Sinterability of Y2O3-Al2O3 particulate stainless steel matrix composites

P/M 316L austenitic stainless steel has been reinforced with yttria and alumina particles. In order to improve the sintering behaviour of these composite materials, chromium diboride and boron nitride were added. The sinterability of the different materials has been characterised through dilatometry and sintering curves (sintered density vs. sintering temperature). A metallographic study by SEM coupled with microprobe has also been performed. Composites materials present a good densification. Chromium diboride and boron nitride react with the matrix in different manners, but they both greatly improve the sinterability of reinforced materials. The optimum sintering temperature for these composites materials is 1250 °C.     

Production of injection moulded 316L stainless steels reinforced with TiC(N) particles

Abstract

Metal matrix composites, based on 316L stainless steel and reinforced with TiC and TiCN particles, were manufactured following a powder injection moulding route: mixing, preparation of feedstock, moulding, debinding and sintering. The 316L stainless steel and carbide powders were dry mixed and moulded with wax based binder. The critical powder loading for injection moulding was 62·5 vol.-% for all samples. Binder debinding was performed by solvent and thermal method. After debinding, the samples were sintered at 1250 and 1385°C for 1 h in pure H₂. Metallographic studies were conducted to extend densification and the corresponding microstructural changes. The sintered samples were characterised by measuring tensile strength, hardness and wear behaviour. Wear loss was determined for all samples after wear tests. All powder, fracture surfaces of moulded and sintered samples, and worn surfaces of all the samples, were examined using scanning electron microscope. The sintered density of injection moulded 316L stainless steel samples, reinforced and unreinforced, increases with increasing sintering temperature. The addition of TiC and TiCN improves the hardness and wear resistance with increasing sintering temperature.     

Microstructure and abrasive behaviors of TiC-316L composites prepared by warm compaction and microwave sintering

316L stainless steel composites with various weight fractions of TiC particles were prepared using warm compaction and microwave sintering. Abrasion resistance measurements were used to study the abrasive behaviors of TiC-316L stainless steel composites. The effects of TiC content and preparation methods on the microstructure and mechanical properties of 316L stainless steel composites have been investigated. The results showed that the sample prepared by warm compaction and microwave sintering exhibited significantly superior densification, higher hardness, and better abrasion resistance when compared with conventionally processed counterpart. TiC particles reinforcement improved the abrasion resistance of 316L stainless steel, and the abrasion resistance of the composites was considerably better than that of the 316L stainless steel. The volume loss initially decreases with increasing TiC content up to 5 wt.%, it then slightly increases as increase the TiC particles content to 10 and 15 wt.%. In this present abrasion tests, the composites using 5 wt.% TiC addition offers a high abrasion resistance.     

Microstructure and wear properties of spark plasma sintered 316L-30W composites

ABSTRACT

316L-30W composites were successfully fabricated via spark plasma sintering at 1550°C to evaluate their potential as the interlayer between W and 316L stainless steel in fusion reactors. The effect of holding time on the microstructure and its subsequent effects on the fracture morphology and wear properties of the composites were investigated. The results show that more W particles melted and reacted with 316L steel as the holding time increased from 1 to 5 min. The generation of voids was mainly caused by the differences in diffusivity and the coefficient of thermal expansion between W and 316L. The 316L-30W composite held for 3 min had a smaller and more stable friction coefficient, indicating that its interface was firmly bonded and homogeneous.

This paper is part of a thematic issue on Nuclear Materials.     

Nickel-free stainless steel avoids neointima formation following coronary stent implantation

