Dry sliding wear and friction: Laser-treated ductile iron

Wear,

21

173 (1994) 21-29

Dry sliding wear and friction: laser-treated

ductile iron

S.P. Gadag* and M.N. Srinivasan Depatlment

of Mechanical

Engineering,

Indian Institute of Science,

Bangalore

560 012 (India)

(Received March 25, 1993; accepted December 2, 1993)

Abstract Pin-on-disc wear and friction of hypereutectic ductile iron, the type employed for automotive components, was investigated at sliding speeds of 5 and 7.5 m s-r, before and after laser surface treatment by COa continuouswave and Nd-YAG pulsed lasers. A significant increase in transition load and wear resistance upon laser treatment has been attributed to the ultrafine microstructure and high hardness; laser-melted ledeburite was superior to martensite by transformation hardening. Wear rate at a specific contact pressure and sliding speed bears a log-linear relationship with the harmonic mean of tensile and fatigue stress of ductile irons. The role of lubrication by graphite during mild wear and plastic deformation in severe wear of pearlitic ductile iron, and its enhanced resistance to plastic flow on laser melting, have been confirmed. The coefficient of friction of a ductile iron pin sliding on a steel disc before and after laser melting has been determined and compared with that of white iron of identical composition and structure obtained by conventional chilling.

1. Introduction Wear and friction characteristics of a material improve on laser surface processing. Selective hardening of inaccessible areas such as grooves and re-entrant corners

without undesirable bulk heating of the substrate, minimal distortion and good surface quality are a few of the merits of laser treatment. This makes it superior to conventional treatments and has led to its importance for tribological applications. In laser melting and freezing, localized high freezing rates of the order of lo3 to lo7 “C s-’ correspond to that in splat quenching of Fe-C alloys. This promotes refinement, higher dissolution of the solute and homogenization of the near-surface microstructure, resulting in excellent wear, erosion and abrasion resistance at the surface, retaining the inherent properties of the substrate. These factors have led to increasing and widespread use of laser processing in industry, especially for automotive components. Study of the friction and wear of laser-processed ductile irons used for highspeed internal combustion engine components - crank and cam shafts - has been taken up. 1.1. Dy sliding wear of cast iron Dry sliding wear of materials has been investigated in the laboratory, in the main, by the pin-on-ring or pin-on-disc test methods. *Present address: Institut tir WerkstoEkunde und Werkstofftechnik, D 38678 Clausthal-Zellerfeld,

Elsevier Sequoia SSDZ 0043-1648(93)06396-L

Germany.

Early studies on dry wear of steel and cast iron distinguished between two modes; mild and severe. Mild wear is characterized by a smooth surface due to sliding contact, and wear debris of oxide flakes 100 8, to 1 ym long. On the other hand, severe wear displays a rough surface due to rubbing or sliding at high contact pressures, and mostly metallic debris 10 to 100 pm long [l]. The transition from oxidative mild wear to severe metallic wear in cast iron is often abrupt, at a critical combination of load and sliding speed. From the tribological viewpoint, graphite is desirable in cast iron, its shape playing a significant role in the extent of wear. Free graphite in the form of flakes in grey iron may be pulled out and smeared over the surface to act as a self-lubricant during dry sliding wear [2]. The lubricity of graphite in cast iron was sign&ant at temperatures below 100 “C. The coefficient of friction increases with further rise in temperature owing to hardening of the graphite film [2]. These flakes also contribute to relatively high thermal conductivity. On the other hand, spheroidal graphite, with its lower surface-area-to-volume ratio, imparts lower thermal conductivity to ductile iron and is less effective in lubrication. Dry sliding pin-on-disc wear of grey and ductile cast irons indicates that steady-state weight loss and wear rates increase with load between 50 and 200 N cmA2 for sliding speeds between 1 and 5 m s-l [3]. This is in accordance with basic laws governing the wear of metals.

