Diat Htt Lect-28

8/11/2019 Diat Htt Lect-28

1/12

HeatTreatmentTechnology

Dr.

Santosh

S.

Hosmani

DEPT. METALLURGY & MATERIALS SCIENCE,

COLLEGE OF ENGINEERING, PUNE

Heat

Treatment

of

Issue of Sensitisation in Stainless Steels

,

20wt% Ni and between 0.03 and 0.1wt% C.

The solubility limit of carbon is about 0.05wt% at 800 C, rising to 0.5wt% at

1100 C. Therefore, solution treatment between 1050 C and 1150 C will

take all of the carbon into solution and rapid cooling from this temperature

range will give a supersaturated austenite solid solution at roomtemperature.

However, slow cooling or reheating within the range 550800 C will lead to

the re ection of carbon from solution usuall as the chromiumrich carbide

Cr23C6, even when the carbon content of the steel is very low (


2/12


FIGURE: Complete intergranular cracking of the sensitized type 304

stainless steel in 400 hours of exposure to high purity water at

2800C. The figure shows (a) intergranular facets of the fracture

sur ace an secon ary n ergranu ar ranc es emana ngfrom the main intergranular crack.

Ref.: BARC HIGHLIGHTS. Engineering - Material Research


More grain boundary

martensite is present.

The 1112% Cr-type ferritic stainless steels are susceptibile to sensitization

The ferrite-ferrite grain boundaries are sensitized, whereas the

ferrite-martensite phase boun daries are largely unattacked

.partially to austenite in the high-temperature heat-affected zone (HTHAZ) during

cooling, with the austenite transforming to martensite at lower temperatures. The

ferrite-martensite boundaries were generally observed to be unsensitized. The

results suggest that if enough austenite forms in the HTHAZ during cooling, it actsas acarbon sink to dissolve excess carbon. This prevents supersaturation of the

ferrite phase and subsequent carbide precipitation that could lead to sensitization

.sensitization during low heat input welding.

Ref.:M.L.Greeff andM.duToit:WeldingJournal(Nov.2006) 243.


FIGURE: Sensitized 304 stainless steel

i e after service in dilute nitric acid. Note

stainless steel plate. Chromiumcarbides are indicated by chains of

dark particles along austenite grain

that the weld metal and the HAZ closest

to the weld metal are unaffected, but the

region of the HAZ that reached peak

oun ar es. empera ures n e range o o870C is severely attacked.

Ref.:WeldingHandbook,8thEdition,Vol.4,p.273


Possible Resolution Treatment: after welding, the steel

can be reheated to 9501100 C to allow Cr C to

redissolve, and further precipitation is then prevented by

rapid cooling to avoid the Cshaped curve.


3/12


As shown in the timetemperaturesensitisation curves below, the precipitation

of carbides occurs over time at temperatures in the range of about 450 850 C.

The time for precipitation to occur is highly dependant upon the amount of

carbon present in the steel, so low carbon content increases resistance to this

pro em.

content below 0.03wt% is

possible by modern steel-

making methods involving

oxygen anc ng. or

complete immunity fromintergranular corrosion in

18/8 steels, a carbon level

of 0.02wt% should not beexceeded.

Recall yourself from the earlier lectures that:

Addition of metallic allo in elements exce t Co shifts TTT curves

towards right. But, we made little comment about effect of carbon.However, I believe that, for a constant concentrations of partitioning (or

carbide forming) metallic alloying elements, if we increase the carbon

content, TTT curves forcarbide formation will shift towards left (in the

similar way as mentioned on earlier slide). See also the next slide (this is

same slide which we discussed in the past) in support of this argument.

In the case of alloy steels containing carbide-forming elements such as Cr, Mo,

, an , e prec p a on agrams ave wo c ear y separa e ranges o

pearlite and intermediate transformations. Each of the ranges is characterized

by its own C-shaped curves.

en e car on con en o s ruc ura s ee s s up o . . , e s agetransformation is shifted to the right relative to the stage II transformation;if the

carbon content is higher, stage I is found to the left of stage II.

