Abstract
Effects of metallurgical factors on grain boundary embrittlement phenomena in ultra-low-C low-alloy steels (basic composition; 0.025C-2Mn-0.05Nb-Ti-B) were systematically investigated. Intergranular fracture in the low temperature Charpy test apparently occurred in ultra-low-C Nb-contained steels reheated in the temperature range of 600-700°C, which have the microstructure of martensite and/or bainite with a coarse prior austenite grain size 150μm.
Either decrease of C content or increase of Nb content enhanced the intergranular embrittlement, and intergranular fracture occurred in the C range lower than a critical carbon content which increased with niobium content. Carbon segregation on the prior austenite grain boundaries was revealed by Auger electron spectroscopy for the steels and segregated carbon content was found to decrease remarkably in the lower-carbon steels and the higher-niobium steels. There was a consistent correlation between the fractional increase of intergranular fracture appearance and the decrease of segregated C content.
The same mechanism for intergranular fracture as in pure iron was considered to operate in the steels. The general conditions for the intergranular embrittlement and effects of P content, B content and the prior austenite grain size were also discussed.
