Increasing Die Durability in Cold Stamping by Quenching with Intermediate Tempering

Fig. 2. Dependence of the dislocation density p in U8 steel after heat treatment on the initial quenching temperature T with preliminary tempering at Tte = 200°C, without inter­mediate tempering (1) and with T_int = 200 (2), 300 (3), 350 (4), and 450°C (5).

intermediate tempering at T_int = 450°C. The optimal temperature for preliminary quenching is T_q1 = 1100— 1150°C, since it ensures solution of the refractory impurities: nitrides, oxides, and oxysulfides. Chemical uniformity in the austenite leads, on quenching, to breakdown of the structural blocks and increase in the microstress. Further temperature rise is accompanied by homogenization of the austenite. On quenching, the defect density of the crystal lattice declines.

Solution of impurities at high temperature ensures that they remain in the solid solution after quenching. The impurity atoms pass to dislocations and anchor them.
Thus, we find that the defect content of the crystal­line structure is greatest (for the given heat treatment) at the same temperatures as for the initial quenching, in contrast to the data in [13]. No displacement of the maximum defect content to higher temperatures is observed. Hence, in terms of maximum resistance to plastic deformation in friction, the optimal prelimi­nary quenching temperature for the steel is T_q1 = 1100—1150°C; the optimal intermediate quenching temperatures are T_int = 200 and 450°C.
In the presence of residual austenite, with great structural dispersion, the semimartensitic zone is not the limit of hardenability of tool steels. Accordingly, the hardenability is determined from the thickness of the quenched layer with martensitic structure—that is, from the thickness of the layer with hardness HRC = 60. Regardless of the initial heating temperature, pre-

liminary quenching of the samples has no significant effect on the hardenability of U8 steel in repeated quenching. The results show that the hardenability in terms of the martensitic zone is around 3 mm. That corresponds to the actual critical diameter (10 mm) on cooling in water.
Research shows that there is a direct relation between the wear resistance and the state of the fine structure [6, 9, 13].
Given that there is little change in dislocation den­sity at T_q1 = 1100—1150°C, this range has been recom­mended for heat treatment.
Intermediate tempering at T_int = 450°C is best, since it not only stabilizes the dislocational structure but also ensures greater decrease in internal stress after the initial quenching.
To assess the influence of quenching and interme­diate tempering on the tool deformation in production conditions, we measure the tool before and after heat treatment.
To that end, matrices of the ShMS-12709 piercing tool (AO Uzmetkombinat) for holes of 6-mm diame­ter are produced. The tolerance on the diameter is determined in the last operation (hole reaming). The punches of the ShMS-12709 tool are produced with a margin on the diameter corresponding to the final grinding.
After heat treatment, the punch diameter changes by no more than 0.02 mm; the change in matrix diam­eter is no more than 0.08 mm. These results are within the permissible limits of deformation in one-time heat treatment (heating to 30—50°C above the critical tem­perature Ac₁ with tempering at T_te = 180—200°C).
The life of the dies after standard treatment is 6000—10000 operations. The life of the piercing tool after quenching with intermediate tempering is 27000—34000 operations for a matrix with HRC = 60—62; and 16 000—30 000 operations for a matrix with HRC = 58-60.
Thus, the life of a steel tool after quenching with intermediate tempering is 2-3 times that of the tool after standard heat treatment.
REFERENCES

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