................................... 36 12 Microbiologically Influenced Corrosion (MIC)... 37 13 Coating Stainless Steel.................................... 38 14 Summary.......................................................... 39

Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022

Issue Date: 25 August 2008

Next Planned Update: TBD Stainless Steel Fabrication, Testing and Installation

Page 2 of 40

1 Scope and Purpose

Stainless steels are more expensive than carbon and low alloy steels and are normally

selected for their superior corrosion or heat resistance. For the austenitic stainless

steels, good low temperature toughness is also a key mechanical property. However,

improper fabrication and testing procedures can seriously degrade corrosion resistance

and directly lead to premature failures.

This SABP provides guidelines to optimize performance of stainless steels, particularly

austenitic, duplex and martensitic grades. It identifies specific procedures and

specifications to be followed, during manufacturing, fabrication and commissioning, in

order to achieve optimum performance of stainless steel equipment and piping.

This SABP is based on current industry experiences and is intended for internal Saudi

Aramco operations and maintenance applications. It also provides guidance for

inspection in fabrication facilities.

2 Conflicts and Deviations

If there is a conflict between this Best Practice and a Saudi Aramco standard then the

Standard shall govern. If there is a conflict between this Best Practice and an approved

welding procedure then the approved welding procedure shall govern. If there is a

conflict between this Best Practice and other standards and specifications, please

contact the Coordinator of ME&CCD/CSD.

3 References

3.1 Saudi Aramco References

Saudi Aramco Engineering Standards

SAES-A-007 Hydrostatic Testing Fluids and Lay-Up

Procedures

SAES-G-005 Centrifugal Pumps

SAES-H-001 Coating Selection & Application Requirements for

Industrial Plants and Equipment

SAES-L-132 Material Selection for Piping Systems

SAES-W-010 Welding Requirements for Pressure Vessels

SAES-W-011 Welding Requirements for On-Plot Piping

SAES-W-014 Weld Overlays and welding of Clad Materials

SAES-W-016 Welding of Special Corrosion Resistant Materials

http://standards/docs/Saes/PDF/SAES-A-007.PDF

http://standards/docs/Saes/PDF/SAES-G-005.PDF

http://standards/docs/Saes/PDF/SAES-H-001.PDF

http://standards/docs/Saes/PDF/SAES-L-132.PDF

http://standards/docs/Saes/PDF/SAES-W-010.PDF







Page 3 of 40

Saudi Aramco Standard Drawing

AD-036821 Material Guide for Centrifugal Pumps

Saudi Aramco Best Practice

SABP-A-001 Polythionic Acid SCC Mitigation - Materials

Selection and Effective Protection of Austenitic

Stainless Steels and other Austenitic Alloys

Saudi Aramco Technical Alert

ALERT-93-011 Technical Alert Number 11, Avesta 254SMO

Stainless, issued 11/21/93

3.2 Industry Codes and Standards

American Petroleum Institute

API TR 938-C Use of Duplex Stainless Steels in the Oil Refining

Industry-First Edition, 2005

American Society for Testing and Materials

ASTM A380 Standard Practice for Cleaning, Descaling and

Passivation of Stainless Steel Parts, Equipment,

and Systems

ASTM A967 Standard Specification for Chemical Passivation

Treatments for Stainless Steel Parts

ASTM C871 Standard Test Methods for Chemical Analysis of

Thermal Insulation Materials for Leachable

Chloride, Fluoride, Silicate, and Sodium Ions

American Welding Society

AWS D10.18:2008 Guide for Welding Ferritic/Austenitic Duplex

Stainless Steel Piping and Tubing

AWS D18.2:1999 Guide to Weld Discoloration Levels on Inside of

Austenitic Stainless Steel Tube

International Standards Organization

ISO 15156-3 Petroleum, Petrochemical and Natural Gas

Industries Materials for Use in H2S-Containing

Environments in Oil and Gas Production Part

3: Cracking-Resistant CRAs (Corrosion-

Resistant Alloys) and Other Alloys

http://standards/docs/Standard_Drawings/PDF/AD-036821-002.pdf

http://standards/docs/Best_Practices/PDF/SABP-A-001.pdf

http://standards/docs/Technical_Alerts/PDF/ALERT-93-011.pdf




Page 4 of 40

National Association of Corrosion Engineers

NACE MR103-2007 Materials Resistant to Sulfide Stress Cracking in

Corrosive Petroleum Refining Environments

NACE RP0198 The Control of Corrosion Under Thermal

Insulation and Fireproofing Materials - A

Systems Approach

New Zealand Stainless Steel Development Association

NZSSDA Code of Practice for the Fabrication of Stainless

Steel Plant & Equipment, 2001

3.3 Publications

Optimising Stainless Steel Piping Fabrication Practice, Dr. Liane Smith, North

Scottish Branch of the Welding & Joining Society, Aberdeen, 26th

May 2005

NiDI Publication 11003: Nickel Stainless Steels for Marine Environments,

Natural Waters and Brines (1987)

Guidelines for Successful Use of Stainless Steel in Potable Water Treatment.

Plants (PWTP), R. E. Avery, S. Lamb, A. H. Tuthill, Nickel Development

Institute, April 1998

Fabricating Stainless Steels for the Water Industry Nickel Development Institute

Reference Book 11026, C. Powell, D. Jordan, October 2005

Microbiologically Influenced Corrosion – Case Studies in Australasia, ACCA

2007, Paper 21, L. H. Boulton

Influence of Sulfate-Reducing Bacteria on Corrosion of 2205-Type Duplex

Stainless Steel in Chloride Medium, ACCA 2007, Paper 72, P. J. Antony, R.K.

Singh Raman, et. al.

“Welding of Stainless Steels and other Joining Methods,” A Designers’

Handbook Series No. 9002, NiDI, Nickel Development Institute

American Welding Society, “AWS Welding Handbook, Eighth Edition Volume 4,

Materials and Applications Part 2”, 1998

ASM Handbook, Volume 6, “Welding, Brazing, and Soldering,” 1993

Stainless Steels, Properties, How to Weld Them, Where to Use Them, Kotecki,

D. and Armao, F., 2003, The Lincoln Electric Company

Welding of Austenitic Stainless Steels - a Guide to Best Practice, TWI Members

Website




Page 5 of 40

4 Definitions and Abbreviations

API American Petroleum Institute

ASSDA Australian Stainless Steel Development Association

ASTM American Society for Testing & Materials

CRA Corrosion Resistant Alloy

CUI Corrosion under Insulation

DSS Duplex Stainless steel

Free Iron Surface contamination of SS with carbon or ferritic steels

HAZ Heat Affected Zone

ISO International Standards Organization

MIC Microbiologically Influenced Corrosion

NACE National Association of Corrosion Engineers

NZSSDA New Zealand Stainless Steel Development Association

PASCC SCC which can occur rapidly under refinery shutdown or T&I

conditions. Cracking is due to sulfur acids forming from sulfide scale,

air and moisture acting on sensitized austenitic stainless steels.

Phase Balance Percentage of delta ferrite in duplex stainless steel

PMI Positive Material Identification is a chemical analysis that ideally

includes all alloying elements, to determine conformance to specified

alloy composition. It can be used to check all parts, including weld

consumables and fabricated components.

PWHT Postweld heat treated: Solution Anneal and rapid cool

SCC Stress Corrosion Cracking

SDSS Superduplex Stainless Steel

Sensitization Formation of chromium carbides along grain boundaries which causes

loss in corrosion resistance. Sensitization is dependent on the

composition (see sections 9 and 10.2).

Stabilization Stabilization is an alloying method of preventing sensitization.

Stabilized grades with Ti (grade 321) or Nb (grade 347) are commonly

used. Ti and Nb additions preferentially combine with the carbon,

preventing the depletion in chromium.

SSC Sulfide Stress Cracking

SS Stainless Steel




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SSS Super Austenitic Stainless Steel

5 Introduction to Stainless Steels

Stainless steels are selected for service in oil, gas and downstream systems because of

their high level of corrosion resistance in typical producing and refinery environments.

The protective thin chromium-rich passive film forms spontaneously in the presence of

oxygen. Stainless steel grades are specified for ambient, cryogenic and high

temperature services.

The stainless steels addressed in this best practice (BP) are primarily the 300-series

austenitic stainless steels and the duplex stainless steels. Unfortunately, these materials

are not totally resistant to corrosion in other environmental conditions. In particular,

they are sensitive to pitting and crevice corrosion in aerated water environments with

chloride ions present at certain temperature conditions.

Stainless steel piping systems are typically hydrotested with water of varying quality

from potable water through to raw water with or without various chemicals present.

