Influences of post-weld heat treatment on fatigue crack growth behaviour of strength mismatched HSLA steel welds

Abstract

Welding of high strength low alloy steels (HSLA) involves usage of low, even and high strength filler materials (electrodes) compared with the parent material depending on the application of the welded structures and the availability of the filler material. In the present investigation, the influences of post-weld heat treatment (PWHT) on fatigue crack growth behaviour of under matched (UM), equal matched (EM) and over matched (OM) weld metals has been studied. The base material used in this investigation is HSLA-80 steel of weldable grade. The Shielded Metal Arc Welding (SMAW) process has been used to fabricate the single 'V' butt joints. Centre Cracked Tension (CCT) specimens have been used to evaluate the fatigue crack growth behaviour of the welded joints. Fatigue crack growth experiments have been conducted using servo hydraulic controlled fatigue testing machine at constant amplitude loading (R = 0). From this investigation, it has been found that the fatigue performance of over matched joints is superior compared to under matched and equal matched joints. Moreover, PWHT reduced the magnitude of the tensile residual stress field in the weld region and subsequently enhanced the fatigue performance of the joints irrespective of weld metal strength mismatch.     

Secondary austenite and chromium nitride precipitation in simulated heat affected zones of duplex stainless steels

Abstract

Primary and secondary intragranular austenite precipitation and its relationship with chromium nitride (Cr₂N) were studied in a simulated multipass heat affected zone (HAZ) of five duplex stainless steel alloys (UNS S32304, S32205, S32550, S32750, and S32760). The Gleeble thermal-mechanical simulator was used to perform short duration and high cooling rate ferritisation and reheating heat treatments. TEM and FEG-SEM analysis, coupled with a specially developed electrolytic etching technique, revealed the cooperative growth of secondary austenite and Cr₂N precipitation along the ferrite/austenite (α/γ) interfaces. Additionally, the observed close coexistence of intragranular nitride (Cr₂N) and intragranular secondary austenite suggests the heterogeneous nucleation of secondary austenite from the nitrides as supported by previously reported low energy nitride/austenite (Cr₂N/γ) interfaces for the observed orientation relationship between both phases. Based on these observations, a new mechanism is proposed for intragranular secondary austenite nucleation related to the intragranular nitride precipitates.     

Microstructure,hardness and residual stress distribution in maraging steel gas tungsten arc weldments

Abstract

The distribution of residual stresses due to welding has been studied in maraging steel welds. Gas tungsten arc welding process was used and the effect of filler metal composition on the nature of residual stress distribution has been investigated using X-ray diffraction technique with Cr K_α radiation. Three types of filler materials were used, they include: maraging filler, austenitic stainless steel and medium alloy medium carbon steel filler metal. In the case of maraging steel weld, medium alloy medium carbon filler, the residual stress at the centre of the weld zone was more compressive while, less compressive stresses have been identified in the heat affected zone of the parent metal adjacent to the weld metal. But, in the case of austenitic stainless steel filler the residual stresses at the centre of the weld and heat affected zone were tensile. Post-weld aging treatment reduced the magnitude of stresses. The observed residual stress distribution across the weldments has been correlated with microstructure and hardness distribution across the weld.     

Effect of solution annealing on weldability of aged alloy 800/25Cr–35Ni steel dissimilar welds

Abstract

Dissimilar welding of the aged alloy 800 and the as cast 25Cr–35Ni (wt-%) heat resistant steel was investigated. Microstructures, mechanical properties and weldability of the dissimilar welds were characterised using optical microscopy, scanning electron microscopy and transition electron microscopy equipped by energy dispersive X-ray spectroscopy, and Varestraint test. Since such dissimilar welding was susceptible to crack formation in the heat affected zone of the aged part, the effects of a preweld solution annealing, heat input, interpass temperature and type of filler metal on the weldability of two alloys were investigated. It was found that during the solution treatment, the precipitates produced in the service stage were decomposed and that TiC was formed. In addition, tensile strength and hardness were reduced, but ductility and toughness increased. It was concluded that the most important step to improve weldability and to reduce cracking susceptibility was solution annealing. A suitable annealing treatment was then proposed. The best weldability was found under conditions of solution annealing, low heat input, low interpass temperature and using Inconel 82 or 617 for filler metals.     

Numerical simulation of spot welding for galvanised sheet steels

Abstract

Galvanised sheet steels are now widely used to be the substrate for body in white (BIW) construction in the automotive industry. Weldability of galvanised sheet steels much worsened compared to spot welding of low carbon steels. The present paper develops a 2D axisymmetric model and employs an incremental coupled thermal–electrical–mechanical analysis to predict the nugget development during resistance spot welding (RSW) of galvanised sheet steels. Temperature dependent contact resistance for faying surfaces was determined to take into account of the influence of zinc coat for spot welding galvanised sheet steels. The effect of dynamic contact radii on temperature distribution was studied and compared with results under constant contact area assumption. The predicted nugget shape and size agreed well with the experimental data. Higher current and longer welding time should be applied for galvanised sheet steels compared to low carbon steel spot welding. The proposed model can be applied to predict weld quality and choose optimal welding conditions for spot welding galvanised sheet steels.     

