Repair Welding Research Articles

Wire-Based Friction Stir Additive Manufacturing toward Field Repairing

A new process is introduced for repair of structural components of aluminum alloys. Additive manufacturing, maintenance and repair welding, Al alloy, mechanical properties.

Effect of Overlaying Welding Repair on Stress Corrosion Cracking Sensitivity of Low-Pressure Nuclear Steam Turbine Rotor in the Simulated Environment

Effect of Overlaying Welding Repair on Stress Corrosion Cracking Sensitivity of Low-Pressure Nuclear Steam Turbine Rotor in the Simulated Environment

Influence of weld repair on the residual stresses induced in austenitic stainless steel weld joints

Influence of weld repair on the residual stresses induced in austenitic stainless steel weld joints

Production of Filler Rod for Repair Welding of ZK51 (ML12) Magnesium Alloy Castings

Production of Filler Rod for Repair Welding of ZK51 (ML12) Magnesium Alloy Castings

Impact of Overlay Welding Repair on Bulges during Normal and Short Cycles in Coke Drums

Impact of Overlay Welding Repair on Bulges during Normal and Short Cycles in Coke Drums

Beneficial use of hyperbaric process conditions on the welding of high-strength low alloy steels

Hyperbaric welding is used for different steels in many underwater applications as a repair welding process. A difference between wet and dry welding processes can be made. Due to the increased ambient pressure, these processes have special features inherent in the process that influence the cooling and penetration behaviour. The positive use of these effects outside underwater applications is currently rarely addressed in science and application. The presented work establishes these advantages on the basis of a higher strength structural steel and characterizes the effects on the microstructure of a joined S700MC steel and on the mechanical properties of the joint. It will be shown that a hyperbaric environment can be used to orient the weld more towards the depth of the sheet. Furthermore, it will be shown that this change leads to modified cooling, which in itself influences the mechanical properties of the weld seam.

Effect of Preheat Temperature and Welding Sequence on the Temperature Distribution and Residual Stress in the Weld Overlay Repair of Hydroturbine Runner.

The hydroturbine runner is the core of the whole hydroelectric generating unit, which is employed to transform water energy into mechanical energy. In the process of service, the runner frequently suffers from abrasion due to erosion and cavitation. Defects are usually repaired by welding. To acquire suitable weld cladding repair process parameters, a combination of experimental and numerical simulation was applied to investigate the temperature and weld residual stress distribution in the repair zone under the different welding repair approaches. The results illustrate that the temperature and welding residual stress distribution of the blade and the shroud are out of symmetry, the temperature conduction rate is faster on the blade side, and the high-stress zone is predominantly concentrated in the weld and its adjacent area. When the preheating temperature is up to 150 °C, the peak value of welding residual stress reaches a minimum of 796.29 MPa. The welding sequence can adjust the distribution trend of welding residual stresses. The welding sequence of three-stage welding can effectively reduce the welding residual stresses near the shroud at the water outlet side of the blade. The results of the study will provide theoretical guidance for the welding repair of hydraulic turbine runners.

Superior tensile and fatigue properties of Incoloy 901 repair welds produced by direct energy deposition

A comparative study of the microstructure and mechanical properties (tensile and high-cycle-fatigue) of Incoloy 901 repair welds produced via direct-energy-deposition (DED) and conventional tungsten-inert-gas (TIG) welding was conducted. The initial microstructure of the cast Incoloy 901 substrate exhibited a γ-matrix (an average grain size of ∼235 μm) and spherical γ′ particles (a size of <20 nm). Most of the grain boundaries were not decorated with precipitates, and bulky Mo-rich and Ti-rich MC carbides were precipitated in the γ-matrix. Following DED, the fusion zone exhibited finer dendritic grains and a lower degree of microsegregation than the TIG repair coupon; this resulted from the higher cooling rate associated with lower heat input than TIG. In addition, the width of heat-affected zone (HAZ) was much narrower in the DED repair coupon (0.23 mm) than that in the TIG repair coupon (2.5 mm) in which the dissolution of γ′ precipitates occurred during TIG weld thermal cycles. This microstructural characteristic resulted in different hardness traverses revealing overmatching and undermatching in the DED and TIG repair coupons, respectively. The DED repair coupons exhibited superior tensile and high-cycle-fatigue properties to the TIG repair coupons. The wide HAZ of the TIG repair coupon was preferentially fractured by tensile and cyclic loading because of the easy gliding of the dislocations. Meanwhile, the DED repair coupon exhibited refined slip bands, which facilitated homogeneous deformation, leading to base-metal fracture. These results demonstrate the potential of the DED repair process to achieve excellent mechanical properties comparable to those of the base metal.

