Showing papers in "Welding in The World in 2010"

S-N Lines for Welded Thin Joints — Suggested Slopes and FAT Values for Applying the Notch Stress Concept with Various Reference Radii

Abstract: An assessment of thin welded structures with actual IIW design lines results in an overestimation of fatigue lives (strengths) at high load levels and a conservative estimation at low load levels, in many cases, independently of the applied fatigue assessment approach (nominal, structural or notch stress). This is mainly due to the slopes of the design S-N line k = 3.0 for normal and k = 5.0 for shear stresses, which are valid for thick and stiff structures. To overcome this inconsistency for welded thin and flexible structures, the slopes k = 5.0 for normal and k = 7.0 for shear stresses are suggested, keeping the already known FAT values derived for the notch stress concept variants with rref = 1.0, 0.3 or 0.05 mm, respectively. However, the slope is not only determined by plate thickness; it is the result of an interaction between thickness, local geometry (stress concentration), structural stiffness, loading mode and last but not least residual stresses. This complexity makes it difficult to identify the driving parameters and to predict the slope of the S-N line in many cases.

Comparison of Post-Weld Treatment of High-Strength Steel Welded Joints in Medium Cycle Fatigue

Abstract: This paper presents a comparison of three post-weld treatments for fatigue life improvement of welded joints. The objective is to determine the most suitable post-weld treatment for implementation in mass production of certain crane components manufactured from very high-strength steel. The processes investigated are: burr grinding, TIG dressing and ultrasonic impact treatment. The focus of this investigation is on the so-called medium cycle area, i.e. 10 000–500 000 cycles and very high stress ranges. In this area of fatigue design, the use of very high-strength steel becomes necessary, since the stress range can exceed the yield-strength of ordinary structural steel, especially when considering positive stress ratios (R > 0). Fatigue experiments and qualitative evaluation of the different post-weld treatments leads to the selection of TIG dressing. The process of implementing TIG dressing in mass production and some inherent initial problems are discussed. The treatment of a few critical welds leads to a significant increase in fatigue performance of the entire structure and the possibility for better utilization of very high-strength steel.

Welding Thick Steel Plates with Fibre Lasers and GMAW

Abstract: The results of a collaborative research project on laser beam weldability of carbon steels of high sheet thickness are presented. That includes single and multiple pass welding of 16 mm and 20 mm thick plates, as well as the investigation of acceptable tolerances i.e. gap bridgeability and edge misalignment. For the welding experiments fibre lasers with 8 kW, 20 kW laser power and different GMAW-techniques were used in various applications. With the 20 kW fibre laser 16 mm plates could be welded with a single pass, 20 mm required a seam preparation or alternatively preheating of the material. For multi pass welding with 8 kW laser power a joint preparation with a single V-butt joint with a broad root face (Y-groove) was applied. The root pass was always welded with a hybrid process, the filler passes with a hybrid process as well as a GMAW process which produced the best results.

Effect of Ms Temperature on Residual Stress in Welded Joints of High-Strength Steels

Abstract: Thermal stress in the weld joints of high-strength steels after welding receives a great influence of dilatation stress during phase transformation, especially martensitic transformation of the low temperature region in weld metal. In order to quantify the amount of residual stress linked with thermal stress, neutron diffraction analyses were carried out in the welded joints having weld metals with very reduced Ms temperatures. In the butt joint of weld metal with an Ms temperature of about 60 °C, compressive residual stresses of about −400 MPa at the weld centre and of about −75 MPa at the toe were observed. The residual stress distributions were quite different from those of conventional weld metals, which have a tensile residual stress of about 400–500 MPa. The formation mechanism of compressive residual stress and the effect of restraint stress and stress-induced transformation on it are discussed, including previous data. It can be concluded that the reduction amount of residual stress induced by low Ms weld metal is about 11–15 MPa per unit length within about 70 mm weld length.

