Low-alloy Weld Research Articles

Modeling of Hydrogen Diffusion in Inhomogeneous Steel Welded Joints.

Hydrogen is a main factor in cold cracking or hydrogen-induced cracking. The most crack susceptible region of a steel welded joint is the heat affected zone (HAZ). The formulation and functional-analytical solution of the one-dimensional problem of hydrogen diffusion in an inhomogeneous butt-welded joint considering weld and joint dimensions and initial hydrogen distribution as well as hydrogen diffusion coefficients and solubilities are presented. It is shown that the peak hydrogen concentration in the HAZ of inhomogeneous joints varies in direct proportion to the initial hydrogen concentration in the weld metal. It is inversely proportional to the ratio of hydrogen solubilities in the weld metal and the HAZ metal and is nonlinear in the diffusion coefficient ratio of these metals. The peak hydrogen concentration in the HAZ can exceed 170% of the average initial concentration in the joint if martensitic steel is welded using low-carbon low-alloy welding consumables. The utilization of austenitic consumables leads to a dramatic reduction in the hydrogen concentration in the HAZ in comparison with the non-austenitic consumables. No direct relationship was found between the hydrogen concentration in the HAZ and the hydrogen evolution from the joint surface.

Microstructure and the fatigue crack propagation in the dissimilar low alloy/stainless steel GMAW welded joints

Joining dissimilar metals is a frequent task for engineers in the power industry. The resulting dissimilar joints are then critical parts of the constructions and have to be able to withstand various loads during the operation of power devices. Due to the metals dissimilarity, the interface between ferritic side and the austenitic side of the joint is considered to be a critical area regarding the joint resistance to failure under static or cyclic loading. Proper understanding of the microstructures formed at the interface is key for production of solid and reliable dissimilar joints. In the present study, two dissimilar joints were prepared by gas metal arc welding (GMAW) using different low alloy steels. The microstructure of both joints is extensively studied using electron microscopy methods, with the emphasis taken on the interface between low alloy steel and austenitic weld metal. It was shown, that the interface between low alloy steel and weld metal is not a preferential crack path in case of non-environmental fatigue crack propagation, and the cracks are rather propagating through to the heat-affected zones of the low alloy steel side of the dissimilar joint. There were recorded significant differences in the threshold stress intensity factor between two tested welded joints (5.8 MPa × m1/2 vs. 8.7 MPa × m1/2. Therefore, the choice of the low alloy steel has a significant effect on the resistance to the fatigue crack propagation, and even small differences in the microstructure of low alloy steel may affect the resistance to the fatigue crack propagation at low stress ratios of the whole dissimilar joint.

Weld Metal Dendritic Structure Modification by Dispersed Refractory Oxide Particles

The paper deals with the modification effect of dispersed oxides particles (Al2O3, MgO, ZrO) on the dendrite structure in low-alloy weld metal. The flux-core wire 1.6 mm diameter for inoculating of oxides powder to weld pool was purposed. Obtained results confirmed an influence of inoculated refractory oxides on dendrites size and morphology, microstructure and mechanical properties of the weld metal.

The Expanded Lever Rule on the Phase Transformation Rate Curve for Phase Volume Fraction Calculation in Thermal Simulations

The expanded lever rule is more efficient for calculating phase volume fraction when two phases are partly overlapped. It can make up the deficiency of metallography analysis when phases are similar in morphology. In this study, the expanded lever rule on the phase transformation rate curve is introduced, the approximate symmetry principle in applying the expanded lever rule is modified, and the resulting measurement uncertainty is discussed. Results show that (i) the calculated volume fraction of pearlite by the expanded lever rule in an overlapped two-phase process of a high strength low alloy (HSLA) weld metal is in accordance with the result by metallography analysis, and (ii) the expanded lever rule has an advantage over metallography analysis as it can calculate upper bainite, lower bainite, and lath-like martensite volume fractions in a three-phase process with two overlaps of a HSLA steel. The expanded lever rule is applicable to overlapped two-phase processes of any material theoretically since each phase process can be described by Johnson–Mehl–Avrami–Kolmogorov equation.

