Effects of welding wire composition and welding process on the weld metal toughness of submerged arc welded pipeline steel

The effects of alloying elements in welding wires and submerged arc welding process on the microstructures and low-temperature impact toughness of weld metals have been investigated. The results indicate that the optimal contents of alloying elements in welding wires can improve the low-temperature impact toughness of weld metals because the proeutectoid ferrite and bainite formations can be suppressed, and the fraction of acicular ferrite increases. However, the contents of alloying elements need to vary along with the welding heat input. With the increase in welding heat input, the contents of alloying elements in welding wires need to be increased accordingly. The microstructures mainly consisting of acicular ferrite can be obtained in weld metals after four-wire submerged arc welding using the wires with a low carbon content and appropriate contents of Mn, Mo, Ti-B, Cu, Ni, and RE, resulting in the high low-temperature impact toughness of weld metals.

MODEL OF LASER-TIG HYBRID WELDING HEAT SOURCE

The welding mechanism of laser-TIG hybrid welding process is analyzed. Withthe variation of arc current, the welding process is divided into two patterns: deep-penetrationwelding and heat conductive welding. The heat flow model of hybrid welding is presented. As todeep-penetration welding, the heat source includes a surface heat flux and a volume heat flux. Theheat source of heat conductive welding is composed of two Gaussian distribute surface heat sources.With this heat source model, a temperature field is calculated. The finite element code MARC isemployed for this purpose. The calculation results show a good agreement with the experimental data.

Prediction of welding residual stress in multipass narrow gap welded pipes with large thickness

The objective of this study is to investigate the influence of post weld heat treatment （PWHT） on the distribution of residual stress in welded pipes with large thickness. The detailed pass-by-pass finite element simulation was developed to study the residual stress in narrow gap multipass welding of pipes with a wall thickness of 150 mm and 89 weld beads. The effect of PWHT on welding residual stress was also investigated by means of numerical analysis. The simulated results show that the hoop stress is tensile stress in the weld region and compressive stress in the parent metal areas adjacent to weld seam. After heat treatment, the residual stresses decrease substantially, and the simulated residual stress is validated by the experimental one. The distribution of the through-wall axial residual stress along the weld center line is a self-equilibrating type.

The mechanical measuring method of welding heat source efficiency

Based on the principle of residual deformation induced by superposition of the welding residual stress and working stress, the welding heat source efficiency has been determined by measuring displacement changes of specimens under loading and unloading in tensile tests, and combining with calculating welding parameters. Meanwhile, the welding heat source eficiencies obtained are compared with those of the measuring-calculating method. The research results show that the welding heat source efficiencies are almost the same as those obtained by the measuring-calculating method. Therefore, the welding heat source efficiency can be determined accurately by this method, and a new determining method of the heat source efficiency for the welding heat process calculating has been provided.

Investigation on automated loading of dynamic 3D heat source model for welding simulation

Since programing complex and dynamic heat source model for welding simulation is a complex job,the parametric methods are studied in this paper.Firstly,an overall flow to achieve automatically modeling welding was introduced.Secondly,an expert module rule for selecting welding heat source model was founded,which is based on simulation knowledge and experiences.Thirdly,a modularity routine method was investigated using writing with C++programing,which automatically creates subroutines of 3D dynamic heat source model for user.To realize the dynamic weld path,the local weld path coordinate system was moved in the global coordinate system and it is used to model the direction of weld gun,welding path and welding pose.The weld path data file was prepared by the automatic tool for the welding heat source subroutines.All above functions were integrated in the user interface and the connection with architecture was introduced.At last,a laser beam welding heat source modeling was automatically modeled and the weld pool geometry was compared with the reported literature.It demonstrated that the automated tool is valid for welding simulation.Since modeling became convenient for welding simulation using the tool proposed,it could be easy and useful for welding engineers to acquire the needed information.

Experimental and Simulation Studies on Cold Welding Sealing Process of Heat Pipes

Sealing quality strongly affects heat pipe performance, but few studies focus on the process of heat pipe sealing. Cold welding sealing technology based on a stamping process is applied for heat pipe sealing. The bonding mechanism of the cold welding sealing process （CWSP） is investigated and compared with the experimental results obtained from the bonding interface analysis. An orthogonal experiment is conducted to observe the effects of various parameters, including the sealing gap, sealing length, sealing diameter, and sealing velocity on bonding strength. A method with the utilization of saturated vapor pressure inside a copper tube is proposed to evaluate bonding strength. A corresponding finite element model is developed to investigate the effects of sealing gap and sealing velocity on plastic deformation during the cold welding process. Effects of various parameters on the bonding strength are determined and it is found that the sealing gap is the most critical factor and that the sealing velocity contributes the least effect. The best parameter combination （AIB3CID3, with a 0.5 mm sealing gap, 6 mm sealing length, 3.8 mm sealing diameter, and 50 mm/s sealing velocity） is derived within the experimental parameters. Plastic deformation results derived from the finite element model are consistent with those from the experiment. The instruction for the CWSP of heat pipes and the design of sealing dies of heat pipes are provided.