22

S.P. Gadag

M.N. Srinivasan I Laser-treated

Flake graphite could be detrimental to wear resistance during severe wear owing to its notch effect on the matrix [2]. The importance of carbon content and the flake size on the nature and rate of wear of a pearlitic iron by pin-on-ring tests has been demonstrated [4] over a range of sliding speeds from 0.5 to 4 m SC’. Wear rate decreased with decrease in the mean distance between the graphite flakes [5]. Severe wear rates were observed, and the effect of carbon content and flake size was a maximum at 20 kgf load, between the sliding speeds of 1 and 3 m SC’. It increased with increase in carbon content and reduction in the flake size. A sharp transition in wear rate was found at an interflake distance of 0.055 mm. The mild-to-severe transition load at 2 m s-’ increased with increasing inter-flake distance (Table 1).

TABLE

ductile iron

In the dry sliding wear investigations, a layer of martensite 20 pm thick was observed [6] on the surface of ductile iron at speeds higher than 3 m SC’. Ferritic ductile iron was highly prone to adhesive wear, compared to pearlitic iron. In general, ductile iron has lower running-in wear and wear rate in the mild region compared to grey iron, although the transition from mild to severe wear occurs at a slightly lower load than that for grey iron. Severe wear takes place by some amount of bulk softening of the material, which leads to a wear rate higher by nearly two to three orders of magnitude [7]. 1.2. Wear resistance on laser treatment Laser treatment at critical areas of automotive cast iron components such as piston rings and ring grooves [8], cylinder liners of locomotive diesel engines [9],

1. Dry sliding wear at 2 m s-‘: effect of laser treatment

Material

Test rig

Wear rate

Transition

type” Mild (X 1o-8 cm-‘) Grey iron 0.15% P Pearlite Ferrite Grey iron, 0.15% P SG Iron Grey iron 0.27% P SG Iron, Perlite Ferrite SG Iron Pearlite, 210 HV 0.3 Bainite, 310 HV 0.3 Martensite, 310 I-IV 0.3 Grey iron %C Flake size, mm 3.35 0.20-0.26 3.20 0.36-0.46 3.17 0.36-0.60 2.90 0.72-1.00 Grey iron, 230 HV 0.3 SG iron, 270 HV 0.3 Grey iron, 220 HB Pearlitic, lubricated Speed, 1.47 m s-’ Grey iron (G) SG iron (D) Laser-treated LT(G) LT(D) Grey iron (G) SG iron (D) Laser-treated LT(G) LT(D)

P.O.R.

0.5

to 1

Severe (X

lo-’

cm-‘)

Load (kgf)

Pressure (kgf cm-‘)

Computed at load value

100

Reference

21 20

0.2 0.5

90 25

P.O.R.

25 25 50

10 3.75 500

P.O.R.

4.0 0.1 0.3

75 75 75

P.O.R.

20 18

6

10 15 15

55 42 40 09

05 05 06 13

10 10 12 26

P.O.R.

1.25 1.75

7.50 0.05

15 10.0

20

P.O.R.

88

l-3

50

29

4-10

5-35 5-10

6-12 13-15

12-25 28-31

0.5-I

915 <20 15-25 20-25

<47 662 47-78 62-78

P.O.R.

7-10 3-6 3-5

P.O.D.

;:t 10 kg@ l-10 l-10

“P.O.R. pin on ring; P.O.D. pin on disc.

21

33 50 50

5-10 5-10 5-10 5-10

P.O.R.

