Nickel-free Austenitic Stainless Steel Research Articles

Impact toughness properties of nickel-free austenitic stainless steels

The Charpy-V impact toughness of six interstitially-alloyed nickel-free austenitic stainless steels was investigated in the temperature range between −196 and 150 °C. Unlike traditional face-centered-cubic steels, the investigated alloys show a marked decrease in impact toughness with decreasing temperature and cleavage-like fracture surfaces at low temperatures. The decrease in impact toughness is due to the combined effects of high strain rate and temperature sensitivity of plastic flow and the strong planarity of the deformation mechanisms of nitrogen-alloyed nickel-free alloys at low temperatures. The effects of the high strain rate and temperature sensitivity of the flow stress on the fracture behavior are discussed based on an analysis of the critical stress for fracture and plastic flow.

Preparation of high nitrogen and nickel-free austenitic stainless steel by powder injection molding

High nitrogen and nickel-free austenitic stainless steel has received much recognition worldwide because it can solve the problem of “nickel-allergy” and has outstanding mechanical and physical properties. In this article, 0Cr17Mn11Mo3N was prepared by powder injection molding (PIM) technique accompanied with solid-nitriding. The results show that the critical solid loading can achieve up to 64 vol% by use of gas-atomized powders with the average size of 17.4 μm. The optimized sintering conditions are determined to be 1300°C, 2 h in flowing nitrogen atmosphere, at which the relative density reaches to 99% and the N content is as high as 0.78wt%. After solution annealing at 1150°C for 90 min and water quench, the 0.2% yield strength, ultimate tensile strength (UTS), elongation, reduction in area, and hardness can reach as high as 580 MPa, 885 MPa, 26.0%, 29.1%, and Hv 222, respectively.

Weldability of Nickel-Free Austenitic Stainless Steel Thin Sheet by Small-Scale Resistance Spot Welding

The feasibility and microstructural development of high-nitrogen-containing nickel-free austenitic stainless steel by small-scale resistance spot welding were studied. Almost fully austenitic cellular microstructure was developed in weld nuggets because the cooling rate of approximately 10 5 Ks � 1 was much faster than that in conventional fusion welding processes. Only a small amount of delta ferrite was formed at the gamma grain boundary in the weld, and chromium nitride precipitation was observed both in the gamma grains and at the grain boundary. No significant defects and sensitization were observed in the weld nugget and in the heat affected zone, respectively, and an adequate joint strength was obtained. [doi:10.2320/matertrans.MRA2008285]

The Effect of Metal Materials on Heat Shock Protein 70B' Gene Expression

To avoid the toxic effect of released nickel ions and compounds from conventional stainless steels, nickel-free austenitic stainless steels have been developed. We previously established a new manufacturing process to produce nickel-free austenitic stainless steel that involves nitrogen adsorption treatment. Although the cytocompatibility of nickelfree austenitic stainless steel produced using this method has been evaluated using two viability assay, molecular level analysis, such as gene expression analysis, has not been previously performed. In the present study, the cytotoxicity of our nickel-free austenitic stainless steel, as well as of commercially available metal materials, was evaluated by analysis of heat shock protein 70B’ (HSP70B’) gene expression as a stress response marker. Furthermore, to investigate the effect of metal materials on cytotoxicity, HSP70B’ gene expression was quantified using human osteoblast-like SaOS-2 cells, human monocyte THP-1 cells and the mouse macrophage cell line J774A.1. We found no significant differences in HSP70B’ expression among the various metal materials, including the nickel-free austenitic stainless steel, indicating that the nickel-free austenitic stainless steel produced using our nitrogen adsorption method has the same cytocompatibility as commercially available metal materials.