Depending upon the duration of exposure, any of these environments may result in

pitting attack. The resistance to pitting is strongly affected by the stability of the

protective passive film on the stainless steel, which is deleteriously affected in various

ways by welding. Microbes introduced in the hydrotest water can promote MIC as

another form of pitting. This usually occurs at weak points such as heat tint oxides, that

are associated with welding cycles.

Two approaches are therefore used for preventing pitting attack of stainless steels

during hydrotesting and prior to service. The fabricator should make every effort to

optimize the quality of the weld and minimize the reduction of passive film quality.

Also, the commissioning procedures should be optimized to minimize the corrosivity of

the environment for the hydrotest period.

The purpose of this BP is to provide guidance on optimum practice at every step in

order to give the very best confidence in the quality of fabrications, castings and other

products.

6 Classes of Stainless Steel

Stainless steels are broadly defined as steels that contain at least 11% Cr. On the basis

of microstructure, five major families of stainless steels are recognized: ferritic,

martensitic, austenitic, duplex and precipitation-hardenable (PH).

The simplified Alloy Tree (Figure 1) shows the relationship of these types of stainless,

building from a base case of 405 ferritic stainless steel (composition 11.5 – 14.5% Cr,

≤0.08% C). Adding C gives a martensitic structure, adding more Cr and > 8%Ni gives

an austenitic microstructure, adding less Ni and some Mo gives a duplex structure,




Page 7 of 40

finally, adding Cr, Ni and Cu gives a precipitation hardening stainless steel. Austenitic

steels include 316 with Mo to increase pitting resistance, low C and weld stabilized

grades (with Ti [321] or Cb [347]). Higher %C and heat resisting grades are specified

for the highest temperatures, where creep and oxidation are the principal damage

mechanisms (examples 304H and 310).

Ferritic stainless steels are named because their body-centered-cubic (bcc) crystal

structure is the same as iron at room temperature. Their Cr content usually range from

11 to 18%. They are magnetic, have moderate corrosion resistance and are not

susceptible to SCC. Generally, ferritic stainless steels do not have particularly high

strength. Their annealed yield strengths range from 275 to 350 MPa (40 to 50 ksi), and

their poor toughness limits their fabricability and the usable section size

Martensitic stainless steels are similar to Fe-C alloys that are austenitized, hardened

by quenching, and then tempered for increased ductility and toughness. These alloys

are strongly magnetic and their heat-treated structure is body-centered tetragonal. In the

annealed condition, they have a tensile yield strength of about 275 MPa (40 ksi) and are

generally machined, cold formed, and cold worked in this condition.

Austenitic stainless steels comprise the largest stainless family, in terms of number of

alloys and usage. Like the ferritic alloys, they cannot be hardened by heat treatment.

Austenitic alloys are nonmagnetic in the solution annealed condition. Castings, welds

and cold-worked austenitic materials can show appreciable magnetism, due to the

presence of second phases such as delta ferrite or martensite. The austenite structure is

face-centered-cubic (fcc), like the high-temperature (900 to 1400°C) austenite form of

iron. They possess excellent ductility, formability, and toughness, even at cryogenic

temperatures. In addition, they can be substantially hardened by cold work.




Page 8 of 40

Figure 1 – Stainless Steel Alloy Tree

Austenitic stainless steels of the 300 series are the most commonly used, such as 304,

316, 321 and 347 grades (see Table 1). Higher Mn-containing alloys, with Mn

exceeding 4%, are covered under 200 series stainless steel specifications. Examples

used in rotating equipment include UNS S20910 (Nitronic 50) and UNS S21800

(Nitronic 60) for shafts, wear rings, etc. The higher Mn levels greatly improve

resistance to wear including galling (a common problem with 300 series steels).

Duplex stainless steels are Cr-Ni-Mo alloys that contain a balanced mixture of

austenite and ferrite, and are therefore significantly magnetic. Duplex stainless steels

combine the optimum properties of austenitic and ferritic types. Typically, they contain

18 - 26% Cr plus 4.5 to 6.5% Ni. Their duplex structure results in improved SCC

resistance, compared with the austenitic stainless steels and improved toughness and

ductility, compared with the ferritic stainless steels. They can have yield strengths

ranging from 550 to 690 MPa (80 to 100 ksi) in the annealed condition, which is

approximately twice the strength level of either phase alone. Typical applications

include handling chlorinated seawater and for some heat exchanger tubing.

Temperature limits typically range from -50 to +300ºC.

Duplex stainless steels have been utilized for seawater applications, such as in Seawater

Treatment Plants and some refinery heat exchanger tubing. Applications have been

limited by two factors: (1) weldability concerns due to formation of deleterious

Ferritic

405 base: 11 – 12% Cr

< 0.03 %C

Martensitic

410 base:

12 %Cr, 0.1 - 0.2%C

Austenitic

304 base:

18 %Cr, 8% Ni, 0.03 -0.08%C

Duplex

2205 base:

22%Cr, 5%Ni, 3%Mo, N

Precipitation

Hardening

15-5 PH 17-4 PH

316 (Mo-alloyed)

Stabilized (Ti or Cb)

or low C (≤ 0.03) Heat Resistant (310)

& high C grades

Austenitic

304 base:

18 %Cr, 8% Ni, 0.03 -0.08%C

13 Cr

13%Cr, 6%Ni, Mo

A486 CA6NM

Superduplex

PREN ⋝ 40

2507 & UNS S32760

2205 base:

22%Cr, 5%Ni, 3%Mo, N

Superaustenitic

PREN ⋝ 40

UNS S31254

0.03 -0.08%C




Page 9 of 40

intermetallic phases leading to loss of toughness and corrosion resistance, especially

with the superduplex grades; (2) low-to-moderate resistance to SSC.

PH stainless steels may be martensitic, semi-austenitic or austenitic. They combine

the heat treatability of normal martensitic grades with the corrosion resistance of

austenitics. They are available in bar form for the production of heavy duty engineering

components. Various alloying elements, such as Al, Ti, Nb or Cu, are used to achieve

age hardening. They generally form intermetallic compounds, but in S17400 (17-4PH),

fine Cu precipitates are formed. Typical uses of 17-4PH include valve stems, drive

shafts and control valve parts.

“L”, Dual Certified and Stabilized Grades

Types 304L or 316L are the standard grades of stainless steel used for welded

applications. Type 316L with 2 - 3% Mo is more resistant to pitting and crevice

corrosion and is preferred over Type 304L for more severe services. The low C "L"

grade which has a maximum of 0.03% C is required for welded structures to minimize

risk of sensitization. Stabilized grades 321 and 347 are also specified for welded

structures.

Where mechanical strength is important for design purposes, the slightly lower tensile

and yield strengths of the "L" grades should be recognized. It is increasingly common

to encounter Dual Certified Type 304/304L and Type 316/316L stainless steel in

warehouse stock. Dual certification means that it meets the 0.03% C maximum

requirement for the "L" grades and also meets the higher mechanical properties of the

regular grades.

7 Composition, Corrosion Resistance and Sensitization

The resistance of stainless steels to pitting and crevice corrosion in aerated waters is

strongly related to the chemical composition. Steels with higher levels of chromium

(Cr), molybdenum (Mo) and nitrogen (N) are more resistant. The resistance to pitting is

related to the composition using the empirical Pitting Resistance Equivalent Number

(PREN) as follows:

PREN = %Cr + 3.3 x %Mo + 16 x %N

Generally, stainless steels with a PREN value above 40 are resistant to pitting corrosion

in ambient temperature seawater. These steels are sometimes referred to as

superstainless steels (SSSs). Superstainless cast and wrought grades are specified with

PREN ≥ 40 in SAES-G-005, Standard Drawing AD-036821 for highly corrosive

services, such as for pumps handling produced brine water, with high TDS and Cl

levels.

http://standards/docs/Saes/PDF/SAES-G-005.PDF

http://standards/docs/Standard_Drawings/PDF/AD-036821-002.pdf




Page 10 of 40

PREN data relates bulk chemistry to corrosion performance, but this is only a ranking

guideline to compare different alloys. ISO 15156-3 includes PREN ranges for stainless

steels in the austenitic and duplex alloy classes. The PREN does not factor alloy

microstructure, microsegregation or surface condition, all of which strongly influence

alloy performance in practice. Thermal history, including original heat treatment and

welding, are major factors influencing ultimate performance. Consequently, SSS

casting alloys must be in the solution annealed condition after weld repairs.

It is important to ensure that the grades of stainless steels which are used in piping

systems are not mixed up. A section of piping with lower resistance to pitting attack, as

a consequence of its lower PREN value, may be more likely to initiate pitting when

connected to higher alloyed stainless steel.