49

0.8%C steel 1.13%C steel


4/12

0.8%C steel 0.38%C alloy steel

But, for En 8 (0.4%C) steel, time indicated by dashed line

is smaller than 0.8%C steel why??

0.8%C steel 0.4%C steel (En 8)

The possible reasons could be as follows (this is as per my thought /

opinion):

It is known that the position of TTT diagram can be affected by prior

austenitic grain-size (i.e. grain-boundary-area per unit volume). In

case of 0.4% C steel there is a formation of ferrite rains in the. ,

austenite grains prior to the carbide formation (here, carbide is

cementite, Fe3C). Due to the formation of ferrite, total grain-boundary-

area per unit volume increases. Therefore, available sites for the

formation of carbide (here, Fe3C) increase, i.e. kinetics of the carbide

formation accelerates.

, . ,

side (compared to eutectoid steel) is possibly due to the other metallic

alloying elements in the steel (and may be not due to the decrease in

.

I strongly think that: for the alloy steel with increase in carbon content(without disturbing other alloying elements concentrations), if there is

no other phase formation (like, above mentioned ferrite formation prior

to carbide formation), TTT curve should move towards left


elements, Nb, Ti niobium and

titanium form carbides which are

much more stable than Cr C so

they preferentially combine with the

available carbon and thus lessen the

opportunity for Cr23C6to nucleate.

TiC and NbC are much less soluble in

austenite than is chromium carbide,

so the will form at much hi her

temperatures as relatively stable

particles. These should remain

relatively inert during commercial

heat treatments involving solution

temperatures no higher than 1050

C, and thus minimize the possible

nucleation of Cr23C6.


5/12

HT of Austenitic Stainless Steels

HT of Austenitic Stainless Steels

Before going into the different heat treatments for austenitic stainless

steels, it is important to remember two particularly important

properties of these steels: (i) low thermal conductivity and (ii) high

thermal expansion coefficient.

Thethermal conductivityof the austenitic stainless steels is low; about

one fifthof the value for pure iron andone thirdof the conductivity of an

AISI 1025 carbon steel.

higherthan for pure iron or of an AISI 1025 carbon steel.

speeds, the high thermal expansion coefficient requires special careconcerning the spacing between pieces to be treated.

FIGURE: Main thermal

rea men s an

transformations that

occur in austenitic

between room

temperature and the

.

(Ref.: A.F. Padilha, R.L. Plaut,

and P.R. Rios: ISIJ

n erna ona apan , ,

143, 2003.)

SOLUTION ANNEALING

Solution annealin is the heat treatment mostfrequently specified for austenitic stainless steels,

before their actual usage. The main objective of this

treatment, as the name implies, is todissolve the

FIGURE: Grain boundary M23C6

p ases a ave prec p a e ur ng e

thermomechanical processing of the material,

especially the chromium-rich carbides of the

M23C6-type, where M = Cr, Fe, Mo. prec p a es n an aus en cstainless steel observed using

transmission electron microscopy. As the precipitation of M23C6 occurs in the 450 to

900 C temperature range, the lower temperature

limit for solution annealing should be over 900 C.

Carbides should be completely dissolved but they

dissolve slowly. Grain growth limits the maximumsolution-annealing temperature. In particular,

,

recrystallization, must be avoided.

FIGURE: Optical micrograph showing secondary

recrystallizationstart in a titanium-stabilized austenitic

stainless steel after solution annealing


6/12


7/12

Martensitic Transformation during Cooling:

s

chemical composition is:

This E uation su ests that man austenitic stainless steels, when

cooled to cryogenic temperatures, will form alpha prime () martensite.

The ability to form alpha prime () martensite becomes more significant

during cooling after sensitization. M23C6 precipitation at grain

boundaries causes depletion of chromium, carbon, and other alloying

elements in the vicinity of the grain boundaries. This leads to a higher

Mstemperature, making the material more susceptible to the formation

o a p a pr me mar ens e c ose o gra n oun ar es ur ng coo ng.

