Nitrogen Containing Austenitic Stainless Steels

Nickel and nitrogen are the two most widely used alloying elements which can impart the face‐centered‐cubic crystal lattice to stainless steels. With the recent price increases and the price volatility of nickel, nitrogen is ever more important as an alloying element for a number of reasons. First, nitrogen is easily available everywhere and thus is not subject to speculation at the Metal Exchange. Second, in addition to making stainless steels austenitic, nitrogen can also make them stronger and more corrosion resistant. It is also a well and clearly established fact since many years, that nitrogen in solid solution makes austenitic stainless steels more wear resistant and more fatigue resistant. Austenitic stainless steel alloy design with nitrogen has for many years now taken account of the role of carbon. This is not only because carbon is just a useful austenite former, but also because nitrogen reduces the temperature where carbides begin to form. Thus there is always an optimum carbon to nitrogen ratio. Finally it is now well established that carbon in solid solution helps to increase the strength, the corrosion resistance and the wear resistance of austenitic stainless steels. A number of quantitative correlations between alloy composition and materials properties are presented and their useful role in alloy design is pointed out. This will further help to lower the nickel content in austenitic stainless steels or even replace nickel altogether.     

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.     

A review on nickel-free nitrogen containing austenitic stainless steels for biomedical applications

The field of biomaterials has become a vital area, as these materials can enhance the quality and longevity of human life. Metallic materials are often used as biomaterials to replace structural components of the human body. Stainless steels, cobalt–chromium alloys, commercially pure titanium and its alloys are typical metallic biomaterials that are being used for implant devices. Stainless steels have been widely used as biomaterials because of their very low cost as compared to other metallic materials, good mechanical and corrosion resistant properties and adequate biocompatibility. However, the adverse effects of nickel ions being released into the human body have promoted the development of “nickel-free nitrogen containing 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 and emphatically the advantages of nitrogen in stainless steel, as well as the development of nickel-free nitrogen containing stainless steels for medical applications. By combining the benefits of stable austenitic structure, high strength, better corrosion and wear resistance and superior biocompatibility in comparison to the currently used austenitic stainless steel (e.g. 316L), the newly developed nickel-free high nitrogen austenitic stainless steel is a reliable substitute for the conventionally used medical stainless steels.     

Nickel-free Stainless Steel for Medical Applications

BIOSS4 steel is essentially a nickel-free austenitic stainless steel developed by the Institute of Metal Research, Chinese Academy of Sciences, in response to nickel allergy problems associated with nickel-containing stainless steels that are widely used in medical applications. The high nitrogen content of this steel effectively maintains the austenitic stability and also contributes to the high levels of corrosion resistance and strength. BIOSS4 steel possesses a good combination of high strength and toughness, better corrosion resistance, and better blood compatibility, in comparison with the medical 316L stainless steel. Potential applications of BIOSS4 steel can include medical implantation material and orthodontic or orthopedic devices, as well as jewelries and other decorations.     

Super austenitic stainless steels — a promising replacement for the currently used type 316L stainless steel as the construction material for flue-gas desulphurization plant

Potentiodynamic anodic cyclic polarization experiments on type 316L stainless steel and 6Mo super austenitic stainless steels were carried out in simulated flue-gas desulphurization (FGD) environment in order to assess the localized corrosion resistance. The pitting corrosion resistance was higher in the case of the super austenitic stainless steel containing 6Mo and a higher amount of nitrogen. The pit-protection potential of these alloys was more noble than the corrosion potential, indicating the higher repassivation tendency of actively growing pits in these alloys. The accelerated leaching study conducted for the above alloys showed that the super austenitic stainless steels have a little tendency for leaching of metal ions such as iron, chromium and nickel at different impressed potentials. This may be due to surface segregation of nitrogen as CrN, which would, in turn, enrich a chromium and molybdenum mixed oxide film and thus impedes the release of metal ions. The present study indicates that the 6Mo super austenitics can be adopted as a promising replacement for the currently used type 316L stainless steel as the construction material for FGD plants.     