Mechanisms of Residual Stress Relaxation and Redistribution in Welded High-Strength Steel Specimens under Mechanical Loading

Abstract: In this article, the relaxation of surface and near-surface welding residual stress fields in S690QL under static and cyclic loadings has been studied. The influence of local mechanical properties in the weld metal, heat-affected zone and in the base metal upon relaxation and redistribution of residual stress were considered. Welded specimens were loaded statically and cyclically. Under static loading, the variation of the residual stress field under tension and compression was investigated and it was observed that, by increasing the load, residual stress relaxes continuously. However, the relaxation under compression sets in with delay, since the tensile residual stresses should be first overcome. Under cyclic loading, with or without mean stress, the behaviour of the residual stresses depends on the initial residual stress, local yield strength and maximum applied load. If the von Mises stress, which is a function of residual stress, load and mean stress, exceeds the local yield strength, plastic deformation and thus a relaxation occurs. In the case of relaxation, the first load cycle is of importance, in which a considerable amount of the initial residual stress field is relaxed. After the first relaxation, no significant variation of the residual stress field in the fatigue crack free phase could be observed. When no residual stress relaxation in the first load cycle occurred, the number of load cycles did not change the residual stress field. The specimens were loaded until failure or maximum 2×106 cycles.

Statistics of Weld Geometry for Laser-Hybrid welded Joints and Its Application within Notch Stress Approach

Abstract: During the past few decades the shipbuilding industry has shown an increasing interest in laser welding In particular, laser-hybrid welding offers special advantages over arc welding such as low levels of welding distortion because of the different welding processes used In the present study, the influence of weld geometry on notch stresses in butt-welded joints was studied experimentally and numerically using a notch stress approach The results of hybrid-welded joints were compared to those of laser- and arc-welded joints In the case of laser-based joints the weld height, weld width and notch depth are the dominant geometrical parameters The flank angle of the weld has a minor influence, unlike in an arc-welded joint, because of the difference in the weld bead size The weld dimensions showed a large statistical variation, and this resulted in significant differences in the stress concentration factor values between the critical and mean weld geometries The analysis of fatigue test results indicated that the statistical variation of the weld dimensions should be included in the notch stress analysis in order to improve the accuracy of fatigue strength prediction

Thermo-Mechanical Investigations during Friction Stir Spot Welding (FSSW) of AA6082-T6

Abstract: Friction Stir Spot Welding (FSSW) is a variant of the Friction Stir Welding (FSW) process and has been successfully used in industrial applications. During the FSSW process, thermal inputs due to friction and deformation are commenced simultaneously, as the non-consumable rotating tool plunges into the workpiece to be welded. Various assumptions and hypotheses for mechanisms of heat generation and material deformation during FSW/FSSW process are reported, but a consensus is still to be reached. The joining quality is mainly dependent upon the material flow in this solid state joining technique. The material flow and deformations in the near and far fields of the weld are directly affected by the temperature-sensitive mechanical properties. Therefore, a comprehension of thermo-mechanical responses are of high importance from the viewpoints of parameter optimization and understanding of the mechanisms. The FSSW process is experimentally and theoretically studied to address these issues of the mechanism of heat generation and coupled thermo-mechanical response of the workpiece, as well as the effects of tool rotation and plunge speeds. For theoretical studies, a 3-dimensional, physical-based FEM (Finite Element Method) model is developed using commercial code. For heat generation, friction and deformation-based formulations are used. For material responses, thermal and strain rate-sensitive, elastic-plastic data are employed by a constitutive Johnson Cook material model and thermo-mechanical behaviour is analyzed with respect to experimental observations. To cope with high calculation time and distortion of the mesh, built-in features of the code, mass scaling, ALE (Arbitrary Lagrangian Eulerian) and mesh re-mapping were used. As a result of this work, a basic platform in the form of a physical-based, coupled, thermo-mechanical model is developed. With the help of this model, effects of process parameters on the temperature — displacement behaviour of the workpiece are studied. The role of interaction conditions at the tool-workpiece interface is emphasized and a simplified conceptual mechanism for effects of process variables on the physical phenomena is presented.

Verification of FAT Values for the Application of the Notch Stress Concept with the Reference Radii Rref = 1.00 and 0.05 mm

Abstract: For the verification of the FAT225 value suggested in the IIW Recommendations for the notch stress concept with the reference radius rref = 1.00 mm and of FAT630 for rref = 0.05 mm, investigations were carried out with fully and non-fully penetrated welds, and with non-fully penetrated welds with circular and long holes. The FAT values were confirmed by fatigue tests carried out under the stress ratio R = 0.5. However, in the case of low stress concentrations, e.g. weld toes of butt welds, when rref = 0.05 mm is applied, the FAT class should be downgraded from 630 to 400 MPa to account for the size effect of larger maximum stressed material volumes.

