chapter surface modification and surface...
TRANSCRIPT
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CHAPTER -5
Surface Modification and Surface Melting
5.1 Introduetiori
Engineering the surfaces of components used in automotive and aerospace
engineering to improve their lives and perfoi-mances is the active area of research
Suitable Ther~nali Mechan~ca1,"Thenno chemical surface engineering treatineilts will
produce extensive rea~~angement of atoms in metals and alloys and corresponding
~narked variations 111 Physical, Chemical and Mechanical propelt~es. .Among the more
linportant of these treatments are heat treatment processes such as hardening by
Quenching, Illduclion hardening and Case Carburizing which rely on phase
tlnnsformations to producc dcslrcd changes in mechanical piopel-tles. Other processes
\vhe~-e phase transformations occur are cast~ng, welding and machining etc.
In industry, different heat trzatinent methods are used for production of reilulred
surface layer properties. A classification of the surface liardenlng methods based on three
main mechanisms viz., mechanical, thermal and thei-ino chenlical alterations of surface
layers is given in figure 5.1. It is clear from the classification that surface layer hardening
by lnachining remains an unestablished method of obtaining the required mechanical
propelties on the component surfaces (Brinksmeier et al.. 1997). With the exceptioil of
some investigations into surface layer hardening by friction hardening, a gap exists in the
development and use of surface layer hardening by machining.
Based on the chip removal and chip formation mechanisms, it is understood that a
substantial pait of cutting energy in i~lachining is transformed into thermal energy. The
dependence of theimal loading of the workpiece due to the development of heatflux at the
surface layer on the machining conditions and type of machining process was first
described by Snoeys and Maris (1978). The process parameters and the thermo-physical
properties of the work and tool inaterials principally influence the effective work surface
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temperature. if the material in f h i s~lsface layer is heated above the chasacreristics
temperature (720°C - 910°C) ciuring lnachiliing operation, diffusion and phase
transformation take place. Thus. most of the research findings show that the mechanical
and metallurgical characteristics of machi~ied surfaces can be contl-olled by controlling
tile effective \vol.k si~rface temperatut.o.
Established methods
studied methods
O unestablished methods
Figure 5.1 Surface strengthening processes,
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With this in mind. in this WOI-li a study has been made in using the the]-ma1 energy
generated in grinding for obtaining required hardness ievei at the surface layer without
having adverse effect on the quality of the job. In this study. the surface metallurgy o f the
ground hardened con~ponents. turn hardened and conventionally surface hardened
components have also been analyzed and compared to get a better understanding on this
phenomenon.
5.2 Sul-face hardening processes
4 thorougl~ knoti8iedge of'\ nrious su~.i'aci. hardriling PI-uccsscs hslps the engineers
and researchers in deciding the type o f process to be employeti to obtain requirctl surface
characteristics for the efficient and smooth p e r h n n a ~ ~ c e o f the engineering con1poilents
in their allotted functions. Surface hardening can basically be achieved by two ciifiirent
methods. The ail11 of both the methods is same. The first method is thel~nochemical
surface hardening, in which the surface composition o f the steel changes by ditiusion o f
carbon and or nitrogen 01- sometimes by other elements. Carburizing. Cyaniding,
Carbonitriding. Plasmanitriding. Boronizing. Chromizing and Toyota diffusion process
are some o f them. The Second method is Theinla1 impact surface hardening which
involves phase transfo~mation by rapid heating and cooling of the outer surface (Flame
hardening, Induction hardening, Electron Bean1 hardening, Laser surface hardening and
Salt bath hardening). In Majority of the industries, the gas carburising and induction
hardening processes are very common (Rajan et al., 1991). Hence, the objectives and
principles of the Inductioa hardening and gas carburising processes are discussed below:
5.2.1 Inductiorl hardening
Generally this process is used to surface harden crank shaft, call1 shall, gears,
crank pins and axles. In this process heating o f the components is achieved b y
electromagnetic induction. A conductor (coil) c a l ~ i e s an altelnating current of high
frequency which is then induced in the enclosed steel part within the magnetic field o f the
coil. As a result, induction heating taltes place. The heat so generated affects only the
outer surface of the steel components due to skin effect. T h e component is heated usually
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for a few seconds only. immediately the surfhce i.s quenched b!. thejet of cold water. Due
to cluenching a martensitic strucl~ire is Ihl-med \ihich makes tile outer surfhce hard and
\\,ear resistance. The hardening temperature ranges from 760 to 930. OC. Accorciing to
the Carbon content and alloy addition. the temperature is fixed (Palaniradja et al.. 2005).
5.2.2 Gas carburising
Carburising is the niost widely used process for surface hardening of steels. In this
process, carbon is diffused into steel by heating above the transformation temperature and
holding the steel in contact with a carbonaceous niaterial. wliich may be a solid medium.
liquid. or a gas. Under such condition, carbon is absorbed in solid solution in austenite.
As the solubility of carbon is more in the austenitic state than i l l fen-itic state. fully
austenitic steel is essential for carburising. Carburising can be divided into Pack
carburising, Liquid Carburising and Gas Carburising. Among them Gas Carburising is
the nlost widely used industrial heat treatment process.