Microstructure and Mechanical Properties of Ni-Free High Nitrogen Austenite Stainless Steels Fabricated by Mechanical Alloying Method

Particular attention is called to nickel-free or lower nickel austenitic stainless steels containing high nitrogen from the viewpoint of protections against earth resources and Ni allergy for human being. We have made high nitrogen steels (HNS) that were containing 0.7-2.0mass%N and 23Cr-2Mo by mechanically alloying method and the microstructure and mechanical properties have been examined. HNS containing 1.0-1.6mass%N have nearly full austenite phase and the mean grain diameter was about 2-3μm for every HNS. The strength and work hardening rate increased and the elongation decreased with increasing nitrogen content. Especially, HNS containing 1.0mass%N exhibited superior balance of strength-ductile properties, that is, yield strength of 1200MPa and fracture elongation of 36%. The yield point phenomena and ductile-brittle transition behavior can be seen in spite of austenitic stainless steels. The planar dislocation arrays were seen on the deformed structure. These results suggest that the formation of planar dislocation structure, high work hardening rate for high nitrogen steels and temperature dependence of mechanical properties might be caused by the formation of the Mo-N interstitial-substitutional atom couples.

Metal Injection Molding of Nickel-Free Austenitic Stainless-Steels I-Manufacturing Process

Ni-free austenitic steels containing high nitrogen have been developed to protect against earth resource. High nitrogen steels (HNS) have a lot of advantages, e.g., HNS have high strength, corrosion resistance, toughness, work hardening rate and large rocking parameter in the Hall-Petch equation. On the other hand, it is difficult to fabricate HNS by IM method under 0.1 MPa and to work at room temperature. We have tried to make HNS by combined use of metal injection molding method (MIM) and nitrogen absorption method. Powder compositions used was Fe-17Cr-12Mn-3Mo.The benefit of this method is to make metal parts in near net shape. In order to use this method, we should know the sintering heat schedule, timing for introducing nitrogen gas, gas pressure and setter material etc. Therefore, the shrinkage rate, density and the solution-treated microstructure of MIM compacts were examined to find out the optimum conditions.

In Vitro Biocompatibility of a New High Nitrogen Nickel Free Austenitic Stainless Steel

The in vitro blood compatibility of a new nickel free high nitrogen austenitic stainless steel Fe-Cr-Mn-Mo-N (BIOSSN4) was studied by the kinetic clotting time test and the platelet rich plasma adhesion test in this paper. In comparison with 316L stainless steel, the kinetic clotting time of BIOSSN4 steel are longer, and only causes less activation of platelets in platelet adhesion test, which was indicated by their morphology and low spreading. The experimental results reveals that the BIOSSN4 stainless steel has better blood compatibility, the blood compatibility mechanism of steels was analyzed based on surface tension and interfacial tension between the steels and blood.

The Effect of Metal Materials on Heat Shock Protein 70B’ Gene Expression

To avoid the toxic effect of released nickel ions and compounds from conventional stainless steels, nickel-free austenitic stainless steels have been developed. We previously established a new manufacturing process to produce nickel-free austenitic stainless steel that involves nitrogen adsorption treatment. Although the cytocompatibility of nickelfree austenitic stainless steel produced using this method has been evaluated using two viability assay, molecular level analysis, such as gene expression analysis, has not been previously performed. In the present study, the cytotoxicity of our nickel-free austenitic stainless steel, as well as of commercially available metal materials, was evaluated by analysis of heat shock protein 70B’ (HSP70B’) gene expression as a stress response marker. Furthermore, to investigate the effect of metal materials on cytotoxicity, HSP70B’ gene expression was quantified using human osteoblast-like SaOS-2 cells, human monocyte THP-1 cells and the mouse macrophage cell line J774A.1. We found no significant differences in HSP70B’ expression among the various metal materials, including the nickel-free austenitic stainless steel, indicating that the nickel-free austenitic stainless steel produced using our nitrogen adsorption method has the same cytocompatibility as commercially available metal materials.

Grain refinement of nickel-free high nitrogen austenitic stainless steel by reversion of eutectoid structure

For the purpose of austenite grain refinement, a two-step phase transformation was applied to nickel-free austenitic stainless steel (Fe–25Cr–1 N mass% alloy): first, the austenitic steel was subjected to isothermal heat treatment at around 1200 K to make the austenite decompose, which results in the formation of (ferrite + Cr2N) lamellar structure. The specimen with eutectoid structure was then reversed to austenite by heating. In addition, the grain refinement mechanism was discussed in terms of the nucleation of the reversed austenite.

