medium carbon bainitic steels

WEAR E LS EVI E R

Wear200 (1996) 74-.82

Unlubricated sliding and rolling/sliding wear behavior of continuously cooled, low/medium carbon bainitic steels P. Clayton, N. Jin Oregon GraduateInstitute of'Scienceand Technology, Departmentof MaterialsScienceand Engineering. PO Box 91000, Portland, OR 97291.1000. USA

Abstract Tie developmentof low and mediumcarbon bainitie steels for railroad track applicationsis traced through investigatio~tsto understand wear behavior.Carbide-freeUrainileconsistingof bainitic ferritelaths, with or withoutlath boundaryretainedaustenite,ha: emergedas the best microstructure.Ca~t and wrought0.25%C, 1.75%Si,Mo--Bsteegshave exhibited wear resistancecomparable with ~hatof Hadfield's austeniticsteel under severerolling/slidingcontact. Keywords: Rollinglslidingwe~,r,Bainiticsteels;Lowlmediumcarbon

1. Introduction Deformation, wear, fatigue and fr~:ture, in their various form.,i, have provided the incentive to experiment with new rail .,r,aterials since the b:;ginning of railways. The suspicion of ant' resistance to the ir,troduetion of new alloys are a direct consequence ef the critical nature of the components involved. The fi~i Bessemer steel rail was used by the Midland Rallw~:yat Derby Station. England, in 1857 [ 1] and proved vastly superior to iron, bt-;tvariable quality kept the controversy of steel versus iron fueled until the end of the century. With e~peri~nce, steelmaters learned how to maintain aphosphoros level below 0. 1% enabling carbon contents as high as 0.5% to be utilized for wear resistance [2]. The subsequent challec,ge has been to maintain mechanical integrity in the face of increasing wheel loads and vehicle speeds. Apart from isolated forays into tougher steels [3,4], the subsequent mainstream development of rail steel can be viewed as the reduction of free fertile, interlamellar spacing and inclusion content to produce cleaner, pearlitic steel of higl~:r strength and hardness. Recent years have seen the development of online .',~ea4hardening and significant innovaf,ons in clean steel practice. With interlamellar spacings as low as 100 nm, it is conceivable that this approach to strength has almost run its full course with other options, such as precipitation hardening, being studied [5] hut little used. Hadfield's manganese steel offers an unusual combination of strength and toughness. In wrought and cast forms, it has become the universally accepted standard for switch and 0043-1642/96/$15.00 © 1996ElsevierScienceS.A.Allrightsreserved PIi $0043-1648 {90) 07249-3

crossing components olzrating under severe conditions. It is difficult to cast and machine, however, and only achieves full resistance to deformation and wear as a r;~sultof significant plastic strain Furthevnore, the high alloy content significantly affects weldability. The desire to have a steel which can be welded into track, and weld repaired with ease, surfaces periodically [6]. Low carbon bainiti,: microstructures produced by continuous cooling were first proposed as alternatives to pearlite [7] and austenite [81] twenty ye~xs ago. This paper traces part of the history of the work on bainitic steels by focusing o~ wear behavior.

2. Bainite The term bainite refers to microstructures formed by the decomposition of austenite below the austenite to pearlite transformation temperature and above the austenite to martensile temp~:rature,typically within the broad range of 250650 *C. A homogeneous bainite transformation is difficult to achieve in most low alloy steels because the bainite reaction is masked by the fertile and pearlite reactions, as shown by the schematic continuous cooling transformation diagram in Fig. 1. Much of the early work on bainitie steels focused on isothermally treated steels in which austenit¢ is cooled below the pearlite transformation nose and held at temperature to form bainite. This produces two forms of balnitic microstructure termed upper and lower bainite, with reference to the

