Weld Centerline Research Articles

Effect of self-support friction stir welding on microstructure and microhardness of 6082-T6 aluminum alloy joint

In this paper, 5-mm-thick 6082-T6 aluminum alloy was joined by means of self-support friction stir welding (SSFSW). Here we report the grain structure and second phase particles in various regions including the welding nugget zone (WNZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ). In the upper part of the joint, microhardness in the TMAZ in proximity of the UWNZ was the highest (average 89.4HV) due to the severe plastic deformation. The similar result was also found in the lower part of the SSFSW joint. The microstructural development in each region was a strong function of the local thermo-mechanical cycle experienced during welding. Some coarse equiaxed grains which were produced in incomplete dynamic recrystallization process and dissolution of some precipitates have been observed in TMAZ. The HAZ retained the same grain structure as the base material, however, the grain size decreased with increasing distance of the weld centerline.

Effect of joint design on ballistic performance of quenched and tempered steel welded joints

A study was carried out to evaluate the effect of joint design on ballistic performance of armour grade quenched and tempered steel welded joints. Equal double Vee and unequal double Vee joint configuration were considered in this study. Targets were fabricated using 4mm thick tungsten carbide hardfaced middle layer; above and below which austenitic stainless steel layers were deposited on both sides of the hardfaced interlayer in both joint configurations. Shielded metal arc welding process was used to deposit for all layers. The fabricated targets were evaluated for its ballistic performance and the results were compared in terms of depth of penetration on weld metal. From the ballistic test results, it was observed that both the targets successfully stopped the bullet penetration at weld center line. Of the two targets, the target made with unequal double Vee joint configuration offered maximum resistance to the bullet penetration at weld metal location without any bulge at the rear side. The higher volume of austenitic stainless steel front layer and the presence of hardfaced interlayer after some depth of soft austenitic stainless steel front layer is the primary reason for the superior ballistic performance of this joint.

Effect of post-weld heat treatment and electrolytic plasma processing on tungsten inert gas welded AISI 4140 alloy steel

Post-weld heat treatment (PWHT) is commonly adopted on welded joints and structures to relieve post-weld residual stresses; and restore the mechanical properties and structural integrity. An electrolytic plasma process (EPP) has been developed to improve corrosion behavior and wear resistance of structural materials; and can be employed in other applications and surface modifications aspects. In this study the effects of PWHT and EPP on the residual stresses, micro-hardness, microstructures, and uniaxial tensile properties are explored on tungsten inert gas (TIG) welded AISI-4140 alloys steel with SAE-4130 chromium–molybdenum alloy welding filler rod. For rational comparison all of the welded samples are checked with nondestructive Phased Array Ultrasonic Testing (PAUT) and to ensure defect-free samples before testing. Residual stresses are assessed with ultrasonic testing at different distances from weld center line. PWHT resulted in relief of tensile residual stress due to grain refinement. As a consequence higher ductility but lower strength existed in PWHT samples. In comparison, EPP-treated samples revealed lower residual stresses, but no significant variation on the grain refinement. Consequently, EPP-treated specimens exhibited higher tensile strength but lower ductility and toughness for the martensitic formation due to the rapid heating and quenching effects. EPP was also applied on PWHT samples, but which did not reveal any substantial effect on the tensile properties after PWHT at 650°C. Finally the microstructures and fracture morphology are analyzed using scanning electron microscopy (SEM) and optical microscope to study the evolution of microstructures.

Effects of specimen geometry and loading mode on crack growth resistance curves of a high-strength pipeline girth weld

This work presents an investigation of the ductile tearing properties for a girth weld made of an API 5L X80 pipeline steel using experimentally measured crack growth resistance curves. Use of these materials is motivated by the increasing demand in the number of applications for manufacturing high strength pipes for the oil and gas industry including marine applications and steel catenary risers. Testing of the pipeline girth welds employed side-grooved, clamped SE(T) specimens and shallow crack bend SE(B) specimens with a weld centerline notch to determine the crack growth resistance curves based upon the unloading compliance (UC) method using the single specimen technique. Recently developed compliance functions and η-factors applicable for SE(T) and SE(B) fracture specimens with homogeneous material and overmatched welds are introduced to determine crack growth resistance data from laboratory measurements of load-displacement records.

