Not found null

Default Latest Most Read
Please wait a minute...
For Selected: Toggle Thumbnails
Effect of Nitrogen Content on Precipitation Behavior and Mechanical Properties of Mn18Cr18NAustenitic Stainless Steel
Fengming QIN, Yajie LI, Xiaodong ZHAO, Wenwu HE, Huiqin CHEN
Acta Metall Sin    2018, 54 (1): 55-64.   doi:10.11900/0412.1961.2017.00291
Accepted: 31 October 2017

Abstract108)   HTML0)    PDF (1226KB)(607)      

Mn18Cr18N austenitic stainless steel with excellent mechanical properties and corrosion resistance is widely used in nuclear industries, power plants and medicine field. However, precipitation of the second phases during hot deformation deteriorates the mechanical properties and hot formability. In order to clarify the precipitation behavior of this steel, the precipitation behavior and its influence on mechanical properties of Mn18Cr18N austenitic stainless steel with different nitrogen contents were investigated by JmatPro software, OM, SEM and TEM analytical methods. The results indicate that precipitation phases consist of Cr2N and a few M23C6, in which Cr2N preferentially precipitates along grain bound aries and then grows up to the interior of austenite grain by discontinuous cellular. With increasing of ageing temperature, the precipitation of Cr2N became more sensitive. When the nitrogen content increases to 0.7%, the most sensitive precipitation temperature of Cr2N is 750 ℃ with an incubation period of 10 min. However, M23C6 mainly precipitates by granular at austenitic grain boundaries and maintains cube-on-cube orientation relationship with adjacent austenite grain. The results of mechanical property test indicate that the precipitation of Cr2N has a negligible effect on strength and obvious deterioration on plasticity of Mn18Cr18N austenitic stainless steel. The precipitation of Cr2N after ageing treatment leads to remarkable decrease in elongation and reduction of area, and the elongation reduced from 52.9% to 27.7%. Meanwhile, fracture mode also transformed from ductile fracture to intergranular fracture and transgranular fracture with the increasing of Cr2N. TEM analysis shows that solution treatment sample reveals good plastic deformation ability and coordinates deformation by slip and twinning, simultaneously. Nevertheless, dislocations slipped, propagated and eventually piled up between lamellas of Cr2N and around granular M23C6 after ageing treatment, which induce the degeneration of the plastic deformation capacity of Mn18Cr18N austenitic stainless steel.

Table and Figures | Reference | Related Articles | Metrics
Research on Hot Working Behavior of Low-NickelDuplex Stainless Steel 2101
Yusen SU, Yinhui YANG, Jianchun CAO, Yuliang BAI
Acta Metall Sin    2018, 54 (4): 485-493.   doi:10.11900/0412.1961.2017.00151
Accepted: 31 August 2017

Abstract88)   HTML2)    PDF (7433KB)(565)      

The thermal deformation difference of two phases for duplex stainless steel (DSS) makes hot working difficult, 2101 DSS substitute Mn, N for Ni to stabilize austenite phase, which will significantly affect hot deformation behavior. Hot compression tests in the temperature ranging from 1123 to 1423 K and strain rate ranging from 0.001 to 10 s-1 were carried out on a Gleeble-3800 thermal simulator for 2101 DSS. At the same strain rate, the flow curve characteristics of 2101 DSS changed from dynamic recrystallization (DRX) to dynamic recovery with increasing deformation temperature. Increasing deformation stain rate from 0.001 s-1 to 0.01 and 0.1 s-1 increased DRX temperature range, but higher strain rate of 1 and 10 s-1 is not beneficial to DRX occurrence. In the deformation temperature region of 1253~1323 K and low strain rate of 0.001~0.1 s-1, the smaller strain value corresponding to the peak stress, the austenite DRX is more likely to occur, which is beneficial to the equiaxed recrystallized grains formation. At low strain rate, the recrystallization grain grows up with the increase of deformation temperature, the worse effect of austenite DRX is related to weakened austenite stabilized ability of Mn substitution for Ni at high Zener-Hollomon parameter values. Based on the thermal deformation equation, the apparent activation energy Q was calculated as 464.49 kJ/mol, which is slightly higher than that of 2205 DSS, and the constitutive equation of the peak flow stress was established. By combining with flow curve and microstructure analysis, the processing map exhibits the optimum processing conditions are in deformation temperature ranging from 1220 to 1350 K and strain rate ranging from 0.001 to 0.1 s-1 with high power dissipation coefficient of 0.40~0.47, under which the austenite DRX obviously occurred.