21

66 60

20 kgf

5

200 N cm-*

3

70 kgf

16

20 kgf

20

19

S.P. Gadag, MA? Srinivasan / Laser-treated

lobes of cams [IO] and fillets of crankshafts [II] has demonstrated improved resistance to wear during normal engine operation. Laser hardening of a power steering gear housing in ferritic malleable iron gave a ten-fold increase in wear resistance over that of an untreated casting [12,13]. Dry sliding wear of laser surface-hardened grey and ductile irons was studied by pm-on-disc test at a constant speed of 1.33 m s-l, load ranging from 10 to 30 kgf, over a period of 10 to 90 min [14]. Mild wear of grey iron after 7.2 km of sliding at approximately 1.2~ 10T7 -I, reduced to 4.2 X 10m9g cm-l after laser surfaceia:ening. Severe wear rates of grey iron after 4.8 km of sliding reduced from 4X 10y7 g cm-’ to mild rates of approximately 1X lo-* g cm-’ on laser surfacehardening. Laser-melted grey and ductile irons showed essentially mild wear, approx~ately 5.5 X lo-’ g cm-*, over 2 to 20 kgf load, at sliding speeds less than or equal to 4 m s-’ in a pin-on-ring test [15]. Laser treatment reduced mild wear by a factor of two or three compared to untreated irons; wear was entirely in the mild region, and transition from mild to severe wear was absent over the load range at these speeds. The laser-melted ledeburitic structure was superior in resistance to abrasive wear compared with martensite, or martensite with retained austenite, obtained by laser surface-hardening [16]. The latter in turn has higher resistance than a pearlitic structure. Here, a pad-onplate reciprocatary test was used, and the blocks were oscillated at a frequency of 2.5 Hz at 5 kgf load beneath a stationary Sic wear pad of 8 mm diameter. Wear resistance of laser-melted grey iron evaluated by pin-on-ring test was comparable to that of inductionhardened nodular iron 1141.Laser-melted nodular iron was far superior to ~nventiona~ly hardened plain carbon steel in rolling-sliding wear of ductile iron discs with up to 5% slip, as the wear resistance ratio was nearly lOO:l, depending on counter-surface and slip conditions

P71. Enhanced wear resistance of laser-processed ductile irons, assessed from scratches made on the surface by a diamond indenter (100-2500 gf) at speeds ranging from 5 to 35 mm s-’ was attributed to refined microstructure and wear-induced hardening [18]. A wear test of laser-melted chromium steel by pressing a ruby ball of 5 mm diameter against a rotating disc (speed, SO mm s-‘; pressure, 2.3 N) showed an appreciable increase in the hardness of laser-treated samples after wear testing. This method seems to be insufficiently sensitive to differentiate the abrasive wear rates of steel samples before and after laser melting P9]A dry sliding pin-on-ring wear test of Meehanite grey iron, laser surface-alloyed with 14-40% chromium [20], showed an increment in wear resistance of the laser-

ductile iron

23

mehed layer by at least three times that of an alloyed coating. This suggests that surface alloying is suitable for withstanding corrosive wear conditions. Frictional heating increased on increasing the depth of the laser-transformed case 1141. In Sudan, earlier work on wear of grey and ductile irons shows significant reduction in the rate of wear after laser treatment. Comparison of the results of different investigators has not been feasible owing to the diversity of test procedures employed. The condition for transition from mild to severe wear has not been assessed so far in laser-melted cast irons. Wear studies in laser-processed cast irons have been in the mild region with respect to loads and speeds up to 4 m s-l. Wear at or above 5 m s-l, often encountered in automotive applications, has not been examined so far. Also wear in laser-hardened, as distinct from lasermelted, ductile iron requires investigation. The coefficient of friction of laser-treated grey or ductile irons during sliding wear has not been assessed. Considerable work has been done on laser treatment with continuous-wave (CW) COz gas lasers; there is little published literature on scientific investigations relating to the wear of cast irons treated by pulsed Nd-YAG lasers. Correlation between mechanical properties and wear resistance has not been attempted for ductile iron.

