No abstract

Overlapping tracks were processed by melting preplaced titanium carbide (TiC) powder on steel surfaces using a tungsten inert gas torch. The tracks produced ~1.0 mm melt depth free from cracks, but occasional pores were observed. The microstructure consisted of unmelted and partially melted TiC particulates together with reprecipitated TiC particles, which were prominent in tracks processed in the initial stage. A greater number of reprecipitated globular and cubic TiC particles were observed in tracks processed in the later stages, indicating more dissolution of TiC particulates from the overlapping operation. Those multitracks processed in the initial stage developed a maximum hardness of 850-1000 HV, which was lower in most other tracks, although comparable hardness values were recorded in the last track.

Surface cladding utilizes a high energy input to deposit a layer on substrate surfaces providing protection against wear and corrosion. In this work, TiC particulates were incorporated by meltingsingle tracksin powder preplaced onto AISI 4340 low alloy steel surfaces using a Tungsten Inert Gas (TIG) torch with a range of processing conditions. The effects of energy input and powder content on the melt geometry, microstructure and hardness were investigated. The highest energy input (1680 J/mm) under theTIG torchproduced deeper (1.0 mm) and wider melt pools, associated with increased dilution, compared to that processed at the lowest energy (1008 J/mm). The melt microstructure contained partially meltedTiC particulatesassociated with dendritic, cubic and globular typecarbidesprecipitated upon solidification of TiC dissolved in the melt; TiC accumulated more near to the melt-matrix interface and at the track edges. Addition of 0.4, 0.5 and 1.0 mg/mm2TiC gave hardness values in the resolidified melt pools between 750 to over 1100Hv, against a base hardness of 300 Hv; hardness values are higher in tracks processed with a greater TiC addition and reduced energy input.

Surface modification by reinforcing ceramic particulates can give protection against wear and corrosion of metal. In this work, two different amounts of TiC powder of nominal size 45 to 100 µm were embedded on AISI 4340 steel surfaces by melting under a Tungsten Inert Gas (TIG) welding torch with an energy input of 2640 J/mm. The microstructure, geometry and hardness of the single track composite layers were investigated. The resolidified melt tracks were hemispherical in shape. With increasing TiC content, the melt dimensions reduced a little but the microstructure had a remarkable change. The track with 1.5 mg/mm2 TiC gave more un-melted TiC, partially melted TiC and agglomeration of ceramic particulates while the 1.0 mg/mm2 track dissolved most TiC particulates and precipitated carbides in the form of dendrite, globular and flower type particles; dendrites are almost absent in the 1.5 mg/mm2 track. A reduced TiC addition created more fluid melt which accelerated dissolution of TiC and that caused more carbide precipitation in the 1.0 mg/mm2 track compared to that with 1.5 mg/mm2 track. The 1.0 mg/mm2 track produced lower hardness of 1065 Hv compared to 1350 Hv for the 1.5 mg/mm2 track.

Wear is a common problem for engineering components subjected to dynamic loading. Surface modification is mostly applied to reduce the wear. An exploratory research is conducted to form a composite coating on AISI 4340 steel surfaces by incorporating a mixture of TiC and hexagonal Boron Nitride (h-BN) particulates using powder placement and TIG torch melting techniques. Initial results show the evidence of TiC incorporation in all tracks but the presence of h-BN is limited in a few tracks. However, processing conditions are identified that can produce composite coatings incorporating both TiC and h-BN particulates. The melt microstructure consists of a small amount of un-melted TiC and h-BN, partially melted TiC particulates with eutectic structure containing precipitated TiC and TiB2 particles. Hardness of the coating is found to fluctuate along the melt depth. However, the maximum hardness of the coating is about 3 times the base hardness of 250 HV.

A comparison of the room temperature wear behaviour of untreated low alloy steel surfaces with those containing TiC powders was conducted against an alumina ball. The coefficient of friction, the wear rate and the severity of the damage on the surface were assessed. Incorporation of powders produced a hardness 2.6 times greater and a wear rate 21 times less than the untreated steel. Friction from the third body abrasion and protruding carbides of the processed steel resulted in mild wear with a steady state coefficient friction of 0.4. Both samples showed surface chemical reactivity with the environment as a result of the generation of flash temperature producing an oxide layer, which influenced wear.

A comparison of a coating of Fe–Cr–B alloy powder processed on a plain carbon steel by surface melting using either laser or gas tungsten arc welding (GTAW) torch was made. The alloy powder was injected in laser melted surface, while the preplaced powder was melted using the GTAW technique. The topography of the GTAW tracks showed a relatively smooth surface, but the laser track surface was very rough. Because of high energy input, the GTAW method produced very wide and deep tracks with a high dilution compared to those of the laser tracks. Pores and fine cracks were seen in laser tracks; the cracks were elongated across the track width, and some of them propagated down to the melt depth. The GTAW track produced at a low energy was free from any cracking, but a centreline crack was present when processed at a high energy input.

A comparison has been made of the relationship between microstructure and microhardness developed by surface melting Nanosteel SHS 7170 Fe-Cr-B alloy powder onto a plain carbon steel surface. This powder was initially developed as a high velocity oxy fuel sprayed coating giving a strength ten times that of mild steel, and is particularly suitable for surface protection against wear and corrosion. In this study, the alloy powder was injected into the laser melted surface, while a preplaced powder was melted using the gas tungsten arc welding (GTAW) technique. The laser track consisted of fine dendrites and needle-like microstructures which produced a maximum hardness value of over 800 HV, while the GTAW track produced a mixture of equiaxed and columnar grain microstructures with a maximum hardness value of 2 670 HV. The lower hardness values are considered to be associated with dilution and grain size.