Table and Figures | Reference | Related Articles | Metrics
Morphological Characteristics and Size Distributions of Three-Dimensional Grains and Grain Boundaries in 316L Stainless Steel
Tingguang LIU, Shuang XIA, Qin BAI, Bangxin ZHOU
Acta Metall Sin    2018, 54 (6): 868-876.   doi:10.11900/0412.1961.2017.00318
Accepted: 23 January 2018

Abstract123)   HTML10)    PDF (2969KB)(808)      

Three-dimensional characterization of grains and grain boundaries is significant to study the microstructure of polycrystalline materials, and is the key to advance the subject of three-dimensional materials science (3DMS). In this work, the technique of serial sectioning by mechanical polishing coupled with 3D electron backscatter diffraction (3D-EBSD) mapping was used to measure the microstructure of a 316L stainless steel in 3D. Volume of the collected 3D-EBSD microstructure is 600 μm×600 μm×257.5 μm, which is quite large to study the 3D microstructure of structural materials with conventional grain size (20~60 μm). Dream3D and in-house developed Matlab programs were used to process the 3D-EBSD data, and subsequently ParaView was used to visualize the grains and grain boundaries in 3D. Combined usage of these tools and in-house programs make the possibility that not only 3D grains but also 3D grain boundaries can be studied in both morphology and quantification. In total, 1840 grains and 9177 grain boundaries are included in the measured 3D-EBSD microstructure. The 3D morphological characteristics and size distributions of grains and grain boundaries in the 316L stainless steel were investigated, including 3D grain size, grain surface area, boundary quantity per grain, grain boundary size and the average boundary size per grain, as well as relationships between these morphological parameters were discussed. Results showed that distributions of all of these morphological parameters of 3D grains and grain boundaries in the polycrystalline 316L steel can be well represented by log-normal distribution, and all relationships of these parameters versus grain size can be well represented by power function. Additionally, the 3D morphologies of most grains in the 316L stainless steel deviate from the ideal equiaxed grains, having complex shapes due to existing of twins, such as semi-sphere shaped, plate shaped and some very complex grains. In many ways, the larger grains have more complex morphology with greater number of faces, larger surface area and larger deviation from equiaxed grains.

Table and Figures | Reference | Related Articles | Metrics
Direct Synthesis of NiCo2O4 Nanoneedles and MoS2 Nanoflakes Grown on 316L Stainless Steel Meshes by Two Step Hydrothermal Method for HER
Dan LI, Yang LI, Rongsheng CHEN, Hongwei NI
Acta Metall Sin    2018, 54 (8): 1179-1186.   doi:10.11900/0412.1961.2018.00001
Accepted: 09 May 2018

Abstract110)   HTML3)    PDF (5085KB)(547)      

The synthesis of nanostructures catalytic electrode for hydrogen evolution reaction (HER) plays an important role in national economy such as chlor-alkali industry, chemical power supply and fuel cell. Electro-splitting of water powered by electric energy has attracted extensive attention because this process can convert electric energy into chemical energy for easier storage and delivery. In this work, a facile and direct synthesis of NiCo2O4 nanoneedles and MoS2 nanoflakes grown on 316L stainless steel meshes substrate by two step hydrothermal method was reported. Initially MoS2 nanoflakes grown on the stainless steel (SS) meshes, and then NiCo2O4 nanoneedles were grown on MoS2/SS meshes at optimum conditions using hydrothermal method. The prepared nanostructures were characterized by SEM, TEM and XRD. Then a three-electrode system was used to test the property of HER. The results show that the as-prepared electrode exhibits good catalytic behavior towards HER. The onset overpotential and Tafel slope are 65 mV and 108 mV/dec respectively. When the current density reaches 100 mA/cm2, the overpotential is 219.6 mV. Furthermore, the composite structure exhibits good cycle stability in the same experimental conditions.

Table and Figures | Reference | Related Articles | Metrics
Evaluation of Fatigue Properties of CA6NM Martensite Stainless Steel Using Miniature Specimens
Yefei MA, Zhuman SONG, Siqian ZHANG, Lijia CHEN, Guangping ZHANG
Acta Metall Sin    2018, 54 (10): 1359-1367.   doi:10.11900/0412.1961.2018.00023
Accepted: 23 April 2018

Abstract119)   HTML9)    PDF (6624KB)(519)      

Since structural components in the nuclear power plant are unable to be disassembled during their in service process, it is an urgent and key problem how to quickly and non-destructively evaluate fatigue reliability of these key structural components by using miniature specimens. Fatigue properties of miniature specimens of CA6NM martensite stainless steel for impellers in the nuclear pump were obtained by using symmetrically bending fatigue loading and uniaxial tension-tension fatigue loading, respectively. A comparison of fatigue properties between the miniature specimens and bulk specimens was conducted to examine feasibility for the evaluation of fatigue reliability of the CA6NM steel using miniature specimens. The results show that tensile strength of the 40 μm-thick CA6NM specimens is slightly higher than that of the bulk specimens, but elongation of the 40 μm-thick specimens is lower than that of the bulk counterparts. In low cycle fatigue regime, fatigue strength of the 40 μm-thick specimens subjected to uniaxial tension-tension fatigue loading is lower than that of the standard bulk counterparts. With decreasing the applied stress amplitude, the difference in fatigue properties gradually decreases, and the fatigue limit of the miniature specimen is close to that of the bulk counterparts. Fatigue strength of the 40 μm-thick specimens subjected to bending fatigue loading is much higher than that subjected to uniaxial tension-tension fatigue loading, and also higher than that of the bulk counterparts. Fatigue strength of the miniature specimens is related to the loading mode. The difference in the fatigue mechanism between the miniature specimens and the bulk counterparts is discussed, and the feasibility to evaluate fatigue reliability of the steel using miniature specimens is addressed.