Gas carburizing is a process in which the surface of the conlponents is saturated
with carbon in a gaseous atmosphere containing carbon. To accomplish this. first the
components are heated in a gas tight funlace in a neutral atmosphere to a predetermined
temperature in the range of 870 O C to 940 "C. Then the furnace is flooded with a suitable
gas such as Propane, Butane, and Kerosene etc. Finally the conlponents are held at this
temperature to allow diffusion of carbon into the case. After the carburizing treatment is
over the components are quenched to get the required hardness, wear resistance and
fatigue resistance on the surface, supported by a tougher core. A striking feature of gas
carburizing process is that in this process the original toughness and the ductility
remains unaffected even after the heat treatnlent (Palaniradja et al., 2005).
5.3 Grinding and heat-treating
Many research findings indicate that there is definite evidence that during
grinding the workmaterial is subjected to conditions akin to the heat treatment process. It
is because of the fact that in grinding the surface temperature of the work lnaterial rises in
the range of 1000°C and 1700 "C that too with shai-p heat gradient. This can have
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considerable effect on the surface integrity of the workpiece. As the wheel abrasivss are
insulators. most of the heat generated during gsinding goes into the work piece. M%en
fine grinding with A120;. without a coolant. about 80% of the total heat energy ends up in
the workpiece (Sato, 1961; Malkin. 1978). This is because of the lllaterinl removal
mechanism involved in gl-inding. The ploughing aclion before the separation of chip
causes higher late of strain due to deformation and which in turn colltributes to the
generation of heat that too at a distance ecjual to the depth of cut from the s1;in of he
\iorkpiece.
111 grinding, virtually all mechanical energy is converted into the thermal tnergy.
heat. The heat conducted into the worl
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5.4.1 Experimentation in Gas carburising and Induction hardening
To compare the surface metallurgy of the conventionally surface ha-dened
matesial with the machine-hardened material. trials are cai-ried out in Gas Carburising and
Induction hardening furnaces. The conditions undei-m hich tlie trials have been ca~r ied out
and the results obtained fi-om the above-ment~oned trials are given in Tables 5. I . 5.2 and
Table 5.1 Details of Gas carburising --
I Material Used . AISI 33 10 - Lou Carbon atcel Diameter : 1 jmm ; Length : 200 inln I Fuinace Details :
I ! Methanol - Acetone Unithe~m Gas Carburising Furnace of 3 /z in depth ~ Electi-ical rating : I30 K W
I Temperature : 870°C to 940 "C
Operating conditions: I Holding time - 200 minutes (maximum) Quenching time - 90 ininutes (maximum) Carbon potential - I 1 1 OmV.
Table 5.2 Details of Induction hardening
Material Used : AISI 4140 - hledium Carbon steel
Diameter : 25mm ; Heating length : 1501nin I
Furnace Details :
440V, 31nm coupling distance Inducto Heat induction harclening device
Frequency : 1000 to 10.000 cycles per second
Temperature : 750 to 800°C. I
Operating conditions: i Power Potential - 5.5 kw/inch2 Scan speed - 1.72 mlminutes I
Quench Flow rate - 15 Litei-slminutes I
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Table 5.3 Micro hardncss \values of sul-face hardened materials
, Depth beneath the surface 1 Average Value of Micro ha1-dness in VHN ! in mm
/ Gas Carburized (33 10) Induction hardened (4140)
*Even though. a total case depth of upto 4 mm is possible in the con\entionaI
surface hardening processes, the measurement of microhardness is restricted to 1.92mm
only. Since higher case depth is not possible with inachine hardening.
5.4.2 Experimentation in Rough turning (turn hardening)
Tu~ning is a process that removes excess material by a wedge -shaped tool to
generate specified geometry and surfaces. Turning process itself is uniquely characterized
by high stresses, high strain rate, high temperature, and generally short interaction time
with the workpiece malerial encountered during chip formation. Thus, tunling process
alwaj~s results in some metallurgical changes at the workpiece surface, such as
niicrostructural alteration, microhardness changes. and residual stresses. These changes,
which are coming under surface integrity classification, are impol-tant, especially in
finishing processes: for they affect the performance of the workpiece in its allotted
function. An investigation has been conducted to identify the potential of applying heat
energy generated in turning to modify the surface layer of steel parts. The conditions
undemlhich the tu1-n hardening experimentation are carried out given in the Table 5.4 and
5.5. The results are reported in the Table 5.6.
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Table 5.4 Details about materials subjected for rough ti lrnit~g test
Table 5.5 Lathe Machine details and the operating parameters
S.So
I 1
7
1 i .I L
Machine : HMT Lathc Model : NH 26 Cutting tool signature : 0.7mm radius, 5" Clearance angle
and -30" Rake angle Cutting conditions : Depth of cut 1000,1200 and 1400pm Cutting speed : 3-5m:sec Feed rate : 0.15 -0.25inni/rel~. Length of cut per pass : 90- 100mm
Material
Medium carbon st~uctural steel Diameter
=3 51111n Medium carbon spring steel I
High carbon liigb specd ~ A I S I T ~ 1 O R O
I
I 5On1m steel I
Table 5.6 Micro liard~less values of the turn-hardened materials
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5.4.3 Experimentation in Rough grinding (Grind hardening)
Rough grinding is carried out as per tile conditions given. under Table 5.7 and 5.8
and the results are tabulated in the Table 5.9.
