Manidipto Mukherjee

Influence of Heat Input on Martensite Formation and Impact Property of Ferritic-Austenitic Dissimilar Weld Metals

The effect of heat input on martensite formation and impact properties of gas metal arc welded modified ferritic stainless steel (409M) sheets (as received) with thickness of 4 mm was described in detail in this work. The welded joints were prepared under three heat input conditions, i.e. 0.4, 0.5 and 0.6 kJ/mm using two different austenitic filler wires (308L and 316L) and shielding gas composition of Ar + 5% CO2. The welded joints were evaluated by microstructure and charpy impact toughness. The dependence of weld metal microstructure on heat input and filler wires were determined by dilution calculation, Creq/Nieq ratio, stacking fault energy (SFE), optical microscopy (OM) and transmission electron microscopy (TEM). It was observed that the microstructure as well as impact property of weld metal was significantly affected by the heat input and filler wire. Weld metals prepared by high heat input exhibited higher amount of martensite laths and toughness compared with those prepared by medium and low heat inputs, which was true for both the filler wires. Furthermore, 308L weld metals in general provided higher amount of martensite laths and toughness than 316L weld metals.

Role of microstructural constituents on surface crack formation during hot rolling of standard and low nickel austenitic stainless steels

The effect of alloy segregation and delta (δ) ferrite contents on surface cracking of three standard (i.e. AISI 304L, AISI 310S and AISI 321) and two low nickel (i.e. LNi-1 and LNi-0.3) austenitic stainless steels (ASS) during hot rolling was investigated using optical microscopy (OM), automatic image analyzer, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electron probe micro analyzer (EPMA). It was observed that the amount of δ-ferrite varied among different grades and also distributed heterogeneously across the width of the steel plates. In general, low nickel ASS showed higher amount of δ-ferrite compared to the standard ASS grades. The tendency to surface cracking during hot rolling gradually increased with increasing δ-ferrite content. Interestingly, carbon and nitrogen exerted maximum effect on δ-ferrite formation. The higher carbon and nitrogen content in the steel decreased δ-ferrite content. In addition, the segregation of Cu and Mn plays significant role in low nickel ASS and Ni-Cr in case of standard ASS has profound effect on surface cracking of the steel plates. A possible cause of surface crack formation/origination in steel plates during hot rolling was discussed.

Development of a Direct Correlation of Bead Geometry, Grain Size and HAZ Width with the GMAW Process Parameters on Bead-on-plate Welds of Mild Steel

In this study, an experimental work was conducted to correlate the effect of gas metal arc welding (GMAW) process parameters such as wire feed speed, voltage, and contact tip to work piece distance along with interactive variables on bead-on-plate weld characteristics using multiple linear regression analysis and ANOVA (analysis of variance). The different responses such as convexity index, depth of penetration, reinforcement area, deposition rate, width of heat affected zone (HAZ), weld metal grain size and HAZ grain size were studied. The aim of the present investigation is to develop multiple linear regression equations to predict different responses (outputs) as a function of multiple input variables for ‘bead-on-plate’ type GMAW process. Multiple linear regression equations were first developed for weld bead geometry as a function of several individual and interactive variables. Then an effort was made to effectively predict the grain structure of the weldments as a function of multiple variables. Predicted responses are very close and sometime superimposed on the actual responses which clearly indicate the adequacy of the regression equations.

Effect of modes of metal transfer and microstructure on corrosion behavior of welded modified ferritic stainless steel in acidic environments

The effects of modes of metal transfer, i.e., short-circuit (SC) and spray (S) modes in single-pass gas metal arc welding of modified ferritic stainless steel using two types of austenitic stainless steel filler metals on microstructure as well as corrosion behavior of weld metal and high-temperature heat-affected zone (HTHAZ), were investigated. The results show that primary solidification modes (PSM) of the welds were exclusively dependent upon the Creq/Nieq ratio of the respective welds. However, the amount of grain boundary austenite and martensite transformation in the welds were solely dependent upon the mode of metal transfer and the extent of cooling rate. Regarding the corrosion mechanism, grain boundary corrosion (GBC) behavior of welds and HTHAZ relied on the microstructural changes along the grain boundary due to the variation in mode of metal transfer. The results show that S-mode resisted grain boundary corrosion of the welds and both GBC as well as pitting corrosion of the HTHAZ. On the other hand, SC-mode improved only pitting corrosion resistance of the welds. Between the filler wires used, 316L welds, in general, provided better corrosion resistance compared with 308L welds.

Effect of Cu Addition on Microstructure and Hardness of As-Cast and Heat-Treated High-Cr Cast Iron

The objective of the present study is to investigate the effect of Cu addition on microstructure and hardness of hypoeutectic high-Cr cast irons (26 wt% Cr) in as-cast and heat-treated conditions. As-cast specimens with varying amount of copper (Cu) addition were produced using an industrial grade high-frequency induction furnace. As-cast specimens were heat-treated in a box furnace using a typical reversed heat treatment sequence where subcritical heat treatment was followed by destabilization heat treatment. Microstructural analysis of the as-cast and heat-treated specimens was carried out using an optical microscope and Feritscope to understand the effect of Cu addition on the microstructure. The bulk hardness of all the specimens was measured using a Brinell hardness tester. The obtained result shows that the microstructures of the as-cast specimens mostly consist of austenitic dendrite matrix with γ+ M7C3 eutectic colony morphology. The eutectic colony consists of fine carbides in the central region and coarse carbides at the boundary region. High Cu containing specimen has a higher amount of thin rod-like M7C3 carbides, whereas low Cu containing specimen has a higher fraction of massive carbide morphology. Heat-treated specimens with various Cu content show mixed microstructure mostly consisting of retained austenite, transformed martensite, eutectic carbides, and secondary carbides. However, their relative volume fraction changes with the change in Cu content. Hardness result shows that higher Cu containing specimen has a lower hardness than the lower Cu contain specimens for both as-cast and heat-treated conditions. Although heat-treated specimens, in general, have higher hardness values than the as-cast specimens.

Study on Microstructure and Mechanical Properties of Thick Low-Alloy Quench and Tempered Steel Welded Joint

Low-alloy quench and tempered (Q&T) steel plates having yield strength of a minimum of 670 MPa are used extensively for the fabrication of impellers, penstocks, excavators, dumpers, and raw material handling devices, where welding is an important and probably most economic and productive process of joining. Thick low-alloy Q&T steel plates having a relatively high carbon equivalent (∼0.6) are frequently susceptible to a crack-sensitive microstructure leading to cold cracking. Thus, the welding parameters need to be carefully selected for these grades of steel to obtain the desired microstructure and properties. The present study deals with the characterization of microstructure evolution and mechanical properties of thick (38 mm) welded low-alloy quench and tempered steel plates (A 517 Grade F by ASTM) with the shielded metal arc welding process using a low hydrogen covered electrode (E 11018 M). The microstructural characterization was done for top (P1), middle (P2), and bottom (P3) regions of the weldment by optical microscope and transmission electron microscope to understand the microstructural features. The micrographs of the weld and coarse-grain heat-affected zone (CGHAZ) of the three regions mainly show variation in lath martensite and different morphologies of ferrite. Interestingly, the weld metal and CGHAZ in the middle (P2) region show coarse grains compared to the other two regions where grain size is more or less the same and finer. Furthermore, among the three regions, the P3 region, in general, has comparatively higher hardness and toughness. The high cycle fatigue test under two load ratios (i.e., R = 0.1 and −1) conducted only on specimens extracted from the P3 region, because of its better properties, shows better fatigue performance of weld joint under R = 0.1.