Material scientists from RUDN University have established the cause of the anomaly in martensitic transformation, which is observed in steels of some structural classes during quenching cooling. The results of the study allowed them to propose a way to eliminate this anomaly. The results are published in Materials.

The optimal properties of steels are formed as a result of heat treatment, consisting of quenching and tempering. The mode of heat treatment, its temperature, and exposure significantly affect the structure of steel and its characteristics. The martensite structure appears during quenching and is partially decayed during tempering. One of the methods of studying phase transformations during heat treatment is the determination of temperature dependence of the thermal expansion coefficient (TEC). The temperature intervals of phase transitions can be found by the maximum of TEC. For the ChS-139 alloy, an anomaly of martensitic transformation was detected by this method: they found two peaks corresponding to the formation of the martensite structure on the TEC temperature dependence graph, where there should be only one. The causes of this anomaly remained unknown. RUDN materials scientists investigated this effect, explained its cause, and proposed a way to eliminate the anomaly of martensitic transformation.

The leading engineer-technologist of the Institute of Innovative Engineering Technologies of the RUDN University, PhD Kseniya Bazaleeva together with her colleagues, suggested that the observed anomaly is associated with the "splitting" of the martensitic transformation into two, and the nature of this splitting has a diffusion character: during the heat treatment of steel, the homogenization of the alloy composition does not have time to occur.

By constructing the TEC temperature dependence, material scientists investigated the anomaly of martensitic transformation on three types of steel: EK-181, EP-823, and ChS-139. All these steels are the same ferritic-martensitic structural class but with different concentrations of alloying elements. The significant anomaly of martensitic transformation was found only in ChS-139 steel, which has a higher degree of alloying.

It is known that the degree of alloying affects the rate of diffusion processes in steel. The higher the degree of alloying, the lower the diffusion rate. An increase of the concentration of the alloying elements slows down the homogenization of the solid solution at high temperatures. The temperature of the martensitic transformation depends on the composition of the solid solution. Due to the heterogeneity of its composition, transformations of solid solutions of different compositions overlap during quenching cooling, each of them has its own temperature range.

“An increase in the exposure time and heating temperature, made it possible to establish that the effect of the splitting of the martensitic peak can be eliminated by homogenization of the austenitic solid solution”, said Ksenya Bazaleeva from the RUDN University.

Journal Link: Materials 2021, 14(16), 4503