Effect of microstructural inhomogeneity on hydrogen diffusion in welded joints

Hydrogen is a main factor in cold cracking or hydrogen-induced cracking. The most crack susceptible region of a steel welded joint is the heat affected zone (HAZ). Formulation and functional-analytical solution of the one-dimensional problem of hydrogen diffusion in an inhomogeneous butt-welded joint taking into account weld and joint dimensions as well as hydrogen diffusion coefficients and solubilities are presented. It is shown that the peak hydrogen concentration in the HAZ of inhomogeneous joints varies in direct proportion to the initial hydrogen concentration in the weld metal. Peak hydrogen concentration is inversely proportional to the ratio of hydrogen solubilities in the weld and HAZ and the square root of diffusion coefficient ratio of the HAZ and weld. The peak hydrogen concentration in the HAZ can exceed 80% of the initial concentration in the weld if martensitic steel is welded using low-carbon low-alloy welding consumables. Utilization of austenitic consumables leads to reduction in hydrogen concentration in the HAZ by 75–85% in comparison with non-austenitic consumables.

Narrow gap welding of low alloy and austenitic stainless steels using different Inconel alloys: comparison of microstructure and properties

In the present investigation, SA508 low alloy steel and 304LN austenitic stainless steel were joined by hot wire narrow gap gas tungsten arc welding. Inconel 82 and 52 were used as filler alloys. Evaluation of microstructure, micro-hardness, and tensile strength through in situ test in scanning electron microscope were attempted for the welded assemblies. Low alloy steel consisted of a heat affected zone adjacent to interface. A thin layer of martensite and Type-I boundary were revealed near the fusion boundary between low alloy steel and weld metal. Micro-hardness distribution showed a peak near the fusion boundary between low alloy steel and filler alloy owing to the presence of lath martensite, Type-I boundary, and complex alloy carbides. The micro-hardness was further dropped within the filler alloy. During the in situ tensile test, deformation was confined within the filler alloy, as it was the weakest link across the fusion boundary. It was revealed that the joint produced with Inconel 82 filler alloy has a better mechanical strength than the assembly fabricated with Inconel 52 filler alloy. Inferior quality of the latter was attributed to the relatively coarser grain structure and the presence of larger alloy carbides within filler alloy when compared with the former. Improved strength of the weld with Inconel 82 filler alloy was further endorsed by the lower slip distance for dislocation movement and higher strain hardening rate in comparison to the weld fabricated with Inconel 52 filler alloy.

Weld Repair of Steam Turbine Rotor with 12 Cr Weld to Mitigate Corrosion Issues

Weld repair of steam turbine rotors has become an acceptable practice to extend the life of rotors. Depending on the type of damage, the extent of weld repair can range from weld build-up of integral discs to stubbing a new forging to replace the damaged portion of an old rotor. Steam turbine rotors made of low alloy steels experience corrosion related damages such as stress corrosion cracking, pitting corrosion, corrosion-erosion etc., Traditional weld repairs have been using low alloy steel welds. While the low alloy welds may have mechanical properties comparable to or even slightly exceeding that of the low alloy steel rotor alloys, the corrosion resistance of the low alloy welds are not great. 12 Chromium weld will provide better resistance to corrosion related damages than typical low alloy welds. However using 12 Chromium weld on low alloy steel rotors provides some additional challenges and limitations. These include selection of optimal combination of weld wire and flux for rotor welding, selection of optimal post weld stress relieve temperature and dealing with abrupt change in chemical composition at the weld interface. This paper discusses development of such a weld procedure and provides some examples where the 12 Chromium weld was successfully applied on steam turbine rotors that experienced corrosion related damages.

Influence of the atmosphere corrosion on the fatigue life of welded T-joints treated by high frequency mechanical impact