Table and Figures | Reference | Related Articles | Metrics
Distribution Characteristics of Twin-Boundaries in Three-Dimensional Grain Boundary Network of 316L Stainless Steel
Tingguang LIU, Shuang XIA, Qin BAI, Bangxin ZHOU, Yonghao LU
Acta Metall Sin    2018, 54 (10): 1377-1386.   doi:10.11900/0412.1961.2018.00062
Accepted: 27 June 2018

Abstract120)   HTML6)    PDF (5151KB)(392)      

Grain boundaries are sources of failure and weakness due to their relatively excess free volume compared to the lattice of polycrystalline materials exposed to aggressive environment. The control of grain boundary degradation has become one of the key issues of materials science and engineering. It has been found that the coincidence site lattice (CSL) boundaries, especially Σ3 (the twin boundaries), have stronger resistance to intergranular degradation than random boundaries. Materials with a high proportion of CSL boundaries that could disrupt the connectivity of random boundaries have better performance to resist intergranular failures. However, the distribution characteristics of twin boundaries in grain boundary network are still unclear. In this work, three-dimensional electron backscatter diffraction (3D-EBSD) was used to map the 3D grain boundary network of a 316L stainless steel. The topological characteristics of triple junction and quadruple junction in the presence of twin boundaries were investigated. The distribution of twin boundaries around grains and grain boundaries was analyzed. The results show that the twin boundary number fraction in the 3D grain boundary network is lower than the measured twin boundary area fraction, indicating that the average area per twin boundary is larger than random boundary. Most of triple junctions in the 316L stainless steel have one twin boundary. The proportion of triple junctions with two twin boundaries is about 9.4%. A quadruple junction has three twin boundaries at most. Most of quadruple junctions have one or two twin boundaries. About 7.9% of quadruple junctions have three twin boundaries. The 3D-EBSD data of 316L includes 1840 grains, 7353 random boundaries and 1824 twin boundaries. On average, a 3D grain in the 3D microstructure has 11 faces (39.85 neighboring faces that includes all boundaries of the grain and all boundaries that connected with the grain by lines or points), in which the number of twin boundaries is 2.03 (8.02) on average. A 3D grain boundary has 9.35 neighboring boundaries, in which the number of twin boundaries is 1.99 on average.

Table and Figures | Reference | Related Articles | Metrics
Research and Development of Maraging Stainless Steel Used for New Generation Landing Gear
Ke YANG, Mengchao U, Jialong AN, Wei NG
Acta Metall Sin    2018, 54 (11): 1567-1585.   doi:10.11900/0412.1961.2018.00356
Accepted: 30 August 2018

Abstract120)   HTML22)    PDF (11433KB)(730)      

Properties of landing gear are closely related to the service safety of aircraft. Thus, it is essential to improve the comprehensive properties of the material used for landing gear. This article briefly introduces the application status and existing problems of currently used landing gear materials, and then proposes future developing directions of landing gear materials. Finally, a new maraging stainless steel with high strength, high toughness and good corrosion resistance, which can be a promising steel for the new generation landing gear material, is introduced.

Table and Figures | Reference | Related Articles | Metrics
Precipitate Evolution in a Modified 25Cr-20Ni Austenitic Heat Resistant Stainless Steel During CreepRupture Test at 750 ℃
Guodong HU, Pei WANG, Dianzhong LI, Yiyi LI
Acta Metall Sin    2018, 54 (11): 1705-1714.   doi:10.11900/0412.1961.2018.00361
Accepted: 13 September 2018

Abstract151)   HTML2)    PDF (9763KB)(533)      

25Cr-20Ni austenitic heat resistant stainless steels are widely used as structural materials in nuclear industries and power plants for their excellent corrosion resistance and creep properties at elevated temperature. It is generally accepted that the precipitation during creep is a key factor influencing the creep properties. However, the evolution of precipitates is complicated due to the interaction of the alloy elements. To investigate the precipitation behaviors, a modified 25Cr-20Ni austenitic heat resistant stainless steel has been crept at 750 ℃ under different stresses varying from 100 MPa to 180 MPa. The microstructure observation indicates that M23C6 and (Nb, V)(C, N) precipitates are formed during 32.6 h creeping deformation under 180 MPa. M23C6 precipitates are mainly generated at grain boundaries and (Nb, V)(C, N) particles are dispersively distributed in austenitic matrix. The grain boundary M23C6 carbides are significantly coarsened and Ostwald ripening process happens during 98.1 h creeping deformation under the stress of 150 MPa and 353.0 h creeping deformation under stress of 120 MPa, while (Nb, V)(C, N) carbonitrides show high dimensional stability. With the creep rupture time further prolonging to 353.0 h and 752.3 h under the creep stress of 120 and 100 MPa, respectively, σ-phases are generated first at grain boundaries and then at inner grains. Meanwhile, large amounts of σ-phases are formed around (Nb, V)(C, N) particles, indicating the σ-phase precipitation is accelerated by (Nb, V)(C, N) carbonitrides. Composition analysis and thermodynamic calculation are subsequently performed to elucidate the precipitation mechanism of σ-phase. Carbon and nitrogen depleted zone is detected at the interface between (Nb, V)(C, N) precipitates and austenitic matrix. A correlation between σ-phase and C/N contents has been calculated by Thermo-Calc, which shows that the mass fraction of σ-phase increases with the decreasing C/N contents. According to the thermodynamic calculations and experimental results, it is reasonably inferred that the formation of σ-phase is induced by the carbon and nitrogen depletion in austenitic matrix. Additionally, the fracture surfaces of creep specimens show intergranular fracture under all creep stresses. When the creep time is comparatively short, cracks are inclined to propagate along grain boundaries owing to the low cohesion between grain boundary M23C6 precipitates and austenitic matrix, resulting in intergranular creep fracture. With the precipitation of σ-phase at grain boundaries after long time creep, the cracks are primarily generated from σ-phase, further deteriorating the creep elongation.

