Influence of Shallow and Deep Cryogenic Treatment on the Residual State of Stress of 4140 Steel

8/10/2019 Influence of Shallow and Deep Cryogenic Treatment on the Residual State of Stress of 4140 Steel

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Journal of Materials Processing Technology 211 (2011) 396401

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Journal of Materials Processing Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c

Influence of shallow and deep cryogenic treatment on the residual state ofstress of 4140 steel

D. Senthilkumar a,1, I. Rajendran a,, M. Pellizzari b, Juha Siiriainen c,2

a Department of Mechanical Engineering, Dr. Mahalingam College of Engineering and Technology, Udumalai Road, Pollachi 642 003, Tamil Nadu, Indiab Department of Materials Engineering and Industrial Technologies, University of Trento 38050, Italyc Stresstech Oy, Tikkutehtaantie 1, 40800 Vaajakoski, Finland

a r t i c l e i n f o

Article history:Received 8 April 2010

Received in revised form 13 October 2010

Accepted 21 October 2010

Keywords:

Cryogenic treatment

Residual stress

Steel

a b s t r a c t

The present research work studies the effect of cryogenic treatment on the residual stress state in4140 steel. Two kinds of cryogenic treatment, namely shallow (SCT, 80 C5 h) and deep cryogenic

treatment (DCT, 196 C24 h) were carried out between quenching and tempering in conventional

heat treatment process. The results evidenced an increase in the compressive residual stress in steel

are subjected to cryogenic treatment before tempering. X-ray diffractometry revealed that residual

stresses are relieved during tempering, according to the redistribution of carbon in martensite and

the precipitation of transition carbides. While conventional heat treatment (CHT) and shallow cryo-

genic treatment (SCT) promote a tensile state of residual stress, DCT shows a compressive residual

stress.

2010 Elsevier B.V. All rights reserved.

1. Introduction

In recent years material scientists and engineers devoted their

efforts in enhancing the fatigue and impact properties of met-

als by deliberately producing compressive residual stresses in to

the surface of engineering materials. Residual stresses are the

stresses that remain within a part after the original cause of

the stresses (external forces, heat gradient) has been removed.

Kalpakjian (1985) pointed out that the residual stresses remain

along a cross-section of the component, even without the exter-

nal cause. Samant and Dahotre (2008) described thatthese internal

stresses become evenly balanced by themselves. They existed in a

free body that had no external forces or constraints acting on its

boundary.

Residual stresses are caused by means of load or thermal

gradients or both. These stresses are developed during differ-

ent processes like non uniform plastic deformation during cold

working, shot peening, surface hammering, grinding, welding,phase transformations, and high thermal gradients. Over the past

few years, much interest has been shown in the properties and

improvement of compressive residual stress. Knowledge of resid-

Corresponding author. Tel.:+91 04259236030/236040; fax:+91 04259236070.

E-mail addresses:kumarsen [email protected](D. Senthilkumar),

irus [email protected](I. Rajendran),[email protected]

(M. Pellizzari),[email protected](J. Siiriainen).1 Tel.: +91 04259 236030/236040; fax: +91 04259 236070.2 Tel.: +358 014 333 0037; fax: +358 014 333 0099.

ual stress in steels is important in the component design field.

It not only leads to improve fatigue resistance but also improves

the dimensional stability. It can also lead to improve the con-

temporary drop in resistance against stress corrosion cracking.

In polycrystalline and/or multiphase materials, residual stresses

can be classified as microstresses and macrostresses. Almer et al.

(1998)stated that the microstresses are formed due to incompat-

ibilities between grains or between phases and the macrostresses

are formed by differential plastic deformation over a large scale

relative to microstructure. Prevey (1996) explained thatthe macro-

scopic stresses or macrostresses are extended over large distances

relative to thegrainsize. Macrostresses vary within the body of the

component over a range larger than the grain size of the material.

These stresses are of general interest in design and failure analysis.