P. Clayton,N, lin l Wear200(1996) 74-82

75

encing the su'ucture obtaiaed. Further control of the austenite to balnite transform:,tion is exercised through the use of the alloying elements C, Ni, Cr and Mn, which act to depress the bainite transformation start temperature (B..). As this temperature is lowered, the strength properties are increased without necessarily affecting ductility. Of the alloying additions commonlypresent, carbon has by far the f,trongesteffeet on the bainite start tentperature as evidenced by the equations for Bs developed by Steven and Haynes [ 12]: Bs = 830 ° C - 270%C- 90%Mn- 37%Ni - 70%Cr- 83%Mo

lime

Rg. !. Schematic~lneSanlalionof the effectof Mo+B on continuously cooledtransformationdiagram.

for isothermally heat-treated boron-free steels, and Devanathan [13]:

temperature of transformation. These have become known as classical bainitic microstructures. The addition of 0.5%Mo and 0.003%B is one way of retarding the austenite to ferrite and pearlite reactions, Fig. 1. This approach opens up the possibility of producing bainite by continuous cooling over a wide range of cooling rates. Under such continuous cooling conditions additional balnitic structures can be formed. There have been several attempts to classify bainitic microstrectures [9-11 ] and four types are recognized in this paper; the classical forms plus carbide-free and granular bainite, shown schematically in Fig. 2. Upper bainite consists of packets of heavily dislocated ferrite laths and intedath cementite. Lower bainite contains packets of heavily dislocated ferrite laths with cementitepresent within the laths at an angle of 55--60° to the longitudinal axis. Carbide-free bainite has packets of heavily dislocated ferrite laths and, usually, interlath films of retained anstenite or martensite-austenite (M-A) islands. Granular bainite appears as non-lath bainitic ferrite usually with M-A islands. While there is little documented evidence on the relationships between cooling rate, microstructure and chemical composition, the control of cooling rate is one way of influ-

B~-- 721°C- 598%C- 85%Mn- 43%Cr

up~t balnite

r~rbide-fraebalr~

lower bain~te

for a set of M o B steels under continuously cooled conditions. Much of the early work on the wear of bainitic steels, in the context of wheel and rail applications, produced disappointing results [ 14-18] in the laboratory and in track. This sent a strong message that bainitic steels had poor wear resistance and could not be candidates for new materials development. These investigations had not, however, systematically examined wear resistance with respect to test conditions, chemical composition and other mechanical properties and the microstructural characterization was vague at best. There was often no way of knowing which form of bainite had been tested and it was clear that a more detailed examination of the wear behavior of bainitic steels was justified.

3. Sliding wear investigation [19] It is necessary to distinguish between wear on the top running surface of a rail and the gage face, Fig. 3. The mechanisms of deterioration are noticeably different [20] because the degree of sliding is typically much higher in the interaction between a wheel flange and the inside gage face of the rail in curved track. The significance of the two is also separated in economic terms. While the wear of a gage face can lead to rail replacement, there is a perception that head wear is of iittl¢ financial consequence. Nearly all recent studies of rail wear have been focused on the former.

granularbaintte

]Fig.2. Schematicof the four bainitic microstructures.

Fig. 3. Schematicof rail gagefaceand headwear.