Effect of Zr-Ti combined deoxidation on impact toughness of coarse-grained heat-affected zone with high heat input welding

In this study, the effects of Zr-Ti combined deoxidation and AI deoxidation on the impact toughness of coarse- grained heat-affected zone in high-strength low-alloy steels were investigated. More fine oxides were formed in the Zr-Ti-killed steel than in Al-killed steel. It was also found that more acicular ferrite grains were formed in the coarse-grained heat-affected zone in the Zr-Ti-killed steel than in Al-killed steel. The impact toughness of coarse-grained heat-affected zone of Zr-Ti-kiUed steel was higher than that of Al-killed steel. The good impact toughness was attributable to the pinning effect of fine oxides and the formation of acicular ferrite grains on fine oxides.

Welding mode transition and process stability in high power laser welding

For high-power CO2 laser welding, besides two well known stable welding processes, i.e. stable deep penetration welding (DPW) and stable heat conduction welding (HCW), the authors have found the third welding process, i.e. unstable-mode welding (UMW) under the certain condition. UMW has its basic feature that the two welding modes (DPW and HCW) appear intermittently, with jumping of penetration depth and weld width between large and small levels. In this paper, effects of welding parameters (focal position, laser power and traveling speed) on laser welding mode and weld appearance have been comprehensively studied. Double-U curves of laser welding mode transition have been obtained, which indicate the ranges of the three mentioned welding processes. This work affords science foundation for selecting the welding process parameters correctly and obtaining stable laser welding.

Fast Welding of Chromium Carbide and Nickel Alloy by Spark Plasma Sintering

Spark plasma sintering（SPS） was used to weld the ceramics,eg,Cr3C2 and metal,Ni in this paper.It is found that the SPS can weld the Cr3C2 and Ni plates at lower temperatures and shorter holding time comparing with that of hot-pressing（HP）.The binding strength was 113 MPa when the temperature was 1000 ℃ by SPS,compared with 10 MPa by HP at the same temperature.SPS remarkably enhances the atom diffusion in welding.Thermodynamics analysis at different welding temperatures and holding times of SPS or HP shows that the local temperature gradient,different from the past effects of by-passing current,is the dominative mechanism of the SPS welding.

Temperature field analysis of main steam pipe under local post weld heat treatment

In local post weld heat treatment, the temperature difference is the criterion of the process. The temperature field in the main stream pipe under local post weld heat treatment is simulated by finite element method. A close-loop control program is designed to simulate the temperature field of two different pipes. Both the skin effect of induction heating and electro-thermal coupled effect are considered in the heating model. The local heat treatment temperature difference at the inner and outer side of the pipe is analyzed and the different convection conditions are also considered. The simulation results show that in appropriate induction heating process, the temperature difference in the pipe can be controlled within 30 ℃.

Microstructure and fracture toughness of electron beam welded joints of 30CrMnSiNi2A steel

Two post weld heat treatments (PWHT), 900 ℃ oil quenched and low temperature tempered (PWHTA) and high temperature tempered and then 900 ℃ oil quenched and low temperature tempered (PWHTB), are employed to treat the weldment. Then the effect of two post weld heat treatment processes on the microstructure,mechanical properties and fracture toughness of electron beam welded joints of 30CrMnSiNi2A steel have been discussed. The results show that, after two kinds of PWHT the microstructure and hardness at every zones of EBW joints are nearly same. Although the welds have good mechanical properties, fracture toughness of both weld and heat affected zone (HAZ) is low, the CTOD values of welds are comparatively higher than that of HAZ. Microstructure and fracture toughness of two EBW joints have no evident differences.

Evolution behavior of phase and performance in Ni-based coating layer based on high temperature thermal field

Effect of post-welding heat treatment on the microstructure and properties of BNi-2 brazing coating was analyzed. The phase transformation in the heat treatment process was analyzed by high temperature metallographic microscope, scanning electron microscope and X-ray diffraction, and the microhardness after the post heat treatment was tested by microhardness tester. The results show that the microstructure of brazing BNi-2 brazing coating is mainly CrB phase, Ni solid solution phase and eutectic phase of Ni_(3)B and Ni_(3)Si. After brazing,the overall hardness value is higher, generally higher than 510 HV, and the microhardness in some areas is higher than 700 HV. After quenching, the solid solution phase of Ni increased, and the eutectic phase of Ni and Ni_(3)Si decreased, leading to the softening of the brazing coating, the area whose microhardness in the range of 342 HV–454 HV increased. After tempering the brazed coating samples, the eutectic phases of Ni and Ni_(3)Si grew and the hardness recovered, but the overall hardness distribution was still lower than the brazing one. The results of the brazing post-heat treatment test show that the post-welding heat treatment will bring adverse effects on the hardness distribution of the coating, which may affect the wear resistance of the coating.