Weld metal must have a pitting resistance equivalent to or better than that of the parent

metal. This means that even in the root region, with some dilution of the filler metal

composition by the parent metal, the pitting resistance will still be acceptable. For this

reason, superstainless steels such as UNS S31254 are fusion welded using overalloyed

filler metal (Inconel 625). ALERT-93-011 (Saudi Aramco Technical Alert Number 11)

provides additional information and advice concerning seawater service, as follows:

Corrosion damage initiated in the root pass of the UNS S31254 butt-welds.

Field and laboratory investigations indicate the most likely cause of failure was

insufficient (or no) addition of the recommended Inconel 625 filler material during

welding of the root pass.

Table 1 – Some Commonly Used Stainless Steel Grades

Material Nominal Composition Example

Specification (ASTM/SAE)

UNS # / Grade

PREN

Ferritic – 400 series

11 Cr-C≤0.08 13 Cr-C≤0.08

A240 A240

S40500 S41008

10.5 – 12.5 11.5 – 14.5

Martensitic – 400 series

12 Cr 13Cr-4Ni-0.5Mo 13Cr-4Ni-0.5Mo

A240-410 A487 CA6NM A276

S41000 J91540 S41500

11.5 – 14.0 12.8 – 17.3 12.8 – 17.3

Austenitic - 300 series 200 series Superaustenitic

18Cr-9Ni 18Cr-8Ni-2.5Mo 18Cr-8Ni-3.5Mo 18Cr-10Ni-Ti 18Cr-10Ni-Nb 25Cr-20Ni 22Cr-12.5Ni-5Mn-2.5Mo-0.3N 17Cr-8.5Ni-8Mn-4Si-0.15N 20Cr-18Ni-6Mo-N (254SMO) 21Cr-24Ni-6Mo-N (AL-6XN)

A240 S30400 A240 S31600 A240 S31700 A240 S32100 A240 S34700 A240 S31000 A240 XM-19 A240 S21800 A240-S31254 B688

S30400 S31600 S31700 S32100 S34700 S31000 S20910 S21800 S31254 N08367

18 - 20 23 - 28 28 - 33 17 - 20 17 - 19 24 - 26 29 -38 17.3 - 20.9 42 - 45 43 – 49

http://standards/docs/Technical_Alerts/PDF/ALERT-93-011.pdf




Page 11 of 40

Material Nominal Composition Example

Specification (ASTM/SAE)

UNS # / Grade

PREN

Other grades

20Cr-35Ni-2.5Mo-3.5Cu 21Cr-25Ni-4.5Mo-1.5Cu

B464 B673

N08020 (20Cb-3) N08904 (904L)

26 - 31 32 - 40

Duplex Superduplex

22Cr-5Ni-3.5Mo-N 25Cr-7Ni-3.5Mo-N-W 25Cr-7Ni-3.5Mo-N-W 25Cr-7Ni-4Mo-0.3N

A182-F51 A890 Gr 4A A276/A182-F55

A890 Gr 6A A240-S32750

S31803 S32205 J92205 S32760 J93380 S32750

31 - 38 40 - 46 39 - 46 38 - 44

Precipitation Hardening

17Cr-4Ni-3Cu A564 Type 630 S17400 15 – 17.5

Note: The international UNS numbering system uses a letter prefix and numbers to designate specific alloy

grades. In this system: S = a regular stainless steel, N = a Ni-based alloy (including some higher grade stainless steels) and J = a casting specification.

Autogenous welds in UNS S31254 (without Inconel 625 filler) lack sufficient

corrosion resistance to withstand seawater - particularly chlorinated seawater.

Therefore, welding must be performed with the continuous addition of filler.

Sensitization is the formation of chromium carbides along grain boundaries that causes

loss in corrosion resistance. The degree of sensitization is dependent on the

composition of the steel and the time it spends in the temperature range 370°C to 815°C

(700°F to 1500°F). If a steel has been sensitized, corrosion resistance may be recovered

only by the use of a full anneal (and suitably rapid quench) to dissolve the carbides.

Use of low carbon or stabilized grades are methods of preventing sensitization. The (L

grade) low carbon level limits the amounts of chromium carbides that form, thereby

limiting chromium depletion at the grain boundaries. Sensitization can occur in the

HAZ adjacent to the weldment. Sensitization gives rise to corrosion in the HAZ and is

a prerequisite for polythionic acid stress corrosion cracking.

8 Materials Selection

Guidelines and standards that cover selection and specifications of stainless steels

include international standards, company standards and technical guidelines.

ISO 15156-3 presents materials selection for CRAs for sour chloride containing

environments for upstream applications. Sour corrosion issues are different for refinery

applications and NACE MR103 presents materials selection for CRAs for sour refinery

environments. SABP-A-001 (Polythionic Acid SCC Mitigation - Materials Selection

and Effective Protection of Austenitic Stainless Steels and other Austenitic Alloys)

provides practical advice on mitigation of PASCC in refinery environments. This

http://standards/docs/Best_Practices/PDF/SABP-A-001.pdf




Page 12 of 40

damage mechanism may also affect burner tips and other combustion equipment which

are fired on sour hydrocarbons.

For sour services, UNS S41000 stainless steel (410 stainless steel) and other martensitic

grades must be quenched and double tempered to a maximum allowable hardness level

of 22 HRC. PWHT is also required, since this material is strongly air-hardenable.

Austenitic stainless steel grades offer a combination of good mechanical properties and

material performance at both low (cryogenic) and high temperatures (heat resisting

grades), as well as for a range of corrosive environments. Duplex stainless are

attractive since they offer improved mechanical properties compared to austenitics and

better corrosion resistance in saline environments. Duplex limitations include lower

subzero impact toughness (-50ºC limit) and generally lower tolerance to sour

environments. API TR 938-C (Use of Duplex Stainless Steels in the Oil Refining

Industry-First Edition, 2005) provides advice on duplex steels in refining.

Wrought S17400 stainless steel is permitted for sour service but must be carefully

processed to prevent SSC. ISO 15156-3 specifies two different acceptable heat

treatments for S17400. The maximum hardness level is 33 HRC for both conditions.

Solid austenitic stainless steels are used for cryogenic and other applications, but should

normally be avoided for services above 60ºC, when internal stainless steel cladding is

recommended. Above 60ºC, there is an increased risk of either process-side or external

chloride SCC.

9 Castings

The chemical composition of cast stainless grades is adjusted to increase fluidity and

minimize defects such as porosity and hot tears. Sensitization can occur in the HAZ of

weld repaired areas of higher carbon austenitic grades especially, lowering corrosion

resistance by intergranular attack, which can also promote PASCC.

ASTM A744 requires that castings shall be PWHT (solution anneal and rapid cool)

after all major weld repairs and after those minor weld repairs involving either of the

following conditions: (1) welding on a wetted surface, or (2) welding that heats a wetted

surface to or above 800°F [425°C]. PWHT is omitted from ASTM A743 but is

available as a Supplementary Requirement. ASTM A744 is preferred if non-L grade

steels are specified or allowed. Foundries often make “minor” weld repairs without

PWHT and this practice is generally satisfactory if L (lower carbon) grades are used.

PWHT (solution anneal and rapid cool) should be considered for duplex stainless steel

castings that have been weld repaired. Solution treatment requirements are provided in

ASTM A890.




Page 13 of 40

10 Fabrication

10.1 Welding and Corrosion Resistance

When stainless steel piping or components are manufactured they are normally

subjected to a final high temperature solution heat treatment (typically at

1050ºC) followed by water quenching. This leaves the material in the solution

annealed condition which is the optimum for corrosion resistance. This heat

treatment gives a poor quality high temperature oxide film on the surface which

is quite thick, porous and cracked. During manufacturing, this oxide film is

normally removed by pickling and passivating the surface in oxidizing acids.

The oxide film which is formed at low temperature, either in the passivating

solution or just by exposure to air, is thin and dense, giving good protection to

the stainless steel. The as-delivered piping is therefore in an optimum condition,

with a solution heat treated microstructure and a well-passivated surface oxide

finish.

When welding is carried out, the metal close to the weld is reheated to high

temperatures, from the melting point at the fusion line to lower temperatures

through the HAZ. An unprotected HAZ which is exposed to air during welding

will therefore show a high level of oxidation. Characteristically, it would be

black in color and in the worst condition may be visibly thick and porous. This

oxide is far more easily broken down than the original passivated surface and

there is a zone of metal under this oxide which is lower in chromium content

relative to the bulk metal.