Table 5.9 Micro hardness values of grind hardened materials Depth beneath the I .4\erage Value of Mlci-o haidness of ground specimen for various surface in mill depth of cut , ~n VHN
AISI 4140 I .&IS1 9255 AISI T4 I
1000 1200 1400 1 1000 1200 1400 1000 1200 1400
0 48 1
Table 5.7 Details about materials subjected for rough grinding test
S.No
I
Type of wheel used
Size
Mediuln carbon spring 2 ~ steel
Material 9'0 of
P.120: Diameter
I =35mm
AlSl 9255 0.55 X1203 and Lengtli =
Medium carbon structural
-...- steel
, High carbon high speed 1 7
I steel .41SI T4 150mrn
Table 5.8 Grinding conditions (Rough grinding) 1 Machine: CGU 500 Vlodel His11 Precision plunge type c);lindrical I
grinding machine. Grindii~g wheel speciiicarioli :WPsel diaineter -35Omni and \vidth- 60mni ~ Cutting conditions : Depth of' cut 1000.1200 and 1400pm Cutting speed of the wheel : 251n!sec
1 Cutting speed of the work piece : 1 misec L Number of passes : Ranges from 5 - 10
' Designation Carbon
AISI 4110 0.40 _-
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+ In tul-n hardening and grind l~nsilening tile 1~1ic1.o liarclness \,ariation is upto a niasi~iiun~ of2111111 beneath the surface and 11ence i t is measul.ed upto 1.921iirn. Table 5. 10
s h o ~ s the maxin~uiil and minimu~n hardness obtained in the different surface
modification processes for different AISI steel materials subjected for experimentation
Table 5.10 Maximum and Mininlum micro hardness obtained in the different surface modification processes
1 3 Induction ~ 630 1 210 hardened I 1 1.4mm I
/ Nameof the 1 Maximum S.NO. i material micro hardness , inVHN
1 AIS14140 236
Minimum micro hardness in \'HN
210
i
.At n depth of 1
Remarks
At a depth of 1400 pin T u ~ n hardened
AISI 4140 I Grind hardened
3 15
AISI4140 I
1 AISI 9255 1 241 1 I 215 Tum hardened I I
cut 1200 p1n 1 At a depth of cut 1400 pm At a depth of cut 1200 p 1
210 At a depth of
, ' Case depth AISI 3310
54 1 220 1 maximum Gas Carburised 1 5611-1111
I
215
230
230
5
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5.5 Surface modification -A comprehensive study
Long back, grinding was named as an unintentional heat treatment method.
AISI 9255 365
Grind hardened 1 AISI T4 305
Turn hardened
Recently, the idea was taken up and fundamental investigarions were call-ied out to
AISI T4 Grind hardened
develop suitable strategies of process control for well-aimed modifications of surface
3 86
layer (Komandurai, 1993)
In finishing operatio11 of engineering components, grinding process tinds an
important place because they allow the machining of both hard and brittle materials to
high dimensional and geo~netric accuracies with a good surface finish (Hahn: 1962).
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In general. engineering colnponents have to be subjected to a hardening heat
treatment process at the surface layer to ha\,e both fatigue strength and wear resistance
properties. Highly loaded components subjected to relative inotions are often surface
hardened using a variety of processes based on thel-ma1 01- the]-mo-chemical impact. e.g.,
Indllction or Case hardening. Such heat treatment processes. which are restricted to the
surface layer. arc connected with some typical ad\,antagcs compared to ti111 hardening
lieat treatments. In industry, dif'ferent heat treatlnent methods arc used for the production
of required surface layer properties. The problem is that these processes cannot simply
be integrated into the production line causing economical disad\.antages.
Further, surface hardening is a critical process due to its effects on production cost
and part quality. It also has greater influence on energy efficiency and inipact on the
environment that receive fast growing attention. Depending on part dimension,
geometry, and batch size, the cost of surface hardening varies and it can have
considerable effect in its applications. Moreover. the cycle time and the associated down
time of surface heat treatment substantially lower productivity v/hich may rum out to be
the bottleneck to process flow optim.ization. The proposed new approach is to utilize
grinding process for surface hardening also while it is pertbrming its rllajor function as a
material removal process. The propoxci surface tnodification approach by grinding
provides possible benefits such as higher productivity and lo\ver production cost.
The superimposition of theirnal and mechanical load is able to cause alterations in
the surface layer like, tempered zones, white etching areas etc. These effects were
investigated and discussed by Field and Kahles (I97 1).
The process parameters and the thei-mo physical properties of the work and tool
materials principally influence the effective work surface temperature. If the material in
the surface layer is heated above the characteristics temperature (870°C - 9 10 "C) during
machining operation, diffusion and phase tiansfoimatio~l take place. So. most of the
research findings show that the surface integrity of machined surfaces can be controlled
by controlling the effective work surface temperature.
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In any case, [lie generated lleat in grinding is considzrecl as a restricting Ikctor that
necessitates the use of coolants and the ssjection of moderate grinding conditions. The
abo\.e said limitation causes one to investigate how this process energy can be effectively
controlled and utilized to improve the surface integrity and also prevent thelmal dainages.
