Microstructural evolution of a Ni-based alloy overlay on a2.25Cr-1Mo steel during thermal ageing

The microstructural changes during therma l ageing at 650 ºC in the interfacial region of Alloy625 weld overlay on a 2.25Cr1Mo steel was evaluated. Different analytical methods wereconducted in an effort to identify evolutions ca n occur during service life. O pticalmicroscopy, scanning electron microscopy in SE and BSE modes, electron probemicroanalysis and hardness mapping were all used to characterize as-welded microstructureand its c hanges during thermal ageingand identifying precipitates in the interfac ial region ofthis dissimilar alloy combination.Different regions were identified in heat affected zone of the 2.25Cr1Mo steel and weldoverlay and carbon migration across interface was recognised as the most importantmicrostructural evolution. A region of 300 μm wide adjacent to fusion line termed near-HAZwhich was found harder in the as-welded condition decarburized during thermal exposure.The microstructural studies revealed that this region transformed to a soft region containinglarge ferrite grains and micron sized M6C carbides.On the other hand, in the weld overlay region there are two changes in the microstructureincluding formation of a 50 μm w ide hard band adjacent to fusion line with over 650 Kgf mm-2 and carbide precipitation along grain boundaries. Metallographic studies revealed thatcarbide precipitation mostly occur in the 150 μm wide region close to fusion line and longerexposure time has a limited effect on the length of this region. The paper describes themetallurgical basis for these microstructural changes in the interfacial region.

Keywords:

Welding , Cladding , Microstructure , Alloy 625 , 2.25Cr-1Mo , Thermal exposure Carbon migration