Dry and lubricated wear of rail steel under rolling contact fatigue - Wear mechanisms and crack growth

doi:10.1016/j.wear.2017.03.025

Wear

Volumes 380–381, 15 June 2017, Pages 240-250

https://doi.org/10.1016/j.wear.2017.03.025 Get rights and content

Highlights

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R370CrHT fully pearlitic rail steel was studied under Rolling Contact Fatigue in two conditions, dry, and lubricated with a friction modifier.
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The effect of adding a friction modifier after a certain number of dry fatigue cycles was discussed.
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Correlations between the non-metallic inclusions and the crack growth were found.
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The crack depth, and crack angle were measured for three different numbers of fatigue cycles.
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The mass loss of wheel and rail were measured, observing higher mass losses in the pre-fatigued rail samples after the addition of a friction modifier.

Abstract

In this work, the dry and lubricated wear behavior of rail steels (R370CrHT) under Rolling Contact Fatigue (RCF) conditions was studied in laboratory. A twin-disc rolling-sliding machine was used to reproduce contact conditions under high contact pressure (1.1 GPa) and high creepage (5%) with the aim of accelerating RCF failure. Two different types of tests were performed, namely dry and lubricated with a friction modifier, to simulate the mechanisms that cause RCF. The results showed an increase of the wear rate in the tests where a friction modifier was added after an initial stage of 4,000 dry cycles. The damage of the surface proceeded by cracks formation during the dry stage followed by accelerated crack growth and flaking due to the effects induced by the friction modifier entering into the original cracks. After several thousand cycles, the flakes were detached from the surface causing high wear rates. The cracks morphology was observed under SEM. The cracks depth and angle with respect to the contact surface were reported and some correlations were made with the mass loss results.

Introduction

The wear on rail systems is a serious problem for the competitiveness of transport industry. A global trend to address wear issues is the use of friction modifiers (FMs) at the interface between wheel and rail. Bower (1988) [1], Kaneta and Murakami (1985) [2], [3], [4], and others have studied rolling contact fatigue (RCF) to gain understanding of the behavior of the FM when entering the cracks and to evaluate its influence on wear and cracks growth [5], [6], [7], [8], [9]. Bower [1] proposed three mechanisms of crack growth assisted by fluids based on the experimental results of S. Way (1935): (i) the cracks only propagate if a fluid lubricant is applied to the surfaces in contact, (ii) the cracks always propagate in the direction of motion of the load over the surface, and (iii) if there is some relative sliding between the two contacting surfaces, cracks only propagate in the driven surface. Although it is acknowledged that a FM can minimize friction between the crack faces and promote crack growth through the Mode II (shear), it is also well known these days that no fluid is required to grow a crack; regarding (iii), it has been found that the cracks grow in both wheel and rail.

A research in Australia found that the total annual maintenance cost of a 12 Million Gross Tons (MGT) railway system can be as high as USD $ 34 per meter of rail. The costs and the wear rates are influenced by the curvature radius of the railroad, the planning intervals of rail grinding (Rail grinding refers to a controlled wear operation used to eliminate cracks before they reach a critical length at which its growth rate sharply increases) and the use of a suitable FM at the interface, among others. The total annual cost/meter of the rail maintenance in a non-lubricated 12 MGT transport systems is USD $ 54, whereas with lubrication savings could amount around 60% of the total value [10].

A survey released by the Engineering Research Programme in 2003 [11], where the information gathered from 35 rail networks was analyzed, concluded that effective lubrication can reduce wear on wheel and rail and the noise levels. The rails life can be increased by a factor of two, the wheel life by a factor of five and in some cases depending on the radius of curvature and grinding intervals by a factor of four. However, today many of the steps and principles that govern the phenomenon of RCF in the presence of a FM are not entirely known. Several studies carried all over the world have helped understand some of the principles that govern the phenomenon of RCF and other problems related to the tribology of wheel/rail contact such as the type of predominant defects, nondestructive techniques for inspecting rails, among others [12], [13], [14], [15], [16], [17], [18].

In this work, the dry and lubricated wear behavior of rail steels under RCF conditions was studied in laboratory. A twin-disc rolling-sliding machine was used to impose controlled contact pressure and creepage conditions that led the tribological pair to a severe wear regime. The study was done to evaluate the effect of the application of a FM to the interface when cracks are already formed at the surface of the rail.

Section snippets

Materials and methods

Rail specimens were extracted from R370CrHT rail sections, manufactured by the company Voestalpine Schienen GMBH-Austria, and wheel specimens were extracted from a railway wheel grade ER8. The chemical composition measured by optical emission spectroscopy and relevant mechanical properties of the rail and wheel, in line with European standards EN 13674-1:2011 [19] and EN 13262:2004 [20], are shown in Table 1 and Table 2 respectively. The hardness of the wheel was 291±17 HV_31.25kgf.

The samples

Results and discussion

The nomenclature used hereinafter has the first letter related to the interfacial condition (H for HLK and D for dry), followed by the slip percentage and the number of wet cycles. As an example, a test run with the addition of HLK with 1.1 GPa, 5% creepage and 10,000 wet cycles, is named “H5% 10 K”.

Conclusions

Dry and lubricated tests were carried out to evaluate the performance of a friction modifier (FM) on preexisting cracks and its effects on the wear rate and the wear mechanisms of an R370CrHT fully pearlitic rail steel.

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The FM, Sintono Terra HLK, was able to maintain the friction at the expected levels of twin-disk tests, i.e., COT < 0.1.
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With 5% of creepage, the addition of a FM in presence of preexisting cracks increases the mass loss in the rail in comparison to the dry tests. However,

Acknowledgements

The authors wish to acknowledge the support of the Medellín City Hall with his program “Enlazamundos 2012”, co-supported by Colciencias and the Metro system of Medellín-Colombia; contract No 111850227421. Also thank to the LME at the Brazilian Nanotechnology National Laboratory (LNNano). Research supported by LNNano - Brazilian Nanotechnology National Laboratory, CNPEM/MCTI.

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Dry and lubricated wear of rail steel under rolling contact fatigue - Wear mechanisms and crack growth

Highlights

Abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Conclusions

Acknowledgements

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