Micro-Resistance Spot Welding of Nickel Free Austenitic Stainless Steel

Micro-resistance spot weldability of nickel free stainless steel sheet was studied to fabricate medical implants. Weld lobe was established and the effect of welding parameters on joint strength was investigated. Weld nugget represented a rapidly quenched austenitic cellular structure whose cell size is several μm with a little ferrite. The amount of δ-ferrite in weld nugget is smaller than that predicted by some Schaefller diagrams due to rapid solidification during micro-resistance spot welding.

Decomposition of Austenite in Fe-25Cr-1N Alloy Produced by Solution Nitriding

The nickel-free austenitic stainless steel produced by solution nitriding (Fe-25%Cr-1%N alloy) was subjected to isothermal heat treatment, and then the microstructure formed through the decomposition of austenite was investigated in terms of the morphology of eutectoid structure and the size of eutectoid block. On the isothermal heat treatment at 873K~1223K for the solution-nitrided steel, the austenite decomposed to eutectoid structure composed of ferrite and Cr2N nitride. This transformation could be completely finished after long time heat treatment in the above temperature range. The nose temperature of T.T.T. curve was around 1173K, and the time to start the eutectoid transformation was only 100~200s. The eutectoid structure was formed mainly along austenite grain boundaries and then grew into the untransformed austenite region. Finally, the austenite was completely decomposed into ferrite and Cr2N nitride. As a result of OIM observation for the specimen after isothermal heat treatment, the eutectoid structure was found to be divided into small-sized ferrite blocks, in which lamellar Cr2N plates were finely distributed. The block size and the mean ferrite path of eutectoid structure were decreased with lowering the heat treatment temperature. In the 873K heat-treated material, these values were estimated at 20 microns and 0.1 microns, respectively.

Recent Development of New Alloys for Biomedical Use

Metallic materials are widely used in medicine not only for orthopedic implants but also for cardiovascular devices and other purposes. New alloys for biomedical use are developed all over the world continuously to decrease corrosion, toxicity and fracture during implantation and increase interfacial and dynamical tissue compatibility. Most of efforts are made to develop titanium alloys, especially in β-type alloys whose Young’s modulus is as low as cortical bone. Nickel-free alloy is also necessary to prevent nickel allergy: nickel-free austenitic stainless steels and shape memory alloys are developed. To increase iocompatibility, the controls of surface morphology and surface treatment or modification are necessary.

窒素吸収処理によるNiフリーステンレス鋼の新製造技術

Ingots of ferritic stainless steels, Fe-24Cr-2Mo in mass%, were worked to various dimensions for test specimens. Nitrogen was absorbed by the specimens in a furnace filled with nitrogen gas with a pressure of 101.3 kPa at 1473 K to develop a simple and convenient manufacturing process of nickel-free austenitic stainless steels. Changes in the mechanical properties of the alloy with nitrogen absorption treatment are discussed on the basis of the resultant microstructure. Ferritic Fe-24Cr-2Mo was austenitized with 129.6-ks nitrogen absorption to a 2-mm depth from the surface. On the other hand, Fe-24Cr-2Mo was completely austenitized with 7.2-ks nitrogen absorption to a 0.5-mm depth from the surface. The hardness, tensile strength, 0.2% proof stress, and elongation to fracture increased, and the reduction of area decreased in Fe-24Cr-2Mo by austenitization due to nitrogen absorption. Tensile properties of the alloy after nitrogen absorption were improved by grain refinement process before nitrogen absorption. The tensile strength and 0.2% proof stress of the alloy with nitrogen absorption for 129.6 ks were much larger than those of 316L steel, while the elongation to fracture was much smaller than that of 316L steel. Therefore, small devices and parts with a maximum thickness or diameter of 4 mm were manufactured with this process in this study.

