ARTICLE - Relieving Stress in Stainless Steels (2013)

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WORLD PUMPS January 2013

As the name suggests, stress

relieving is employed to relieve

internal residual stresses that

remain locked in a structure as a conse-

quence of a manufacturing sequence.

Causes of residual stresses are thermal

factors (e.g. thermal stresses caused by

temperature gradients during heating or

cooling), mechanical factors (e.g.

machining) and metallurgical factors (e.g.

transformation of microstructure). Under

certain service conditions, internal

stresses have adverse eff ects. For

example, steels with residual stresses

under a corrosive service environment

may fail by stress corrosion cracking

(SCC). Failure by SCC occurs, in general,

under the combined action of corrosion

and externally applied stresses in suscep-

tible materials.

Conventional austenitic stainless steels –

alloys containing chromium and nickel that

are commonly used in castings and fabri-

cated welds in pumps – are especially

susceptible to corrosion cracking. These

alloys are not hardened by heat treatment

(as no phase changes occur on heating or

cooling), but may be hardened by cold

working. Heat treating is performed by full

solution annealing, which includes heating

in the 1,040–1,120°C (1,900–2,050°F) range

depending upon the alloy, followed by

rapid cooling. This restores the material to

its optimal condition, removing the eff ects

of alloy segregation, sensitization and

sigma phase; and restoring ductility after

cold working. Unfortunately, the rapid

Cold working

When, for example, a metal ingot is rolled,

centre sections receive greater reduction

compared to the surface, due to greater

elongation. As the elongated central

section pulls from within the surface layers,

internal tensile stresses in the surface layers

and internal compressive stresses in the

central portion are developed.

Machining

Heavy machining creates stress within

cold worked surfaces, which may induce

internal stresses. These may cause

cracking during subsequent heat treat-

ment or further processing. Machining

processes such as turning, grinding,

drilling, milling and broaching involve

removal of metal by force, which intro-

duces stress.

Heat treatment process

Fast heating during heat treatment results

in thermal temperature gradients that

cause diff erential expansion across the

part cross-section. This often results in

tensile stress in surface layers and interior

compressive stress. Fast cooling results in

the opposite stresses compared to fast

cooling will generally reintroduce residual

stresses, which can be suffi cient to provide

the necessary stress condition to promote

SCC near welds and other regions of a

component that have been cold-strained

during processing. Furthermore, cold strain

can produce a reduction in creep strength

at elevated temperatures and reduce

fatigue strength. Distortion can also occur

if the object is not properly supported

during the solution annealing process.

Stress-relief heat treatments may be used

to reduce distortion and other stress

eff ects that can have detrimental eff ects

on service performance.

Purpose of stress relief

Cold working, machining, heat treatment,

casting, welding, shot peening, surface

hammering, electroplated coatings,

thermal cutting and other processes can

all induce residual stress in metal. The

nature of the residual stress, its distribu-

tion and eff ects within a metal is complex

and not a completely understood

phenomenon, but one can be sure of its

presence in complex or fabricated parts.

Residual internal stresses can cause cracking of metal casings or

components in corrosive environments, leading to pump failure.

Here, Vikas Panchal and Stephen Morrow discuss two primary

treatments to reduce such stresses in stainless steels: low-

temperature stress relief with slow cooling, and solution annealing.

Relieving stress in stainless steels

Materials

0262 1762/13 © 2013 Elsevier Ltd. All rights reserved

“Stress-relief heat treatments may be used to reduce distortion and other stress eff ects that can have detrimental eff ects on service performance.”

WOPU0113_Feat_ITT_StainlessSteel 28 03-01-13 10:59:14

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Cast Stainless Steel (CF8M)Impeller Failure Photomicrograph 500X

Ferrite Islands

AusteniteMatrix

Non-metallic Inclusion

Chloride Ion Stress Corrosion Cracking

heating. Quenching stresses may be suffi -

ciently high to cause the development of

quench cracks. Heat treatment can also

produce dimensional changes and the

distortion of components.

Castings

During solidifi cation, stresses are invariably

present in castings due to non-uniform

surface cooling compared to the centre.

Welding

In welding, a very high heat source is

applied to a small area relative to the

cooler surrounding area. As the molten

weld pool solidifi es within the joint, it

creates resistance to shrinkage by the

already solidifi ed weld metal and

unmelted base metal adjacent to the

weld. This resistance creates a tensile

strain in the weld. Because of the rapid

thermal expansion and contraction

created within a very localized area,

residual tensile stress is introduced.

Distortion, buckling and cracking often

result, and shortened fatigue life is

possible. The introduction of residual

stresses by welding is due to diff erential

expansion and contraction of the heat

aff ected zone (HAZ), and the weld metal

itself. Heat input, base metal thickness,

cooling rate, restraint of the weldment

and the welding process used all aff ect

the level of residual stress induced by

welding.