Therefore, the HAZ has to be protected from re-oxidising at high temperature

during the welding process. This is done by shielding around the weld using an

inert gas to exclude air, thus preventing oxidation whilst the weld metal and

HAZ is hot. Once the weld is cooler, the HAZ will re-oxidise slightly at lower

temperatures but this oxide film is a more protective film than the high

temperature oxide film. This „heat tint‟ oxide formed at the HAZ at lower

temperatures is so thin that it forms interference colors, forming characteristic

blue or rainbow colors. AWS D18.2:1999 addresses factors affecting weld

discoloration inside a 316L austenitic stainless steel tube. It presents a color

guide (Figure 2) relating degree of discoloration to oxygen content in the

backing shielding gas. The heat tint oxide can be identified by a number

corresponding to the oxygen level in the shielding gas. A straw-to-yellow heat

tint (chart no. 3) is considered desirable but a blue tint up to chart no.6 is

acceptable for standard applications.

Heat tint exceeding no. 6 is normally removed by grinding, pickling and

passivating the surface in oxidizing acids. Where welds are accessible,

excessive heat tint can be removed by pickling and passivating the surface using




Page 14 of 40

standard products. Surfaces should be fully rinsed with clean water to ensure

removal of any residual acids on the surface.

Figure 2 – Weld Discoloration and Heat Tint Numbers

Scanning electron microscopy examination of heavily heat tinted surfaces has

revealed a craze-cracked oxide surface that is enriched in Cr (Figure 3).

Stainless steel lines have failed prematurely in service at heavily heat-tinted

HAZ locations, which was attributed to the heat tint plus poor quality hydrotest

and contributory MIC factors. The craze-cracked surface topography may

promote bacterial colonization.

10.2 Welding of Stainless Steels

Introduction

The purpose of this section is to give information on the correct welding of

stainless steels. It is intended for internal Saudi Aramco operations and

maintenance applications. It also provides guidance for inspection in fabrication

facilities. This section is divided into two parts. The first part gives information

and techniques that are pertinent to the general classifications of stainless steels.

The second part gives consumables and techniques that are specific to individual

grades of stainless steel. This best practice gives general information. If there is

a conflict between this Best Practice and an approved welding procedure then

the approved welding procedure shall govern. All conflicts shall be brought to

the attention of CSD.

10.2.1 Welding Processes

The welding of stainless steels may be carried out using almost any

welding process including all arc welding (Shielded Metal Arc, Gas

Tungsten Arc, Gas Metal Arc, Flux-cored Arc, Submerged Arc and

Plasma Arc), friction, resistance, laser and electron-beam welding

techniques. Stainless steels should not be welded with the oxy-fuel




Page 15 of 40

welding processes. Within Saudi Aramco we only use two welding

processes: Shielded Metal Arc (SMAW) and Gas Tungsten Arc

(GTAW). Fabricators and contractors may use a wider range of welding

processes depending on their needs. The welding consumables for the

commonly found stainless steels are given in Table 2. The GTAW

process is always used for root passes on butt welds and for butt welds

on pipes smaller than 2" diameter. The GTAW process can also be used

for socket welds. SMAW may be used for the fill passes on butt welds

in pipes larger than 2" diameter and all socket welds.

SMAW Welding

The SMAW welding electrodes for stainless steels are given in AWS

A5.4 specification. Generally, the number of the SMAW welding

electrode is equivalent to the type of stainless steel. For example E316L

electrode is for type 316L stainless steel. SMAW electrodes for stainless

steels come with four usability classifications. These are indicated by

the two numbers following the alloy designation -15, -16. -17 and -26.

For example, E316L-16. The significance of these usability

classifications are as follows:

-15. This is a lime (also called basic) coated electrode. These electrodes

are usable with DCEP (electrode positive) only. Electrode sizes

5⁄32 in. [4.0 mm] and smaller may be used in all positions of

welding. These electrodes tend to weld easier out of position but

are not as smooth. They give superior impact properties at

cryogenic temperatures.

-16. This is a lime - titania (also called basic–rutile) covered electrode.

Electrode sizes 5⁄32 in. [4.0 mm] and smaller may be used in all

positions of welding. This produces a smoother weld bead

appearance than the -15 but is more difficult to weld out-of-

position and a smaller diameter should be selected.

-17. This is a lime - silica - titania covered electrode (also called Acid–

Rutile). This electrode produces a smooth, concave weld bead. It

is generally is not as weldable out-of-position as the -16.

-26. This designation is for those electrodes that are designed for flat

and horizontal fillet welding and that have limited out of position

characteristics. Electrodes with the -26 designation are

recommended for welding only in the flat and horizontal fillet

positions.




Page 16 of 40

Electrode care, storage and conditioning

Although hydrogen cracking is not a major problem in austenitic steels,

electrodes should not be exposed to humid environments. It may be

possible to recover moist electrodes by baking, however, this is not a

general rule and the electrode supplier should be contacted for advice on

a case by case basis. It is very important to keep electrodes for

dissimilar metal welds in a low hydrogen condition. Failure to do this

can result in hydrogen cracking on the non-austenitic side of the joint.

Dissimilar metal welds are not permitted in sour service exposure.

Dissimilar metal welds in hydrocarbon service must be made with a

nickel based consumable.

SAW Welding

The high heat input from SAW leads to larger weld beads and slow

cooling rates. This may lead to problems with hot cracking. Hot

cracking is caused by the segregation of minor elements to the liquid

phase during freezing. One of the elements that can cause this

phenomenon is silicon and, due to the slow cooling rates from SAW,

silicon pickup from the slag can be quite significant. Ferrite control in

submerged arc welds is therefore very important, but this may be

problematic due to the difficulty of control over dilution in the weld.

Base metal dilution during SAW can vary between 10 and 75%, more

than any other arc welding process. Careful control must be maintained

over the arc during welding as small variations in penetration can greatly

affect the dilution levels. Electrode wire is readily available from a

number of different suppliers. Flux for SAW is usually designed to be

either basic or neutral with respect to the steel; however, no AWS

specification exists to cover these fluxes. Neutral fluxes give sound weld

deposits and permit some oxidation and loss of alloy to the flux. Basic

fluxes contain additions of alloying elements that are imparted to the

weld when molten. Fluxes can be susceptible to pickup of moisture and

should be baked as per the manufacturer's instructions prior to use.

Moisture in the flux can lead to defects such as wormholes and porosity.

GTAW Welding

GTAW (Gas Tungsten Arc Welding also referred to as TIG) is similar to

Gas Metal Arc Welding (GMAW) in that the arc is protected by an inert

gas. An arc is established between a tungsten electrode and the work

piece. The filler material is added separately into the molten weld

puddle. Since the filler material does not form part of the arc, there will

be extremely low loss of alloying elements during welding. All grades

of weldable stainless steel may be welded using GTAW welding, giving




Page 17 of 40

a high quality weld. GTA welding has extremely wide applicability and

can be used to weld in all positions. It may be used to weld any

thickness of material; but is most suited to thin sections. Only inert gases

are used during GTA welding. Argon is primarily used for both

shielding the arc and back-purging. Helium shielding provides deeper

penetration, which is useful for thicker sections. Contact between the

electrode and the weld pool is to be avoided to avoid tungsten

contamination. Back-purging to protect the root of the weld is important

when GTA welding stainless steels to control oxidation of the weld area.

Argon is typically used for this purpose.

FCAW Welding

Electrodes for FCAW are designated according to AWS specification

A5.22. This designation takes the form EXXXTY-Z wherein XXX

denotes the chemical specification of the steel (e.g., 308), Y denotes the

applicable positions for welding (1 for all position and 0 for flat or

horizontal only) and Z the shielding gas (1 for CO2; 3 for self-shielding,

(i.e., no gas); 4 for 75 - 80% Ar in CO2 , and G for unspecified). It is not

unusual for bismuth to be added to the flux of flux cored arc welds; this

facilitates ease of flux removal after welding. However, bismuth

additions should not be employed for components which will operate at

high temperatures or require a postweld heat treatment. This is due to

the link between bismuth and reheat cracking susceptibility. Saudi

Aramco does not approve the use of self-shielded (No shielding gas)

FCAW welding for stainless steel.

Electroslag Welding

Electroslag is frequently used for overlay welding in fabrication shops.

It has very low dilution and can be used for single or multi-pass overlays.

Austenitic Stainless Steel Properties Affecting Welding

The coefficient of thermal expansion for the austenitic stainless steels is

almost 40% greater than that of carbon steel. This means that distortion

is generally more of a problem with austenitic stainless steels and must

be considered during welding.

The thermal and electrical conductivity of austenitic stainless steel is

lower than that of carbon steel. Less welding heat is required to make a

weld because the heat is not conducted away from a joint as rapidly as in

carbon steel. This also makes the weld puddle more fluid and difficult to

control.




