Steels for bearings

Chemical stability, mechanical behaviour and biocompatibility in body fluids and tissues are the basic requirements for successful application of implant materials in bone fractures and replacements. Corrosion is one of the major processes affecting the life and service of orthopaedic devices made of metals and alloys used as implants in the body. Among the metals and alloys known, stainless steels (SS), Co-Cr alloys and titanium and its alloys are the most widely used for the making of biodevices for extended life in human body. Incidences of failure of stainless steel implant devices reveal the occurrence of significant localised corroding viz., pitting and crevice corrosion. Titanium forms a stable TiO2 film which can release titanium particles under wear into the body environment. To reduce corrosion and achieve better biocompatibility, bulk alloying of stainless steels with titanium and nitrogen, surface alloying by ion implantation of stainless steels and titanium and its alloys, and surface modification of stainless steel with bioceramic coatings are considered potential methods for improving the performance of orthopaedic devices. This review discusses these issues in depth and examines emerging directions.

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Corrosion of bio implants

Electrochemical investigation of the influence of nitrogen alloying on pitting corrosion of austenitic stainless steels

Solidification cracking is a significant problem during the welding of austenitic stainless steels, particularly in fully austenitic and stabilized compositions. Hot cracking in stainless steel welds is caused by low-melting eutectics containing impurities such as S, P and alloy elements such as Ti, Nb. The WRC-92 diagram can be used as a general guide to maintain a desirable solidification mode during welding. Nitrogen has complex effects on weld-metal microstructure and cracking. In stabilized stainless steels, Ti and Nb react with S, N and C to form low-melting eutectics. Nitrogen picked up during welding significantly enhances cracking, which is reduced by minimizing the ratio of Ti or Nb to that of C and N present. The metallurgical propensity to solidification cracking is determined by elemental segregation, which manifests itself as a brittleness temperature range or BTR, that can be determined using the varestraint test. Total crack length (TCL), used extensively in hot cracking assessment, exhibits greater variability due to extraneous factors as compared to BTR. In austenitic stainless steels, segregation plays an overwhelming role in determining cracking susceptibility.

Solidification cracking in austenitic stainless steel welds

Pitting and stress corrosion cracking behavior in welded austenitic stainless steel

Influence of nitrogen addition on microstructure and pitting corrosion resistance of austenitic weld metals.

Austenitic stainless steel weld metals have, in general, inferior corrosion resistance compared with the base metals. This is due to the fact that the weld metal has an inhomogeneous and dendritic microstructure with microsegregation of major elements (i.e., Cr, Mo, and Ni) as well as minor elements (i.e., S and P) at the δ-γ interface boundaries. The nonuniform alloying element concentration around ferrite particles plays a major role in determining the electrochemical corrosion behavior of such weld metals. Although the presence of ferrite is considered to be detrimental as far as the localized corrosion is considered, its exact role in uniform corrosion is still not clear. The uniform corrosion behavior of an alloy is determined by the fundamental electrochemical parameters of the major alloying elements. In this study, an attempt has been made to correlate the microstructure and uniform corrosion behavior of type 316 stainless steel weld metals with varying concentrations of Cr and Mo, and different ferrite contents. From the empirical equations obtained during the analysis of the electrochemical corrosion data, an attempt has been made to understand the role of Cr, Mo, and ferrite in altering the electrochemical corrosion parameters of the weld metal. Based on the extensive microstructural characterization, a dissolution model for the weld metal in the moderately oxidizing medium has been proposed.

Evaluation of microstructure and electrochemical corrosion behavior of austenitic 316 stainless steel weld metals with varying chemical compositions

The pitting resistance of AISI 316L stainless steel (SS) weld metal in the as-welded and stress-relieved conditions was evaluated as a function of heat input. The results obtained indicated a decrease in the value of critical pitting potential in acidic chloride medium as the heat input was increased. However, on stress-relief heat treatment, the critical pitting potential remained unchanged with varying heat input, though its magnitude was much lower than that of the as-welded alloy. The deterioration in the pitting resistance on increasing heat input is discussed on the basis of enhanced segregation of solute elements like chromium and molybdenum to delta-ferrite and the presence of M23C6 carbides at the austenite/ferrite interface.

Effect of Heat Input and Microstructure on Pitting Corrosion in AISI 316L Submerged Arc Welds

Role of Delta-Ferrite in the Dissolution of Passive Films on the Austenitic Stainless-Steel Weld Metals

In this study, an attempt was made to investigate the pitting and intergranular corrosion susceptibility of as-deposited and thermally aged weldments of type 316LN stainless steel (SS) made by shielded metallic arc (SMA) welding process using 316 SS filler wires having a carbon content of 0059 wt% The delta-ferrite content, which was 45 FN (Ferrite Number) for the as-welded samples, changed to 03 FN after the aging treatment (1023 K/5 h) Initial delta-ferrite on thermal aging decomposed to various secondary phases like sigma and M23 C6 and chi These phases were confirmed metallographically and identified by the powder x-ray diffraction method Pitting corrosion resistance was determined at ambient temperature in neutral chloride and acidic chloride media on both as-welded and thermally aged weld metal samples by anodic polarization technique The results indicated that in neutral and acidic chloride media, the pitting corrosion resistance of thermally aged samples was lower compared to that

T. P. S. Gill

Papers

Influence of nitrogen addition on microstructure and pitting corrosion resistance of austenitic weld metals.

Evaluation of microstructure and electrochemical corrosion behavior of austenitic 316 stainless steel weld metals with varying chemical compositions

Effect of Heat Input and Microstructure on Pitting Corrosion in AISI 316L Submerged Arc Welds

Role of Delta-Ferrite in the Dissolution of Passive Films on the Austenitic Stainless-Steel Weld Metals

Susceptibility of As-Welded and Thermally Aged Type 316LN Weldments Toward Pitting and Intergranular Corrosion