For epsilon () martensite no equations like above Equation are

.

The most frequent case of martensite formation at room temperature in

Stain-Induced Martensitic Transformation:

- .

Widely used empirical equations that relate the Mdtemperature with the

chemical composition is:

dtrue tensile strain of 30%. For the majority of austenitic stainless steels,

the Md temperature is above room temperature. For epsilon ()

martensite such em irical e uations are not available..

Susceptibility of the austenite to form martensite and the amount of

martensite formed increases with decreasing deformation temperature.

When stainless steels containing deformation-induced martensite areannealed, the martensite may revert to austenite. This reversion usually

occurs at temperatures about 100 C lower and for shorter times than

those required for the recrystallization of the deformed stainless steel.

The formability of the austenitic alloys is influenced greatly by

martensitic transformation during straining.

HT of Ferritic Stainless Steels

HT of Ferritic Stainless Steels

Ferritic stainless steel ingots have a coarse grain size, are relatively

brittle, and should not be submitted to thermal shocks.

Cast plates are ground, slowly heated, and hot rolled into strips.

- , , .

Cold rolling, used to obtain intermediate gages, is followed by

- .


8/12

SOLUTION ANNEALING

Ferritic stainless steels are normally used only after solution annealing.

The solution-annealing temperature varies substantially according to the

steel type.

The first-generation steels are treated at lower temperatures, forexample, AISI 430 steel is treated in the 705 to 790 C temperature

ran e and the AISI 446 in the 760 to 830 C ran e..

The second-generation steels are treated at somewhat higher

temperatures, for example, AISI 409 is treated in the 870 to 925 C

range.

The third-generation steels, such as the AISI 444, are treated at even

, .

Steels with higher chromium contents, such as the super-ferritic, should

be water cooled in order to avoid the 475 C embrittlement due to the

formation of intermetallic alpha prime () phase.

crograp

h

Optical

rphase.

osed

mi

ss

steel.

he

darke

nalcom

x

stainl

rrite

is

t

haraII.

-dimensi

lled

dup

l

scopy.F

dwithBe

Three

of

rol

micro

Etche

HT of Duplex Stainless Steels

Ferriticaustenitic stainless steels with a duplex microstructure can be

classified into two subgroups:

. oys w ow-car on con en . w . w , requen y

mechanically worked and heat treatable

2. Alloys with high-carbon content (0.3 wt% C 0.5 wt%), used in theas-cast condition or after solution annealing.

Duplex steels of higher carbon content show lower toughness and ductility

u ave an exce en wear res s ance.

Duplex steels of lower carbon content have better formability and

.

Duplex stainless steels are susceptible tothree types of embrittlement:

HT of Duplex Stainless Steels

1. Embrittlement caused by the presence of a carbide network,

particularly in the austenite, in alloys with higher carbon content

. m r emen cause y prec p a on o e n erme a c -p ase,

475 C embrittlement of ferrite

3. Embrittlement caused by precipitation of the -phase, particularlyin the ferrite

FIGURE: Schematic TTT diagram showing precipitation of sigma (), alpha prime (),

and other phases in duplex stainless steels.


9/12

-

SOLUTION ANNEALING

. -

annealing temperature should be sufficiently fast, generally into water, in

order to avoid precipitation (see schematic TTT diagram on previous

slide es eciall the 475 C embrittlement due to intermetallic al ha,

prime () phase.After welding, the solution-annealing treatment is recommended, followed

.

FIGURE: Ferrite brittle cleavage

fracture (475 C embrittlement)

duplex stainless steel. Scanningelectron microscopy with secondary

electrons. A=austenite; F=ferrite.

