Influence of the Carbo-Chromization Process on the Microstructural,Hardness, and Corrosion Properties of 316L Sintered Stainless Steel

We report on the changes on the microstructural, hardness, and corrosion properties induced by carbo-chromization of 316L stainless steel prepared by Spark Plasma Sintering technique. The thermo-chemical treatments have been performed using pack cementation. The carburizing and chromization were carried out between 1153 K (880 °C)/4 h to 1253 K (980 °C)/12 h and 1223 K (950 °C)/6 h to 1273 K (1000 °C)/12 h in a solid powder mixture of charcoal/BaCO₃ and ferrochromium/alumina/NH₄Cl, respectively. The obtained layers were investigated using X-ray and electron diffraction, optical and scanning electron microscopies, Vickers micro-hardness, and potentiodynamic measurements. The thickness of the carbo-chromized layer ranges between 300 and 500 μm. Besides the host γ-phase, the layers are mainly constituted of carbides (Fe₇C₃, Cr₂₃C₆, Cr₇C₃, and Fe₃C) and traces of α′-martensite. The average hardness values decrease smoothly from 650 HV at the sample surface down to 200 HV at the center of the sample. The potentiodynamic tests revealed that the carbo-chromized samples have smaller corrosion resistance with respect to the untreated material. For strong chromization regimes, the corrosion rate is increased by a factor of four with respect to that of the untreated material, while the micro-hardness of the layer is three times larger. Such materials are suited to be used in environments where good corrosion resistance and wear properties are required.     

316L奥氏体不锈钢的氮合金化

采用金相显微镜、XRD、拉伸试验机及高低温冲击试验机等,并结合Thermo-calc软件计算研究了氮对316L奥氏体不锈钢微观组织、析出相、力学性能和耐点蚀性能的影响.结果表明:氮合金化能够抑制316L不锈钢中σ相和Chi相的析出,增加Cr2N的析出倾向,对奥氏体晶粒细化不明显;氮的添加能够提高316L不锈钢的室温强度和-100℃以上温度的夏比冲击功,降低-100℃以下的夏比冲击功,但对室温拉伸塑性影响不明显.此外,氮能够改善316L不锈钢的耐点蚀能力.     

Influence of Inclusion and Specimen Orientations on Intergranular Corrosion Testing of AISI 316LN Stainless Steel

Fabricated components must be free from sensitization for using these in critical applications in aggressive environments. During fabrication of a hollow bar from solid bar, deep hole drilling was employed which introduces residual stresses. Stress-relieving heat treatment was employed by heating the hollow bar from room temperature to 1,065°C @ 40°C/h and soaking at 1,065°C for 1 h followed by cooling @ 40°C/h as well as 70°C/h. To detect the susceptibility to IGC, specimens were taken from both circumferential direction as well as longitudinal direction and subjected to ASTM A 262 Practice E test. In U bend, the specimens from the circumferential direction failed whereas longitudinal specimens did not fail. However specimens of both orientations showed Step structure in Practice A test indicating that no carbide has nucleated during the stress-relieving heat treatment ensuring that the cooling rates are faster than the critical cooling rates and the material is not susceptible to IGC. EDAX studies indicated the presence of numerous MnS inclusions enriched in chromium which might have led to chromium depletion around the inclusions resulting in poor passivity at these locations. This study presents the influence of orientation of MnS inclusions in causing failure in U bend test. The need to select specimens of correct orientation during IGC testing is emphasized in this investigation.     

316L不锈钢换热器的失效原因分析

利用化学法、金相法、SEM和EDX等方法,分析了苯酚中间再沸器316L不锈钢换热器管束和筒体失效材料的化学成分、显微组织、夹杂物水平、腐蚀产物的成分和裂纹形貌等.结果表明316L不锈钢的化学成分、显微组织和夹杂物都符合标准,换热器管束和筒体失效的主要原因是Cl-诱导的应力腐蚀开裂,此外,苯酚在裂纹处的结晶与溶解加速了应力腐蚀裂纹的扩展.针对上述应力腐蚀问题提出了相应的改进措施.     

Effect of Surface Mechanical Attrition Treatment on Corrosion Behavior of 316 Stainless Steel

 The nanocrystalline microstructure of the surface of 316 stainless steel (316SS) induced by surface mechanical attrition treatment (SMAT) was determined by X ray diffraction (XRD) and scanning electron microscopy (SEM). The technique of hydrogen embrittlement was first used to obtain the information of the brittleness cleavage plane. The effects of SMAT and the following annealing process on the corrosion behavior of 316SS were investigated by potentiodynamic polarization curves and potentiostatic critical pitting temperature measurements. The results demonstrated that the nanocrystalline layer with an average grain size of 19 nm was produced. However, there were lots of cracks on the surface, which led to the degradation in the corrosion resistance of 316SS after SMAT. Nevertheless, after annealing treatment, the corrosion resistance of the nanocrystalline surface had been improved greatly. The higher the annealing temperature, the better was the corrosion resistance.     