Production of filler rods for repair welding of ML12 (ZK51) magnesium alloy castings

In magnesium alloys castings, the casting defects such as shrinkage porosity are often occur. Such defects can be suppressed by repair welding or surfacing using a special filler rod. Unfortunately, in Russia, the low amount of filler rod is consumed. Therefore, native enterprises do not manufacture it, limiting themselves to imports or homemade low-quality substitutes. Nevertheless, there is a need for filler rod, and recently it has become unprofitable to replace them with imported materials due to a significantly increased price. Therefore, there is a need to study the technology of its production to replace imported filler rod with native material. Magnesium alloys based on the Mg–Zn–Zr (La, Nd) system: SV1, SV122, and ML12 (ZK51) that used as a filler rod for repair welding of ZK51 alloy castings were studied in this work. The samples were obtained by permanent mold casting into aluminum molds followed by hot extrusion into a filler rod with a diameter of 4 mm. It was shown that all the investigated alloys could be obtained in the form of a rod with a diameter of 4 mm. Therefore, the investigated rod samples from the SV122 alloy were used as filler material for repair welding of ZK51 magnesium alloy castings. The weld seam in the T1 condition has an ultimate tensile strength (UTS) about 80 % of the UTS of the casting material.

Acceptability assessment of casting defects under transient thermal loading

Weld repairs of castings during manufacture is a well-known and acceptable practice if conducted in accordance with approved standards and procedures. However, either original casting induced cracks or cracking of these weld repairs in high temperature and aging plant is not uncommon. Structural integrity assessments of such components must not only consider reduced material toughness, due to temper embrittlement, but also the stress intensities generated during transient thermal events such as start/stops and quenching incidents. In the first of two case studies the acceptability of weld repair defects in a high pressure turbine outer casing is presented. Metallurgical assessments concluded significant temper embrittlement had occurred which was taken into account in a finite element based structural integrity assessment that considered design operating conditions as well as a hypothetical quenching event. Low pressure turbine bypass valves are considered in the second case study. In this case the material temper embrittlement was found not to be as severe but the criticality of pre-warming to reduce transient thermal stress and by extension crack stress intensities during trips or shutdowns was clearly demonstrated. Remaining life assessments concluded the defects to be acceptable for operation to the next planned outage.

On the Accurate Prediction of Residual Stress in a Three-Pass Slot Nickel-Base Repair Weld by Numerical Simulations

The activities within a European network to develop accurate experimental and numerical methods to assess residual stresses in structural weldments are reported. The NeT Task Group 6 or NeT-TG6 project examined an Alloy 600 plate containing a three-pass slot weld made with Alloy 82 consumables. A number of identical specimens were fabricated and detailed records of the manufacturing history were kept. Parallel measurement and simulation round robins were performed. Residual stresses were measured using neutron diffraction via five different instruments. The acquired database is large enough to generate reliable mean profiles, to identify clear outliers, and to establish the systematic uncertainty associated with this non-destructive technique. NeT-TG6 gives a valuable insight into the real-world variability of diffraction-based residual stress measurements, and forms a reliable foundation against which to benchmark other measurement methods. The mean measured profiles were used to validate the accuracy achieved by the network in the prediction of residual stresses.

Molecular dynamics study on the microscopic mechanism of in-service welding damage and failure

The risk of burn-through is a major concern when conducting in-service welding repair for oil-gas pipelines. The governing mechanism of burn-through has not hitherto been studied systematically. During in-service welding, different micro-zones of the pipe wall metal below the molten pool have different crack initiation mechanisms due to different temperature and stress conditions. This paper uses molecular dynamics to study the initiation and evolution of micro-defects in different micro-zones of the pipe wall under the in-service welding pool and reveals the microscopic mechanism of in-service welding damage and failure. The research found that the pipe wall under the in-service welding pool is divided into a fusion zone and a coarse grain zone. For the coarse grain zone, the grain boundary region is subjected to large deformations and causes high strain. The stress concentration at the grain boundary (especially the triple junctions) is caused by the arrest of grain boundary slip, which leads to the initiation and propagation of microcracks in the grain boundary region. The displacement vector of atoms at the grain boundaries is generally larger than that of the atoms in the crystal, further indicating that grain boundary slip is the primary mechanism of microcrack initiation in the coarse grain zone. Compared with the coarse grain zone, the grain boundary width of the fusion zone increases, and the grain boundary atoms show apparent disorder. The peak value of the radial distribution function decreases, and the peak becomes wider, indicating that the grain boundary pre-melting phenomenon occurs in the fusion zone. In addition, as the tensile strain increases, the instability of the pre-melting zone expands rapidly and advances into the crystal grain. During in-service welding, the high-temperature deformation of the coarse grain zone and the fusion zone of the tube wall are mainly controlled by the intergranular mechanism. Through molecular dynamics, the mechanism of grain boundary slip in coarse grain zone and grain boundary pre-melting in fusion zone were studied from the microscopic scale. Thereby the microscopic mechanism of in-service welding damage failure was further clarified.

