Dening Zou

Influence of Tempering Process on Mechanical Properties of 00Cr13Ni4Mo Supermartensitic Stainless Steel

To investigate the influence of tempering process on microstructural evolutions and mechanical properties of 00Cr13Ni4Mo supermartensitic stainless steel (SMSS), specimens were tempered in the temperature range of 520– 720 °C for 3 h followed by air cooling and an optimized tempering temperature was chosen to prolong holding time from 3 to 12 h. After heat treatments, microstructure examination was conducted by scanning electron microscope, X-ray diffraction examinations, hardness measurements and tensile tests. The results revealed that the superior mechanical properties were achieved by quenching at 1040 °C for 1 h+water cooling and tempering at 600 °C for 3 h+ air cooling. Increasing isothermal tempering time could improve the toughness notably. It was believed that the property was correlated with the microstructure of tempered lath martensite and retained austenite. More retained austenite content is beneficial to the higher toughness of the SMSS.

Effect of Rare Earth on Microstructures and Properties of High Speed Steel With High Carbon Content

The effects of rare earth (RE) on the microstructures and properties of high carbon high speed steel (HCHSS) were investigated. The results show that when suitable RE is added to the HCHSS, the effect of RE on the austenite and eutectic carbides is obvious. The austenite grain and coarse eutectic structure are refined, and flake carbides in the eutectic structures become short and fine. After heat treatment, most of the eutectic carbides are spheroidized and distributed in a uniform manner. The hardness and red hardness of modified HCHSS are slightly increased; impact toughness is greatly increased by 37. 81% and reaches 10. 17 J/cm2. The mechanism by which RE improves the structures and properties of HCHSS is also analyzed.

Investigation on hot deformation of 20Cr–25Ni superaustenitic stainless steel with starting columnar dendritic microstructure based on kinetic analysis and processing map

Abstract The deformation behaviour of a 20Cr–25Ni superaustenitic stainless steel (SASS) with initial microstructure of columnar dendrites was investigated using the hot compression method at temperatures of 1000–1200°C and strain rates of 0·01–10 s−1. It was found that the flow stress was strongly dependent on the applied temperature and strain rate. The constitutive equation relating to the flow stress, temperature and stain rate was proposed for hot deformation of this material, and the apparent activation energy of deformation was calculated to be 516·7 kJ mol−1. Based on the dynamic materials model and the Murty’s instability criterion, the variations of dissipation efficiency and instability factor with processing parameters were studied. The processing map, combined with the instability map and the dissipation map, was constructed to demonstrate the relationship between hot workability and microstructural evolution. The stability region for hot processing was inferred accurately from the map. The optimum hot working domains were identified in the respective ranges of the temperature and the strain rate of 1025–1120°C and 0·01–0·03 s−1 or 1140–1200°C and 0·08–1 s−1, where the material produced many more equiaxed recrystallised grains. Moreover, instability regimes that should be avoided in the actual working were also identified by the processing map. The corresponding instability was associated with localised flow, adiabatic shear band, microcracking and free surface cracks.

Phase Transformation and Its Effects on Mechanical Properties and Pitting Corrosion Resistance of 2205 Duplex Stainless Steel

The effects of phase transformation on mechanical properties and pitting corrosion of 2205 duplex stainless steel were investigated. The amount of σ phase in the test specimen varied up to a maximum of 6% by thermal treatment at 850 °C for up to 60 min. The results showed that σ phase markedly increased the hardness and decreased the impact toughness of the test steel. But the increasing tendency of the ultimate tensile strength and the yield strength was not obvious, while the total elongation abruptly decreased with the aging time from 5 to 60 min. SEM impact microfractograph analysis revealed that the types of impact fracture changed from ductile mode to transcrystalline mode when the specimens were aged for 5—60 min. Furthermore, the extent of pitting potential reducing was found to be strongly temperature dependent, more pronounced at the higher temperature. During the incubation period of σ phase nucleation, the pitting corrosion test temperature and the aging time had collaborative effects on evidently displacing the pitting potential towards less noble values. After 15 min, the higher temperature contributed more to decreasing the pitting potential than the aging time.

