Tara Chandra

Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar 3, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar 3, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar 3 temperature, decreased the time of transformation, and increased the hardness.

Microstructural Characterisation of Railhead Damage in Insulated Rail Joints

As an integral part the railway network infrastructure, insulated rail joints (IRJs) electrically isolate track segments providing critical feedback to both track signaling and train position detection systems. Because of the discontinuous nature of IRJs, accumulated damage at the railhead is high. Failure modes include plastic flow of metal across joints, bolt and fishplate failures, delamination of insulated material and, as a result of rolling contact fatigue, end post and endpost surface damage. In the current investigation, microstructural changes in the vicinity of endposts of IRJs made from both surface coated and uncoated rail are investigated using techniques of optical and scanning electron microscopy. Damaged IRJs made from pearlitic head hardened rail steel are compared with head hardened rail steel laser coated with martensitic stainless steel, the latter having an increased service life. Problems associated with the surface coating are identified and approaches to further improving IRJ resistance to rolling contact fatigue suggested. Keywords: Insulated rail joints, rail, head hardened, surface coated rail

The Effect of Boron on the Hardness and Fracture Toughness of WC-FeAl-B and WC-Ni3Al-B Composites

The hardness and indentation fracture toughness of sub micron WC composites based on aluminide and cobalt binders were investigated. Doped Fe60Al40 and Ni3Al alloys with boron levels ranging from 0 to 0.1 wt%, were used as the aluminide binders. The composite materials were processed by uniaxial hot pressing of milled powder samples at 1500 °C under argon atmosphere. The hardness of WC-40vol%(FeAl-B) was found to be higher than that of WC-40vol%(Ni3Al-B), and it approached to the hardness level of the commercial grade of WC-10wt%Co (H10F). The fracture toughness of both WC-40vol%(FeAl-B) and WC-40vol%(Ni3Al-B) cermets was higher than that of WC-40vol%Co and the toughness increased with increasing boron content. It is believed that boron addition to the aluminide binders leads to improvement in the fracture toughness of the intermetallic matrix composites as a result of increase in the ductility and toughness of the aluminides and also due to increase in WC solubility in the aluminide binders in presence of boron.

Effect of Boron on the WC Morphology in Sub Micron Tungsten Carbide-Aluminide Composites

The effect of boron on the WC morphology and on the grain size of binders in sub micron WC composites containing Fe60Al40 and Ni3Al binders was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composites were prepared under uniaxial hot pressing of milled powder samples at 1500 °C in inert argon atmosphere. Doped aluminides with boron levels ranging from 0 to 0.1 wt% were used as the binders. It was found that the microstructural characteristics of boron doped aluminide WC composites were similar to those of hot pressed WC-Co and commercial grade WC-10wt%Co (H10F) hardmetals. The contiguity of WC particles (WC/WC contact) and the grain sizes of aluminides decreased and the extent of faceting of tungsten carbide increased in the aluminide tungsten carbide composites in presence of boron.

In Situ Observations of the Densification Behavior of WC-FeAl-B Composites during Liquid Phase Sintering

The densification behavior of WC composites based on iron aluminide binder was investigated using laser scanning confocal mi¬croscopy (LSCM). Doped Fe60Al40 alloys with boron levels ranging from 0 to 0.1 wt% were used as the aluminide binders. The aluminide binders were prepared using controlled atmosphere ring grinding and then blended with WC powder. The composite powder compacted in an alumina crucible and held in a platinum holder in the confocal microscope. The temperature increased from ambient temperature up to 1500 °C under high purity argon. The presence of boron was found to facilitate compaction of the composites and improve the wetting between WC and FeAl binder during liquid phase sintering. Increasing the amount of boron in the binder resulted in the melting of binder at lower temperature and increasing of the compacting of the intermetallic tungsten carbide composites.