Macrostresses are tensor quantities. These stresses are determined

for a given location and direction by measuring the strain in that

directionat a single point. Microscopic stresses or microstresses are

treated as scalarproperties of the material.These microstresses arerelated to the degree of cold working or hardness, and the result of

imperfections in the crystal lattice. Microstresses arise from vari-

ations in strain between the crystallites bound by dislocation

tangles within the grains. They are acting over distances less than

the dimensions of the crystals. Hoffmann et al. (1997)pointed out

that the microstresses vary from point to point within the crystals.

They are producinga range of lattice spacing andbroadening of the

diffraction peak. These micro-residual stresses are generated dur-

ing diffusionless martensitic transformation by dislocations and by

solute carbon atoms remaining in their octahedral sites without

diffusion.

0924-0136/$ see front matter 2010 Elsevier B.V. All rights reserved.

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D. Senthilkumar et al. / Journal of Materials Processing Technology211 (2011) 396401 397

During the last decade, cryogenic treatment techniques have

been developed and are now broadly used by industry to improve

the mechanical properties of steel components. Barron Randall

(1974)andHarish et al. (2009)studied that deep cryogenic treat-

ment of SAE 52100 bearing steel enhances wear resistance.Collins

and Dormer (1997) investigated the influence of deep cryogenic

treatment on D2 cold work tool steel. Dong et al. (1998) stud-

ied the effect of DCT with respect to the microstructure of T1

high speed steels. It was proved that deep cryogenic treatment

can improve wear resistance by the precipitation of nano-sized

eta-carbides in the primary martensite. It was also observed by

Stratton (2007). Tamas Reti (2002) found that the amount of

retained austenite present in steel plays a significant influence

on the magnitude of the residual stresses and dimensional sta-

bility. They also pointed out that the effect of retained austenite

on component performance is still a controversial issue. Some of

the key factors influencing the retained austenite transformation

include grain size,quenching temperature, hardening temperature,

chemical composition, quenching cooling rates, and stress reliev-

ing or tempering. Retained austenite causes a decrease in tensile

and yield strength in steels and reduces the maximum achiev-

able surface compressive stresses relative to the amount of this

phase. Tempering at sufficiently high temperature promotes the

transformation of retained austenite, accompanied by increasedhardness. The contemporary loss in hardness due to the tempering

of primary martensite partially hides the positive effect of former

transformation.Alexandru and Bulancea (2002)have pointed out

that cryogenic treatments have been proposed as a useful method

to transformretained austeniteprior to temperingand to overcome

the problems related to austenite stabilization. The transformation

of retained austenite into martensite influences the residual stress,

whichwill have an effecton theperformance of thematerial. How-

ever, Preciado et al. (2006) statedthat because of rather lowamount

of retainedaustenite (lessthan 15%)left by conventionalquenching

in the microstructure of alloy steels, it appears that the cryogenic

cooling would not cause additional microstructure improvements

comparedto ordinary quenching. So, cryogenic treatments are nec-

essaryto createa molecularchange in alloy steels, making the mostretained austenite into martensite, a denser, refined mix, smaller

and a more uniform than austenite. Besides, cryogenic treatment

would induce the precipitation of very fine carbides of dimensions

lessthan1m, which occupiesthe microvoids so thatit contributes

to the increase of both coherence and density within the metal.

Molinari et al. (2001) studied thatcarbide precipitationoccurs with

a higher activation energy thus leading to a higher nucleation rate

which in turn leads to finer dimensions and a more homogenous

distribution. A new phenomenon referred as tempered martensite

detwinningoccurred in AISI M2 steel, which showed a reduction

of twins after soaking at 196 C for 35 h. Deep cryogenic treat-

ment reduces the wear rate of the hot work tool steel. This result

was interpreted on the basis of increased toughness, because in

the presence of delamination, the ability of materials to opposecrack propagation can really increase the mechanical stability on

the wear surface and load bearing capacity. Therefore, even if the

deep cryogenic treatment does not influence hardness, it increases

both toughness and wear resistance. This is usual with no or low

amount of retained austenite present in steel.