To obtain this. attempt is made with different steel materiais for different grinding
conditions. The experilnental obsz~liations are tabulated (Table 5.1 1 to 5.28) and the
corresponding characteristics curves are drawn (Figures j . l i to 5.30). The
~nicrostructures of \:arious surface ~nadified AlSI steel materials which are sul~jected to
grinding process are also reported (Figures 5.2 t05.10).
In this experiment, the forniation ot'a heat-trtateci zone (,HTZ) beneath the surface
is characterized by a significant hardness increase. This heat treated zone is compclscd of
t\vo different boundary layers - The surface upto a few micrometer thick ~vhite etclling
area with an extremely high hardness followed by a hardened st~ucture consisting of
martensite and carbides. The interesting observation or fact is that in tile experi~nents
conducted no time surface cracks have been developed on the grind hardened parts.
.Table 5.11 Details about materials subjected for grind hardening test
3
4
Medium c a ~ b o n st~uctulal steel
Med~uin carbon s t ~ u c r u ~ a l steel
1 5 Med~urn carbon spring steel AISI 9255 0 55 and 1 Lengrh = 15 Omni
8 High carbon tool steel AISI 1095 0 90 A1203 1
AlSI 1040
1 High carbon non shnnklng
die steel
0 35 A120:, I
AISI D2
Diameter ~ =35mm 1
AiSl 4140 , 40 I
1 70 A1203
A120;
1
A1203
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Table 5.12 Hardness a t various depths for different depth of cut for AISI 3310
beneath the Total depth of cut with varying number of passes, in s.No. surface in mm 1 I 'HN H 1400
I
Table 5.13 Hardness a t various depths for different depth of cut for AlSI H21
Depth I
Rlicro hardness values for different
1 1 Depth I Micro hardness values for different 1 1 beneath the Total depth of cut ~ v i t h varying number of passes, in
S.No. 1
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Table 5.14 Hardness a t various depths for different depth of cut for AlSI 1040
8
Micro hardness values for different Total depth of cut with varying number of passes, in ~
S.Uo. surface in rnrn / V H N I I Dc / 100 600 SOU 1 1000 I 1200 / 1100 1
Table 5.15 Hardness a t various depths for different depth of cut for AISl 1095
Depth Micro hardness alues for different I beneath the Total depth of cut with varying number of passes, in
S.No. surface in mm 'VHN I ' Dc 400 600 800 1 1000 1200 1400
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Table 5.16 Hardness a t ~ a r i o u s depths for different depth of cut for AISI 01
Table 5.17 Hardness a t various depths for different depth of cut for AISI D2
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S.YO. I
I Micro hardness values for different 1
depth of cut with varying number of passes, in VHK
400-T00 800 1 1000 1200 1 1400
Depth beneath the
surface in rnrn DC
Micro hardness values for different Total depth of cut with varying number of passes, in
VHN I 3 4 T I
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Table 5.18 Influence of number of passes on hardness and surface roughness for AISI 3310
Table 5.19 Influence of number of passes on hardness and surface roughness for AISI H21
Table 5.20 Influence of number of passes on hardness and surface roughness for AISI 1040
S,No.
1
/ S.No. 1 Number of 1 Maximum / Ra ,pm ! Rt, pm I Rz, p111 1
Number of passes
5
Maximum hardness
3 14
Ra, pin ' Rt, 1-lm 0.21 2 . 2 0
Rz ,pi11
2.50
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Table 5.21 Influence of number of passes on hardness and surface roughness for AISI 1095
Table 5.22 Influence of number of passes on hardness and surface roughness for AISI 0 1
S.No.
/ S.No. 1 Number of I Il.laximum / Ra, IJm / Rt, pln 1 Rz, pm 1
Number of passes
Table 5.23 Influence of number of passes on hardness and surface roughness for AISI D2
I J 1 J lJV ( CI.AI 1 . 0 I L.00
2 7 3 17 / 0.19 2.30 2.52 , '
passes / hardness 1
1 S.No. 1 Number of / Maximum 1 Ra, pm Rf, lim : Rz, Pm 1
Nlaximurn hardness
4 5 6
I
Ra, pm
1 1 13 I5
1
1 2 3 4 5 6
Rt ,pm / Rz ,pm
5 270 I 0.18 / 1.89 / 2.62 1 I
3 54 367
passes -- 5 7 9 11 13 15
0.17 0. 16
hardness 34 1 3 74
-- - 2.80 _ 2.89 2.63
I
319 / 0.25 / 3.02 / 3 .11
2.24
7 0.19 2.22
396 0.17 2.68
- 2.57 ~ 2.32 2.84
434 , 0.15 1 3.01 2.24 477 0 . 1 7 2.48 , 2.74 424 0.22 3.05 3.30
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Table 5.24 Influence of infeed rate on hardness a t various depths for different depth of cut AISI 4140
Table 5.25 Influence of infeed rate on hardness a t various depths for different depth of cut AISI 9255
I Depth beneath S.No. the surface in
Micro hardness a t different total depths of cut with varying number of passes, in VHN
I
I Micro hardness at different total depths of cut with I
Depth beneath varying number of passes, in VHN S.No. the surface in 1
I m m 2.3n1mimin
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Table 5.26 Influence of infeed rate on hardness a t various depths for different depth of cut AISI T4
/ Micro hardness at different total depths of cut with Depth beneath 1 varying number of passes, in VHN the surface in
Table 5.27 Influence of infeed rate on hardness penetration depths for different AISI materials
Table 5.28 Influence of % of carbon on hardness for the AISI steel materials
I I I