The investigation deals with the effectiveness of high frequency mechanical impact (HFMI) treatment technology to increase the corrosion resistance of a base metal and the fatigue life of as-welded T-shaped joints and after their long-time exploitation in conditions modelling moderate climate. The conditions of moderate climate were simulated by exposing the specimens in a moisture chamber of G4 at a temperature of 40°C and air humidity of 98%. The microstructure of low alloy 15KhSND steel, weld zone and heat affected zone (HAZ) before and after the HFMI treatment was investigated. The corrosion tests of the base metal were carried out during 240, 480, 720, 960, 1200 and 2400 h, respectively. After exposure the specimens were removed and their weight reduction was defined. The duration of the corrosion test of the welded joints before fatigue tests was 1200 h. Corrosion-mechanical fracture of the hardened surface layer of the base metal, weld zone and HAZ after 1200 h of exposure is observed. The fatigue tests were carried out on six series of the welded T-joints of low alloy steel: as-welded and treated by HFMI on air; as-welded and treated by HFMI states after exposing in the chamber of G4; after accumulation 2·106 cycles and exposing in the chamber G4; after accumulation 2·106 cycles, exposing in the chamber G4 and subsequent treated by HFMI. It was established by the experiment that the use of HFMI technology can significantly increase the fatigue life of the welded T-joints of steel structures.

Impact Toughness of Steel WMD After TIG Welding

AbstractThe material selected for this investigation was low alloy weld metal deposit after TIG welding with various amount of oxygen in weld metal deposit (WMD). After TIG process it is difficult to get proper amount of oxygen in WMD on the level much lower than 350 ppm. The highest impact toughness of low alloy WMD corresponds with the amount of oxygen in WMD above 350 ppm. In the paper focuses on low alloy steel after innovate welding method with micro-jet cooling that could be treated as a chance on rising amount of oxygen in weld. Weld metal deposit (WMD) was carried out for TIG welding with micro-jet cooling with various amount of oxygen in WMD. In that paper various gas mixtures (gas mixtures Ar-O2and Ar-CO2) were tested for micro-jet cooling after TIG welding. An important role in the interpretation of the results can give methods of artificial intelligence.

In situ neutron diffraction measurement of strain relaxation in welds during heat treatment

ABSTRACTNeutron diffraction (ND) is commonly used to investigate the stress redistribution before and after post-weld heat treatment (PWHT) in welded structures. However, there is a lack of information on the evaluations of strains during PWHT. The present work employed in situ ND to measure the relaxation of residual strains during conventional PWHT in multi-pass high-strength low-alloy steel welds. It was found that strain relaxation occurs principally during the heating stage of the heat treatment. The findings have important economic bearings and can be used to characterise comparable material combinations and optimise the PWHT process for high-strength low-alloy weld joints. This unique information also provides a valuable benchmark for the finite element modelling of this complex process.

Sources of Variability and Lower Values in Toughness Measurements of Weld Metals

In this paper, the sources of variability and lower values in toughness measurements of a high-strength low alloy weld metal are investigated by detailed observations of fracture surfaces and the microstructures at crack initiation. The results reveal that the variability of the notch Charpy toughness is related to the location of the cleavage initiation origins. The three factors jointly contribute to give rise to variability and lower value of toughness. That is, (1) the location that the notch is sampled at deposited weld metal or reheated weld metal, (2) the location that a large grain region is appeared on the path ahead of the notch root, (3) the distributed location of the defect or the brittle second-phase particle. When three factors were simultaneously satisfied, the lower value of the Charpy toughness is appeared. The notch is sampled at the largest grain region at deposited weld metal, and the defect or the brittle second-phase particle is close to the notch root and sampled by its high stress field, the lowest Charpy toughness is obtained.

Control of hydrogen cracking in the welded steel using microstructural traps

Hydrogen diffusion into steel can embrittle the material in H2S environments, but this effect can be offset by suitable hydrogen trapping sites in the microstructure. Fine Ti(C,N) inclusions have been studied as the trapping sites in high strength low alloy (API X-70) welds, with Ti additions ranging from 0.004 to 0.16 wt.%. The trapping sites were investigated by electron microscopy and thermal desorption spectroscopy. Manganese sulphide particles were the main initiation sites for hydrogen induced cracking as expected. The optimum Ti addition was around 0.02% with no evidence of cracking in the weld. The estimated values of trapping activation energy for dislocations, microvoids, MnS and Ti(C, N) were approximately 25.9, 34.6, 65.1 and 87.6 kJ mol−1, respectively.

