Shahjahan Mridha

The role of SiC particulate and Al2O3 (Saffil) fibers in several alloys during the formation of in situ MMCs developed by laser processing

Abstract Laser surface treatments have recently been developed for the incorporation of ceramic particulate into metals or alloys, to produce a surface in situ MMC (metal matrix composite) layer. Based on the laser processes of alloy-ceramic systems carried out in the present work, some characteristics of the ceramic, such as the energy absorption, the thermal conductivity, the dissolution, and the relative density, are found to be very important in the process. The energy absorption and the thermal conductivity together can be used as one criterion for the selection of the ceramic to be used in the process described in this work, and the extent of dissolution and the relative density of ceramic should be given particular consideration in designing the processing conditions. The purpose of this work is to investigate the basic influences of SiC particulate and Al2O3 fibre on the formation of an MMC layer by laser processing, and it is believed that the discussions in the present work will be applicable for a wider use of this laser process.

Incorporation of TiC Particulates on AISI 4340 Low Alloy Steel Surfaces via Tungsten Inert Gas Arc Melting

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 melting single tracks in 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 the TIG torch produced 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 melted TiC particulates associated with dendritic, cubic and globular type carbides precipitated 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/mm2 TiC 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.

An Overview of Sensitization Dynamics in Ferritic Stainless Steel Welds

Besides the problem of low ductility and poor notch toughness of ferritic stainless steel welds due to the microstructure characteristics of the weld section as a result of the weld heat input rate and the heat transfer factor, susceptibility to intergranular corrosion caused by the depletion of the chromium content of the weld matrix particularly in the HAZ is a major concern limiting the full deployment of the material in certain engineering applications regardless of its attractive economics combined with moderate strength and excellent corrosion resistance in alkali and acidic environments. Several attempts had been made to solve the problem. In the present work, a generic review of the sensitization problem in ferritic stainless steel welds as well as remediation techniques is presented. While stabilization is the most practiced prevention technique, it appears that the control of weld heat input and by extension the cooling rate is the ultimate option to prevent the onset of sensitization and control susceptibility to intergranular corrosion; however, the specific range of welding current and speeds that forms the given range of weld heat input needs to be determined.

Influence of shielding gases on preheat produced in surface coatings incorporating SiC particulates into microalloy steel using TIG technique

Abstract The use of a tungsten inert gas (TIG) welding torch has resulted in the development of an economical route for surface engineering of alloys, giving similar results to the more expensive high power laser. Owing to the preheating generated by both techniques, the extent of the temperature rise is sufficient to produce significant changes to the melt dimensions, microstructure and properties between the first and last tracks melted during the coating of a complete surface. The present study examines if similar changes can occur between the start and finish locations of a single track of 50 mm length. The results show that for a TIG melted surface of a microalloy steel substrate, with or without incorporating preplaced SiC particles, in either argon or argon–helium environments, a maximum temperature of 375°C developed in the second third of the track. Even over this short distance, a hardness decrease of >300 HV was recorded in the resolidified SiC coated substrate melt zone, microstructure of a cast iron with cracks was observed. In addition, porosity was found in all the tracks, with and without preplaced SiC powders.

Grain Refinement in Ferritic Stainless Steel Welds: The Journey so Far

The ferritic stainless steel is a low cost alternative to the most often adopted austenitic stainless steel due to its higher strength, better ductility and superior corrosion resistance in caustic and chloride environments. However, the application of ferritic steel is limited because of poor ductility and notch impact toughness of its weld section with differential grain structures. Several techniques have been explored to control the grain features of the weld to minimize these problems. In the present effort, a review of these options in relation to the degree of grain refinement in ferritic stainless steel weld is conducted in order to have a better understanding about the grain refining phenomenon in the weld microstructure. So far, the most effective technique is found to be the pulse AC TIG welding which can produce weld with mechanical properties equivalent to 65% to those of the base metal. The refinement in this process occurred through dendrite fragmentation and grain detachment in the weld pool producing small-grained microstructures with a large fraction of equiaxed grains. However, in friction welding process where heat input and heat transfer are effectively controlled, the strength can be as high as 95% of the parent metal. This suggests that the total energy input for welding and heat transfer phenomenon mainly control the development of microstructural feature in the weld pool and hence the strength.

Melting of multipass surface tracks in steel incorporating titanium carbide powders

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.

Overlapping tracks processed by TIG melting TiC preplaced powder on low alloy steel surfaces

Abstract An overlapping composite track coating was produced on a steel surface by preplacing a 0·5 mm thick layer of TiC powder and then melting using a tungsten inert gas torch of constant energy input. The influence of the overlapping operation on preheating of the substrate, the dissolution of TiC particulates and the subsequent depth and hardness of the composite layer was analysed. The melt microstructure consisted of both undissolved and partially dissolved TiC particulates, together with a variety of morphologies and sizes of TiC particles precipitated during solidification. Preheating, resulting from the overlapping operation, occurred, producing additional melting of the TiC particulates and deeper melt depths but with a reduced volume fraction of TiC precipitates in the subsequent tracks. A maximum hardness of over 800 HV was developed in the composite layer. The high hardness was unevenly distributed in tracks melted at the initial and final stages, while it varied across the melt depths in other tracks.

Incorporation of TiC on Alsi 4340 low alloy steel surfaces via tungsten inert gas arc melting

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 melting single tracks in 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 the TIG torch produced 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 melted TiC particulates associated with dendritic, cubic and globular type carbides precipitated 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/mm2 TiC 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.

Low Temperature Fluidized Bed Nitriding of Austenitic Stainless Steel

In the present investigation, low temperature nitriding has been attempted on AISI 316L austenitic stainless steel by using a laboratory fluidized bed furnace. The nitriding was performed in temperature range between 400°C and 500°C. X-ray diffraction, metallography, and corrosion tests were used to characterize the resultant nitrided surface and layers. The results showed that fluidized bed process can be used to produce a precipitation-free nitrided layer characterized by the S phase or expanded austenite on austenitic stainless steel at temperatures below 500°C. But there exists a critical temperature and an incubation time for effective nitriding, below which nitriding is ineffective. The corrosion behaviour of the as-nitrided surfaces is significantly different from that previously reported for low temperature plasma nitriding. This anomaly is explained by the formation of iron oxide products and surface contamination during the fluidized process.

Wear Behavior of Modified Surface Layer Produced by TIG Melting of Preplaced Ti Powder in Nitrogen Environment

The formation of hard surface layer on steel provides a protective coating against wear, thermal loads and corrosion. In the present work a hard composite layer is formed on steel surfaces by preplacement of titanium powder and melted under nitrogen environment. Surface melting was conducted using TIG torch with different energy inputs. The microstructure and the morphology of the melt tracks were investigated using SEM and X-ray diffraction. The in-situ melting of titanium powder in nitrogen atmosphere produced dendritic microstructure of titanium nitride. The melt layer contained dispersed TiN, Ti2N dendrites highly populated at the surface compared to the deeper melt and gave a maximum surface hardness of around 1927 Hv. The wear property of the melt track was investigated using pin-on-disk tribometer at room temperature. The modified surface layer gave a low friction value of 0.12 and wear rate of 0.007895 ×10-4 compared to 1.648 × 10-4 mm3/N/m for the uncoated steel surface.