Stress relieving plays an important role in

avoiding and/or minimizing all the above-

mentioned problems.

Austenitic stainless steels are commonly

used as castings and in fabricated weld-

ments but are susceptible to SCC. SCC is

a localized attack that results from the

combined action of tensile stresses and

corrosive liquid in a susceptible material.

The stability of residual stresses depends

on the amount of cold working employed

during processing. The cold working can

lead to unexpected, sudden failure of

normally ductile metals when subjected

to tensile stress, especially at elevated

temperatures.

SCC damage in an austenitic stainless

steel are illustrated in Figures 1 and 2.

Figures 3 and 4 show the eff ects of SCC

damage related to stress on an impeller

and pump casing.

Role of stress relieving

Stress relieving plays a major role in

removing or reducing the eff ects of stress.

Stress relieving reduces residual stresses,

helps avoid SCC, improves notch tough-

ness, improves the dimensional stability

Figure 1. Scanning electron micrograph of stress corrosion cracking (SCC) in austenitic

stainless steel; magnifi cation 100× (© ITT Goulds Pumps).

Figure 2. Higher magnifi cation (500×) photograph showing the details of SCC

(© ITT Goulds Pumps).

Figure 3. Pump impeller showing the eff ects of SCC damage (© ITT Goulds Pumps). Figure 4. The eff ects of SCC on a pump casing (© ITT Goulds Pumps).


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M 23 C 6

Chromium depleted zone

The metallurgical characteristics of auste-

nitic stainless steel that may aff ect the

selection of a stress-relieving treatment

are discussed in the following sections.

Heating in the 480–815°C range

For austenitic stainless steel, stress

relieving at temperatures below 400°C is

an acceptable practice, but it results in

only modest stress relief. Temperatures up

to 425°C may be used if resistance to

inter-granular corrosion is not required.

Heating to below 480°C, followed by slow

cooling, is used to remove peak stress

only and to improve dimensional stability.

Higher temperatures will reduce strength

and sensitize the metal, and are not

generally used for stress relieving.

In partially ferritic cast grades such as

duplex stainless steel, carbides will precip-

itate initially in the discontinuous ferrite

pools rather than in the continuous grain

boundary network. After prolonged

heating, which is necessary for heavy

sections, precipitation of grain boundary

carbides may occur. For cold-worked

stainless steel, carbide precipitation may

occur at temperatures as low as 425°C.

For types 309 and 310, the upper limit for

carbide precipitation may be as high as

900°C. In this condition, steel will be

susceptible to inter-granular corrosion.

By using stabilized or extra-low carbon

grades, this inter-granular precipitation

can be avoided.

Heating in the 540–925°C range

Stress relieving at 540°C to 925°C signifi -

cantly reduces residual stresses that

otherwise might lead to SCC or dimen-

sional instability in service. By heating

the steel in this region, the formation of

a hard, brittle sigma phase may result,

which can decrease corrosion resistance

and ductility. During the necessary

stress-relieving time, the sigma phase will

not form in fully austenitic wrought, cast

or welded stainless steel. However, if the

stainless steel is partially ferritic, the

ferrite may transform to sigma phase

steel by the method of post-weld heat

treatment (PWHT) tempering.

Stress relieving of austenitic grades

Low-temperature stress relieving can

increase the proportional limit and yield

strength (particularly compressive yield

strength) of austenitic stainless steel that

has been cold worked to develop high

strength2.

To produce adequate stress relief, auste-

nitic stainless steel must be heated

above 900°C. In some instances, heating

to the annealing temperature may be

required. Holding at a temperature lower

than about 870°C results in only partial

stress relief, which may be suffi cient for

certain applications. Slow cooling results

in the most eff ective stress relieving.

Quenching or other rapid cooling, as is

normal in the annealing of austenitic

stainless steel, will usually reintroduce

residual stresses and should be avoided

if trying to eliminate stress. Stress

relieving is usually only necessary when

austenitic stainless steel parts are

subjected to corrosive conditions and

the intent is to reduce risks associated

with stress-corrosion failures.

of complex geometries, reduces distortion

during machining and reduces the eff ects

of cold working.

Stress relieving of stainless steel

There are two primary stress-relief treat-

ments for stainless steel1. The 400°C, 8–10

hour stress-relief treatment is below the

temperature–time spectrum at which

sensitization (see later section) is likely to

occur. This treatment is usually adequate

to maintain dimensional stability during

machining. If this stress relief does not

maintain dimensional stability within

the limits imposed, stress relief at high

temperature within the range of

sensitization may be necessary. Such treat-

ments should be designed with full

knowledge of the alloy chemistry (actual

carbon content), section thickness,

temperature and time utilized. They should

be designed specifi cally for each alloy and

individual case situation reviewed.

