Z-Phase formation in welded joints of high chromium ferritic steels after long-term creep

The precipitation behavior of Z phase in welded joints was investigated after long-term creep tests in ASME Gr.92 and Gr.122 type steels. Z phase was distinguished from MX carbonitrides by comparing elemental maps for Cr and V. The creep strength of welded joints was smaller than that of the base metal for times longer than 1000 h at 923 K for both steels. Z-phase particles were observed at prior austenite grain boundaries in the fine-grained heat-affected zones of both steels after long-term creep testing. The size of the Z-phase particles was 70–400 nm, which is similar to those of the M23C6 and Laves phases. In the fine-grained heat-affected zone, the number density of Z phase in Gr.122 was five times higher than that in Gr.92. The formation of a large number of Z-phase particles causes the disappearance of MX carbonitrides in Gr.122, compared with Gr.92.