Residual Stress Relaxation of Quasi-Statically and Cyclically-Loaded Steel Welds

Abstract: Weld fatigue strength is currently the bottleneck to designing high performance and lightweight welded structures using advanced materials. In addition to loading conditions, environmental aspects, geometrical features and defects, it has been proven that studying the influence of residual stresses on fatigue performance is indispensible. The extent of the influence is, however, a matter of discussion. In this work, residual stress behaviour in welded S355J2G3 and S1100QL steel specimens under quasi-static and cyclic loading were studied and the correlation between the relaxation of residual stress field and mechanical properties was studied. Residual stress measurements were performed using the X-ray diffraction technique. The relaxation behaviour in S355J2G3 at weld metal, weld toe and in the base metal could be described by Von Mises criteria. This was not the case for S1100QL. In the case of relaxation, it was observed that if the Von Mises stress, as a function of residual, load and mean stresses, exceeds the monotonic yield strength of the base metal, relaxation takes place. Otherwise, the internal elastic stresses remain stable regardless of the number of loading cycles and could contribute to fatigue damage.

Laser Welding of Dissimilar Aluminium Alloys with Filler Materials

Abstract: Aluminium alloys have great potential in industrial applications due to their high strength and low weight, specially the AA5xxx and AA6xxx series that have particular applications in the aerospace, military and information technology industries. Laser welding is quite used for joining these materials despite their high reflectivity against laser beam radiation. Nd:YAG lasers are used in pulsed wave mode to overcome a set of difficulties, such as the material absorptivity, its thermal expansion coefficient and thermal conductivity. Autogenous welding of dissimilar aluminium alloys brings additional difficulties due to chemical, thermal and mechanical incompatibility, so the use of a correctly selected filler material improves the compatibility of both alloys being joined. This paper describes the work performed aiming to study laser welding of dissimilar alloys of AA6xxx series and of AA5xxx to AA6xxx series with filler wire. A Nd:YAG laser was used in a pulsed wave mode and a set of filler wires was tested aiming to select the filler presenting better performance. The quality of the welds was evaluated by macro and microstructural analysis. Scanning electron microscopy and hardness measurements were also performed. The results indicate that filler wires with low magnesium and high silicon contents allow for a better weld quality, free of porosity (individual or in clusters). However, the mechanical strength has to be considered in order to assure adequate service performance of the components or structures.

An Integrated Model to Simulate Laser Cladding Manufacturing Process for Engine Repair Applications

Abstract: An integrated thermal, microstructure, and thermomechanical model was developed to predict temperature, thermomechanical strain, stress and distortion evolution during laser cladding of engine blade repair. The model was used to analyze a simplified geometry to evaluate the effect of process conditions on solidification cracking tendency. Two cases with and without cooling to room temperature between welded layers were studied. Simulation results suggested that laser cladding without cooling down between welded layers produces higher temperature, higher thermomechanical tensile strain during cooling after solidification than with cooling down between welded layers. This indicates that the solidification cracking tendency is higher without cooling down between welded layers than with cooling down between welded layers. Because of the preheating effect resulted from the continual laser cladding without cooling down between welded layers, the residual stress is lower, but the distortion is higher than the case with cooling down between welded layers.

Comprehensive Diagnostics of Parent Metal and Welded Joints of Steam Pipeline Bends

Abstract: Experience with carrying out comprehensive diagnostics of steam pipelines bends and welded joints using the methods of metal magnetic memory, ultrasonic testing and microstructure analysis is described. It is shown that stress concentration zones caused by additional working loads — not planned and expected in the design phase of the components — are sources of which pipeline damage emerges and develops.