高氮低镍奥氏体不锈钢的研究进展

高氮低镍奥氏体不锈钢是一种以氮代镍来获得稳定奥氏体组织的新钢种,它不但可以提高不锈钢的综合性能、节约镍资源,而且可以解决含镍较高的不锈钢用于人体时造成的镍过敏问题,在生物医学领域应用潜力巨大.综述了高氮低镍奥氏体不锈钢的发展历史和现状、不锈钢中氮的作用及高氮钢的主要制备工艺.     

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.     

Development of high nitrogen, low nickel, 18%Cr austenitic stainless steels

Two high nitrogen stainless steels are studied through metallographic, mechanical and corrosionistic tests and the results are compared with those shown by a standard AISI 304. These high nitrogen steels show a significantly higher mechanical strength than usual AISI 304 while their corrosion resistance lie among that of standard austenitic and that of standard ferritic stainless steels.     

Fatigue and Corrosion Fatigue of High-Nitrogen Austenitic Stainless Steel

High nitrogen contents in solid solution as well as appropriate strengthening mechanisms in austenitic stainless steels can result in very high corrosion resistance. This is true in both air environment and in simulated human body fluids (corrosion fatigue). High cycle corrosion fatigue data are listed and compared with similar data for titanium base and cobalt base implant materials. Thus high nitrogen austenitic stainless steels are candidates to replace other stainless steels as implant materials.     

Mechanisch-technologische Eigenschaften nickelhaltiger Superferrite

Mechanical and Technological Properties of Nickel Containing Superferrites High chromium ferritic stainless steels with molybdenum and nickel additions, containing low amounts of interstitials, show remarkably good mechanical properties besides their excellent corrosion behaviour. Yield strengths of these materials can be more than the twofold, compared with that of austenitic stainless steels. In contrary to commercial ferritic stainless chromium steels, the Superferrites exhibit remarkable notch impact toughness also at temperatures below 0 °C. These properties as well as their permanent toughness after a welding process permit a good technological workability and, because of the high strength properties, the application of thinner dimensions in construction.     

The influence of the nitrogen/nickel‐ratio on the cyclic behavior of austenitic high strength steels with twinning‐induced plasticity and transformation‐induced plasticity effects

Austenitic high nitrogen (AHNS) and austenitic high interstitial steels (AHIS) are of interest for mechanical engineering applications because of their unique combination of mechanical (strength, ductility), chemical (corrosion resistance) and physical (non‐ferromagnetic) properties. But despite their high strength values e. g. after cold deformation up to 2 GPa in combination with an elongation to fracture of 30 %, which is based on twinning‐induced plasticity (TWIP) mechanisms and transformation‐induced plasticity (TRIP) mechanisms, the fatigue limit remains relatively small. While for chromium‐nickel steels the fatigue limit rises with about 0.5‐times the elastic limit it does not at all for austenitic high‐nitrogen steels or only to a much smaller extent for nickel‐free austenitic high‐interstitial steels. The reasons are still not fully understood but this behavior can roughly be related to the tendency for planar or wavy slip. Now the latter is hindered by nitrogen and promoted by nickel. This contribution shows the fatigue behavior of chromium‐manganese‐carbon‐nitrogen (CrMnCn) steels with carbon+nitrogen‐contents up to 1.07 wt.%. Beside the governing influence of these interstitials on fatigue this study displays, how the nitrogen/nickel‐ratio might be another important parameter for the fatigue behavior of such steels.     