P. Clayton, IV. Jin / Wear 200 (1996) 74-82

76

Table l Chemical compositionof nine bainiticsteels (wt.%) Cast number

C

Mn

Cr

Me

B

Si

AI

Ti

P

S

1 2 3 4 5 6 7 8 9

0.09 0.21 0.30 0.09 0.19 0.29 0.09 0.19 0,29

1.0t 1.99 1.49 1.53 0.98 1.98 2.01 1.52 1,20

0 0 0 0.95 0.95 1,02 i.96 2.00 1.98

0.50 0.50 0.50 0,4~,' 0A9 0,50 0.50 0.50 0.50

0.0029 0.0026 0.0027 0,0028 0.0028 0,0030 0.0029 0.0025 0.0030

0.21 0.22 0.21 0.21 0.20 0.21 0+24 0,22 0.23

0.029 0.030 0.02"/ 0.024 0.023 0.030 0.028 0+028 0.030

0.030 0.024 0.025 0.028 0,024 0.030 0.028 0.030 0.027

<0.005 <0.005 <0.005 <0.005 <0,005 <0.005 <0.005 <0.005 <0,005

<0.01 <0.01 <0.01 <0.01 <0.0t <0.01 <0.01 <0.0l <0.01

The gage face wear of rails (and wear of some parts of switch and crossing components) resembles severe metallic wear in pure sliding [21]. In evaluating rail steels this was the wear regime aimed for in laboratory studies of sliding wear. The testing procedure incorporated a blast of dry compressed air to minimize surface heating and discourage oxidation. Nine M o B steels containing different levels of carbon, manganese and chromium, Table 1, were produced by a vacuum melting process to give small ingots of 20 kg which were hot rolled to 12 mm diameter rod. The small amounts of titanium and aluminum were used to react with any free nitrogen and prevent the formation of bore nitrides which reduce the hardenability of steel. The steels formed a one third full factorial experiment. Bainitic steel pins 6 mm in diameter were run against a pearlitic steel ring, of hardness 230 HV30, in a pin-on:ring wear testing machine [21 ]. A new pin and an unworn ring surface were used foreach test with both being freshly ground on grade 400 silicon carbide paper and washed in isopropanol. Thc specimens were tested at a constant sliding speed of 40 cm s - I over a range of loads 75 to 150 kg. Throughout each test, pin height loss and cumulative ring revolutions were monitored and the data used to construct plots of wear against sliding, distance. The slopes of these linear relations were measured to give the wear rate in terms of volume loss per unit of sliding distance, mm 3 c m - ~. In keeping with previous investigations of steels [22,23], two types of wear were observed. At lower loads, an initial period of severe metallic wear causes sufficient work hardening that the applied load can be resisted with little damage to the pin and a mild oxidative wear typically ensues. Beyond a certain transition load the material does not run in and a linear pattern of severe metallic wear occurs, following the terminology of Archard and Hirst [23]. At loads of 60 kg or greater, this was the case for all nine bainitic steels. The highest wear resistance was exhibited by steel 8 which contained 0.2%C, had a hardness of 433 HV and a mixed microstructure of eat'bide-free bainite and lower bainite. Transmission electron microscopy of carbon replicas was used to evaluate the microstructare at high magnification, an advance on previous wear investigations.

An analysis of variance showed that the strongest correlation was between wear rate and a chemical composition term of % C + % C r + % M n / 3 3 . The importance of Cr appeared obvious but why Cr should have 33 times the effect of Mn and be equal to that of C could not be explained. It was not possible to compare the wear results directly with previous data on pearlific steels [21] because some of the more resistant pearlitic steels did not exhibit the transition at 60 kg. For some it occurred at more than 120 kg and for two steels it did not occur at 200 kg. To put all data points on the sanre plot, Fig. 4, it is necessary to use the initial wear rate of those pearlitie st~ls that resisted equilibrium severe metallic wear at 100 kg load. There is some justification for this since it is not known what load in the pin-on-ring test corresponds to the service situation. Even with this approach, at hardnesses greater than 300 HV the pearlitic steels were much more wear resistant to severe metallic wear than the bainitic steels. But for the pearlitic steels at hardnesses lower than 300 HV it is possible to directly contrast the two, and the bainitic steels compare favorably with the softer pearlitic steels. This was the first evidence that any bainitic steel could be compared with a pearlitic steel of the same hardness and was sufficiently interesting to encourage further investigation. The laboratory wear studies under review were discontinued for ten years. In the meantime, British Rail had success-

1,0el

+:I

zoo

++-++++'

z~o

zoo

z.~

see

3so

4~

4,~

Hsnl,,m (fl~JO) Fig. 4. Sliding wear rate versus hardness for bainitic and pearlitic steels.

P. Clayton,N. Jinl Wear 200(1996) 74-.82 Table2 Chemical compositionof two low carbon bainidc steels (wt.%)

DI D2

C

bin

Cr

Ni

Me

B

Si

AI

Ti

P

S

0.04 0.l !

