Effect of grain size and cooling rate on 475° embrittlement of steel 15Kh25T

Conclusions

• 1. The kinetics of embrittlement is affected by the grain size and the chemical heterogeneity of the α solid solution. It was not possible to separate these effects, and therefore the combined influence was determined in these experiments. With increasing preliminary annealing temperatures, leading to grain growth and an increase in the homogeneity of the solid solution, the rate of the embrittlement process decreases somewhat. Heterogeneity also leads to a difference in the brittleness of microvolumes: sections rich in chromium, primarily in the grain boundaries, are more highly embrittled.

• 2. Continuous cooling of high-chromium steels at relatively high rates (∼10 deg/sec) induces some embrittlement in the range of 475°. With very slow cooling the embrittlement range shifts to higher temperatures, and a notable reduction of the notch toughness is observed with slow cooling in the range of 550–600°. The highest rate of intraphase decomposition occurs at 450–550°.

• 3. By proper selection of the cooling rate and the temperature of final slow cooling it is possible to eliminate 475° embrittlement almost completely with continuous cooling of high-chromium steel 15Kh25T. To prevent embrittlement we recommend continuous cooling at rates not less than 10 deg/sec. In those cases where cooling is conducted at a lower rate it should be stopped at temperatures no lower than 600°, followed by cooling at a high rate.