The Role of Alloy Composition on the Stability of Nitrides in Ti-Microalloyed Steels during Weld Thermal Cycles

The effect of weld thermal cycling on titanium nitride stability in a range of normalized Timicroalloyed steels containing various alloying additions of V, Al, and N has been investigated. Nitride dispersions and the chemical analysis of individual particles are studied using transmission electron microscopy and a quantitative 200 kV STEM-EDX-EELS microanalysis system. It is found that whereas the normalized material contains various nitrides of mixed compositions, only nitrides based on TiN survive high energy simulated weld cycles. Grain growth in weld cycled material is highly dependent upon the stability of nitrides during the weld cycle, and this stability depends on the original composition of the nitrides and hence of the steel itself. The presence of aluminum in the particles is particularly detrimental in this respect. The best grain growth control and highest toughness are found in steels based on optimum ratios of Ti/V/N and on low Al levels. The possible way in which the steel’s composition affects particle stability and hence grain growth during weld thermal cycles is discussed.