Effects of Solution Treatment on High-Temperature Properties of 18Cr-8Ni-Ti Steel

Abstract

The present study was carried out to make clear the effects of solution treatment on the creep rupture properties of type 321 stainless steel.
To prepare test pieces, three kinds of stainless steel bars, 17Cr-9Ni, 17Cr-9Ni-Ti and 18 Cr-11Ni-Ti with 25mm diameter were reduced to 15mm diameter by “hot-cold working” at 500°C after water-cooling from 1050°C. And then these specimens were solution-treated at various temperatures between 800°C and 1200°C for 1 hour. Some of these were reheated at 1000°C for 2 hours, 900°C for 5 hours and 800°C for 100 hours after water-cooling from 1100°C.
The obtained results were summarized as follows.
(1) Low-temperature heat-treated material had very fine grains and a low rupture strength. However, with increase of solution temperature it showed coarser grains and a higher strength. but a decreased rupture elongation. In general, rupture elongation of 18Cr-8Ni steel was larger than that of 18Cr-8Ni-Ti steel.
The effect of solution-treating temperature on the creep rupture strength of 18Cr-8Ni-Ti steel was very larger than that of 18Cr-8Ni steel.
(2) With the 18Cr-8Ni-Ti steel which had been water-cooled or furnace-cooled from 1100°C, reheated at 1000°C and 900°C after solution treatment at 1100°C, there was a considerable difference among their carbide precipitation in microstructure, although there was little difference among their creep rupture strength. Specimens which had been reheated at 800°C for 100 hours after water-cooling from 1100°C, showed a lower creep rupture strength than that of water-cooled specimens.
(3) It is presumed that the variation of strength with solution-treating temperature and time was related to both grain size and solubility of carbide.