EFFECT OF COOLING AFTER WELDING ON MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF 12 PCT CR STEEL WELD METALS

The microstructure of three 12 pet cr steel weld metals with different nickel and nitrogen contents was studied in as-welded condition and a fter postweld heat treatment with and without intercooling. Tensile st rength and impact toughness of the weld metals were investigated in di fferent postweld heat treatment conditions. In weld metals heat treate d without intercooling, austenite decomposed by a eutectoid reaction t hat resulted in M23C6 aggregates around retained delta ferrite. Two mo rphologies of M2N and MN precipitates were found in a low-dislocation alpha-ferrite. It was concluded that these phases were also transforme d from austenite. In weld metals heat treated with intercooling, M23C6 precipitates were smaller and more homogeneously distributed. Differe nt MN precipitates were found in the tempered martensite. The fracture mode of the weld metals at room temperature was mainly transgranular cleavage with some fibrous fracture. Intercooling treatment improved C harpy impact toughness of the 12 pet Cr steel weld metals substantiall y. It was found that the important microstructural factors affecting t he impact toughness of the weld metals which were heat treated without intercooling were the sizes of the cu-ferrite grains, nonmetallic inc lusions, and M23C6 aggregates. For the weld metals heat treated with i ntercooling, the factors which affect the toughness of the weld metals were the sizes of martensite packets and nonmetallic inclusions.