Development of oxide metallurgy technology at Baosteel

Oxide metallurgy technology has been developed at Baosteel for improving the heat affected zone toughness of steel plates for high heat input welding. After deoxidation with strong deoxidizers of Mg alloy, the complex inclusions containing parts of compounds of MgO,Ti2 O3,MnO, Al2 O3 ,MnS, CaS and TiN are formed. These kinds of inclusions are beneficial for promoting the formation of intragranular ferrite. After two-electrode vibratory electrogas arc welding with the V-type groove and a high heat input of 395 kJ/cm in a single pass,the former austenite grain is very fine in size,with an average grain size of only 85 μm. Excellent heat affected zone toughness is obtained for the developed steel plates with a thickness up to 68 mm.

Laboratory development of high performance steel plates with excellent HAZ toughness at large heat inputs

Presented in this study is the result of steel plates developed at laboratory by using the technique of chemistry design based on microstructure evolution.It has been shown that the produced 50mm thickness steel plates with yield and tensile strength being 420 MPa and 530 MPa respectively exhibit excellent large heat input weldability:the Charpy impact tests in the whole range of heat affected zone(HAZ) including the fusion line at the welded joint with large heat input of 100 -300 kJ/cm showed uniform impact toughness of above 140 J at -40℃.Welding simulations were also performed for heat inputs of 200-600 kJ/cm,which showed far better toughness at -20℃.Analysis on the results of the simulations and the practical welding tests were done and the microstructure evolution mechanisms were proposed.Finally suggestions were given to improve the simulation processes as well as chemistry modification.

Effect of Post Weld Heat Treatment on Microstructure and Fracture Toughness of Friction Welded Joint

The effect of post weld heat treatment on the microstructure and fracture toughness of friction welded joints of Ti-6.5Al-1Mo-1V-2Zr alloy was studied. The experimental results show that equiaxial grains were formed at the center of the weld metal while highly deformed grains were observed in the thermomechanically affected zone. The fracture toughness of the weld metal was lower than that of the thermomechanically affected zone under as-weld and post weld heat treatment conditions. With increasing temperature of post weld heat treatment, the fracture toughness of weld center and thermomechanically affected zone increased. The fractographic observation revealed that the friction welded joints fractured in a ductile mode.

Effect of Post-weld Heat Treatment on Mechanical Characteristics of AZ31 Magnesium Alloy Welded Joints

Tungsten inert gas（TIG） welding was performed on 2.7 mm thick commercial extruded AZ31 B magnesium alloy plates. We investigated the effect of post-weld heat treatment（PWHT） on the microstructure, mechanical properties and precipitated phase of the weld joints. The results showed that during the annealing treatment（200 ℃-1 h, 250 ℃-1 h, 300 ℃-1 h, 350 ℃-1 h, 400 ℃-1 h, and 450 ℃-1 h）, the average grain size in the weld seam was the minimum after annealing at 400 ℃ for 1 hour, and then abnormally grew up after annealing at 450 ℃ for 1 hour. The mechanical properties enhanced when the joints were processed from 200 ℃-1 h to 400 ℃-1 h but sharply decreased with increasing annealing temperature. In contrast to the annealing treatment, solution treatment（250 ℃-10 h, 300 ℃-10 h, 350 ℃-10 h, 400 ℃-10 h, and 450 ℃-10 h） exhibited a better ductility but a slight deterioration in tensile strength. Especially speaking, no eutectic compounds（such as Mg17 Al12） were observed in the weld seam. The supersaturated Al atoms were precipitated in a coarse spherical shape dispersed in the weld seam. The precipitated Al atoms dissolved in the matrix substances at the condition（400 ℃-1 h） or（250 ℃-10 h）. The solution treatment caused grain coarsening and precipitated Al atoms dissolved in the weld seam substantially, which resulted in a drop in micro-hardness at the weld seam compared to the area of the annealed joints.

