Papers by Author: S. Mridha

Abstract: Other than grain coarsening, the loss of corrosion resistance in ferritic stainless steel (FSS) welds due to intergranular precipitation of chromium carbides restricts the use of the alloy for structural application. The use of cryogenic cooling offers dual opportunities for the control of weld geometry and grain structure in FSS. This results in improved mechanical properties but the effect on carbide precipitation was not investigated. In this paper, the effect of heat flux, welding speed and flow rate of cryogenic liquid on carbide precipitation in 16% chromium FSS welds are discussed. The use of cryogenic cooling reduces the size of the sensitized zone but this is not significantly affected by the flow rate of the cryogenic fluid. Compared to the conventional welding, the cryogenic cooling increases the cooling rate and reduces the martensite content in the high temperature heat affected zone (HTHAZ) by about 20%. This results in wider ditched-structure in welds made with flow rates lower than 0.052L/min. Cryogenic cooling produces more ditched weld microstructure revealed by electrolytic etching in oxalic acid; however, the structure is acceptable since no single grain boundary is completely surrounded by ditches.

Abstract: Grain refinement in medium chromium ferritic stainless steel weld was attempted via elemental (aluminum) powder pre-placement technique prior to melting under a TIG torch. A Box-Behnken experimental design was adopted with current, travel speed and the amount of aluminum powder added as the process factors for producing weld pool. The resolidified weld tracks were characterized using microscopy, microhardness and mechanical testing. The degree of grain refinement achieved was evaluated using a scaling index known as Grain Refinement Index (GRI). The findings showed that the GRI is influenced by the concentration of the aluminum powder introduced into the melt pool. Furthermore, high GRI does not necessarily translate to better mechanical properties relative to the conventional weld. This suggests that the grain size effect might not be the only factor influencing the property of weld metal. However, weld track treated with 0.08mg/mm² of aluminum powder exhibited about 20% improvement in properties relative to the conventional weld made under the same energy conditions.

Abstract: 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.

Abstract: Hybrid jute-carbon/ epoxy composites, fabricated by hand lay-up method with fiber volume fractions of 0.47, 0.58 and 0.68 were used to investigate water absorption behavior as a function of immersion time and fiber content. The effect of moisture content on impact strength and failure modes was also studied. Results show that the moisture absorption increased with increasing the immersion time in water and it was more with higher fiber fraction specimens. Maximum moisture contents of 0.45%, 0.52% and 0.61% were recorded for the specimens containing fiber volume fractions of 0.47, 0.58 and 0.68, respectively. The impact strength reduced with increasing moisture absorption in all specimens containing different fiber fractions. Composites with higher fiber content gave reduced impact strength under all test conditions. Composites of different fiber fractions and of highest moisture content produced impact strengths about 20 to 28% less than those strengths obtained without water immersion. The 47 vol% fiber specimen was least affected by water immersion and impact strength reduction was only 17% after immersion till saturation. Failure occurred by mainly by delamination and it was evident in all fractured specimens. Results of the effect of impact energy on moisture content have been evaluated using ANOVA ANALYSIS and the results gave errors of 1%, 0.6 % and 0.8 % for 0.47, 0.58 and 0.68 fiber volume fraction specimens, respectively.

Abstract: Charpy impact tests were conducted on carbon reinforced epoxy composites fabricated by hand lay-up method using 0.47, 0.56 and 0.66 carbon fiber volume fractions; tests were conducted at temperatures between -60oC to 60oC. The impact strength was found, in general, to increase when the samples were fractured at temperatures above 0oC and the impact strength decreased with the increase of fiber content. The impact energy absorption was highest of 270 KJm-2 with 47 vol% fiber when fractured at +60oC and it reduced to 130 KJm-2 at -60oC. With decreasing the fracture temperature and increasing the fiber content the impact strength reduced significantly. The reduction of impact energy was from 235 KJm-2 to 107 KJm-2 for 56 vol% fiber and from 196 KJm-2 to 90 KJm-2 for 66 vol% fiber when fractured at +60oC and -60oC, respectively. Failure occurred mostly by fiber delamination; fiber splitting and matrix cracking were also present. Delamination was more in specimens tested at -60oC while fiber splitting and matrix cracking were more when fractured at +60oC.

Abstract: Studies on the weldability of ferritic stainless steel grades suggests that low heat input rate and better heat transfer dynamics are appropriate for the control of grain size and microstructural feature in thin sections. However, the optimal welding conditions to achieve combination of such characteristics are yet to be established. In the present investigation, AISI 430 ferritic stainless steel is TIG welded using energy input between 0.205 and 2.05kj/mm and characterized in terms of microstructure and hardness. The microstructural characterization of the welds with varying heat input rates suggests the presence of interdendritic martensite in the fusion zone and grain boundary martensite in the HAZ in conjunction with some intermetallics in varying proportion. The hardness values across the welds indicate that grain growth and the presence of intermetallics are minimized when welded with increased heat input rates that permits transformation within the dual phase regions. The study provided a new insight into the contribution of heat input rate in the production of unwanted weld microstructural features and assisted in the design of methods and techniques for tailoring weld microstructures with optimum properties.

Abstract: In this work, the preliminary result on the effect of cryogenic cooling on grain growth in weld is reported. Ferritic stainless steel weld produced under TIG torch in argon environment is cooled in liquid nitrogen. The weld structure is characterized using LOM, SEM and EDX spectroscopy. The results suggest that cryogenic cooling reduced the weld width within 2% to 5% and HAZ to 39% relative to those cooled in normal condition. This ensures that the area of the base metal affected and exposed to the weld thermal cycle is reduced and hence probably generates less metallurgical distortion. The cryogenic cooling also generated 14% to 36% grain refinement compared to welds cooled in normal condition.

Abstract: In this study an attempt has been made to produce titanium-aluminium dispersed hard nitride layer on mild steel surfaces by preplacement of 50 % Ti and 50 % Al powder mixture and then melting with TIG torch under nitrogen environment. Parameter such as heat input of the torch was varied between 540, 608 and 675 J/mm and its effect on the resolidified melt pool was studied. Glazing under all energy inputs produced more than 1mm thick resolidified clad layer. The microstructural analysis revealed the clad layer with dispersion of dendrites of Ti-Al nitrides and Ti- Al intermetallic in ferrite matrix. The concentration of dendrites were found to be maximum near the surface and decreased at deeper depths.The maximum hardness of the modified surface layer was found to be 900 Hv compared to180 Hv of the mild steel substrate.

Abstract: 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.

Abstract: In order to modify surface structure titanium powder was preplaced on steel surface and melted under TIG (tungsten inert gas) torch in a pure nitrogen environment which formed a resolidified layer of around 1 mm thickness. The preplaced titanium powder content was varied between 1.3 and 1.8 mg/mm2 and melting was conducted with energy inputs of 324 J/mm to 540 J/mm. The modified surface layer was analyzed in terms of microstructure, hardness, surface defects such as cracks and pores. The resolidified layer consisted of dispersion of TiN, Ti2N and TiN.88 dendrites in a ferrite matrix containing titanium. The modified layer showed some defects when melting were performed with low energy inputs. A maximum surface hardness of around 2000 Hv was developed in most of the tracks and this hardness corresponds to high concentration of TiN dendrites within the modified layer.