Laser scanning confocal microscopy(LSCM) was used to study the inhibition of austenite grain growth by the inclusions and the effect of cerium on the trend of acicular ferrite(AF) and ferrite side plate(FSP) transformation temperature in coarse grain heat affected zone(CGHAZ) of Ce-alloyed weld metals. The results showed there were lots of tiny cerium oxides and sulfides inclusions in the CGHAZ of Ce-alloyed weld metals. When the concentration of Ce was 0.021%, the volume fraction of inclusions in weld metal CGHAZ was higher and the inclusion size was smaller, therefore austenite grain size was smaller with the increase of hightemperature residence time. Cerium tended to segregate at austenite grain boundaries, so FSP transformation temperature decreased and FSP transformation was suppressed. On the contrary, AF transformation temperature increased because AF transformation was promoted in CGHAZ of the Ce-alloyed weld metal, especially when the concentration of Ce was 0.021%.
In this study,the welding thermal cycle curve exhibited two temperature peaks in high heat-input twin-wire separate-pool submerged-arc welding and coarse-grained heat affected zone existed in the welded joint. The inclusions of primary weld metal and coarse-grained heat affected zone of Ce-added SAW should be Al_2O_3,MnO,SiO_2,TiO,Ce_2S_3,CeS,Ce_2O_2S and Ce_2O_3. Under the effect of welding thermal cycle,oxy-sulfides inclusions of Ce,the diameter of which was less than 2. 0 μm,slightly grew larger,but the composition and type of the inclusions didn't change. The microstructure of the large heat input weld metal had acicular ferrite that Ce oxide sulphide particles induced nucleation and proeutectoid ferrite. In the coarse-grained heat affected zone of weld metal,home-position precipitation of acicular ferrite and sympathetic acicular ferrite were both observed. It was supposed that previous crystal cells of acicular ferrite in austenite grain promoted home-position precipitation of acicular ferrite. Meanwhile,sympathetic acicular ferrite tended to nucleate at the primary acicular ferrite grain boundaries,where high dislocation density was located,and grew inside the neighboring carbon-depleted austenitic regions. The granular bainite nucleated in the austenitic zone with high carbon content close to acicular ferrite and sympathetic acicular ferrite.
