Interior crack initiation during the very-high-cycle fatigue of railway wheel steel under axial loading and rolling contact loading

In this paper, a comparative study of the very-high-cycle fatigue (VHCF) behavior of railway wheel steel under axial loading and rolling contact loading was conducted. Fatigue tests were performed with an ultrasonic fatigue test machine under axial loading, and the fracture surfaces from the fatigue tests and shattered rims taken from the failed railway wheels were observed. The wheel steel under axial loading presents a VHCF behavior with Mode I crack, and that under rolling contact loading is a VHCF behavior with mix Mode II-III crack. For the VHCF behavior with Mode I crack ,surface and interior crack initiation occurred with equal probability at both low and high stress levels and produced a dual linear S-N curve since the value of fatigue limits for the surface and interior crack initiation are close. For the VHCF behavior with mix Mode II-III crack, cracks were initiated from the interior Al O inclusion and the fatigue life was beyond 10 cycles. Fatigue bands were observed on the fracture surface under rolling contact loading. The ferrite nanograins formed due to the stress state of shear plastic strain with a large compressive stress. The formed nanograins were softer than the matrix caused by the redistribution of the carbon.

The Degassing Laws for Railway Wheel Steel in a Vacuum Tank Degasser

The research presents the results of data analysis on degassing of wheel grades of steels in a tank degasser with a capacity of 120 tons, operated at the JSC “Ural Steel”. The volume of the analyzed sample included 754 steels for railway wheels (steel grades “2” and “T” according to State standard GOST 10791-2011) weighing more than 80 thousand tons received in November-December 2019.It was established that in order to guarantee the production of hydrogen content of less than 1.5 ppm and nitrogen before 0.007%, it is necessary to carry out vacuum treatment of metal with overheating of 110-130°C at the residual pressure of up to 3 mbar for 20-25 minutes and argon flow rate of at least 0.05 m3/ton. The regression equation was obtained, which allows to predict the results of degassing, as well as select the values of vacuum treatment parameters in order to achieve a given content of dissolved gases - hydrogen and nitrogen.

Effect of Tempering Temperature on Microstructure and Mechanical Properties of Bainitic Railway Wheel Steel with Thermal Damage Resistance by Alloy Design

Thermal damage is one of the principle modes of wagon railway wheels. A new bainitic railway wheel steel with high thermal damage resistance and good combination of strength, plasticity, and toughness was developed. Microstructure and mechanical properties of the new steels in a tempered condition at different temperatures were examined. Microstructures were observed using scanning electron microscope and transmission electron microscope. Mechanical properties were evaluated by tensile, hardness, and Charpy impact tests with a simultaneous comparison to pearlitic railway wheel steel. The characteristic of retain austenite and V(C,N) were measured through X-ray diffractometry and energy disperse spectroscopy. The results indicate that this new bainitic wheel steel presents a submicron-sized carbide-free bainite morphology and preferable integrated mechanical properties when tempered at 280–360 °C. Precipitation strengthening plays an important role for the high strength, since a two-time-strengthening mechanism of the yield strength led by precipitation has been found at 280–360 and 480–560 °C, respectively. Compared with a pearlitic railway wheel steel, bainitic wheel steel tempered at 320 °C has a 10% higher yield strength, five times higher impact toughness, and much better thermal damage resistance, which is a promising railway wheel material for higher speed or heavier axle-load service conditions.