Effect of initial ferrite content on phase transformations in niobium stabilized austenitic clad metals

Austenitic stainless steel welds which normally contain some ferrite may be exposed to high temperatures either during stress relief or service exposure. Ferrite being an unstable phase is transformed to various secondary phases during high temperature exposure. Data on the affect of temperature and time at temperature on the transformation products of austenitic stainless steel welds are meagre, also few reports are available on the affect of initial ferrite content on phase transformations. The present study included cladding on low carbon steel with two niobium-stabilized austenitic stainless strips using submerged arc welding to obtain clads with two ferrite contents, low (4 FN) and high (12 FN). Clads were PWHT at 600, 800, 1000 ° C for 1, 10 and 100 h. Ferrite was measured using a ferritescope. Electrochemical dissolution in 10%HCl-CH3OH solution was done to separate out secondary phases which were analysed using the X-ray diffraction powder technique. The results of the study indicated that the as-weld samples contained both M23C6 and NbC. The PWHT samples were found to contain M23C6, NbC and sigma phases. NbC was found in all the PWHT samples. Both 4 FN and 12 FN samples showed M23C6 after PWHT at 600 °C. After 800 °C PWHT 4 FN samples did not show M23C6, while 12 FN samples showed weak M23C6 lines. After 1000 °C PWHT, was absent in both cases. Sigma was absent in the 4 FN sample PWHT at 600 °C, while it was present in the 800 °C PWHT samples. In contrast, samples containing 12 FN samples showed sigma both after 600 and 800 °C PWHT. Sigma was absent in samples PWHT at 1000 °C. The interesting observation of the study was the effect of initial ferrite content on sigma phase formation. It was observed that ferrite transformed to sigma at 600 °C itself when ferrite content is relatively high (12 FN) and not when it is less (4 FN). This behaviour suggests that sigma formation is not only a function of temperature but also initial ferrite content and readily forms even at lower temperatures like 600 °C when initial ferrite content is high. This in contrast to the earlier studies where it was noticed that short time exposures at temperatures up to 700 °C did not show sigma formation. The observations based on percentage ferrite transformed indicated that the kinetics of sigma at 600 and 800 °C are very fast. Also, ferrite dissolution is faster at 1000 °C when the ferrite content is lower. © 1991 Chapman and Hall Ltd.