S.No. 1
3 4
6
Material
AIS14140
I I 0.6 I 1.12
AISI 9255 ~ 1.25 1.12 2.3
7
9
Infeed in mmlmin 0.6 1.25
1.26
Hardness penetration depth, in mm 0.98 1.26
2.3
I 0.6
1.26
1.26
AISI T4 I 1.25 I 1.26 2 3 1.40
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Figure 5.3 Microstructure of AISI H21
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Figure 5.6 Microstructure of AlSI 9255
Figure 5.7 Microstructure of AISI T4
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Figure 5.8 Microstructure of AISI 1095
Figure 5.10 Microstructure of AISI D2
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Depth beneath the surface in rnm
Z 245 1 240 -
zxlnnn"1200 1 4 0 7
-5 235 - % 230 - a,
225 - k 220 - .c 2 215 - .Y 210 --- I I
I 0 0.5 1 1.5 2 2.5 Depth beneath the surface in mm
Depth beneath the surface in mm
Figure 5.11,5.12,5.13 Micro hardness at various depths for different total depth of cut of turn hardened materials
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Depth beneath the surface in rnm
Depth beneath the surface in mm
Depth beneath the surface in mm
Figure 5.14,5.15,5.16 Micro hardness at various depths for different total depth of cut of grind hardened materials
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800 -- > --- &Gas C a r a ~ r l s e d I
600 A - 2 - m -d-= > a-z-z Depth beneath the surface in mm
Figure 5.18 Micro hardness Vs depth beneath the surface for different total depth of cut (AISI 3310)
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Depth beneath the surface in mm
Figure 5.19 Micro hardness Vs depth beneath the surface for different total depth of cut (AISI H 21)
Depth beneath the surface in mm
Figure 5.20 Micro hardness Vs depth beneath the surface for different total depth of cut (AISI 1040)
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Depth beneath the surface in mrn
Figure 5.21 Micro hardness Vs depth beneath the surface for different total depth of cut (AISI 1095)
Depth beneath the surface in mm
Figure 5.22 Micro hardness Vs depth beneath the surface for different total depth of cut (AISI 01)
-
Depth beneath the surface in rnm
Figure 5.23 Micro hardness Vs depth beneath the surface for different total depth of cut (AISI D2)
' + AISI 3310 1
Number of Passes
Figure 5.24 Influence of number of passes on hardness
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Total Depth of cut in mm
Figure 5.25 surface roughness values for different depth ground (AISI 1040)
0 0.5 1 1.5
Depth beneath the surface in mm
Figure 5.26 Hardness at various depths for different total depth of cut for an infeed rate of 0.6 mmlmin (AISI 4140)
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Depth beneath the surface in mm
Figure 5.27 Hardness at various depths for different total depth of cut for an infeed rate of 1.25 mm/min (AISI 9255)
Depth beneath the surface in mm
Figure 5.28 Hardness at various depths for different total depth of for an infeed rate of 2.3 mmlmin (AISI T4)
-
& 1.5 -1 E 1 C
e - 1 1 -- - - .- 2 1 'a El
-"AIS14 140 0 .e I u
+AISI 9255 0.5 -
L AISI T4
u Q)
E P, rn 0 -I - --- - --- - - Y) : 0 0.5 1 1.5 2 3 2.5
Feed rate in mm/minutes
Figure 5.29 Influence of federate on Hardness penetration depth for the materials AISI 4140,9255, T4
_I AlSl D2 1 AISI 9255 AlSl 1095
1 AISI 3310 AISI 4140
Carbon percentage
Figure 5.30 Influence of percentage of carbon on Hardness for the materials AISI 3310, H21,1040,4140,9255, T4,1095,01, and D2
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5.6 Surface melting in grinding
Heat generated in the contact area between a wheel and work piece (grinding
zone) is the main cause for the deterioration in the ~netallurgical propel-ties of the work
piece, dimensional accuracy and wheel life ( Shaw, 1984a) In order to estimate and
predict these damages due to thermal effect, nolnlal and ab~loimal grinding conditions
have been investigated experi~nentally and reported hel-e.
One of the first attelnpts to calculate grinding temperatures \vas rt'portzii by
Out\vater and Shaw (1952). 111 this analysis it was assunled that all the grinding energy
uJent into chip formation.
4 somewhat different approach to the calculation of grinding tempcl-ati11.e was
taken by Sato (1961). The surface tenlperature was calculated using the concept of an
Instantaneous heat source.
Malkin (1984) combined the concept of both a local and average gi-inding
temperature in order to evaluate the surface temperature distribution in grinding. The
local grinding temperature refers to the temperature rise on the workpiece surface due to
the cutting action of an individual abrasive grain; there is also an additional temperature
rise due to the grinding action of all the other abrasive grains in the grinding zone. This is
called grinding zone temperature.
The grinding temperature is calculated as the sum of a local temperature in the
vicinity of an abrasive grain and a grinding zone temperature over the apparent area of
contact between the grinding wheel and the work piece (Shaw, 1990). The peak local
temperature at the cutting edge of an abrasive grain is found to be close to the melting
point of the work piece. It reveals that, work piece bum occurs at a critical grinding zone
temperature.
