Abstract
Tempered martensite embrittlement has been studied in a commercial purity AISI 4340 steel and the results are given in the present paper. In particular the role of carbides in the embrittlement process is examined. It is shown by the results that the standard tempered martensite embrittlement is observed in this steel. The fracture energy after 1 h tempering treatments decreases with increasing tempering temperature between 200 and 350°C. However, a significant amount of intergranular fracture was found only on the surface of the specimen that was tempered at 350°C. Initially, M3C carbides are precipitated after tempering at 250°C. As the tempering temperature increases, both the number density and thickness of the carbides increase. Also, the length of the grain boundary carbides increases significantly with increasing tempering temperature. The carbides are all ribbon-shaped and are essentially pure Fe3C. It is shown using Auger electron spectroscopy that the grain boundaries were enriched in phosphorus and sulphur. When these results are combined, it becomes obvious that the initial decrease in fracture energy after tempering at 250 and 300°C is caused by carbides aiding in ductile fracture. However, after tempering at 350°C for 1h, the carbide density and size on the grain boundaries become sufficiently large for intergranular fracture to occur along the boundaries already weakened by impurities. When these results are combined with others in the literature, it becomes obvious that carbides can decrease the fracture energy by aiding in ductile fracture, intergranular fracture, or cleavage. All these fracture modes can cause a decrease in fracture energy when specimens are tempered in the range of 200–350°C; which fracture mode occurs will depend on the total composition of the specimen, the matrix strength, and the test temperature.
MST/815
Get full access to this article
View all access options for this article.