Deformation and fracture in martensitic carbon steels tempered at low temperatures

This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described. The Edward DeMille Campbell Memorial Lecture was established in 1926 as an annual lecture in memory of and in recognition of the outstanding scientific contributions to the metallurgical profession by a distinguished educator who was blind for all but two years of his professional life. It recognizes demonstrated ability in metallurgical science and engineering. Dr. George Krauss is currently University Emeritus Professor at the Colorado School of Mines. He received the B.S. in Metallurgical Engineering from Lehigh University in 1955 and the M.S. and Sc.D. degrees in Metallurgy from the Massachusetts Institute of Technology in 1958 and 1961, respectively, after working at the Superior Tube Company as a Development Engineer in 1956. In 1962–63, he was an NSF Postdoctoral Fellow at the Max-Planck-Institut für Eisenforshung (Düsseldorf, Germany). He served at Lehigh University as Assistant Professor, Associate Professor, and Professor of Metallurgy and Materials Science from 1963 to 1975 and, in 1975, joined the faculty of the Colorado School of Mines as the AMAX Professor of Physical Metallurgy. He was the John Henry Moore Professor of Metallurgical and Materials Engineering at the time of his retirement from the Colorado School of Mines in 1997. In 1984, Dr. Krauss was a principal in the establishment of the Advanced Steel Processing and Products Research Center, an NSF industry-university cooperative research center at the Colorado School of Mines, and served as its first director until 1993. He has authored the book Steels: Heat Treatment and Processing Principles, ASM International, 1990, coauthored the book Tool Steels, Fifth Edition, ASM International, 1998, and edited or coedited several conference volumes on topics including tempering of steel, carburizing, zinc-based coatings on steel, and microalloyed forging steels. He has published over 280 papers and lectured widely at technical conferences, universities, corporations, and ASM chapters, including a number of keynote, invited, and honorary lectures. Dr. Krauss has served as the President of the International Federation of Heat Treatment and Surface Modification, 1989–91, and as President of ASM International, 1996–97. He is a Fellow of ASM International and has received the Adolf Martens Medal of the German Society for Heat Treatment and Materials Technology, the Charles S. Barrett Silver Medal of the Rocky Mountain Chapter ASM, the George Brown Gold Medal of the Colorado School of Mines, and several other professional and teaching awards, including the ASM Albert Easton White Distinguished Teacher Award in 1999. He is an Honorary Member of the Iron and Steel Institute of Japan and a Distinguished Member of the Iron and Steel Society of AIME.