Full solution treatment (annealing), gener-

ally by heating to about 1,080°C followed

by rapid cooling, removes most residual

stresses. However, the rapid cooling may

reintroduce additional quenching stress.

When full annealing is not possible, such

as on large fabricated components or

intricate shapes, weldments can be

heated to an intermediate temperature to

decrease high residual stresses.

Stress relieving is usually performed

when joining dissimilar metals such as

austenitic stainless steel and low alloy

Figure 5. Chromium carbide precipitates (© ITT Goulds Pumps).

"Stress relieving is usually only necessary when austenitic stainless steel parts are subjected to corrosive conditions...to reduce risks associated with stress-corrosion failures."


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dissolve some of the ferrite present and

further reduce the probability of sigma

phase reforming upon slow cooling.

Quench/solution annealing

In this process austenitic stainless steel is

heated to a temperature of 1,040°C. After

holding at this temperature it is rapidly

cooled to below 600°C, and preferably

below 480°C, to prevent precipitation of

carbides at the grain boundaries (sensiti-

zation); the exception is for stabilized and

extra-low carbon grades. Because of the

rapid cooling this process is called

quench annealing. It can be achieved by

very rapid fan-accelerated gas or water

quenching. SCC of welded austenitic

stainless steels may be of concern in

certain environments.

Austenitic stainless steels that contain

surface residual tensile stress induced

during fabrication or welding strain may

crack by chloride stress corrosion in

certain chloride (e.g. saline) environ-

ments. Welded joints and fabrications

may be particularly susceptible to SCC

due to residual tensile stress in welded

regions, unless an eff ective post-weld

stress relief is carried out. Post-weld

stress relief can be eff ective to remove

the peak stress but is not fully eff ective

in removing all the residual stress due to

the lower temperatures used for the

required stress relief.

Recommendations

For the selection of the proper stress-

relieving treatment, factors such as the

specifi c material used, the fabrication

procedures involved, the design and

expected service operating condition of

the equipment, and related metallurgical

during stress relief. This is not generally a

problem in wrought stainless steels,

because they are fully austenitic. By

heating to temperatures in the

815–925°C range, the formation of chro-

mium carbide precipitates, or sigma

phase, will occur. These temperatures

should therefore be avoided.

Sensitization

When stress relief is carried out at 870°C

for about one hour, it relieves about 85%

of residual stress, but there is a risk that

chromium will form chromium carbides

with any carbon present in the steel. This

reduces the chromium available to

provide corrosion resistance within the

passive fi lm, as shown in Figure 5. This

may lead to preferential corrosion along

the grain boundaries, which can be

severe. This is referred to as sensitization.

Hence, it is advisable to avoid this

temperature or, when welding stainless

steel, to use low heat input and restrict

the maximum interpass temperature to

about 175°C or lower. To avoid these

eff ects, it is recommended that stabilized

stainless steel (grade 321 or 347) or an

extra-low-carbon type (304L or 316L) be

used, particularly when any stress

relieving is required. Sensitization in a

stainless steel pump component can be

seen in Figure 6, with the grain bounda-

ries clearly revealed.

Sigma phase

If steel is heated above 550°C for any

signifi cant periods, the delta ferrite can

transform to a very brittle phase known

as sigma (σ) phase (a chromium/molyb-

denum-rich intermetallic phase), and

material will crack readily as a result of

lost ductility from prolonged exposure to

temperatures of 660–872°C. This will

decrease corrosion resistance and ductility.

For type 316 or 316L stainless steels, the

recommended annealing procedure is to

heat treat between 1,038°C and 1,120°C,

followed by rapid water quench or rapid

cooling by other means within the limits

of distortion. Whenever considerations of

distortion permit, water quenching is

used, thus ensuring dissolved carbides

remain in solution. Where practical

considerations of distortion rule out such

fast cooling, cooling in an accelerated air

blast is generally used.

Alloys that are stabilized or of low carbon

content are much less likely to precipitate

detrimental carbides and detrimental

phases in suffi cient quantities at the grain

boundaries. Low carbon (0.03% maximum)

austenitic grades such as 316L are interme-

diate in tendency to precipitate chromium

carbides, compared to the stabilized and

higher carbon unstabilized grades. This

characteristic of limited sensitization is of

particular value in welding fabrications,

fl ame cutting and other hot-working oper-

ations. Such grades do not require the

water quenching treatment that unstabi-

lized grades require to retain carbon in

solid solution.

Full solution treatment (annealing), gener-

ally by heating to about 1,040°C followed

by rapid cooling, removes all residual

stresses, but is not a practical treatment

for most large or complex fabrications.