Cytotoxicity study of plasma-sprayed hydroxyapatite coating on high nitrogen austenitic stainless steels

Stainless steel has been frequently used for temporary implants but its use as permanent implants is restricted due to its low pitting corrosion resistance. Nitrogen additions to these steels improve both mechanical properties and corrosion resistance, particularly the pitting and crevice corrosion resistance. Many reports concerning allergic reactions caused by nickel led to the development of nickel free stainless steel; it has excellent mechanical properties and very high corrosion resistance. On the other hand, stainless steels are biologically tolerated and no chemical bonds are formed between the steel and the bone tissue. Hydroxyapatite coatings deposited on stainless steels improve osseointegration, due their capacity to form chemical bonds (bioactive fixation) with the bone tissue. In this work hydroxyapatite coatings were plasma-sprayed on three austenitic stainless steels: ASTM-F138, ASTM-F1586 and the nickel-free Böhler-P558. The coatings were analyzed by SEM and XDR. The cytotoxicity of the coatings/steels was studied using the neutral red uptake method by quantitative evaluation of cell viability. The three uncoated stainless steels and the hydroxyapatite coated Böhler-P558 did not have any toxic effect on the cell culture. The hydroxyapatite coated ASTM-F138 and ASTM-F1586 stainless steels presented cytotoxicity indexes (IC_50%) lower than 50% and high nickel contents in the extracts.     

Effect of nitrogen and nickel on the microstructure and mechanical properties of plasma welded UNS S32760 super-duplex stainless steels

Super-duplex stainless steels present excellent combination of mechanical and corrosion resistance, due to their strict composition control and ferrite–austenite phase balance. This balance may, however, be disturbed during welding in both the fusion and HAZ due to the rapid cooling rates and may lead to loss of the good corrosion and mechanical properties of the weldments. The present investigation is studying the effect of nitrogen addition in the plasma operation gases and of the increase of nickel in the filler metal, on the microstructure and on the mechanical properties of super-duplex stainless steels welded by the plasma transferred arc (PTA) technique. Results have shown that nitrogen addition in the plasma operation gas affects the mechanical properties of the weldments. It is shown that nitrogen addition in the plasma and protective gas and higher nickel content in the filler metal have both a positive effect on the elongation of the welded specimens and after optimization of the welding parameters very good results may be obtained in terms of tensile strength.     

Stainless steels suited for solution nitriding

Solution nitriding is a new heat treatment to yield a high nitrogen case on stainless steels at 1100 ± 50°C. Combining experimental results and thermodynamic calculation steels are selected to give a hard martensitic or high strength austenitic case. Especially developed steels are discussed as well as the suitability of standard grades. A martensitic case is combined with a martensitic core in steel Cr13C0.2 and with a softer ferritic‐martensitic core in steel Cr13C0.1. The nitrogen content of an austenitic case increases with the Cr/Ni ratio, e.g. in the order of Cr17Ni12Mo2, Cr18Ni10, Cr22Ni5Mo3N0.2. The duplex microstructure of the latter provides the highest yield strength in the core. It is essential to stay clear of the austenite/austenite + M₂N boundary and avoid precipitates which deteriorate the fatigue and corrosion resistance. Seventeen steels are assessed in this report.     

Mechanical properties and corrosion behaviour of developed high nitrogen high manganese stainless steels

Influence of composition, specifically manganese and nitrogen content, on the microstructure associated corrosion resistance property of newly developed stainless steel has been studied. The developed steels have been characterised for their microstructure, mechanical and electrochemical properties. The results indicate that the addition of manganese and nitrogen as a substitute for nickel favours the austenite microstructure, higher yield strength (>350 MPa), tensile strength (>700 MPa), elongation and superior Charpy V-notch impact toughness properties. The results obtained from electrochemical tests such as potentiodynamic polarisation and electrochemical impedance spectroscopy of manganese stainless steel show remarkable improvement (about 4 times) in corrosion resistance exhibiting passivity behaviour like that of commercial stainless steel (316L).     