Hydrogen Measurements in SiNx: H/Si Thin Films by ERDA

Thin SiN film deposited on Si by plasma enhanced chemical vapour deposition (PECVD) is used for surface passivation of Si. During the PECVD process Hydrogen is incorporated into the SiN film, and the passivation properties of the resulting SiNx:H layers play an important role in enhancing the energy conversion efficiency of solar cells. It is believed that the Hydrogen present in SiNx:H is responsible for this enhancement, and therefore its concentration in the passivating layer is an important parameter. The Hydrogen composition and its depth profile in thin SiNx:H films of 20nm to 200nm was measured by elastic recoil detection analysis (ERDA), using a 1.7MeV He+ ion beam of (1x2)mm2, generated by a high stability 2MV Tandetron ion beam accelerator. Simultaneously, Rutherford backscattering (RBS) spectra were recorded for each sample. The results show that the Hydrogen concentration in the SiNx:H layers is dependent of the deposition conditions. Also, Hydrogen was found to be homogenously distributed across the SiNx:H layer thickness, and the SiNx:H/Si interfaces were well defined.

Recrystallization of Ferrite and Austenite

A combination of recrystallization controlled processing, optimization of steel chemical composition is leading the way for achieving fine-grained and ultrafine-grained steels. These steels have desired combination of most sought-after properties such as ultra high strength, high toughness, good formability, and weldability at a reasonable cost. An understanding of recrystallization of ferrite and austenite is thus of crucial importance as these phases undergo thermomechanical processing (TMP) of steel. As steel is deformed at high temperature internal stresses of deformation are relieved through several competing softening (restoration) processes. This article summarizes the concepts involved, and the kinetics, characteristics, and industrial applications of recrystallization as it occurs in ferrite and austenite as a function of complex combinations of several of these factors.

Microstructural Features of Plasma Nitrided Molybdenum Alloy Steel

This research is part of a larger project to investigate the wear and friction of the centre bearing of a rail freight truck. Existing centre bearing surfaces include flame hardened AISI 1030 steel and AISI 1053 cast steel top centres mating against un-greased and/or greased Hadfield steel centre bowl liners, and polyethylene centre bowl liners. The wear life of the unlubricated steels against Hadfield steel is short, greasing the bearings is costly, and industry reports some failures of polyethylene centre bowl liners due to excessive plastic flow and cracking of the rim wall.

Insulated Rail Joints for Signalling Applications

Insulated rail joint assemblies provide electrical insulation between two sections of rail for signalling purposes. In this work, rail steel was successfully bonded to PSZ ceramic using an active brazing technique. In order to increase the wettability of the PSZ ceramics, titanium coating was deposited on the ceramic surface using a filtered arc deposition system. A filler metal called BVAg-18 (60%Ag-30%Cu-10%Sn) was used and the joining was performed at a temperature of 750 °C. Bonding between partially stabilised zirconia and rail steel with BVAg-18 filler metal was not achieved using a standard brazing method. Bonding did occur with the BVAg-18 filler metal using the advanced brazing technique of active metal brazing, with best results obtained using a brazing temperature of 750oC and a dwell time of 10 minutes. The microstructure of the coating and joint interface were characterised by XRD, SEM and EDS.

Continuous wave Nd:YAG laser cladding of stellite 6 multi-track layers on mild and AISI 4016 steel

The level of sliding friction between the mating top centre and centre bowl liner components of a rail freight truck centre bearing provides both positive and negative effects on rail vehicle dynamics. Currently used centre bearing surfaces include AISI 1053 steel top centres mating against unlubricated and lubricated Hadfield steel, and polyethylene centre bowl liners. The wear life of the unlubricated AISI 1053 steel top centre against Hadfield steel is short, whilst the low friction of the lubricated and polyethylene surfaces can be detrimental to vehicle dynamics. It is proposed to use laser clad Stellite 6 as one of the bearing surfaces. Stellite 6 multi-track layers were laser cladded on mild and AISI 4016 steel substrates with a continuous wave Nd:YAG laser at 1800 W laser source power using four different processing speeds: 600, 900, 1200, and 1500 mm/min, whilst the laser power, defocused laser spot size, and powder feed rate were constant. The cladding samples were characterised using optical microscopy and scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS). Microhardness profiles of the cladded layer and heat affected zone were determined. The effect of processing speed on cladded layer height, depth of penetration and dilution has been examined. The main difference between the samples clad onto the two different substrates was the microstructure and hardness of the heat affected zone. Untempered martensite was observed in the heat affected zone of the AISI 4016 steel substrate at 1500 mm/min. For both substrates the optimum processing speed is between 600 and 900 mm/min.