studies on the grindability of some alloy steels

Studies on the grindability of some alloy steels

J.K.N. Murthya, A.B. Chattopadhyayb, A.K. Chakrabartic,*

aIndian Institute of Technology, Kharagpur 721302, IndiabDepartment of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721302, India

cDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India

Received 10 February 2000

Abstract

The grindability of an austenitic manganese steel has been compared with that of a microalloyed steel and a low alloy steel, both having

tempered martensitic matrices. Grinding has been conducted in both dry and liquid nitrogen environments. The results indicate that for all

the steels the grinding forces required at cryo-temperature are in general lower than that needed for grinding at ambient temperature. In case

of both the die steel and the microalloyed steel, metal removal mechanism did not change very much due to cryogenic cooling. In case of

the had®eld steel, however, the metal removal mechanism changed in cryogenic grinding. Such difference is caused presumably due to

transformation of manganese-stabilised austenite to martensite at cryo-temperature. It is also known that cryogenic cooling reduces in

general the wheel loading and deterioration of the surface integrity of the steels. # 2000 Elsevier Science S.A. All rights reserved.

Keywords: Grindability; Microalloyed; Cryogenic; Transformation

1. Introduction

The present study has been conducted on a forged quality

niobium microalloyed steel, a low alloy steel and an aus-

tenitic manganese steel. These steels represent contrasts in

metallurgical characteristics. While the heat treated micro-

alloyed steel and the low alloy steel are not likely to be work

hardened too much during grinding at either ambient tem-

perature or cryogenic temperature, the austenitic manganese

steel work hardens rapidly under stress at ambient tempera-

ture and is likely to transform to martensite at cryo-tem-

perature. The present study was designed to understand the

effects of such distinctive metallurgical characteristics on

the grindability of the steels.

2. Experimental procedure

The grinding experiments were conducted on three steels

Ð a forged microalloyed steel, a rolled low alloy steel and a

cast had®eld steel. The microalloyed steel was of an air-

hardening grade and did not need any further treatment.

Grinding test specimens (Fig. 1) were prepared by machin-

ing directly from the as-received material. The semi-®nished

low alloy steel specimens were subjected to a quenching and

tempering treatment and then ®nish machined. The had®eld

steel was ®rst solution treated at 10508C and water

quenched. The grinding test specimens were then prepared

by hot machining. The chemical compositions, microstruc-

tural details and hardnesses of the three experimental steels

are given in Table 1. In order to detect the presence of

retained austenite, if any, specimens of the as-received

microalloyed steel and the heat treated low alloy steel were

also subjected to X-ray diffraction analysis using Co Karadiation.

All the grinding tests were performed with alumina

wheels in a surface grinding machine under `up-cut' grind-

ing mode. The grinding was done under two different

environments, viz., under (i) dry and (ii) cryo-cooling by

liquid nitrogen jet with constant ¯ow rate. As the dressing of

the wheel in¯uences the grinding behaviour, it was standar-

dised. The normal (Pv) and the tangential (Ph) components

of the grinding forces were measured by a KISTLER

dynamometer at varying infeeds but ®xed table traverse

speed. The forces were recorded after a few passes when

the grinding process attained steady state.

From the force and metal removal data, the speci®c

energy consumption was determined. Wheel loading tests

have been carried out for an infeed of 300 mm for all the

steels undertaken. The details of the basic experimental

conditions are given in Table 2. The grinding chips of all

Journal of Materials Processing Technology 104 (2000) 59±66

* Corresponding author.

0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 5 1 6 - 1

the steels for 40 mm infeed under both dry and cryogenic

conditions were collected by placing a glass slide coated

with petroleum jelly on the spark stream side during grind-

ing. The collection of the chips were carried out after 10

passes. The chips were washed with iso-propyl alcohol to

dissolve the jelly and the cleaned chips were separated by a

®lter paper. The chips so collected were dried and their

morphology was examined under SEM. The topography of

the corresponding ground surfaces of all the steels were

examined. The surface ®nish of the ground surfaces were

determined by a pro®lometer. The microhardness of the

specimens across the transverse section near the ground

surface was also determined.

3. Results and discussion

Grindability of any material under industrial speed±feed

conditions is generally judged by various grinding responses

[1±5]. For the present study, the following grinding

responses have been taken into consideration:

1. magnitude of the grinding forces and speci®c energy

requirement,

2. chip formation mode,

3. grinding zone temperature,

4. wheel loading,

5. surface characteristics.

