1990 Volume 30 Issue 1 Pages 58-63
Study of the microstructural changes and the embrittlement behaviour of a 21/4Cr-1Mo steel with low impurity content by isothermal aging for periods of up to 10 000 h at 823, 873 and 898 K showed that although grain boundary segregation of phosphorus or other impurity elements were not recognized, considerable reduction in notch toughness occurred due to the microstructural changes associated with the precipitation and coarsening of carbides. It has been shown that the toughness degradation due to long term aging has a linear relationship with the parameter T(log t+8) where T is the aging temperature and t, the aging time. It has also been suggested that step cooling alone is not a viable method of estimating long term embrittlement.
Using the above parameter, it ought to be possible to formulate a method by which the microstructural changes and toughness degradation occurring due to long term service of the order 100 000 h could be effectively simulated in the laboratory by heat treatment of a few thousand hours. An illustrative example of simulation of the changes in carbide morphologies, hardness and toughness that occurred in a 21/4Cr-1Mo steel due to 88 000 h service at 813 K has been presented.
