Precipitation Behaviors And Hot Deformation Processes Of TP347H Austenitic Heat-Resistant Steel

TP347H austenitic heat-resistant steel is a kind of austenitic steel with excellent thermal properties.It is developed on the basis of 18Cr-18 Ni type austenite steels through compositional redesign.TP347 H austenite heat-resistant steel is a type of niobium-stabilized steel,the purpose of increasing the content of niobium is to facilitate the precipitation of Nb(C,N)carbonitrides during aging.The Nb(C,N)precipitates stabilizes the austenite steel against the precipitation of M23C6 carbides.Therefore,the number of the harmful phases decreases.TP347 H austenitic heat-resistant steels have been widely used for super-heater or re-heater tubes in the USC power plants due to their low costs,excellent steam oxidation resistance and good creep properties at high temperatures.Nowadays,a great deal of research activities about TP347 H austenitic heat-resistant steels have been focused on creep performance and weld-ability at high temperatures,whereas little attention has been paid to the carbides precipitation behavior during aging,thermal deformation techniques and its microstructure evalution.Against this background,this paper was mainly targeted at analyzing the precipitation behavior and the coarsening mechanism of carbonitrides during aging.Meanwhile,the appropriate hot deformation parameters were investigated on the basis of the thermal processing map.The following conclusions were achieved:(1)The hot deformation behaviors of the TP347 H austenitic heat-resistant steel were investigated under the deformation temperatures from 800? to 1100? and the strain rates from 0.01s-1 to 10s-1.The secure zone of TP347 H austenitic heat-resistant steel mainly consists of the deformation conditions about 1000? with the strain rate of 1s-1 or less.While the unstable zone mainly consists of the deformation conditions with lower deformation temperatures and higher strain rates.Compared with the samples without thermal deformation,more MX phase will precipitate in the austenite matrix.(2)Various precipitates,MX,M23C6 and Z phase(NbCrN),were identified during the long-time aging at 700?C.MX phases are nanosized Nb(C,N),the amount of these phases increases with the increase of holding time,however,the size of Nb(C,N)is not increased obviously.MX phases are the most predominant strengthening phase in TP347 H austenitic steel.The growth mechanism of Nb(C,N)carbonitrides is controlled by the diffusion niobium atoms in the temperature range between 800 and 900oC.The size and amount of M23C6 phases increase apparently with increase of holding time and temperatures.M23C6 phases can result in the formation of chromium depleted zone at grain boundaries and intergranular corrosion.Accordingly,the thermal performance of the steels will decrease.M23C6 phase is a primary harmful precipitate during aging at 700?C.Nanosized Z phases are identified as NbCrN,they are capable of strengthening austenitic steel due to its stability.The possible precipitation sequence during aging: fine Nb(C,N)precipitates first,then,M23C6 carbides precipitate along the grain boundaries.Finally,nanosized Nb(C,N)transform to Z phase at the grain boundaries.