The variation of the grinding forces and speci®c energy

requirements for different steels under different environ-

ments are given in Fig. 2. Fig. 3a and b shows the variation of

microhardness with infeed for the experimental steels under

different environments. X-ray diffraction analysis revealed

the presence of a small amount of retained austenite in both

the as-forged microalloyed steel and the quenched and

tempered low alloy steel. The surface roughness variations

after grinding are shown in Fig. 4. The SEM photographs of

the ground surface characteristics and morphology of chips

for different experimental steels are shown in Fig. 5. Fig. 6

shows the variation of grinding force (normal) with the

number of passes in order to understand the wheel loading

phenomenon.

Grinding of any material may be assessed in terms of

several responses, for example the mode of chip formation,

the grinding force required, the speci®c energy consumption

and the surface integrity. However, all these responses are

substantially in¯uenced by the work material characteristics.

Fig. 1. Grinding test specimens for different aspects of grinding.

Table 1

Composition (wt.%), microstructure and hardness of the experimental steels

Steel

no.

Steel type C Mn Si Nb Ni Cr Mo Microstructure Hardness

(HRc)

1 Microalloyed steel 0.18 1.81 1.01 0.17 0.24 0.60 ± Tempered martensitic structure 40

2 Low alloy steel 0.35 0.5 0.3 ± 0.5 1.5 0.3 Tempered martensitic structure 53

3 Hadfield manganese steel 1.2 10.0 1.0 ± ± ± ± Austenitic matrix free from any carbides 20

Table 2

Basic experimental conditions

Machine Jung horizontal surface machine, 2.2 kW

Wheel A50K5V10

Spindle speed 3000 rpm

Wheel speed 30 m/min

Table speed 5 m/min

Infeed 10±40 mm in steps of 10 mm

Environment Dry

Liquid N2 (stand-off distance of nozzle:

50 mm, jet pressure: 0.35 mPa, nozzle

orientation: 358)

Wheel dressing conditions

Dresser 1 carat, single point diamond dresser

Dressing depth 10 mm

Dressing lead 160 mm

Spindle speed 3000 rpm

Environment Dry

60 J.K.N. Murthy et al. / Journal of Materials Processing Technology 104 (2000) 59±66

Hence, for proper assessment of grindability of any steel, its

metallurgical properties such as initial hardness, suscept-

ibility to work hardening, possible phase transformations

during grinding, etc. must be considered. In the present

study, an attempt has been made to ®nd a correlation

between the metallurgical characteristics of the steels and

experimental observations on several grindability responses.

The steels selected in the present work represent two

opposite extremes from metallurgical standpoint. The

microalloyed steel and the low alloy steel are basically

hardened materials (HRc between 40 and 53). The hardness

of these steels are likely to drop sharply with rise in

temperature beyond 400±5008C due to overtempering. Such

temperature rise is quite common during dry grinding. On

the other hand, the austenitic manganese steel is quite soft to

start with but its surface gets work hardened during grinding

and the hardness rises sharply. Under cryogenic tempera-

ture, however, the austenitic steel is likely to transform to

Fig. 2. Variation of grinding forces and speci®c energy consumption.

J.K.N. Murthy et al. / Journal of Materials Processing Technology 104 (2000) 59±66 61

martensite. The martensite layer has little plasticity and is

unlikely to work harden further. Hence, the behaviour of the

austenitic manganese steel is likely to change during cryo-

grinding. Such drastic change is not expected in the case of

other steels, because the retained austenite content in such

steels is generally quite small (<5% by vol.). Thus, the

selection of the steels was done in a manner that the effect of

work hardening and phase transformations on grindability

may be identi®ed by comparing their grinding behaviour.

The results indicate that there is indeed a distinct effect of

metallurgical characteristics on grindability. First, a com-

parison is made on the basis of the grinding forces required.