2. Experimental details Unlubricated pin-on-disc wear and friction of ductile iron at high speeds, 5 and 7.5 m s-l, typical in automotive engines were investigated over a range of loads. A stationa~ preloaded specimen (5 mm diameter x 10 mm long) of test material was in sliding contact with a rotating hardened (Rockwell hardness, 62-64 HRC) and ground (0.1 pm centre line average (c.1.a.)) EN31 low-alloy steel disc. The weight loss of the pin was measured to 0.1 mg accuracy and 0.25 mg reprodu~bi~~ at predetermined time intervals, up to a sliding length of 5 to 6 km. Load on the specimen was adjusted to obtain pressures ranging from 5 to 35 kgf cmw2. Wear pins were carefully prepared by machining cast Y-blocks poured under standardized conditions; the grade, chemical ~om~sition, structure (Fig. l(a)) and mechanical properties are given in Tables 2 and 3. Ductile iron was remelted in a medium-frequency induction furnace and poured into a special chill mould to produce white iron structure (Fig. l(b)) at the face, for comparison of the wear characteristics. Two linear displacement transducers ~ntinuously monitored the changes in the pin; one of its height and the other tangential displacement of the axis of the pin. From changes in pin weight or height, the

S.P. Gadag, M.N. Srirzivasan i Laser-treated ductile iron

24 TABLE 2. Chemical Grade

composition

of various grades of ductile iron

Carbon equivalent

wt.% -

C.E.

7OW3 6001’3 400/W

4.7 417 412

TABLE 3. Mechanical Grade ductife iron

properties

Si

Ni

Mn

Mg

P

S

CU

Ti

3.8 3.8 3.46

2.6 2.6 2.1

0.9 0.9 0.9

0.8 0.8 0.4

0.04 0.04 0.045

0.02 0.02 0.02

0.016 0.016 0.025

0.35 _ --

0.02 0.02 0.02

of ductile irons

Hardness

Tensite properties

Micro

70012 600/3 400/12 LSTb

_---~_

C

(HB)

Macro (HV 0.2)

$Pa)

(MPa)

(Gpa)

iw

Endurance ratioa

257 239 126 650

399 383 201 900

726 661 414 445

485 439 250 _

175 174 167 207

2 3 12
0.38 0.39 0.33 0.69

ffY

E

“Un-notched. bLaser surface treated.

delineation adopted by Rat [3] and Leech [15]. The tangential displacement of the specimen axis was directly converted to read the friction force on a digital display meter. The ratio of tangential force FT to the applied normal load FN on the specimen gave the coefficient of friction: ,u = F&F,. 2.1. Laser treatment of wear pin

::,

Lril:ied

w?ite

iron

Fig. 1. As-cast optical micrcostructures: (b) chilled white iron.

corresponding rate of computed. Wear rate sidered to be in the was treated as severe.

(a) pearlitic ductile iron;

weight loss of the specimen was less than 10V7 g cm-’ was conmild region. At higher rates it This was in accordance with the

After sandblasting and cleaning in an ultrasonic bath, laser treatment of the face of the cylindrical test specimen was carried out by successive laser stripes by lateral translation after each stripe, producing an overlap of approximately HI%, until the face was completely covered. Several wear pins, located adjacent to each other on a computer numerically-controlled five-axis work table, were reciprocated under the focused CO, CW laser beam at predetermined scan rates to give treated stripes on the face. Laser processing parameters were: power, 1.2 kW; beam diameter, 2.1 mm; scan rate, 10 mm s-‘; resulting in an average melt depth of 0.25 mm. In the case of the pulsed Nd-YAG laser, the specimen on a rotating work table under the beam was indexed after each laser pulse to cover the face of the specimen with an overlap of appro~mately 25% between consecutive pulses. Each specimen was processed with several pulses of the laser beam of about 1 mm diameter, at two power levels of 50 and 100 W. The energy of each pulse was 10 or 20 J, with 10 or 15 ms pulse width respectively at a pulse rate of 5 s-‘. Transformation hardening to a depth of 0.25 mm and melting to a depth of about 0.25 mm were obtained at 50 W

25

S.P. Gadag, M.N. Srinivasan / Laser-treated ductile iron

a) Melt

zone

Nominal

Pressure,

kg/cm++2

--b)

b)

Heat

affected

m

0.15

zone

Fig. 2. SEM images of laser-treated (b) beat-affected zone.

ductile iron: (a) melt zone;

Nominal

Pressure,

kg/cm**2

Fig. 3. Pearlitic ductile iron: (a) wear and (b) friction, as influenced by sliding speed and contact pressure.

and 100 W of average power, respectively. Typical SEM microstructures of laser-melted and -hardened ductile iron are shown in Figs. 2(a) and 2(b) respectively.

