CHAPTER 6

Phase transformation and Residual stress analysis

6.1 Introduction

Cylindrical grinding is a manilfacturing process rvith a relatively liigh power

density input (Kruszynski and Luttelvelt, 1991). This results in the predominance of

thermal pheno~nena in the grinding process and in the creation of the surface layer of

ground components. The grinding temperature, the temperature gradients and the rate of

lieatflow are the major factors which influence surface integl-ity.

Of all the propel-ties that describe the surface layer chalxteristics, residual

stresses may be regarded as the inost representative one. On the one hand. they give a

good estimate of the surface integrity of the workpiece and on the other hand they afyect

the functional propel-ties of the components \Iery sig~lificantly. Residual stresses are also

useful in quantitatively deteill1ining the grinding conditions. I t is dificult to describe the

grinding conditions quantitatively by ~lsiilg other surface layer parameters (e.g..

Microstrnctu~-a1 changes), as they occur only in a specific range of grinding conditions or

have an ONiOFF characteristic like burns i.e.. they occur or do llot occur. All reasons

nlentioned above inake the residual stress as one of the most impoi-tant surface layer

properties.

With the recent improvement on ~ixichines to measure the residual stress through

XRD, the interest on the knowledge to control such stl-esses has increased. This interest

has its importance due to the fact that the presence of the residual stress interferes with

the fatigue strength of the Materials.

6.2 Residual stress formation

Grinding removes the illeta1 from the workpiece in the form of sniall chips by

mechanical action of abrasive particles bonded together 111 a grinding wheel. Compared

to other machining processes such as tuining, milling etc., grinding requires x r y high

energy input per unit volume of material removal. The frictional resistance encountered

between work illaterial and the tool and rhe chip tool interface and the resistance to

defoimation during shearing of chips contribute to the rise in temperature at the cutting

zone. The temperature generated is not only high but the thermal actions are also severe.

The thei-ma1 and mechanical actions can affect 111e surface integrity of the ground

surface to a depth of'about 2111in. These actions result in rlic de\.elopltie~i~ of high residual

stresses (either compressive or ~eiisiie).lf the developeci residual stresses arc. of tensile

nature \vith considerably highel- values they will initiate surhce cracks.

6.3 Phase transformation

The heat generated in grinding and the consequent high temperatures are of

primary importance since they affect the tool life, diineilsioilal accuracy and surface

integrity of the machined part and ultiiuately economics ofmachining.

Excessive grinding temperature causes thelnlal damage to the work piece. As per

Malltin (1978), grinding temperature generated during grinding is a direct coi~sequeilce

of the energy input to the process. One of the coinman types of thermal damage is called

grinding burn.

Littrnann and Wulff (1955) have found that for hardened steels which have been

burned during gnnding, the work piece subsuiface consists of a rehardened zone near the

surface and a softened tempered zone beneath it. This would suggest that the onset of

burning is characterized by the formation of austenite over some portion of the work

piece subsurface. Rehardening at the surface occurs by martensite foimation as the cooler

material in the bulk of the workpiece quenches the surface. This refers to phase

transformation in grinding. If the phase transformation is mai-tensite to ferritelpearlite the

volume decreases hindered by the bulk matenal produces tensile residual stresses If the

phase transformation is ferrite;pearlite to martensite the volulnt increases hindered by the

bulk material produces comprsssi\;e rcsiduai stresses.

With this in mind as a pal? of the research. the effect of heat generation!'work

surface temperature on the residual stress of various percentage carbon steels subjected to

cylindrical grinding process is srudied and discussed belolv.

6.4 Residual stress on the surface

The surface residual stress and the sub-surface residual stress are of great

importance on the fatigue resistance of the materials. Ki~mbcl- of researchers report that

if those stresses are of compressive 11atures impro\.e thz resistance to fatigue whereas if

those stresses are of tc~lsile ~iatul-e depending on their magnitude they contribute to a

decline in the fatigue resistance. In ordcr to verify t h ~ bella\,iour of the residual stress

for the fine grinding and rough &I-inding. some experiments are conducted fbllo\ving the

methodology presented below.