In vitro study of platelet adhesion on medical nickel-free stainless steel surface

A new nickel-free austenitic stainless steel, named BIOSSN4, has been recently developed to avoid the sensitivity to Ni ions. In the present study, the blood compatibility of the nickel-free stainless steel was evaluated in vitro by the platelet-rich plasma adhesion test with comparison to 316L stainless steel. The result shows that the nickel-free stainless steel only causes less activation of platelets, which was indicated by their morphology and low spreading. The result suggests that the nickel-free stainless steel should have better blood compatibility compared with 316L stainless steel and, as a bio-material, it should have obvious advantage and potential applications.

Development of nitrogen-containing nickel-free austenitic stainless steels for metallic biomaterials—review

Metallic biomaterials—such as 316L stainless steel and cobalt-based alloys—have been used as biomaterials mainly because of their excellent mechanical and corrosion properties. However, the release of nickel trace elements—which cause toxicity—has prompted the development of nitrogen-containing nickel-free austenitic stainless steels. This paper reviews their development, traces the history of 316L stainless steel, and the improvement of properties by nitrogen addition. These steels are now available for production of implant devices such as bone plates and screws. Such production requires special techniques with nitrogen absorption treatment.

Cytocompatibility evaluation of Ni-free stainless steel manufactured by nitrogen adsorption treatment

Cytocompatibility of nickel-free austenitic stainless steel manufactured by nitrogen adsorption treatment, Fe–Cr–Mo–N, was evaluated and compared with a conventional austenitic stainless steel, 316L, and nickel-free ferrite stainless steel, Fe–Cr–Mo, before nitrogen adsorption treatment. Two types of cytotoxicity tests were performed; static and dynamic conditions. In static conditions, human normal diploid fibroblast HEL299 was cultured on the disks of the three materials and cell growth was examined. In dynamic conditions, the disks were rotated in extracting medium with zirconia balls at 37 °C for 14 days, and the collected extracting medium was added into HEL299 culture to examine its inhibitive effect on cell growth. Quantification of metallic elements in collected extracting medium was also performed. As a result, Fe–Cr–Mo–N had higher cell growth than 316L in static and dynamic conditions. Nickel was detected in the extracting media of 316L and Fe–Cr–Mo, whereas no nickel was detected in that of Fe–Cr–Mo–N. These facts indicate that Fe–Cr–Mo–N has higher cytocompatibility than 316L and that the nitrogen adsorption treatment contributes to the higher corrosion resistance of Fe–Cr–Mo–N in the presence of wear.

New Manufacturing Process of Nickel-Free Austenitic Stainless Steel with Nitrogen Absorption Treatment

Ingots of ferritic stainless steels, Fe-24Cr and Fe-24Cr-2Mo in mass%, were worked to various dimensions for test specimens. Nitrogen was absorbed by the specimens in a furnace filled with nitrogen gas with a pressure of 101.3 kPa at 1473 K to develop a simple and convenient manufacturing process of nickel-free austenitic stainless steels. Ferritic Fe-24Cr and Fe-24Cr-2Mo were austenitized with nitrogen absorption to a 2-mm depth from the surface. The hardness, tensile strength, 0.2% proof stress, and elongation to fracture increased, and the reduction of area decreased in the alloys by austenitization due to nitrogen absorption. The tensile strength and 0.2% proof stress of these alloys with nitrogen absorption for 129.6 ks were much larger than those of 316L steel, while the elongation to fracture was much smaller than that of 316L steel. Therefore, small devices and parts with a maximum thickness or diameter of 4 mm were manufactured with this process in this study

Torsion and Tensile Properties of Thin Wires of Nickel-Free Stainless Steel with Nitrogen Absorption Treatment