For grinding of various steels, burning has been found to occur when a critical
wear flat area is reached. The magnitude of which depends upon the operating conditions
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and the particular steel being ground. Before continuing to grind, it is necessary to redress
the wheel. In this way work piece burn can detelmine the grinding wheel tool life (Tool
life refers the time period between successive dressings). Therefore, the grinding
conditions under which work piece bum will occur are of practical significance. In this
connection four AISI steel materials shown in table 5.29 are subjected for surface melting
study by grinding them under abusive grinding conditions (Total depth of cut - between
1200 microns and 1600 microns with number of passes 4 to 6).
Table 5.29 Details about materials subjected for Surface melting study
Under abusive grinding conditions, thermal damages occur. However, under
optimal conditions, process energy generated can be utilized for surface strengthening
purpose. The grind hardened component shows that there is a phase transformation of
fenite to pearlite to martensite (figure5.31). SEM reveals that there is no sign of melting
of layer of metals even in the thermally damaged components (figure -5.32).
Figure531 Microscopic views of phase transformation of ferrite to pearlite to martensite
S.No
1
2
3
3
Designation Material
Low carbon casehardening steel ---
Medium carbon hot work steel
Medium carbon spring steel
High carbon non shrinking die steel
% of ' Type of tool Carbon ! used Size
AISI 8620 0.18
AISI H2 I
).IS1 61 50
AISI O2
0.30
0.50
0.90
A1203
A1203
A1203
Diameter =35mm
and Length = 150mm
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a) No noticeable microstructural surface alterations (Fine grinding) in AISl 8620
b) Presence of discrete pieces of metals and smeared surface layers i n AISl H21 (no melting of metals)
c) Phase transformed structures a t a grind hardened AISI 1040 steel surface (Grind hardening)
d) Rehardened primary martensitic layer with over tempered Sub- surface zone in AISI TI (Thermal damage with visible burn)
Figure 5.32 SEM photographs of grounded components
5.7 Results and discussion
The comparative study shows that machine hardening is a suitable alternative to
conventional surface modification processes. Further. Grind hardening is more effective
than turn hardening in all aspects. To justify this statement a detailed n~icrostiuctui-a1
analysis, microhardness analysis is presented here.
5.7.1 hIicrostructura1 analysis
Microstructural analysis of a finished surface can provide important information
regarding material properties, reliability and nature of machining. The photomicrographs
of the grind hardened specimens are studied The hardness at depth beneath the surface
and total hardness penetration depth for the various materials also determined and
reported. From the microstructures of Low carbon steel AISI 33 10 and nledium carbon
steels AISI H21. AISI 1040, AISI 4140 and .4ISI 9255 (Figures 5.2. 5.3, 5.4, 5 . 5 and 5.6)
it is evident that the bulk of chip produced during machilung has etched darkly but white
etched bands are also present. This means that the carbide atoms are almobt fully
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segregated all along the ferritic matrix, i.e.. ma~-tensitic structure is fortned which are
visible or~ly at sufficient magnification. Beneath this white layel- is a thin transition zone
in u.hich the prior austenitz grain bou~ldaries etched ~vhite and the grain centers tiark. due
to the presence of Carbon (0.2 to O.5wt. %) ill the materials. The structure consists of
mai-tensite that appears in the foim of parallel needles within the folmer austenitc grains.
The retention of austenite is due to the prese~ice alloying elements like Cr, W, V and Co
etc.
From the microstiuctures o f high carbon steels AISI T4, AISl 1095 .AISI 01.
and AISI D2 (Figures 5.7,5.8.5.9 and 5.10) it is inferred that the bulk csf the cliip
produced during machining has etched darkly bur white etched areas are also present b u ~
dispersed in fewite matrix. Beneath the transition zone there is a dark zone gradually
merging with the stmcture of the substrate. Fulther~ilore, the iirst lath mastensites occul-
forming so called mixed mar-tensite(which are visible at higher illagnification only)
besides the acicular martensite. Generally, this lath i~lartensite will only be obtained if
the carbon content exceeds 0.6wt%.
This etching response suggests that about 800°C has been reached in the bulk of
the chip and about 900°C or more has been reached it1 the white etched bands. which may
be expected as reported by Doyle and Dean (1980). The tenlperatui-e developed at the
surface of the workpiece has an i~iipoi-tant influence in the metal removal. However,
heating and cooling of the surface would occur rapidly and this may have the influence of
producing phase changes in the surface.
5.7.2 Microhardness analysis
The rnicrohardness of the various specimens is found by using Vickers's
microhardness tester. Here. the grinding wheel is assumed to be a moving heat source.
the temperature at the contact zone between the grinding wheel and the work piece is of
very high order, The higher hardness resulted from the outer surface is due to the
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hi-~nation of martensite, which is obtained by short t i~ne austenizatio~l of sur.f>ce layers
lvith self-quenching.
The hardness of the ground materials at various depths for different depth of cut
(~vith different number of passes )are shown in figures 5.18. 5.19. 5.20 , 5.2 1.5.22 and
5.23 for the ~naterials XIS1 3310. .4ISI H 21. AISI 1040, AISI I095.AISI OI and AISI
D2 respectively.