Page 18 of 40

Purging and Shielding Gas

A chromium oxide layer forms when stainless steel is exposed to air and

normally acts as a thin passive layer. This layer becomes thicker at

higher temperatures. This chromium oxide can adversely affect the

corrosion resistance of the weld and heat affected zone. The chromium

oxide has a very high melting point (2435°C). This is much higher than

the stainless steel base or weld material which is nominally 1450°C. It

will interfere with the fusion of the welding process and cause an

unacceptable weld profile. For these reasons the weld and Heat Affected

Zone (HAZ) must be protected from the air by an inert gas. This gas

protection is known as a shielding gas and a purge gas.

Areas that have been exposed to high temperatures but did not have, or

had insufficient, gas shielding will form a 'heat tint‟. This heat-tinted

area will have comparatively poor corrosion resistance when compared

with standard plate due to the heat tint oxide giving little protection to

the base metal underneath, along with the base material being depleted of

chromium immediately under the tint. The corrosion resistance of the

component can only be restored by removing the oxide and chromium

depleted layer. A three-step process of grinding, pickling and rinsing is

best. (section 10.5) Pickling pastes or baths should be used at such

strength and duration so as to remove fully the oxide and chromium

depleted layer, but not stain or unduly corrode the surface. Rinsing

solutions should, preferably, be de-mineralized water.

Argon is normally used as the shielding gas on the torch for the GTA

welding process. Argon/Helium and helium gas can also be used but are

not common.

The SMA welding process does not require a shielding gas because it

generates its own protective shielding from the flux during welding.

For groove welds in pipe the back side of the weld must be protected by

a purging gas. Argon is normally used as the purging gas.

Safety Caution

Argon is heavier than air. Though it is not toxic it can cause death by

asphyxiation. Always be aware of this when purging in tanks, vessels or

other areas where argon can accumulate. Never enter a pipe if it has a

purge running.

For the purge to be effective it must reduce the oxygen content on the

back-side of the weld to less than 0.05% (500 ppm). Purge dams must




Page 19 of 40

be installed to contain the purge gas. See the sketch below. There are

many types of purging dams (i.e., cardboard, plastic, wood, balloons, and

water soluble paper.) There are commercially available purging devices

which are recommended because they seal well and reduce the purging

volume. It is critical that the purge dam is tight and can retain the

purging gas. It will not be possible to achieve a good purge if the purge

dam is porous or leaks. The purging volume should be kept as small as

possible.

Figure 3 – General Purge Dam Configuration

Water soluble paper is porous and several layers of it must be glued

together to make it gas tight. Water soluble paper must be installed with

water soluble tape and water soluble glue. Water soluble dams should be

the last choice for purging dam material.

If a purge oxygen monitor is not available then the following table can

be used as a guideline for minimum purging time. This table is only

applicable if there is no leakage of gas through the purge dam. If this

table is used, then the inside of the first portion of root welded must be

examined through the root gap to make sure that there is no excessive

oxidation.

The flow rate shown for this table is 50 CFH (22.5 L/H). Once the purge

has been established the flow rate should be reduced to 10CFH or just

enough to maintain the purge level. If there is excessive purge flow it




Page 20 of 40

can buildup pressure on the inside of the pipe and cause the root to be

defective or unweldable.

Purge Times for Stainless Steel Pipe

Diameter Purging time

Minute

2 and less 0.5

4 1

6 2

8 3

10 4

12 6

16 10

18 12

24 22

Minimum purging time based on pipe size and 6 volume changes of gas

The above table assumes use of argon gas at a flow rate of 50 CFH

(22.5 L/H). Listed times are for each 300 mm of pipe length to be

purged (multiply by actual length). Use the above values for 300 mm for

any shorter length.

Preheat, Interpass Temperature, Heat Input and Post Weld Heat Treatment (PWHT)

The following section provides general information on the preheat,

interpass temperature, heat input and PWHT for stainless steel materials.

The governing code, Saudi Aramco Standards and approved welding

procedure must be followed.

Austenitic, Super-Austenitic, Duplex and Precipitation Hardening Stainless

Austenitic, super-austenitic, duplex and precipitation hardening stainless

steels are susceptible to sensitization and secondary phase embrittlement

when exposed to temperatures in the range of 450°C to 850°C. For this

reason preheat is not used for these materials. They can be heated to

remove moisture prior to welding if required.

To limit sensitization and embrittlement, the maximum interpass

temperature is restricted for these materials. For types 304L and 316L




Page 21 of 40

the interpass temperature shall not exceed 177°C. For all other stainless

steels in this group the interpass temperature shall not exceed 100°C.

The heat input for these materials must also be kept as low as possible.

For the duplex materials there is a heat input range that is maintained in

order to obtain the “duplex” microstructure in the weld. This range will

be indicated on the welding procedure. GTA Welding without the

addition of filler material is expressly prohibited for this material. This

can reduce the corrosion resistance and lead to cracking. Sometimes

welders will want to weld without filler consumable to smooth a rough

weld profile. This should never be done. Filler material must always be

added or the area should be ground.

Post weld heat treatment (PWHT) is not normally recommended for

these materials because it can cause sensitization. However, the “L”

grades and types 321 and 347 will not be sensitized by a normal PWHT.

If heat treatment must be performed it is often specified as a Solution

Anneal heat treatment. This treatment involves heating the material to a

temperature of approximately 1000°C to 1200°C. This relieves the

stresses from forming and welding and is above the temperature at which

sensitization occurs. The material is then quickly cooled through the

sensitization range.

1. Ferritic Stainless Steels

Preheat must be used for the ferritic stainless steels. This is because

these materials can be hardened due to the fast cooling from the

welding temperature. 250°C preheat should be used. The Interpass

temperature is restricted to 315°C. The heat input is not a serious

concern for these materials. PWHT is not normally required for

these materials unless the thickness exceeds 38 mm.

2. Martensitic Stainless Steels

These materials will readily form martensite when air cooled. The

preheat temperature range is from 250°C to 450°C. After welding

they should be slow cooled to 120°C (martensitic transformation

temperature) and then tempered at 750°C. The Interpass temperature

is restricted to 315°C. The heat input is not a serious concern for

these materials.

Contamination

As discussed further in section 10.4, it is important to avoid

contamination of the stainless steel with particles of carbon steel or

elements which can adversely affect the corrosion resistance or cause




Page 22 of 40

cracking. Stainless steel wire brushes must always be used. All grinding

discs must be halogen, sulfur and iron free as indicated by a statement

“for stainless steel.” Brushes and grinding wheels used on stainless

steels should either be new or only previously used on similar stainless

steels. Brushes and grinding disks that have been used on carbon steel

must never be used on stainless steel. Stainless steel must be fabricated

in a separate shop from carbon steel. One of the greatest sources of

carbon steel contamination is through grinding dust from the carbon

steel. This can be embedded into the stainless steel by walking on the

stainless steel with contaminated shoes, contaminated lifting equipment

and contaminated tools.

Other contamination can be from grease, paint and dirt. These can cause

significant pickup of carbon and other elements which may affect the

corrosion resistance. Before any welding operation, it is essential to

ensure that all of the components are clean. If a multipass weld is being

produced, all slag and surface oxide must be removed between passes.

Copper, zinc, tin, lead and other metals may be picked up from

fabrication tools and can lead to cracking. These elements are strongly

linked to hot cracking due to the formation of low melting point alloys.

Ferrite In Weld Deposits

One of the problems associated with welding of austenitic stainless steels

is that of solidification cracking. This problem is often referred to as 'hot

cracking' since it occurs before the weldment has cooled. The presence

of ferrite in the weld reduces the susceptibility of the metal to hot

cracking. This ferrite is referred to as “Delta ferrite” and can be

observed in the as-deposited microstructure of austenitic stainless steels

welds. This is a product of the solidification and transformation

sequence experienced at elevated temperature. For certain alloys it is not

essential to have ferrite in the weld deposit but normally 3 – 7FN ferrite

will prevent cracking. Cracking is more of a problem when the welds

are restrained or the joints are large. Ferrite increases the weld strength

level. Ferrite may have a detrimental effect on corrosion resistance in

some environments. It also is generally regarded as detrimental to

toughness in cryogenic service. In high-temperature service delta ferrite

can transform into the brittle sigma phase. Materials with a ferrite

number greater than 10 will be susceptible to the formation of excessive

quantities of sigma phase and should not be used for elevated

temperature service (i.e., at temperatures exceeding 650°C).

Ferrite can be measured on a relative scale by means of various magnetic

instruments. Ferrite percentage can also be determined by




Page 23 of 40

metallographic examination. The amount of ferrite in austenitic stainless

steel welds can also be predicted from the chemical composition of the

weld deposit using one of several constitution diagrams. These diagrams

use nickel and chrome equivalence empirical relationships. These

diagrams are known as the Schaeffler, DeLong, and the Welding

Research Council (WRC). The WRC diagram should be used for ferrite

predictions and gives a ferrite number. Ferrite can be expressed as a

“percent ferrite” or a ferrite number (FN). The relationship between the

percent ferrite and ferrite number is complex. It is based on the cooling

rate and alloy interaction. TWI has weld simulation software online that

can predict the percentage ferrite

http://www.twi.co.uk/j32k/protected/toolkits/Ferrite/IntroInstructions.html.

