Medical Grade Austenitic Stainless Steel Research Articles

S-Phase Layer Development on 316 LVM Using Low Temperature Hybrid Thermochemical Treatment Process

This investigation focuses on the improvement of surface properties of medical grade austenitic stainless steel (AISI 316LVM). The aim is to develop a homogenous supersaturated hard layer of expanded austenite (s-phase) at the surface of AISI 316LVM using low temperature hybrid thermochemical heat treatment process. The s-phase layer produced by this process is able to improve the surface properties of AISI 316LVM, overcoming its drawback of low surface hardness and wear resistance, without impairing the corrosion resistance of the steel. During the heat treatment process, ammonia (NH3) and methane (CH4) gasses were introduced into the furnace with temperatures of 425°C and 475°C, at 6 and 12 hours with gas composition of 75% of NH3, 10% of CH4, and 15% of Nitrogen (N2). Characterization on the microstructure showed the formation of the S-phase layer with variation of thickness according to parameters used. The S-phase formation was confirmed with phase analysis using XRD. Besides, the surface hardness also significantly increased from 210.9 HV to 1170.0 HV. In conclusion, low temperature hybrid heat treatment process is able to produce a homogenous hard s-phase layer.

The effect of surface mechanical attrition treatment on low temperature plasma nitriding of an austenitic stainless steel

The combined effect of superficial nanocrystallisation by SMAT (Surface Mechanical Attrition Treatment) followed by plasma nitriding on the mechanical properties of a medical grade austenitic stainless steel was studied. SMAT conditions were optimised to enhance nitrogen diffusion. Experimental observations (energy dispersive X-ray spectroscopy profiles, cross-sectional optical micrographs, phase analysis by X-ray diffraction and micro-hardness profiles) show that polishing away a very thin layer after SMAT and before nitriding significantly improves nitrogen diffusion into the substrate, yielding a 50% thicker nitrided layer. Possible causes for this improvement are discussed.

Augmentation of crevice corrosion resistance of medical grade 316LVM stainless steel by plasma carburising

Precipitate free carbon S-phase was produced on the surface of AISI 316LVM medical grade austenitic stainless steel with the use of low temperature direct current and active screen plasma carburising. The treated and untreated alloy was characterised and tested for pitting and crevice corrosion resistance. From this work it can be concluded that when compared to the untreated material, both treatments augment the pitting and crevice corrosion resistance. Using an active screen set-up results in a better surface composition and a higher crevice corrosion resistance than that produced using the direct current plasma carburising treatment.

Corrosion properties of S-phase layers formed on medical grade austenitic stainless steel

The corrosion properties of S-phase surface layers formed in AISI 316LVM (ASTM F138) and High-N (ASTM F1586) medical grade austenitic stainless steels by plasma surface alloying with nitrogen (at 430°C), carbon (at 500°C) and both carbon and nitrogen (at 430°C) has been investigated. The corrosion behaviour of the S-phase layers in Ringer's solutions was evaluated using potentiodynamic and immersion corrosion tests. The corrosion damage was evaluated using microscopy, hardness testing, inductive coupled plasma mass spectroscopy and X-ray diffraction. The experimental results have demonstrated that low-temperature nitriding, carburising and carbonitriding can improve the localised corrosion resistance of both industrial and medical grade austenitic stainless steels as long as the threshold sensitisation temperature is not reached. Carburising at 500°C has proved to be the best hardening treatment with the least effect on the corrosion resistance of the parent alloy.

Effect of low-temperature plasma carbonitriding on the fretting behaviour of 316LVM medical grade austenitic stainless steels

