Kamaraj M

In the present investigation, a nickel based hardfacing alloy (AWS NiCr-B) was deposited on an austenitic stainless steel substrate 316LN using the plasma transferred arc welding process. The deposit was characterised by hardness measurements, microstructural examination and sliding wear assessment. Identification of precipitates was carried out using X-ray diffraction and SEM/ EDAX. These studies revealed the presence of chromium rich carbides and borides in a γ-Ni matrix. Down to a distance of 1 mm from the interface, the hardness of the deposit was found to be 52 HRC. The sliding wear behaviour of the hardfacing alloy was investigated in air in the room temperature to 550°C range, with a pin on disk configuration using a cylindrical pin with tip radius of 3 mm under loads of 30, 40 and 50 N. Wear experiments were conducted up to a sliding distance of 180 m at a sliding speed of 0-1 m s -1. The elastic modulus and Poisson's ratio of the hardfaced deposits were evaluated by the ultrasonic method and these values were used for calculating initial Hertzian contact stress. The study showed that, while significant wear loss occurred at room temperature, there was practically no measurable weight loss at temperatures of 300 and 550°C. This could be attributed to the formation of an oxide layer at the surface during wear testing. © 2006 Institute of Materials, Minerals and Mining.

Fretting is a form of adhesive wear normally occurring at the contact points gradually leading to premature of load bearing medical implants made of titanium alloys. The aim of this work is to characterize the fretting fatigue damage features of PVD TiN coated, plasma nitrided and thermally oxidized Ti-6Al-4V and Ti-6Al-7Nb contact pairs. Fretting damage is applied with calibrated proof ring and contact pad arrangement. The results are compared with fretting damage of uncoated alloys. The damage progression during fretting process is apparently explained with friction coefficient curves. Plasma nitrided pairs performed better in terms of fretting fatigue lives with low friction coefficient of friction. PVD TiN coated pairs have experienced early failures due to third body mode of contact interaction with irregular friction coefficient pattern. Thermally oxidized pairs have experienced early failures due to high case thickness as well as irregular development of modified layer. © 2005 Elsevier B.V. All rights reserved.

Modified 9Cr-1Mo steel, designated as P91, is widely used in the construction of power plants and other sectors involving temperatures higher than 500 °C. Although the creep strength is the prime consideration for elevated temperature applications, notch toughness is also important, especially for welded components, as it is essential to meet the pressure test and other requirements at room temperature. P91 steel weld fusion zone toughness depends on factors such as welding process, chemical composition, and flux composition. Niobium and vanadium are the main alloying elements that significantly influence the toughness as well as creep strength. In the current work, weld metals were produced with varying amounts of niobium and vanadium by dissimilar joints involving P9 and P91 base metals as well as filler materials. Microstructural studies and Charpy V-notch impact testing were carried out on welds to understand the factors influencing toughness. Based on the results, it can be concluded that by reducing vanadium and niobium weld metal toughness can be improved. © ASM International.

The present work describes an investigation of the effect of 3 different parameters of laser surface alloying-i.e., laser scanning speed (LSS), nozzle stand-off distance (NSOD) and laser beam scan-off distance (LBSOD) on coating height, depth and width. Nickel-based Colmonoy 88 alloy powder has' been deposited on 13Cr-4Ni steel by single-step process of laser surface alloying. Laser power and powder feed rate were maintained at 3kW and 25 g/min, respectively. L8 orthogonal array has been designed to study these 3 parameters at 2 levels each. The results of single pass with extent of dilution, surface hardness and microstructures produced by different conditions are presented and discussed. For a specified NSOD and LBSOD, there was a decrease in coating height and depth with increase in LSS. Coating height and depth were not affected much by increase in NSOD. From the present investigation, optimized parameters were identified for enhanced hardness, minimum dilution and desired coating height and coating depth.

In the current investigation, nickel-base hardfacing alloy AWS NiCr-B was deposited on austenitic stainless steel substrate 316 LN using the plasma transferred arc-welding process. The deposit was characterized by hardness measurements, microstructural examination and sliding wear assessment. Identification of precipitates was carried out using X-ray diffraction, scanning electron microscope/energy dispersive X-ray analysis (SEM/EDAX), electron probe micro analyzer (EPMA) mappings and line-scan profiles. The microstructure of the hardfacing deposit predominantly consists of the γ-Ni phase and the interdendritic eutectic mixture comprised of γ-nickel and nickel-rich borides. These studies also revealed the presence of chromium-rich carbides and borides in a γ-nickel matrix. The sliding wear behaviour of the hardfacing alloy was investigated in air at three different temperatures viz., room temperature, 300 and 500 °C. The study revealed significant weight loss at room temperature and abrupt decrease at high temperatures. This behaviour at high temperatures has been attributed to the formation of a wear protective oxide layer at the surface during sliding. To evaluate the microstructural stability of the deposit, ageing studies were carried out at 650 °C for 250 h. Microstructural examination and hardness testing revealed that there is no deterioration in the microstructure and that the hardness remains intact. Sliding wear tests at room temperature and at high temperatures also demonstrated that there is no significant change in the weight loss or the wear behaviour after the thermal exposure. © 2007 Elsevier B.V. All rights reserved.