MohanLal etal. (2001) analyzedthe influenceof cryogenic treat-

ment on T1 type-high speed steel andconcluded that thecryogenic

RAW MATERIAL

CHEMICAL COMPOSITION ANALYSIS

HARDENING (875C, 1 hour)

QUENCHING IN

OIL (30C)

SHALLOW

CRYOGENIC

TREATMENT

(-80C, 5 hours)

TEMPERING (200C, 1

hour)

DEEP

CRYOGENIC

TREATMENT

(-196C, 24

hours)

CHT

SCT

DCT

Fig. 1. Research methodology.

treatment at 93K, soaking for 24 h, imparts 110% improvement in

tool life of T1 type high speed steel.

The main objective of the present work is to evaluate the

influence of two cryogenic treatments, namely shallow cryogenictreatment (SCT, 80 C for 5 h) and deep cryogenic treatment

(DCT, 196 C for 24 h) on surface residual stress, hardness and

impact toughness of 4140 steel. It finds application in axle shafts,

crankshafts, connecting rods, gears, and many other automotive

components.

2. Material and experimental procedure

The material considered in study was obtained in the form of

20 mm diameter rod. The composition of the 4140 chrome molyb-

denum steel was obtained by optical emission spectroscopy (OES).

The chemical composition of the alloy considered is reported in

Table 1.The experimental procedure adopted in the present study

is schematically shown inFig. 1.Samples were subjected to con-

ventional heat treatment (CHT) consisting of quench hardening in

oil at 875 C for 1 h. Part of samples was then subjected to shal-

low cryogenic treatment (SCT)and deep cryogenic treatment(DCT)

as indicated in Bensely et al. (2007).By shallow cryogenic treat-

ment the conventionally quench hardened samples were directly

put in a freezer kept at 80 C and soaked for 5 h to attain ther-

mal equilibrium. Samples were then extracted and left to reach

room temperature in air. By deep cryogenictreatment, theconven-

tionally quench hardened samples were slowly cooled from room

temperature to196 Cin3h,soakedat196 Cfor24handfinally

heated back to room temperature in 6 h. All samples were finally

subjected to tempering or stress relieving at 200 C for 60min.

X-ray diffraction techniques exploit the fact that when a metal

is under stress, applied or residual, the resulting elastic strainscause the atomic planes in the metallic crystal structure to change

their spacing. When a beam of X-rays is incident on a polycrys-

talline material, crystographic planes diffract X-rays and Braggs

law n= 2dh k l sinis satisfied, which was put forward by Martinez

et al. (2003). Here n is an integer indicating the order of diffraction,

is the X-ray wave length,dh k lis lattice spacing of theh k lplanes,

and is the diffraction angle on the h k lplanes.

Table 1

Chemical composition of 4140 steel (wt%).

Sample description % C % Si % Mn % P % S % Cr % Mo

Raw material 0.45 0.35 0.75 0.017 0.019 1.19 0.21

Uncertainty 0.010 0.013 0.012 0.003 0.007 0.007 0.018


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398 D. Senthilkumar et al. / Journal of Materials Processing Technology211 (2011) 396401

Table

2

Residualstressmeasurements.

Sampledescription

Transversemacrostress

Phi=0

(MPa)

Averagetransverse

macrostress(MPa)

Longitudinal

macrostressPhi=90

(MPa)

Averagelongitudinal

macrostress(MPa)

FWHM

Phi=0

Average

FWHM

FWHM

Phi=90

AverageFWHM

Hardness(HRC)