Two rule-of-thumb conversions from FN to percentage are:

1. For low percentages of ferrite (less than 10) the FN is the same as

the percentage.

2. For higher percentages (duplex)

0.70(FN) = % ferrite

Measurement of ferrite content in DSS, however, is currently performed

with a magnetic measuring technique and is more commonly referred to

in terms of percentage.

SAES-W-016 gives ferrite measuring requirements for welding

procedure qualification and production and special requirements for

duplex materials. SAES-W-014 gives ferrite requirements for overlays.

http://www.twi.co.uk/j32k/protected/toolkits/Ferrite/IntroInstructions.html






Page 24 of 40

Figure 4 – WRC Ferrite Number

Weld Overlay Cladding

Weld overlay cladding is the process of depositing a layer (or layers) of

austenitic stainless steel (or other corrosion or wear resistant material)

onto the surface of a base material (such as mild steel). This layered

component can provide corrosion resistance and/or wear resistance

without the expense of producing it entirely from a higher alloy material.

Submerged arc welding is by far the most common method of cladding

with austenitic stainless steel. SAW is ideally suited to the task since

relatively large areas may be deposited with high deposition rates.

Electroslag welding is often frequently used. Less frequently are GTAW

and GMAW and FCAW welding processes. There is a variety of

different equipment available, employing either strip or between one and

six wires. Dilution of the weld pool with molten base material may be a

problem during cladding since the underlying materials may contain

elements detrimental to the performance of the stainless steel (e.g.,

carbon). In these instances, dilution could be minimized but this may be

difficult since it is very sensitive to arc length and voltage.

Alternatively, multiple layers can be deposited so that dilution with the

parent metal is reduced in each progressive pass. Multiple layer overlays

are preferred and single layer overlays require special approval.

Joining of Clad Materials

There are two techniques for joining of carbon steel which is clad with




Page 25 of 40

stainless steel: Single-sided and double-sided:

1. Single-sided

When welding clad materials from one side (for example a small

diameter pipe joint) the entire weld will be made with a

consumable that is compatible with the dissimilar metal welding of

the base material and the CRA.

Figure 5 – Single-sided Cladding

2. Double-sided

The entire weld is made from both sides (See Saudi Aramco

Drawing AB-036367). The stainless steel must be “stripped back”

by grinding or machining to no closer than 10mm from the edge of

the carbon steel weld. The “stripped back” area shall be checked

with a copper sulfate solution to verify that all of the stainless steel

has been removed.

Copper sulfate solution preparation:

4 gm copper sulfate pentahydrate CuSO4.5H20 (use reagent grade).

250 ml of distilled water.

1 ml sulfuric acid H2SO4 (specific gravity 1.84).

The solution is swabbed on the part, and left to stand for 6 minutes

minimum, the part should be carefully rinsed and dried. If copper

deposit is observed, then free iron is on the surface of part.

(Reference ASTM A967)

The carbon steel shall be completely welded and NDE inspected

prior to welding the cladding portion.




Page 26 of 40

Figure 6 – Double-sided Cladding

Caution: Never weld carbon steel or low-alloy steel into high alloy, stainless steel or nickel base material. Never weld stainless steel electrode into nickel base material.

Welding of Duplex and Superduplex

Duplex stainless steels (DSS) are broadly divided into standard and

superduplex stainless steels. These materials are characterized by high

strength, good corrosion resistance, good resistance to chloride stress

corrosion cracking and relatively good notch toughness. Duplex

stainless steels (DSS) contain 35 to 65% ferrite and the remainder is

austenite. The percentage of ferrite is also called the phase balance.

Duplex stainless steels that are currently produced tend to have slightly

more austenite that ferrite. The table below gives an approximate

comparison between the chemistry of the Duplex and the Superduplex.

Figure 7 – Duplex Microstructure, magnification approximately 200X




Page 27 of 40

Table 3 – Typical Chemistry of Duplex

and Superduplex Stainless Steels

Cr (%) Ni (%) N (%) Mo (%) Fe

Standard Duplex Chemistry

22 5 0.12 3.2 Balance

Superduplex Chemistry 25 7 0.25 3.5 Balance

Besides the phase balance, there is a second major concern with duplex

stainless steels. Intermetallic phases (Sigma and chi) form at the

temperature range of 540 to 950°C. These phases can significantly

reduce the toughness and corrosion resistance of these materials. These

phases can be eliminated and the original properties restored if the

material receives a solution anneal heat treatment at 1040°C and is

rapidly cooled through the critical temperature range. The addition of

nitrogen significantly delays formation of these phases. Therefore, it is

critical that sufficient nitrogen be present in these alloys.

The characteristics of welding duplex stainless steel are very similar to

welding 300 series stainless steels. The same welding processes, joint

details and techniques are generally followed. Some of the differences

are noted below.

There is an increased emphasis on heat input control. Both the upper and

lower ends of the heat input range must be established on the welding

procedures in order to achieve the proper phase balance. Welding at

lower heat inputs promotes higher ferrite levels; whereas welding at

higher heat input levels promotes lower ferrite levels. The heat input is

based on the following formula. Generally, the range on heat input for

duplex stainless steel is 0.5 to 2.5 KJ/mm but the heat input restrictions

on the welding procedure must be followed.

dTravelSpee

voltsampsHeatInput

60**

Preheating is not recommended with duplex stainless steels except to dry

the surface or when the temperature is below 5°C [40°F], or when

welding heavy sections under restraint.

The maximum interpass temperature for duplex stainless steels is 100°C

per the Saudi Aramco specification SAES-W-016. The low interpass

temperature reduces the amount of sigma formation in the HAZ.





Page 28 of 40

Autogenous welding (welding without filler material) should not be

performed on duplex stainless steels.

DSS welds tend to be higher in ferrite than the base metal being welded.

There are near matching composition proprietary filler metals for many

of the standard and super DSSs. Suggested filler metals are shown in

Table 2. It is also advisable to consult the alloy producer of proprietary

alloys for their filler metal recommendations and filler metal availability.

Nickel alloy filler metals have also been used to weld both the standard

and super DSSs. One such nickel alloy filler metal for which data is

available is AWS A5.14 ERNiCrMo-3. Since the nickel alloy welds are

fully austenitic, there is no concern regarding obtaining a balance of

austenite and ferrite. This material meets the strength, corrosion

resistance and impact resistance of the base materials.

Dissimilar metal welds between DSS and austenitic stainless steels such

as Type 304 or 316 or for welding to carbon steel should be made with

ENiCrMo-3 or ERNiCrMo-3.

Because of the ferrite present in the duplex stainless steels there is a

possibility of delayed hydrogen cracking. For this reason all welding

electrodes must be handled to insure that they are low hydrogen. All

surfaces to be welded must be free from contaminants that could cause

hydrogen pickup.

Dissimilar Metal Joints

Dissimilar metal weld joints are welds between stainless steel and

another type of alloy. This is frequently stainless steel welded to carbon

steel or a low alloy steel. Consideration must be given to the possibility

of galvanic corrosion in this type of joint. It is very important to keep

electrodes for dissimilar metal welds in a low hydrogen condition.

Failure to do this can result in hydrogen cracking on the non-austenitic

side of the joint. Dissimilar metal welds are not permitted in sour service

exposure. Dissimilar metal welds in hydrocarbon service must be made

with a nickel base consumable, such as E/ERNiCrMo-3. Austenitic

stainless steel (i.e., type 309) may only be used for dissimilar metal

welds in non-hydrocarbon, non-sour service (i.e. water, air or steam) or

external attachments.

Sensitization

Sensitization occurs in the HAZ adjacent to the weldment and may be

minimized by using either low carbon or stabilized grades of stainless




Page 29 of 40

steel. If a steel has been sensitized, corrosion resistance may be

recovered only by the use of a full anneal (and suitably rapid quench) to

dissolve the carbides. Some degree of corrosion resistance may be

recovered by a stabilization anneal, typically in the region of 900°C.

Sensitization gives rise to corrosion in the HAZ and is a prerequisite for

polythionic acid stress corrosion cracking. Low carbon grades or grades

with Ti or Nb stabilization can prevent or reduce sensitization.

Embrittling Phases

Another phenomenon similar to sensitization is the precipitation of Chi

and Sigma intermetallic phases on the grain boundaries in the HAZ.

The precipitation of any secondary, chromium-rich phase will deplete the

local area of chromium.