Fretting wear resistance of uncoated and surface modified biomedical titanium alloys (Ti-6Al-7Nb) in air and Ringer fluid has been investigated in the present work. Laser nitrided specimen has shown superior performance with minimum surface damage and wear rate (<0.1 × 10-6 mm3/Nm) despite high friction coefficient in air (0.6) compared to uncoated and plasma nitrided (>12 × 10-6 mm3/Nm) specimens. This is due to presence of TiN dendrites (60-80%) in the laser nitrided layer. Plasma nitrided surface is relatively softer and hence wear rates and surface damage are comparable with uncoated alloys. Friction coefficient is high for uncoated (0.8) and plasma nitrided alloys (0.6) in air as well as Ringer fluid. Fretting induced electrochemical dissolution is responsible for higher wear rates in uncoated and plasma nitrided specimens. The fretting damage resistance is primarily governed by relative hardness and modified layer thickness of the mating components. © 2006 Elsevier B.V. All rights reserved.

Modified 9Cr-1Mo steel is widely used in the construction of power plants. Flux-shielded processes result in inadequate weld metal toughness due to the presence of inclusions. An acidic-coated electrode with primary constituent of rutile in flux coating tends to produce inferior toughness due to the presence of coarse microinclusions in the resultant weld. In the present study, welds produced using acidic-coated electrodes were given a surface melting using TIG process to refine the inclusions. There was significant reduction in number of coarse microinclusions and increase in number of fine microinclusions. Charpy V-notch test was conducted at room temperature to evaluate the toughness of weld. Surface melted welds have superior toughness compared to unmelted welds. Fractographic features correlate well with the observed impact energy values of welds. © 2008 Elsevier B.V. All rights reserved.

The use of metals and materials for replacement and repair of human body parts are attracting more attention in recent times. Like any other components in service, biomaterials also undergo degradation due to fretting, wear and corrosion. Fretting wear, fretting fatigue and fretting corrosion are the three main areas of concern for the orthopedic surgeons. This paper reviews fretting fatigue along with various methodologies and mechanisms. Fretting of materials is controlled by several sets of variables working synergistically, making the process difficult to quantify. A fretting test rig for biomaterials has been developed simulating the conditions of the actual implants as close as possible. Fretting fatigue life is also controlled by contact geometries, which delay or accelerate the crack initiation. Several contact geometries have been mentioned which can influence the fretting life of the materials. Fretting conditions are also governed by normal pressure and slip amplitude regime in fretting maps. Physiological medium may aggravate or reduce the fretting failures depending on the nature of surface and the medium. Titanium alloys have been established as the most suitable materials for bio implants due to their attractive properties within the body environment. Some important aspects of the fretting damage of these alloys are mentioned in this paper. Fretting fatigue life of these alloys can be significantly improved by surface modification with specialized techniques such as plasma nitriding, ion implantation and Physical Vapour Deposited TiN coatings. The paper describes details of these methods as well.

Fretting fatigue is a form of adhesive wear damage due to small oscillatory movement between two contacting bodies under the action of uniform or non-uniform cyclic loads. Cyclic loads may be experienced due to vibration of one or both the bodies eventually leading to failure at the contact area. Fretting damage is also experienced by load bearing implants within the body environment such as hip joints, knee joints, bone plates, etc. Damage characterization is important from the view of minimizing in-vivo failures. Titanium alloys are frequently used as bioimplants due to its excellent biocompatibility and low modulus of elasticity compared to stainless steel or Co-Cr-Mo alloys. Fretting wear damage of load bearing implants can be minimized through suitable surface modification process. Ti-6Al-4V and Ti-6Al-7Nb are commonly used for biomedical applications and PVD TiN coated alloys are used for our fretting fatigue studies. Fretting fatigue life of PVD TiN coated alloys improved compared to uncoated alloys. © 2005 Elsevier B.V. All rights reserved.

Currently nickel-base single crystal (SX) superalloys are considered for the manufacture of critical components such as turbine blades, vanes etc., for aircraft engines as well as land-based power generation applications. Microstructure and high temperature mechanical properties are the major factors controlling the performance of SX superalloys. Rafting is an important phenomenon in these alloys which occurs during high temperature creep. It is essential to understand the rafting mechanism, and its characteristics on high temperature properties before considering the advanced applications. In this review article, the thermodynamic driving force for rafting with and without stress is explained. The nature and influence of rafting on creep properties including pre-rafted conditions are discussed. In addition, the effect of stress state on γ/γ′ rafting, kinetics and morphological evolution are discussed with the recent experimental results.