Study On The Microstructures And Mechanical Properties Of Grade 91 Heat Resistant Steel And Joint Using Special Welding Technology

Grade 91 steel is one of the widely used materials in ultra-supercritical(USC)power generator unit.The investigations on heat treatment process and special welding technology are meaningful to the development of USC technology.Three aspects have been carefully studied in this thesis.(1)The effect of cooling rate of normalizing process on the microstructure and mechanical properties of Grade 91steel.It was common to the heat treatment of thick-wall P91 parts or large-scale stacked Grade 91 parts.(2)The investigation of application of laser beam welding for the manufacturing of Grade 91 steel.(3)The investigation of application of welding repair technology for Grade 91 steel.Aiming the above three aspects,the effects of different thermal process on microstructure and mechanical properties of Grade 91 steel,special welding joint,and cladding metal were discussed.For heat treatment process,the effects of cooling rate(normalizing process)and tempering process on the microstructure and mechanical properties of P91 steel were investigated.For the laser beam welding,the generation mechanism and depressing mechanism of?-Fe in fusion zone(FZ)was investigated.The depressing mechanism of the width of HAZ was also investigated.The cracking mechanism of FZ was investigated.The effect of post-weld heat treatment(PWHT)on the microstructure and mechanical properties of P91 LBW joint was discussed.For the welding repair,the depressing mechanism of CMT process on the generation of?-Fe was discussed.The PWHT for cladding metal was investigated.The mechanical properties of Grade 91 cladding metal and standard P91 steel were compared.Based on the above experiments and discussion,the following conclusions were derived.Three cooling rates,including cooled in air(normally normalized,NN),200°C/h(critically normalized,CN),25°C/h(completely annealed,CA),were employed during the normalizing process of P91 steel.The results showed that the slower cooling rate contributed to the transformation of microstructure from martensite to bulk ferrite.As the loss of solid solution strengthening effect and dislocation strengthening effect,the hardness and strengthen of P91 steel gradually declined.The longer martensitic transformation time induced the carbon redistribution during CN process and contributed to the mixture microstructure of low-carbon martensite and high-carbon twinned martensite.The high-carbon twinned martensite also induced the embrittlement of CN P91 steel.The continuous M₂₃C₆ generated on the grain boundary of bulk ferrite also contributed to the brittleness of CA P91 steel.The tempering process of three P91 steels was conducted.The solid solution strengthening effect and dislocation strengthening effect of tempered martensite was greatly declined.Therefore,the hardness and strength were greatly declined.The plasticity and toughness were enhanced.The low-carbon martensite and high-carbon twinned martensite in the CN+tempered(CNT)P91 steel converted into the tempered martensite with a lower dislocation density.It reinforced the toughness and damaged the creep performance of CNT P91 steel.After tempering process,the continuous GB's M₂₃C₆ converted into the un-continuous GB's M₂₃C₆ as its dissolution and spheroidization.It did not remarkably change the hardness,but enhanced the toughness greatly.The self-fusion laser beam welding(LBW)was the ideal technology for P91steel.Firstly,the super high cooling rate of FZ depressed the formation of?-Fe.Secondly,the width of HAZ was depressed as the focus heat source and high heating/cooling rate of HAZ.The type-IV cracking would also be suppressed as the narrow HAZ.Thirdly,the P91 LBW joint with a good complex mechanical properties was obtained after PWHT.The results showed that the higher cooling rate suppressed the transformation from liquid phase to?-Fe phase and suppressed the retaining of?-Fe phase.The carbon atom migrated towards some certain?-Fe/martensite interface during PWHT and contributed to the coarsening of M₂₃C₆ on phase interface.The coarse M₂₃C₆ on the interface damaged the toughness.The larger shrinkage of FZ during high temperature contributed to the hot crack when welding the P91 steel with a thickness of 10mm using a large heat input.The CMT technology was the ideal way to repair the Grade 91 steel parts.The low heat input and high cooling rate could depress the formation of?-Fe and the width of HAZ.As the effect of high cooling speed and alloying element of C,Ni,and Mn,?-Fe phase was not observed in the cladding metal.The cladding metal was consisted of solidification region and transition region.The results from multi-layers&single-pass cladding metal showed the weaker dislocation strengthening effect and grain-refine strengthening effect of transition region.Moreover,part of the transition region has a peak temperature between Ac₁ and Ac₃.Therefore,the hardness of transition region was lower than that of solidification region.The microstructure became tempered martensite after the PWHT process of 760°C+1h.The hardness and strength greatly declined.The plasticity slightly improved.For the PWHT process of multi-layers&multi-passes cladding metal,the results showed that the hardness and strength declined gradually when the PWHT temperature was lower than Ac1temperature.The plasticity and toughness improved gradually when the PWHT temperature was lower than Ac1 temperature.When the PWHT temperature was higher than Ac1 temperature,the hardness and strength improved again.The plasticity and toughness remarkable damaged.The hardness and strength of Grade 91 cladding metal was closed to that of standard P91 steel.However,the flaws in the cladding metal induced the poorer plasticity and toughness of cladding metal.Based on the above conclusions,the cooling rate of P91 steel should be monitored as its large effect on microstructure and mechanical properties.The self-fusion LBW technology could be applied in the manufacturing of P91 parts.There was a large potential application of CMT in the repairing of P91 steel part.However,the formation mechanism of flaw should be investigated deeply.