Influence of the Modes of Heat Treatment on the Strength and Cyclic Crack-Growth Resistance of 65G Steel
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INFLUENCE OF THE MODES OF HEAT TREATMENT ON THE STRENGTH AND CYCLIC CRACK-GROWTH RESISTANCE OF 65G STEEL О. P. Ostash,1, 2 V. V. Kulyk,3 V. D. Poznyakov,4 О. А. Gaivorons’kyi,4 and V. V. Vira3
UDC 621.791.7-4:669.539.4
To find the ways of increasing the brittle-fracture resistance of the metal in the heat-affected zone formed as a result of repair surfacing of the rolling surface of railroad wheels, we study the influence of various modes of heat treatment, including the traditional and modified (by the authors) Q-n-Ptreatments on the mechanical properties of 65G steel regarded as a model wheel steel. It is shown that, after the modified Q-n-P-treatment, the mechanical characteristics of steel increase to a greater extent than after the traditional treatment. However, according to the parameter of structural strength P = [ΔU ⋅ ΔK th ⋅ ΔK fc ] , where ΔU is the ultimate strength, ΔK th is the fatigue threshold, and ΔK fc is the cyclic fracture toughness of the material, the mode of treatment proposed earlier, which leads to the relaxation of stresses of the second kind in the bulk of martensite and bainite lathes, proves to be more efficient. Keywords: high-strength steel, bainite–martensite structure, deformation martensite transformation, internal stresses of the second kind, strength, cyclic crack-growth resistance.
The solid-rolled wheels and wheel tires for the railroad transport are produced from high-strength steels with carbon contents of 0.55–0.70% [1, 2]. In these steels, the main alloying elements are manganese (up to 1.2%) and silicon (up to 0.6%). As a result of long-term operation, the wheels are worn out, which leads to the appearance of defects of various types on their rolling surfaces [3]. This is why the wheels are turned and their profile is restored by electric-arc surfacing [4]. However, after this, a structure susceptible to brittle fracture caused by the facilitated initiation and growth of cracks is formed in the heat-affected zone (HAZ) of the wheel depending on the cooling rate [4]. This is why it is necessary to determine efficient modes of surfacing and cooling of the restored wheels guaranteeing the required structural strength of the metal in the HAZ and, in particular, its elevated crack-growth resistance in the high-strength state. It is shown that, in order to increase the brittle-fracture resistance, it is necessary to create conditions under which a bainite–martensite structure without upper bainite is formed in the metal of HAZ, and the fraction of martensite does not exceed the amount of lower bainite. For wheel steels with elevated carbon contents, this is guaranteed by a cooling rate W6/5 = 5–6°С/sec
within the temperature range 600–500°С. However, even in this case, the brittle-fracture resistance of the HAZ is 1.5 times lower than of the base metal of the wheel rim [5]. A noticeable increase in the crack-growth resistance of steel with this bainite–martensite structure under cyclic loading was attained after holding for 2–3 h by interrupting the process of cooling at 100°С
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