Modified Railway Wheel Steels: Production and Evaluation of Mechanical Properties with Emphasis on Low-Cycle Fatigue Beh
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INTRODUCTION
RAILWAY wheels are subjected to mechanical and thermal loads, and as train speeds and weights are increased, these loads increase. The mechanical loads originate from axle loads but are much affected by the contact geometry and dynamical additions that depend on the speed and evenness of wheels and rails, spring stiffnesses, and unsprung mass. Thermal loads arise, for example, during braking, especially on the block-braked wheels that are dominant on freight carriages. During controlled block braking, the surface layer of the wheel is heated by the friction against the brake blocks and cooled in the wheel-rail contact. This gives rise to cyclic thermal stresses in the surface of the wheels, which can generate thermal fatigue cracks.[1] The dynamic stressand-strain fields developed in a rotating wheel in service can create other fatigue phenomena. These may incorporate cracking from existing slag inclusions, but a ‘‘pure’’ material will also gradually be ruined by plastic deformation in the surface, leading to crack initiation or so-called rolling-contact fatigue (RCF). Sometimes the braking action is disturbed. A common example is when the friction between the wheel and the rail is very low due to mixtures of wet leaves and dust on the rails. This often leads to wheel slide, which is treated in earlier articles by the present authors.[2–4] In the slide, the temperature in the material close to the contact surface rises rapidly to levels high enough to transform a thin layer of the wheel steel to austenite.
JOHAN AHLSTRU¨M, Associate Professor, and BIRGER KARLSSON, Professor, are with the Department of Materials and Manufacturing Technology, Chalmers University of Technology, G¼teborg, Sweden. Contact e-mail: [email protected] Manuscript submitted February 2, 2009. Article published online May 23, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
When brakes are released, the austenitized material is rapidly cooled and transforms into brittle martensite. Cracks can form in this layer that can later grow by RCF mechanisms. It is well known that carbon steels become more brittle at low temperatures. This is substantiated to some extent by the observation that more equipment failures occur in the winter months. Earlier studies on the low-temperature fatigue properties of steels have indicated some general trends. The lifetimes of un-notched samples run at constant amplitude appear superior at low temperatures compared to ambient conditions. However, with higher loading amplitudes, the room-temperature behavior is more favorable. With decreasing temperatures, the material becomes more sensitive to notches or inclusions. The more brittle behavior at lower temperatures gives shorter critical crack lengths. There are results that indicate that components that are loaded at both high and low temperatures tend to initiate cracks at high temperatures, when the material is less strong, and then propagate the cracks when the temperature is lower.[5–7] This is, of course, detrimental for railway operation an
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