CHTprior

tempering

141.9

136.9

107.9

107.9

7

4.3

7

4.4

1

4.4

6

4.4

8

60

134.6

109.0

4.4

3

4.4

7

134.2

107.0

4.4

3

4.5

0

SCTprior

tempering

137.2

125.6

7

80.7

65.2

6

5.1

1

5.1

8

5.1

8

5.2

0

64

123.9

67.4

5.2

9

5.2

2

115.9

47.7

5.1

6

5.1

9

DCTprior

tempering

148.3

184.0

6

146.5

175.0

0

5.9

1

5.7

5

5.8

1

5.6

7

66

193.3

208.1

5.6

6

5.6

2

210.6

170.4

5.6

7

5.5

8

CHTafter

tempering

119.6

+108.1

(tensile)

134.7

+148.7

6(tensile)

2.5

2

2.5

4

2.5

8

2.5

9

55

107.6

159.8

2.5

5

2.5

9

97.1

151.8

2.5

4

2.6

0

SCTafter

tempering

19.4

+19.4

3(tensile)

52.3

+49.8

3(tensile)

2.8

0

2.7

4

2.7

9

2.8

0

56.7

18.7

45.3

2.6

6

2.7

8

20.2

51.9

2.7

6

2.8

2

DCTafter

tempering

68.8

69.1

89.9

88.5

3

4.7

6

4.7

3

4.6

9

4.7

1

60.3

66.6

89

4.7

2

4.7

2

71.9

86.7

4.7

2

4.7

1

In this analysis, X-ray diffraction measurements were car-

ried out on X-stress 3000 diffractometer (Stresstech Oy/Finland)

to find out the residual micro- and macro-stresses. This anal-

ysis has been conducted by using solid-state linear sensor

technique (MOS, Dual 512 pixels) with a gonimeter in modi-

fied psi geometry (symmetry side inclination). According to the

instructions given by Stresstech (2006), X-ray diffraction tests

were carried out at room temperature in ambient air using

Cr K radiation for residual micro- and macro-stress measure-

ments. Cylindrical samples of size 15 mm diameter and 10 mm

thick were used for the measurements. The purpose of the

present investigation is to determine surface residual stress after

CHT, SCT andDCT conditions. Theresidualstresses canbe classified

into three types based on the length scale. These stresses remain

homogenous over a large number of grains and equilibrium forces

are assumed over a large number of crystals. Here uniform strain

occurs overlarge distances. Macrostresses are stronglyrelated with

macroscopic structures such as grain boundary area, grain shape,

grain size, cracks, porosity, and several other features.

De Oliveira et al. (2007)pointed out that the microstresses are

given by two distincts. The first one remains homogenous within

onegrain andtheforces areassumedto bein balance amongadjoin-

ing grains. The second one is homogenous over some inter atomic

distances and the internal forces remain in equilibrium aroundcrystalline defects. Microstresses are formed by non-uniform strain

occurs overshort distances. Thisstrain is typically within few grains

or within a single grain of type 2 and type 3, respectively. The

present study measures the micro- and macro-stresses in both lon-

gitudinal and transverse directions with respect to the bar axis

by the X-ray diffraction techniques. This technique measures the

changes in interplanar spacing caused by the residual stress. These

changes in the crystals (grains) are corresponding to the elastic

constants and the residual stress of the material. Prevey (1996)

mentioned that macrostresses produce uniform distortion of many

crystals simultaneously, shifting the angular position of the diffrac-

tion peak selected for residual stress measurement.

The data were obtained at beam angle in the range comprising

between 45 and 45 and the residual stresses were found outby using the Chi-method (-method) which is derived from the

classical sin2() method. The test method for residual stress anal-

ysis by X-ray diffraction is described in SFS-EN 15305 (2008).The

diffraction peak position was found out by using cross-correlation

algorithm. Macroscopic stresses are displayed in MPa. Micro-

scopic stresses are presented as a relative result, by expressing the

peak broadening. With this kind of measurements, the absolute

microstress value in MPa would not be obtained. The behavior of

microstresses is more or less a scalar quantity, apart from vector

form macroscopic stresses.

