Wear behaviour of bainitic rail and wheel steels

Abstract

The paper presents a short review on the past research and recent trends in the development of rail and wheel steels using bainitic morphology from the angle of mechanical and wear performances. The enhancement in mechanical properties as well wear resistance of the bainitic rail and wheel steels is very promising. In addition, the mechanical behaviour of a bainitic rail steel and a wheel steel along with their linearly reciprocating dry sliding wear characteristics in comparison to the existing pearlitic rail and ferritic–pearlitic wheel steels used in Indian Railways is discussed. The steels were subjected to austempering heat treatment at different temperatures and durations in the bainitic region as determined from the TTT diagrams of the respective steels. Bainitic steels show better mechanical properties and wear resistance due to the typical morphology of bainitic microstructural.

Keywords

Bainitic steel Austempering Wear Mechanical properties Rail and wheel

Introduction

Since the first work on bainitic steels,¹ it was thought to be a possible and prospective replacement for the pearlitic steels, where strength and wear resistance were of principal requirements. Bainitic steels with lower carbon content yielded higher hardness than the pearlitic steels.² However, the early works carried out for investigating the wear performance of bainitic steels were quite discouraging.^3–5 Later, Clayton discovered that it was only a particular variant of bainite that accounted for the poor wear performance,⁶ and the results obtained in the low carbon carbide free morphology of bainite were surprisingly favourable. Researchers, thereafter, became very optimistic about the wear performance of this variant of bainitic steel.^6–8

The historical development of rail and wheel steels has witnessed an increase in the hardness of the steels for achieving higher wear resistance. Increasing the carbon content and suitable heat treatments to attain finer pearlite have been the most adopted ways to increase the hardness of the conventional pearlitic rail steels. When the conventional pearlitic steels reached almost to their limit, there was a need for other alternative materials. Bainitic steels, possessing number of mechanical properties, were then looked upon as a promising potential candidate for the rails and wheels. Figure 1 shows the trend of this development.⁹ Over the years, it has been observed that the growth heads towards achieving higher tensile strength and hardness of the steels. In the recent development, it has been found that the high carbon bainitic steels austempered at lower transformation temperatures show excellent wear resistance.^10,11 Hence, these steels are expected to play an important role in development of rail steels in future. This is also indicated in Fig. 1. With specific alloy design and heat treatment, the properties attained by this variant of steels surpass those offered by the conventional pearlitic rail steels. It is also evident from Fig. 1 that bainitic steels are definitely next generation rail steels.

Yokoyama et al.¹² have made some general conclusions based on comparison data of the conventional pearlitic rail steels and novel bainitic rail steels. They are listed as follows:

The wear behaviour of most of the bainitic rail steels was concluded to be inferior to that of the pearlitic steels.^{3–5,13–16}

The bainitic rail steels were found to have higher fatigue strength than the pearlitic rail steels.⁹

The bainitic rail steels possessed fracture toughness almost double than that of the pearlitic rail steels.^17,18

The wear resistance of pearlitic steels has a simple correlation with the contact pressure and hardness described by a Holm's-type equation (W = kP/H, where W, P, H and k are the wear volume, contact pressure, hardness and a constant, respectively). However, the bainitic steels showed complex dependencies on the strength and contact pressures.¹⁸

It is apparent that except for the wear performance of the bainitic steels, these steels offer excellent material properties (strength, ductility and toughness), to be a suitable replacement of the conventional rail steels. Consequently, the improvement of wear resistance of bainitic steels should make it a prospective rail material candidate, which will offer higher damage resistance and longer service life.

Beside improving the wear resistance, there has also been a need for increasing the strength and toughness of the rail steels on account of the increasing axle load, railway traffic and the ever increasing speed of the trains. The increase in strength by most of the routes is accompanied with decrease in toughness. However, bainitic steels usually offer both, increased strength and toughness, simultaneously.

In the history of development of bainitic steel for its possible application as rail and wheel steels, ‘Titan’ was the first bainitic steel to be used in a railway crossing in UK in 1980s.¹⁹ This steel was applied to a high-speed railway system. J9 was another successful Mn–Si–Cr bainitic steel railway crossing reported in 1996.⁶ Again, a novel low carbon bainitic steel was also developed at Cambridge University in 1996. This steel exhibited dramatic improvement in the toughness in comparison to that of the pearlitic rail steel.¹⁷ Impact toughness of this steel had increased 5–10 folds when tested at 0°C.

