Mahadev Shome

Continuous cooling transformation diagrams applicable to the heat-affected zone of HSLA-80 and HSLA-100 steels

Continuous cooling transformation (CCT) diagrams for HSLA-80 and HSLA-100 steels pertaining to fusion welding with heat inputs of 10 to 40 kJ/cm, and peak temperatures of 1000 °C to 1400 °C have been developed. The corresponding nonlinear cooling profiles and related γ → α phase transformation start and finish temperatures for various peak temperature conditions have been taken into account. The martensite start (Ms) temperature for each of the grades and ambient temperature microstructures were considered for mapping the CCT diagrams. The austenite condition and cooling rate are found to influence the phase transformation temperatures, transformation kinetics, and morphology of the transformed products. In the fine-grain heat-affected zone (FGHAZ) of HSLA-80 steel, the transformation during cooling begins at temperatures of 550 °C to 560 °C, and in the HSLA-100 steel at 470 °C to 490 °C. In comparison, the transformation temperature is lower by 120 °C and 30 °C in the coarse-grain heat-affected zone (CGHAZ) of HSLA-80 steel and HSLA-100 steel, respectively. At these temperatures, acicular ferrite (AF) and lath martensite (LM) phases are formed. While the FGHAZ contains a greater proportion of acicular ferrite, the CGHAZ has a higher volume fraction of LM. Cooling profiles from the same peak temperature influence the transformation kinetics with slower cooling rates producing a higher volume fraction of acicular ferrite at the expense of LM. The CCT diagrams produced can predict the microstructure of the entire HAZ and have overcome the limitations of the conventional CCT diagrams, primarily with respect to the CGHAZ.

Microstructures and properties of friction stir spot welded DP590 dual phase steel sheets

Abstract DP590 steel sheets were joined by friction stir spot welding using polycrystalline cubic boron nitride tool with an objective to produce bond diameters similar to conventional spot welding nuggets. A range of spindle rotation (400–2400 rev min−1) and plunge speeds (0·03–3·8 mm s−1) were exercised to attain defect free welds in 1·6 mm thick sheets. A bond diameter of 4t1/2, alike minimum nugget diameter criteria for resistance spot welds, resulted in superior mechanical properties than conventional spot welds. The heat inputs corresponding to different welding parameters influenced the weld microstructure, including grain size, phases and their morphology. The bond diameter was higher for higher heat inputs. However, low heat input welds with weld time cycles ∼4 s produced more refined microstructure and exhibited similar strengths even with reduced bond size. Plug type failure was associated with larger bond diameters (∼7·1 mm), while interfacial failure was observed with smaller welds (∼5·4 mm).

Joining of galvannealed steel and aluminium alloy using controlled short circuiting gas metal arc welding process

Abstract Controlling the Fe–Al intermetallic layer thickness along the joint interface has remained a critical challenge in gas metal arc welding of galvanised steel and aluminium alloys. An attempt is presented here to join galvannealed steel and aluminium alloy sheets using a novel gas metal arc welding process that allows controlled short circuiting to reduce the rate of heat input significantly. The real time current and voltage transients during the process are monitored to estimate the rate of heat input and its influence on the formation of intermetallic phases and layer thickness. The results show that the intermetallic layer thickness can be controlled and good joint strengths can be achieved when the rate of heat input remains lesser than 130 J mm− 1 for the typical lap joint configuration considered here.

Wetting length in gas metal arc brazing of galvanised steel

Appropriate substrate surface wetting by a molten filler ensures a sound bead profile during brazing. A theory to estimate the wetting length considering gradual cooling of liquid filler and loss in its ability to spread is currently unavailable. We present here a methodology to estimate wetting length based on the minimisation of energy for spreading of a liquid filler considering its gradual cooling and tested the computed results in gas metal arc brazing of galvanised sheets for a wide range of conditions. The computed wetting lengths are used further to estimate bead height and toe angle of joint profile. GRAPHICAL ABSTRACT

Gas metal arc brazing of galvannealed steel sheets

An advanced gas metal arc welding technique with controlled short-circuiting mode of metal transfer and arcing at low power was employed for brazing of galvannealed steels using CuSi3 filler wire. The brazed joints showed the presence of an interface layer with Fe–Si intermetallic compounds (IMC). Increase in heat input resulted in an increase of the interface layer thickness. The maximum failure load of 2.5 kN was achieved for a heat input of 105 J mm−1 with the corresponding interface layer thickness of 2.7 µm. Heat inputs beyond 105 J mm−1 led to excessive spatter of zinc and increase in interface layer thickness with hard Si-rich IMC. In contrast, heat input values lower than 80 J mm−1 produced inadequate wetting of the steel surface by the molten filler wire deposit and resulted in low joint strength.