3. Results Wear rate of pearlitic ductile iron at sliding speeds of 5 and 7.5 m s-’ in a pin-on-hardened-steel-disc test is shown in Fig. 3(a). Wear rate increased with increase in contact pressure; the slope of the wear diagram is particularly steep between 7 and 14 kgf cm-‘. Transition from mild to severe wear occurred at about 10 to 12 kgf cm-’ pressure. At low applied pressures, a thin brownish iron oxide layer formed on the worn surface of the specimen, indicative of oxidative wear. At high contact pressures, the disc was smeared with graphite, characteristic of lubricated wear. This is in conformity with the findings of earlier investigators [4-71. Wear rates were slightly higher at 7.5 m s-l than at 5 m s-l. Critical wear rates at the transition were between 5 x lop7 and 7~ 10m7 g cm-’ (Fig. 3(a)). Wear rates of pearlitic ductile iron in the severe regime varied from 8 X low7 to 1.5 X lo-’ g cm-’ at 7.5 m s-l, and 7X 10e7 to 3 X 10e6 g cm-’ at 5 m s-l. Wear of chilled white iron of the same composition as the ductile iron is shown in Fig. 4(a). Critical pressure

10 Nominal

20

25

kg/cm*+2

Pressure,

kg/cm**2

30

,

0.45

0.39

15 Pressure,

3

% g 0.33 x * 0.27 .-s g 0.21 I

m

0.15

Nominal

Fig. 4. Chilled white iron: (a) wear and (b) friction, as influenced by sliding speed and contact pressure.

26

S.P. Gadag,

h4.N. Srinivasan I Laser-treated

at the transition from mild to severe wear, corresponding to low7 g cm-’ of wear rate was 14 to 15 kgf cm-’ at a sliding speed of 5 m s-l, and 21 to 22 kgf cm-’ at 7.5 m s-l. The maximum mild wear rate of chilled white iron was 2x10-’ g cm-’ at 5 m SK’. In the mild regime, wear rate ranged from 4 X lo-’ to 1 X lo-’ g cm-l at 7.5 m s-‘. The coefficient of friction of ductile iron in dry sliding contact increased linearly with increase in applied pressure at a given speed. The friction coefficient was higher at the velocity 5 m s-’ than at 7.5 m s-l at all pressures (Fig. 3(b)). The coefficient of friction of pearlitic ductile iron varied from 0.19 to 0.23 at 5 m s-l, and 0.15 to 0.185 at 7.5 m s-l. The coefficient of friction of chilled white iron ranged from 0.23 to 0.29 at 7.5 m s--l, and 0.34 to 0.44 at 5 m s-’ (Fig. 4(b)). In the ductile iron, transformation-hardened at 50 W average power with a Nd-YAG laser with a martensitic structure, mild-to-severe wear transition occurred at a pressure of 17 kgf cme2 at 5 m s-’ (Fig. 5(a)). The wear diagram for the laser-melted ledeburitic surface obtained by treatment at 100 W average power with a Nd-YAG pulsed laser is also shown in the figure. The wear rate at 7.5 m s-l was less than 1O-7 g cm-l at all test pressures. This suggests that mild wear prevails over the entire range of pressures tested, and that the

Pressure,

kg/cm**2

0.30 4

b)

Melted

at

Hordsnsd

Nominal

Pressure,

5

15

25

35

Nominal

Pressure,

kg/cm**2

Nominal

Pressure. kg/cm**2

5

Fig. 6. Effect of CO2 and Nd-YAG laser treatment iron: (a) wear and (b) friction at 7.5 m s-‘.

