Microstructural evolution of a superaustenitic stainless steel during a two-step deformation process

Single-and two-step hot compression experiments were carried out on 16Cr25Ni6Mo superaustenitic stainless steel in the temperature range from 950 to 1150°C and at a strain rate of 0.1 s^（-1）. In the two-step tests, the first pass was interrupted at a strain of 0.2; after an interpass time of 5, 20, 40, 60, or 80 s, the test was resumed. The progress of dynamic recrystallization at the interruption strain was less than 10%. The static softening in the interpass period increased with increasing deformation temperature and increasing interpass time. The static recrystallization was found to be responsible for fast static softening in the temperature range from 950 to 1050°C. However, the gentle static softening at 1100 and 1150°C was attributed to the combination of static and metadynamic recrystallizations. The correlation between calculated fractional softening and microstructural observations showed that approximately 30% of interpass softening could be attributed to the static recovery. The microstructural observations illustrated the formation of fine recrystallized grains at the grain boundaries at longer interpass time. The Avrami kinetics equation was used to establish a relationship between the fractional softening and the interpass period. The activation energy for static softening was determined as 276 kJ/mol.

Constitutive Modeling for Flow Behaviors of Superaustenitic Stainless Steel S32654 during Hot Deformation

Hot deformation behavior of superaustenitic stainless steel S32654 was investigated with hot compression tests at temperatures of 950-1250 C and strain rates of 0. 001-10 s-1. Above 1150 ℃, with strain rate lower than 0.1 s -1 , the flow curves exhibit nearly steady state behavior, while at higher strain rate, continuous flow softening occurs. To provide a precise prediction of flow behavior for the alloy, the constitutive modeling considering effect of strain was derived on the basis of the obtained experimental data and constitutive relationship which incorporated Ar- rhenius term and hyperbolic sine type equation. The material constants α, n, Q and lnA are found to be functions of the strain and can be fitted employing eighth-order polynomial. The developed constitutive model can be employed to describe the deformation behavior of superaustenitic stainless steel S32654.

Hot Working Characteristic of Superaustenitic Stainless Steel 254SMO

Abstract Hot deformation characteristic of superaustenitic stainless steel 254SMO has been studied by isothermal compression testing in the temperature range of 950-1,200 ℃ and strain rate range of 0.01-10 s^-1. The activation energy of 496 kJ/mol was calculated by a hyperbolic-sine type equation over the entire range of strain rates and temperatures. In order to obtain optimum hot working conditions, processing maps consisting of power dissipation map and instability map were constructed at different strains. The power dissipation map exhibits two domains with relatively high efficiencies of power dissipation. The first domain occurs in the temperature range of 990-1,070 ~C and the strain rate range of 0.01-0.1 s^-1. Microstructure observation in this domain indicates the partial dynamic recrystallization （DRX） accompanied with precipitation of tetragonal sigma phase. The second domain occurs in the temperature range of 1,140-1,200 ℃ and the strain rate range of 0.01-1 s^-1 with a peak efficiency of power dissipation of 39%, and in this domain, the microstructure observation reveals the full DRX. The instability map shows that flow instability occurs at the temperatures below 1,140 ℃ and the strain rates above 0.1 s^-1.

Effect of Thermomechanical Parameters on Σ3~n Grain Boundaries and Grain Boundary Networks of a New Superaustenitic Stainless Steel

Hot compression tests were conducted in a temperature range of 800--1100 ℃and strain rate range of 0. 1- 10 s^-1 using a Gleeble 3500 thermomechanical simulator to investigate the influence of hot deformation parameters （temperatures, strain rates and strains） on the grain boundary network evolution of a new grade Fe-Cr-Ni superaustenitic stainless steel. The results showed that a dominant effect of deformed temperature is ∑3^n （n = 0, 1, 2, 3） boundaries population increased with decreasing temperature, while they first increased and then reduced with in- creasing strain and strain rate. Interestingly, besides E3n （n = 1, 2, 3） twin grain boundaries, some El boundaries could interrupt grain boundaries network effectively, which enhance material performances. But they are scarcely re- ported. The misorientation of some segments LAGBs in the deformed microstructure （pancaked grains） increased and slid to high angle grain boundaries with increasing the fraction of reerystallized grains during hot deformation.

Metadynamic Recrystallization Behavior of As-cast 904L Superaustenitic Stainless Steel

The metadynamic recrystallization （MDRX） behavior of as-cast 904L superaustenitic stainless steel was in- vestigated by double pass isothermal compression tests at temperatures of 950-1 150 ℃, strain rates of 0.05-5 s 1 and interval of 1-100 s. The effects of working parameters （deformation temperature, strain rate, pre-strain and in- terval time） on the flow curves and microstructural evolution were discussed. The MDRX fraction increased obvious- ly with the increase of deformation temperature, strain rate and interval time. The MDRX softening was controlled by the migration of grain boundary, annihilation of dislocation and dynamic recrystallization. Moreover, the kinetic model was established for the prediction of MDRX behavior of as-cast 904L superaustenitic stainless steel based on the experimental data. A good agreement between the predicted and the experimental values was achieved （correla- tion coefficient R2= 0.98）, indicating a satisfactory accuracy.

Precipitated phases of superaustenitic stainless steel 654SMO

The phase diagram of superaustenitic stainless steel 654SMO was calculated by thermodynamic software and the precipitated phases in the specimens aged at 800-1100°C for 1hwere studied by methods of physicochemical phase analysis,scanning electron microscopy and transmission electron microscopy.The results showed that the size of precipitated particles increased with increasing the temperature.The amount of second phases reached the maximum value at 900°C,but decreased above 900°C.There were about eight kinds of precipitated phases in 654SMO includingσphase,Cr_2N,μphase,χphase,Laves phase,M_（23）C_6,M_6C and M_3C,in which theσphase and Cr_2N were the dominant precipitated phases.

Precipitation Behavior and Mechanical Properties of a 16Cr-25Ni Superaustenitic Stainless Steel Weld Metal During Post-weld Heat Treatment

A 16Cr-25Ni superaustenitic stainless steel weld metal for austenitic stainless steel/ferrite heat-resistance steel dissimilar metal weld was designed and prepared through tungsten inert-gas welding.The precipitate evolution and its correlation with mechanical properties were investigated during post-weld heat treatment(PWHT)at 690℃ for up to 12 h.The primary precipitates in the as-welded weld metal were identified as Mo-rich M6C carbides in the interdendritic region and semicontinuous fine-sized M23C6 carbides along grain boundary.After PWHT,three types of precipitates coexisted in the interdendritic region:primary M6C carbides,newly precipitated Mo-rich M2X carbonitrides and some of the secondary M23C6 carbides.Additionally,mass secondary M23C6 carbides formed and coarsened along grain boundary.No undesirable intermetallic phases formed during the whole period.The M2X and interdendritic M23C6 improved the strength of the weld metal after PWHT,but the elongation and impact toughness degraded,which were mainly owing to the intergranular M23C6 carbides that changed the fracture mode from ductile transgranular mode to mixed mode of transgranular and intergranular fracture.Meanwhile,the coarsening of M2X carbonitrides may lead to the elongation loss during 8 h to 12 h.Evolution of impact toughness was also related to the M2X carbonitrides,which made the crack easier to propagate compared with austenitic matrix and contributed to the decline of impact toughness.However,due to the sluggish precipitation of M2X carbonitrides with longer holding time,the decreasing trend became slow from 4 to 12 h.The results showed that PWHT should be controlled within 8 h to obtain better combination of strength and ductility.