Finite element simulation of multi-layer repair welding and experimental investigation of the residual stress fields in steel welded components

Finite element simulation of multi-layer repair welding and experimental investigation of the residual stress fields in steel welded components

Evaluation of Liquation Cracking Susceptibility of CM247LC Superalloy Repair Welds via Pre-Weld Varestraint Test

In this study, the liquation cracking susceptibility in the heat-affected zone of CM247LC superalloy gas turbine blades during repair welding was quantitatively evaluated using a newly developed pre-weld Varestraint test method. The repair welding geometry was replicated through Varestraint tests for the pre-weld bead. The liquation cracking susceptibility, that is, the liquation cracking temperature range (LCTR), could be evaluated through temperature visualization at the time of crack formation during the Varestraint test. The LCTR of CM247LC alloy repair welds (heat-affected zone of the second layer weld) was 280 K. Compared to the LCTR of as-cast (620 K) and aged (65 K) CM247LC, metallurgical mechanisms for controlling the LCTR of repair welds were examined based on the microstructural characterization and Scheil’s solidification calculations for the pre-weld. The LCTR of the CM247LC alloy repair weld was influenced by the MC carbide fraction and the segregated concentration of trace and impurity elements, such as B and S in pre-weld solidification path. A process design capable of reducing the fraction of MC carbides and solidification segregation of trace impurity elements is required. Based on the experimental and theoretical results, the proposed modified Varestraint testing method for dissimilar welds is expected to analyze the solidification cracking behavior effectively in manufacturing high-soundness CM247LC superalloy welds.

Failure mode and mechanism of a blast furnace tuyere

In this study, the failure modes of a blast furnace tuyere after service damage were systematically investigated. The high ambient temperature caused grain growth and hardness reduction at the front end of the tuyere. Owing to scraping by gyratory coke in the blast furnace, a perforation formed at the front end of the tuyere, causing damage. Owing to the uneven distribution of components caused by repair welding, cracks occurred in the front coating of the inner wall of the tuyere. The wear of the inner wall of the tuyere was related to the angle and depth of the coal injection gun, and the wear mechanism differed at different positions. Thus, tuyere failure is mainly attributed to the wear of the front end and inner wall. Therefore, it is necessary to develop a coating with excellent high-temperature wear resistance to protect the inner wall and the front end of tuyeres, thereby extending their service life.

Effect of Repeated Weld Repairs on Microstructure and Mechanical Properties of Heat-Affected Zone in CA6NM Stainless Steel

The low-carbon martensitic stainless steel CA6NM is widely used in the impellers of hydroelectric and nuclear power units due to its advantages of high hardness, corrosion fatigue strength, and good fracture toughness. In order to analyze the number of repair welding times on the properties of heat-affected zone (HAZ, the most unstable area of welded joint property) in CA6NM, the microstructure and mechanical properties of HAZ with once-repaired welds (1R) and twice-repaired welds (2R) were tested. Through data analysis, the following conclusions are drawn for welded joints: (1) the grain size of HAZ with a different number of repair welding processes is smaller than that of the base material; (2) the density of texture concentration is reduced and the directionality is not obvious; (3) the density of twins and dislocations are increased; (4) the hardness and impact energy of HAZ of the 1R and 2R specimens are higher than that of the base material.

Numerical Analysis Growth Kinetics of Dendrite Tip during Laser Welding Nickel-Based Single-Crystal Superalloy Part I: Supersaturation-Driven Dendrite Growth