Abstract

SUS316L stainless steel and cobalt–chromium and platinum–chromium alloys are widely used platforms for coronary stents. These alloys also contain nickel (Ni), which reportedly induces allergic reactions in some subjects and is known to have various cellular effects. The effects of Ni on neointima formation after stent implantation remain unknown, however. We developed coronary stents made of Ni-free high-nitrogen austenitic stainless steel prepared using a N₂-gas pressurized electroslag remelting (P-ESR) process. Neointima formation and inflammatory responses following stent implantation in porcine coronary arteries were then compared between the Ni-free and SUS316L stainless steel stents. We found significantly less neointima formation and inflammation in arteries implanted with Ni-free stents, as compared to SUS316L stents. Notably, Ni²⁺ was eluted into the medium from SUS316L but not from Ni-free stainless steel. Mechanistically, Ni²⁺ increased levels of hypoxia inducible factor protein-1α (HIF-1α) and its target genes in cultured smooth muscle cells. HIF-1α and their target gene levels were also increased in the vascular wall at SUS316L stent sites but not at Ni-free stent sites. The Ni-free stainless steel coronary stent reduces neointima formation, in part by avoiding activation of inflammatory processes via the Ni-HIF pathway. The Ni-free-stainless steel stent is a promising new coronary stent platform.     

Si3N4和52100钢对磨副材料对CrN薄膜干摩擦学行为的影响

利用阴极电弧离子镀技术在316L不锈钢基体上制备了CrN薄膜。采用扫描电子显微镜(SEM)、X射线衍射仪(XRD)、纳米压痕仪对CrN薄膜的形貌、成分和力学性能进行了表征。为了研究Si_3N_4和52100钢对磨副材料对CrN薄膜和316L不锈钢干摩擦行为的影响,在2N、5N、8N三种载荷下,将CrN薄膜和316L不锈钢基体与Si_3N_4陶瓷球和52100钢球分别进行了往复式滑动干摩擦实验。采用扫描电子显微镜观察了磨痕的微观形貌,并对CrN薄膜和316L不锈钢基体的磨损机制进行了分析。结果表明:CrN薄膜表面平整,缺陷较少;CrN薄膜的纳米硬度约为28GPa,弹性模量约为350GPa;与Si_3N_4陶瓷球相比,CrN薄膜与52100钢球摩擦时摩擦因数相对较小(保持在0.7左右)且更加稳定;316L不锈钢的摩擦因数远大于CrN薄膜且波动较大;对磨球为Si_3N_4陶瓷球时,CrN薄膜的主要磨损机制为磨粒磨损,伴有少量的氧化和黏着磨损,316L不锈钢的磨损机制主要为磨粒磨损和塑性变形,伴有少量的氧化和黏着磨损;对磨球为52100钢球时,CrN薄膜的主要磨损机制为黏着磨损,伴有少量的氧化,316L不锈钢的磨损机制主要为黏着磨损,伴有少量的氧化和磨粒磨损。CrN薄膜与两种对磨球的磨损量均小于316L不锈钢基体的磨损量,说明CrN薄膜有效提高了基体的耐磨性。     

Synthesis and properties of metal matrix composite foams based on austenitic stainless steels –titanium carbonitrides

Metal matrix composite foams based on 316L stainless steel and reinforced with TiC0.7N0.3 were produced by the replication method using polyurethane sponge as a template. The rheological properties of the slurry appeared to be the key issue in the preparation of the composite foams. A homogeneous distribution of TiC0.7N0.3 particles throughout the 316L matrix and a good interaction between the 316L matrix and TiC0.7N0.3 reinforcement particles were obtained. Compression strength results showed that TiC0.7N0.3 particles acted as the real reinforcement medium. The values of the compressive yield strength and the elastic modulus of the metal matrix composite foams increased significantly with increasing TiC0.7N0.3 content when compared to the open cell 316L stainless steel foams.     

Evaluation of tensile properties of austenitic stainless steel 316L with linear hardening by modified indentation method

Abstract

The instrumented indentation test has been widely used for the non-destructive evaluation of the tensile properties of metal materials. The true stress–strain curve, yield strength and tensile strength can be obtained by this method. In the present study, a new modified indentation algorithm was used to determine the tensile properties of austenitic stainless steel 316L by taking into account its linear hardening characteristic. As received 316L was solution treated at four different temperatures in the range of 1223–1473K for 2 h followed by water quenching. The effect of solution treatment temperature on the tensile properties of 316L was investigated by the instrumented indentation test using the modified indentation algorithm. Results reveal that the new modified indentation algorithm can be used to estimate the tensile properties of austenitic stainless steel with linear hardening.     