Numerical Analysis of Forming Mechanism of Hump Bead in High Speed GMAW

Abstract: A thorough understanding of the physical mechanisms responsible for humping bead formation in high speed GMAW (gas metal arc welding) is essential to improve the welding productivity in today’s competitive manufacturing industries. Based on the experimental observing results, a mathematical model is developed to quantitatively analyze the forming mechanism of humping bead for high speed GMAW, considering both the momentum and heat content of the backward flowing molten metal in weld pools. One term related to the momentum of backward flowing molten metal is added to the equation of the weld pool surface deformation, and the heat content of overheated droplets is distributed within the layer covering the whole pool. The humping bead forming process and its dimension and 3-D geometry are numerically simulated under some welding conditions. It is found that the model can describe and characterize the humping formation in high speed GMAW quantitatively.

Numerical Investigations of the Influence of Design Parameters, Gas Composition and Electric Current in Plasma Arc Welding (PAW)

Abstract: Plasma Tungsten Arc Welding (PTAW) compared with TIG Welding enables an increased welding speed, a reduced energy input per unit length and butt joint welding of plates without preparation of the welded seam due to keyhole effect. However, because of missed profound understanding of effects in plasma arcs, the indisputable advantages of this process increase demands on education and especially on the experience of developers and operators. In this paper, process parameters, properties of the plasma jet and the molten pool are derived from the numerical modelling of arc and sheath layer under consideration of the process gas properties and the effects of demixing. Basics of the model and the testing site are introduced in this paper. Additionally, influences of current intensity, plasma gas quantity, process gases and their specific properties, torch geometry on plasma jet and energy input into work piece is shown by an exemplary torch. Model and numerical results have been validated by impact pressure measurements at the surface of the work piece and penetration profiles (cross section).

Improving the Life of Continuous Casting Rolls Through Submerged Arc Cladding with Nitrogen-Alloyed Martensitic Stainless Steel

Abstract: Continuous casting rolls are subjected to extreme temperature fluctuations and harsh environmental conditions during service. High slab contact pressures, high surface temperatures, adhesive or abrasive wear by slag or oxides and thermal fatigue all contribute towards deterioration of the roll surface. Rolls may also suffer surface degradation as a result of corrosion. The slab is cooled using water spray, often containing hydrofluoric acid as product of the reaction between water and the mould flux powder used to keep the molten metal from adhering to the copper mould plate. This results in an aggressive low pH environment that may promote pitting or crevice corrosion and stress corrosion cracking failures. In order to prolong the life of continuous casting rolls in service, the rolls are surfaced using martensitic stainless steel weld cladding. After surfacing, the roll may be given a post-weld heat treatment to temper the martensite to the required hardness level, or to relieve thermally-induced stresses. This investigation examined the submerged arc cladding of continuous casting rolls using nitrogen-alloyed martensitic stainless steel, and focused on the effect of the weld thermal cycle and post-weld heat treatment on carbide precipitation in the clad layer. The precipitation of chromium-rich carbides during welding or post-weld heat treatment may lead to weld decay due to chromium depletion, rendering the roll surface susceptible to pit nucleation and crack initiation.

Fatigue Tests and Numerical Analyses of Partial-Load and Full-Load Carrying Fillet Welds at Cover Plates and Lap Joints

Abstract: The fatigue strength of fillet welds is dependent on the amount of loads transferred. This is considered in the approaches for fatigue assessment in different ways. Particularly in the structural stress approaches, modified stress distributions in thickness direction of the parent plate have recently been proposed to consider the amount of load carried by the fillet weld in the fatigue assessment. In order to validate the approaches, fatigue tests have been performed with cover plates having partial-load carrying fillet welds and lap joints having full-load carrying fillet welds. The thickness of all plates was 12 mm, while two throat thicknesses with 3 and 7 mm were realized. The approaches for fatigue strength assessment include different structural stress approaches and the effective notch stress approach. Conclusions are drawn from a comparison of the approaches and from the fatigue lives obtained with respect to the consideration of the amount of load carried by fillet welds.

