Quality of Rolled Steel for Cold Bulk Stamping

TLDR: In this article, the authors obtained standard mechanical characteristics and failure criteria for 40Kh steel components and determined the optimal preparation of the rolled steel in terms of its structural and mechanical characteristics after cold bulk stamping.

Abstract: Metal products obtained from rolled blanks by cold upsetting are used in every branch of manufacturing. The quality of such components is assessed in terms of the required chemical composition and the plasticity, the consistency of the mechanical characteristics over the whole length, and the lack of internal and surface defects. If such metal components are to be competitive, all the steps in the production chain must be optimized: from smelting of the steel to cold upsetting of the product. In attempting to minimize costs and ensure the required quality, it is important to ensure safety and to decrease energy and labor costs in manufacturing. In this chain, the preparation of the material for cold bulk stamping is a key step. The high-strength fasteners obtained by cold upsetting are most often made of chrome steel. Recently, alternative boron-bearing steels have been actively introduced. However, their thermal hardening in quenching is unstable because boron oxides and nitrides may be formed, with decrease in their hardenability. In addition, chrome steels are 12–16% cheaper, as a rule. Also, since foreign supplies of boron steels are associated with additional costs, fasteners made of boron steels are even more expensive. That is a further impetus to using chrome steels. In the present work, we obtain standard mechanical characteristics and failure criteria for 40Kh steel components patented in a saltpeter bath at different temperatures, with subsequent drawing. The optimal preparation of the rolled steel in terms of its structural and mechanical characteristics after cold bulk stamping is determined: patenting (in a saltpeter bath at 400°C) and drawing (with 5–10% reduction). Such treatment yields products of the required quality and is preferable to the existing technology.