Corrosion Resistance and Semiconducting Properties of Passive Films Formed on 00Cr13Ni5Mo2 Supermartensitic Stainless Steel in Cl− Environment

The semiconducting properties of passive films grown on 00Cr13Ni5Mo2 supermartensitic stainless steel were investigated in comparison with conventional 2Cr13 martensitic stainless steel. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were used for the studies. 00Cr13Ni5Mo2 steel exhibited a good corrosion resistance performance, attributing to its passive capability. The results of Mott-Schottky analysis demonstrated n-type semiconductors for the passive films with doping densities of about 1020–1021 cm−3, and the thickness of space-charge layers was also calculated. The experimental results confirmed that Mo plays an important role in improving the corrosion resistance of 00Cr13Ni5Mo2 steel due to its impact on the doping density.

On dynamic recrystallisation under hot working of superaustenitic stainless steel

Abstract Dynamic recrystallisation (DRX) behaviour in 904L superaustenitic stainless steel was studied by isothermal hot compression tests conducted in the temperature range of 900–1200°C and strain rate range of 0·001–10 s−1. The critical conditions for initiation of DRX are identified from the stress dependence of the derivative of strain hardening rate versus stress curves (dθ/dσ versus σ curves). The Z parameters combined with the temperature and strain rate effects during hot deformation are obtained by the regression analysis. Moreover, the kinetics model of DRX is established and the complete DRX grain size of 904L superaustenitic stainless steel is a power-law function of Z parameter with an exponent of –0·27.

Influence of Heat Treatment Temperature on Microstructure and Property of 00Crl3Ni5Mo2 Supermartensitic Stainless Steel

The microstructural evolution and mechanical property of 00Cr13Ni5Mo2 supermartensitic stainless steel (SMSS) subjected to different heat treatments were investigated Room tensile tests, hardness tests, scanning electron microscopy, transmission electron microscopy and X-ray diffraction were conducted on the heat-treated steels. It is found that the microstructure of the heat-treated steel is composed of tempered lath martensite, retained austenite and δ-ferrite. The austenitizing temperature and tempering temperature have a significant effect on the microstructural changes, which leads to the complex variations of mechanical properties. The fine tempered lath martensite and more dispersed reversed austenite in the microstructure facilitate improving the comprehensive mechanical properties of the studied steel. The optimal heat treatment process of 00CrNi5Mo2 SMSS is obtained by austenitizing at 1000 °C for 0.5 h+air cooling followed by tempering at 630 °C for 2 h+ air cooling, where the excellent combination of tensile strength, elongation and hardness can be achieved.

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

The metadynamic recrystallization (MDRX) behavior of as-cast 904L superaustenitic stainless steel was investigated by double pass isothermal compression tests at temperatures of 950 – 1150 °C, 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 interval time) on the flow curves and microstructural evolution were discussed. The MDRX fraction increased obviously 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 (correlation coefficient R2 =0. 98), indicating a satisfactory accuracy.

Formation Mechanism of Oxide Scales Grown on X10CrAlSi18 and 310S Heat-Resistant Stainless Steels Under High Temperature in Air

To promote the application of resource-saving heat-resistant stainless steels, the study on high temperature oxidation behavior and oxide-scales formation mechanism of X10CrAlSi18 ferritic heat-resistant stainless steels (FHSSs) and 310S austenitic heat-resistant stainless steels (AHSSs) was carried out in air up to 140 h, and the microstructure of oxide scales, oxidation kinetics and thermodynamics theories, and the classical hypothesis of the third-element effect analyzed. The results showed that the cost-effective X10CrAlSi18 FHSSs presented good oxidation resistance at 800 and 900 °C due to the compact multicomponent oxide scales composed of Al2O3, Cr2O3, MnCr2O4 and MnFe2O4 and good adhesion to the matrix. However, the oxidation resistance of the alloy at 1000 °C was deteriorated, which is mainly due to the non-protective Fe2O3. The oxide scales of 310S AHSSs containing Cr2O3, MnCr2O4 and inner SiO2 exhibited good oxidation resistance, while the internal oxidation of silicon weakened the adhesion to the substrate.