Table and Figures | Reference | Related Articles | Metrics
Study on Irradiation Assisted Stress Corrosion Cracking of Nuclear Grade 304 Stainless Steel
Ping DENG,Chen SUN,Qunjia PENG,En-Hou HAN,Wei KE,Zhijie JIAO
Acta Metall Sin    2019, 55 (3): 349-361.   doi:10.11900/0412.1961.2018.00359
Accepted: 14 November 2018

Abstract122)   HTML9)    PDF (24156KB)(390)      

Irradiation assisted stress corrosion cracking (IASCC) of austenitic stainless steel core components is one major concern for maintenance of nuclear power plants. Previous studies on the IASCC had mainly focused on the effect of irradiation on changes in deformation modes and interaction of dislocation channels with grain boundary. The role of corrosion in IASCC, however, has not received sufficient attentions. In the process of stress corrosion cracking (SCC), corrosion occurs simultaneously with localized deformation in the vicinity of the crack tip. This indicates that corrosion is one of the potential contributors to IASCC. In this work, IASCC of proton-irradiated nuclear grade 304 stainless steel (304SS) was investigated. The IASCC tests were conducted by interrupted slow strain rate tensile (SSRT) tests at 320 ℃ in simulated primary water of pressurized water reactor containing 1200 mg/L B as H3BO3 and 2.3 mg/L Li as LiOH·H2O, with a dissolved hydrogen concentration of 2.6 mg/L. Following the SSRT tests, the localized deformation, corrosion and IASCC of the specimens were characterized. The results revealed that increasing the irradiation dose promoted residual strain accumulation at slip steps and grain boundaries of nuclear grade 304SS. Since the slip step usually transmitted or terminated at the grain boundary, it eventually promoted localized deformation at the grain boundary. Specially, the slip step transmitted at grain boundary led to slip continuity at the grain boundary. In contrast, a slip discontinuity was observed at the grain boundary where the slip step terminated, which caused a much higher strain accumulation by feeding dislocations to the grain boundary region. Further, formation of the slip discontinuity was related to the Schmidt factor pair type of the adjacent grains. The irradiation resulted in a depletion of Cr and an enrichment of Ni at grain boundary, while the magnitude of Cr depletion and Ni enrichment increased with increasing the irradiation dose. Following the SSRT tests, intergranular cracking was observed on surfaces of the irradiated specimens, while the number of the cracks was increased by a higher irradiation dose and applied strain. This suggested a higher IASCC susceptibility of nuclear grade 304SS in the primary water. Meanwhile, significant intergranular oxidation ahead of the crack tip was observed, while both the width and length of the oxide were larger at a higher irradiation dose. The synergic effect of irradiation-promoted deformation and intergranular corrosion was the primary cause for the IASCC of the irradiated steel.

Table and Figures | Reference | Related Articles | Metrics
Effect of Solution Temperature on Tensile Deformation Behavior of Mn-N Bearing Duplex Stainless Steel
Miao JIN, Wenquan LI, Shuo HAO, Ruixue MEI, Na LI, Lei CHEN
Acta Metall Sin    2019, 55 (4): 436-444.   doi:10.11900/0412.1961.2018.00276
Abstract85)   HTML7)    PDF (19176KB)(316)      