The classic sigma-phase is nominally FeCr composition, but it can have

a more complex, variable composition. High molybdenum high strength

stainless steels can contain the Chi phase (Fe36Cr12Mo10). The presence

of this phase, which normally occurs at grain boundaries, depletes the

chromium content leading to intergranular corrosion. This may cause

alloy embrittlement during long term use. The presence of such phase

has proven to be highly sensitive to alloy processing parameters such as

the cooling rate after a final heat treatment. Both and phases can be

easily formed by the decomposition of ferrite, at 540-950°C for

phase and 650-950°C for phase. If these precipitates form a

continuous network there can be a corresponding reduction in ductility,

toughness and corrosion resistance. Recent alloy developments have

included the addition of significant amounts of nitrogen to high alloy

stainless steels, this acts to retard the nucleation of both and phases,

meaning that thick plate can be successfully multi-pass welded.

Microsegregation

Pitting of the weldment, independent of any precipitation, may occur

where micro-segregation and coring within dendrites is particularly

severe. This can often be the case if a matching filler, autogenous weld

or high molybdenum stainless steel (4-6% Mo) is used or if there are

large surface-lying dendrites. This is the reason that high molybdenum

grades must be welded with nickel base fillers.

Crevice Corrosion

Crevice corrosion is another frequent cause of failure in weldments.

Possibly the most frequent cause of crevice corrosion are solidification

cracks and microfissures. Smaller microfissures are often invisible to the




Page 30 of 40

naked eye and therefore microfissure crevice corrosion is often mistaken

for simple weldment pitting. As previously stated, solidification cracks

may be avoided by proper control of weldment delta ferrite content. Any

welding defect that causes a crevice or confined feature can also lead to

crevice corrosion. Such defects include flux layers or inclusions, pores,

and weld start/stop cracks and craters. Moreover, poorly adherent weld

spatter may also create crevices. Bad joint design and/or poor welding

practice can also cause crevice corrosion; for example, non-removable

backing rings are to be avoided or, if essential, should be fully consumed

during welding. Failure to do this may result in crevices being formed

between the component and the backing ring. Microfissuring and

crevice corrosion in high molybdenum alloys (4-6%) is best avoided by

the use of nickel base fillers.

10.2.2 Fume and Welding Safety

Normal safety considerations should be taken when welding austenitic

stainless steels. Austenitic stainless steels also generally contain nickel

and this may lead to sensitization of the skin. Sensitization may be as

minor as mild irritation, or as major as gross swelling. Particular care

should be taken to minimize exposure to weld fumes. Fumes from the

welding of stainless steels may also contain significant quantities of

nickel and hexavalent and trivalent chromium. Chromium is toxic by

inhalation; therefore, welding should only be carried out using proper

fume extraction. Chromium and nickel are both known carcinogens and

typically have maximum exposure limits as specified in the MSSG. It

should be noted that MIG and TIG welding can also give rise to ozone;

this also necessitates suitable ventilation.

Table 2 – SMAW And GTAW Consumable Selection, Preheat and PWHT

Austenitic Stainless Steels

Parent Material Type

Filler Material

SMAW Electrode

GTAW Bare Wire

Preheat PWHT Comments

304 and 304L E304L ER304L 20° and dry none

304H E304H ER304H 20° and dry none

316 and 316L E316L ER316L 20° and dry none

317L E317L ER317L 20° and dry none

310 E310 ER310L 20° and dry none

321 E347 ER347 20° and dry none

330 E330 ER330 20° and dry none ENiCrFe-3 and ERNiCr-3 can also be used




Page 31 of 40

Austenitic Stainless Steels


Filler Material

SMAW Electrode

GTAW Bare Wire


347 E347 ER347 20° and dry none ENiCrFe-3 and ERNiCr-3 can also be used

Martensitic Stainless Steels


Filler Material

SMAW Electrode

MIG, TIG, SAW

Bare Wire Preheat PWHT Comments

410 E410 ER410 250°C required

430 E430 ER430 250°C required

Ferritic Stainless Steels


Filler Material

SMAW Electrode

GTAW Bare Wire


405 E309 ER309 250°C Not normally

required

410S E309 ER309 250°C Not normally

required

Superaustenitic, Precipitation Hardening


Filler Material

SMAW Electrode

GTAW Bare Wire


904L E385 ER385 20° and dry none E/ER NiCrMo-3 may be used

254SMO ENiCrMo-3 ERNiCrMo-3 20° and dry none

Alloy 20 ENiCrMo-3 ERNiCrMo-3 20° and dry none

17-4 PH ENiCrMo-3 ERNiCrMo-3 20° and dry none

Duplex and Super Duplex


Filler Material

SMAW Electrode

GTAW Bare Wire


S31803 S32205 J92205

E2209 ER2209 20° and dry none

S32550 S32750 S32760

CD-4MCu (cast)

E2595 ER2594 20° and dry none E/ERNiCrMo-3 may be used




Page 32 of 40

10.3 Storage

Storage areas must be clean, dry and well ventilated. Absorbent wrappings such

as interleaving paper, cardboard and timber should be kept dry to prevent

surface staining. Plates and sheets should be stored vertically in racks and not

be dragged out of the racks or over one another. Racks should be protected to

prevent iron contamination. Outdoor storage of stainless steels adjacent to

carbon steels should be avoided (Figure 9).

Figure 8 – Microcracked Surface at Heat Tint

Oxide Films on 316L Stainless Steel, x200.

Figure 9 – Unsuitable Materials

Separation and Outdoor Storage

10.4 Shop Fabrication

The number one problem with unsuccessful fabrication of stainless steel is

surface contamination. The shop should have a separate area where only

stainless steel is fabricated. This will prevent the cross-contamination of the

stainless steel surface with iron particles.

NiDI publication 11003 highlights that: “experience has shown that (surface)

cleanliness and weld quality are far more critical to successful performance than

Cl-ion concentration”. Some suggested methods for the removal of surface

contamination and defects are presented in Table 1. NZSSDA (Code of Practice

for the Fabrication of Stainless Steel Plant & Equipment, 2001) stresses that the

fabricator must ensure that the fabrication is clearly identified and protected

from damage and contamination. “Mild steel lifting forks, hooks, chains and

wire ropes shall be kept from coming into contact with stainless steel

equipment”. Plastic-coated or SS weld overlayed parts should be used for

handling SS. Cleaners which can be used include stainless steel wool and

stainless steel wire brushes, provided that they have not previously been used on

non-stainless steels.




Page 33 of 40

Figure 10 shows an unacceptable example of poor material segregation, where

carbon and stainless steels are being processed together. Figure 11 shows an

example rust spotting caused by iron contamination.

Figure 10 – Poor Shop Material Segregation

Figure 11 – Iron Contamination Caused by

Operator Grinding Carbon Steel Nearby

10.5 Pickling, Passivation and Iron Removal

Chemical passivation is normally only used to remove “free iron”, high

temperature oxides and other surface contamination arising from processing and

handling. Pickling treatments provided in ASTM A380 include the following:

Typical austenitic stainless steel pickle solution: 10% HNO3 + 2% HF at

50ºC

Removes oxide film plus 25 – 40 μm of surface

Effectively removes welding heat tint

Removes embedded iron particles

Can improve corrosion resistance of ground, wire brushed and blasted

surfaces by removing surface contamination and exposed impurities in the

metal, such as sulphides

Exposes a new, clean surface which can then be passivated

Air passivate or be chemically passivated

Pickling is normally performed during manufacturing processes, such as

following solution heat treatment of welded stainless pipe. The parts are picked

by immersion in a bath and thoroughly rinsed and blown air dried afterwards.




Page 34 of 40

Pickling can also be performed after shop or site welding operations, using

proprietary pickle pastes or swabbing followed by thorough rinsing in clean

water. Chemical passivation, using nitric acid treatments, etc., is also normally

used during manufacturing after pickling treatments.

Pickling with nitric-hydrofluoric acid removes free iron and a thin surface layer

of metal that may contain surface defects. The metal surface is then passive and

in the most corrosion resistant state. If a nitric-hydrofluoric acid pickle is not

practical, the free iron can be removed mechanically. Acceptable methods

include the use of medium to fine-grit abrasives such as clean flapper wheels,

flexible disks or blasting with clean abrasives such as glass beads, garnet or

walnut shells. Free iron and heat tint can also be removed by a hand-held

electropolishing probe.

The “Rust Bloom” water wetting and drying procedure described in ASTM

A380 para 7.2.5.1 is an effective test to check for the removal of free iron. The

procedure calls for wetting the surface with distilled or deionized water or fresh

water followed by drying. Formation of rust stains may be accelerated by

periodically wetting the surface with preferably distilled or deionized water or

clean, fresh, potable tap water. The wet-dry cycles should be such that the

sample remains dry for a total of 8 h in a 24-h test period. After completion of

this test, the surface should show no evidence of rust stains or other corrosion

products.

