Abstract
In the previous paper, the authors conducted the coil-spring loading 3-point bending COD test and the large sacle tests on the 9% nickel steel weldments and determined that the weld-HAZ itself had satisfactorily high toughness to avoid the initiation of brittle fracture at cryogenic temperatures down to -196°C.
This paper describes further studies conducted on the 9% nickel steels including the extra low impurities one to bear out the superior weld-HAZ toughness from the metallurgical view points. Fracture toughness expressed in terms of CTOD and Charpy-V energy value was revealed to be significantly improved with the decreasing of phosphorous content in the region below 0.005%.
The relation of fracture toughness and the microstructures which form the weld-HAZ structure were evaluated qualitatively by paying special attention to the reverse martensitic transformation mode during heating. As a result, it was evidenced that the superior weld-HAZ toughness depended on mainly its matrix structure, as low carbon Ni contained cellular martensite. The island-like martensite observed in microstructure of specimens subjected to heating to the temperatures, 700 to 900°C was identified as high carbon twinned martensite by the electron diffraction pattern analysis. The microstructure of specimens subjected to heating to 1350°C contains a little upper bainite.
