Hot Working Technology Basis And Production Practice Of 321 Stainless Steel Up Head For Nuclear Reactor

AISI 321 austenitic stainless steel is widely used in the manufacture of large up head of pressure vessel for nuclear reactor（hereinafter referred to as reactor）.Due to the high content of alloy elements and the huge size of steel ingot used,such products are prone to surface cracking,coarse grain and mixed grain structure during hot forging.These problems affect the ultrasonic flaw detection and mechanical properties.In serious cases,they also lead to the scrapping of head forgings,reduce the production efficiency,increase the production cost,and pose a great threat to the safety of nuclear reactors.It is difficult to control the microstructure and the hot working ability is poor,these issues have become the bottleneck restricting the production of large heap head forgings.At present,there are few reports on the thermal deformation behavior of 321 stainless steel for reactor,but most of the materials used in these studies are as forged or as cast microstructure of small ingot.This is very different from the coarse as cast structure of large head ingot and the corresponding hot working properties.The solidification rate of large ingots is very slow,and component segregation is often caused δ Ferrite formation,and δThe effect of ferrite on the hot working properties of 321 stainless steel is not very clear.The production process of large-scale stacking head forgings is various and the cost is extremely expensive.Domestic mass production cannot be carried out independently,and there is no relevant public technical report in the world.In view of this,it is of great significance to systematically study the mechanical behavior,microstructure evolution law and hot forging surface cracking mechanism of 321 stainless steel head for reactor in the forging process,so as to obtain the optimal hot working process window,so as to realize the effective control of head microstructure and properties,so as to improve the quality of large stainless steel forgings and the safety of nuclear reactor in China.AISI 321 stainless steel for reactor head was selected as the research object,the pseudo binary equilibrium phase diagram is calculated by thermodynamic software,and the structural properties and stability of the initial Y phase in the steel are calculated by the first principle method.The flow behavior,microstructure evolution,instability characteristics and cracking mechanism of the steel under different processing conditions are studied by thermal/mechanical physical simulation system.The main research results are as follows:The pseudo binary equilibrium phase diagram of 321 stainless steel for reactor is obtained.Its Y-phase composition is close to Ti₂SC,in strip shape and approximately parallel arrangement,and has high thermal stability;However,fracture and fragmentation will occur in the forging process,especially y phase with large size.The first principle calculation results show that Ti₂SC can not spontaneously become TiS and TiC,but Ti₄S₂C₂（Z=2）can spontaneously become these two phases;The mechanical stability of Ti₄S₂C₂ is poor;Ti₂SC has high structural stability,belongs to hard brittle phase,is not easy to plastic deformation,and is easy to brittle fracture under stress.At 900～1200℃ and 0.01～10 s^-1,the thermal deformation equation of 321 stainless steel under transverse compression along dendrite is ε=5.81 ×10¹⁹[sinh（0.0095σ）]^5.05·exp（-478000/RT）,when compressed along the longitudinal direction of dendrite,it is ε=3.89 × 10¹⁸[sinh（0.0093σ）]^5.38 · exp（-477147/RT）;the activation energy during thermal deformation under the two conditions is basically the same.The flow stress of 321 stainless steel decreases with the increase of deformation temperature and the decrease of strain rate,which is the critical stress for dynamic recrystallization σ The quantitative relationship between C and Z parameters is:σ_c=13.4 ln Z-457（MPa）;Dynamic recrystallization occurs when the strain rate is high.The recrystallized grain size increases with the increase of deformation temperature and decreases with the increase of strain rate.The relationship between dynamic recrystallized grain size（D）and Z and A is D=1.301 ×（Z/A）^-0.355（μm）.The dynamic microstructure diagram and hot working diagram of 321 stainless steel were obtained.At the initial stage of deformation,flow instability occurs at low temperature（900～1000℃）,and develops to high temperature and high strain rate with the increase of strain;the essence of instability is the local rheology at the grain boundary.When the tested steel is compressed along the longitudinal direction of dendrite,it is more prone to dynamic recrystallization,the dynamic softening effect is more obvious,and the instability area in the hot working diagram is smaller.For the hot working of as cast 321 stainless steel,the deformation process window is recommended to be 1000～1200℃ and 0.01～0.1 s^-1.During the hot tensile deformation of 321 stainless steel,the cracks mainly originate from δ At the phase interface between ferrite and austenite,a small number of cracks originate at the phase interface between primary y phase and austenite matrix,and a small number of cracks are directly at the interface δ Ferrite forms,expands and causes it to fall off.Usually,dynamic recrystallization will consume the damage accumulated by deformation,inhibit the propagation of crack,and then reduce the risk of deformation and cracking.Finally,the forging and heat treatment process of 321 stainless steel head for reactor with independent intellectual property rights was put forward,and the large head forgings with geometric size,performance and multi-position grain size met the technical requirements of special reactor were manufactured.