0,80 0.57

2.76 !.68

1.93 4.09

0.25 0~58

0.0023 0.0023

0.19 0.27

0,03 0

0.03 0

0.009 0.008

0.0023 0.026

fully tested crossings [6] which contained 0.1%C and 3%Ni with the alloy design being influenced by the work of Callender [ 24 ] particularly with regard to impact resistance. It was never envisaged that this steel would be suitable for the heavy axle loads used in North America. A final point to arise from Fig. 4 is the different response of wear resistance to increasing hardness. There is a quite dramatic increase in the wear resistance of pearlitic steel as the hardness is increased while the bainitic steels exhibit a far smaller dependency.

4, Rolling/sliding wear of wrought and cast steels An invesgigation of the wear behavior of wheel and rail steels over the full range of operating conditions of the Amsler machine revealed three different wear regimes [25]. These were designated types I, II and HI and occurred in ascending order of contact pressure and slide/roll ratio. Type I involves acombination of two modes of wear resulting in debris containing oxide and metal particles. Type II is characterized by completely metallic wear debris, the occurrence of ripples on the roller surfaces and some metal transfer. Type III involves an initial break-in period that leads to the production of large pieces of wear debris by a profess that is similar to galling and scuffing. These debris become embedded in the specimen surfaces and cause wear of both rollers by a process akin to gouging abrasion. Type III wear most closely resembles severe metallic wear in pure sliding, and Devanathan [ 13] has provided a compelling energy analysis to suggest that they are the same wear mechanism, The wear process experienced by an unlubricated gage face in a 5 ° curve has been investigated by Danks and Clayton [26]. They demonstrated that type III wear closely simulates the rail wear profess resulting from heavy axle loads. This view is supported by a comparison of the relative wear rates of four rail steels, in full scale tests at the Department of Transportation's Facility for Accelerated Service Testing (FAST) and in the laboratory [27]. Devanathan and Clayton [28] examined the behavior of two low carbon bainitic steels under type IH wear conditions. One was taken from an experimental cast frog while the other was as-rolled, Table 2. The 0.04%C steel was also tested in two heat-treated conditions [ 13]. The material was austenitized at 816 *(2 and cooled at 2 and 200 °C s -1 to generate hardness values of 223 and 293 HB, respectively. Wear terSts wero conducted with a 35% slide/roll ratio, contact pressures of 500 to 1645 MPa and a speed of 200

r.p.m. All tests were run at room temperature with a blast of dry, compressed air directed at the contact zone to minimize heating effects. The geometry of the rollers is shown in Fig. 5. The top rollers were made out ofbainitic steels and the bottom rollers from a wheel steel. The roller configuration provided a constant contact w idth throughout the tests. Wear was measured by removing both rollers periodically and recording weight loss and the number of revolutions. From these data, wear rates, in micrograms per meter rolled per millimeter of roller contact width, were calculated. The most modern head hardened rail steels exhibit a resistance to break-in at pressures greater than 1220 MPa [29], showing a superiority to the low carbon bainitic steels. However, the steady state wear rate of the 0.04%C steel, heat treated to a hardness of 223 HB, compared favorably with the current standard carbon rail steels of 300 HB used in North America. Gamham and Beynon [30] tested similar bainitic steels under type H conditions with results no better than BS 11 rail steel of about 260 HB. Sawley et al [31] also tested a low carbon bainitic steel under type II conditions and showed that it did not perform as well as a normal wheel steel. The North American research continued with five steels produced by Bethlehem Steel Corporation, Table 3. The goals were to find out more about the significance of carbon and whether Mn could be used to replace the more expensive alloying elements Ni and Cr. Previous experience indicated that a carbon content as high as 0.2% would produce carbides and the evidence of the pure sliding wear results [ 19] suggested that carbides were undesirable. The steel pairs of J2/J3 and J4/J5 were designed to investigate the effects of Mn as a replacement for Ni and Cr at the lowat i

Smm Fig, 5. Test specimen geometry.