Table 6.1 and 6.2 shows the details about materials subjected for fine grinding and rough

grinding test respectively.

Table 6.3 and 6.4 shows the operating parameters and their levels for fine grinding and

rough grinding test respectively.

Table 6.5 shows the experimental design matrix used in both the tests.

Table 6.6, 6.7, 6.8 (fine grinding) and 6.9, 6.10, 6.1 1 (rough grinding) shows the test

results.

Table 6.1 Details about the materials subjected for fine Grinding test (Residual stress analysis)

%, of Type of aheel 1

Carbon used

0 15

I ShJo 1 Matenal Des~gnanon -- 7

I I AIS1 33 10

steel Medium carbon I 2 1 '4ISI 4340 , 0 45 structural steel I

1 and I

1501n1n 1

=351nm AllO;

A120, 1 75 1 3 Length = Hlgh carbon hlgh speed

Table 6.2 Details about the materials subjected for rough Grinding test (Residual stress analysis)

I I Matel-ial Type of\vheel Size 1 1 used -

1

2 F

3

Table 6.3 Operating parameters and their le\ els for fine grinding (Residual stress analysis)

Number of' passes Np 3

Med~uni carbon structural steel

High carbon high speed steel

Higli carbon Non-shl-inking d & ~ steel

S ho

1,- iC ,,,,, -La*, Total depth of cut 50 75 I 100

1 3 Wlieel speed ' Ns r p m 3 5 5 0 ' 1730 - 1 1 2 1 3 1

- n~edium 1 high i

I I 3 I 3 1

; 2 4 Work spezd 3 I

AISI 4340

~a r r lne t e i notatloo 1 l i l l l r ~ Levei\/act~~ai

Table 6.4 Operating parameters and their levels for rough grinding (Residual tress)

Lei elitcode

.-.A- ..-I 7 -r - -- ow m e d i u ~ - ~ ~ i ~ ~ ~ medium 1 liigli Total depth of rut I Dc pni I I 1 I

0.45

Table 6.5 Experimental design matrix (Residual stress Analysis)

AlrO; Dialnetel- =351iim

1 Ex.No.in standard order / Depth of cut I Number of passes MJlieel speed Work speed 1

A 2 0 1 1 and i .,,,, k n g t l l = 1 150nirn

AISI M7 0.80

.US1 0 2 - -

Table 6.6 Fine grinding test results of AISI 3310 steel material

Table 6.7 Fine grinding test results of AlSI 4340 steel material (Residual stress analysis)

Table 6 .8 Fine grinding test results of AlSI T5 stecl material (Residual stress analvsisl

SA - Surface appearance, Li - Light appearance, Da - Dark appearance DB - Degree of burn, nb - no burn, b - burn HE - Hardening effect, Sh - Selective hardening, Oh - Over hardening Td - Thermal damage, VHN- Surface Hardness in Vickers's Hardness Number

Table 6.9 Rough grinding test results of AISI 4340 steel material

Table 6.10 Rough grinding test results of AISI M2 steel material (Residual stress analysis)

Table 6.11 Rough grinding test results of AISI 0 2 steel material (Residual stress analysis)

1 Ex. KO.

1 2 3 4 5

7

Ra, Rt and Rz - Surface roughness in prn T, - Grinding Temperature in " C Q, - Amount of heat entering into the workpiece in W/mm UQ, - Residual stress in axial direction , MPa {Tensile (+)and Compressive(-) ) UQ, - Residual stress in hoop direction, MPa {Tensile (+) and Compressive(-) )

SA

Li Li Li Da

0.27 2.9 0.23 3.1

DB FIE VHN I nb j Sh 287

L i i n b 6LinbSh314

-232 -219

2.7 : 932 265 2.9 1 820 1 242

of,,, ~ o,,,, I

-222 ! -21 I

I Rs ' Rr 1 Kz ; -SS 1 Q.