The bainitic steels used in all the above cases^6,19^,¹⁷ were carbide free bainitic steels. However, other studies conducted on the performance of bainitic rail steels reported the inferior wear resistance of the bainitic rail steels as compared to that of the pearlitic rail steels.^3,4^,⁵^,¹⁶ A general observation drawn from various studies carried out for the wear performance of bainitic steels showed that while the carbide free bainitic steels emerged to perform well, high carbon steels failed to do so. However, in spite of several advantages offered by the carbide free morphology of bainitic steels, they had few disadvantages as listed below.²⁰

Firstly, in the gauge face of a rail road, greater resistance to plastic deformation was equalised by higher wear rates.

Under contact conditions seen in the rail curves, low carbon steels would fail to give longer service life.

This clearly indicates that under severe loading conditions in heavy haul rail network, high carbon bainitic steels are expected to offer greater resistance to plastic deformation and high wear resistance. High carbon bainitic steels made by suitable heat treatment were also known to possess better mechanical properties than the carbide free bainite.^21,22 The development and investigation of the rail steel with high carbon bainitic morphology were focussed by the researchers past decades. However, the results were scattered, and there was no common conclusion on the wear performance of high carbon bainitic steels. Most of the studies showed poor wear resistance under ambient conditions.^3–5,23^,²⁴ But some results were positive, and hence optimistic.^8,25^,²⁶ However, a definite conclusion on the suitability of the bainitic steels needs a systematic investigation of the wear performance of the high carbon bainitic steels based on the morphological dependence, initial hardness and strength of the steel.

The contact stresses generated at the wheel–rail contact interface also complicate the wear phenomena of wheels. Lubrication at the gauge face reduces the wear rate by 8–20 times.²⁷ However, plastic shear causes repeated deformation on the wheel surface and leads to the development of surface cracks. Depending on the contact stress conditions, these surface cracks generally result into either wear or rolling contact fatigue.²⁸ Hence, high strength and wear resistant wheel steels are required and being developed in order to extend the life of railroad wheels. Thus, the suitability of bainitic steels for possible application in railway wheels has also been explored.

At present, most of the rail materials are made of high carbon grade steels (C wt- ranging between 0.7 and 0.8), and wheel steels are made of medium to high carbon steel grades with C ranging from 0.45–0.8 wt-. Microstructure is typically pearlitic in case of rail and ferritic-pearlitic for wheels. This review presents the results of some studies based on the wear performance of two steels, which have similar composition to that of the conventional rail steels and wheel steels.

Recent worldwide trends of bainitic steels

Bainitic steels with right composition and appropriate thermo-mechanical treatments are being used in the heavy haul rail networks worldwide. Nippon Steel Corporation has successfully produced bainitic rail steels that are used in railway crossings, points and level crossings.²⁹ It has been reported that the bainitic steel is also being used in British Railways and in London tube.²⁹ These rails were installed in curved railway tracks. This steel yielded only slightly lesser lifetime than the conventional pearlitic rail steels used under same conditions.

Bhadeshia²⁵ has presented that one of the major applications of the bainitic steels has been in the development of rail steels. A bainitic steel rail developed by them possesses excellent wear resistance, imparted reduced wear on the wheels and showed good toughness and weldability. The commercial application of this steel was also verified and proven to perform well in-service. They also reported excellent strength and toughness combination of these steels, which matched and even outperformed those of the expensive maraging and quenched and tempered steels.²⁵

Another bainitic rail steel (S49) has been successfully implemented since 2004 in the main exit track from Arcelor Mittal Steel Plant of Poland in Dabrowa Gornicza.²⁹ The rails are operative in straight and curved rail tracks as well. This is a nickel free steel with a hardness of 370 HBW with a trade name RB370. It was developed and implemented by Rolls–Rolls, Inc. A second grade with a trade name RB390 was also developed. As it contained Ni as alloying element, this steel was expensive and used only for railway frogs. However, this steel became a prospective candidate for heavy duty rails. This steel was under test for its possible application in mono-block of railway frogs. All the above mentioned steels are performing well in their in-service conditions, and reported to be failure free. The compositions of some of these steels are given in Table 1.^24,25

Table 1

Compositions of some of the steels studied in literature for rail and wheel applications