Free Iron Contamination is detected using a ferroxyl test. A solution of nitric

acid and potassium ferricyanide is sprayed onto the surface and free iron

contamination is disclosed by the development of a dark blue color within 30 s.

The solution should be removed after a few minutes with a damp cloth or water

spray. ASTM A967 Practice E (Potassium Ferricyanide–Nitric Acid Test)

provides further details of this type of test, which is recommended for

application only on 200 and 300 series stainless steels.

SAES-W-014 recommends that only stainless steel brushes, ceramic (glass)

beads, iron-free grit, or stainless steel grit shall be used to mechanically clean

the weld overlay surfaces.

10.6 Field Fabrication

Shop fabrication is normally better controlled and potentially much more serious

field fabrication errors could be overlooked. Some contractors apparently lack

knowledge and/or experience of appropriate handling techniques for stainless

and other CRAs.

Storage racks, forklift truck forks and handling tools, etc., must be coated with

suitable materials such as plastic, rubber or weld overlayed with SS. Alternative





Page 35 of 40

lifting equipment materials to those used in carbon steel fabrications shops are

also recommended. Fabric or rope slings should be used rather than steel

chains. Conveyor tables should be designed and operated to avoid damage and

contamination. Where hardened ferrous tools (such as roll, presses and angle

rolls) must be used, they must be completely cleaned (solvent/steam cleaned).

Heavy paper sheets are sometimes used to prevent direct contact between tool

and stainless steel.

Packaging materials and methods used must help prevent surface damage.

Carbon steel strapping must not be allowed to come into contact with the

stainless steel surfaces. If used, wooden bearers should be inserted between the

carbon steel strapping and the stainless steel surfaces.

Table 4 – Suggested Removal Methods for Various Surface Defects and Contamination

The following photographs (Figures 12 to 15) show improper handling of SS

and are from an actual jobsite. Contractors seemed totally unaware of

appropriate techniques for good fabrication practice for stainless steels.

Examples included: contractors off loading stainless steel pipe with carbon steel

hooks, using carbon steel wedges to support stainless steel during tank

fabrication, using wire brushes and grinding wheels that were first used on

carbon steel, using carbon steel lifting brackets to place shell plates, not

covering the storage facilities so the stainless steel does not rest on carbon steel

and other items.




Page 36 of 40

11 Hydrostatic Testing

SAES-A-007 paragraph 6 limits the chloride content of hydrotest water to 50 ppm to

minimize the risk of chloride pitting or SCC during startup. Hydrotest water should

also be verified as low chloride and low SRB bacteria count, or be biocide treated. For

example, use Kitagawa or Draeger tubes or similar field techniques, to verify chloride

level on-site (Figure 16).

The key actions are summarized:

Hydrotesting should be carried out with clean treated water

Do not use untreated raw water, seawater, or contaminated recycled water

Drain water from inside plant promptly after hydrotesting

After draining hydrotest water, check that there are no areas of ponding (stagnant

water) – if necessary, wipe / mop to dry

Figure 12 – Carbon Steel Wedges

on Stainless Steel Tank Bottom

Figure 13 – Carbon Steel Fitting Plates

Used to Fit Stainless Steel Shell Plates





Page 37 of 40

Figure 14 – Carbon Steel “Key Nut”

on Stainless Tank

Figure 15 – Carbon steel Fixturing

(Wedges and Key Shims) on Stainless Tank

Figure 16 – Kitagawa Chloride Test Tube # 201SB

12 Microbiologically Influenced Corrosion (MIC)

MIC is a fairly common problem with stainless steels, including duplex stainless grades.

Failures can occur rapidly in microbially contaminated waters containing relatively low

chlorides (<100 ppm). These failures can occur too rapidly (in terms of weeks to

months) to be attributed to conventional chloride crevice corrosion. The morphology of

MIC failed components is usually consistent with localized corrosion and pitting at or

adjacent to austenitic stainless welds (Figures 17, 18). According to the ASSDA, MIC

can be avoided as follows:

Ideally, ensure all internal welds are polished smooth – bacteria can form colonies

on rough weld surfaces. Though this may be impractical in many engineering

applications, weld spatter should definitely be removed or controlled (Table 4).

Remove heat tints, as these favor bacterial colonization

Use only treated water for hydrotesting and for cooling water: microorganisms

cannot live in treated (chlorinated) water

Introduce a biocide such as chlorine into the water system




Page 38 of 40

Do not leave stainless equipment, such as tanks, pipes and vessels, filled with

stagnant water

Note frequently there is confusion between the “chloride” that causes stress corrosion

cracking and the “chlorine” found in the biocides used to treat water. “Chloride” is the

ion of Chlorine (Cl-) that is present in water from dissolving NaCl, MgCl2, or other

salts. The “chlorine” that is present in water from some of the biocides is HOCl and

OCl-. These forms of “chlorine” are sometimes referred to as “free chlorine.” They do

not break down or change into the harmful “chloride” ion that causes stress corrosion

cracking.

Figure 17 – Stainless steel water tank after

8 months, showing preferential corrosion

of welds by MIC, where heat tints were not

removed.

Figure 18 – Microsection through a SS tank

weld showing undercut pitting due to MIC

in weld filler metal. 316L plate and filler

metal were damaged in 6 weeks due to MIC

in raw water

Remedial measures include: drain promptly and completely after hydrostatic testing, or

circulate water for about one hour daily if the water cannot be drained. Regarding

draining and drying, valves, for example, should not be overlooked and must be drained

fully.

13 Coating Stainless Steel

13.1 Coating Under Insulation

To mitigate CUI, stainless steels operating at low and high temperatures ranges

are insulated and normally the SS is coated. For SSs, CUI damage can

potentially occur as SCC, pitting and crevice corrosion. Insulation is normally

encased in aluminum sheet wrapping. Causes of CUI-related failures can




Page 39 of 40

include moisture entrapment under insulation, chloride contamination of

insulation and damaged or non-weatherproof casings.

NACE RP0198 (The Control of Corrosion Under Thermal Insulation and

Fireproofing Materials - A Systems Approach) provides practical guidelines for

managing CUI. It is also very important to specify and install low leachable

chloride insulation materials for SSs (conforming to ASTM C871). External

coating selection shall be reviewed and approved by the CSD Coatings Team.

13.2 Coating above Grade Piping Exposed to Atmospheric Conditions

Exposure to chloride contamination due to washing down with saline or raw

water and proximity to seawater breeze or splash can cause significant damage

to plant piping and equipment made of stainless steel. To prevent chloride

induced stress corrosion cracking caused, protective coating shall be used.

Select halide free coating materials which need minimum surface preparation

either by solvent cleaning or by abrasive blasting using aluminum oxide

abrasive without causing damage to the passivation layer.

13.3 Coating Buried Pipes

All buried stainless steel pipes shall be coated to prevent adverse interference

with the cathodic protection system. In addition, stainless steel pipes are

susceptible chloride stress corrosion cracking due to exposure to soil chloride

contamination and soil stress, and hence, halide free external coating shall be

applied on stainless steel pipes after appropriate surface preparation with solvent

cleaning and/or abrasive blasting using aluminum oxide abrasive.

14 Summary

The flowchart (Figure 19) summarizes five principal process steps for the specification,

fabrication, hyrotesting and protection of stainless steels.

The controls associated with each step should be carefully followed and documented

where required, to ensure an optimum performance and reliable installation using

stainless steels. The controls should be used as a checklist to document the use of

correct materials and proper storage, fabrication, hydrotesting and insulation and

external coating (where appropriate).




Page 40 of 40

Process Step Control

Figure 19 – Flowchart summarizing principal process steps and associated controls in the

specification, fabrication, hydrotesting and protection of stainless steels Revision Summary

25 August 2008 New Saudi Aramco Best Practice.

Storage and

Segregation

Stainless Steel

Selection and

Specification

Consider service, PREN

Review heat treatment of

wrought and cast parts

Perform PMI

Store SSs separately in dry conditions

Avoid free iron contamination

Use non-chlorinated solvents and

chloride-free marking inks

Fabrication and

Assembly –

Shop and Field

Hydrotest

Procedure and

Documentation

Protection

Insulation and

External Coating

Verify hydrotest water chloride (≤ 50 ppm)

and low SRBs (SAES-A-007)

Completely drain and dry water after test

Don’t leave trapped stagnant water

Verify low leachable chloride

insulation

External coating holiday check

Water tightness of sheathing

Control or remove welding heat tints

Check for and remove any iron contamination

Check appropriate handling, tools and equipment

Welding procedure control – PMI of consumables

& fabricated parts


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