Advanced duplex stainless steels (DSSs) in which Ni is mostly or completely replaced by Mn and N have been developed in recent years. Such Mn-N bearing DSSs can readily achieve exceptional room-temperature tensile properties through the transformation-induced plasticity (TRIP) effect of metastable austenite. During the processing of DSSs, solution treatment is a critical step that tailors the phase fraction and the overall properties. In particular, the phase chemistry can change due to different element partitioning between two constituents, resulting in a different TRIP kinetics, when DSS is solution treated at different temperature. In this work, the effect of solution temperature on tensile deformation behavior of a new Mn-N bearing DSS was studied. The mechanical properties and work-hardening characteristic of the steels solution treated at different solution temperature (1000~1200 ℃) were investigated by thermal modeling test, and the effects of solution temperature on the deformation substructure and fracture characteristics were analyzed by OM, SEM and EBSD. The results show that as the solution temperature increases, the yield strength and tensile strength of the steels decrease, while the elongation (uniform elongation and total elongation) increases firstly and then decreases. The steel solution treated at 1100 ℃ shows the optimum uniform elongation of 46.7%, and a better combination of ultimate tensile strength and ductility of approximately 44.6 GPa·%. The work-hardening rate of the steel shows a three-stage characteristic, namely it declines firstly and then increases and subsequently declines again as the strain increases. However, the increasing extent of the work-hardening rate decreases as the solution temperature increases. The strain-induced martensitic transformation (SIMT) of metastable austenite which causes the TRIP effect has two evolution mechanisms of γεα' and γα'. But SIMT can be suppressed when the solution temperature increases. The fracture surfaces of specimens solution treated at different temperatures show a quasi-cleavage mode, in which both ferrite and strain-induced martensite exhibit cleavage fracture while the residual austenite displays a dimple-mode fracture. Furthermore, the Md30 which can characterize the stability of metastable austenite was calculated, which decreases from 81 ℃ to 38 ℃ as the solution temperature increases from 1000 ℃ to 1200 ℃, indicating that the TRIP effect gets weakening at a higher solution temperature, and the work-hardening and plasticity therefore decrease.

Table and Figures | Reference | Related Articles | Metrics
Research on Dynamic Recrystallization Behavior of 23Cr-2.2Ni-6.3Mn-0.26N Low Nickel TypeDuplex Stainless Steel
Yahui DENG,Yinhui YANG,Jianchun CAO,Hao QIAN
Acta Metall Sin    2019, 55 (4): 445-456.   doi:10.11900/0412.1961.2018.00449
Accepted: 18 December 2018

Abstract74)   HTML2)    PDF (13546KB)(360)      

The difference of crystal structure and stacking fault energy (SFE) of two phases in duplex stainless steels (DSS) make different softening mechanism during hot deformation. Due to different austenite stability of Mn and Ni, the substitution of Mn for Ni will significantly affect dynamic recrystallization (DRX) behavior of compression deformation. The DRX behaviors of 23Cr-2.2Ni-6.3Mn-0.26N low nickel type DSS were studied in the deformation temperatures of 1073~1423 K and strain rates of 0.01~10 s-1 by using a thermal simulator. The results showed that the deformation procedure of samples are mainly softened by dynamic recovery (DRV) of two phases at low temperature and high deformation strain rate, and mainly softened by austenite DRX at high temperature and low deformation strain rate. At the low strain rates of 0.01 and 0.1 s-1, the grain size of austenite DRX increased with the increase of deformation temperature. The softening mechanism of samples are related to the Z parameter, and the deformation softening is mainly caused by austenite DRX under the condition of low Z value. Based on the thermal deformation equation, the apparent stress index of samples were calculated as 5.18, and the apparent activation energy of thermal deformation was calculated as 391.16 kJ/mol. The constitutive equation of the relationship between the peak flow stress and the Z parameter was established by hyperbolic sinusoidal model proposed by Sellars. The critical stress of DRX increases with increasing strain rate and decreasing deformation temperature, while the critical strain of DRX increases with the decrease of deformation temperature, and increases at first and then decreases with increasing strain rate (0.1~10 s-1) at low deformation temperature. The relationship between the DRX critical stress (strain) and peak stress (strain), as well as DRX characteristic parameters and Z parameter correlation models, and the austenite phase DRX volume fraction models were determined. Moreover, the DRX volume fraction models predict that the increase of strain rate and the decrease of deformation temperature can delay occurrence of DRX.

Table and Figures | Reference | Related Articles | Metrics
Effect of Annealing on Microstructure of Thermally Aged 308L Stainless Steel Weld Metal
Xiaodong LIN,Qunjia PENG,En-Hou HAN,Wei KE
Acta Metall Sin    2019, 55 (5): 555-565.   doi:10.11900/0412.1961.2018.00365
Accepted: 17 December 2018

Abstract96)   HTML3)    PDF (22244KB)(405)      

Austenitic stainless steel weld metal has been widely used as nozzle/safe-end joint and inner surface cladding of reactor pressure vessel, due to its good mechanical property and corrosion resistance. However, long-term thermal ageing at the service temperature (280~330 ℃) could induce hardening and embrittlement of the weld metal. To recover the thermal ageing embrittlement, the annealing treatment has been proposed since the annealing could affect the ageing-induced microstructural changes such as spinodal decomposition and G-phase precipitation in ferrite. However, there is still an incomplete understanding as well as a lack of nanoscale investigation about the annealing effect on the microstructural change of the weld metal. In this work, 308L stainless steel weld metal was thermally aged at 410 ℃ for 7000 h, followed by an annealing treatment at 550 ℃ for 1 h. Since the weld metal has a dual-phase structure of austenite and δ-ferrite, the phase transformation of austenite and δ-ferrite as well as the element segregation at the δ-ferrite/austenite phase boundary were investigated by TEM and atom probe tomography. The results revealed that austenite was unaffected by annealing while the ageing-induced spinodal decomposition of δ-ferrite was completely recovered. In addition, the number density of G phase in δ-ferrite was significantly reduced following annealing. This indicates that austenite has a higher stability compared with δ-ferrite. As for the δ-ferrite/austenite phase boundary, thermal ageing induced the segregation of Ni, Mn and C at the phase boundary, while the contents of Cr, Si and P remained almost unchanged. Following the annealing treatment, the segregation of all elements was eliminated. Further, only a small quantity of Ni and Mn was enriched in austenite near the phase boundary. The results suggested that the microstructure of the annealed specimen was similar to that of the unaged specimen, indicating a good recovery of the microstructure by annealing.

