High temperature tempering brittleness is A517 alloy steel Grade S between 500 ~ 550 ℃ tempering after quenching, or at the temperature above 600 ℃ after tempering at a slow cooling rate through 500 ~ 550 ℃ range embrittlement phenomenon occurs. If the A517 Grade S to rapid cooling after heated to above 600 ℃ temperature, can restore toughness, therefore is also called reversible temper brittleness.
It has been proved that A517 Grade S of P, Sn, Sb, As, and other impurity elements in 500 ~ 550 ℃ temperature together with the original austenite grain boundary, lead to high temperature tempering brittleness. Ni, Mn and other elements can be and P, Sb impurity elements such as poly (cosegregation) grain boundary synergy, Cr element and promote this kind of gather together, so all the elements are intensifying A517 Grade S steel high temperature tempering brittleness.
On the contrary, the interaction of molybdenum and phosphorus, hinder the phosphorus gathered in the grain boundary, can reduce the high temperature tempering brittleness. Rare earth elements have a similar effect. A517 Grade S steel at above 600 ℃ temperature tempering after rapid cooling can suppress the segregation of phosphorus, in the heat treatment operation is often used to avoid high temperature tempering brittleness.
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