The Research Of Domestic Super304H Steel Pipes On Mechanical Properties, Microstructure And Creep Rupture Strength

Super304H steel is novel heat resistant steel,developed by Japan,which was mainly applied to make superheat pipes and reheat pipes of ultra supercritical fossil fired boilers.It has extremely high allowable stress,creep rupture strength,excellent steam oxidation resistance,high-temperature corrosion resistance and moderate price. At present,only Sumitomo Metal Industries and DMV Pipe Company can manufacture Super304H steel pipes abroad,but which have limited production capacity.Domestic Super304H steel pipes were still in the stage of industry trial production,the similar products didn't appear in the domestic markets.Therefore the results of the study and development of domestic Super304H steel pipes have important applicable value in engineering.The tests were completed in terms of the room temperature mechanical properties,high temperature tensile properties,processing properties and high temperature creep rupture properties,the microstructure and fracture surface were analyzed by means of optical microscope(OM)and scanning electron microscope (SEM).The results showed that,domestic Super304H steel pipes possess good room temperature mechanical properties and processing properties;High temperature tensile strength of domestic Super304H steel pipes reached similar that of Japanese(slightly lower by about 6 percent),which was significantly higher than values provided by GB5310-××××——High-pressure Boiler Seamless Steel Tube(approval),the tensile strength of steel was still much enough,which was higher about 30 percent than the values at 600C,reached 434MPa;In addition to higher inclusion content and more concentrated distribution,microstructure basically reached similar product requirements.The creep fracture characteristics and fracture mechanism of Super304H steel at different temperature and stress were studied.The results showed that,at 650℃in higher stress the rupture of Super304H steel was a mixture of trans-granular and inter-granular fracture,while it emerged ductility fracture along the grain boundary in lower stress,the surface of creep fracture had a large number of tear ridge,materials showed high plastic.But for long time creeping at 700℃the tear ridge on creep fracture surface decreased significantly,the fracture surface showed the grain-shaped contours clearly,its plastic was low.Meanwhile,high temperature creep rupture strength estimation of pipes has been carried out by three kinds of mature extrapolation methods such as isothermal method,Larson-Miller parameter method and Ge ting-sui -Torn method.After discussion of the effect factors on constant Cin Larson-Miller method,the revision has been refereed.Furthermore factors influencing on creep activation energy Q of Ge ting-sui-Torn method has been analyzed,and it has been brought forward how to choose the number of hot strong parameter polynomial P(σ).The analysis showed that,in three extrapolation methods,Ge ting-sui-Torn method has more precision,and high temperature creep rupture strength were extrapolated by this methods,σ105650of Super304H steel was 122.74MPa,which was slightly higher than the value provided by GB5310-××××——High-pressure Boiler Seamless Steel Tube(approval),σ105700was 60.78MPa,which was only 80.6%of the specified value.So it can't be used in the ultra-high-temperature(＞650℃).