Structural changes in overheating zone of HAZ metal of railway wheels in arc surfacing

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Structural changes in overheating zone of HAZ metal of railway wheels in arc surfacing

Gajvoronsky, A.A. / Zhukov, V.V. / Vasiliev, V.G. / Zuber, T.A. / Shishkevich, A.S.

The objective of the work is investigation of the parameters of welding thermal cycle (WTC) in arc surfacing, including heating and cooling stages, on formation of metal structure and properties in overheating zone of the HAZ of high-strength wheel steel of grade 2, containing 0.58 % C. Gleeble 3800 complex was used to conduct heat treatment of model samples at heating rate of 25 to 210 °C/s up to the temperature of 1250 °C with subsequent cooling by WTC (ω_6/5 = 2.5-64 °C/s), perform metallographic investigations of the structure and plot diagrams of overcooled austenite transformation. At testing by the Implant method influence of cooling rate and structural state of metal in overheating zone of HAZ on critical stress values at delayed cracking of wheel steel was assessed. It is established that incompleteness of the processes of metal austenite homogenizing in arc surfacing, because of its fast heating and limited time of soaking at the temperature above A_c3, has an essential influence on subsequent γ-α transformation in the HAZ metal. This leads to lowering of the critical cooling rate to 20 °C/s, at which not more than 50 % of martensite forms in the structure (ω_50M). It is shown that the high delayed fracture resistance of HAZ metal on the level of σ_cr ≥ 0.45σ_0.2 can be ensured, provided ω_6/5 ≤ ω_50M. Investigation results can be used at specifying the technology of building-up by surfacing of items from high-strength steels.