Dynamic Recrystallization Behavior of a Fe-Cr-Ni Super-Austenitic Stainless Steel

The super-austenitic stainless steels are extensively utilized in the seamless tubes production for oil extraction industries. Due to the importance of thermo-mechanical processing in the production of these tubes, the dynamic recrystallization （DRX） characteristics of a Cr-Ni super austenitic stainless steel （1.4563） were investigated in the present study. This was performed using the hot compression testing method in the temperature range of 950-1150℃ and the strain rate of 10^-3-10^-1 s^-1. The initiation and evolution of DRX were examined through microstructural analysis. The results indicated that the recrystallized grain formed a necklace type structure at the prior austenite grain boundaries at higher strain rates. In addition, DRX nucleation occurs by bulging and successive strain induced boundary migration （SIBM）.

Effects of B segregation on Mo-rich phase precipitation in S31254 super-austenitic stainless steels:Experimental and first-principles study

Precipitation in super-austenitic stainless steels will significantly affect their corrosion resistance and hot workability.The effects of Cr and Mo on precipitation behaviors were mainly achieved by affecting the driving force for precipitation,especially Mo has a more substantial promotion effect on the formation of theσphase than Cr.In the present study,B addition to the S31254 super-austenitic stainless steels shows an excellent ability to inhibit precipitation.The effect of B on the precipitation behaviors was investigated by microstructure characterization and theoretical calculations.The experimental observation shows that the small addition of B inhibits the formation of theσphase along grain boundaries and changes from continuous to intermittent distribution.Moreover,the inhibitory effect increased obviously with the increase of B content.The influence of B addition was theoretically analyzed from the atomic level,and the calculation results demonstrate that B can inhibit the formation ofσphase precipitates by suppressing Mo migration to grain boundaries.It is found that B and Mo are inclined to segregate atΣ5 andΣ9 grain boundaries,with B showing the most severe grain boundary segregation tendency.While B distribution at the grain boundary before precipitation begins,the segregation of Mo and Cr will be restrained.Additionally,B’s occupation will induce a high potential barrier,making it difficult for Mo to diffuse towards grain boundaries.

Static recrystallization behaviors and mechanisms of 7Mo super-austenitic stainless steel with undissolved sigma precipitates during double-stage hot deformation

Static recrystallization(SRX)behaviors and corresponding recrystallization mechanisms of 7Mo super-austenitic stainless steel were studied under different deformation conditions.The order of influence of deformation parameters on static recrystallization behaviors,from high to low,is followed by temperature,first-stage strain and strain rate.Meanwhile,the effect of holding time on static recrystallization behaviors is significantly controlled by temperature.In addition,with the increase in temperature from 1000 to 1200°C,the static recrystallization mechanism evolves from discontinuous static recrystallization and continuous static recrystallization(cSRX)to metadynamic recrystallization and cSRX,and finally to cSRX.The cSRX exists at all temperatures.This is because high stacking fault energy(56 mJ m−2)promotes the movement of dislocations,making the deformation mechanism of this steel is dominated by planar slip of dislocation.Large undissolved sigma precipitates promote static recrystallization through particle-stimulated nucleation.However,small strain-induced precipitates at grain boundaries hinder the nucleation of conventional SRX and the growth of recrystallized grains,while the hindering effect decreases with the increase in temperature.

Effect of boron on dissolution and repairing behavior of passive film on S31254 super-austenitic stainless steel immersed in H_(2)SO_(4) solution

The impact of boron on the dissolution and repairing behavior of passive films formed on S31254 super-austenitic stainless steel(SASS)was investigated.SASS was immersed in 0.5 mol/L of H_(2)SO_(4) for 0,2,6,10,and 14 days to explore the evolution of the passive film.The electrochemical impedance spectroscope(EIS),the Mott–Schottky analysis,and X-ray photoelectron spectroscope were utilized to analyze the semiconductor properties and compositions of the passive films.EIS showed a decrease and consequent increase over 14 days;the same pattern was observed for Cr_(2)O_(3) and Cr/Fe.However,the defect density of the passive film exhibited a reverse trend.The variation in film thicknesses indicated that the passive films possessed dissolution and repairing behavior.SASS passive film had a double-layer structure;the outer layer was found to be rich in Fe3+and Cr(OH)3,but low on Mo6+,while the inner layer was rich in Cr_(2)O_(3) and low in Mo4+.The addition of boron increased the corrosion resistance and could promote the efficiency of the passive film repair,likely by promoting the migration of Mox+,which promoted the repairing of the passive film.