The properties and parameters chosen for the experimen-

tation are Poissons ratio= 0.3, Youngs modulus = 211,000 MPa,

diffraction angle = 156.4, and exposure time= 5 s. Due torestricted

penetration of Cr K radiation in steel (4m), only the stresses inthe outermost surface region could be determined. Hence, stresses

at 3 locations approximately 120 degree apart were determined

on the surface of the outer diameter of the samples. Microstruc-

ture analysis on the CHT, SCT and DCT samples (before and after

tempering) were examined by using scanning electron micro-

scope. Furthermore, the tempered CHT, SCT and DCT samples were

also subjected to Charpy test at room temperature as per ASTM

standards E23 (2002).

3. Results and discussions

The results of the residual stress measurements and the tough-

ness values for CHT, SCT and DCT conditions are discussed below.


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3.1. Residual stress

The micro- and macro-stresses measured in both, longitudi-

nal and transverse directions, for CHT, SCT and DCT samples are

tabulated inTable 2.

1. Residual stress after CHT: Compressive stresses after quenching

(1372MPa).

2. Residual stress after SCT (influence of 80 C5 h): No influ-

ence of SCT in view of the measured experimental scatter

(12510 MPa).

3. Residual stress after DCT(influence of196 C24 h): The lower

temperature and longer soaking time in DCT promote higher

compressive stresses (18430 MPa).

Bhadeshia (2002) pointed out that the strains due to phase

transformations can alter the state of residual stress. It is known

that the martensitic transformation of a steel component puts the

surface under compression. It is argued that this is because of the

expansion at the surface due to formation of the lower density bct

martensite from fcc austenite.

Present results highlight that hardness is higher in SCT and

DCT than in CHT, of 4 and 6HRC, respectively, given inTable 2.As

proposed byCollins and Dormer (1997),the increase in hardnessproduced by subzero treatment is given by the transformation of

retained austenite to martensite. After studying most reliable the-

ories, it is inferred that through the carbon redistribution at low

temperature, DCT further promotes the formation of nucleation

sites for subsequent precipitation of large numbers of very fine

carbide particles, during tempering. This effect has been claimed

as low temperature conditioning of martensite, which was inves-

tigated byMeng et al. (1994). Our previous study reports about

retained austenitepresent in thesamples of 4140 subjectedto CHT,

SCT and DCT after tempering, respectively, by X-ray diffraction

techniques. It was reported that 6.5% austenite is retained in the

CHT sample. After SCT, the retained austenite is reduced from 6.5%

to 5.1%. Further, after DCT results in a further reduction of retained

austenite from 6.5% (CHT) to 2.7%. It is found that the SCT andDCT promoted the transformation of retained austenite to marten-

site and cause an increase in compressive residual stresses. The

reductionof retained austenitefrom CHTsamples does notdevelop

compressive residual stressin SCT samples (prior and aftertemper-

ing) whereas an increase in compressive residual stress is observed

in DCT samples (prior and after tempering). This is contributed to

the conversion of available retained austenite to martensite. The

process of treatingsampleswith shallow cryogenic freezer is totally

different from deep cryogenic system from the level of temper-

ature reduction. Cryogenic treatment reduces retained austenite

and the volume decreases as treatment temperature is lowered.

Thedecreasein the temperature of cryogenic treatmentwill lead to

more transformation of austenite to martensite and hence, greater

compressive residual stress will be developed. The temperatureto which the metal has been cooled is more important than the

holding time, because of the athermal character of the marten-

sitic transformation. The transformation of retained austenite to

martensite is depends on the cooling temperature.Bozidar Liscic

(1997)pointed out that the further transformation will take place

only if the temperature is lowered further. However, martensite

finish temperature (Mf) for 4140 steel should not be lower than

80 C, so that no difference should be observed between SCT and

DCT sample; i.e., both treatment temperatures are lower thanMf.

In light of the present difference stabilization phenomenon has

to be taken into account.