When three welding conditions, including welding configuration, welding speed and laser power, are sequentially altered, supersaturation of liquid aluminum is crystallography-dependently determined to limit growth kinetics of dendrite tip with decease of thermo-metallurgical factors for solidification cracking to salvage the weld properties during emerging laser welding repair of multicomponent nickel-based single-crystal superalloy. After comparing growth crystallography between right side and left side of weld, the distribution of supersaturation of liquid aluminum is axis- symmetrically developed by favorite (001)/[100] dendrite growth kinetics, while the distribution is nonaxisymmetrically developed by detrimental (001)/[110] dendrite growth kinetics. High heat input is inappropriately procured by either high laser power or slow welding speed to insidiously boost supersaturation of liquid aluminum, worsen alloying partition and solute copiousness, heterogeneously exacerbate morphology and size of dendrite growth, whereby stray grain formation is strongly produced. In order to ameliorate dendrite growth, low heat input is usefully rendered by either low laser power or high welding speed to attenuate supersaturation of liquid aluminum, narrow solidification temperature range and obviate dendrite tip undercooling alongside columnar interface in order to augment crack-resistant dendrite of epitaxial growth. The overall supersaturation of liquid aluminum is crystallography-dependent. During across whole melt-pool solidification interface, supersaturation of nonsymmetric (001)/[110] welding configuration is not as small as symmetric (001)/[100] welding configuration under well-controlled heat input, thereby fosters the kinetic factors for stray grain development to obstruct columnar dendrite and consequently weaken unidirectional morphology. It is crystallographically favorable for relief of supersaturation with axis- symmetrical dendrite growth. The mechanism of crack-vulnerable dendrite growth elimination through circumspect improvement of supersaturation-controlled growth kinetics is constructively proposed for feasible crackless rejuvenation of laser welding or laser cladding. The theoretical predictions are scrupulously supported by corroborative metallograph observation.

Effect of Repeated Repair Welding on the Mechanical Properties and Pitting Corrosion Resistance of Super Duplex Stainless Steel

The study investigated the effect of repeated repair welding on the mechanical properties and pitting corrosion resistance of 25% Cr super duplex stainless steels (UNS S32750). The test coupons were prepared by gas tungsten arc welding (GTAW) with ER2594 filler metal and repaired 3-times at the same weld locations, where the remaining previous welds were 2-3 mm in depth. The number of repairs provided various reheating to the original welds (as-welded condition) and to the previously repaired welds. The effect of repair welding reheating on the microstructure, ferrite content, hardness, impact toughness and pitting corrosion was investigated. The pitting corrosion occurred in the root region of the first repair weld (R1). Secondary phases such as austenite (γ2) and sigma (σ) were observed in the reheated regions with a consumption of ferrite content, where impact toughness and pitting corrosion resistance were significantly reduced. The vicinity of the pitting corrosion region was analyzed, and showed that sigma (σ) was precipitated in the secondary austenite (γ2) and at the boundary between ferrite and austenite. The fraction of the sigma phase was determined by electron back scatter diffraction (EBSD) to be 0.3% in the vicinity of the pitting corrosion region. This study also discusses the applicability of 3-times repair welding in the offshore industry.

Effects of welding wire composition on the repair welds of sand-cast Mg–Gd–Y alloy: Microstructure and mechanical properties

Cast defects could severely deteriorate the quality of magnesium alloy component. Repair welding is an efficient and economic method to improve the casting qualification. In this study, the impact of welding wire composition on the microstructures and mechanical properties of Mg–Gd–Y repair welds was investigated. The results illustrated that, the Zr in the Mg–6Gd–3Y–0.5Zr (VW63K) welding wire promoted the grain refinement in the fusion zone (FZ) of the repair weld, and VW63K FZ performed better thermal stability when subjected to 500 °C for 6 h, because the grain boundaries migration in VW63K FZ was hindered by Zr-contained particles, while the Mg–6Gd–3Y (VW63) FZ presented dramatic grain coarsening. Under as-welded, T4 and T6 conditions, the joint efficiency of VW63K repair weld was 127.64%, 113.14% and 107.37% respectively, while that of VW63 repair weld was 121.61%, 94.92% and 91.67%, respectively.

Determination of residual stress evolution during repair welding of high-strength steel components

During the assembly of steel structures, unacceptable weld defects may be found. An economical solution is local thermal gouging of the affected areas and re-welding. Due to high shrinkage restraints of repair weld and surrounding structure, high global and local welding stresses superimpose, and may lead to cracking and component failure, especially in connection with the degraded microstructure and mechanical properties of high-strength steels during the repair process. Component-related investigations of high-strength steels (FOSTA P1311/IGF20162N) focus on welding residual stress evolution during local thermal gouging and rewelding. In this study, repair welding of S500MLO (EN 10225) is carried out using in-situ digital image correlation (DIC) and ex-situ X-ray diffraction (XRD) to analyse strains and stresses. Self-restrained slit specimen geometries were identified representing defined rigidity conditions of repair welds of real components, which were quantified using the restraint intensity concept. The specimens were rewelded with constant welding heat control and parameters. Weld specimens exhibited significantly increased transverse residual stresses with higher transverse restraint intensities, in the weld metal, and in the heat affected zone. Transverse stresses along the weld seam decrease at the weld seam ends leading to different stress state during gouging and welding. XRD analysis of the longitudinal and transverse local residual stresses after cooling to RT showed a good comparability with global DIC analyses.