on ductile

transition load cannot be determined owing to the 35 kgf cmF2 limit for applied test pressures. The wear diagram for the ductile iron treated by CO, CW laser is shown in Fig. 6(a). Diagrams for the wear of untreated ductile iron and the Nd-YAG lasermelted iron have been superimposed on the same diagram for comparison. In the mild wear region, the wear rates of the iron treated by the two different lasers are nearly the same; considerably less than for the untreated iron. Mild-to-severe transition for a CO, CW laser-treated ductile iron occurs at 33 kgf cm-‘. The coefficient of friction of laser-treated iron varies linearly with contact pressure (Figs. 5(b) and 6(b)); slightly lower for the CO, CW laser-treated iron in comparison with Nd-YAG laser-treated iron.

10

Nominal

ductile iron

IOOW

at

SOW

kg/cm**2

Fig. 5. Nd-YAG laser-treated ductile iron: {a) wear and (b) friction, as influenced by siiding speed and contact pressure.

4. Discussion This investigation has reconfirmed that the wear of ductile iron in the mild regime was predominantly due to oxidation: the surface of the pin specimen turned light brown at mild wear loads, and wear debris consisted of agglomerates of Fe,O, oxide. Adhesion of graphite film to the wear tracks on the disc material was clearly visible, and the wear rate was iow. This indicates the lubricating action of graphite film. It can be concluded

27

S.P. Gadag, MN. S~niva~an I Laser-treated ductile irm

that the mild wear of untreated ductile iron was controlled mainly by oxidation of iron forming a protective light brown film of Fe,O,, and by self-lubrication due to graphite. In untreated ductile iron, at loads above 3 to 4 kgf and at the sliding speeds employed in this investigation, severe wear greater than 10m7 g cm-l was observed. The severe wear was characterized by a thick dark brown layer over the surface of the pin specimens, extending to l-2 mm depth. This could be att~buted to the high interfacial temperatures caused by friction of the mating surfaces at the high speeds of sliding. Fine pearlite obtained at the surface, extending to a depth of 25 to 30 pm, clearly indicates that the interfacial temperatures due to frictional heating are higher than the critical transformation (&) temperature. This is typical for high-sliding-speed severe wear, involving frictional heating. Leech and Borland [4] observed a white layer on the top surface of the grey iron samples subjected to severe wear. The hardness of the surface of the worn samples after test did not vary appreciably from that of the untreated ductile iron substrate. The deformed surface layer showed only a trace of associated fine pearlite. Worn samples of untreated ductile iron did not show any trace of deformed martensite after severe wear at high-speed sliding contact as found in steels. High carbon and silicon contents in the ductile iron have suppressed MS - the temperature for the start of transformation to martensite. Severe wear in ductile iron (Fig. 7(a)) occurs by either abrasion or plastic ploughing, resulting in chip formation around the circumference of the specimen, while subsequent material removal at a later stage could be by either delamination or the fatigue process proposed earlier [14,15]. Fatigue cracks might initiate in such a case at the stressed pearlite matrix beneath the surface of untreated ductile iron. Sometimes the graphite nodule acts as a potential site for crack initiation and propagation [14]. In laser surface-melted ductile iron, mostly mild wear has occurred over the load range (up to 35 kgf cmw2 pressure) examined at the sliding speeds employed. Ledeburite eutectic of laser-melted ductile iron had mild wear rates between 2 x lo-’ g cm-’ and 5 x lo-’ g cm-’ over a pressure range of 15 to 32 kgf crne2 at a sliding speed of 5 m s-l (Fig. 5(a)). Transition from mild to severe wear for ledeburite occurs between 31 to 33 kgf cm-2, and the corresponding severe wear ratesareoftheorderof8x10-7gcm-1t01.25x10-6 g cm-” at a sliding speed of 7.5 m s-l (Fig. 6(a)), An exception to this was the transformation-hardened iron obtained by treatment at 50 W average power by the Nd-YAG laser, when subjected to sliding wear at a speed of 5 m s-l. The critical pressure for transition from mild to severe wear of martensite of laser-hardened

r

a)

b)

