Ni-free Austenitic Stainless Steel Research Articles

Surface property enhancement of Ni-free medical grade austenitic stainless steel by low-temperature plasma carburising

Based on the success of the feasibility study reported, the surface properties of low-temperature plasma carburised P558 Ni-free medical grade (ASTM F2581) austenitic stainless steel have been fully evaluated in terms of electrochemical corrosion, dry- and corrosion-wear and fretting-wear in Ringer's solution. Anodic polarization tests demonstrated that the precipitate-free S-phase generated by low-temperature plasma carburising at 500 °C for 15 h can retain the good corrosion resistance of the untreated ASTM F2581 Ni-Free material in Ringer's solution. The wear resistance of the Ni-free austenitic stainless steel can be improved by 700% and 140% when reciprocating against a WC ball in air (dry-wear) and in Ringer's solution (corrosion-wear) respectively. In addition, the low-temperature plasma carburising treatment can considerably reduce the friction coefficient and improve the fretting-wear resistance of the Ni-free austenitic stainless steel in Ringer's solution.

Formation and microstructural characterisation of S-phase layers in Ni-free austenitic stainless steels by low-temperature plasma surface alloying

The feasibility of generating S-phase surface layers in nickel-free austenitic stainless steels by plasma surface alloying with nitrogen (at 430 °C), carbon (at 430 °C and 500 °C) and both carbon and nitrogen (at 430 °C) has been investigated. The structure, microstructure and composition of the plasma-alloyed surfaces were characterised by X-ray Diffraction (XRD), microscopy, Glow Discharge Optical Emission Spectroscopy (GDOES) and Transmission Electron Microscopy (TEM). The experimental results have demonstrated for the first time that the S-phase can be produced in the surface of nickel-free austenitic stainless steel by low-temperature plasma surface alloying. TEM analysis has revealed that when alloyed with carbon no precipitates can be found within the carbon-rich S-phase layer; however, when alloyed with nitrogen or both carbon and nitrogen some nitride precipitates (Mn 3N 2 and Cr 2N) were found within the nitrogen-rich S-phase layer. Based on experimental results, the response of Ni-free austenitic stainless steel to plasma surface alloying has been compared to the Ni-containing counterpart, and the role of nickel in the formation of S-phase in austenitic stainless steels has been discussed.

Fatigue Property of Nickel-Free High-Nitrogen Austenitic Stainless Steels in Simulated Body Fluid

Two kinds of high nitrogen-containing Ni-free austenitic stainless steels for medical applications were used for evaluation of fatigue behavior. One was an Fe-23%Cr-1%N stainless steel heat-treated in a N 2 gas atmosphere (NA alloy) and the other was an Fe 24%Cr-2%Mo-1 %N stainless steel fabricated with an electro-slag remelting method in a pressurized N 2 gas atmosphere (P-ESR alloy). Fatigue tests were carried out both in air and in the phosphate-buffered saline solution, PBS(-). Cyclic stress with a sinusoidal waveform was applied to the specimen in a tension-to-tension mode with a stress ratio of 0.1 at a frequency of 20 Hz in air and 2 Hz in PBS(-). During testing, the saline solution was kept at 310 K and at the pH value of 7.5. A nitrogen-4% oxygen gas mixture was bubbled into the PBS(-). The results obtained are as follows. There was no difference between S-N (stress-number of cycles to failure) curves in air and in PBS(-) for each stainless steel. The fatigue strength at 10 7 cycles for the NA alloy was 245 MPa. No difference was found in the fatigue strength between the NA specimens heat-treated for 259.2 ks and 129.6 ks. The fatigue strength at 10 7 cycles for the P-ESR alloy was 320 MPa. The fatigue life at various stress amplitudes depended on preparation of the test specimens for the P-ESR alloy. The fatigue life was decreased by the plastic deformation layer and/or cracks on the specimen surface caused by machining and increased by elimination of the deformation layer and/or cracks. The fracture surface exhibited a cleavage-like appearance at the fatigue crack initiation site for both alloys.

Ni フリー高窒素オーステナイト系ステンレス鋼の擬似体液中疲労特性

Two kinds of high nitrogen-containing Ni-free austenitic stainless steels for medical applications were used for evaluation of fatigue behavior. One was a Fe-23%Cr-2%Mo-1%N stainless steel heat-treated in a N2 gas atmosphere (NA) and the other was a Fe 23%Cr-1%Mo-1%N stainless steel fabricated with an electro-slag remelting method in a pressurized N2 gas atmosphere (P-ESR). Fatigue tests were carried out both in air and in the phosphate-buffered saline solution, PBS (−). Cyclic stress with a sine waveform was applied to the specimen in a tension-to-tension mode with a stress ratio of 0.1 at a frequency of 20 Hz in air and 2 Hz in PBS (−). During testing, the saline solution was kept at 310 K and at the pH of 7.5. A nitrogen-4% oxygen gas mixture was bubbled into the PBS (−). The results obtained are as follows. There was no difference between S-N (stress-number of cycles to failure) curves in air and in PBS (−) for each stainless steel. The fatigue strength at 107 cycles for NA was 245 MPa. No difference was found in the fatigue strength between the NA specimens heat- treated for 259.2 ks and 129.6 ks. The fatigue strength at 107 cycles for P-ESR was 320 MPa. The fatigue life at various stress amplitudes depended on preparation of the test specimens for P-ESR. The fatigue life was decreased by the plastic deformation layer and/or cracks on the specimen surface caused by machining and increased by elimination of the deformation layer and/or cracks. The fracture surface exhibited a cleavage-like appearance at the fatigue crack initiation site for both the stainless steels.