Solidification and microstructure modelling of welds in aluminium alloys 5754 and 6111

Abstract

A solidification and microstructure modelling approach has been developed to predict weld metal and heat affected zone (HAZ) characteristics. The freezing range and phase evolution in the weld metal region were predicted using thermodynamic and diffusion controlled growth calculations. The calculated freezing range was correlated with the weld solidification cracking tendency. A simplified analytical model was suggested to describe thermal cycles that are experienced by the HAZ. This analytical model was coupled with a published microstructure model for age hardenable alloys to predict the hardness variations across the HAZ. The above integrated approach was evaluated using experimental welds made on non­age hardenable 5754 (Al–Mg) and age hardenable 6111 (Al–Mg–Si) alloys using gas tungsten arc, electron beam, and gas metal arc welding processes.     

Dynamic characteristics of adhesive bonded high strength steel joints

Abstract

With an increase in the use of advanced high strength steels in vehicle architectures, materials joining issues have become increasingly important. Among the various joining methods, adhesive bonding is increasingly used in automobile manufacturing. Successful implementation of adhesive bonding to improve structural crashworthiness and reduce vehicle weight requires the knowledge of issues related not only to processing but also to joint performance. In this study, the impact strength of adhesive bonded high strength steel joints is evaluated with the split Hopkinson tension bar (SHTB) technique. The influences of loading speed and thickness of the steels on the shear strength of the joints were examined. Comparative quasi-static lap shear tests were also conducted on a tensile testing machine. Test results showed that strength and energy absorption of bonded steel joints increase with loading speed, and is greatly affected by the thickness of the steels. As the loading rates are increased to 1100 s^–1 (i.e. 20 m s^–1), bonded 0·75 mm thick DP600 steel shows a 152% increase in strength and an 83% increase in energy absorption when compared to its quasi-static values. Examination of the impact tested specimens showed the failure mode changes from coarse cohesive mode to fine cohesive mode with increasing loading speed. The results from this study will provide the information for a better understanding of impact failure mechanisms of adhesive bonded high strength steels.     

Ferrite to austenite reverse transformation process in B containing 9%Cr heat resistant steel HAZ

Abstract

Creep properties of the high Cr heat resistant steel welded joint can be improved by adding B due to prevention of the grain refinement in heat affected zone (HAZ). In the present study, phase transformation behaviour of the B steel HAZ has been investigated to understand suppression mechanism of the grain refinement. During reverse transformation, fine austenite was formed through diffusional transformation at the prior austenite grain boundary in the first stage, and then coarse austenite was formed at the same location of the original austenite. The volume fraction of the fine austenite increased with increasing perk temperature of the weld thermal cycle. This phenomenon can be explained if the coarse austenite contains high density of dislocations. Clear surface relief was observed during the reverse transformation by a confocal laser microscope. These results indicate that martensitic or displacive reverse transformation takes place during welding and it prevents the grain refinement in HAZ.     

Effects of torch configuration and welding current on weld bead formation in high speed tandem pulsed gas metal arc welding of steel sheets

Abstract

Undercut and humping bead are the common defects that limit the maximum welding speed of tandem pulsed gas metal arc (GMA) welding. In order to increase the maximum welding speed, effects of the inclination angle, interwire distance and welding current ratio between the leading wire and trailing wire on bead formation in high speed welding are investigated. The undercut and humping bead is attributed to the irregular flow of molten metal towards the rear part of the weld pool. This irregular flow can be prevented by the trailing wire with a push angle from 5° to 13° , which provides an appropriate component of arc force in the welding direction. The irregular flow is also related to the distance between the leading wire and the trailing wire, and the flow becomes regular when the distance is in the range 9–12 mm. Moreover, the stabilisation of the bulge of the weld pool between the two wires, the presence of enough molten metal below the trailing arc, and the reduced velocity of molten metal flow towards the rear part of the weld pool, are essential to increase the maximum welding speed. These conditions can be obtained by adjusting the ratio of the leading arc current to the trailing arc current. A maximum welding speed as high as 4–4·5 m min^?1 is achieved by setting the current ratio to a value ranging from 0·31 to 0·5.     

Properties and improvement of super fine grained steel friction welded joint

Abstract

This paper describes friction welded joint properties of super fine grained steel (SFGS) and discusses improvements in these joint properties. The average grain size diameter of the SFGS base metal is ～0·6 μm, and its ultimate tensile strength is 660 MPa. The joint, made by a continuous drive friction welding machine (conventional method), fractured at the welded interface even though it possessed 100% joint efficiency. This was due to both the coarsening of the grain size and the softening of the welded interface with its adjacent region caused by heat input during braking times. The authors developed a joining method using a continuous drive friction welding machine that has an electromagnetic clutch to eliminate heat input during braking time, which was called the 'low heat input friction welding method' (LHI method). The joint obtained by the LHI method had the same tensile strength as the base metal at the friction time when the friction torque reached the initial peak. That is, the joint obtained 100% joint efficiency and fractured at the base metal, although the adjacent region of the welded interface softened only slightly. The grain size of this joint was smaller than that obtained by the conventional method. It was clarified that the optimum friction welded joint of the SFGS could be obtained by the LHI method in comparison with the conventional method.