The low alloy steel, which had the highest initial hardness

(HRc�53), naturally recorded a rise in grinding force

requirement with increase in infeed up to 30 mm (Fig. 2),

but the grinding force decreased with further increase in

infeed to 40 mm. This obviously suggests that the tempera-

ture rise at higher infeed was so much that its surface

Fig. 3. (a) Variation of microhardness with infeed; (b) Variation of microhardness with depth below the ground surface for an infeed of 40 mm.


hardness dropped through overtempering and hence grind-

ing was facilitated. On the contrary, the microalloyed steel

required more force (Fig. 2) after 30 mm infeed. This

suggests that the retained austenite in the as-forged steel

transformed to stress induced martensite at higher infeed,

resulting in hardness rise. These conjectures are validated by

actual measurement of the microhardness of the surface

(Fig. 3a). The surface hardness of the low alloy steel dropped

and that of the as-forged microalloyed steel slightly

increased on increasing the infeed from 30 to 40 mm.

The had®eld manganese steel behaved differently. Its grind-

ing force requirement (Fig. 2) increased appreciably beyond

20 mm infeed. The surface hardness of this steel also

increased sharply at high infeed (Fig. 3a). Con¯icting

behaviour of the two groups of steels was noticed during

cryo-grinding as well. Paul and Chattopadhyay [6,7] had

reported positive improvement of grindability of carbon

steels in cryogenic environment. In this study, the low alloy

steel behaved more or less in an identical manner during

cryo-grinding and dry grinding. In the case of the had®eld

manganese steel, grinding forces required during cryo-

grinding were less than that during dry grinding up to

300 mm infeed (Fig. 2). Beyond that the force required

increased sharply. The corresponding surface hardness also

increased (Fig. 3a). However, it is interesting that the

microalloyed steel behaved similar to that of had®eld steel

during cryo-grinding. This indicates total transformation of

retained austenite to martensite at cryo-temperature caused

hardening of the surface and boosted up cutting force

requirement. X-ray diffraction analysis of the microalloyed

steel has shown the presence of retained austenite in the as-

received material. On the contrary, the XRD of the low alloy

steel showed that there was practically no retained austenite.

This explains the phenomena observed.

When grindability of the steels are compared in terms of

speci®c energy consumption (Fig. 2), similar effect of

metallurgical characteristics on grindability is noted. Spe-

ci®c energy is governed mainly by the grinding forces which

depend on the metallurgical characteristics of the work

material and wheel loading. In general, speci®c energy

consumption decreased with increase in infeed both during

dry and cryo-grinding in case of all the steels. Compared to

the inner layer, the top layer remains hardened and hence

requires larger forces and higher speci®c energy. During

cryo-grinding speci®c energy consumption was lower in all

cases but the drop in speci®c energy requirement at low feed

(10 mm) during cryo-grinding was conspicuous in the case of

the had®eld manganese steel. This is indicative of appreci-

able embrittlement of the steel surface at cryo-temperature.

In general, however, the hardness distribution from the

surface to the core was more uniform and of higher magni-

tude after cryo-grinding than that after dry grinding (Fig. 3b).

The roughness of the ground surface depends very much

on the mode of chip formation and the intrinsic toughness of

the steel. The surface roughness varies with infeed as shown

in Fig. 4. However, the variations does not represent any

de®nite pattern. Even then some general trends may be

noted. For example, the ®gure indicates that the roughness

was signi®cantly lower after cryo-grinding compared to that

after dry grinding for all steels and at all the infeeds. Of

course, the surface ®nish does not always indicate surface

integrity. The surface roughness reached a peak value at a

particular infeed. In the austenitic manganese steel, the

surface roughness increased steeply at 40 mm infeed. The

force requirement was also highest at this feed, but in the

case of other steels, the roughest surface was not developed

at the infeed where the force requirement was highest.

Further experimental evidence was sought through SEM

study of chips and ground surfaces. An analysis of the SEM

photographs of the ground surfaces and chips provide an

insight into the mechanisms of metal removal from the steels

Fig. 4. Variation of surface roughness.


in dry and cryo-grinding (Fig. 5a±c). The metal removal

mechanism is directly related to the phase transformations

during grinding of the low alloy steel. The initial hardness

was high (HRc�53). The metal removal occurred by shear-

ing, ploughing and delamination. This is evident from

Fig. 5b. Delamination normally occurs by nucleation and

propagation of subsurface cracks. Both long ribbon and

delaminated fragments are observed as shown in Fig. 5b

Fig. 5. (a) SEM photographs of ground surface characteristics and chip morphology for microalloyed steel for an infeed of 40 mm. (i) and (ii) dry, (iii) and