Interface microstructures in diffusion bonding of titanium alloys to stainless and low alloy steels

Abstract

Commercial purity Ti and a Ti 6242 alloy have been diffusion bonded to an AISI 316L stainless steel and an AISI 4130 low alloy steel. The microstructures of the as processed products have been analysed using optical metallography, scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) techniques. The interdiffusion of the different elements through the interface has been determined using energy dispersive spectroscopy microanalysis in both a SEM and a STEM. For the combinations AISI 316L–commercially pure Ti and AISI 316L–Ti 6242 several regions surrounding the original interface have been observed. Starting from the 316L side, first a α phase is observed, followed by an Fe₂ Ti intermetallic, an FeTi intermetallic, and finally an Fe₂Ti₄O oxide just before the Ti and Ti 6242. Because the diffusion ofTi in Fe is faster than the diffusion of Fe in Ti, a Kirkendall effect is produced. In the AISI 4130–Ti 6242 combination a thin layer of TiC is observed at the interface, limiting the interdiffusion of elements.

MST/1746     

Effects of aging on impact behaviour of different stainless steel weldments

Abstract

Instrumented impact testing was used to investigate the effects of aging on the impact deformation and fracture of different weldments of an AISI type 316L stainless steel plate and a superduplex stainless steel plate. Aging at 800C of metal inert gas MIG and laser beam LB weldments of the 316L stainless steel plate produced a precipitation of sigma phase, which was more intense in the MIG weldments. Correspondingly, the MIG weldments showed a noticeable decrease in impact energy and in impact yield and maximum loads as the aging time was increased. The LB weldments, on the other hand, showed an initial decrease in absorbed energy followed by a noticeable increase, because of the appearance of delaminations during the fracture event. Aging at 900C of the parent metal and of LB and plasma arc PA weldments of the superduplex stainless steel plate produced a precipitation of sigma phase which was much more intense in the parent metal. Both the parent metal and the weldments showed a noticeable decrease in impact energy and in impact yield and maximum loads as the aging time was increased. These weldments were found to be more sensitive to the precipitation of sigma phase than the austenitic stainless steel weldments.     

Studies on the mechanical properties of steel–TiB2 composites obtained by high pressure sintering

The currently used composites produced by classical sintering methods are characterised by numerous limitations due to the difficulties in combining different materials with extreme properties. One of the ways to overcome these limitations is in the use of modern sintering methods, including the high pressure-high temperature process. This study describes the composite materials based on 316L austenitic steel reinforced with titanium diboride and examines the effect of sintering conditions on the mechanical properties and microstructure of sintered composites. It has been found that the key parameter in the manufacture of composites with optimal properties is the sintering time and temperature, while martensitic transformation taking place in the composite matrix can be controlled by the properly selected pressure applied during the sintering process.     

Multiple heat isothermal stress rupture correlations for type 316L(N) stainless steel and their use Part 2 – Applications for nuclear grade type 316L(N) stainless steel base metal and its weld metal

Abstract

The 'reference' multiple heat isothermal stress rupture correlations for stainless steel types 316 and 316L(N) base metals derived in Part 1 are used for establishing those for a specific 316L(N) stainless steel base metal and also its weld, both candidates for the forthcoming prototype fast breeder reactor at Kalpakkam. The phases that form in the weld metal during creep are the same as those in the base metal; however, the uniformly distributed δ ferrite ( ~ 7 ferrite number) in vermicular morphology present in the initial microstructure accelerates their formation and increases their quantities, resulting in poorer stress rupture properties. A simple modification allows for correlating and extrapolating the weld data to long rupture lives using the multiple heat isothermal correlations developed for the base metal.     