Abstract An optimized low-temperature plasma carbonitriding process at 430 °C for 15 h was used to treat the surface of 316LVM medical grade austenitic stainless steel. By using a servo-hydraulic dynamic test machine, the fretting behaviour of both as-received and the surface-treated 316LVM plate samples against martensitic stainless steel balls was studied in the Ringer's solution at various displacement amplitudes. The experimental results demonstrate that the optimized low-temperature plasma surface treatment can produce a precipitate-free, C/N supersaturated S-phase layer. Such layer revealed a hardness of 5 times that of 316LVM substrate. The formation of S-phase can protect the 316LVM from corrosion in the Ringer's solution, and the hard surface-treated layer can significantly improve the fretting wear resistance of 316LVM material. When fretted in a mixed regime, a sharp drop of friction force was observed for untreated 316LVM after fretting for about 1000 cycles, which was mainly caused by the penetration of Ringer's solution into the contact area of the fretting pairs. As a comparison, no friction drop was observed through the fretting process on the surface-treated samples. Due to the high ratio of elastic modulus to hardness (E/H) of untreated sample, the main damage mechanism on untreated sample was adhesive and abrasive wear during the fretting process. However, because of the high hardness and low E/H ratio of the surface-treated sample, the main damage mechanism was mild abrasive wear. In conclusion, the low temperature plasma surface treatment can effectively improve the fretting wear properties of 316LVM stainless steel, which could pave the way to develop the high-performance, long-life body implants.

Evaluation of the biocompatibility of S-phase layers on medical grade austenitic stainless steels

S-phase surface layers were formed in AISI 316LVM (ASTM F138) and High-N (ASTM F1586) medical grade austenitic stainless steels by plasma surface alloying with nitrogen (at 430°C), carbon (at 500°C) and both carbon and nitrogen (at 430°C). The presence of the S-phase was confirmed by microscopy, hardness testing, depth-profile analysis of chemical composition and X-ray Diffraction. Attachment and proliferation of mouse osteoblast MC3T3-E1 cells were tested on S-phase and untreated controls and the results demonstrated that all the S-phase layers formed were biocompatible under the conditions used. Cells adhered equally well to all samples but proliferation was enhanced on the treated materials.

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.

Low temperature plasma carbonitriding of ASTM F138 and ASTM F1586 biomedical stainless steels

Low temperature carbonitriding has been reported to improve the wear resistance of engineering grade AISI 316 through the formation of a hybrid S phase, but little or no research work has been applied to medical grade austenitic stainless steels. In this study, the aforementioned hybrid S phase has been successfully implemented in the surface of two medical grade austenitic stainless steels, ASTM F138 and ASTM F1586, as well as on engineering grade AISI 316 for comparison. The wear behaviour of the plasma carbonitrided and the untreated samples was compared using reciprocating sliding wear tests in air, Ringer's solution and distilled water. Experimental results have demonstrated that low temperature plasma carbonitriding can effectively reduce the wear of these two medical grade materials independent of the testing conditions. Wear results from the reciprocating wear test have shown that for the ASTM F138 alloy an improvement over the untreated material of 450% (in air) and 100% (in Ringer's solution) was registered; for the ASTM F1586 stainless steel, the improvements over the untreated materials were 380% (in air) and 40% (in Ringer's solution). It is proposed that the improved performance of the treated samples can be attributed to a desired combination of the significantly improved hardness which provides a better support to the surface oxide layer and the retained good corrosion resistance.

Low-Temperature Plasma Surface Modification of Medical Grade Austenitic Stainless Steel to Combat Wear and Corrosion

The novel low temperature plasma alloying technique that simultaneously introduces both nitrogen and carbon into the surface of austenitic stainless steel has been used in the past to create a hybrid N-C S-Phase. This S-Phase layer boasts of high hardness and wear resistance without any detriment to corrosion resistance. In this study, the afore mentioned hybrid N-C S-Phase was successfully implemented in the surface of two medical grade austenitic stainless steels: ASTM F138 and F1586. At an optimum process temperature of 430°C a very hard, 20μm precipitate-free S-Phase layer was created. Anodic Polarization tests in Ringer’s solution showed that the corrosion resistance of this layer was similar to that of the untreated alloys. Both dry-wear and corrosion-wear (Ringer’s) behaviour of the surface treated alloys showed an improvement of more than 350% and 40% respectively when compared to the untreated material.

