Quench embrittlement of hardened 5160 steel as a function of austenitizing temperature
- PDF / 419,518 Bytes
- 10 Pages / 684 x 858 pts Page_size
- 105 Downloads / 279 Views
INTRODUCTION
HARDENED carbon and low-alloy carbon steels are susceptible, depending on chemical composition and heat treatment, to intergranular fracture along prior austenite grain boundaries. Tempered martensite embrittlement and temper embrittlement are examples of well-studied, intergranular embrittlement phenomena. Both types of embrittlement are in part related to grain-boundary segregation of impurity elements and both develop during thermal processing after austenitizing and quenching to martensite.[1–4] Tempered martensite embrittlement develops during tempering around 350 °C and temper embrittlement after tempering around 500 °C. Hardness and strength decrease continuously with increasing tempering, and therefore, hardened steels that require high strengths and good toughness are generally tempered at temperatures below those that produce tempered martensite embrittlement. Recently, however, a transition from ductile to intergranular fracture when steel carbon content exceeds 0.5 mass pct has been characterized in martensitic steels tempered at low temperatures.[5,6] Under tensile or bending stress states, the higher carbon steels are highly susceptible to intergranular fracture in both the as-quenched condition and after tempering at low temperatures generally considered to be safe from embrittlement phenomena. In view of the fact that tempering is not required to render the microstructure susceptible to intergranular fracture, the latter embrittlement phenomenon is referred to as quench embrittlement.[7] A. REGULY, Assistant Professor, and T.R. STROHAECKER, Professor, are with the Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil. G. KRAUSS, University Emeritus Professor, and D.K. MATLOCK, Armco Foundation Fogarty Professor, are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] Manuscript submitted October 7, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
The role that phosphorus plays in various intergranular embrittlement phenomena in hardened carbon steels is well known.[1–4,8] In tempered martensite embrittlement, cementite formation during tempering as a result of austenite transformation is also a critical factor.[2,9] Phosphorus segregation and cementite formation are also related to quench embrittlement.[6,7] However, instead of cementite formation during tempering, the cementite forms during austenitizing or quenching. As in the temper-driven embrittlement mechanisms, the grain-boundary structures are extremely fine and cannot be resolved by light microscopy. The presence of the cementite therefore is primarily documented by Auger electron spectroscopy of intergranular fracture surfaces.[10,11] Phosphorus segregation exacerbates intergranular fracture and lowers the steel carbon content at which quench embrittlement occurs, especially in view of the fact that phosphorus segregation has been shown to occur during austenitizing.[1] Figure 1 shows a composition map that defines re
Data Loading...