Modeling transient fluid flow and heat transfer phenothena in stationary pulsed current TIG weld pool

A mathematical model is presented to describe transient behavior of heat transfer and fluid flow in stationary pulsed current tungsten inert gas (PC-TIG) weld pool, which considers three kinds of driving, forces for weld pool convection, i,e. buoyancyforce, electromagnetic force and surface tension force. furthermore. the effect of vaporization heat flux at the free surface of weld pool and the temperature coefficient of surface tenston which is a function of temperatuer and composition are considered in the model In order to accelerate the convergence of iteration the AST(additive source term)method which concerns with the thermal energv boundary conditions is extended successfully to deal with the momentum boundary conditions by which the transient momentum equation and energy equation are mutually coupled. At the same time. ADI (Alternating direction implicit) method and DBC (double blocks correction) technque are employed to solve the finite difference equations. The results of numerical simulation demonstrate the transient behavior of PC-TIG weld pool, as well as the periodic variation of fluid flow and heat transfer with the periodic variation of welding current in stationary PC-TIG weld pool. The theoretical predictions based on this model are, shown to be in good accordance with the experimental measurements.

A COMPARISON OF PUBLISHED HAZ THERMAL SIMULATION METHODS USED TO DERIVEE WELD HAZ THERMAL CYCLES

Accurate thermal simulation is the key in the simulation approach to defining the unique microstructure and properties of the HAZ regions. Simulation enables an expansion of the volume of material char- acteristic of each H4Z region to a sufficient size for property determination.The combined influence of heat input,preheat temperature, plate thickness, and widing process on HAZ microstructure and properties relics on the accurate simulation of thermal cycles corresponding to different peak temperatures using an HAZ simulator like the Gleeble.Several computer programs have been developed to predict thermal excursions in various HAZ regions during welding. Some were developed based simply on the thermal propertics of a material;whereas, others were generated on the basis of actual experimental data.A suitable HAZ thermal simulation program must be ons that can authntically duplicate an ac- tual thermal cycle experienced during welding within reasonable limies. Therefore, the similarities and differences among the HAZ thermal cycles predicted by various methodologies should be fully under- stood. A total of sts thermal cycle prediction methodologies were compared in this evaluation. It was de- termined that some HAZ simulation programs have sever limitations due to the idealized assumptions considered in their development.According to the experieare at The University of Tennessee and the comparison results in this study, the HAZ thermal simulation programs; ' HAZ Calculator' and F(s, d); are recommended for accurate HAZ thermal cycle duplication.

Influence of Ti on Weld Microstructure and Mechanical Properties in Large Heat Input Welding of High Strength Low Alloy Steels

The influence of Ti on weld microstructure and mechanical properties in large heat input welding of high strength low alloy steels is investigated. The results indicate that a moderate amount of Ti is still effective for grain refinement even under larger heat input and a large amount of acicular ferrite （AF） is formed in the weld metal when Ti content is within 0. 028%--0. 038%. With increasing Ti content, proeutectoid ferrite in the weld metal decreases, whereas bainite and M-A constituent increase. The type of inclusion in the welds varies from Mn-Si-AI-O to Ti-Mn- A1-O and finally to Ti-A1-O as Ti content increases from 0 up to 0. 064%. As for adding 0. 028%--0. 038% Ti, high weld toughness could be attained since most inclusions less than 2 tim which contain Ti20s provide the effective nu- clei for aeicular ferrite formation. However, the toughness of the weld metals severely reduces when Ti content is over the optimum ranRe of 0. 028%--0. 038%.

Computational fluid dynamics simulation of friction stir welding:A comparative study on different frictional boundary conditions

Numerical simulation based on computational fluid dynamics （CFD） is a useful approach for quantitatively investigating the underlying thermal-mechanical conditions during FSW, such as temperature field and material deformation field. One of the critical issues in CFD simulation of FSW is the use of the frictional boundary condition, which represents the friction between the welding tool and the workpiece in the numerical models. In this study, three-dimensional numerical simulation is conducted to analyze the heat transfer and plastic deformation behaviors during the FSW of AA2024. For comparison purposes, both the boundary velocity （BV） models and the boundary shear stress （BSS） models are employed in order to assess their performances in predicting the temperature and material deformation in FSW. It is interesting to note that different boundary conditions yield similar predictions on temperature, but quite different predictions on material deformation. The numerical predictions are compared with the experimental results. The predicted deformation zone geometry by the BSS model is consistent with the experimental results while there is large difference between the predictions by the BV models and the experimental measurements. The fact that the BSS model yields more reasonable predictions on the deformation zone geometry is attributed to its capacity to automatically adjust the contact state at the tool/workpiece interface. Based on the favorable predictions on both the temperature field and the material deformation field, the BSS model is suggested to have a better performance in numerical simulation of FSW than the BV model.