78

P. Clayton. N. Jin/Wear 200 f1996)74-82

Table 3 Chemicalcompositionof sevenfoiledsteels(wt.%)

JI J2 J3 .14. J5 J6 J8

C

Mn

Cr

l~i

Mo

B

Si

AI

Ti

P

S

0.181 0.115 0.077

2.01 3.97 2.03 2.02 4.04 2.00 2.03

1.94 0.017 1.97 1.96 0.018 !.93 0.01

0.008 0.015 !.93 1.93 0.019 0 0

0A82 0.474 0.475 0.475 0.469 OA9 0.5

0.0027 0.0027 0.0031 0.0030 0.0030 0.003 0.003

!.13 0.270 0.270 0.268 0.272 !.81 1.76

0.029 0.028 0.028 0.026 0,025 0.040 0.049

0.025 0.038 0.026 0.023 0.037 0.042 0.050

0.010 0,009 0.009 0.009 0.009 0.013 0.0(39

0.0131 0.0133 0.0082 0.0071 0.0137 0.009 0.009

0.023

0.026 0.258 0,244

0.1 and 0.04%C levels, respectively. The carbon level of J2 was, unfortunately, significantly higher than that of J3, precluding a direct comparison. The Jl steel resulted from a suggestion by Bhadeshia [32]. He recommended testing a 0.2%C with l%Si steel on the basis of work on isothermally produced bainitic steels investigated for tensile and impact p~operties [33,34]. The wear tests were conducted in the same manneroutlined above. The results for the five J steels [35] showed carbidefree bainite gave the best wear properties. This microstrucrural form was enhanced by higher alloy content and faster cooling rates. In the as-quenched condition the 0.18%C steel performed best. It was clear that carbon content was extremely important. Jin showed [ 36] that the 4%Mn alloying addition achieved the same effect as the 2%Mn, 2%Cr and 2%1qi primarily because the mechanical properties were dominated by carbon. He developed equations to predict tensile strength of bainitic steels in the as-received condition:

o'~(ksi)= 480 + 2 2 6 % C - 105%Cr- 11%Ni -.-9 7 % M n o',,= -421 + 556%C + 133%Cr + 9 % N i + 133%Mn and the waterquenched condition:

cr~ = 267 + 320%C - 42%Cr + 3 % N i - 37%Mn or. = 18L + 6 0 1 % C - 15%Cr + 4 % N i - 1 l%Mn The next step [36] involved taking the Jl variation to a higher carbon content of 0.26 and silicon of 1.8%, Table 3, keeping all other elements the same as in J1. The as-received I6 steel was in the hot-rolled condition and had a microstructure that consisted of carbide-free bainite, Fig. 6(a), with areas of fine twinned martensite, Fig. 6(b). The hardness of the steel, 423 FIB, was considerably higher than the as-rolled Jl. Table 4 shows that Jin's predictions were quite accurate. The ductility of the as-received steel was lower than anticipated but improved on re-austenitization followed by water quenching. The steel was tested in the as-roiled condition and the wear data were comparable to those obtained with wrought Hadfield's manganese ste~l, Fig. 7. One final wrought steel, J$, was produced to check a log. ;cal, but unlikely, conclusion from the analysis of the Jl-J6 series of materials. Everything pointed to the mechanical properties* depending on carbon content with little influence of Mn, Cr or Ni. In the as.rezeived condition J8, containing

,

,,i ........................

.........................

.... : :i! ::::.::ii¸ii!i:i:::::!.!ii:.::i!:!i:: ¸: :ii::::i.:!.!i::i:• :

.........

.......... I .....................

J5 rolled J/cast J8 ruUed J9 dbbt Mnwrt •~ ¢nst JT.~q JSwq Jgwq Mn~t

i 1 2 2 0 MYa[ ] 1700MPa i Fig. 6. TransmissionelectronmicrographsofJ6 (a) carbide-freebeiniteand (b) twinnedmalterts!~.

