Proceedings of COBEM 2005 18th International Congress of Mechanical Engineering Copyright © 2005 by ABCM November 6-11, 2005, Ouro Preto, MG EFFECTS OF COPPER ADDITION ON DELAYED CRACKING PHENOMENON OF DEEP-DRAWN AND REDRAWN CUPS OF AUSTENITIC STAINLESS STEELS Marta Ribeiro dos Santos Federal University of Minas Gerais - Metallurgical and Material Engineering Department – Rua Espírito Santo, 35 - Centro - Belo Horizonte - Minas Gerais marta@demet.ufmg.br Hélio José Batista Alves Cia Siderúrgica Acesita S.A. – Centro de Pesquisa – Praça 10 de Maio, 09 – Centro – Timóteo – Minas Gerais helioalves@acesita.com.br Berenice Mendonça Gonzalez Federal University of Minas Gerais - Metallurgical and Material Engineering Department – Rua Espírito Santo, 35 - Centro - Belo Horizonte - Minas Gerais gonzalez@demet.ufmg.br Abstract. Metastable austenitic stainless steels show the Delayed Cracking Phenomenon when they are significantly deformed by deep drawing operation. This phenomenon occurs in those steels that transform substantially to martensite during forming. Copper is an alloying element that can partially inhibit this transformation making austenite more stable against strain induced martensitic transformation. The purpose of this investigation was to evaluate both the effect of copper addition on the amount of α' martensite formed along the cup wall of austenitic stainless steel deep-drawn and redrawn cups and the number of cracks appearing in the cups edges due to the Delayed Cracking Phenomenon. Also in this work, the same features of a conventional metastable austenitic stainless steel are presented in order to compare to copper-added steel. Keywords Delayed Cracking Phenomenon, strain induced martensite, austenitic stainless steel 1. Introduction Metastable austenitic stainless steels, such as AISI type 304, are used in the production of a wide variety of formed and drawn parts for architectural, automotive, industrial, domestic applications and in the food and pharmaceutical industries. Equipments made of these materials are indicated for many applications in the chemical industry, since they show high corrosion resistance and very smooth surfaces, avoiding accumulation of impurities and optmizing cleaning procedures (Acesita, 2002). Most of these applications demand good performance during fabrication steps, since they will be press-formed into a more complicated shape or the material will be deep-drawn to a required depth without intermediate annealing (Ikegami et al., 1999). Austenitic stainless steels attain good ductility and improved stretch formability resulting from strain-induced martensitic transformation (Rintamaa et al., 1982; Gonzalez et al., 2003) which are adequate to press-forming performance. However, their deep drawability is not so good as the others properties. In addition, metastable austenitic stainless steels are susceptible to Delayed Cracking Phenomenon when the sheets of these materials are heavily deep- drawn (Ikegami et al., 1999). The Delayed Cracking Phenomenon, that is observed in successfully deep-drawn cups, is characterized by the appearance of cracks in the top edge of the cups in a matter of hours, or even days or months after the forming (Schaller et al., 1972; Frehn and Bleck, 2003). The phenomenon, which occurs only in those steels that transform substantially to martensite during forming (Schaller et al., 1972), is related to the residual stresses that remain in the parts due to the different strain levels experienced in different locations of the workpiece caused by difference in strength between different but co-existent phases in the material, or different strains in different locations and or else, a possible gradient temperature. (Wang and Gong, 2002). These stresses are largest near the top of the walls and they can lead to splitting of them by cracking unless the cups are stress relieved (Hosford and Caddell, 1983). In austenitic stainless steels, the sensivity to the phenomenon is determined by the volume fraction of martensite (Hoshino, 1977) which in turn is a function of the chemical composition of the steels (Gonzalez et al., 2003). Frehn and Bleck (2003) have reported the existence of an α' martensite gradient through the cup walls. According to these authors, the phenomenon was observed in austenitic stainless steels which have formed high amounts, over 35%, of α' martensite during deep drawing process, or respectively, showed a high gradient of that phase. This gradient was responsible for residual stresses in the cup wall which could lead to the Delayed Cracking Phenomenon. The formation of α' martensite phase is related to the austenite stability: the more stable the austenite phase, the less martensite is formed. The stability of austenite in relation to the α' martensite transformation is determined by the