Heat Treatments of Weld Alloy 625: Influence on the Microstructure, Mechanical Properties and Corrosion Resistance
Superalloys 718, 625, 706 and Various Derivatives (1994)
The effects of heat treatments of the industrial type: 8 h hold times at temperatures comprised between 600°C and lOOO'C, on the structural, mechanical and corrosion resistance characteristics of weld alloy 625 have been studied. During the heat treatments, the mean concentration ratios of Nb, MO, Si, Cr, Ni and Fe elements between the interdendritic spaces and dendrite cores feature but little evolution up to 850°C. Beyond that temperature, this ratio approximates 1 and the composition
... neity has practically disappeared at 1000°C. A 8 h heat treatment at temperatures comprised between 650°C and 750°C results in increased mechanical strength values and reduced ductility and impact strength linked to the precipitation of body-centred tetragonal intermetallic y" Ni3Nb phase in the interdendritic spaces. A 8 h treatment in the temperature range comprised between 750°C and 950°C has catastrophic effects on all mechanical characteristics in relation with the precipitation, in the interdendritric spaces, of the stable orthorhombic intermetallic F Ni3(Nb, MO, Cr, Fe, Ti) phase. At lOOO"C, the ductility and impact strength are restored. However, the higher the heat treatment temperature, the weaker the mechanical strength. Heat treatments have no effect on the pitting resistance of weld alloy 625 in sea-water. The comparison of the results of this study on weld alloy 625 with those previously obtained on forged metal 625 shows that heat treatments below 650°C and above 1000°C are the s'ole treatments allowing to avoid embrittlement and impairment of the corrosion resistance characteristics of alloy 625. btroduction Alloy 625 features good mechanical strength, combined with good corrosion resistance and good weldability characteristics. It can be used for welding low alloyed steels or high yield strength steels with alloy 625 for repairing these steels by welding or by coating process in order to protect them from corrosion. In such cases, post-welding heat treatments are necessary to preserve the characteristics of the heat affected zone (HAZ) of ferritic steels. As few data was avalaible on the evolution of the structure and properties of alloy 625 during industrial heat treatments (l-3) a first study was conducted on a forged bar (4) which has allowed determining the temperature ranges within which the different phase precipitations occur : intermetallic y" phase, carbides M23C6, M6C, MC and establishing a link between the mechanical properties and corrosion resistance versus the microstructure of the wrought metal 625. This paper presents the results obtained concerning the effects of industrial type heat treatments: 8 h hold times at temperatures comprised between 600°C and 1000°C and air cooling, on the microstructure, mechanical characteristics and corrosion resistance in sea-water and chloride environments of weld alloy 62'5.