Fig, 7. Rollin~,/slidingwearbehaviorof Jt.-J9 bedniticsteelsand Hadfield's austenitic manganesesteel at 1220and 1700 MPn contactpressures (wq: waterquenched;wrt:wrought;dbht:doubleheatlreated).

P. Clayton, 31.Jin / Wear 200 (1996) 74-82

Table4 Predictionof the propertiesof J6 basedonJI-J5 analysis

As-received Water-quenched

Ultimatetensilestrength(MPa)

0.2%yield strength (MPa)

Elongation(%)

Reductionofarea (%

Pred.

Meas.

I'~1.

Meas.

Pred.

Me.as.

Pred.

Me.as.

1631 1954

1531 1964

976 1336

1003 1373

12.9 8.8

4,2 10.2

27 20.7

6,7 30,2

Table5 Chemicalcomposition of cast bainitle steels (wt.%)

]7 J9

C

Mn

Cr

Ni

Me

B

Si

AI

Ti

Zr

P

0.269 0.261

1.87 1.81

2.02 0.14

0.21 3.02

0,51 0,47

0.005 0.002

1.87 1.73

0.083 0.015

0.002 0.042

0.112 0.002

0.017 0.008 0.010 0.009

4'

S

the casting from which the roller specimens were taken. In the as-cast and un-heat-treated condition, the cast material did not fare nearly as well as the as-rolled steel of the same composition. In the re-austenitized and water quenched condition, however, the results were much better, Fig, 7, Faced with a deadline to come up with a composition with whtch to make a full size component for testing in the field, a more highly alloyed composition, Jg, was resorted to, Table 5. This cast was heat treated with a double austenitization and forced air quench to give a material consisting of lath ferrite. In this heat-treated condition (the sort of heat treatment considered appropriate for a large casting), J9 produced wear results comparable to the wrought J6 and Hadfield's Mn steel, Fig. 7.

5. Di.~assion

Fig, 8. Keelblock~asfingand specimenlocationfor J7 aud J9. 2%Mn with no Ni or Cr, Table 3, had a hardness of 351 HB and exhibited regions ofpro-eutectoid ferrite along with massive and lath bainitic ferrite. The presence of the pro-eutectoid ferrite could indicate that the alloy content was insufficient to provide the hardenability required. There is an alternative explanation, however. An attempt was made to reduce the prior austenite grain size by rolling at a low temperature. It is possible that the steel was allowed to cool too far resulting in the formation ofproeutectoid ferrite before rolling commenced, This would have prevented the transform,~tion to bainite in those areas, The wear performance in the as-received condition was unexceptional, Fig, 7, but in the as-quenched condition it was comparable with that of the as-rolled J6 when tested at 1220 MPa. The most recent work has focused on the properties of cast bainitic steels with a view to frog applications. The first cast of steel, JT, was made to a composition very similar to that of the J6 wrought material, Table 5. Fig. 8 shows a sketch of

The earliest reports on bainitic steels were very discouraging for rail wear situations until the publication of the sliding wear investigation involving nine different steels [ 19]. Although the results were not overwhelmingly favorable they provided sufficient encouragement to investigate bainitic steels more carefully and challenge some established beliefs. Ia turn, the results indicating that chromium might be a very significant alloying element have not been substantiated by any subsequent work. Devanathan's program suggested [13] that granular bainite might be the optimum micros~ructure but this was overturned by later tests. At one point Devanathan's work indicated a trend of increasing wear resistance with decreasing hardness for bainitic steels. Those results were subsequently seen to be part of a broader trend, Fig. 9, in which the bainitic steel data fall within a wide scatter band. If too narrow a view is taken, premature conclusions ate easily arrived at and difficult to dislodge yet they can eventually be shown to be part of a larger picture. The data of Fig, 9 raise several questions about the wear behavior of bainitic and pearlitic steels. In pure sliding and severe rolling/sliding conditions the improvement in wear resistance with increasing hardness for pearlitic steels is much

P. Clayton, N. .#inI Wear 200 f i gi6j 74--82

80 50

,,

,

.,

,0

".,.