Sh

1 D a -269 -263

0.19

b

-244 -274

11b nb

310 - - - - ~ --

-218 -216

nb

0.25 1 2.7 T d ] - ~ - 1 -

202 21 1 253

2 1 1 7 Sh 288 ! 0.16

2.4 800 1 225 1 -2% /-212 1 - 1 _

775 795 840

2.8 2.4 Sh

Oh

2.4 2.5 290 0.17

343 0.30 1 2.9 2.5

6.5 Residual stress beneath the surface

The residual stress bencatl~ the surface of few grind hardened (Rough grinding)

s17ecimens are measured using Residual stress analyzer. It is found that the stress

developed at the surface is compressive in nature .The results are tabulated in Table 6.12.

Table 6.12 Micro hardness and Residual stress of grind hardened specimens

Depth Residual stress in MPa and hlicrohardness in VHN of g i i l d l~ardcned ~ beneath the ~ specimens for different trials of mug11 ginding test 1

AISI b12((i1" tr~al) AlSI 02(3"' trial) Residual 1 Hardness 1 Resldual I Hardness stress stress 1 -

206 -276 1 292 -364

6.6 Control of Residual stress

The ground components which are critically important and to be used in dynamic

loading need special attention because their functional efficiency and fatigue life are

predon~ina~ltly gove~ned by the existence of the tensile residual stresses and micro cracks.

The grinding temperature could be reduced to some extent by improving the

machinability characteristics of the work material, the wheel abrasives, and by optimizing

the process parameters. Selection and use of superhard abrasives like Cubic Boron

Nitride could also resist the thermal effects. But even after that the temperature problein

remains acute and even profuse cooling with conventional cutting fluids in the fo1111 ofjet

or mist is virtually unable to solve this problem. The main reasons are;

a) Rapid wheel loading by the clogged chips,

b) Inefficient fuilctioning of the coolant, and

c) Inadequate heat carrying capacity of the con.i;entional fluids.

Seine recent techniques have enabled pai-tial control of the gi-inding temperature.

Aoyama and Inasaki (1984). 1.zporlet1 that the quick raise in high temperi~turc due to

wheel loading can be controlled by online ultrasonic cieaiiing o r tlie lodgcd chips from

the wheel surface. I11 convelltional peripheral sul-face grinding, a thin but stiff air film

developed arounti the wheel surface prevents the cutting fluids fiotn desired l'unctioning.

This problem has also been pairially overcome either by adopting z-z cooling technique

or by using scraper board and coating of the unused wheel faces.

Grabner and Tonsoff ( 1984) have recently reported that CBY wheels ~f properly

manufactured, selected and used provides much less cutting forces. temperature and

hence less tensile residual stresses. But CBN wheels are very expensive.

In spite of such developments, it is still felt necessary to develop sollie sinlple

econonlic cooling process to remove the major portion of the heat much effectively.

Under sucli circumstances cryogenic cooling by agents like Liquici nitrogen (-196°C)

which is gradually being made available in plenty and at lov~el- pl.ices may serve the

above purpose. Research has already been undei-taken in this direction to explore the

effectiveness and economic feasibility of cryogenic grinding.

Uhera and Kumagai (1 968, 1969) reported that cryogenic machining w ~ t h liquid

nitrogen resulted in relatively lesser cutting forces. longer tool life and better surface

conditions. Ippolito and Fillippi (1970) also observed similar results in cryogenically

cooled face milling. Paul and Chattopadhyay (1995) investigated into the role of

cryogenic cooling on the different machinability parameters under different conditions of

gl-inding of some steels and obsel-\fed signiticant rtduction i n surfact. dalnages. ciltting

[orczs and cutting tempel-atures.

Proper application of liquid nitrogen jet reduces grinding temperature drastically

and protects the surface from che~nical and galvanic deterioration. which results in

i) Much better finish and other conditions of the g r o u ~ ~ d surfaces,

li) Lesser grinding forces,

iii) Much less tensile residual stresses, and

iv) Longer tool life.

