Thermal stability of TiN in CGHAZ of nitrogen-enhanced HSLA steels
- Colorado School of Mines, Golden, CO (United States)
Light microscopy, electron microscopy, and Charpy-V-notch (CVN) testing were used to characterize two Ti-V microalloyed steels, containing high and low (130 and 30 ppm) levels of nitrogen. In this study, particular emphases have been placed on the understanding of the role of nitrogen on the `heat input-microstructure-toughness` relationship and the kinetics of precipitate dissolution and/or coarsening during welding in the high nitrogen HSLA steels. Within the 1{emdash}3 kJ/mm heat input range, the high nitrogen steel with 130 ppm of nitrogen exhibited coarse-grained heat-affected zone (CGHAZ) toughness superior to that of the low nitrogen steel welds, with transition temperatures 15 to 60{degrees}C lower than those of the low nitrogen HSLA steel welds, at 50 ft-lb (68J) impact energy. Since the CGHAZ of high nitrogen steel welds have fine austenite grain size and large amount of grain boundary ferrite, the HAZ toughness is also related to these two competing factors, and both are controlled by the fine nitride precipitates in the steel. As a result of the nitride precipitate population in the high nitrogen steel, the average austenite grain size was approximately three times smaller than that of the low nitrogen steel, e. g., 50 and 170 {mu}m at 1350{degrees}C, respectively. In summary, the nitride precipitates in the high nitrogen steel did exhibit greater thermal stability than in the low nitrogen steel. At hyperstoichiometric N/Ti conditions, TiN can resist coarsening and dissolution even at temperatures over 1350{degrees}C and retard the austenite grain growth effectively during welding thermal cycles. As a result of the austenite grain size control, a CGHAZ microstructure consisted of 70 vol% of ferrite with aligned second phase and 30 vol% of grain boundary ferrite with improved toughness is developed.
- OSTI ID:
- 115441
- Report Number(s):
- CONF-9404233-; TRN: 95:002927-0019
- Resource Relation:
- Conference: 75. American Welding Society (AWS) annual meeting, Philadelphia, PA (United States), 10-15 Apr 1994; Other Information: PBD: 1994; Related Information: Is Part Of 75th Diamond anniversary American Welding Society annual meeting; PB: 273 p.
- Country of Publication:
- United States
- Language:
- English
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