A Study of the Effect of Two-Stage Tempering on Mechanical Properties of Steel 30CrMnSi Using Analysis on Response Surface in Design of Experiment

Abstract

The mechanical properties of steel 30CrMnSi (30KhGSA) are studied upon optimization of the mode of heat treatment with respect to four parameters [the duration of austenitization (15 -40 min), the temperature of the first tempering (480 -530°C), the temperature of the second tempering (the temperature of the first tempering ± 50°C), the duration of the second tempering (60 -100 min)] and upon changing the cooling medium in quenching, first tempering, second tempering, and second refinement. The parameters are optimized using the method of analysis on the response surface for 30 tests. The structure of the steel after the treatment by the optimized modes is studied by scanning electron microscopy, including the methods of back-scattered electrons and energy dispersive analysis. INTRODUCTIONSteel 30CrMnSi (30KhGSA) is a well known representative of high-strength low-alloy steels. Various variants of heat treatment of this steel provide a wide range of mechanical properties, which makes this grade suitable for various operating conditions. The composition of steel 30CrMnSi fully matches that of the Russian 30KhGSA counterpart; the western counterparts are grades AISI 4130 and 5130 [1]. Table 1 presents the chemical compositions of steel 30KhGSA prescribed by the Russian Standard and of steel 30CrMnSi determined by a spark emission spectroscopic analysis. The design critical points for 30CrMnSi are as follows: Ac 1 = 718°C, Ac 3 = 846°C (reported values are Ac 1 = 760°C and Ac 3 = 830°C). The preferred temperature of austenitizing is about 890°C [2].Repeated tempering of tool steels is a well known method, but such tempering is applied rarely to low-alloy steels. As a rule, the required properties are obtained by varying the parameters of one-stage (first) tempering. The available data show that two-stage tempering affects little the rupture strength. The dependence of the ductility and of the impact toughness on the composition of the steel and on the tempering temperature and time is more complex [3]. The effect of two-stage tempering is stronger in silicon-alloyed steels due to the action of silicon on formation of carbides during tempering. At a high Si : C proportion in the solid solution an e-carbide may precipitate due to low-temperature tempering of martensite. At higher tempering temperatures carbon becomes steadier in the solid solution, and the volume fraction of the carbide phase decreases. The impact toughness of a steel can be raised by choosing the temperatures of the first and second tempering, if the steel contains alloying elements decelerating the decomposition of martensite. The appropriate choice provides optimum distribution of the carbide phase, admissible level of stresses in the martensite, and hence creation of optimum properties in the metal [3]. In [1] the impact toughness of steel 30KhGSA was measured after 3-h tempering at 200 -600°C and cooling in air. Tempering at 250 -400°C lowers the impact toughness, i.e., the steel becomes susceptible to temper brittleness. In [4] the microstructur...

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