Weakening affinity of SUS304 asperities to die surface with TiN coating for preventing delayed cracks in deep-drawn cups

Abstract

SUS304 stainless steel surface asperities have a strong affinity to tool steel surfaces. Therefore, the flow of material in the flange portion during the deep drawing process is retarded due to adhesive wear, leading to increase in amount of wall thicknening along the cup edge and rising risk for delayed cracks. In this paper, TiN coating is applied to the drawing die surface to weaken the affinity. Under elevated blank holding forces (BHF), the experimental results showed that the crack-free BHF range for the TiN coated and the uncoated dies are 5–10 and 12 kN, respectively. The crack-free BHF magnitude is successfully lowered and the range is enlarged with the coated die. The weakened affinity is evidenced by the low estimated coefficient of friction (COF) obtained from the finite element (FE) simulation. In contrast, the estimated COF of the uncoated die is high even at low BHF due to adhesive wear. Therefore, delayed cracks are observed under BHF range of 7–11 kN. At BHF 12 kN, the amount of wear fragments formed in the boundary layer increases as a result of the continuous polishing of the SUS304 asperities by the uncoated die surface asperities. The COF is sharpyly decreased due to the smooth relative movement of contacting surfaces facilitated by the fragments. Therefore, the cracks are prevented. However, the fragments tend to penetrate into the SUS304 surface under excessive BHF of 13 kN and above resulting in the formation of the cracks again. To eliminate the cracks, drawn cups must achieve threshold values of less than 32.5% wall thickening and more than 33.3 mm cup height in the valley points.