An Investigation of Failure Types in High-Strength Steel Resistance Spot Welds

Abstract: On the shop floor, as in laboratories, destructive testing remains the main means of quality control of spot welds. After hand or mechanized weld destruction, the so-called “plug diameter” is measured and is usually considered a good indicator of the weld quality. However, it turns out that the “plug” failure of spot welds is far from being the rule. Moreover, fracture may occur in different zones of the weld, leading to very different meanings of the plug diameter. Therefore, the plug diameter is most of the time not well-correlated with the weld strength, which is the main value of the spot weld. Partial or full interfacial failures (through the weld nugget) exhibit equivalent mechanical strengths and therefore should not be rejected. Through this work, it is shown that the failure type depends on various parameters (nugget diameter, sheet thickness, loading mode, …), and consequently, it is not an intrinsic property of the steel grade. In this way, recommended quality criteria are based on weld strength, weld diameter (including the interfacial fracture area if any), or absorbed energy independently of failure occurrence. In addition, non-destructive techniques might be key investigation methods and have to be developed further.

Henry Granjon Prize Competition 2009 Winner Category B: “Materials Behaviour and Weldability” Pitting Corrosion Resistance of GTA Welded Lean Duplex Stainless Steel

Abstract: The use of lean duplex stainless steel increases steadily worldwide and the fluctuations in nickel price in recent years have contributed to further growth. The weldability of these grades is diffe ...

Measurements of Diffusible Hydrogen Contents at Elevated Temperatures using Different Hot Extraction Techniques — An International Round Robin Test

Abstract: This international round robin test served to scrutinize the procedures specified in ISO/DIS 3690:2009 for determining the diffusible hydrogen content in weld metals with bcc-lattice structure. It was specifically intended to check in what respect the specifications defined in the indicated standards for specimen preparation, storage and hydrogen analysis provide comparable measurement results. The round robin test is presented comprising comparative measurements at various degassing temperatures using hot extraction techniques and a thermal conductivity detector (TCD). A major focus of this investigation was the examination of the maximum degassing temperature for analysing the diffusible hydrogen in materials with bcc-lattice structure. The analyses were performed using two different stick electrodes and three different filler wires. As a significant result it was found that no deviations or increases, were detected in the measured contents of diffusible hydrogen for the investigated degassing temperatures ranging between 45 °C and 400 °C. Hydrogen analyses for contents below HD = 1.5 ml/100 g with the hot extraction techniques in conjunction with TCD applied in this study led to considerable relative standard deviations.

Verification of Validity and Generality of Dominant Factors in High Accuracy Prediction of Welding Distortion

Abstract: The two dominant factors, satisfying the yield condition (especially at high temperature above 700 °C) and regarding the weld metal shape as the deposited metal, in the high accuracy prediction of welding distortion had been previously determined using a bead-on-plate specimen with a groove. To verify the validity of the dominant factors, the experiment and thermal elastic-plastic analysis by FEM were conducted using a specimen for butt welding and fillet welding. The validity of the dominant factors in the high accuracy prediction of welding distortion using the bead-on-plate specimen with the groove was verified by the results of butt welding and fillet welding, and its generality was confirmed. It was confirmed that residual stress was also accurately predicted because welding distortion was accurately predicted.

Fatigue Design of Welded Components of Railway Vehicles — Influence of Manufacturing Conditions and Weld Quality

Abstract: The weld quality has a significant influence on the fatigue strength of welded joints and components. In the industrial field, there are different manufacturing conditions and as a consequence of this, the weld quality can vary considerably. Therefore, fatigue tests of different joint types were performed, analysing the influence of manufacturing conditions and of weld quality on the fatigue strength of welded components of railway vehicles. For these fatigue tests, typical welded joints with the significantly different notch effect were selected, i.e. butt weld specimens with relatively low notch effect and cruciform joints with relatively high notch effect. These specimens were made under typical industrial conditions by three different companies from the railway industry (Bombardier, Siemens, and ALSTOM). Thus, they represented the weld quality level of this industrial field. Prior to the fatigue testing of these specimens, the weld quality was checked in accordance with the criteria of the DIN EN 15085, which is based upon the general weld quality standard DIN EN ISO 5817. On the basis of the result of this weld quality inspection, most specimens have the quality level B relating to DIN EN ISO 5817. The fatigue tests under axial loading show a significantly higher fatigue strength level for the nominal approach in comparison with IIW Recommendations. However, the determined fatigue stress range of both joint types investigated is conservative, in comparison with the allowable fatigue strength of the actual guidelines or standards applied in the railway industry. For both joint types investigated the relevant notch stress range was determined based on the results for nominal fatigue strength. The fatigue strength differences between the notch stress range of the test results and the notch fatigue resistance of the IIW Recommendations are significantly less than the differences in the nominal strength comparison. The determined nominal fatigue strength values of the investigated welded joints correspond to the weld quality level B, as defined by DIN EN ISO 5817, and additionally, to the requirement of angular distortion ≤ 1°. Furthermore, the determined nominal fatigue strength values of the cruciform joints is related to the throat thickness ≥ 0.75 × plate thickness and to the root face length ≤ 0.75 × plate thickness. In summary, this investigation shows the fatigue strength potential for welded components in the field of the railway industry. The results provide useful information for the economical and safe fatigue design of welded components, depending on the manufacturing conditions and the real weld quality.