The influence of numbel. of passes for obtaining higl~er hardness is shown in
tables 5.18. 5.19. 5.20. 5.21. 5.22 and 5.23 for the material AISI33 10. .41SI H21.
AISI1040. AlSI 1095. AISI O l a ~ i d AISl D2 respectively. Figure 5.23 si~o\\.s the
intluence of number of passes 011 11al.ciness at various numbers of passes.
The hardness obtained on the g ~ ~ u n d specimen compared with turned specimen is
shown in figures 5.1 1. 5.12, 5.13. 5.14. 5.15 and 5.16 hr. the materials AISI 4140, AISI
9255. and AISI T4 respectively.
The hardness obtained on the gas carbul-ized and induction hardened materials are
compared with the nlachine hardened specimens in figure 5.17.
It is inferred from the graphs (Figures 5.14. 5.15, 5.16, 5.18, 5.19, 5.20, 5.2 1 , 5.22
and 5.23) that the total depth of cut!number of passes increases the hardness at the surface
to certain depth of cut only. Further increase in total depth of cut (increase in number of
passes) decreases the hardness. This is due to the increase in total depth of cut after
cel-tain period decreases the specific cutting energy, which inturn decreases the
temperature developed at the surface. Thus. the teinperature at the surface of the work
piece has an important influence on the metal renloval at higher depth of cut. The
increase in total depth of cut above the critical value decreases specific energy (Us).
However, beyond a particular depth of cut the rate of change of pailition energy(R) is
much lesser than the rate of change of specific energy. Hence, the surface temperature
decreases with increase in depth of cut. Thus, for cuts where depth of cut is above the
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critical value. increasing the deptii of cut may rfduce the surfact te~nperatul-c. Tlius, in
all cases, the specific energy per unit volu~nc is thc ~najor quantity which drtzl-mines the
level of surface temperature in~o1vt.d in a grinding proccss.
The hardness of the ground sl-recirnen AISI 33 10 at various depths for different
numbers of passes is shown in Figure 5.18. The maximum llardness is between 307VHK
and 309 VHN is obtained at a total depth of cut of 1200pm at the hardness peneti-ation
depth of 1.2mn1.
The l~ardness of tlie ground specimen AISI H21 at various depths foi- different
numbers of passes is shown in Figure 5.19. The inaxirnum hardness is between 371VHN
and 373VHN is obtained at a total depth of cut of 1200pm at the hardness penetration
depth of 1.4mm. AISI H21 is a hot work steel having more chromium, tungsten and
vanadiuln content. This will be tlie reason for higher hardness formation.
The hardness of the ground specimen 41S1 I040 at various depths for different
depth of cut shown in figure 5.20 .The higher 1ia1-dness is obtained at 13"' pass for the
total depth of cut of 1.2 lnln at the hardness penetration depth of 1.21nm.
When comparing, the rnaxinlum hardness for AISI 1040 (C 0.35%) and AISI
4140 (C 0.40%) there is not much variation in the hardness obtained, even though the
percentage of carbon in AISI 4140 is higher. But, there is considerable increase in
hardness penetration depth (1.44mm). This is because of increase in percentage of
chromium (0.9%) in AISI 4140. This increases the hardenability of the material and also
retains the hardness to some extent.
The maximum hardness of the ground material (AISI 9255) is 365 VHN, for a
total depth of cut of 1.2mm. The hardness penetration depth is 1.321nn1.
The maximuill hardness of ground mater~al AISI T4 (C 0.80%) is 386 \'HY for a
total depth of cut of 1.20mm. The hardness penetration depth is l.56mm. This AISI T4 is
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High speed steel (C1' 0.8%. V 1.6%. Co 9.5%. blo 0.8%) imparts high hardenability.
Thus. for a total depth of cut of 1.20mm. thc maximum hat.dness penetration depth
obtained is 1.56mm because of higher I~ardenability.
The maximum hardness of the ground material AISI 1095 (C O.C)?/o) is 367 VHN
at 13"' pass for a total depth of cut of 1.20n1m. The hardness penet~xion depth is 1.21121n.
Here, there is a decrease in hardness penetration depth (HPD) and hardness value. even
though the carbon percentage is higher when compared with AISI T4 because of he
absence of other alloying elements like Cs. V. W and Mn etc.
The nlaxilnunl hardness of the ground material AISI 01 (C0.95%) is 387 VHN
for a total depth of cut of 1.20mm. Tile hardness peileti-ation depth is 1.Omm. When
comparing tlie HPD in AISI T4 and AISI 0 1 , the HPD in AISI T4 is more because of
presence of W 18%. V1% and Cr 4.3%. These alloying elenlents in AISI T4 increase the
hardenability quality and also retain higher hardness even at red hot conditions. (i.e.. the
retention of austenite at higher temperatures (9 1 ODC)).
Similarly, the rnaximuill hardness of ground material AISI D2(C 1.7%) is
477VHK at 1 3 ~ " pass for a total depth of clepth cut 1.2111n-1. The HPD is 1.2111111 11ence
there is considerable increase in hardness because of higher percentage of Cr 12%: V
0.1%, \V 0.5% Si and hln. These alloying eleillents not oiily increase the hardenability
and also retain the hardness even at elevated temperatures (9 10°C).