Effect of Capping Front Layer Materials on the Penetration Resistance of Q&T Steel Welded Joints Against 7.62-mm Armor-Piercing Projectile

In the present investigation, an attempt has been made to study the effect of capping front layers on the ballistic performance of shielded metal arc-welded armor steel joints which were fabricated with a chromium carbide-rich hardfaced middle layer on the buttered/beveled edge. Two different capping front layer materials were chosen for achieving better ballistic performance, namely, low hydrogen ferritic (LHF) and austenitic stainless steel (SS) fillers. On the other hand, the bottom layers are welded with SS filler for both joints. The consequent sandwiched joint served the dual purpose of weld integrity and penetration resistance of the bullet. It is observed that the penetration resistance is due to the high hardness of the hardfacing layer on the one hand and the energy-absorbing capacity of the soft backing SS weld deposits on the other hand. The complementary effect of layers successfully provided resistance to the projectile penetration. On a comparative analysis, the joint fabricated using the LHF filler capping front layer offered superior ballistic performance with respect to depth of penetration. This is mainly due to the presence of acicular ferrite along the bainitic structure in the LHF capping front layer, which caused a shallow hardness gradient along the weld center line.

Materials flow and phase transformation in friction stir welding of Al 6013/Mg

Material flow and phase transformation were studied at the interface of dissimilar joint between Al 6013 and Mg, produced by stir friction welding (FSW) experiments. Defect-free weld was obtained when aluminum and magnesium were placed in the advancing side and retreating side respectively and the tool was placed 1 mm off the weld centerline into the aluminum side. In order to understand the material flow during FSW, steel shots were implanted as indexes into the welding path. After welding, using X-ray images, secondary positions of the steel shots were evaluated. It was revealed that steel shots implanted in advancing side were penetrated from the advancing side into the retreating side, whereas the shots implanted in the retreating side remained in the retreating side, without penetrating into the advancing side. The welded specimens were also heat treated. The effects of heat treatment on the mechanical properties of the welds and the formation of new intermetallic layers were investigated. Two intermetallic compounds, Al3Mg2 and Al12Mg17, were formed sequentially at Al6013/Mg interface.

Repair welding influence on offshore pipelines residual stress fields: An experimental study

Repair welds, are frequently used in steel structures either to remedy initial fabrication defects, or to rectify in-service degradations of the components. Some previous investigations indicated that repair welding is likely to pose adverse effects on the long-term integrity of the structure exposed to high pressure and temperature actions. It is believed that high residual stresses, associated with the repair process, most probably play an important role in many of subsequent failures. Repair welds might aggravate the size, magnitude and distribution of the tensile residual stresses in the weldments. These adversely affect the component structural integrity and remaining life. So far, no generally accepted guideline is available to provide reliable evaluations on the possible side effects from the repair welding in offshore oil/gas pipelines. This paper reports the result of residual stress measurement on single/double and partial/full repair welds in offshore pipelines. The semi destructive blind hole drilling and destructive sectioning methods have been employed to measure the residual stress fields in each case. In general, the results of the two measurement methods are in reasonable agreement. Residual stresses which are caused by full and partial repairs in the studied samples slightly increased the residual stress distribution when compared to the as-welded condition. Repetition of repair welding in same area influenced the residual stresses' magnitude and distribution especially in areas close to the weld centre line.

Through-thickness distributions of residual stresses in two extreme heat-input thick welds: A neutron diffraction, contour method and deep hole drilling study

Spatial variations of residual stresses were determined through the thickness of 70mm thick ferritic steel welds created using low (1.7kJmm−1) and high (56kJmm−1) heat inputs. Two-dimensional maps of the longitudinal residual stress were obtained by using the contour method. The results were compared to neutron diffraction measurements through the thickness at different locations from the weld centerline. The deep hole drilling technique was utilized to confirm the maximum stress locations and magnitudes. The results show that significant tensile stresses (∼90% of yield strength) occur along the weld centerline near the top surface (within 10% of the depth) in the low heat-input specimen. Meanwhile, in the high heat-input weld, the peak stress moved towards the heat-affected zone at a depth of ∼40% of the thickness. Finally, the influence of residual stresses on potential fracture behavior of the welded joints is discussed.