Nickel-free stainless steel avoids neointima formation following coronary stent implantation

SUS316L stainless steel and cobalt–chromium and platinum–chromium alloys are widely used platforms for coronary stents. These alloys also contain nickel (Ni), which reportedly induces allergic reactions in some subjects and is known to have various cellular effects. The effects of Ni on neointima formation after stent implantation remain unknown, however. We developed coronary stents made of Ni-free high-nitrogen austenitic stainless steel prepared using a N₂-gas pressurized electroslag remelting (P-ESR) process. Neointima formation and inflammatory responses following stent implantation in porcine coronary arteries were then compared between the Ni-free and SUS316L stainless steel stents. We found significantly less neointima formation and inflammation in arteries implanted with Ni-free stents, as compared to SUS316L stents. Notably, Ni²⁺ was eluted into the medium from SUS316L but not from Ni-free stainless steel. Mechanistically, Ni²⁺ increased levels of hypoxia inducible factor protein-1α (HIF-1α) and its target genes in cultured smooth muscle cells. HIF-1α and their target gene levels were also increased in the vascular wall at SUS316L stent sites but not at Ni-free stent sites. The Ni-free stainless steel coronary stent reduces neointima formation, in part by avoiding activation of inflammatory processes via the Ni-HIF pathway. The Ni-free-stainless steel stent is a promising new coronary stent platform.     

Nickel-free austenitic stainless steels for medical applications

Abstract

The adverse effects of nickel ions being released into the human body have prompted the development of high-nitrogen nickel-free austenitic stainless steels for medical applications. Nitrogen not only replaces nickel for austenitic structure stability but also much improves steel properties. Here we review the harmful effects associated with nickel in medical stainless steels, the advantages of nitrogen in stainless steels, and emphatically, the development of high-nitrogen nickel-free stainless steels for medical applications. By combining the benefits of stable austenitic structure, high strength and good plasticity, better corrosion and wear resistances, and superior biocompatibility compared to the currently used 316L stainless steel, the newly developed high-nitrogen nickel-free stainless steel is a reliable substitute for the conventional medical stainless steels.     

Mitigation of chloride driven stress corrosion cracking susceptibility of 316L austenitic stainless steel using plasma sprayed TiO2 coating

The present study proposes a protective TiO₂ coating against chloride driven stress corrosion cracking problem of 316L austenitic stainless steel. To test the performance of the proposed coating, the severe chloride-based boiling magnesium chloride solution at 155 °C was chosen. For experimentation, the constant strain-based U-bend specimens were coated with TiO₂ using atmospheric plasma spray method. The results indicated higher resistance by TiO₂ coated specimens against stress corrosion cracking problem, while the bare specimens experienced severe damage in the boiling magnesium chloride solution under various strain loading configurations. The coating-electrolyte system of TiO₂ coated sample demonstrated over seven times higher resistance, eventually led to reduction in corrosion rate over fifteen times compared to the bare 316L stainless steel in the boiling magnesium chloride solution. This improved performance of the coated 316L stainless steel is attributed to inhibition of outward diffusion of iron-chromium-nickel in the corrosive environment and the high chemical stability of TiO₂.     

A new high temperature life correlation model for austenitic and ferritic stainless steels

Low-cycle fatigue tests on 429EM ferritic stainless steel and 316L austenitic stainless steel were carried out in a wide range of temperatures from room temperature to 750 °C. The Tomkins fatigue life model was applied to correlate the fatigue life with crack propagation rate and this model matched well with the fatigue life of 429EM stainless steel but not for the 316L stainless steel. A new life prediction model was developed to consider the temperature effect on fatigue life. The predictions show good agreement with experimental results for both materials. The predicted lives were within a±2X scatter band at all test temperatures.     

Mechanical properties of phosphate glass-ceramic-316 L stainless steel composites

Thermal, elastic and mechanical properties of phosphate glass-ceramic-316 L stainless steel particulate composites, prepared by flash-pressing, have been measured. Results have then been explained using various theoretical models. It is shown that particles partly shrink away from the matrix on cooling; this is due both to the slight thermal mismatch between glass-ceramic matrix and 316 L stainless steel particles and to poor bonding between both phases. This small partial shrinkage could explain both the fracture characteristics and the fair agreement between theoretical and experimental values of elastic moduli.     