5.7.3 Other factors
Role of surface roughness
On engineering applications, the surface roughness parameters like Ra, Rt and Rz
are important. Hence, the surface roughness values are measured by using Surftest 402
instrument. The specimens are cleaned by using benzene to renlove any dirt and other
foreign matters and then placed on the platform of the Surftest 40'2.
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As the stylus is moved over the specimen, the roughness \.slues of Ra, Rt, and Rz are
automatically indicated.
The rougl~ness values for the various materials AISI 33 10. AISI H2 1 . AISI 1040,
AIS 4140. AISI 9255, AISI T4. AISI 1095, AISI 0 1 . and AISI D2 are measured and the
results are plotted. From the graph (Figure 5.25) and tables 5.18 to 5.23 it is infei~ed that
the values are consistent and also at acceptable level (within 0 8 microns).
The effect of feed rate on hardness
The effect of feed rate is also studitlcl for different materials. Increasing feed rates
are generally coiinected with process forces. ~vhich arc. also found for grind hiirdening
process. The influence of feed rate 011 hardness and hardness penetl.atio11 depth For the
materials AISI 4140, AISI 9255, and AISI T4 21-2 sho~,~ti i n iigures 5.26. 5.27 and 5.28.
For the material AISI 4140. the maximum hardness obtained for an infeed of
0.6mnlimin is 218 VHN, maximurn hardness obtained for the infeed of 1.25mmimin is
228 VHK and the maximum hardness obtained for the infeed of 2.3mmimin is 231 VHN.
For the material AISI 9255. the maximum hardness obtained for an infeed of
0.6min/min is 223 VHN, maximum hardness obtained for the infeed of 1.25ininimin is
229 VHN and the rnaximuln hardness obtained for the infeed of 2.3mmImin is 234 VHN.
Similarly, for the material AISI T4 at an infeed of 0.6 nlmimin the illaximunl
hardness obtained at a total depth of cut of 1.2 Inn1 is 290 VHN. For the infeed of 1.25
nnmimin, the maximuin hardness is 330 VHN and the maximum hardness for the infeed
of 2.3inminlin is 323 VHN.
From the above discussion, it is inferred that the at lo~vei- infeed of 0.6mmimin.
the traveling energy is high but due to lower cutting power the extent of hardened layer is
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reciuced. The hardness penetration ciep~h i.01. AlSl 4110 is 0.98111m. \Vhc.i.eas, tile
harciness penetration depth for thc material .41SI 9255 and AlSl T1 is 1.26mmi.mi1l.
For higher infeed of 2.3nim/lnin tlie hardness psnetration depth of AISI 4 130 and
XIS1 9255 is 1.2611~11 and for AISI T4, i t is I .40mm. This is duo to highel. infeed. the
cutting power increases but tlie contact time decreases. IS the con~uct tinlc decreases then
the traveling energy also decreases. Thus the extent of hardened layer is reduced.
At moderate or medium infeed of 1.25mm/min the traveling energy and contact
tiins is high, so the hardness penetration depth and hardness is unilbrm. The hardness
penetration deptli of AISI 1040 is 1.26 mill and for AISI M2 i t is 1.4niin (Figure 5.29).
The influence of percentage of carbon and white etched area on hardness
As regards to tlic influence of percentage of cal-boil, higher hardness is obtained
for the matel-ial AISI H2 lwl~en compared to other materials 41SI 33 10. .4ISI 1040. and
AISI 4 140.Similarly. the material A1S1 D2 exhibits higher hardness as compared to other
materials AISI 9255, AISI T 4 AISI 1095, and AISI 0 I (Figure 5.30).
Froin the ~nici-ostructural and mlcrohardness analysis it is evident that at the white
etched area the hardness is increased. This is due to the presence of etchable martensite
in the surface layers. bIoren\.er, it can be seen that grind hardening of material is
reachable at relatively soft grinding conditions. This hardness progression is similar to
surface treatments like laser and induction hardening. The short time metallurgical
effects in grind-hardening are also comparable to those of laser or electron beam
hardening.
Further the metallographic examination (Figure 5.3 1 ) reveals that in this work the
grinding parameters are being optimized to iiiduce fine ma~tensitic phase transfor~ilatioli
in the surface layers of steels as it is achieved by other surface strengthening processes.
SEM stluctures (Figure 5.32) grain flow pattern shows that there is no sign of melting in
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grounded components even if it is thri-mally daniaged. Hence, in this study it is pi-zdicted
that there is insufficient timc for n-iclting to occur. I t clearly gives an idea that, in
grinding, high magnitude thermal pulses of \:cry short duration is involved. This may not
meit a layer o f steel of one nlicron thickness from the cl-ystalline to the amoiphous state.
So, melting does not occur on the ground surface.
Concluding I-emarks
*3 The possibility of' int i~~str ia l application oi' the grinding PI-oczss 1.01- surface
strengthening is identified.
4 T11e surface strengthened parts by utilizing the in-pl-ocess energy ycnzl-ated in
grinding are chai-acterized by tine rna~tensitic structure which is obtained by
phase transfolmations (short time austenisation of surtice/sub-surface layers with
self quenching).
4:. Micro and nlacro cracks are not found in the coniponents which are surface
hardened by grinding process.
0:. There is no surface melting o f metal.