Improvement on Age Hardening Properties and Creep Rupture Strength of P-Containing Austenitic Heat Resisting Steel due to Various Cooling Procedures

Abstract

he effect of cooling rate from solution temperature on the age hardening properties of a P-containing austenitic heat resisting steel was investigated.
Little difference in the age hardening properties was detected between a water-quenched specimen and a air-cooled one, while at a slower cooling rate as in furnace cooling, the aged hardness was lower than that of a water-quenched or a air-cooled specimen.
Transmission electron microscopic examination revealed that in specimens aged afer water quenching or air cooling very fine precipitates were formed in uniform dispersion almost independently of dislocation of which density would vary with cooling rate. The cause of this very fine and uniform dispersion of precipitates will probably be attributed to P contained in this steel. It will be also assumed that this independence of nucleation on dislocation leads to the quench insensitivity of this steel. On the toher hand, it was detected that in a furnace-cooled specimen precipitates were already nucleated during cooling from solution temperature, and these pre-formed precipitates affected appreciably the dispersion of precipitates formed at subsequent aging.
On the basis of above results, two step cooling process from solution temperature was investigated in order to obtain high aged hardness as well as marked grain boundary serration. It involved following processes:(1) furnace cooling through a higher temperature region where grain boundary serration due to coarse precipitates occurred markedly but precipitates were not yet nucleated in the matrix, and (2) subsequent air cooling. This two step cooling process resulted in a marked age hardenability of the steel during aging.
It was found that the creep rupture sterngth of this steel was improved significantly by the above heat treatment; e. g. 1000hr rupture strength reached to 28-29 Kg/mm² at 700°C.