On microstructure characterization of Fe-Cr-Ni-Mo-N super-austenitic stainless steel during hot deformation

Hot compression tests of Fe-Cr-Ni-Mo-N super-austenitic stainless steel were carried out in the temperature range of 950-1150℃with a strain rate of 0.01-10 s-1,in which dynamic recrystallization(DRX)mechanisms and coincident site lattice(CSL)boundary evolution in consideration of adiabatic heating were investigated.The results show that discontinuous DRX was the main DRX mechanism.Due to the high stacking fault energy(162-173 mJ/m2),subgrain evolution occurring in dynamic recovery process was stimulated at high temperatures and high strain rates.DRX behavior was accelerated by higher strain rate and adiabatic heating.Also,amounts of fine annealing twin boundaries were observed in the specimens deformed at higher strain rates and higher temperatures.By analyzing the length fractions of∑3 boundary in overall boundaries and in CSL boundaries,the results indicate that∑3 regeneration mechanism and new twinning mechanism take effect concurrently for twin-related grain boundary when the specimens were deformed at 950℃with a strain rate of 0.01-10 s-1.With increasing strain rate or deformation temperature,the propagation of∑3 boundaries was mainly dominated by new twinning mechanism.

Microstructure Evolution and Strain-Dependent Constitutive Modeling to Predict the Flow Behavior of 20Cr–24Ni–6Mo Super-Austenitic Stainless Steel During Hot Deformation

Hot compression tests were carried out with specimens of 20Cr-24Ni-6Mo super-austenitic stainless steel at strain rate from 0.01 to 10 s^-1 in the temperature range from 950 to 1150 ℃,and flow behavior was analyzed.Microstructure analysis indicated that dynamic recrystallization（DRX）behavior was more sensitive to the temperature than strain rate,and full DRX was obtained when the specimen deformed at 1150℃.When the temperature reduced to 1050 ℃,full DRX was completed at the highest strain rate 10 s-l rather than at the lowest strain rate 0.01 s-1 because the adiabatic heating was pronounced at higher strain rate.In addition,flow behavior reflected in flow curves was inconsistent with the actual microstructural evolution during hot deformation,especially at higher strain rates and lower temperatures.Therefore,flow curves were revised in consideration of the effects of adiabatic heating and friction during hot deformation.The results showed that adiabatic heating became greater with the increase of strain level,strain rate and the decrease of temperature,while the frictional effect cannot be neglected at high strain level.Moreover,based on the revised flow curves,strain-dependent constitutive modeling was developed and verified by comparing the predicted data with the experimental data and the modified data.The result suggested that the developed constitutive modeling can more adequately predict the flow behavior reflected by corrected flow curves than that reflected by experimental flow curves,even though some difference existed at 950℃ and 0.01 s^-1.The main reason was that plenty of precipitates generated at this deformation condition and affected the DRX behavior and deformation behavior,eventually resulted in dramatic increase of deformation resistance.

Unveiling the mechanism of yttrium significantly improving high-temperature oxidation resistance of super-austenitic stainless steel S32654

Aiming at serious catastrophic oxidation problem of super-austenitic stainless steel S32654,the influence of different rare earth elements on its oxidation behavior was comparatively investigated at 1200℃.The mechanism of Y significantly improving high-temperature oxidation resistance of S32654 was unveiled.The results demonstrated that Y played much better beneficial roles than Ce and La in the initial formation of oxide layer:(1)Y promoted Cr segregation to steel surface to combine with O;(2)its preferential oxidation provided nucleation cores for Cr_(2)O_(3).Both roles jointly promoted the selective oxidation of Cr and then the formation of protective Cr-rich oxide layer.This provided good prerequisites for inhibiting the formation and volatilization of MoO_(3).Additionally,Y cation segregation to oxide grain boundaries further promoted the selective oxidation of Cr and Si to form more protective oxide layer.These beneficial roles of Y essentially eliminated the synergistic effects of MoO_(3) volatilization and lamellar Cr_(2)N precipitation on catastrophic oxidation.Accordingly,the oxidation resistance of Y-bearing S32654 was improved by 22%–45%.

Effect of Grain Boundary Engineering on the Work Hardening Behavior of AL6XN Super‑Austenitic Stainless Steel

To examine the influence of grain boundary engineering(GBE)on the work hardening behavior,the tensile tests were carried out on the non-GBE and GBE AL6XN super-austenitic stainless steel(ASS)samples with a comparable grain size at two strain rates of 10^(-2)s^(-1)and 10^(-4)s^(-1).The evolution of deformation microstructures was revealed by transmission electron microscopy(TEM)and quasi-in situ electron backscatter diffraction(EBSD)observations.The results show that the influence of GBE on the mechanical properties of AL6XN super-ASS is mainly manifested in the change of work hardening behavior.At the early stage of plastic deformation,GBE samples show a slightly lowered work hardening rate,since the special grain boundaries(SBs)of a high fraction induce a higher dislocation free path and a weaker back stress;however,with increasing plastic deformation amount,the work hardening rate of GBE samples gradually surpasses that of non-GBE samples due to the better capacity of maintainable work hardening that is profited from the inhibited dislocation annihilation by SBs.In a word,the enhanced capacity of sustained work hardening effectively postpones the appearance of necking point and thus efficaciously ameliorates the ductility of GBE samples under the premise of little changes in yield strength and ultimate tensile strength.