Investigation of Dissolution and Precipitation Behaviours of Precipitates in Welded Joints of High Cr Ferritic Heat Resistant Steels

Abstract

This paper is aimed at investigating dissolution and precipitation behaviour in fine grained heat-affected zones (FGHAZ), coarse grained heat-affected zones (CGHAZ), and base metals for the welded joints of high Cr ferrite heat resistant steels. The simulated HAZ (SHAZ) specimens were used, whose thermal cycles were controlled to be the same as those in the actual welded joint with peak temperatures of 1 173 and 1 523 K to represent FGHAZ and CGHAZ, respectively. The behaviours of dissolution and precipitation during welding, PWHT and ageing processes were observed. SEM observation showed that after long time ageing the precipitates in FGHAZ specimens (1 173 K) were fewer and larger than those in CGHAZ (1 523 K) specimens and base metal specimens. This phenomenon implied that growth and coarsening of precipitates in FGHAZ may play a role in the deterioration of creep property and Type IV cracking, which was observed in previous creep tests. X-ray diffraction analysis for the electrolytic extraction showed that the types of precipitates were M23C6, M7C3, MX, Laves phase, and μ phase. By using software of Thermo-Calc and Dictra, a 3-D model was performed to simulate the dissolution process of M23C6 during welding. It was found that the dissolution of M23C6 can lead to high Cr and high W concentration in a local region. On the other hand, the precipitation behaviour of M23C6 was simulated, and it was found that the growth of M23C6 was fast and in the period of PWHT and creep process it can reach to the equilibrium concentration in a short time of about 2 000 seconds. At last, the growth of Laves phase was simulated and it was found that it grew very slowly and it reached the equilibrium concentration in about 3 000 hours. As an important point, the simulation result showed that Laves phase was easy to grow in the region where M23C6 had dissolved due to the high residual concentration of Cr and W.