Welding – Post weld heat treatment parameters for steels
Soudage — Lignes directrices concernant les exigences de qualité relatives au traitement thermique ensoudage et techniques connexes
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ISO/NP 14745
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Welding – Post weld heat treatment parameters for steels
1 Scope
This Technical Report provides recommendations for post weld heat treatment (PWHT) of steels with recommendations for holding temperatures and holding times for different materials and material thicknesses. These recommendations are limited to stress relieving and independent of type of product or location. The recommendations do not supersede any guidance from the material supplier specifications, e. g. thermomechanically treated fine-grain steels.
This Technical Report does not specify when PWHT is required. Such requirements are given in product standards, material specifications or material data sheets.
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 17663, Quality requirements for heat treatment in connection with welding and allied processes
EN 10052, Vocabulary of heat treatment terms for ferrous products
ISO/TR 15608, Welding — Guidelines for a metallic materials grouping system
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 10052 and the following apply.
3.1 cooling rate
variation in temperature as a function of time during cooling
[ISO 4885]
3.2 heating rate
variation in temperature as a function of time during heating
[ISO 4885]
3.3
holding temperature
temperature in which the product or component is kept in order to achieve specified properties
NOTE 1 The holding temperature depends on the type of heat treatment, type of material and material thickness.
NOTE 2 Normally the holding temperature is expressed as a temperature range.
3.4
holding time
time the product or component is kept in the holding temperature
NOTE 1 The holding time starts when the temperature in all measuring points has reached the minimum value of the range of the holding temperature and stops when one of the measuring points falls below that temperature.
NOTE 2 The holding time depends on the type of heat treatment, material and material thickness.
NOTE Alternative symbols may be used in other standards
5 General information
PWHT shall be performed in accordance with a written heat treatment procedure specification which includes the essential parameters for the heat treatment process.
Equipment used for the heat treatment shall be suitable according to ISO 17663. It shall permit the temperature control of the component with adequate accuracy and uniformity especially for materials having a small temperature range.
The heat treatment of products or components shall be recorded by the manufacturer indicating the holding temperature, holding time, the heating- and cooling-rate.
Heat treatment as per Table 1 particularity in the upper range of holding temperature and/or holding time may unduly impair the physical properties, e. g. yield strength, tensile strength and toughness of the material. The additional effect of multiple heating cycles should be considered. This effect is explained and published as Hollomon-Jaffe Parameter P as given in equation (1):
P = Ts (20 + lg t)10- 3 (1)
where
Ts is the holding temperature, in Kelvin;
t is the holding time, in hours.
A higher PWHT temperature than tempering temperature for NT- and QT-materials could impair the mechanical properties of the material.
6 Heat treatment conditions
Post weld heat treatment, with the exception of those materials mentioned in clause 8, shall be applied to steels in accordance with Table 1 on completion of welding. If PWHT of materials, not listed in Table 1, is considered necessary, the PWHT holding time and holding temperature shall take into account recommendations from the material manufacturer as well as the welding consumable manufacturer to achieve the required material properties.
NOTE PWHT may be carried out on steels of thickness lower than specified by Table 1. Such instances will include products intended for service with media liable to cause stress corrosion cracking or prevention of brittle fracture.
Where welded joints are connecting parts different in thickness, the thickness to be used in applying the requirements for post weld heat treatment shall be:
a) for butt welds the thicker of the two parts;
b) for butt welded set-on nozzles the thickness of the nozzle
c) for fillet welded set-on nozzles the thickness of the weld;
d) for set-through nozzles the thickness of the shell;
e) for set-in nozzles the thickness of the weld;
f) for fillet welded attachments the thickness of the weld.
ISO/NP 14745When additional welds or weld repairs have been made after post weld heat treatment, a further heat treatment is normally carried out. The thickness to be used in defining the holding time required at temperature shall be the thickness of the weld applied after the PWHT.
In Table 1 a Pcrit-value is introduced. This parameter, a critical Hollomon-Jaffe value, shall not be exceeded without proving the mechanical properties.
Table 1 Post weld heat treatment parameters for steel
Mate-rial
Group
ISO/TR 15608
Material Holding tempera-
ture
(°C)
Material thickness
e
(mm) 1)
Holding time
(min)
Pcrit
(see
Clause 6)
1.1 Steels with ReH ≤ 275 MPa 550-600 e ≤ 35 1)
35 < e ≤ 90
e > 90
30
e – 5
40 + 0,5e
17,5
1.2 Steels with 275 MPa < ReH ≤ 360 MPa
• delivery condition M 530-580
e ≤ 35 1)
35 < e ≤ 90
e > 90
30
e – 5
40 + 0,5e
17,3
• delivery condition QT 550-600 2) 17,5
• delivery condition N (except 16Mo3) 550-600 17,5
• 16Mo3, 18MnMo4-5, 18Mo5 550-620 17,5
1.3 Normalized fine grain steels with ReH > 360 MPa 530-580 17,3
2.1 Thermomechanically treated fine grain steels with 360 MPa< ReH ≤ 460 MPa
530-580 17,3
2.2 Thermomechanically treated fine grain steels with ReH > 460 MPa
4)
3.1 Quenched and tempered steels with ReH > 360 ≤ 690 MPa
• 15NiCuMoNb5-6-4 +NT or +QT 580-620 2) e ≤ 15
15 < e ≤ 60
e > 60
30
2e
120
17,5
• 20MnMoNi4-5 3) 550-620 2) 17,5
• 360 MPa < ReH ≤ 500 MPa1) 530-580 2) -
• 500 MPa < ReH ≤ 690 MPa 4)
3.2 Quenched and tempered steels with ReH > 690 MPa
4)
4.1 Low vanadium alloyed Cr-Mo-(Ni) steels with Mo ≤ 0,7 % and V ≤ 0,1 % and Cr ≤ 0,3 % and Ni ≤ 0,7%
1) For thickness ≤ 35 mm post weld heat treatment is normally only necessary in special cases (e.g. to reduce the danger of stress corrosion cracking or hydrogen-induced cracking (sour gas)).
