DEFORMATION AND FRACTURE OF STRENGTHENED HIGH-CARBON STEEL AFTER TREATMENT IN TEMPERATURE CONDITIONS OF PHASE PRE-TRANSFORMATION AND TRANSFORMATION

Traditional methods of heat treatment are energy-intensive and time-consuming, so the task of increasing their efficiency is very relevant. The process of repeated high-speed heating with short-term aging in the temperature range of polymorphic pre-transformation and transformation from the viewpoint of evolution of structure, properties and character of the fracture of quenched high-carbon steels was investigated. In particular, it was found that high-speed heating (600  –  700  °C/s) and short-term holding (0.5  sec) followed by cooling in salted water (6  °C) leads to the formation of a structure not differing from the structure of low-drawn (200  °C, 2  hours) martensite of traditionally hardened steel with 4 times increase in elongation and 2  times contraction while maintaining strength during the tensile test. Short-term aging of 8  –  15  –  25  seconds with repeated quenching of high-carbon steels from 820  °С in cold salted water (6  °С) leads to formation of the structure of ultra-fine-lamellar, submicroplast, globular perlite. There is a three-dimensional nanostructuring of steel that differs from traditional hardening with high temperature tempering by the structure and properties: magnitude of the applied stresses, both at deformation stages and at fracture (increase in σ by 55  %, in σ0.2 – by 17  %, in ψ  –  8  times, in αn  –  by 80  %). Increase in short-time holding up to 40  –  50  sec, with repeated quenching from 820  °C leads, in contrast to the traditional one, to formation of the structure of ultra-small-malt martensite and to appearance in fracture on the slip planes of the patching structure, resembling a honeycomb in shape. After the low temperature tempering during the tensile test, all stages of deformation are presented. In fracture the crushing of pit structure and the absence of brittle tunnel pits are observed, plasticity is improved (δ  ~  1.5  times, ψ  –  3  times) at strength preservation.