Steel Welding with Micro-Jet Injector

The article focuses on mild steel welding and covers the new possibilities of that method. Since 2011 innovate welding technology based on micro-jet cooling just after welding is being checked. Weld metal deposit (WMD) was carried out for standard MIG welding and for new welding method with micro-jet cooling. A very high percentage of acicular ferrite (AF) in WMD was gettable (55-75%) for low alloy welding with micro-jet cooling injector. This beneficial structure (very high amount of AF) is unusual to observe in WMD in other welding methods. This method is very promising mainly due to the significant improvement of weld quality and reduces costs. Furthermore impact toughness and strengths of WMD were carried out. The present paper aims at outlining same of the recent innovations in MIG welding which represent steps ahead to achieve the objectives outlined above.

Microstructure and Mechanical Properties of Submerged Arc Welding Filled with Ni Wire Dissimilar Joints Comprising SUS304 and Q245R

Aim at the issue that the bending crack is easy to generate inside dissimilar welded joint of low alloy steel and stainless steel, a new process of submerged arc welding filled with Ni wire is put forward. The influence mechanism of filling quantity of Ni wire to the joint of microstructure evolutions and mechanical properties is mainly explored based on the contrast test of non-filling Ni wire and three different filling speeds of Ni wire. The experiment results indicate that, the filling process of Ni wire supplies austenite forming element, and alters heat allocation process of molten pool. The filling quantity of Ni wire is the quantitative parameter, it is used to control the transition width between low alloy steel and weld metal, also to adjust the peak temperature and holding time at elevated temperature of molten pool. When the value of the filling quantity of Ni wire is determined, the problem of bending crack in the fusion zone of low alloy steel can be solved, and also the overheat damage can be eliminated. At the same time, the grains of weld metal and heat affected zone are refined. And on this basis the plasticity, toughness and microhardness of welded joint can be guaranteed.

Role of pulsed current on metallurgical and mechanical properties of dissimilar metal gas tungsten arc welding of maraging steel to low alloy steel

This research work encompasses the investigations carried out on the mechanical and metallurgical properties of maraging steel and AISI 4340 aeronautical steel weldments. The materials were joined by continuous current gas tungsten arc welding (CCGTA) and pulse current (PCGTA) gas tungsten arc welding processes using ErNiCrMo-3 filler wire. Cross sectional macrostructures confirmed proper deposition of the fillers and lack of discontinuities. Optical microscopy studies revealed that at the maraging steel–weld interface, martensite in distorted and block forms prevailed in CCGTA and PCGTA weldments whereas tempered martensite was predominant at the low alloy–weld interfaces of both the welds. Scanning electron microscopy (SEM) with energy dispersive analysis of X-rays (EDAX) analysis apparently showed less elemental migration in PCGTA weldments as compared to the other. Results of X-ray diffraction analysis recorded possible phase formations in various zones of the weldments. Microhardness profiles in either weld zones followed a constant trend whereas it showed a downtrend in the heat affected zones (HAZ) of the maraging steel and very high hardness profiles were observed in the low alloy steel side. Tensile studies on various factors and impact testing showed that PCGTA weldments outperformed the continuous ones in terms of strength, ductility and toughness. Fractograph analysis depicted the nature of failures of tensile and impact tested specimens. Comparison analyses involving influence and nature of pulsed current welds over continuous ones were done to determine the possibility of implementing these joining processes in aerospace applications. Weldments fabricated using PCGTA technique proved to be superior to the other, resulting in exceptional mechanical properties.

An investigation of effects of welding residual stresses on creep crack growth for a low alloy butt weld

R5 is an assessment procedure for the high temperature response of structures and R5 Volume 7 specifies the creep crack growth assessment for low alloy welds. In power plants, almost all welds operating in creeping regime have been post weld heat treated (PWHT). However, low levels of welding residual stresses may still exist. It is known that the welding residual stresses in weldments can be difficult to estimate, as their magnitude and distribution depend on materials, geometry, weld procedure and heat treatment.For high temperature low alloy steel, CrMoV, butt welds, R5 Volume 7 suggests that a bending welding residual stress of 60 MPa should be assumed for a circumferential defect following a post weld heat treatment. For conservatism, the relaxation of this low level of the welding residual stress in a service component operating at high temperatures for a long period time has been discounted in R5. As the actual welding residual stresses can be difficult to estimate, it is necessary to carry out a detailed investigation for the effects of the welding residual stresses on the creep crack growth and a number of associated crack growth parameters.In this paper, for a typical low alloy steel butt weld in ½Cr½Mo¼V (‘½CMV’) pipework with 2¼Cr1Mo weld metal under a given set of internal pressure, temperature and system moment, a series of creep crack growth analyses have been conducted using the R5 Volume 7 assessment procedure, assuming the welding residual stresses of 10 MPa–100 MPa and three different crack initiation times. A postulated service-initiated internal circumferential defect (semi-elliptical surface-breaking surface defect) of 2.3 mm deep by 15 mm long has been used.The investigation results are presented by a series of graphs which show the effects of the welding residual stresses and different crack initiation times on the creep crack growth for the weld studied. After a detailed discussion, conclusions can be drawn.

