Research On Grain Refining Mechanism And Control In Heat-affected Zone Of SUPER304H

SUPER304H is a kind of stainless steel using for the overheating and reheating tube in ultra-super critical boiler developed by Sumitomo Metal Industries.Most of tube used in boiler in China was imported at present,some factories in China produced this kind of steel also,but with lots of problems.One of the most important problems is the coarse grain in welding heat affected zone(HAZ),so the detailed research on grain size in HAZ have significant meaning.The information and report have not been seen about the research on grain size in HAZ of this steel yet.In this paper,the research methods on grain size in HAZ of other similar steel is introduced,the factor affected grain size in HAZ is analyzed.By means of optical microscope(OM),scanning electron microscope(SEM) with X-ray energy dispersive spectrum(EDS),electronic probe microscopy analyzer(EPMA),The grain size, microstructure and precipitates at welding joint HAZ of SUPER304H suffered different welding methods,produced by different countries and both original and mended produced in China is detailing analyzed,the most important factor affected grain size in HAZ is obtained. Base on the heat treating experiments with different temperature and different times,the rules of grain growth of this steel on different conditions is obtained.It is recognized that the main affected factor of grain growth at HAZ is the minute precipitates.As to the grain size in HAZ of SUPER304H,the advantaged precipitates are: NbC,(Nb,Ti)(C,N),TiN,while the disadvantaged ones are:(Fe₃Nb₃) N,(Fe,Cr, Mo) ₆C,Fe₂Nb intermetallic phase,Cr₂₃C₆.In every individual heat or cool process,note that measures that could diffusely precipitate the minuteness phases should be adopted,the temperature intervals that could formed the disadvantage phases should be avoided or stayed at minimum time.In the welding process,the suitable welding materials and welding procedure should be preferred,and it is suitable to use the lowest welding input power.The mended kind of SUPER304H is testing produced in a factory according to the conclusion of this paper.Thermal analysis experiments on SUPER304H produced by Japan, German,and China is carried out by STA449C thermal analyzer(TA),the result show:The melting,solidifying and solid-state phases transition have some differences for products of different factories.The welding HAZ simulate experiments on different conditions is performed by Gleeble-3800 thermo-mechanic simulator(TMS),the treated samples is analyzed by OM and SEM with EDS,the result show:the advantage precipitates is found in welding HAZ and the grain size is fine.The actual welding test show that the grain size number in HAZ is above NO.6.In sum,the coarse grain in HAZ problem is resolved in mended SUPER304H.