However, retention of grit shal-pness and reduction oj'plastic flow are expected to

be able to reduce the cutting fol-ces and increase tool lit& due to extreme cooling.

6.7 Results and discussion

For a worlcpiece subjected to grinding. mechanical pinstic deformation, thelnlal

plastic defollnatioll and irreversible deformation due to phase transfomlation are the

major cause of residual stress generation (Zhang et al..I992). The magnitude and nature

of the residual stresses left after grinding at different operating conditions have been

measured by X -ray diffi-action technique using the Residual stress analyzer. The

measurements have been made on ground specimens along the axial, hoop direction and

beneath the surface as shown in figure 6.l.The results are shown in tables 6.6, 6.7, 6.8,

6.9.6.10,6.11 and6.12.

Figure 6.1 Measurement of residual stress in axial (A), hoop (B) direction and depth

beneath the surface (dl) on the grind-hardened specimen

In fine grinding the resitlual stl-ess values obtained c1ea1-ly, sho~v that the

magnitude of the harmful tensile 1.csidua1 stress 11as heen reduced cjuite substantially

(Table 6.6, 6.7. 6.8). Fui-tlier, in rough grinding . i t is found thal the stress developed at the

surface are conmpressive in nature (Table 6.9. 6.10.6.11).

In the grinding process, the induced residual stresseb in a ground specin~en are

due to tlie combined action of mechanical, themmal and transfolmational effccts.

Mechanical forces during grinding expand the surface of a sanlple so that the plastically

deformed surface experiences a compressive residual stress state while the elastically

deformed layer next to the surface layer is left in a state of residual tension. Dui-iiig tlie

grinding process. the temperature dramatically increases in a thin surface layer of the

specimen, while the bulk inner parts remain cool. .4s tlie plastically defonned surface

layer cools, its thern1al contraction is aided by the part's interior, generating tensile

residual stresses at the surface. At certain grindir-ig conditions. the grinding zone

temperature is so high that it can cause transformation of'the phases. Due to the phase

transformations. change in the specitic \.olume of tlic different phases reduces the tensile

residual stresses or improves the coiimpressii~e residual stresses.

In fine grinding. at 50 p111 depth of cut. mechanical effects are predominant and

at 75 pnl depth of cut the~mal effects influence the tensile residual stress to decrease. At

100 pm depth of cut. the grinding zone tenlperature is above the Acl line of the iron

carbon equilibrium diagram. Partial phase transforn~ation from ferrite to austeiiite could

take place at this conditioil and reduce the already induced tensile residual stresses. This

call be the reason for the reduction in the residual stresses at 100 pm depth of cut as

indicated in figure 6.2.

A significant obsei-vation made froni the figure 6.3 is that, in low stress grinding

(low metal removal rate) tlie thermal effect is less important and the mechanical effect

overtakes the thermal aspect and the superimposition of the two results in a low tensile

residual stress profile in the A,1 - A,3 tenlperature zone ( i.e. 730CC -9 10°C).

- 1

I - AIS133 10 + AISI 4340 + AISI T5

Depth of cut in microns

Figure 6.2 Depth of cut Vs Average Tensile Residual stress

I + AISI 33 1& AISI 434WAISI T5 1

Grinding temperature in Degree Celsius

Figure 6.3 Grinding Temperature Vs Tensile Residual stress

From tables 6.9,6.10 and 6.1 1, it is evident that the stress developed at the surface

for the materials AISI 4340, AISI M2, AISI 0 2 (subjected for high metal removal rate

grinding) are compressive in nature. This shows that the mechanical effect overtakes the

thermal aspects and the superimposition of the hvo results in a residual profile that is

predominantly in the compressive zone.

The compressive residual stress developed on the surface of the grind hardened

components shows that there is phase transformation of ferrite or pearlite to martensite.

The compressive state of residual stress enhances the fatigue behaviour of the grind

hardened components.