Steel	C	Si	Mn	P	S	V	Mo	Nb	B	Cr
A²⁴	0.28	0.3	1.21	0.13	0.009	0.1	…	…	…	…
B²⁴	0.31	0.31	1.32	0.13	0.008	…	0.26	…	…	…
C²⁴	0.29	0.55	1.10	0.01	0.006	…	…	0.040	…	…
D²⁴	0.34	0.32	0.70	0.011	0.007	…	…	…	0.0015	…
Swiss Railways steel²⁵	0.3	1.25	1.55	…	…	…	0.15	…	…	0.5
French National Railway steel²⁵	0.2	1.25	1.55	…	…	0.15	0.15	…	…	0.5

Studies have also indicated superior mechanical properties and fracture toughness of the bainitic steels in comparison with the pearlitic rail steels. A brief summary of the mechanical properties of some of the bainitic and pearlitic steels reported in the literature are given in Table 2.^10,29–35 Pseudo-toughness values calculated from the area under the stress–strain curve are also mentioned in Table 2. It is evident that the bainitic steels have superior mechanical properties and toughness in comparison with those of the pearlitic rail steels.

Table 2

A brief summary of the mechanical properties of some of the bainitic and pearlitic rail and wheel steels reported in the literature

	Steel	Tensile strength/MPa	Yield strength/MPa	Elongation/	Hardness	Calculated toughness, U_T/MPa
Garnham and Beynon³⁰	B04 (B)	924	638	30	275 HV	234
	B20 (B)	1235	750	27	378 HV	268
	B52 (B)	1321	853	13	355 HV	141
	R52 (P)	781	443	22	220 HV	135
	W64 (P wheel)	857	367	20	245 HV	122
Tyfour et al.³¹	BS11 (P rail)	781	406	21.9	240 HV	130
Tyfour et al.³¹	W8A (P wheel)	857	325	20.3	275 HV	120
Su and Clayton³²	J1 (B)	1360	846	14	40 HRC	154
	J2 (B)	1151	830	16	37 HRC	158
	J4 (B)	945	721	18	28 HRC	150
	HH	1320	790	13	37 HRC	137
Yokoyama et al.³³	Rail 1 (P)	855	401	15.3	26 HRC	96
	Rail 2 (P)	1303	900	13.3	40 HRC	146
	Rail 1 (B)	823	645	19.6	43 HRC	144
	Rail 2 (B)	1421	1079	14	37 HRC	175
Singh et al.³⁴	B grade rail steels	1150–1350	720–820	15–20	390–400 HV	140–217
Pacyna²⁹	RB370 (B)	1197–1211	843–858	12.2–14.1	371–378 HV	124–146
	RB390 (B)	1347–1353	825–832	13.0–14.9	390–398 HV	141–163
	R350HT (P rail)	1080–1098	700–712	10.3–11.1	350–390 HV	92–100
Sharma et al.¹⁰	NCC (B rail)	1808	1566	14.5	485 HV	245
Sharma et al.¹⁰	C–Mn (P rail)	893	420	12	330 HV	79
Sharma et al.³⁵	MAS2 (B wheel)	1075	725	18	302 HV	162
Sharma et al.³⁵	Wheel-R19 (F-P steel)	820–940	520	14	245 HV	94–102

State of current research

High carbon bainitic steels for rails

At present, most of the steels used in Indian Railways are high carbon pearlitic steels with carbon content close to 0.7–0.8 wt-. Steel with similar composition was investigated by the authors to understand the wear behaviour.¹⁰ The approach was to carry out a systematic investigation based on the variations in morphologies resulting from the different heat treatment parameters employed. The investigation carried out in this manner led to identify a particular bainitic variant of this steel that offered superior wear resistance at heavy loading conditions in the laboratory investigation. The other variants of this steel were not competent enough to be recommended as wear resistant steels for rails. This investigation concluded that for any particular composition of steel, the different variants of bainitic steels obtained in the different temperature zones of austempering transformation responded differently to the tribological tests. Not all the bainitic steels have inferior wear resistance as compared to the conventional pearlitic rail steels. High carbon bainitic steels transformed at lower bainitic transformation temperature zones can outperform the wear resistance of the pearlitic rail steels.

Alloy design and heat treatment

Composition having carbon content similar to that of the conventional rail steel was chosen in this investigation. Alloying additions of hardenability enhancing elements, Cr, Cu and Ni were made in order to escape the diffusive transformation zone, to allow the steel to enter into the bainitic zone. The compositions of the steel tested and the conventional steel used as reference for comparison are shown in Table 3.