Table and Figures | Reference | Related Articles | Metrics
Effect of Hot Band Annealing Processes on Texture and Formability of 19Cr2Mo1W Ferritic Stainless Steel
Houlong LIU,Mingyu MA,Lingling LIU,Liangliang WEI,Liqing CHEN
Acta Metall Sin    2019, 55 (5): 566-574.   doi:10.11900/0412.1961.2018.00540
Accepted: 04 March 2019

Abstract71)   HTML3)    PDF (13885KB)(385)      

Low-cost ferritic stainless steels with excellent oxidation resistance and anti-corrosion ability are widely used in the fields of household appliances, hardware decoration, architectural structures, fuel cells and automobile exhaust systems. In order to achieve good formability of the ferritic stainless steel, the annealing process of hot-rolled sheet is crucial. As a newly developed 444-type heat-resistant ferritic stainless steel containing W and Ce, however, the influence of hot band annealing process of 19Cr2Mo1W ferritic stainless steel on its formability is not clear and need to have a deep understanding. In this work, the effect of annealing temperature of hot band on the microstructure, texture and formability of this steel was studied by means of XRD, EBSD, roughness measurement and formability test. The results indicated that although annealing processes were carried out at different temperatures after hot rolling, the characteristic of texture in the hot-rolled and annealed sheet was inherited to the cold-rolled sheet to some extent. The increased intensities of {223}<11ˉ0> and {111}<01ˉ1> texture components in the hot-rolled and annealed sheet were beneficial to improvement of the γ-fiber texture in the cold-rolled and annealed sheet. The extent of deviation from γ-fiber texture in the cold-rolled and annealed sheet was increased with increasing the intensities of {001}<11ˉ0>~{115}<11ˉ0> texture components in the cold-rolled sheet. An increased annealing temperature of the hot-rolled sheet could effectively weaken the intensities of {001}<11ˉ0>~{115}<11ˉ0> texture components in the cold-rolled sheet. In addition, the banded microstructures in the hot-rolled and annealed sheet were significantly reduced by increasing annealing temperature of the hot-rolled sheet, which improved the microstructure uniformity and formability of the cold-rolled and annealed sheet.

Table and Figures | Reference | Related Articles | Metrics
Fatigue and Cycle Plastic Behavior of 316L Austenitic Stainless Steel Under Asymmetric Load
Jian PENG,Yi GAO,Qiao DAI,Ying WANG,Kaishang LI
Acta Metall Sin    2019, 55 (6): 773-782.   doi:10.11900/0412.1961.2018.00377
Accepted: 06 March 2019

Abstract150)   HTML3)    PDF (11728KB)(552)      

Due to excellent mechanical property and corrosion resistance of 316L austenitic stainless steel, it is widely used in chemical industry, but its fatigue behavior under asymmetric cycle load is not well understood. In this work, the fatigue and cyclic plastic deformation behavior of 316L austenitic stainless steel under asymmetric tensile-tensile cycle loading are studied, focusing on the variations of fatigue life, cycle plastic deformation and fracture mechanism with applied cycle load. The high and low stress regions can be clearly divided based on the differences of fatigue life, cyclic strain amplitude, mean strain, mean strain rate and failure strain. In the high stress region, mean strain, mean strain rate and failure strain are large, resulting in the significant cyclic plastic deformation, and the fatigue life is short. In the low stress region, the cyclic plastic deformation accumulation is limited, and the fatigue life is significantly increased. Through microstructural observations near fracture area and fracture surface analyses, the differences between large stress region and low stress region can be found. In the high stress region, a large number of voids are generated near the fracture surface, and the fracture surface is mainly featured by dimples. In contrast, in the low stress region, the fatigue crack is found near the fracture surface, and its propagation direction is perpendicular to the loading direction. The fatigue crack initiation site, the fatigue crack propagation zone, transition zone and rapid fracture zone are found on the fracture surface. Results of fracture mechanism analyses suggest that, the high stress region of 316L austenitic stainless steel is the cyclic plastic deformation dominant region, and the failure mechanism is the ductile failure caused by the accumulation of cyclic plastic deformation; while the low stress region is the fatigue dominant zone, and the failure mechanism is the fatigue crack propagation failure.