(iv) cryo grinding. (b) SEM photographs of ground surface characteristics and chip morphology for low alloy steel for an infeed of 40 mm. (i) and (ii) dry, (iii)

and (iv) cryo grinding. (c) SEM photographs of ground surface characteristics and chip morphology for hand®eld steel for an infeed of 40 mm. (i) and (ii) dry,

(iii) and (iv) cryo grinding.


of the grinding chips. At cryo-temperature, the ductility of

the material decreased. As a result, the proportion of the long

ribbon type chips decreased and the tendency for fragmen-

tation of chips increased. But the basic grinding mechanism

remained the same. A comparison of the appearance of the

surfaces produced by dry and cryo-grinding (Fig. 5b) also

con®rm such similarity. The microalloyed steel developed

ground surfaces (Fig. 5a) similar to those of the low alloy

steel. Both ploughing marks and cratering could be

observed. But a distinct effect of cryo-grinding could be

observed after cryo-grinding. The proportion of ¯aky chips

produced by delamination increased after cryo-grinding.

This suggests that the material became more brittle after

cryo-grinding. A possible reason could be conversion of

retained austenite to martensite under cryo-cooling. Since

the cushioning effect of retained austenite is lost, the nuclea-

tion and propagation of subsurface cracks at the cryo-

temperature is likely to increase, which in turn will facilitate

the delamination process. The sharp increase in cutting force

requirement at 40 mm infeed also lends support to such a

possibility. The behaviour of the austenitic manganese steel,

however, was unique. The surface of the steel after dry

grinding shows numerous cracks. The ploughing marks were

also not very prominent in Fig. 5c. Subsurface crack nuclea-

tion and its subsequent propagation is expected to produce

more of ¯aky chips by delamination. This was exactly the

case in dry grinding of had®eld steel. The proportion of

ribbon type chips was very small. Cryo-grinding produced a

very de®nite change in its grinding behaviour. The propor-

tion of leafy chips decreased and that of ribbon type and

spherical chips increased. On examination of the ground

surface, it was further observed that a large number of chip

particles were actually redeposited on the surface. A pos-

sible explanation for this may be provided if the transforma-

tion of manganese-stabilised austenite to martensite at cryo-

temperature is considered. The ®rst visible effect of such

transformation should be a rise in grinding force. This in fact

was so. The removal of this martensite top layer by the grits

of the grinding wheel progressed in the same manner as in

the low alloy steel. The sticky overheated chips were

redeposited on the job surface (Fig. 5c). Another evidence

of (Fig. 5c) excessive heating is the presence of a larger

number of spherical chips in the debris. The spherical chips

are produced by complete melting and decarburisation of the

chips.

Fig. 6 shows the pattern of increase in the grinding forces

with the passage of number of grinding passes. Generally,

the forces increase and then may ¯uctuate in grinding ductile

metals due to several reasons such as spring-back action,

change in metallurgical characteristics of the work surface,

wheel loading and self-sharpening effects. In the present

work it has been observed that cryo-cooling had predomi-

nant effect on such increase/change in the grinding forces in

cases of had®eld steel and the low alloy steel.

The force might have increased sharply in case of had®eld

steel mainly due to quick hardening effect with metal

working. In other steels, autosharpening might have main-

tained the forces uniform for a longer time.

Fig. 5. (Continued ).


4. Conclusions

1. The grinding forces for dry grinding was more when

compared to cryo-grinding except in the case of the

had®eld steel at maximum infeed due to the transforma-

tion of retained austenite to martensite at cryo-

temperature which led to hardening of the surface, thus

increasing the cutting force.

2. The speci®c energy required for cryo-grinding was

lower than that for dry grinding in case of all the steels.

3. Materials removal during grinding of both low alloy

steel and microalloyed steel having a tempered marten-

site matrix occurs by shearing, ploughing and delamina-

tion. No signi®cant change in this basic mechanism occurs

on changeover from dry grinding to cryo-grinding.

4. In had®eld manganese steel delamination predominates

during dry grinding. But during cryo-grinding, shearing

also accounts for a signi®cant proportion of metal removal.

References

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[3] E.D. Doyle, S.K. Dean, An inside into grinding from materials view

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30 mm.


studies on the grindability of some alloy steels

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