Heating in the 955–1,120°C range

Annealing treatment in the 955–1,120°C

range causes all grain-boundary chromium

carbide precipitates to re-dissolve, and

transforms sigma phase back to ferrite.

Long heating times (>1 hour) may even

Figure 6. Sensitization in a stainless steel pump component; magnifi cation 1×


Figure 7. Microstructure illustrating sigma phase; magnifi cation 500×



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Contact

Vikas D. Panchal, material specialist, and Stephen Morrow, chief metallurgist & global mgr materials technology

ITT Industries, Goulds Pumps

240 Fall St

Seneca Falls, NY 13148, USA

Tel: +1 315 568 2811

Email: [email protected]

www.gouldspumps.com

factors must be taken into consideration.

Stress relieving is not generally advisable

for stainless steels unless the service envi-

ronment is known or suspected to cause

stress corrosion and there is an associated

benefi t. The treatment selections listed

should be used with appropriate caution.

In order of decreasing preference, these

thermal treatments are:

A. Anneal at 1,065–1,120°C, then slow

cool.

B. Stress relieve at 900°C, then slow cool.

C. Anneal at 1,065–1,120°C, then quench

or cool rapidly.

D. Stress relieve at 900°C, then quench or

cool rapidly.

E. Stress relieve at 480–650°C, then slow

cool.

F. Stress relieve at <480°C, then slow cool.

G. Stress relieve at 205–480°C, then slow

cool (usually four hours per inch of

section).

With reference to this list, Table 1 recom-

mends the best stress-relieving treatments

Table 2. Treatments for dimensional stability (adapted from Ref. 1)

AlloyTemperature/type of

treatmentComment

Austenitic stainless steel400°C, 8–10 hours (typical treatment)

Designed to stay below sensitization temperature. Treatments B, F and G can be used for dimensional stability.

Duplex stainless steel F, GDuplex stainless steel is subject to 475°C embrittlement. F and G can be used for dimensional stability.

to use for three diff erent categories of

austenitic stainless steels to optimize relief

for ten diff erent applications or desired

characteristics.

In addition, recommended treatments for

dimensional stability are given in Table 2.

Summary

All types of metalworking introduce

residual stresses that can cause distortion

and aff ect pump performance. In corro-

sive environments, these stresses can

cause pump failure from cracking of

metal casings or components. Austenitic

stainless steels are especially susceptible

to corrosion cracking.

For pumps that will be used in corrosive

environments, manufacturers can employ

a variety of stress-relief methods to reduce

the residual, internal stresses locked in the

metal after manufacturing. They are

usually performed when joining dissimilar

metals such as austenitic stainless steel

and low alloy steel by way of PWHT

tempering. The two primary stress-relief

treatments for stainless steel are: low-

temperature stress relief with slow cooling;

and annealing, or full solution treatment,

which involves higher temperatures and

rapid cooling. This article has described

the impact of stress relieving in diff erent

temperature ranges for diff erent alloys,

and identifi ed temperature ranges that

should be avoided, concluding with a

table of prioritized recommendations iden-

tifying the best stress-relieving methods to

use in a variety of circumstances.

References

[1] Nickel Development Institute, Materials

for saline water, desalination and oilfi eld

brine pumps, A Nickel Development

Institute Reference Book, Series No. 11

004, 2nd edition, p 12, (1995).

[2] ASM Handbook Volume 04: Heat

Treating, pp. 774–776, ASM Interna-

tional (1991) [ISBN: 978-0871703798].

Table 1. Recommended stress-relieving treatments (from Ref. 2)

Application or desired characteristics

Suggested thermal treatments

Extra-low carbon

grades such as 304 L

and 316 L

Stabilized grades,

such as 318, 321 and

347

Unstabilized grades,

such as 304 and 316

Severe stress corrosion A, B B, A (a)Moderate stress corrosion A, B, C B, A, C C (a)Mild stress relief A, B, C, E, F B, A, C, E, F C, FRemove peak stress only F F FNo stress corrosion None required None required None requiredInter-granular corrosion A, C (b) A, C, B (b) CStress relief after severe forming A, C A, C CRelief between forming operations A, B, C B, A, C C (c)Structural soundness (d) A, C, B A, C, B CDimensional stability G G G

(a) The use of stabilized or extra-low carbon grades is recommended.(b) In most instances no heat treatment is required, except where fabrication procedures may have sensitized the stainless steel.(c) Treatment A, B or D also may be used, if followed by treatment C when forming is completed.(d) Where severe fabricating stresses, coupled with high service loading, may cause cracking. Also after welding heavy sections.


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ARTICLE - Relieving Stress in Stainless Steels (2013)