Low temperature plasma surface alloying of medical grade austenitic stainless steel with carbon and nitrogen

Low temperature surface alloying with either nitrogen (nitriding) or carbon (carburising) has been successfully employed in hardening AISI 316. However, little work has been directed towards low temperature plasma surface alloying with both nitrogen and carbon simultaneously. In addition, little or no research has been conducted on the surface modification of medical grade austenitic stainless steels, such as ASTM F138 and F1586. In the present study, plasma surface alloying treatments have been conducted on medical grade ASTM F138 and ASTM F1586 as well as on engineering grade AISI 316 for comparison. Systematic materials characterisation was carried out using optical microscopy, glow discharge optical emission spectroscopy (GDOES) and X-ray diffraction (XRD). The three stainless steels had similar response to the plasma alloying treatments. At a temperature of 425°C plasma surface alloying with both carbon and nitrogen can effectively increase the surface hardness and wear resistance of the three austenitic stainless steels without compromising the corrosion resistance of the alloy.

The damage tolerance of S-phase coated biomedical grade stainless steel

There is an increasing demand to manufacture total joint replacements (TJRs) with damage tolerant bearing surfaces that minimise the creation of wear debris in vivo. To this end there is potential for exploiting hard coatings. The work reported here investigates the potential of using S-phase coatings as scratch resistant bearing surfaces. A series of these coatings with nitrogen contents [N] ranging from 8 to 29 at.% were synthesised by magnetron sputtering and applied to polished medical grade austenitic stainless steel (Ortron 90). The load invariant hardness (LIH) was proportional to [N]. Damage tolerance of the coated and uncoated Ortron 90 was assessed using both Rockwell C hardness indentation and scratch testing. Nine modes of scratch response were identified which fell into three broad groupings: (i) tough; (ii) intermediate; (iii) brittle. These were subsequently presented in the form of a scratch mode map. The toughest coating contained 8 at.% N with an LIH ∼ 900 kg/mm 2, and did not crack, even after scratching with a 5 kg load or being indented by Rockwell C hardness testing using a 150 kg load.

Low temperature plasma surface alloying of medical grade austenitic stainless steel with carbon and nitrogen

Low temperature surface alloying with either nitrogen (nitriding) or carbon (carburizing) has been successfully employed in hardening AISI 316. However, little work has been directed towards low temperature plasma surface alloying with both nitrogen and carbon simultaneously. In addition, little or no research has been conducted on the surface modification of medical grade austenitic stainless steels, such as ASTM F138 and F1586. In this study, plasma surface alloying treatments have been conducted on medical grade ASTM F138 and ASTM F1586 as well as on engineering grade AISI 316 for comparison. Systematic materials characterization was carried out using optical microscopy, GDOES and XRD. The surface properties such as hardness, wear and corrosion of the modified surface layers were evaluated. Based on the results, the response of these materials to simultaneous plasma surface alloying with both nitrogen and carbon is compared with nitriding and carburizing processes.

Creep and fatigue interaction in a nickel-based alloy: Wang, X., Wang, D.N. and Kong, Q.P. Scr. Metall. Mater. (15 Feb. 1993) 28 (4), 401–404

An optimized low-temperature plasma carbonitriding process at 430 °C for 15 h was used to treat the surface of 316LVM medical grade austenitic stainless steel. By using a servo-hydraulic dynamic test machine, the fretting behaviour of both as-received and the surface-treated 316LVM plate samples against martensitic stainless steel balls was studied in the Ringer's solution at various displacement amplitudes. The experimental results demonstrate that the optimized low-temperature plasma surface treatment can produce a precipitate-free, C/N supersaturated S-phase layer. Such layer revealed a hardness of 5 times that of 316LVM substrate. The formation of S-phase can protect the 316LVM from corrosion in the Ringer's solution, and the hard surface-treated layer can significantly improve the fretting wear resistance of 316LVM material. When fretted in a mixed regime, a sharp drop of friction force was observed for untreated 316LVM after fretting for about 1000 cycles, which was mainly caused by the penetration of Ringer's solution into the contact area of the fretting pairs. As a comparison, no friction drop was observed through the fretting process on the surface-treated samples. Due to the high ratio of elastic modulus to hardness (E/H) of untreated sample, the main damage mechanism on untreated sample was adhesive and abrasive wear during the fretting process. However, because of the high hardness and low E/H ratio of the surface-treated sample, the main damage mechanism was mild abrasive wear. In conclusion, the low temperature plasma surface treatment can effectively improve the fretting wear properties of 316LVM stainless steel, which could pave the way to develop the high-performance, long-life body implants.