Research On Fatigue Crack Growth Of Typical Steels At High Temperature

Many equipments experience elevated temperature in the powergeneration, petroleum and chemical industry. High temperature fatigue failures often take place in these equipments and structures. Therefore, the study on fatigue crack growth laws at high temperature is very useful for safety of high temperature equipments. As the part of tenth five-year national science and technology project, the laws of fatigue crack growth at high temperature for four typical materials (316L, 2(1/4)Cr1Mo, 2(1/4)Cr1MoV , 16MnR) used in the petrochemical industry are studied.(1) The data of fatigue crack growth rates at high temperature for four materials are gained. The laws of fatigue crack growth with temperatureare researched.(1) Fatigue crack growth rates of 316L, 2(1/4)Cr1Mo and 2(1/4)Cr1MoVincrease with temperature. The exponent n , in the Paris law, da/dN = C( K)", changes little with increasing temperature. Effect of temperature on the fatigue crack growth rate of austenitic material ismore evident than that of bainite material.(2) The fatigue crack growth laws of 2(1/4)CrlMo are nearly the same asthat of 2(1/4)Cr1Mo V at high temperature.(3) The crack growth rates of 16MnR at 150 and 300 are lowerthan those at room temperature and 425 . The crack growth rate at 425 is highest.(2) The fatigue crack growth mechanism about four materials is researched.(1) Oxidation on the crack surface of 16MnR increase with temperature. Oxidation mainly takes place on the crack surfaces at 150 ~300 ,which it is serious both on the crack surfaces and in the grain boundaries at 425 . Oxide-induced closure at 150 ~300 results in the drop of crack growth rates. Embrittled grain boundaries results in the increase of crack growth rates at 425 .(2) There is little oxidation in 316L at high temperature. Increase of fatigue crack growth rates of 316L at elevated temperature mainly results from the drop of material intensity and elastic modulus.(3) Both oxide-induced closure and embrittlement of grain boundariestake place for 2-Cr1Mo and 2-Cr1MoV at high temperature. The change of fatigue crack growth rates with temperature is not so evident as 316Land 16MnR.(4) The fatigue cracks propagate mainly transgranularly at room temperature, while those propagate mainly intergranularly at high temperature.