Comparative Study between Hot Extraction Methods and Mercury Method — A National Round Robin Test

Abstract: A round robin test is presented comprising comparative measurements using hot extraction at different degassing temperatures as well as the mercury method. A major focus of the investigation was verification of the maximum degassing temperature for analysing the diffusible hydrogen in weld metals with bcc-lattice structure. The analyses were executed using a basic stick electrode with high weld metal cracking, a high-alloyed supermartensitic filler wire with different hydrogen contents in the shielding gas and a high-strength solid wire. The results show that degassing temperatures of 150 °C and 400 °C do not lead to an increase in the measured contents of diffusible hydrogen as compared to measurements at room temperature. The measuring techniques and procedures specified in ISO/DIS 3690:2009 for determining the diffusible hydrogen content in weld metals with bcc-lattice structure yield approximately the same results. This is to say that the mercury method and the hot extraction methods with thermal conductivity detector (TCD) can be regarded as equivalent reference methods.

Metal vapour in MIG arcs can cause (1) minima in central arc temperatures and (2) increased arc voltages

Abstract: Radiation emission coefficients of plasmas from high temperature metal iron vapour are about a factor of a hundred larger than emission coefficients of high temperature argon plasmas as occur in argon welding arcs. Iron vapour can be easily introduced into the central region of such a welding arc due to melting of an electrode containing iron. The high radiation emission can produce several effects contrary to the usual behaviour of arcs in pure argon. Firstly, a central cylindrical core of iron vapour will be highly radiating and calculations show that the central core can consequently be at a lower temperature than the surrounding argon plasma. Because this central core has higher radiation emission it can give the appearance of having a higher central temperature. Secondly, it is predicted that radiation cooling of the arc due to iron vapour reduces the arc temperature and has the effect of increasing the arc voltage. The effect of increased radiation in the main arc column outweighs the effect of iron vapour increasing the electrical conductivity of the plasma at low temperatures which has the effect of reducing the arc voltage. This latter effect is limited to a small region near the electrodes.

Effects of alloying concepts on ferrite morphology and toughness of lean duplex stainless steel weld metals

Abstract: Alloying concepts for lean duplex weld metals have been explored. Focus was on compositions with 21.5–24 % Cr with different levels of Ni and Mn in combination with N producing a minimum PREn value of 26. Microstructures and properties of weld metals produced with experimental covered electrodes or metal-cored wires were evaluated. The electron backscattered diffraction (EBSD) technique combined with scanning electron microscopy (SEM) was used to explore relations between weld metal morphology and crystallographic orientation relationships between ferrite and austenite. A duplex microstructure with Widmanstatten type ferrite characteristic of a ferritic solidification was found throughout most weld metals. Regions with a morphology suggesting a mixed ferritic-austenitic solidification occurred particularly for higher Ni- and N-contents. EBSD studies showed that most of the austenite formed following a ferritic solidification was near either the Nishiyama-Wasserman (NW) or Kurjdumov-Sachs (KS) relationships with adjacent ferrite. More phase boundaries in the mixed mode solidification regions had a random orientation relationship. Strength, ductility and PREn requirements were readily matched but the combined Mn, Ni and N alloying levels and the ferrite morphology affected toughness significantly. Both a fully ferritic solidification and a sufficient Ni-content were necessary to produce acceptable and reproducible impact toughness at room and sub-zero temperatures. It was concluded that a composition of 23–24 % Cr, 7–8 % Ni and 0.12–0.16 % N was well suited to fulfil requirements.

