Phase Transformations in Powder Sintered Steels during Cooling

Introduction. Heat treatment is a common post-processing operation applied to powder steel (PS) after deformation. The fundamental principles of heat treatment theory, developed for solid materials, also apply to PS. However, the specific structure of PS introduces quantitative and qualitative differences in the kinetics of heat treatment processes. Therefore, it is important to understand the effect of heat treatment on the structure and properties of PS when developing new materials. The aim of this study is to investigate phase transformations in sintered powder steels during cooling and to determine their mechanical properties after heat treatment.

Materials and Methods. The study used domestic powders of brands PZHRV 2.200.28 (TU 14-1-5365-98)  and PL-N4D2M (TU 14-5402-2002) produced by PJSC Severstal (Cherepovets). During the mixing process, ultrafine additives of silicon nitride (Si3N4) and nickel oxide (NiO) manufactured by Plasnotherm (Moscow) were added to the charge. Before use, the powders were tested on a universal laser particle size measuring device (FRITSCH ANALYSETTE 22 MicroTec plus) and a submicron particle analyzer (Beckman COULTER No. 5). To prepare the charge, we used a two-cone mixer RT-NM05S (Taiwan) and an ultrasonic station for sieving and mixing powders with ultrafine particles Assonic SPC (China). Static cold pressing was carried out in laboratory molds on a hydraulic press model TS0500-6 (China) with a maximum force of 50 tons. Homogenizing sintering was performed in the laboratory of heat treatment at the Department of Materials Science and Technology of Metals at Don State Technical University in a muffle electric furnace model 6.7/1300 in the temperature range of 900–1150℃, in a protective gas  environment — dissociated ammonia. Sintering time was 15–180 minutes. Heat treatment of sintered powder steels was also performed in these furnaces. Quenching of sintered samples was carried out at a temperature of 800°C. The initial porosity of sintered samples was 10.15.25%. Sintered samples were cooled at a temperature between 100 and 300°C. Tensile testing was conducted in accordance with GOST 18227–851, using a floor-mounted servohydraulic tensile testing machine MGS-V15 in an automatic mode, with the help of a personal computer. Hardness was measured using a Rockwell hardness tester TK-2M with a diamond cone indenter under a total load of 1471 N.

Results. The study conducted allowed us to identify the patterns of phase transformation in powder-sintered steels with ultra-fine particles during cooling after quenching. We experimentally determined the values of critical cooling points for powder-sintered eutectic steels at cooling rates of 60–400°C per minute. Additionally, we determined the mechanical properties of sintered powder steels with ultrafine particles depending on the temperature range of transformations 

Discussion and Conclusion. The research has allowed us to establish the effect of ultrafine particles on the temperature of the critical points of sintered eutectoid PS, to construct diagrams of isothermal transformation of austenite, as well as to determine the mechanical properties of powder-sintered steels containing ultrafine particles. The analysis of the results obtained from the research has shown a multifaceted impact of nickel oxide and silicon nitride particles on phase transformations in powder-sintered steel.

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