Nanoscale Nickel-Free Austenitic Stainless Steel

In this work Ni-free austenitic stainless steels with nanostructure were synthesized by mechanical alloying (MA), heat treatment and nitrogenation of elemental Fe, Cr, Mn and Mo microcrystalline powders. The phase transformation from ferritic to austenitic was confirmed by XRD analysis. The mechanical and corrosion properties of the produced biomaterials were investigated. Additionally, the biocompatibility of nickel-free austenitic stainless steels with nanostructure and microcrystalline 316L steel, were analyzed studying the behaviour of Normal Human Osteoblast (NHOst) cells from Cambrex (CC-2538). An enhancement of the properties due to the nanoscale structures in the bulk consolidated materials was observed.

Metal Injection Molding Method of Ni-Free Austenitic Stainless Steel II - Microstructure and Mechanical Properties

Microstructure and mechanical properties of high nitrogen steels whose chemical composition were Fe-17Cr-12Mn-3Mo and that was produced by using metal injection molding method and nitrogen absorption methods were examined. A compact which is furnace cooled from 1573 K has a brittle surface layer with high chromium and nitrogen contents but the surface layer disappears when the compact is held at 1473 K. The compact which is furnace cooled at 1473 K is observed precipitates in the grains and the grain boundary, while the compact which is quenched at 1473 K shows homogeneous microstructure. In the heat treatments at 1473 K for 2, 5, and 10 h, the nitrogen content becomes higher with increasing of holding time. In the holding times of 5 and 10 h, the microstructure is austenite. In the tensile tests, tensile strength becomes larger with increasing of nitrogen content. In the specimen which is conducted the heat treatment at 1473 K for 10 h, tensile strength is about 1,000 MPa and elongation is 80 %, which shows better balance of strength and elongation than SUS304 and SUS316 steels.

Electrochemical Behavior of High Nitrogen Ni-Free Austenitic Stainless Steels in L929 Cells and Cell Culture Medium

The localized corrosion behavior of high nitrogen Ni-free and Ni-reduced austenitic stainless steels in L929 mouse fibroblasts and cell culture medium (Eagle's minimum essential medium (MEM) supplemented with 10% fetal bovine serum (FBS)) was evaluated by electrochemical techniques in an electrochemical cell culture corrosion cell. The positive effect of nitrogen alloying was evidenced. The examined high nitrogen Ni-free and Ni-reduced alloys resulted to be immune to pitting corrosion even in the presence of cells. Cell culture studies showed that high nitrogen lotus-type porous Ni-free and Ni-reduced stainless steels were non-cytotoxic to mouse fibroblasts cell line. In this sense, they represent a better alternative to other conventional stainless steels.

Electrochemical Behavior of High Nitrogen Ni-Free Austenitic Stainless Steels in L929 Cells and Cell Culture Medium

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In vitro corrosion study by EIS of a nickel-free stainless steel for orthopaedic applications

The electrochemical impedance spectroscopy (EIS) technique was used for the study of the electrochemical behaviour of Ni-free austenitic stainless steel for orthopaedic applications. Experiments were carried out using four different test solutions: (i) phosphate-buffered saline (PBS), (ii) minimum essential medium (MEM), (iii) MEM + 10% fetal calf serum (FCS), (iv) MEM + 10% fetal calf serum + L929 fibroblast cell line (Cell). Bode-phase spectra showed the presence of two maxima and were fitted with an equivalent circuit characterized by two parallel combinations (Resistance, Constant Phase Element). The ( R 1, CPE 1) branch was assigned to the inner compact passive film and the ( R 2, CPE 2) branch to the external porous film. The resistance of the inner film R 1, here directly related to the material's uniform corrosion resistance, raised with the immersion time and increased in the following order: PBS<FCS≈Cell<MEM. In the same test condition the R 1 value of the Ni-free austenitic stainless steel is, as expected, lower than that of titanium.

In vitro corrosion study by EIS of a nickel-free stainless steel for orthopaedic applications

The electrochemical impedance spectroscopy (EIS) technique was used for the study of the electrochemical behaviour of Ni-free austenitic stainless steel for orthopaedic applications. Experiments were carried out using four different test solutions: (i) phosphate-buffered saline (PBS), (ii) minimum essential medium (MEM), (iii) MEM + 10% fetal calf serum (FCS), (iv) MEM + 10% fetal calf serum + L929 fibroblast cell line (Cell). Bode-phase spectra showed the presence of two maxima and were fitted with an equivalent circuit characterized by two parallel combinations (Resistance, Constant Phase Element). The (R1, CPE1) branch was assigned to the inner compact passive film and the (R2, CPE2) branch to the external porous film. The resistance of the inner film R1, here directly related to the material's uniform corrosion resistance, raised with the immersion time and increased in the following order: PBS<FCS≈Cell<MEM. In the same test condition the R1 value of the Ni-free austenitic stainless steel is, as expected, lower than that of titanium.

高窒素含有Niフリーオーステナイト系ステンレス鋼のHAZ靭性に及ぼす窒化物析出の影響

高窒素含有Niフリーオーステナイト系ステンレス鋼のHAZ靭性に及ぼす窒化物析出の影響