高Cr-Niオーステナイト鋼の高温強度

Abstract

The present study was made to examine high temperature strength of high Cr-Ni austenitic steels. Creep rupture tests and short time tensile tests at 650, 800 and 1000°C were conducted for AISI type 309 (25Cr-12Ni), 310 (25Cr-20Ni) and 330 (15Cr-35Ni) specimens to determine the effect of alloying elements, solution treatment and micro-structure on high temperature strength.The results obtained are summarized as follows:(1) Differences of tensile properties and creep rupture strengths between those of three grades of austenitic steels tested are not clear. But grade 309 possesses the highest rupture strength for 1000h at 800°C, while grade 310 has the lowest strength, and the latter shows the highest elongation at any temperature.(2) As to the effect of alloying elements, C and N, which are interstitial atoms, have a remarkable effect for the improvement of high temperature strength, while substitutional atoms such as Cr, Si and Ni are only slightly effective for creep rupture strength.(3) Because of the transformations of ferrite into sigma phase during test, its very effect is not clear at 650 and 800°C tests, while detrimental effect is observed at 1000°C where the transformation does not occur. Elongation at rupture of specimens containing ferrite at 1000°C is higher than that of fully austenitic specimen.(4) Specimens containing 0.16% or more of C show a clear grain growth when solution treatment temperature is increased from 1000 to 1300°C while low C specimens do not show further grain growth over 1100°C. On the tensile properties at room temperature, the higher the solution temperature is, the lower is the strength and the higher ductility. But both of strength and ductility in short time tensile test at 650 and 800°C decrease with the increase of treatment temperature and remarkable embrittlement appears at 800°C.Creep rupture strengths of higher C steels are improved as solution treatment temperature raises up to 1200°C. Lower C specimens do not show definite improvement of creep rupture strength, while elongation decreases remarkably. In this respect, creep rupture strengths are influenced more by behavior of C and N than by grain size.(5) Type of fracture is transgranular at room temperature test and intergranular at 650°C and above, independent of steel grades and heat treatment temperatures.