ボイラ管用炭素鋼のクリープ破断強さのばらつきの要因

Abstract

Long-time creep rupture tests up to 40000hr have been carried out at 400°-500°C on nine commercial heats of silicon-killed carbon steel for boiler tube (JIS STB42). The data shows a large scatter from heat to heat and from product to product. The main factors affecting the scatter have been studied and the results obtained are as follows:(1) The scatter of ±20per cent in rupture strength has been observed. By taking into consideration that the nine heats are from nominally same steel, the scatter is considerably large. In addition, the scatter doesn't become smaller at higher temperatures nor with increasing time to rupture.(2) All the 100000hr rupture stresses extrapolated by the Larson-Miller method are higher than the average stress calculated from the allowable stress in JIS.(3) Rupture strength at higher temperatures and for longer time becomes higher in proportion to the increase of molybdenum content from 0.005 to 0.019per cent.(4) At lower temperatures and for shorter time, the variations in the molybdenum content and the nitrogen content in solution are the main causes of the scatter. With increasing testing time the nitrogen content in solution is considerably reduced owing to the formation of nitride with a corresponding deterioration in rupture strength.(5) Although the effect of normalizing temperature on rupture strength is slight, tempering causes the rupture strength at lower temperatures and for shorter time to deteriorate due to the decrease in the nitrogen content in solution.