Table and Figures | Reference | Related Articles | Metrics
Influence of Size Factor on Calculation Accuracy of Welding Residual Stress of Stainless Steel Pipe by 2D Axisymmetric Model
Peiyuan DAI,Xing HU,Shijie LU,Yifeng WANG,Dean DENG
Acta Metall Sin    2019, 55 (8): 1058-1066.   doi:10.11900/0412.1961.2018.00567
Accepted: 27 May 2019

Abstract124)   HTML2)    PDF (7704KB)(297)      

Austenitic stainless steel, owing to its good mechanical properties and excellent corrosion resistance, is widely used in petroleum, chemical, nuclear power and other fields. Welding is an extremely important manufacturing method in industrial production. When the thermal elastic-plastic finite element method (TEP-FEM) is used to simulate welding residual stress, especially in thick welded joints, a long calculation time is generally needed. Therefore, it has become an urgent problem to develop an efficient and high-precision computational approach to simulate welding residual stress. In this work, numerical simulation and experimental methods were combined to explore the effect of size on the calculation precision of welding residual stress of SUS316 stainless steel by the 2D axisymmetric model, in order to clarify the applicability of 2D axisymmetric model in the prediction of welding residual stress in pipe butt joints. This research can provide theoretical support for the development of computational methods suitable for engineering applications. Based on the finite element software MSC. Marc, the temperature field and welding residual stress distribution of three different sizes of pipes were calculated by 2D axisymmetric model and 3D model. The calculated residual stress distributions in the thin pipe model are compared with the experimental measurements. The results show that calculated residual stress by the 2D axisymmetric model agrees well with the 3D model. However, in the weld seam near the inner surface and the areas near the weld seam, a deviation on the residual stress distribution between in the 2D axisymmetric model and in the 3D model was observed, which is significant as the pipe size increases. For practical engineering applications, with the regardless of the stress problems at the beginning and end positions, the 2D axisymmetric model can be used instead of the 3D model to calculate the residual stress of the girth weld, which is very beneficial to calculation time saving.

Table and Figures | Reference | Related Articles | Metrics
Intrinsic Increment of Plasticity Induced by TRIP and Its Dependence on the Annealing Temperature in a Lean Duplex Stainless Steel
CHEN Lei , HAO Shuo , MEI Ruixue , JIA Wei , LI Wenquan , GUO Baofeng
Acta Metall Sin    2019, 55 (11): 1359-1366.   doi:10.11900/0412.1961.2019.00108
Accepted: 02 September 2019

Abstract58)   HTML0)    PDF (6212KB)(290)      

Recently, advanced lean duplex stainless steels (LDXs) with exceptionally good tensile properties by transformation-induced plasticity (TRIP) have been developed to respond to the skyrocketing raw material cost. In these new alloys, TRIP in the metastable austenite phase is expected to dominate overall deformation of the steels. Solution annealing, as a critical step of production processing, affects the austenite characteristics in LDXs, such as volume fraction and mechanical stability of austenite, which in turn influences its TRIP behavior. In order to further develop advanced LDXs, an assessment in the plastic increment of TRIP and its dependence on solution treatment are necessary. In this work, the tensile deformation test of a LDX which was annealed in the range of 1000~1200 ℃ was carried out on a Gleeble-3800 machine. The microstructural mechanism of work hardening characteristics was characterized by TEM, and the saturation of strain-induced martensite (SIM) under different conditions was calculated by XRD. Some quantitative indicators which can characterize the plastic increment of TRIP were proposed, including apparent plastic increment (Δe), average plastic increment (Δeˉ) induced by unit volume SIM and intrinsic plastic increment (Δe*) related only to mechanical stability of austenite. Meanwhile, their dependences on annealing temperature were discussed. The results show that SIM can develop in two ways of γεα′ and γα′ whereby the work hardening of the LDX exhibit a "three-stage" characteristic. There is a critical deformation temperature (Md) where the TRIP is absent at every annealing temperatures. The higher the annealing temperature is, the smaller the Md and the Δeare. As annealing temperature increases, Δeˉ increases, while Δe* decreases, indicating a fact that the more stable the austenite is, the smaller the intrinsic plastic increment of TRIP is. In addition, both Δeˉ and Δe* show a linear relationship with the austenite stability coefficient (k).

Table and Figures | Reference | Related Articles | Metrics
Annealing Process Optimization of High Frequency Longitudinal Resistance Welded Low-CarbonFerritic Stainless Steel Pipe
SHAO Yi , LI Yanmo , LIU Chenxi , YAN Zesheng , LIU Yongchang
Acta Metall Sin    2019, 55 (11): 1367-1378.   doi:10.11900/0412.1961.2019.00051
Accepted: 22 April 2019

Abstract109)   HTML0)    PDF (35775KB)(225)      