Loser

Untreated

treated

ductile

iron

Fig. 7. Wear scar topographs for ductile iron (load, 5 kgf, speed, 7.5 m ‘-‘): (a) untreated; (b) laser-treated.

ductile iron was approximately 17 kgf cmm2. The corresponding value for ledeburite was nearly double that for the martensite structure (Figs. 5(a) and 6(a)). The typical topograph of the laser-melted ductile iron pins subjected to wear test (Fig. 7(b)) shows that the surface has undergone negligible wear. Also there was hardly any plastic flow or deformation. The predominant effect of laser surface-melting was in the retention of the regime of mild equilibrium wear and the substantial rise in load for transition from mild to severe wear to occur. This can be attributed to the high hardness and structural stability of the network of cementite in ledeburite eutectic. Dry sliding wear of faser-melted pea&tic ductile iron at 5 m s-’ was better than for chilled white iron of identical composition. The critical pressure for transition to severe wear in laser-treated pearlitic ductile iron was nearly twice that for a chilled white iron of comparable composition. With increasing pressure, there was no evidence of any subsurface cracking during wear testing of laser-treated ductile iron. Increased resistance to subsurface crack formation, resulting in mild wear at high loads and sliding speeds, makes the application of laser surface processing most suitable for improved performance of the internal com-

28

S.P. Gadag

M.N. Srinivasan I Laser-treated

ductile iron

_-. --__-______A-111

b)

-! i

co2cw LAsER 1 Omm/s

1 2kW

SEVERE

WEAR

Al

75m/s

35!q/cm2

/ i’ii

T--T?-=TWO

F.M-1

OA Fig. 8. XRD

data

THETA,

Degree

10

20 TWO

THETA.

40

for laser-melted

ductile

iron:

(a) before

test;

bustion engine. Microhardness at the surface of lasermelted ductile iron increased from 677-700 HV 0.1 to 1000-1140 HV 0.1 after severe wear, with some evidence of martensite structure on the surface. Upon severe wear at high contact loads and sliding speeds, a portion of the laser-melted skin surface attained critical transformation (&) temperature by the high frictional heating due to work hardening. This was the strain-induced martensitic transformation of the austenite dendrites, confirmed by X-ray diffraction (Fig. 8). The coefficient of friction p of the ductile iron in dry sliding contact with a hardened steel disc increased linearly with either increasing pressure, between 15 and 32 kgf cm-*, or decreasing sliding velocity, 7.5 m s-l to 5 m SK’. Laser-melted (0.23 to 0.265) or chilled white iron (0.22 to 0.4) had a slightly higher coefficient of friction compared to the ductile iron (0.15 to 0.23) at the same contact pressure and speed. The coefficient of friction of specimens treated by CW CO, laser ranged from 0.228 to 0.255 at 7.5 m SK’ over a pressure range of 17 to 32 kgf cmP2. The corresponding value of the Nd-YAG laser-melted specimens was slightly (0.7%) higher than for the CW CO, laser, perhaps due to the higher tendency to ripples on the treated surface in the latter.

(b) after

5.

THETA,

severe

1

80

60 TWO

Degree

Degree

wear

test.

Inset:

Conclusions

(1) Experimental investigations on wear by pin-ondisc test of pearlitic ductile iron before and after laser treatment, and chilled white iron, at 5 and 7.5 m SK’ sliding speeds have enabled the delineation of mild and severe wear, in particular the transition between the two for the materials under test. (2) Assessment of the coefficient of friction under dry sliding conditions carried out here as influenced by contact pressure and sliding speed will pave the way for a better understanding in tribological applications. Laser treatment increases the coefficient of friction. (3) The superiority of laser-melted ductile iron over laser transformation-hardened or chilled white iron in terms of wear resistance has been clearly established. (4) Wear rate was a function of contact pressure and sliding speed; lubrication by graphite during mild wear of untreated ductile iron and the role of plastic deformation in severe wear of pearlitic ductile iron and its increased resistance to plastic flow after laser treatment have been reaffirmed. (5) Laser surface treatment imparts resistance to subsurface crack initiation and propagation under surface loading. This should make surface modification an excellent choice for improving performance at high

S.P. Gadag, M.N. S~~~vasa~ I Laser-treated ductile &=m

hertzian stresses and at elevated temperatures, as in internal combustion engines.