2) Higher temperatures may be applied subject to conditions given in application standards.
3) For thickness ≤ 15 mm PWHT is optional.
4) If PWHT is considered necessary, the PWHT holding time and holding temperature shall take into account recommendations from the material manufacturer as well as the welding consumable manufacturer to achieve the required material properties.
5) Normally welded with austenitic filler metal; in view of possible carbon diffusion PWHT should be avoided.
6) According to EN 13480-4 no post weld heat treatment is required if all the following conditions are fulfilled:
– tubes with outside diameter ≤ 114,3 mm;
– nominal wall thickness ≤ 7,1 mm
– minimum pre-heating temperature 200 °C
7) According to EN 13445-4 no post weld heat treatment is required if all the following conditions are fulfilled:
– tubes with nominal diameter < 120 mm;
– nominal wall thickness < 13 mm
8) According to EN 13445-4 no post weld heat treatment required if all the following conditions are fulfilled:
– tubes with nominal diameter < 120 mm
– nominal wall thickness < 13 mm
– design temperature > 480 °C
9) In case of intermediate stress relieving (ISR): 630–650°C
10) Intermediate cooling of the weld below the Mf temperature (typically 90-100 °C) shall take place before PWHT to ensure full transformation into martensite.
11) After the welding of the 3,5 % Ni steels with a thickness over 35 mm, it is permitted to weld, without new heat treatment, components that are not subject to pressure provided the following conditions are met:
– the weld dimensions (weld thickness or corner joint throat) are less or equal to 12 mm;
– a preheat temperature of minimum 100°C is applied during the welding operation.
PWHT can be applied with a furnace or with local heating. Reference should be made to ISO 17663 for the methods of application..
The PWHT temperatures and times shall be in accordance with Table 1.
Materials other than those listed in Table 1 shall be heat treated according to the specification of the material manufacturer.
7.2 Heating and cooling
During the heating and cooling periods, for temperatures up to 500 °C variation in temperature throughout the product or component shall not exceed 150 °C within 4 500 mm and the temperature gradient shall be gradual. Above 500 °C, this variation shall not exceed 100 °C.
During the heating and holding periods, the furnace atmosphere shall be controlled so as to avoid excessive oxidation of the surface of the product or component. There shall be no direct impingement of flame on the product or component.
When the product or component has attained a uniform holding temperature (see Table 1) this temperature shall be held for the period specified in Table 1.
7.3 Heating in a furnace
In the case of furnace PWHT, the temperature of the furnace at the time when the product or component is placed in or taken out of the furnace shall not exceed:
400 °C for products or components of not complicated shape or e < 60 mm thickness;
300 °C for products or components of complicated shape or e ≥ 60 mm thickness.
The rate for heating or cooling of the product or component shall not exceed the following:
for thickness e ≤ 25 mm: 220 K/h;
for thickness 25 mm < e ≤ 100 mm: (5 500/e) K/h;
for thickness e > 100 mm : 55 K/h.
The temperature specified shall be the actual temperature of any part of the product or zone being heat treated.
Where a product is manufactured from dissimilar ferritic steels special consideration with respect to the influence on the mechanical properties shall be given to the post weld heat treatment.
8.2 Holding temperature
If the maximum temperature in Table 1 required of one material in the dissimilar combination is equal to or greater than the minimum of the second, e.g. 16Mo3 to 13CrMo4-5, then PWHT for the material with the highest temperature in Table 1 shall be carried out, but as close to the minimum temperature as practical. In this respect the time shall be measured when the lowest reading thermocouple reaches a temperature 10 °C below the minimum specified in Table 1 (without the need for requalification of the welding procedure). Consideration shall be given to the effect of this higher temperature and possibly longer time on the mechanical properties of the second material in accordance with the requirements stated in application standards.
When the maximum temperature allowed in Table 1 of one material in the dissimilar combination is lower than the minimum temperature allowed for the second, e.g. 13CrMo4-5 to X11CrMo9-1 then one of the following routes shall be followed:
a) PWHT the combination in accordance with the requirements for the material requiring the highest temperature according to Table 1 and carry out a simulated treatment on the other material to determine suitable mechanical properties for design. This shall include the effect of PWHT on impact values and microstructure in addition to strength;
b) redesign the combination to include a transition material that overlaps the temperature range.
EXAMPLE First weld X11CrMo9-1 to 10CrMo9-10 and PWHT at (730 to 750) °C, then later weld 10CrMo9-10 to 13CrMo4-5 and PWHT at (670 to 690) °C.
c) Weld overlay the joint preparations with stainless steel or high nickel weld metal;
PWHT each component separately after welding at the appropriate time and temperature for each material in the combination.
Weld the component/material combination of the weld overlay joints using stainless/high nickel weld and do not carry out further PWHT.
8.3 Dissimilar parts
Where a weld is between a structural part and a pressure part the PWHT should be as for the pressure part.