Welding Procedure Development for Welding of High Strength Carbon Steel Cladded with Austenitic Stainless Steel 316L by Using Overmatching Filler Metal

This work aims to develop welding procedure for small diameter longitudinal welded clad pipe made from clad plate. High strength carbon steel base metal bonded with 316L stainless steel clad layer was used in this study. The dissimilar materials at the weld joint and accessibility limitation of small diameter present difficulty in welding process selection to achieve weld soundness. The joint and welding se¬quence are designed to avoid solidification cracking. Nickel base over matching filler is used on the clad side. Typical joint configuration is double V groove weld without clad peel back to minimize the number of passes inside the pipe. Firstly, welding is done on the carbon steel side by using Shielded Metal Arc Welding (SMAW) and Submerged Arc Welding (SAW) with carbon steel electrodes. Then, welding on the clad side is done by using ERNiCrMo-3 filler metal. Two different procedures for the clad side are studied. The first procedure is to weld the clad side by using Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding pulse current (GMAW-pulse) and another procedure is to weld the clad side by using the SAW procedure. Hot cracking was observed in the case of SAW procedure at the clad weld centerline due to high heat input and high level of dilution. Mechanical properties and microstructure are evaluated. Clad weld by use of GTAW and GMAW-pulse could give sound weld metal. The tensile and yield strength of all weld metal were found to be greater than that of base metal and 100% shear failures were observed. Charpy impact energy of weld and HAZ at -10°C was found to be over 100 joules. Hardness of weld and HAZ area are surveyed over the weld cross section to determine local hardening. Additionally intergranular corrosion testing was carried out on the clad weld side and then bend testing was done. No crack was observed. Therefore, GTAW and GMAW-pulse clad weld procedure could give required properties according to clad line pipe standard, reduce cost of production and increase productivity compared to the peel back method.

Welding residual stress behavior under mechanical loading

For an accurate estimation of the fatigue performance of welded joints, not only the initial residual stress field but also its variation under load is decisive. The portion of residual stress which stays stable can shift the load stress range much the same way as the mean stresses do in fatigue loading. The relaxation of residual stresses during fatigue loading however reduces this hazard to the structural health. That is, before considering the influence of residual stresses in fatigue, the effect of fatigue on residual stresses should be understood. In this work, the influence of the quasi-static and cyclic loading on the relaxation of welding residual stresses in small specimens and large components out of different steels namely S235JRG2, S355J2G3, P460NL, S690QL, S960QL, and S1100QL has been investigated experimentally. X-ray diffraction analysis as a reliable method has been used for the determination of residual stress profiles in surface layers. For residual stress analysis in deeper layers, synchrotron and neutron diffraction techniques were applied as complementary methods. A clear recognition of the difference between initial welding residual stresses in small- and large-scale specimens was observed. Tensile residual stresses as high as the yield strength could appear in large-scale welded specimens. That was not the case for small-scale welds due to the low grade of restraint. Nevertheless, in both types of samples, the highest residual stresses were obtained in the weld centerline. At the weld toe which is critical regarding fatigue crack initiation, the magnitude of the residual stresses is lower. Beside of that, it was observed that in the case of relaxation, the first load cycles especially in components out of low-strength steels are decisive. The influence of loading conditions and local mechanical properties on the relaxation of welding residual stresses was investigated based on principles of solid mechanics. The von Mises failure criterion was able to describe the relaxation behavior.

Friction Stir Welds of AA6351-T6 and AA2024-T6 Dissimiler Aluminium Alloys

Friction stir welding is a novel welding technology, which has been applied widely in aerospace, traffic, marine industries because of its unique mechanical properties, metallurgic structure and environmental friendly. In the present study, the effect of pin rotational speed and traverse speed at constant downward force on microstructure and mechanical properties (Tensile and Hardness) of wrought aluminum alloys AA6351-T6 and AA2024-T6 of dissimilar friction stir welds were studied. Transverse tensile strength and hardness profiles on either side of the weld centre line were evaluated and the results were compared with base materials at room temperature.

Residual stresses in P91 steel electron beam welds

This paper describes the characterisation of residual stress in electron beam welded P91 ferritic–martensitic steel plates (9 mm thick) by neutron diffraction and contour measurement methods. The novelty of the work lies in revealing the residual stress profile at a fine length scale associated with a ∼1 mm wide fusion zone. A characteristic ‘M’ shaped distribution of stresses across the weld line is observed with high tensile peaks situated just beyond the heat affected zone/parent material boundary. Measured stresses close to the weld centreline are significantly less tensile than the adjacent peaks owing to martensitic phase transformation during cool down of the weld region. The effect of applying a second smoothing weld pass is shown to be undesirable from a residual stress standpoint because it increases the tensile magnitude and spread of residual stress. The results are suitable for validating finite element predictions of residual stress in electron beam welds made from ferritic–martensitic steels.