316L不锈钢的疲劳裂纹扩展行为试验

 工程构件普遍承受疲劳载荷,从而导致疲劳失效。针对由316L不锈钢制成的标准紧凑拉伸试样,开展了一系列疲劳裂纹扩展试验。试验内容包括不同应力比下的常幅加载和在常幅加载过程中引入单个拉伸过载峰。试验结果表明：316L不锈钢具有很强的应力比效应,裂纹扩展速率随应力比的增大而增大。在引入单个拉伸过载峰后,观察到出现迟滞效应前发生了短暂的加速扩展现象。通过一种新的双参数模型来描述材料的应力比效应,并使用改进的Wheeler模型对过载后的裂纹扩展行为进行预测。预测结果表明：该方法能够更好地描述不同工况下316L不锈钢的疲劳裂纹扩展行为。     

Gas Tungsten Arc Welding of 316L Austenitic Stainless Steel with UNS S32205 Duplex Stainless Steel

In the present work, dissimilar welding between UNS S32205 duplex stainless steel (DSS) and 316L austenitic stainless steel (ASS) was performed by using gas tungsten arc welding and ER2209 filler at two different heat inputs (0.52 and 0.98 kJ/mm). Microstructures were characterized using reflected light optical microscope and scanning electron microscope. Micro-hardness and tensile properties were measured across the weld for both the heat inputs. The microstructure of the welded region was primarily austenitic (for both heat inputs) with Widmanstätten morphology. The grain size of the heat affected zone on DSS side was very large (~200 µm) for the high heat input sample with the presence of partially transformed austenite and acicular austenite. The precipitation of intermetallic phases and carbides was not observed for both the heat inputs. The proportion of ferrite in the weld metal (as measured by feritscope) was higher for the high heat input sample than the low heat input sample. During the tensile test, fracture occurred in 316L ASS base metal (because of its lower strength) in ductile manner. For high heat input welds, the impact tested sample showed the presence of fine spherical precipitates rich in Cr, Mn and Fe in the fracture surface of weld metal.     

Fe-Mo-B Enhanced Sintering of P/M 316L Stainless Steel

Liquid-phase enhanced sintering of powder metallurgy（P/M）316L stainless steel by addition of sintering aids was studied.2%-8% of pre-alloyed Fe-Mo-B powder with two different particle sizes was added as sintering aids,and the specimens were sintered in vacuum at 1 200-1 350 ℃.The results show that the fine Fe-Mo-B powder（5-10 μm）has stronger activated effect.The sintered density increases with the increase in sintering aid content or sintering temperature.Warm compaction has a better effect on the control of dimensional precision of compacts.The prealloyed Fe-Mo-B powder deviated from Mo2FeB2 component can also be sintering aid of P/M 316L stainless steel.     

Effects of Particle Sizes on Sintering Behavior of 316L Stainless Steel Powder

In rapidly evolving powder injection molding technology, the wide prevalence of various microstructures demands the powders of smaller particle sizes. The effects of particle size on the sintering behavior are critical to not only shape retention of microstructure but also its mechanical properties. This study investigates the effects of three different particle sizes on the sintering behavior of the 316L stainless steel (STS316L) samples, prepared by powder injection molding, via the dilatometry experiments. For this purpose, the STS316L powders of three different mean particle sizes, i.e., 2.97, 4.16, and 8.04 μm, were produced for STS316L. The samples for the dilatometry test were prepared through powder-binder mixing, injection molding, and solvent and thermal debinding. Dilatometry experiments were carried out with the samples in a H₂ atmosphere at three different heating rates of 3, 6, and 10 K/min. The shrinkage data obtained by dilatometry experiments was collected and analyzed to help understand the densification and the sintering behaviors in terms of particles size and heating rate. The master sintering curve (MSC) model was used to quantify the effects of particle sizes. In addition, we investigated the microstructure evolutions in terms of particles sizes.     

Studies on Direct Laser Cladding of SiC Dispersed AISI 316L Stainless Steel

In the present study, attempts have been made to develop SiC dispersed (5 and 20 wt pct) AISI 316L stainless steel matrix composite by direct laser cladding with a high power diode laser. Direct laser cladding has been carried out by melting the powder blends of AISI 316L stainless steel and SiC (5 and 20 wt pct) and, subsequently, depositing it on mild steel (0.15 pct C steel) in a layer by layer fashion to develop a coupon of 100 mm² × 10 mm dimension. A continuous, defect-free (microcracks and micro- or macroporosities), and homogeneous microstructure is formed, which consists of a dispersion of partially dissolved SiC (leading to formation of very low fraction of Cr₃C₂ and Fe₂Si) in grain-refined austenite. The microhardness of the clad layer increases from 155 VHN to 250 to 340 VHN (for 5 wt pct SiC dispersed) and 450 to 825 VHN (for 20 wt pct SiC dispersed) as compared to 155 VHN of commercially available AISI 316L stainless steel. The corrosion rate in 3.56 wt pct NaCl solution is significantly reduced in 5 wt pct SiC dispersed steel; however, 20 wt pct SiC dispersed steel showed a similar behavior as the commercially available AISI 316L stainless steel. The processing zone for the development of a defect-free microstructure with improved properties has been established.     