Acknowledgments

The authors are grateful to Professor C.N.R. Rao, Director, II%, Dr. P, Rama Rao, Secretary, DS?; and Dr. I?. ~bid~bara~, Director, BARC, for providing facilities to carry out this investigation.

References T.H.C. Childs, Sliding wear mechanisms of metals, mainly steels, Tribal. ht., 12 (1980) 285-293. J. Sugishita and S. Fujiyoshi, The effect of cast iron graphites on friction and wear performance; II: Variables in~uen~ing graphite fdm fo~ation,~~~~ 68 (1981) 7-20. A. Rat, Influence of load and speed on wear characteristics of grey cast iron in dry sliding - selection for minimum wear, liibof. Int., 18 (1) (1985) 29-33. P.W. Leach and D.W. Borland, The process of severe wear of graphitic cast iron, Mater. Sci. Eng.,_66 (1984) 167-177, P.W. Leach and D.W. Borland, The unlubricated wear of flake graphite cast iron, Wear, 85 (1983) 257-266. E. Takeuchi, Tlie mechanism of wear of spheroidal graphite cast iron in dry sliding, Wear, 19 (1972) 267-276. E. Takeuchi, The mechanism of wear of cast iron in dry sliding, Wear, II (1968) 201-212.

29

Using the industrial 8 F.D. Seaman and D.S. Gn~amuthu, lasers to surface harden and alloy, Mefaf Pmgms, 108 (1975) 67-72. 9 B.M. Afitashkevich, S-S. Voinov and E.A. Shur, Laser hardening of the cylinder liner diesel locomotive engine, Metalloved. Term. Obmd. Met., 4 (1985) 12-15. 10 D.A. Belforte, Laser Surjace Treatment, AVCO Everett Research Laboratory, Inc., MA, USA. 11 J.E. Miller and J.A. Wineman, Laser hardening of seginaw steering gear, Metals Prqress, 111 (5) (1977) 38-43. for laser treatment in automot~e 12 D,P. Holton, opportunities industries, Metal~u~~ May 1986 183-185. 13 D.N.H. Trafford, T. Bell, J.H.P.C. Megaw and A.S. Bransden, Laser treatment of grey iron, Metal. Tech., IO (2) (1983) 69-77. 14 I-I. de Beurs and J.Th.M. de Hoeson, Wear induced hardening of laser processed chromium carbon steel, Scripta Metall., 21 (1987) 627-632. 15 C.P. Ju, C.H. Chen and J.M. Rigsbee, Laser surface modification of ductile iron: Part 2, Wear mechanism, Mater. Sci. Tech., 4 (2) (1988) 167-172. 16 A. Blarasin, S. Corcoruto, A. Belmonde and D. Bacci, Development of a laser surface melting process for improvement of the wear resistance of grey iron, Weat, 86 (1983) 315-325. 17 H.W. Bergmann, Current status of laser melting of cast iron, surf: &g., f (2) (1985) 137-155. 18 W.J. ‘Fomlinson, M. Cash and AS. Bransden, Dry sliding wear of grey iron laser surface alloyed with 14-&I% chromium Wear, 14.2 (1991) 353-386. 19 P.A. Molian and M. Baldwin, Wear behavior of laser surface hardened gray and ductile cast irons: Part 1 - Sliding wear, Trans. ASME, 108 (6) (1986) 326-333. 20 P.W. Leech, Comparison of the sliding wear process of various cast irons in the laser-surface-melted and as-cast forms, Wear, 113 (1986) 233-245. 21 S.P. Gadag, Laser processing of ductile iron, Ph.D. The&, Indian Institute of Science, India, 1992.