Abstract: Impact toughness tests are used to compare two different kinds of temper heat treatment for the filler metal of 2.25Cr-1Mo steel. In the first temper process, the top temperature is 720 ℃ and the holding time is 2 h. The other temper treatment includes 680 ℃ and 12 h. The first heat treatment process shows a better impact toughness. To study what causes the difference, the joints processed with the first temper treatment are dipped into liquid nitrogen to decompose residual austenite in welds. The later tests show that impact toughness is deceased super cooling, which suggests austenite may be an important factor for welds toughness. To validate the assumption, magneto-polarization intensity and electron backscattered diffraction of different samples are investigated to quantify residual austenite quality and distribution through vibration sample magnetometer (VSM) and electron back-scattering diffraction (EBSD) separately. After the first temper process, the welding metal has about 3% residual austenite quality investigated by VSM, and has 0.2% after the second temper process; On the other hand, it has about 3.49% residual austenite quality investigated by EBSD after the first process ,and has about 0.18% after the second process. The measurement results show the difference of residual austenite content with different heat treatment process, which makes sure the effect of residual austenite content on the impact toughness of the filler metal.

Key words: Filler metal of steel 2.25Cr-1Mo, Heat treatment, Impact toughness, Residual austenite content