*3.4. Weld Microstructure*

The weld microstructure with different heat input is depicted in Figure 7. The weld metal in Test 1 consists of martensite and small amount of bainite, whereas only bainite with larger grain size is observed in the weld metal of Test 7. The same is true for the microstructure in the coarse grain heat-affected zone (CGHAZ) where a combination of martensite with bainite is present in Test 1 and only bainitic microstructure is found in Test 7. When it comes to fine grain heat-affected zone (FGHAZ), the microstructure consists predominantly of martensite. However, it is evident that the fraction of bainite in FGHAZ of Test 7 is higher than that of Test 1.

It is known that microstructure is closely linked to welding thermal cycle and hardenability. During welding thermal cycle, CGHAZ and FGHAZ were heated to austenite phase region. Upon cooling, the reformed austenite would transform to martensite, bainite or ferrite, depending on the cooling rate and hardenability of the base metal. Since the base metal contains large amount of carbon and chromium, the hardenability is relatively high, which favours the formation of martensite and/or bainite instead of ferrite. As the heat input increases, the cooling rate decreases, which favours the formation of bainite rather than martensite. The same is true for the weld metal as it still experiences phase transformation from austenite to various transformation products upon cooling, although it was melted during welding thermal cycle. That is why higher fraction of martensite was formed in the weld and HAZ with lower heat input.