Table 3

Chemical composition of the high carbon bainitic steel and conventional rail steel

	Chemical composition/wt-
Sample name	C	Si	P	S	Mn	Cr	Cu	Ni
Conventional rail steel	0.71	0.21	0.022	0.013	1.04
High C bainitic steel	0.71	0.35	0.027	0.026	1.15	0.59	0.40	0.20

The steels were austempered in a salt bath at different transformation temperatures after austenisation at 900°C. The austempering durations were also varied. This resulted in different microstructures with different length scales, and volume fraction of the constituent phases. Some of the relevant micrographs of the austempered samples are shown in Fig. 2 (treatment conditions, time and temperature are specified along with).

Figure 3 shows TEM micrographs of the samples shown in Fig. 2, revealing the variation in the size of ferritic laths and carbides. It is evident that the fineness of the bainitic ferrite laths and carbides is inversely proportional to the transformation temperature, as it is increasing with the decreasing temperature.

Small cuboid specimens from the heat treated steels were prepared according to ASTM G133 for tribological tests. Linearly reciprocating sliding wear tests were carried out using a ball-on-disk Reciprocatory Friction Monitor TR-287, DUCOM, India. All the tests were carried out in dry un-lubricated condition at a frequency of 5 Hz and the load was varied from 30 to 50 N. Each test was conducted for duration of 1 hour, equivalent to 18 000 cycles. A new 8.3 mm steel ball with hardness of 750–800 VHN was used for each test.

The initial Hertzian contact pressure was 1.7 GPa (calculated using Hertzian contact pressure equation³⁶), which was slightly lower than the actual rail/wheel contact condition (1.8 GPa). The higher load contacts corresponded to 2.5 GPa. The geometry of the wear scar obtained on the specimen surface was used to calculate the wear volume loss. For detailed experimental procedures, references^10,37 can be consulted.

High strength wear resistant rail steels

The steels austempered at different times and temperatures yielded a wide range of the mechanical properties.¹⁰ While the samples transformed at higher transformation temperatures yielded high tensile strength and elongation values, those transformed at lower transformation temperatures yielded further increase in the tensile strength without much compromise in the ductility. The trend of strength and elongation values is shown in Fig. 4. The trend observed in the strength could also be correlated to the microstructural evolution as seen in Fig. 2. With increase in temperature, the sheaf thickening progresses, yielding coarser sheaves at higher temperature. The thickening of sheaves progresses easily at higher temperatures because the bainitic ferrite can easily get accommodated by the advancing bainite/austenite interface into the parent austenite of lower YS (due to higher temperature). However, at lower temperature the higher strength is due to finer bainite morphology.

The TEM images also support this observation. The TEM image (Fig. 3a) of the high carbon steel austempered at 300°C for a holding duration of 60 minutes reveals finest length scale of the ferritic and carbide laths. Besides, the presence of both inter-lath and intra-lath cementite is also evident. Figure 3b and c showing the TEM images of high carbon steels austempered at 350 and 400°C, respectively, have revealed upper bainite morphology as there is no evidence of intra-lath carbides. The intra-lath carbides also contribute towards attaining much better strength. Hence, the strength obtained in case of bainite austempered at 300°C is significantly high.

On wear testing and microstructural examination of the worn bed morphology of these steels, the results were surprisingly favourable for only a single variant of this steel out of 12 different variants tested. However, after testing repeatability of result, this variant of the steel was confirmed to have superior wear resistance over the existing rail steels in Indian Railways. Figure 5a shows the wear resistance of this variant of steel (transformed at 300°C for 120 minutes) along with those transformed at intermediate and higher temperatures transformed for same duration tested at different loads. The wear volume loss of the conventional rail steel is also shown for reference. This steel has also exhibited a remarkable combination of strength and elongation values (1800 MPa UTS and 14.5 total elongation). This result definitely builds newer prospects to realise the enhanced benefits of increased strength and greater wear resistance offered by the high carbon bainitic steels over the carbide free bainitic steel.

Figure 5b shows the specific wear rates of various bainitic steels obtained at different transformation temperatures and times as indicated. The degree of severity of the wear rates could best be represented by this graph. The bainitic steels obtained at lower transformation temperature (300°C – 60 minutes and 300°C – 120 minutes) and those obtained at 350°C – 30 minutes have shown a gentle slope like the conventional pearlitic rail steels. However, the bainitic steel obtained at higher transformation temperature and longer holding durations (350°C – 60 minutes and 350°C – 120 minutes) have exhibited steep slopes indicating higher degree of severity. Another way of interpreting this observation is that when the bainitic sheaf size is allowed to grow either at higher temperature or longer holding duration, the length scale of the microstructure increases resulting into higher specific wear rates.

