In 300-500 degrees C ageing, the H-v value and Ms temperature are strongly affected by the ageing time. The Ms temperature was found to drop during early ageing and then to increase. The H-v value was found to increase during early ageing and then to decrease. However, precipitation hardening is one of the variables that affect the Ms temperature. The precipitates were lined up in a single preferred orientation during the constraint ageing condition. The precipitates with preferred orientation caused a shear stress field which improved the martensitic transformation. The result is that the stress required to induce martensite decreases and the Ms temperature increases.