The Effect of Ageing on the Spot Weld Strength of AHSS and the Consequences for Testing Procedures

Abstract: Market trends within the automotive industry are leading to an ever-increasing use of high-strength and advanced high-strength steels (AHSS). The attraction of these materials is the advantage of excellent formability, combined with increasingly high tensile strength. It is a well-known fact that weld performance can be an issue with AHSS, where susceptible weld microstructures can lead to low strengths and undesirable failure modes. Much research has been conducted and published on this subject. A less well-documented effect in the weld performance of AHSS is ‘ageing’, whereby a weld exhibits poor mechanical performance immediately after welding, but after a certain period of time, the weld properties improve significantly. In the Corus — SMI research cooperation, this ‘ageing’ effect was first observed in weld samples in 2004, since this time ageing has been a major topic of combined research. This presentation is a summary of the Corus — SMI weld ageing study, highlighting the major issues and characteristics of the effect: the influence of process parameters, the susceptible alloying systems and the possible mechanisms that can cause ageing of the weld. The ‘ageing’ effect has serious implications for standardized testing procedures, where the timescale between welding and testing is not specified, ageing can have a huge influence on the welding results obtained in ageing susceptible materials. The final aspect of this report is to consider the consequences of ageing for weld testing procedures.

Effect of flux ratio in flux-cored wire on wire melting behaviour and fume emission rate

Abstract: In GMAW, the welding wire melts itself due to the arc discharges from the electrode. Both the Joule heating and the arc heating of the electrode contribute to the melting of the wire. In general, it is said that flux-cored wire (FCW) melts faster than solid wire as the welding current flows mainly through the sheath of the FCW. FCW structure differs from solid wire and these differences affect not only the wire melting behaviour, but also the fume emission rate (FER). However, at present, no report exists which defines these effects in detail. Therefore, it is very important to clarify the effect of these differences in wire structure on the wire melting behaviour and FER in order to understand how to reduce the latter. In this study, we found that FER increased with the flux ratio of FCW. It is assumed that these results can be attributed to the state of the stability of the flux column in the arc and the heat content of the droplets.

Study of Hot Cracking Behaviour of Nitrogen-Enhanced Austenitic Stainless Steels using Varestraint and Hot Ductility Tests

Abstract: Hot cracking is a major problem in the welding of austenitic stainless steels (SS), particularly the fully austenitic grades. A group of alloys of high nitrogen stainless steel is being developed for structural components of the Indian Fast Reactor programme. Studying the hot cracking behaviour of this nitrogen-enhanced austenitic stainless steel is an important consideration during welding, as this material solidifies without any residual delta-ferrite in the primary austenitic mode. Nitrogen has potent effects on the solidification microstructure; hence, it is expected to have a strong influence on the hot cracking behaviour. Both Varestraint and hot ductility tests were used to evaluate its solidification and liquation cracking susceptibility. Different heats of this material were investigated, which included fully austenitic (high nitrogen stainless steels) containing 0.07–0.22 wt. (%) nitrogen. Varestraint tests were carried out on these alloys using specimens of 3 mm thickness at four strain levels between 0.5 and 4.0%. The Brittleness Temperature Range (BTR) was also evaluated from these tests. The Varestraint test results showed that the solidification cracking susceptibility is higher for 0.22 wt. (%) steel and the liquation (HAZ) crack significantly increases with increasing nitrogen content. Hot ductility tests were conducted on these alloys using a thermomechanical simulator and the Nil Strength Temperature (NST), Nil Ductility Temperature (NDT) and Ductility Recovery Temperature (DRT) were determined. The hot ductility test results showed that the nil ductility range (NDR), the difference between NST and DRT of the nitrogen-enhanced steel containing 0.22 % N, is higher (50 °C) than that of the alloys containing 0.07 % N (40 °C) and 0.14 % N (30 °C), indicating high susceptibility of the 0.22 % N alloy to liquation cracking. This paper presents and discusses the role of high nitrogen content on hot cracking susceptibility of this class of steels using Varestraint and hot ductility tests.