Cryogenic treatment is not a final heat treatment process since

successive stress relieving or tempering is absolutely necessary

to attain the stress relieving of brittle primary martensite. Avner

(2003) and Vanvlack (1998) have explained that the tempering

reduces hardness and residual stress but it increases ductility

and toughness and also it provides dimensional stability. Data in

Table 2,evidence a progressive reduction in compressive stresses

from CHT (+245MPa), to SCT (+145MPa) and DCT (+115MPa),

respectively. In the case of CHT and SCT samples, tensile stresses

develop after tempering. The same does not occur in DCT sample,

where residual macrostresses remain compressive. It is observed

that the different behavior of CHT, SCT and DCT is not causedby the different amount of retained austenite, because the tem-

pering temperature is too low to cause the transformation of

this phase into ferrite and cementite. The influence of cryogenic

treatment on the martensitic microstructure seems to be the

sole factor affecting present value of residual stress after temper-

ing.

Huang et al. (2003)pointed out that the substantial relief of

compressive residual stress occurred in the SCT and DCT sam-

ples after stress relieving or tempering. The reason is due to the

occurrence of finer carbide precipitates throughout the matrix and

the loss of tetragonality of martensite. However, the stress relief

is higher for the CHT sample, which was not subjected to sub

zero treatment.Grachev (2009)explained that the structural and

phase transformations can promote the process of shear forma-

tion and also raise the shear resistance due to hardening of the

alloy in the process of phase transformations that stabilize the

structure, as in decomposition of super saturated solid solutions.

It should be noted that an alloy with less stable structure necessar-

ily relaxes more intensely than an alloy with more stable structure.

The rate of transition of the system to a more stable state has great

importance. Higher relaxation of stresses corresponds to a higher

rate of transition to a more stable state. The mechanism of this

process responsible for stabilization of the structure has substan-

tial significance. This stress is caused by the spatial variation in

composition and microstructure which leads to different thermal

contraction and also by the transformation of retained austenite

to martensite. The martensite needs to be cooled below a certain

temperature to develop internal stress high enough to generate

crystal defects. The required long holding time suggests a local-ized carbondistribution occurringby clustering of carbonatoms to

lattice defects (dislocations). The martensite becomes more super-

saturated with decreasing temperature. This increases the lattice

distortion and thermodynamic instability of the martensite, both

of which drive carbon and alloying atoms to segregate nearby

defects. These clusters act as or grow into nuclei for the forma-

tion of carbides when tempered subsequently. Fig. 2 shows the

martensite microstructure of CHT, SCTand DCTsamples before and

after tempering. There are no microstructural changes observed

between CHT, SCT and DCT samples before and after tempering.

No appreciable differences could be detected by scanning electron

microscopy. However, the changes in lattice parameters are proved

by recent in situ neutron diffraction studyindicatingthat the lattice

parameters a and c of the martensite behave differently duringthe cooling and warming-up processes. The lattice parameter a

changes with temperature almost linearly, following almost the

same curve during the cooling and warming-up process, indicat-

ing that it is not only a pure thermal effect. This was stated by

Huang et al. (2003). It is inferred from the above result that carbon

atoms segregation occurredup to 0.2%, during the deepand shallow

treatment process, which was put forward by Mittemeijer and Van

Doorn (1983). Because carbon atoms predominantly occupy the

octahedral or tetrahedral site to the defect regions mainly affecting

the c lattice parameter. The capacity of carbon atoms to diffuse

increases as the temperature rises back to room temperature. Dur-

ingthis stage carbonatoms move along short distance to segregate

on thetwin crystal surface or on other defects, form fine carbides of

diameter 2660A leading to relief of residual stress in cryogenically


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400 D. Senthilkumar et al. / Journal of Materials Processing Technology211 (2011) 396401

Fig. 2. Microstructure of (a) CHT prior tempering, (b) CHT after tempering, (c) SCT prior tempering, (d) SCT after tempering, (e) DCT prior tempering and (f) DCT after

tempering, at 5000 magnification.

treated samples, as explained inBensely et al. (2008)andHuang

et al. (2003).

