Increasing Die Durability in Cold Stamping by Quenching with Intermediate Tempering

Fig. 1. Dependence of the mean diameter d_me of the aus- tenite grain in U8 steel after heat treatment on the initial quenching temperature T without intermediate tempering (1) and with T_int = 200 (2), 300 (3), 350 (4), and 450°C (5).





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BERDIEV et al.
for X-ray structural analysis [11]. We investigate the fine structure of the steel: specifically, the dislocation density, the quantity of residual austenite, the lattice period, and the carbon content in the phases of the quenched steel. The hardenability of the steel is deter­mined in accordance with State Standard GOST on an end quenching unit [12].
In Fig. 1, we plot the mean diameter of the austen- ite grains in heat-treated U8 steel against the tempera­ture T_q1 of preliminary quenching. We find that the size score of the austenite grains in the heat-treated samples is lower by 1—2 points than that for the metal after furnace heating.
In addition, the initial quenching after heating the sample to 1100°C ensures further decrease by 1— 2 points. The optimal temperatures for intermediate tempering are T_int = 200, 350, and 450°C; that ensures stable austenite grain zine for initial quenching at T_q1 = 1100—1150°C. With initial quenching and high temperatures, we note increase in size of the austenite grains and the martensite needles (by up to 1 point at T_q1 = 1260°C).
Quenching again at T_q2 = 820°C after intermediate tempering at T_int = 450°C yields small martensite nee­dles. The smallest martensite is obtained at T_q1 = 1100°C. No undissolved cement particles are observed; that implies complete transition of the car­bon to the solid solution.
Thus, to obtain the minimum austenite grain size, preliminary quenching at T_q1 = 1100°C and intermedi­ate tempering at T_int = 200, 350, and 450°C are best.
The results of X-ray analysis are shown in Fig. 2. The dislocation density increases on preliminary quenching at 1100—1200°C. The results in this tem­perature range are most stable for samples subjected to

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