Using austenitic-martensitic transformation in low-temperature plastic deformation for stabilizing the dimensions and shape of components

Abstract

Austenitic chrome-nickel and chrome-manganese steels are used widely in special and cryogenic engineering. These materials are relatively expensive and operate under special conditions. Consequently, it is important to increase the efficiency of these steels and utilize more efficiently their possibilities.An integral part of cryogenic systems are closing and regulating fittings, with the least reliable section of fittings being sealing joints of various types. The working characteristics and the service life of these sections are determined not only ,by the design but also service parameters of the material.The working surfaces of cryogenic fittings in individual cases did not make it possible to ensure the required service characteristics over a long period of time under the specific conditions with cryogenic temperatures. For increasing the service properties of the material there are a number of promising methods of restoring the efficiency of the already installed fittings. In particular, the method of low-temperature surface-plastic deformation (SPD) is interesting. SPD results in smoothing of micro-irregularities, changes the distribution of residual stresses in the surface layer, increases its microhardness, and reduces gas release [1].Changes taking place in steels treated by the SPD at room temperatures (293 K) have been studied quite sufficiently. After heat treatment steels contain retained austenite whose amount is determined by the chemical composition of the material and heat treatment conditions. However, for sections of cryogenic equipment operating under conditions of increased wear the surface layer of the material should have a martensitic structure characterised by high physical-mechanical properties (mainly increased hardness and strength, improved thermal conductivity, and higher magnetic characteristics) [2].The most rational method of transforming retained austenite to martensite is deep cooling. An important role in developing a teelmologieal process is played by the temperature at the start and finish of the austenitic-martensitic transformation. This process is influenced most significantly by the chemical composition of the materials [3].For example, when adding 1% of the alloying element to a steel with 0.9-1% C the temperature at the start of transformation of austenite to martensite decreases: in alloying with manganese by 45 K, with nickel by 26 K, with vanadium by 30 K, with molybdenum by 25 K, with chrome by 35 K, with copper by 7 K; when adding cobalt, this temperature increases by 12 K [2].Thus, the temperature of the start and finish of the austenitic-martensitic transformation decreases with increasing content of carbon and alloying elements in steel (with the exception of cobalt). Consequently, to stabilize martensite the material should be subjected to cold treatment.Cold treatment is used in production of tool steels to increase wear resistance and improve the cutting properties of tools; to improve the quality of the surface of steel components subjected to polishing or ...