I-F~-1

.

.

.

.

.

,,

,o t / 0 L

150

~-~"i'~" ~ "t • .......m:m+ =~ ,

200

-

c

250

, =~-

,

....- -

300 350 400 Brinell Hardness

..~

" , ~

450

= ___~

f~o

550

P i g . 9. R o l l i n g / s l i d i n g w e a r rate v e r s u s h a r d n e s s for bainitic a n d ilearlitic steels.

greater than that for the bainitic steels. For the latter the same wear rate can be achieved for a very wide range of hardness levels. Yet even for pear!,'tic steels there is a wide scatterband and the question has to be asked as to whether this reflects natural experimental statistical scatter or some aspect of material properties, or microstrncture, not covered by the blunt mechanical property surrogate of hardness. Given the reprodacibility obtained with different steels under type 1II wear conditions [ 13,36,37] there is a strong case to be made for the fatter. There is still a degree of mystery about the wear behavior of the low/medium carbon bainitic steels. With pearlitic steels, deformation resistance, rolling contact fatigue (RCF) and wear all improve significantly with increasing initial hardness [29]. For bainitic steels this is also true of defermat!on and RCF [ 13,38] but wear does not fit so easily into the pattern. Furthermore, the wear resistance of several different bainitic steels has been disappointing [15,16,30,31 ] under relatively mild test conditions. These results suggest that low/medium carbon bainite produces its best response under high strain conditions indicating that the strain hardening capabilities of the steels may be significant. This aspect of the mechanical behavior needs m be investigated in depth as does the possibility that ductility is a factor in wear resistance. Bainitic ferrite is very hen, ily dis!~cated with respect to pro-eutectoid ferrite which has very poor resistance under severe contact conditions [21]. The dislocation densities have not been measured in the current program but reported figures [39] suggest that they may be as h~.$h as 1.7)< 1014 m-2. Without an extensive investment in further research it has to be assumed that the dislocations give rise to the hardness, strength and work hardening capacity. Other microstructural features such as lath width, quantity and distribution of retained austenite, M-A islands and the presence of martensite islanda will also have an influence.

The quantitative effect of retained austenite could be investigated with the existing steels through X-ray diffraction analysis. The influence of retained austenite on wear resistance has not been studied extensively but there are data which show it can be advantageous in abrasive situations [40-42], The beneficial effect of retained austenite is associated with interlath films [42,43] which arc stable and ductile [33,34,44]. Zhu et al. have observed a positive benefit of retained austenite in the RCF behavior of some highly alloyed steels [45]. In high Si, carbide-free bainite, retained anstenite films between the laths were beneficial to impact resistance [ 33]. Blocky austenite, however, exhibited a greater degree of instability and transformed to untempered martensite. This would not be expected to be a problem in very low carbon steels but the carbon content of the M-A phase can be as high as ten times the average [46]. At the very least it is important to know what impact retained austenite has on wear resistance if austenite were deemed to be vital for toughness. The role of carbon has been shown to be very significant and there could be two reasons for this. The first is that carbon has by far the most significant effect on the B, temperature, and as this is lowered the dislocation density increases [47]. In the absence of carbides, carbon has m be in interstitial solid solution or located on the dislocations, thus greatly impeding their movement and resisting plastic flow. On the b~sis of work on lath martensite, it is reasonable to suggest that the latter location applies to the bainitic steels. Up to 90% of carlrn segregates to structural defects like dislocations and lath boundaries in lath martensiteeven at very high quenching rates [48-501. In the development of frog steels file way forward hinges on determining the maximum level of carbon that can be used without forming e~bides in the presence of silicon. (This may correspond to the maximum level of silicon that can be used without preventing austenite formation.) This would enable the hardest, most deformation resistant, carbide-free