EFFECT OF STRAIN RATE ON DYNAMIC DEFORMATION BEHAVIOR OF LASER WELDED DP780 STEEL JOINTS

Dual phase(DP) steels have good combinations of strength and ductility,and are being increasingly used in vehicle body structures to meet enhanced government regulations and safety standards.The use of DP steels in automotive industries involves laser welding,which would lead to changes in local material properties and create potential safety and reliability issues under dynamic loads.The present work aimed to study the effects of strain rate on tensile properties and deformation behavior of laser welded DP780 steel joints.The results showed that the deformation behavior of laser welded joints was more sensitive to strain rate as compared to base metal of DP780 steel.The strength of DP780 steel joint increased with increasing strain rate,while the ductility decreased first with increasing strain rate from 10~(-3) to 10~1 s~(-1),and then increased up to a strain rate of 10~2 s~(-2).The strain rate sensitivity of the deformation behavior of DP780 steel joints was mainly dependent on the change of deformation behavior and its mechanisms of base metal at various strain rates.The distance of the tensile failure location from the weld centerline decreased obviously with the increase of strain rate.And the failure location changed from the base metal to the softened heat-affected zone(HAZ) as strain rate increased.The mechanism for changing failure location can be related to the strain rate dependence of the plastic deformation behaviors of microstructures in various regions across a joint.

Finite Element Simulation of Carbide Precipitation in Austenitic Stainless Steel 304

A three dimensional transient finite element model with Gaussian heat source was proposed to predict the heat transfer in Gas Tungsten Arc Welding (GTAW) of stainless steel 304 and experimental tests were conducted to study chromium carbide precipitation and to verify the developed model. The position of chromium carbide precipitation band and its relation to welding parameters were studied. The results showed, increasing the heat input of weld, increases the distance in which precipitation in austenite grain boundaries occurs. After measurement of carbide-band distance from weld centre and using obtained time-temperature curves of the developed model, the distance of carbide-band from the weld centre line was simulated. Finally, good agreement was observed between experimental and simulation results in terms of prediction of thermal history and carbide precipitation.

Correlation of Mechanical and Microstructural Properties in SMAW Welded Cr-Mo Boiler Steels Subjected to Different Post Weld Heat Treatment Soaking Times

Abstract: In the present work the effect of different durations of Post Weld Heat Treatment soaking times (0.5,2,10,50 hours) are analyzed on the mechanical and microstructural properties of Cr-Mo alloy steel of ASTM A387 Gr-22,2.25% Cr-1%Mo alloy steel. These are used in high temperature services in steam generation applications, Oil and Gas industries, Thermal and Nuclear power plants etc. The work is focused on different regions of weldment like base metal, HAZ and weld metal to compare and analyze the results obtained in as-welded and different soaking times from hardness test, tensile testing and microstructural examination. Results showed that the hardness varies from weld centre line to base metal and peak hardness was found in the HAZ. The hardness is reduced to large extent when PWHT is done at 50 h. Microstructural changes were observed during different post weld heat treatments and their effect on mechanical properties and best suitable soaking times were reported.

Enhancement of mechanical properties and failure mechanism of electron beam welded 300M ultrahigh strength steel joints

In this study, four post-weld heat treatment (PWHT) schedules were selected to enhance the mechanical properties of electron beam welded 300M ultrahigh strength steel joints. The microstructure, mechanical properties and fractography of specimens under the four post-weld heat treatment (PWHT) conditions were investigated and also compared with the base metal (BM) specimens treated by conventional quenching and tempering (QT). Results of macro and microstructures indicate that all of the four PWHT procedures did not eliminate the coarse columnar dendritic grains in weld metal (WM). Whereas, the morphology of the weld centerline and the boundaries of the columnar dendritic grains in WM of weld joint specimens subjected to the PWHT procedure of normalizing at 970°C for 1h followed by conventional quenching and tempering (W-N2QT) are indistinct. The width of martensite lath in WM of W-N2QT is narrower than that of specimens subjected to other PWHT procedures. Experimental results indicate that the ductility and toughness of conventional quenched and tempered joints are very low compared with the BM specimens treated by conventional QT. However, the strength and impact toughness of the W-N2QT specimens are superior to those of the BM specimen treated by conventional QT, and the ductility is only slightly inferior to that of the latter.