HT of Martensitic Stainless Steels

Martensitic stainless steels are essentially FeCrC alloys, containing

chromium in the range of 11.5 to 18 wt% and carbon in range of the 0.1

o . w . s c rom um eve s ncrease , car on eve as o ncrease

also in order to stabilize austenite.

or e mar ens c s ee s, s, o

course, essential to form austenite

from which martensite is obtained. ,

complete austenitization, steels

containing 13 wt% chromium need

0.15 wt% carbon and to be heated

to at least 950C. Steels containing

more chromium, say 27wt%, need

to have carbon content higher than

0.3 wt% carbon and to be heated to

at least 1100C. Apart from carbon.

both nitrogen and nickel expand

the -loop,


10/12

Other alloying elements frequently observed in martensitic stainless steels

are nickel and molybdenum. Nickel entitles the usage of lower carbon

, ,

resistance may be obtained. For soft but tougher martensitic steels

containing lower carbon levels, nickel content may reach 5 wt%.

o y enum a so mproves corros on res s ance, n a on o

improvement in toughness. It must be remembered that corrosionresistance in martensitic stainless steels is significantly lower compared to

.

Martensitic stainless steels may be subdivided into three subgroups: (a)

low-carbon steels for turbines;(b)medium-carbon steels for cutlery;

and(c)high-carbon wear-resistant steels.

The microstructure of each group is also characteristic: (a)martensitic

-

and (c) ultrafine martensitic microstructure containing primarycarbides, respectively.

(a)

(b)

(c)

Higher carbon steels, such as the AISI 440C, or nickel containing,

such as the AISI 431, may present large amounts (more than 30% in

volume ofretained austenite after uenchin .De endin on tem erin

temperature and chemical composition, especially the Cr/C ratio, several

carbides may precipitate, such as the M2X, M3C, M7C3, M23C6, and MC

types.

FIGURE: AISI 410 martensitic

stainless steel, quenched andtempered to 20 HRC. Microstructure of

FIGURE: AISI 420 martensitic

stainless steel. Microstructure oftempered martensite with intergranular

tempered martensite with fine-carbideprecipitates. Optical microscopy.

Etched with Villela.

and intragranular precipitates. SEMimage using secondary electrons.

Etched with Villela.

martensite laths

retained austenite

Ref.:


11/12

As-quenchedAISI 440C

As-tempered

The Equilibrium Phase Diagram of440C Steel

Ref.: International Journal of Mechanical and MaterialsEngineering (IJMME), Vol. 4 (2009), No. 2, 123-126

Prior to final hardening and

tempering heat treatments,

martensitic stainless steels

areannealed in order to be

machined and cold worked.

For example, an AISI 410 is

annealed in the 750 to 900 Ctemperature range for 2 to 4h

.

high temperatures, their

stable structure is austenitic

is a stable mixture offerrite

and carbide.

FIGURE: Continuous cooling transformation

diagram forAISI 410 steel.

Prior to their final usage, martensitic stainless steels are submitted to the

same heat treatment sequence as that for carbon steels, namely they are

, ,

ductility and toughness.

The formation of a more stable (ferrite + carbides) microstructure is very

sluggish and the tendency toward martensite formation (high

hardenability) is very high.

,

air cooling, even for sections that are up to about 300mm in thickness.

Hardening media may be air or oil.

While oil cooling is preferred in order to avoid carbide precipitation, air

cooling may be needed to avoid distortions in more complex sections.

ar emper ng s a so poss e n s c ass o s ee s. ar ens e

hardness depends essentially on carbon content varying from about

35 HRC for a 0.1 wt% carbon to 60 HRC for 0.5 wt% carbon, thereon. .

For low-carbon martensitic steels, such as the AISI 410, the Msand Mftemperatures are relatively high, 350 and 250 C, respectively, and

decrease with increasin carbon content.

High-carbon steels may present retained austenite (more than about 30

vol.%) and a subzero treatment around -75 C, immediately after

hardenin is recommended. Double tem erin is also ver common., . .

Tempering temperature is determined by the required mechanical

properties.

FIGURE: Tempering

temperature effect on

mechanical properties

.


12/12

Date post:	02-Jun-2018
Category:	Documents
Upload:	prakush01975225403
View:	224 times
Download:	0 times

Diat Htt Lect-28

Documents