P. Clayton, IV.Jin I Wear 200 (1996) 74.82

steel to be produced to enhance wear and rolling contact fatigue resistance. Tribological behavior is, however, only one aspect of the overall properties required of a tail or frog steel. Little mendon has been made of tensile properties, impact resistance, fatigue initiation behavior, fatigue crack propagation rates, residual stresses and weldability, all of which are critical to the development of new materials for these components. While the significance of the alloying elements Mn, Cr and Hi is in some doubt with regards to wear resistance, the influence on hardenability for air cooled steels and other mechanical properties needs to be investigated. Establishing the lowest cost alloy to achieve this has obvious economic implications. While the recent drive has been towards developing new materials for components that the railroad industry is eager to test in trac|: at the earliest possible opportunity, attempts have been made to address some of the more academic questions that have arisen. It may of course prove to be the ease that these academic issues are also of great practical concern. In particular, the quantitative relationships between mechanical properties and microstructure for carbide-free bainite in terms of lath size and width, proportion of lath boundary M-A islands, and retained austenite and dislocation density require further study. A particularly intriguing possibility offered by the bainitie steels is a relatively soft, ductile and weldable steel that responds well to very high surface strains. Although some of the softer steels only produced an apparent high wear resistance because bulk surface flow reduced weight loss [35] this has not always been the case [ 13,37]. An investigation of these occurrences and the difference between wear restslance ,Jnder high and low strains would make an interesting research program.

6. Conclusions

A carbide-free bainite microstructure can be produced in a 0.25%C, 1.75%Si M o B steel, in cast and wrought form, that exhibits a wear behavior that competes with Hadfield's manganese steel under severe rolling/sliding conditions. In the cast condition the material has to receive a suitable heat treatment {as with all steel castings) but with a sufficiently high alloying content can be used in the as-rolled condition. With leaner alloys a surface hardening treatmenl involving forced air or water cooling can produce the properties required. These alloys offer potential for railroad rail and switch and crossing components. One of the alloying di~.ctions appears to be in determining the highest carbon level, with the appropriate addition of St, that can be used without producing carbides. The influence of other alloying eieraents on wear resistance and other key mechanical properties remains unclear.

81

The immediate research needs are to produce full size components out of the most prc~4~4ngalloys and generate field performance data. These are important in terms of furfiler development of the alloy design and in calibrating the methods of laboratory assessment.

Aeknowledgemeats

The program reported here has not E~n continuous, has spanned two decades, taken place on two continen:s and been funded by different organizations. At various stages the work could easily have been terminated and led to spurious conclusions. In the present climate, where industrial organizations are under increasing commercial pressures to produce technologically useful results, long term research has become a real challenge. 'ti~e authors would like to acknowledge all the support they have received from so many people. Some have provided the encouragement and funding environment to enable the work to continue despite periods of inactivity and all have contributed their expertise. Among those should be mentioned E.G. Jones and C.O. Frederick, formerly of British Rail Research; R.K. Steele, formerly of the Association of American Railroads, Research and Test; O. Orringer, formerly of the Volpe National Transportation Systems Center; W. Paxton formerly of the Federal Railroad Administration, Research and Development; H. Lees of Bvrlington Northern Santa Fe Corporation; D. Stone, S. Kalay, K.L Sawley and D. Davis of the Association of American Railroads; J. Deiley cf the Federal Railroad Administration; B. Bramfitt and D. Wirick of Bethlehem Steel. Many former research colleagues have to be recognized: P.I. Bolton and G. Peli, when with British Rail Research; J. Gamham and J. Beynon, formedy with Leicester University; H.K.H.D. Bhadeshia for his insights into the metallurgy of bainitic steels and the suggestion that led to the Jl variation and subsequent theme, and former students D. Danks of ESCO and R. Devanathan of Micropolis. Funding was provided by British Rail, the Association of American Railroads, the Federal Railroad Administration, Burlington Northern Railroad and Bethlehem Steel Corporation. To these institutions, companies and ndividuals we extend our gratitude for their commitment, pa,~nce and time in the hope that the work will help in the dew lopment of a new generation of steels for the railroad industr. '.

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