Numerical modelling of gas metal arc joining of aluminium alloy and galvanised steels in lap joint configuration

Advanced pulsed current gas metal arc based processes are increasingly attempted for the joining of aluminium alloys and galvanised steel sheets. The bead profile and the thickness of the interfacial Fe–Al intermetallic (IMC) layer significantly influence the failure strength of these joints. Although several experimental studies have examined the nature and extent of the IMC phases and consequent joint strength, quantitative efforts to estimate bead profile and the IMC layer thickness as function of process conditions and resulting heat input are scarce. We present here for the first time a coupled theoretical and experimental study to estimate the bead profile and Fe–Al IMC layer thickness for joining of galvannealed steel and aluminium alloy sheets in a typical lap joint configuration. The computed values of bead profile and IMC layer thicknesses are validated with the corresponding experimental results.

Microstructure and Mechanical Properties of Friction Stir Spot-Welded IF/DP Dissimilar Steel Joints

Interstitial-free (IF) and dual-phase (DP) steel sheets of 1-mm thickness were joined by friction stir spot welding with a convex shoulder tool. Two different combinations were used; one with IF as top sheet (IF/DP) and another with DP as top sheet (DP/IF). Material intermixing between the overlapping sheets takes place within the stirred zone. The truncated sheet interface curls upward into the top sheet, more so in case of IF/DP, due to lower resistance offered by the top (IF) sheet to the upward migrating bottom (DP) sheet material. Material from the IF steel contains ferrite phases, while that from the DP steel contains acicular ferrite and lath martensite. Under quasi-static loading, the crack passes along the dissimilar interface and into the top sheet thickness, resulting in pull-out failure. Under cyclic loading, the failure is brought about by the initiation of kinked fatigue cracks and their subsequent propagation through the top and bottom sheet thickness. The dominant fatigue crack moves through the reduced top sheet thickness. The mechanical performance of DP/IF is better than IF/DP owing to higher strength of the stirred zone. The mechanical performances of the dissimilar joints are intermediate to that of the similar material joints.

Probing joint strength and distortion in gas metal arc lap joining of aluminum and steel sheets

Joining of multi-metallic assemblies such as aluminum and steel sheets using fusion joining technologies is prudent although the formation of intermetallic compounds along joint interface has remained a critical challenge. An advanced, low-power input, gas metal arc process was employed here for joining of aluminum and zinc-coated steel sheets of dissimilar thicknesses in lap-joint configuration. The heat input during the process was restricted by fast responsive current and voltage pulses that allowed a synchronized arcing and short circuiting at a low arc power. The effect of heat input and thermophysical properties of base materials on the bead profile, joint strength, and distortion was studied extensively. The results indicated a rational improvement of joint quality with lowering of the heat input within a restrictive range of processing conditions such as wire feed rate and travel speed. Most importantly, the mixed-metal assembly exhibited different thermal distortions with the aluminum top sheet undergoing greater distortion than the bottom steel sheet due to a higher coefficient of thermal expansion.

Effect of upset pressure on texture evolution and its correlation to toughness in flash butt joints

ABSTRACT Effect of upset pressure on the weld zone microstructure, texture evolution and its effect on toughness were investigated. It was observed that an increase in upset pressure leads to a decrease in the heat-affected zone width and also the grain size as a result of recrystallisation. It was also observed that insufficient upset pressure leads to porosity in the joints. Electron backscattered diffraction study on the weld zone showed that an increase in the upset pressure resulted in a sharp rise in (110)[001] and (110) component. Shear deformation at the plate edges due to the constrained flow of material led to the evolution of Goss fibre component parallel to the fracture plane. The presence of Goss fibre components that are generated during welding as a result of upsetting operation played a critical role in decreasing the toughness at the weld zone compared to the parent material.

Dynamic strain aging during wire drawing and its effect on electrochemical behaviour

Second stage dynamic strain aging is observed during cold drawing of low carbon steel wire rods with higher free nitrogen content. The effect is prominent in thinner wires which have experienced higher strain and are thereby heavily stressed. Consequently, these severely deformed wires with higher nitrogen content are prone to early environmental degradation.