Determination of Dominant Factors in High Accuracy Prediction of Welding Distortion

Abstract: The experiment was carried out using a bead-on-plate specimen with the groove placed to accurately predict welding distortion. The experiment was simulated with thermal elastic-plastic analysis based on FEM. Comparing and investigating both results, the dominant factors for high accuracy prediction of welding distortion were determined. In the thermal elastic-plastic analysis, the yield condition should be satisfied regardless of the magnitude of yield stress of the material. It was not known if the predicted results of welding distortion obtained without satisfying the yield condition in the whole temperature range were overestimated or underestimated. The accuracy of prediction of welding distortion was improved by applying heat input to the weld metal bead shape, not just the groove profile. It was determined that satisfying the yield condition in the entire temperature range and considering the weld metal shape as the deposited metal were the dominant factors for high accuracy prediction of welding distortion.

The Effect of Voltage and Metal Transfer Mode on Particulate Fume Size During the GMAW of Plain Carbon Steel

Abstract: Particulate welding fumes can enter unciliated airways, or may even be absorbed through the skin, possibly to the detriment of the health of welders. The size and shape of the particulates will determine their ability to infiltrate the human body. Hence, the sensitivity of particulate fume size to welding parameters such as arc voltage (20–36 V) and metal transfer mode (dip, globular and spray) was assessed. Transmission electron microscopy (TEM) imaging was used for determining particulate fume size and it was found to be an accurate, reproducible, and relatively simple technique. The results revealed a higher percentage of ultra fine particulates (i.e. in < 20 nm and 20–40 nm intervals) in ‘low welding voltage’ fume plume compared with ‘high welding voltage’ fume plume. Fundamentally, the fume plume created during dip metal transfer at low welding voltages (20–26 V) had much lower levels of particulate fume concentration compared with spray metal transfer at high welding voltages (30–36 V). TEM also revealed that for the range of welding voltages and metal transfer modes investigated, the particulates produced were predominantly less than 100 nm in diameter and spherical. Spherical particles (< 100 nm) have been reported elsewhere to reach the alveoli in rats and, hence, may be of relevance with respect to the health of welders. Slightly faceted crystalline particulates and fume particle sizes of up to 800 nm in diameter were also observed with TEM. It should be noted that the agglomeration behaviour of these particulates in the fume plume is considered important but not entirely understood. The particle size results suggest that the key to welder safety is to minimize cumulative exposure to particulate fume over their working life, irrespective of the welding parameters used. Innovative fume extraction techniques, clean workshops, automated welding, and low welding fume consumables should all form part of an integrated solution to help ensure the health and well-being of welding personnel.

Correlation of creep properties of simulated and real weld joints in modified 9 %Cr steels

Abstract: This paper examines the creep properties of modified 9–12% Cr steels, especially the P91 grade. This steel is widely-used in many power plant constructions all over the world. Welding P91 steel components is one of the most important technological procedures. The aim of the investigation performed is to why creep rupture on welded joints at high temperatures is often observed in the heat-affected zone (HAZ), especially in the so-called inter-critical zone (IC-HAZ). The creep rupture strength (CRS) of cross-weld joints is usually about 20 to 30% lower than that of the base material, in the case of creep-resistant modified chromium steels. Precipitation strengthening (PS) depends on the inter-particle spacing (IPS) of secondary phases — MX nanoparticles and/or M23C6 particles. With increasing IPS of secondary phases, the proof stress and creep rupture strength decreases, while creep rate increases. With respect to the small size of each in the HAZ region and to determine the IPS in the HAZ, the simulation technique was used for estimation of the microstructure and properties. The Smitweld machine was used to reproduce the thermal cycles in the HAZ. The microstructure of the HAZ-simulated regions was analysed by transmission electron microscopy. Real creep tests were carried out on specimens with HAZ-simulated regions and also on the P91 real welded joints. It was found that inter-particle spacing of secondary phases in the inter-critical region of the HAZ is much higher than that of the base material. Therefore, the proof stress and hardness, as well as creep rupture strength of the HAZ are lower than that of the base material. Creep test results of the real welded joints and simulated HAZ regions are in good correlation with all theoretical presumptions.