Development of an austenitic chromium-manganese steel with high hydrogen resistance

We compare the results of studying the effect of hydrogen on the characteristics of short-time strength and low-cycle durability of austenitic stainless steels and age-hardening alloys. We show that austenitic stainless 06Kh20N16G6AF steel with solid-solution strengthening and the optimal content of nickel, manganese, and nitrogen possesses high mechanical characteristics and is least inclined to hydrogen degradation. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 5, pp. 95–100, September–October, 2005.     

Some Characteristics of Automotive Exhaust Valve Alloys

The PH stainless steels and the nickel-base superalloys, can be evaluated for exhaust valve applications by considering their metallurgical, environmental and high temperature strength properties. The PH stainless steels are characterized by their good sulfidation and high temperature strength. Good PbO corrosion resistance is achieved with the low silicon, nickel containing alloys. Stable alloys show the greatest high temperature strength which can be improved further by a solution and age treatment. Aging below the optimum temperature of 760°C results in grain boundary sensitization and low impact properties while higher temperatures produce more of the discontinuous phase. The addition of refractory elements can be detrimental to the oxidation resistance of these alloys. The highest elevated temperature strength and best PbO and oxidation resistance is achieved with the nickel-base superalloys. These alloys are completely stable and highest strength at elevated temperatures is achieved with a solution and age treatment. These alloys show lower sulfidation corrosion resistance relative to the PH stainless steels, however this can be improved with higher chromium contents.     

医用不锈钢的研究与发展

不锈钢由于具有优异的力学性能、耐蚀性能和加工性能而被广泛应用于各种医疗器械及手术工具的制造。概述了医用不锈钢的特点和临床应用,以及存在的主要问题,并以高氮无镍奥氏体不锈钢、不锈钢表面改性、抗菌不锈钢为重点,介绍了医用不锈钢近年来在国内外的主要研究进展。表明医用不锈钢的研究与发展,进一步提高或改善了不锈钢的生物安全性、力学性能、耐蚀性能,甚至带来了一些生物功能化,为医用不锈钢的临床应用带来了新的机遇。     

The effect of TiN inclusions and deformation-induced martensite on the corrosion properties of AISI 321 stainless steel

Stainless steel of type 321 is commonly used for the production of exhaust systems because of its temperature resistance and welding properties, which are better than those of AISI 304 or similar steels. AISI 321 is a titanium stabilized austenitic stainless steel, where this element is added to form carbides in order to avoid chromium impoverishment due to chromium carbide formation. Cold shaping can, in the case of austenitic stainless steel, cause the formation of deformation induced martensite, which can improve its mechanical properties, but unfortunately can also spoil its good resistance to corrosion. Titanium nitride inclusions are cathodic with respect to steels, and therefore cause their anodic dissolution. Martensite is, however, more susceptible to the corrosion than austenite in steels. The main aim of this study was to analyze the pitting corrosion and stress corrosion cracking which is initiated on prototype cold-formed outer exhaust sleeves during the testing of different cleaning procedures before chromium plating. Various microscopic methods were used to identify the initiation of corrosion and its propagation.     

Thermal spraying of a nitrogen alloyed austenitic steel

Stainless steel coatings provide an alternative to protect steel surfaces against corrosive attack. The 316 L stainless steel coatings have been conventionally produced by different spraying processes for such applications. Because the nitrogen alloyed stainless steels exhibit not only superior mechanical properties, but also better corrosion behaviour than conventional stainless steels, in this study the coatings of a nitrogen alloyed austenitic steel were produced using a high velocity oxy-fuel (HVOF) spraying process and an atmospheric plasma spraying (APS) process. Due to much stronger deformation strengthening, the coatings deposited by the HVOF spraying process presented a much higher microhardness than the coatings deposited by the APS process. Moreover, the coatings deposited by the HVOF spraying process were also more corrosion resistant than the coatings deposited by the APS process, because the oxidation of the powder material during HVOF spraying was much lower than that during APS. Compared with the coatings of the conventional stainless steel 316 L, the nitrogen alloyed steel coating deposited by the HVOF spraying process showed a much better corrosion performance.