Increasing Die Durability in Cold Stamping by Quenching with Intermediate Tempering

ISSN 1068-798X, Russian Engineering Research, 2022, Vol. 42, No. 10, pp. 1011—1013. © Allerton Press, Inc., 2022. Russian Text © The Author(s), 2022, published in Vestnik Mashinostroeniya, 2022, No. 7, pp. 61—64.

Increasing Die Durability in Cold Stamping by Quenching with Intermediate Tempering
D. M. Berdiev"’ *, A. A. Yusupov^4, ", R. K. Toshmatov", and A. Kh. Abdullaev*
^a Karimov Tashkent State Technical University, Tashkent, Uzbekistan ^bAlmalyk branch of the Islam Karimov Tashkent State Technical University, Tashkent, Uzbekistan
*e-mail: [e-mail] Received September 14, 2021; revised September 17, 2021; accepted September 22, 2021
Abstract—Analysis of the wear for a cold-stamping tool indicates that quenching of the tool with intermediate tempering increase tool life by a factor of 2-3.
Keywords: heat treatment, hardness, dislocation density, intermediate tempering, low-alloy steel, cold stamping, dies
DOI: 10.3103/S1068798X22100057

In cold stamping, the tool (die) experiences very high contact stress (3-5 kPa) [1]. Therefore, the die materials must have appropriate hardness, ductility, strength, and other physical properties [1]. Hardness alone will not ensure the required tool life, since a material of insufficient ductility will be inclined to brittle failure. Effective tool operation requires a com­bination of hardness with sufficient ductility and strength.
Low tempering of steel (150-200°C) after quench­ing to the martensitic state somewhat decreases the hardness (to HRC = 60) but increases its viscosity.
For punching dies, we use carbon steel with low hardenability, characterized by limited hardness (HRC = 58-60) and adequate ductility.
Analysis of the dies at AO Uzmetkombinat shows that the tools for cold stamping are made of U8, U8A, U10, U10A, 9XC, and X12M steel; sometimes, hard- alloy inserts are employed. These dies are used in piercing, punching, chopping, and cold upsetting. Analysis of spent tools shows that their life is deter­mined mainly by wear. However, cold fracture also appears as a result of both inaccurate centering and inadequate tool ductility.
The stamping tool is made of carbon steel, with quenching after heating 30-50°C above the critical temperature Ac₁ and tempering at T_te = 180-200°C. That increases the tool’s wear resistance and strength [2]. Sometimes, however, this is insufficient [3]. Accordingly, various methods are used for additional strengthening, such as chemicothermal treatment and laser treatment [4]. Those methods are expensive.
The most effective strengthening technique is non­standard heat treatment so as to increase the yield point by structural inheritance [5, 6]; that is accompa­nied by the maximum formation of lattice defects [6­8].
Analysis of the relationship between the steel’s structural parameters and its wear resistance indicates that its performance is largely determined by its fine structure [9]. Therefore, in optimizing the heat treat­ment, we use X-ray structural and metallographic analysis. To that end, we investigate industrial melts of eutectoidal U8 carbon steel (State Standard GOST 8559-75), which is widely used in producing tools for cold stamping.
To retain fine grains in the structure during the final heat treatment and to eliminate tempering, the steel is heated in salt baths, with quenching in a saltpe­ter bath at 180°C.
The samples are heated to different temperatures: T_q1 = 820, 900, 1000, 1100, 1150, 1200, and 1260°C; the heating time is 5 min. To form martensitic struc­ture after the initial quenching, the samples are cooled in water and then in oil.
The quenched samples undergo intermediate tem­pering at T_int = 200, 300, 350, and 450°C. The samples are heated in a salt bath at 820°C and held for 5 min. On cooling, a troostite network is formed along the boundaries of the austenite grain.
Sections are produced from the samples by etching in 4% ethanolic nitric acid solution, a saturated etha- nolic solution of nitric acid; and saturated nitric-acid solution with added detergents. The austenite grain size is determined in accordance with State Standard GOST 5639-65.
A MIM-8M microscope is used for metallographic analysis [10]. A DRON-2.0 diffraction system is used

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