Comparison of weld residual stress measurement results in low alloy welds between X-ray diffraction and stress relief methods

In this study, a comparison of weld residual stress measurements between the X-ray diffraction technique and the stress relief technique is performed with the focus on the effect of the thickness on the stress measurement results. As the difference in the thickness becomes smaller, the difference in the stress measurement results also becomes smaller. At the weld center, where the stress gradient is in the thickness direction, the difference in the thickness is not negligible. In other words, it is concluded that the X-ray diffraction method is advantageous because it can evaluate the unique stress generated only in the surface layer.

Effect of long-term stress relief annealing on properties of low-alloy weld deposit of covered electrodes for welding of thick-walled nuclear constructions

Abstract The article discusses the results of tests on the impact of long-term stress relief annealing at 620–650°C and cooling at a rate of 10°C/h on the mechanical properties of low-alloy weld deposits of covered electrodes containing 1.5–3% Ni, 0–0.3% Mo and 0.01–0.03% P. The article also contains information on the most advantageous nickel content for obtaining the highest impact energy (KV) values in subzero temperatures. X-ray structural analysis revealed that such weld deposits contain solely Fe3C carbides, which prevents the impact of molybdenum carbides on the change of KV. The study also confirmed the adverse effect of phosphorus on the weld deposit impact energy after long-lasting heat treatment.

Effect of welding process on the microstructure and properties of dissimilar weld joints between low alloy steel and duplex stainless steel

To obtain high-quality dissimilar weld joints, the processes of metal inert gas (MIG) welding and tungsten inert gas (TIG) welding for duplex stainless steel (DSS) and low alloy steel were compared in this paper. The microstructure and corrosion morphology of dissimilar weld joints were observed by scanning electron microscopy (SEM); the chemical compositions in different zones were detected by energy-dispersive spectroscopy (EDS); the mechanical properties were measured by microhardness test, tensile test, and impact test; the corrosion behavior was evaluated by polarization curves. Obvious concentration gradients of Ni and Cr exist between the fusion boundary and the type II boundary, where the hardness is much higher. The impact toughness of weld metal by MIG welding is higher than that by TIG welding. The corrosion current density of TIG weld metal is higher than that of MIG weld metal in a 3.5wt% NaCl solution. Galvanic corrosion happens between low alloy steel and weld metal, revealing the weakness of low alloy steel in industrial service. The quality of joints produced by MIG welding is better than that by TIG welding in mechanical performance and corrosion resistance. MIG welding with the filler metal ER2009 is the suitable welding process for dissimilar metals jointing between UNS S31803 duplex stainless steel and low alloy steel in practical application.

Microstructure, hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel

The present investigation reports on a study that has been taken up to develop an understanding of the electron beam welding characteristics of similar and dissimilar combination of maraging steel and high strength low alloy steel, which are in the hardened condition, i.e. maraging steel, in a solution that was in treated and aged condition, whereas high strength low alloy steel in a quenched and tempered condition before welding. The joint characterisation studies include microstructural examination, microhardness survey across the weldment and measurement of residual stresses. Maraging steel weld metal is under compressive stress rather than tensile stress as observed in low alloy steel welds because the martensite transformation occurs at a relatively low temperature. It has been observed that, in dissimilar metal welds, tensile stress is observed at the fusion boundary of low alloy steel and weld metal, whereas compressive stress is obtained at the location between weld and maraging steel fusion boundary. Dissimilar weldment contains a soft region beside the interface on maraging steel side because of the diffusion of manganese from low alloy steel towards maraging steel. The observed residual stresses, hardness distribution across the similar and dissimilar metal welds are correlated with the observed microstructures.