Abstract
The effects of phosphorus addition on both the precipitation behavior of M23C6 carbides and the quench sensitivity of the age hardening properties have been investigated in 21Cr-12Ni austenitic heat resisting steels containing carbon up to 0.3%.
It is found that in P containing steels the higher aged hardness is obtained and this is not affected by the cooling rate after solution heating. According to the transmission electron microscopy, this quench rate insensitivity of the aged hardness is revealed to be attributed to the fact that in these steels the matrix precipitation of carbides occurs and the dispersion of carbides is not affected by the cooling procedures unless carbides are pre-formed during cooling. On the other hand, the pre-formed precipitates which have been formed during both slower cooling from the solution temperature and heating up to aging temperatures affect appreciably the dispersion of carbides formed during the subsequent aging.
In directly aged specimens of P containing steels heterogeneous precipitation occur above a certain critical aging temperature. It is suggested that the existence of the critical temperature at which the precipitation mode of carbides changes from matrix-to heterogeneous-precipitation is based on the easilr growth of precipitates associated with dislocations at higher temperature.
On the basis of the above results, the role of phosphorus in M23C6 carbide precipitation is discussed.