The wear mechanisms of these steels after the wear tests were also studied by examining the wear scar bed surface. The severity of wear is usually classified under three categories, namely types I, II and III.³⁸ The size and shape of the chunk of materials removed from the wear bed, presence of gouges and mounds, crack formation and some other phenomena, like rachetting, break-in-surfaces, etc. are some of the indications that suggest the mode of wear mechanisms as predicted from the wear scar bed analysis. The worn surface of the high carbon steel (transformed at 300°C for 120 minutes) showed lesser degree of severity as compared with that of the conventional rail steels studied in this case.¹⁰ The worn scar bed morphologies of different variants of high carbon bainitic steels obtained at higher transformation temperatures are shown in Fig. 6a–c. The worn bed morphology of the conventional wheel steel (Fig. 6d) is also shown in order to compare their wear mechanisms. The presence of break-in-period, which is indicative of material degradation through mutual abrasion, is evident in Fig. 6a, b and d. This feature is absent in the bainite obtained at higher transformation temperature (Fig. 6c). This is attributed to the lower hardness of bainite obtained at 400°C. Surface damage indicated by break-in-period is reflected as gouges and mounds. It is evident that the sizes of these gouges and mounds formed under similar tribological testing condition are smaller in case of bainite formed at 300°C (Fig. 6a) as compared to that of the conventional rail steel. This assessment gives a reasonable representation of the in-service behaviour and expected life of the material examined. Detailed worn morphological examination can be found elsewhere.^10,37

It can be seen from Table 2 that the strength and elongation values achieved for these bainitic rail steels are remarkably high. In addition, the total elongation obtained from the tensile tests is also very high. It can be noted this steel exhibits significantly improved toughness as compared to that of the conventional rail steels. This encourages the prospective use of this bainitic steel as a suitable replacement of the existing conventional pearlitic rail steels.

Medium carbon bainitic steels for wheels

Alloy design and heat treatment

High strength and wear resistant wheels are required to extend the life of railroad wheels under severe loading conditions of railways in the present scenario. To attain wear resistant wheel steel, newer materials and heat treatment routes are being adopted. The wear resistance offered by medium carbon bainitic steel needed to be investigated and some unexplored properties could be realised. Considerable enhancement in strength and wear resistance of the bainitic steels were obtained from these medium carbon steels.

The wheel material used in Indian Railways is a forged coaching wheel with IRS specification R19, which is a medium carbon steel. A micro-alloyed steel was designed keeping the carbon content in this range and adding some suitable alloying elements (Cr, V and Cu). Table 4 shows the composition of the steel tested and the conventional wheel steel (R19).

Table 4

Composition of the medium carbon bainitic steel tested and the conventional wheel steel

	Chemical composition/wt-
Sample name	C	Si	P	S	Mn	Mo	Ni	Cr	Cu	Al	V
MAS2	0.47	0.51	0.033	0.031	0.87	0.02	Tr	0.15	0.029	Tr	0.11
Wheel-R19	0.53	0.22	0.022	0.024	0.81	…	…	…	…	0.14	…

Small specimens were austenised at 900°C and austempered for varied temperature and holding durations. Each treatment resulted into varied morphological length scale and volume fraction. The perceived effect of time was observed in thickening of the sheaves as mentioned before. Detailed dry sliding reciprocating wear tests were also carried out. For detailed experimental procedures, references^10,37 can be consulted.

Figure 7a–c shows the SEM micrographs of the medium carbon steels austempered at different temperatures for 60 minutes. The micrographs consist of mainly bainitic ferrite. The micrographs reveal that the bainite obtained at lower transformation temperature (325°C) is finer. Thickening of sheaves is observed with increasing transformation temperature. Figure 7d shows the SEM micrograph of the conventional wheel steel, wheel R-19. The microstructures reveal primarily ferritic-pearlitic steel. The parallel lamellae of pearlite are clearly revealed in the microstructure.

