Abstract
In the previous report it was proved that the stress releasement mainly occured by transformation superplasticity compared with by transformation expansion under some conditions. For example one of conditions is the case that the much amount of thermal stress is generated in spite of a small transformed area.
In the present study, in order to obtain the suitable conditions which make most use of the stress releasement in welding by transformation superplasticity, the transformation temperature was remarked. Since the transformation temperature changes by the kind and content of alloying elements, the steels with many kinds of transformation temperature were made and the effect of transformation temperature on stress releasement was investigated by use of a special torsion thermal cycle test apparatus. The results are shown as follows;
1) In the region of 4-14%Cr and 3.5-14%Ni in this study, the transformation start temperature (that is a martensite transformation) of the test steel is in proportion to the alloy contents.
Martensite start temperature was shown as follows:
Ms(°C)=719-26.5×Nieq(%)-23.7×Creq(%)
Nieq=30×C+Ni+0.5×Mn, Creq=Cr+Mo+1.5×Si+0.5×Nb (unit: mass%)
2) The deformation stress in the midst of cooling took the minimum value at the temperature 70-80 degrees lower than its transformation start temperature.
3) According to the selected test materials having nearly the same transformation temperature and different chemical compositions, their deformation stress-temperature curves showed a similar characteristic.
4) From the results of 1) and 3) the change of deformation stress during cooling can be estimated from its chemical composition.