The residual stress profiles of the specimens, AISI 4340, M2, 0 2 are shown in

figure 6.4.From the figure it is evident that there is a considerable induced compressive

residual stress (175 MPa) upto 0.6mm. After that it decreases with increase in depth

beneath the surface. But upto 0.85mm the stress developed is compressive in nature.

Further, the magnitude of the compressive residual stress is more for the material which

is having higher percentage of carbon content. This is a significant finding in this study.

1 -- A151 4340. h1SI M2 AISI 0 2 1

Depth beneath the surface in mm

Figure 6.4 Residual stress Vs Depth beneath the surface

Most thermal analysis of grinding processes are based on the moving heat source

theory and according to this theory the Peclect number (PC) of a grinding process ,

reflects the variation of the grinding feed speed and thus the thermal energy or difhsion

rate through the ground surface. If all the other grinding conditions are the same an

increase of Pe results in a decrease in grinding temperature and an ii~crease of cooling

rate (Mofid Mahadi and Liangclii Zhang. 1999).Lower values of grinding temperature

(with in the Acl - Ac3 I-ange) produces increased compsessive residual stresses 01.

educed tensile residual stresses (Tables 6.9. 6.10 and 6. I I ) .

PC - Peclect number, 1;s - Velocity of wheel. 2a - length of the heat source or dialnetel of

wheel, cc - Thennal diffusi\,ity of the wheel

T,- Grinding temperature, R- partition ratio, Q, - Heat generated per unit area per unit

time. k , - Thelma1 conductivity of the wheel

Table 6.12 indicates that Res~dual stress decreases fi-om the surface to the

subsurface; it clearly shows that, at the boundary of the rnai-tensite zone. a rapid change

of residual stress occurs due to the sudden change of workpiece lsroperties Further, this

table reveals that greater the material hardness greater will be coinpressive resldual stress.

The fine grinding and rough grinding experimental results indicate that at higher

depth of cut, either over hardening or thermal damage of components occurs and this

effect is undesirable. The results indicate that if wheel speed increases specific grinding

energy increases, intuin residual stress also increases.

A significant relation bztween tempel.ature and resitiuai stress is founcl fi-om the

results and when the wheel n.ol-1~ contact zone temperature is low, the stress fomiaiion is

collipressive stress or low tensile stress. It is a desirable aspect from the fatigue strength

point of view of the component.

When a ~ ~ ~ o r k p i e c e experiences the critical temperature variation in grinding,

phase change occurs at a certain distance away from the grinding zone as demonstrated

by figures 6.5, 6.6, 6.7 and 6.8.

Figure 6.5 The subsurrace microstrr~ctr~re of AlSI 4340 grind hartlencd component \+ith less white etched areas

Depth beneath the surface in mm

Figure 6.6 The subsurface nlicrostructure of AISI TS grind hardened component with moderate white etched areas.

Depth beneath the surface in 111111

Figure 6.7 The subsurface microstructure of AISI M2 grind hardened component with more white etched areas.

Depth beneath the surface in mill

Figure 6.8 The subsurface microstructure of AISI 0 2 grind hardened component with more white etched areas of moderate conversion of austenite into martensite.

Concluding Remarks

f* The cornpi-essive residual stresses are obtained on the surface of the specimen. It

enhances the fatigue strength of the grounded parts. These negative I-esidual

stresses rnay slow dotvn the crack initiation and propagation in the con~ponents.

Residual stress decreases from the surface to the sub-surface and it is found o u ~

that there is a sig~lificailt relation between temperature and residual stress i.e.,

when the wheel work contact zone temperature is low. the stress formation is

compressive or low tensile. I t is a desirable aspect from the fatigue strength point

of the component.

*:* In fine grinding, i.e., ductile regime grinding, it is observecl that there is a

reduction of teiisile residual stress between the temperature range 720°C to 930°C

(Acl - Ac3) for alloy steels. I t matches with the suggestion given by Brinksmeier

et al. (1982).