氮对316LN奥氏体不锈钢力学性能和耐蚀性的影响

试验用316LN钢(/%:0.015C、0.65Si、0.90Mn、17.3Cr、12.8 Ni、2.6Mo、0.018~0.200N)由50 kg真空感应炉冶炼,破真空后加入氮化铬铁,铸锭锻成Φ20 mm棒材和热轧成4 mm板材,并分别经1 100℃30 min和10min水淬固溶处理。研究结果表明,316LN不锈钢每增加0.010%的氮,抗拉强度提高9 MPa,屈服强度提高7 MPa;伸长率降低0.55%,氮含量对断面收缩率没有影响,约保持在72.5%;氮强烈提高316LN不锈钢的耐点腐蚀性能,每增加0.010%的氮,其点蚀击穿电位提高7 mV;添加适宜的氮(0.079%N),可以改善316LN不锈钢的耐晶间腐蚀性能,过高的氮含量(超过0.120%N)对晶间腐蚀性能有害。     

Characteristics of Mechanical Properties and Microstructure for 316L Austenitic Stainless Steel

 A comparative study on mechanical properties and microstructure of 316L austenitic stainless steel between solution treated specimen and hot rolled specimen was conducted. After a specimen was subjected to solution treatment at 1050 ℃ for 6 min, its mechanical properties were determined through tensile and hardness tests. Based on the true stress vs true strain and engineering stress vs engineering strain flow curves, the work hardening rate has been explored. The results show that the solution treated specimen has an excellent combination of strength and elongation, and that this steel is easy to work-hardening during deformation. Optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffraction examinations were conducted, these reveal that twins in 316L austenitic stainless steel can be divided into suspended twin and transgranular twin which have different formation mechanisms in growth, and that the deformation induced martensite nucleated and grown in the shear band intersections can be observed, and that the fracture surfaces are mainly composed of dimples and exhibit a tough fracture character.     

粉末316L不锈钢热挤压工艺

研究了氮气雾化316L粉的真空碳热还原烧结规律和挤压坯密度对热挤压工艺和挤压材性能的影响。获得了1种粉末316L钢无包套热挤压新工艺,即氮气雾化制粉→粉末振实真空烧结→热挤压。由于在真空烧结中碳对氧化物的还原作用,使316L雾化钢水的碳含量可以适当提高,简化了熔炼工艺。     

Hydrogen Softening in the Thin Plate of Microcrystalline 316L Stainless Steel

The thin‐plate specimen of 316L austenite stainless steel was charged with hydrogen using a cathodic charging technique. Despite the short diffusion distance of hydrogen predicted by the diffusion‐controlled model for a semi‐infinite sheet, the Vickers hardness measurements revealed the full effect of hydrogen in the center of the cross‐sections of thin‐plate specimens as well as in the vicinity of the outer surfaces, which appears to be due to the short‐circuit diffusion mechanism along the grain boundaries. The room‐temperature tensile properties of both undeformed and deformed (20, 40%) samples were examined and compared. Hydrogen softening was apparent in both types of samples. For example, the 40% deformed sample showed an approximately 17 and 7% lower yield and tensile strength, respectively, after H charging at a strain rate of 2 × 10^?4 s^?1 with a concomitant decrease in ductility compared to that without H.     

Microstructure and Texture Evolution During Cold Rolling of 316L Stainless Steel

Metallurgical and Materials Transactions A - The evolution of the deformation microstructure and the local crystallographic orientations are investigated for 10, 30, 50, and 80 pct cold...     

扫描速度对激光快速成型316L不锈钢组织性能的影响

研究了在激光功率900W条件下，扫描速度对激光快速成型不锈钢零件组织，性能的影响，在各扫描速度下，所成型的不锈钢薄壁板件的组织均为枝晶组织，随扫描速度提高，不锈钢薄壁板的定向生长树枝晶组织细化，枝晶间距变小，同时其沿X方向的机械性能（抗拉强度及硬度）降低。     

Stress Corrosion Behavior of Low-temperature Liquid-Nitrided 316 Austenitic Stainless Steel in a Sour Environment

Low-temperature nitridation is a widely used surface heat treatment. Low-temperature liquid nitridation was applied to 316 austenitic stainless steel and an S-phase (expanded austenite) layer was achieved on the alloy surface. The effect of the S-phase layer on corrosion resistance and stress corrosion cracking was investigated in a sour environment. When a bending stress of 164 MPa (80 pct yield stress, YS) was applied, no macroscopic corrosion cracking and pits were observed on the nitrided samples and the S-phase layer stayed intact. Although no macroscopic corrosion cracking was observed on the non-nitrided samples under 205 MPa (100 pct YS), some pits were formed on the alloy surface. This could be attributed to the high stresses and hardness, and the excellent corrosion resistance of the S-phase layer introduced by low-temperature nitridation. Supersaturated nitrogen atoms in the S-phase layer can effectively prevent the decrease in pH of the corrosive medium and accelerate the alloy repassivation kinetics. However, when the bending stress was increased to 205 and 246 MPa (100 pct YS, 120 pct YS), macroscopic cracks were observed in the presence of both tensile stress and a corrosive medium.