18-8 Tiステンレス鋼の高温強度

Abstract

A comprehensive study was carried out to investigate the effects of the chemical compositions, heat treatments and grain size on the creep rupture properties of the 18-8 Ti stainless steels, in wide use for superheater tubes and reheat tubes of power boilers. The conclusions are as follows:(1) Effect of chemical compositionsIt was found that the ferrite-free austenitic steels had the creep rupture strength superior to that with 14% ferrite at 550°C and 650°C, both of which had different compositions even though within the range of the specification (AISI 321 or JIS STB 42 D) and were obtainable to single or duplex structure by selection of proper composition also within the range of the specification.That is speculated to be attributable to occurrence of the precipitation of enormous chromium carbide around the ferrite in duplex austenite-ferrite structure at testing temperature.From these results, it is desirable as for the chemical compositions to offer fully austenitic structure, if possible, for elevated temperature applications. The results of measuring of the creep rupture strength of the 18-8 Ti steels within the range of 3 to 19 of Ti/C ratio, keeping the constant contents of carbon and other alloying elements, revealed that it increased with decrease of Ti/C ratio. It is assumed that increase of Ti/C ratio follows additional insoluble TiC at the prevalent solution treatment temperature (1100°C) and consequently decrease of TiC effective to creep rupture on the contrary, which precipitates finely and dispersedly into the austenite structure at service temperature. Thus it is recommendable to adopt low Ti/C ratio within the range of 3 to 7, to obtain high strength at elevated temperature.(2) Effect of heat treatmentsArising of the solution treatment temperature in the range of 1000°C to 1300°C brought about increase of the creep rupture strength of the steels. We might conclude, however, that to the temperature range of 1100°C to 1150°C is practically preferable, because too high temperature heat treatment bears low toughness.(3) Effects of grain sizeIt has generally been said that the coarsely grained steel has good creep rupture strengtth in comparison with that of the finely grained. However, in case of austenitic stainless steels, as creep rupture strength markedly depends on the condition of the precipitation of carbide along the grain boundaries at the service temperature range of 550°C to 650°C, it was found that the coarsely grained steel does not always show good creep rupture strength. Thus we may consider that the fact described in (2) is due not to the coarse grain but to the fine and uniform distribution of the precipitated carbide at service temperature, which shows the higher solubility to the matrix at higher solution treatment temperature.(4) Long-time creep rupture strengthCreep rupture tests up to 13000 hours are practised for advantageously heat-treated 18-8 Ti stainless steel with desirable chemical compositions. The extraporated 100000 hours rupture strength was 9.3kg/mm2. That is superior to the average value, 5.5kg/mm2, of ASTM STP No. 124. We may probably attribute that to underestimation of 18-8 Ti stainless steel, because the data of ASTM had been obtained before the relations between metallographic behaviors and creep rupture strength in this steel were satisfactorily clarified.