A new manufacturing process for nickel-free austenitic stainless steel has been developed by authors. In combination with machining and a nitrogen absorption treatment, this process makes it possible to form small precise devices with a maximum thickness or diameter of 4 mm. The refinement of grains of Fe-24Cr-2Mo in mass% was attempted by thermo-mechanical treatment before nitrogen absorption treatment in order to increase the mechanical properties after nitrogen absorption treatment. Torsion and tensile properties and microstructures of Fe-24Cr-2Mo before and after nitrogen absorption treatment were evaluated to understand the effects of grain refinement on nitrogen absorption. The thin wire of the alloy is completely austenitized with nitrogen absorption at 1473 K for over 7.2 ks. The mean grain size of the alloy with nitrogen absorption decrease with the grain refinement process attempted in this study. The values of ultimate tensile strength and elongation in the alloy with and without nitrogen absorption increase with the grain refinement process. The torsional stress and rotation angle to fracture of the alloy increase with the grain refinement process and nitrogen absorption. According to the results of the torsion and tensile tests, the thin wire of the alloy with nitrogen absorption is expected to have good mechanical properties than conventional austenitic stainless steels.

Characterization of the Surface Oxide Film on an Fe-Cr-N System Alloy in Environments Simulating the Human Body

The objective of this study was to characterize the surface oxide film on nickel-free austenitic stainless steel with nitrogen absorption, Fe24Cr-1N in mass% (Fe-26.4Cr-3.4N in at%), located in various environments, to estimate the reconstruction of the surface oxide film in the human body. The specimens were metallographically polished in deionized water, autoclaved after polishing, immersed in Hanks’ solution for 7 days after polishing and autoclaving, immersed in Eagle’s minimum essential medium containing fetal bovine serum for 7 days after polishing and autoclaving, and incubated with L929 fibroblasts for 7 days after polishing and autoclaving. L929 fibroblasts cultured on the specimen were removed with a flow of deionized water before surface analysis. X-ray photoelectron spectroscopy was performed to determine the composition of the surface oxide film and substrate and the thickness of the surface oxide film. The depth profiles of elements in the surface region were also characterized using Auger electron spectroscopy in combination with argon-ion-sputtering. The surface oxide film on Fe-24Cr-1N polished mechanically in deionized water consists of iron and chromium oxides. Large amounts of carbon, nitrogen, oxygen, and sulfur originating from proteins were detected after immersion in the medium. Sulfur existed as sulfide or sulfite. Calcium phosphate was formed on the surface oxide film after immersion in the Hanks’ solution and medium. On the other hand, calcium and sulfur were not detected after being incubated with the cells, indicating that a large amount of protein was absorbed on the top of the surface oxide film.

Mechanical Properties and Microstructures of a Thin Plate of Nickel-Free Stainless Steel with Nitrogen Absorption Treatment

We have developed a new manufacturing process for nickel-free austenitic stainless steel. In combination with machining and a nitrogen absorption treatment, this process makes it possible to form small precise devices. However, the temperature for the nitrogen absorption, 1473 K, was sufficiently high for grain growth, and coarsening was observed after nitrogen absorption. Therefore, a nitrogen absorption treatment which allows the retention of strength and ductility was performed with a grain refinement process before nitrogen absorption. In this study, we attempted the refinement of grains by thermo-mechanical treatment before nitrogen absorption treatment. The mechanical properties and microstructures of Fe–24Cr and Fe–24Cr–2Mo with fine grains before and after nitrogen absorption treatment were evaluated to understand the effects of grain refinement on nitrogen absorption. The austenintic phase was only obtained from the surface to a 0.1-mm depth in Fe–24Cr and Fe–24Cr–2Mo with nitrogen absorption at 1473 K for 2.7 ks. The balance between strength and elongation in Fe–24Cr and Fe–24Cr–2Mo with nitrogen absorption at 1473 K for 2.7 ks was better than that in conventional austenitic stainless steel. The values of ultimate tensile strength in both alloys with nitrogen absorption increased with the grain refinement process attempted in this study. The balance between strength and elongation in both alloys with nitrogen absorption over 2.7 ks decreased because of grain growth. Therefore, the process described in this study can be used to manufacture small devices with a great deal of precision and parts with a maximum thickness or diameter of 0.2 mm in a short time. Grain refinement in a nickel-free austenitic stainless steel can increase the balance between the strength and elongation.