Quantifying residual stresses in resistance spot welding of 6061-T6 aluminum alloy sheets via neutron diffraction measurements

Residual strains of resistance spot welded joints of 6061-T6 aluminum alloy sheets were measured in three different directions denoted as in-plane longitudinal (σ11), in-plane transversal (σ22), and normal (σ33). The welding process parameters were established to meet or exceed MIL-W-6858D specifications (i.e., approximately 5.7mm weld nugget and minimum shearing force of 3.8kN per weld confirmed via quasi-static tensile testing). Electron backscatter diffraction (EBSD) and optical microscopy (OM) were performed to determine grain size and orientation. The residual stress measurements were taken at a series of points along the weld centerline at depths corresponding to the weld mid-plane and at both 1mm below the top surface of the plate and 1mm above bottom surface. The residual stresses were captured on the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM) of the resistance spot welded joint. Neutron diffraction results show residual stresses in the weld are approximately 40% lower than yield strength of the parent material. The maximum variation in residual stresses occurs, as expected, in the vertical position of the specimen because of the orientation of electrode clamping forces that produce a non-uniform solidification pattern. Despite the high anisotropy of the welding nugget and surrounding area, a significant result is that σ33 measured stress values are negligible in both the horizontal and vertical directions of the specimen. Consequently, microstructure–property relationships characterized here can indeed inform continuum material models for application in multiscale models.

Further results in J and CTOD estimation procedures for SE(T) fracture specimens – Part II: Weld centerline cracks

This work examines the effect of weld strength mismatch on J and CTOD fracture parameters using single edge notch tension (SE(T)) specimens. A central objective of the study is to enlarge on previous J and CTOD estimation procedures based upon plastic eta factors (η) described in Part I for centerline-cracked welds. The present analyses provide a large set of η-factors for a wide range of crack sizes (a/W-ratio) and material properties, including different levels of weld strength mismatch, which are applicable in improved estimation equations for J and CTOD for weld centerline notched SE(T) specimens.

Friction Stir Welding in HSLA-65 Steel: Part I. Influence of Weld Speed and Tool Material on Microstructural Development

A systematic set of single-pass full penetration friction stir bead-on-plate and butt-welds in HSLA-65 steel were produced using a range of different traverse speeds (50 to 500 mm/min) and two tool materials (W-Re and PCBN). Microstructural analysis of the welds was carried out using optical microscopy, and hardness variations were also mapped across the weld-plate cross sections. The maximum and minimum hardnesses were found to be dependent upon both welding traverse speed and tool material. A maximum hardness of 323 Hv(10) was observed in the mixed martensite/bainite/ferrite microstructure of the weld nugget for a welding traverse speed of 200 mm/min using a PCBN tool. A minimum hardness of 179 Hv(10) was found in the outer heat-affected zone (OHAZ) for welding traverse speed of 50 mm/min using a PCBN tool. The distance from the weld centerline to the OHAZ increased with decreasing weld speed due to the greater heat input into the weld. Likewise for similar energy inputs, the size of the transformed zone and the OHAZ increased on moving from a W-Re tool to a PCBN tool probably due to the poorer thermal conductivity of the PCBN tool. The associated residual stresses are reported in Part II of this series of articles.

Microstructural and Residual Stress Development due to Inertia Friction Welding in Ti-6246

A thorough investigation has been performed to assess the microstructural properties, mechanical properties (hardness and elastic modulus), and residual stress development in Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) inertia friction welds in the as-welded and postweld heat-treated conditions. It was evident that the thermomechanical deformation in the weld region occurred above the β transus, forming dynamically recrystallized β grains and precipitating acicular α within the β grains, which resulted in a localized hardness increase. In the heat-affected zone, a ghost microstructure of the base metal formed because of the absence of sufficient time for diffusion, resulting in Mo segregation in the prior primary α plates. Energy-dispersive synchrotron X-ray diffraction and neutron diffraction were used to assess the residual stress development in the three principal directions. The variation in the unstrained lattice parameters across the weld regions was established by imposing a stress balance on the axial stress component in the radial direction. It was found that the maximum stresses occurred in the hoop direction, with significantly lower stresses present in the radial and axial directions. The maximum tensile hoop stresses were located at ~4 mm from the weld centerline and not at the dynamically recrystallized β-rich weld zone. This was associated with the α → β phase transformation and the subsequent acicular α precipitation within the region surrounding the weld centerline.