Precipitate stability in the heat affected zone of nitrogen-enhanced high strength low alloy steels

A high nitrogen, Ti–V microalloyed steel was found to produce coarse-grained heat affected zone (CGHAZ) with impact toughness superior to a similar Ti–V steel with lower nitrogen content. The CGHAZ microstructures of the high nitrogen steels were investigated to determine that precipitation in this region played an important role for the improvement of the mentioned toughness. The nitrogen-enhanced steel, with 130 parts per million (ppm) nitrogen, yielded a large number of fine nitride precipitates with sizes ranging from 30 to 900 Å (3 to 90 nm). These cubic or rectangular-prism shaped, titanium-rich particles limited the austenite grain growth at high temperatures. The average austenite grain diameter of 50 μm at 1350°C was about three times smaller than that observed in the low nitrogen steel with 30 ppm nitrogen. The small grain size in the CGHAZ of the nitrogen-enhanced steel controlled the toughness. Instead of nitrides, the low nitrogen steel CGHAZ exhibited a large number of titanium or aluminum oxide particles. An important observation was that the particle number density of the nitrogen-enhanced steel decreased slowly, from 4.3×106 to 3.3×106 per mm2, with increasing holding time at 1350°C. Ostwald ripening alone was unable to explain the particle density change and precipitate growth. It was then determined that most TiN particles smaller than 420 Å (42 nm) were located along the austenite grain boundaries. Thus, coarsening of these finer TiN precipitates at the grain boundaries would be determined by grain boundary diffusion and not lattice diffusion. In the case of welding, with thermal cycles characterized by high peak temperatures and short holding times, much of the titanium and nitrogen atoms would be expected to remain in solution, albeit in supersaturation. Hence, nitride particles larger than 420 Å (42 nm) and located inside the austenite grains would receive solute atoms precipitating directly from the supersaturated matrix. Only after this supersaturated concentration reaches the equilibrium solubility that the effect of Ostwald ripening would become predominant in regulating the size distribution of the precipitates. In this paper, a combined diffusion model has been suggested to describe the growth mechanism of titanium nitride precipitates in the CGHAZ of a high nitrogen Ti–V microalloyed steel weld.