The Effect of Hydrogen on Fatigue Properties of Metals used for Fuel Cell System

The effect of hydrogen on the fatigue properties of alloys which are used in fuel cell (FC) systems has been investigated. In a typical FC system, various alloys are used in hydrogen environments and are subjected to cyclic loading due to pressurization, mechanical vibrations, etc. The materials investigated were three austenitic stainless steels (SUS304, SUS316 and SUS316L), one ferritic stainless steel (SUS405), one martensitic stainless steel (0.7C-13Cr), a Cr-Mo martensitic steel (SCM435) and two annealed medium-carbon steels (0.47 and 0.45%C). In order to simulate the pick-up of hydrogen in service, the specimens were charged with hydrogen. The fatigue crack growth behaviour of charged specimens of SUS304, SUS316, SUS316L and SUS405 was compared with that of specimens which had not been hydrogen-charged. The comparison showed that there was a degradation in fatigue crack growth resistance due to hydrogen in the case of SUS304 and SUS316 austenitic stainless steels. However, SUS316L and SUS405 showed little degradation due to hydrogen. A marked increase in the amount of martensitic transformation occurred in the hydrogen-charged SUS304 specimens compared to specimens without hydrogen charge. In case of SUS316L, little martensitic transformation occurred in either specimens with and without hydrogen charge. The results of S-N testing showed that in the case of the 0.7C–13Cr stainless steel and the Cr–Mo steel a marked decrease in fatigue resistance due to hydrogen occurred. In the case of the medium carbon steels hydrogen did not cause a reduction in fatigue behaviour. Examination of the slip band characteristics of a number of the alloys showed that slip was more localized in the case of hydrogen-charged specimens. Thus, it is presumed that a synergetic effect of hydrogen and martensitic structure enhances degradation of fatigue crack resistance.     

HA-316L不锈钢纤维非对称功能梯度生物材料制备与显微组织

用低压热等静压方法在1100℃下制备了HA(ZrO₂)-316L不锈钢纤维非对称FGM,其中316L不锈钢纤维含量按体积比20%→15%→10%→5%呈非对称梯度变化.并通过金相显微镜、SEM、EDXA分析了材料的微观结构和微区元素含量.结果表明,HA(ZrO₂)-316L不锈钢纤维非对称FGM微观上表现为316L不锈钢纤维在FGM中呈无序、均匀分布状态,316L不锈钢纤维包裹于HA(ZrO₂)基体中,两者结合紧密,界面表现为部分凹凸不平,316L不锈钢纤维与HA(ZrO₂)基体紧紧的咬合在一起.在FGM基体中发生了微量的韧化相Fe元素扩散,韧化相316L不锈钢纤维不发生基体相Ca、P元素的扩散,基体与韧化相均相对独立,二者之间不发生任何化学反应.随着HA含量增加,HA(ZrO₂)-316L不锈钢纤维复合材料的断裂韧性和弹性模量逐渐减小,体现了FGM中各梯度层的力学性能缓和设计.HA(ZrO₂)-316L不锈钢纤维FGM中,分析认为,增韧机理主要为纤维拔出增韧和层间裂纹偏转增韧.     

Yield stress of duplex stainless steel specimens estimated using a compound Hall–Petch equation

Abstract

In this study, the 0.2% yield stress of duplex stainless steel was evaluated using a compound Hall–Petch equation. The compound Hall–Petch equation was derived from four types of duplex stainless steel, which contained 0.2–64.4 wt% δ-ferrite phase, had different chemical compositions and were annealed at different temperatures. Intragranular yield stress was measured with an ultra-microhardness tester and evaluated with the yield stress model proposed by Dao et al. Grain size, volume fraction and texture were monitored by electron backscattering diffraction measurement. The k_γ constant in the compound equation for duplex stainless steel agrees well with that for γ-phase SUS316L steel in the temperature range of 1323–1473 K. The derived compound Hall–Petch equation predicts that the yield stress will be in good agreement with the experimental results for the Cr, Mn, Si, Ni and N solid-solution states. We find that the intragranular yield stress of the δ-phase of duplex stainless steel is rather sensitive to the chemical composition and annealing conditions, which is attributed to the size misfit parameter.