With the development of economy and technology, the application of ferritic stainless steel is becoming increasingly wider. 12Cr ferritic stainless steel has low carbon equivalent and good weldability, and it can not only be applied to a variety of conditions, but also reduce the production cost. High frequency longitudinal resistance welding is an advanced welding technology with high quality and efficiency. In this work, low-carbon ferritic stainless steel pipe has been joined successfully by high frequency longitudinal resistance welding. Microstructure characteristics and mechanical properties of the stainless steel pipe joint after annealing at different temperatures for 3 min were investigated by OM, SEM, TEM and mechanical testing. In the process of high frequency longitudinal resistance welding, the weld zone was heated quickly to a high austenization temperature which led to a coarse grain structure in this zone assisted by high pressure. The weld zone presented martenite and ferrite microstructure with irregular grain. As a result, the hardness of the weld zone reached 315 HV and the impact energy dropped to near zero. After annealing at 950 ℃ for 3 min, the decomposition of martensite was the main reason of the decrease of hardness (260 HV) in weld zone. The microstructure of weld zone was composed of ferrite and bainite, resulting in the increase of impact energy from 0 to 23 J.

Table and Figures | Reference | Related Articles | Metrics
Mechanical Characteristics of TRIP-Assisted Duplex Stainless Steel Fe-19.6Cr-2Ni-2.9Mn-1.6Si During Cyclic Deformation
CHEN Lei, HAO Shuo, ZOU Zongyuan, HAN Shuting, ZHANG Rongqiang, GUO Baofeng
Acta Metall Sin    2019, 55 (12): 1495-1502.   doi:10.11900/0412.1961.2019.00220
Accepted: 25 September 2019

Abstract66)   HTML2)    PDF (8552KB)(242)      

Duplex stainless steel (DSS) is a type of steel with ferritic-austenitic duplex structure. It has been widely used in the engineering field such as petrochemicals and oceans. Recently, a series of economical DSSs with TRIP effect have been developed by replacing Ni-Mo with Mn-N. Generally, most structural components are subjected to periodic alternating loads during service, and then cyclic deformation which causes different mechanical responses with monotonous loading condition occurs. In this work, the mechanical properties of a Mn-N bearing DSS Fe-19.6Cr-2Ni-2.9Mn-1.6Si during cyclic deformation condition were studied and the microstructural mechanism was characterized by TEM. The results show that the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel exhibits enhanced mechanical properties and a typical "three-stage" hardening characteristic due to TRIP effect under monotonic loading condition. Cyclic hardening/softening characteristics of the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel are sensitive to strain amplitude and the number of cycle (N). At a small strain amplitude, cyclic hardening occurs firstly when N<5 cyc, then cyclic softening starts and cyclic deformation gradually trends to a stabilization. At a large strain amplitude, after a rapidly cyclic hardening (N<5 cyc), the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel is continuously softened until failure and no stabilization occurs. The dislocation walls form in ferrite during cyclic deformation which responsible for the overall cyclic softening of the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel; While austenite undergoes cyclic induced ε martensite transformation at large strain amplitude whereby the softening is suppressed, so that the cyclic softening rate of the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel increases rapidly with the increase of the (plastic) strain amplitude, followed by a slow increase and a final decrease. Compared with the monotonous loading condition, the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel shows a law of "hardening→softening→ re-hardening" with the increase of strain amplitude. In particular, there is a three-stage linear relationship between logarithmic cyclic stress amplitude and logarithmic plastic strain amplitude (lgσa-lgεa), and the corresponding cyclic hardening index (n') are: 0.16 (stage I), 0.09 (stage II) and 0.17 (stage III), respectively. The change of n' in each stage is related to the coordinated deformation between two phases (I→II) and the cyclic induced ε martensitic transformation (II→III).

Table and Figures | Reference | Related Articles | Metrics
Tensile Properties of Selective Laser Melted 316L Stainless Steel
YU Chenfan, ZHAO Congcong, ZHANG Zhefeng, LIU Wei
Acta Metall Sin    2020, 56 (5): 683-692.   doi:10.11900/0412.1961.2019.00278
Accepted: 25 September 2019

Abstract287)   HTML4)    PDF (3487KB)(413)      

Selective laser melting (SLM), as the most common additive manufacturing (AM) method, is capable of manufacturing metallic components with complex shape layer by layer. Compared with conventional manufacturing technologies such as casting or forging, the SLM technology has the advantages of high degree accuracy, high material utilization rate and environmentally friendly, and has attracted great attention in the fields of aerospace, nuclear power and medicine. The 316L austenitic stainless steel is widely used in the industrial field because of the excellent corrosion resistance and plasticity. It is also one of the commonly used material systems for SLM. In this work, the tensile properties and fracture mechanism of 316L stainless steel fabricated via SLM technology were investigated. The microstructure of the SLMed 316L specimens after tensile fracture was characterized and analyzed. The results show that the SLMed 316L stainless steel has a relatively desirable combination of strength and ductility, and its tensile performance is obviously better than that of 316L stainless steel prepared by traditional methods. The nanometer-scale cell structure inside the grain contributes to the improvement of strength. Deformation twins were observed in the SLMed 316L stainless steel after tensile test. The appearance of twins is oriented-dependent, and it is easy to occur in the grain with the direction near <110>-<111>.

Table and Figures | Reference | Related Articles | Metrics