High strength wear resistant wheel steels

The strength and elongation values of the medium carbon bainitic steel transformed at three different transformation temperatures for different holding durations are shown in Fig. 8. Higher transformation temperatures have yielded lower strength and higher elongation values. Lower transformation temperature resulted into higher strength and lower elongation. However, the compromise in elongation is not very significant as can be observed in case of the sample transformed at 325°C for 30 minutes (1483 MPa UTS and 13 elongation). A representative bar diagram for the wear volume loss of the medium carbon steel transformed to bainite at 325°C for 10 and 120 minutes is shown in Fig. 9b. The wear performances of these samples were much better than those compared with the conventional ferritic-pearlitic wheel steel R19.³⁵ Also, it showed that with the increase in holding duration the hardness decreased and the wear resistance also decreased as shown in Fig. 9a and b, respectively. However, the correlation between hardness and wear volume was not a linear one, and the dependence seemed to be complex. Hence the mechanical properties were correlated to the wear performance by applying a linear fit and the measure of goodness of the fit reflected the correlation between the said properties. The details can be referred elsewhere.³⁵

This study clearly concluded that the wear resistance of medium carbon steels outperformed those of the conventional wheel steels used in railways. However, the wear performance is not only about the wear volume loss. Other factors like the degree of severity of the material removal, wear oxidation, influence to fatigue behaviour due to wear, etc., also need to be considered. The steel studied in this case was examined for their material removal mechanisms too. Figure 10 shows the worn scar bed morphology of the medium carbon steels transformed at various transformation temperatures for 120 minutes and tested at a load of 35 N. The worn surface of medium carbon steel treated at lower transformation temperature (325°C) showed thick patches of detached metallic chunks indicating spalling (Fig. 10a). This was attributed to higher hardness obtained at lower transformation temperature and the shear contact stresses becoming dominant enough under higher contact stresses. For bainite formed at intermediate transformation temperature (350°C) debris containing heavy amount of metallic flakes were obtained, and no oxides could be generated (Fig. 10b). Bainite formed at higher transformation temperature (Fig. 10c) showed large amount of small patchy oxide particles. The mechanism of wear was primarily oxidative. It was expected that abrasion of oxide particles and re-oxidation of the surface resulted into the removal of oxide patches. The worn bed morphology of the conventional wheel steel showed delamination, which is characteristic of the rail/wheel interface wear (Fig. 10d). Such kind of wear was expected to be initiated by the presence of cracks in the subsurface layer due to repeated alternating fatigue loading. This kind of wear mode is typically observed in wheel flange/rail interface wear.³⁹

However, none of the bainitic samples showed catastrophic mode of wear for any of its variant. The wear track morphological examinations suggested milder mode of wear of the steels austempered at relatively higher transformation temperature (400°C) in the bainitic transformation zone than for other cases.³⁵ Higher endurance of the MAS2 bainitic steel samples to withstand higher degree of severity is equivalent to that of the conventional wheel steel tested at lower loads. The details can be referred elsewhere.³⁵ Hence, the bainite obtained at higher transformation temperature was competent enough to be able to perform better than the existing ferritic-pearlitic wheel steels.

Improved strength and toughness offered by the bainitic MAS2 steels over the conventional wheel steel is shown in Table 2. The wear resistance of the bainitic MAS2 steels has also been found to be better than that of the conventional wheel-R19. However, the mechanism of wear is the most important aspect to be considered while judging the suitability of these materials. In the present case, the severity of modes of wear of bainitic wheel steels is compared with the wheel-R19, and characterised on the basis of the wear mechanism models available in the literature. This study³⁵ concluded that the MAS2 medium carbon bainitic sample could withstand higher degree of severity equivalent to that of the conventional wheel-R19 samples indicating superior wear performance of the bainitic steel samples. With such strength, toughness and wear resistance, the bainitic samples are expected to be the next generation materials to be used for railways wheels too.

Conclusions

The development of bainite as wear resistant steel was accredited to a specific type of its microstructure, low carbon carbide free bainite. Low carbon bainitic steels for rail and wheel steels are being used widely and performing well in-service. However, the unexplored properties of high carbon bainitic steels for its prospective use in railways as rail steel and medium carbon bainitic steel for wheel steel were identified to be quite encouraging. Some of the salient outcomes of the present work are listed as follows:

High carbon and medium carbon bainitic steels could also perform well with an added advantage of higher strength and longer service life.

High carbon bainitic steels made by austempering at lower transformation temperatures could outperform the wear resistance of the conventional rail steels.

Medium carbon bainitic steels offered greater strength, toughness and wear resistance for its entire variants. However, the severity of wear is an important factor to be considered for the selection of suitable replacement of the existing conventional wheel steels.

The wear scar bed morphology suggested the mode of wear to be milder in case of the steels isothermally annealed at higher transformation temperature (400°C) than for other cases.

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