Microstresses were also estimated by the prediction of full

width at half-maximum of the peaks (FWHM) from the X-ray

diffraction patterns of processed samples. FWHM illustrate the

microstresses and or hardness and plastic deformation. When

the value of FWHM increases, the hardness of steel also will

be increasing. FWHM values are the average values from psi

angles. The hardness values are also reported in Table 2. The

reduction of retained austenite from CHT samples increases both

compressive microstresses and hardness of SCT samples and DCT

samples. This is attributed to the transformation of austenite to

martensite.The untempered samples have the highest microstresses and

hardness in all the cases but the material is more brittle due to

the untempered martensite. Hence tempering should be done to

reduce the brittleness by sacrificing some hardness and tensile

strength to relieve internal stress and to increase toughness and

ductility. During tempering, martensite rejects carbon in the form

of finely divided carbide phases. The end result of tempering is a

fine dispersion of carbides in the -iron matrix, which bears little

structural stability to the original as-quenched martensite. Hence,

the microstressesand hardness of all thesamples are reduced after

tempering. Itis observedthatthe SCTandDCT ofbothtempered and

un-tempered samples show increased hardness and microstresses

when compared with the conventionally treated samples, respec-

tively.

3.2. Impact test

The impact energy for all the samples is tabulated in Table 3.

In the light of the experimental scatter, CHT, SCT and DCT samples

have practically the same toughness. However, the slightly higher

toughness of SCT and DCT must be considered in view of the lower

Table 3

Impact energy.

Conditions Sample

identification

Absorbed

energyJ

AverageJ

Raw material (GroupA) A 31 31 1.5B 32

C 29

D 32

CHT (Group B) E 8 8.0 1

F 9

G 7

H 8

SCT (Group C) J 9 9.5 0.5

K 10

L 9

M 10

DCT (Group D) N 10 9.5 0.5

O 9

P 10

Q 9


6/6


amount of austenitein thesesamples,whichcannotbe transformed

during tempering. Note thatthe transformationof retained austen-

ite to martensite would eventually cause a reduction in toughness.

Hence, a compensating effect by cryogenic treatment can be

hypothesized as, for example, the finer precipitation of transition

carbidesis promotedby the low temperatureconditioningof marten-

site. A very interesting result is that very similar impact toughness

is observed for samples having different HRC. Present results show

that the higher toughness can be expected after cryogenic treat-

ment, irrespective from the kind of soaking temperature and/or

time for SCT and DCT conditions.

4. Conclusions

In this work the influence of a post-quench shallow cryogenic

treatment (SCT, 80 C5 h) and deep cryogenic treatment (DCT,

196 C24 h) on the residual state of stress of 4140 steel was

evaluated.

Similar values of compressive macrostresses were measured

after quenching and further SCT, while higher stresses devel-

oped after DCT. The reduction in temperature reduces density

of lattice defects (dislocations) and thermodynamic instability of

the martensite, which drives carbon and alloying elements to

nearby defects. These clusters act as nuclei for the formation offine carbides when stress is relieved or tempered subsequently.

The precipitation of carbides in tempered SCT and DCT sam-

ples are responsible for the residual stress relaxation. However,

the decrease in the temperature of cryogenic treatment will lead

to more transformation of austenite into martensite and hence,

greater compressive residual stress willbe developedin the untem-

peredDCT samples whencompared withSCT andCHT, respectively.

This study concludes that in 4140 steel maximum compressive

stresses develop after DCT, before tempering. This was highlighted

to positively influence the state of stressafterstress relieving. Com-

pared to tensile stresses observed in CHT andSCT samples, residual

compressive stresses were found after DCT.

The toughness of the 4140 steel is not significantly influenced

by SCTand DCTsampleswithrespect to CHTsamples.However,theexpected drop in toughness for these samples, due to the reduced

amount of austenite, was not observed. This was interpreted as the

possible evidence of a low temperature conditioning of martensite,

leading to a finer transition carbides precipitation during temper-

ing.

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Influence of Shallow and Deep Cryogenic Treatment on the Residual State of Stress of 4140 Steel