Microcracking in multipass weld metal of alloy 690 Part 2 – Microcracking mechanism in reheated weld metal

Abstract

To elucidate the microcracking (ductility dip cracking) mechanism in the multipass weld metal of alloy 690, the hot ductility of the reheated weld metal was evaluated using three different filler metals with varying contents of impurity elements such as P and S. Hot ductility of the weld metal decreased at temperatures over 1400 K, and the weld metal containing a low quantity of impurity elements showed much higher ductility than that containing a high quantity of impurity elements. Local deformability at high temperature of the alloy 690 reheated weld metal was compared with that of Invar alloy. Grain boundary sliding in alloy 690 occurred not in the intermediate temperature range (800–1000 K), where grain boundary sliding was activated in Invar alloy, but at high temperatures just below the melting temperature of alloy 690. The computer simulation of microsegregation suggested that the deterioration of hot ductility is caused by the grain boundary segregation of impurity elements during the multiple thermal cycling. The ductility dip cracking in the reheated weld metal resulted predominantly from the embrittlement of grain boundaries due to the imbalance between intergranular strength and intragranular strength at high temperature.     

Effect of strong carbide forming elements in hardfacing weld metal

To achieve high carbon hard-facing weld metals with both high hardness and crack resistance, strong carbide forming elements Ti, Nb and V were alloyed into the weld metals, and their effect on the formation of carbides and the matrix microstructure were studied. Electron Probe Microanalysis (EPMA), Energy Dispersive Spectroscopy(EDS) and Transmission Electron Microscopy (TEM) were adopted to investigate the microstructure, then thermodynamics of the formation of carbides was calculated and their effect on the matrix was further discussed. It is revealed that Nb, Ti and V influence strongly the distribution and existing state of carbon, inducing precipitation of carbides accompanying with the depletion of carbon in matrix. But when only V are alloyed as carbide forming element, the carbides are scarce and distributed along grain boundaries, and the hard-facing alloy is too hard, while the using of only Nb or Ti could not reinforce the weld metals effectively. The hard-facing alloy reinforced with Nb, V a     

Microstructural changes in weld metal during isothermal process. Study of reheat cracking in flux‐cored arc welding stainless steel weld metal (5th report)

The soiling of the slag, spatter and the fume, etc., which come into contact with the steel sheet surface with welding, is cleaned making use of steel sphere shot material of large particle diameter, high projection pressure with strong peening processing (below, called strong peening cleaning). In this research, the cleaning state of the soiling with welding and improvement of fatigue strength of the hot galvanized welded joint was inspected, when the surface of a SM490A welded joint was cleaned with strong peening cleaning.

The following experimental results were obtained:

1. The fatigue limit of smooth base metal which received strong peening cleaning at about 320 MPa was remarkably high in comparison with smooth base metal at about 245 MPa.

2. The fatigue limit of a welded joint which received strong peening cleaning at about 300 MPa was remarkably high in comparison with a welded joint at about 170 MPa.

3. The strong peening cleaning was highly efficient and the cleaning state was satisfactory.

4. The cause of the remarkable rise of the fatigue limit (300 MPa) of the welded joint which received strong peening cleaning was because the fatigue limit (about 170 MPa, 57%) of the welded joint was improved (about 130 MPa, 43%) with peening cleaning. It was considered that improvement effects were: a rise (about 68 MPa, 23%) of hardness of the weld toe; relief (about 43 MPa, 14%) of stress concentration; increase (about 136 MPa, 45%) of compressive residual stress; and the decrease (about ? 96 MPa, ? 32%) by increase of surface roughness.

5. The fatigue strength of the hot galvanized welded joint decreased remarkably. This was thought to be due to the decrease (about HV40) of hardness of the surface, the decrease (about 188 MPa) of the compressive residual stress and the influence of many factors which accompanied hot galvanizing.

     

Effect of chemical metallurgical factors on contents of S and P in weld metal during SAW

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Thermal and metal transfer behaviours in pulsed current gas metal arc weld deposition of Al–Mg alloy

Abstract

The control of pulsed current gas metal arc (GMA) welding is highly critical owing to the simultaneous influence of the pulse parameters on thermal and metal transfer behaviours of the process. An analytical model has been developed to provide a theoretical understanding of the influence of pulse parameters on the behaviour of metal transfer and thermal characteristics in pulsed current GMA welding using Al–Mg filler wire. The variations in thermal and metal transfer behaviours with changes in pulse parameters have been satisfactorily analysed considering a summarised influence of pulse parameters defined by a dimensionless factor &phis; = (I _b/I _p)ft _b, proposed previously. A large number of process parameters have been considered, as a result of using four different GMA welding power sources. The hypothesis has been verified using some previously reported experimental results. The theoretical model may be useful in the control of pulse parameters to achieve desired behaviours of thermal and metal transfer under different conditions of weld fabrication, thereby facilitating more universal application of GMA welding.     

Thermodynamic analysis of inclusion in weld

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Effect of weld microstructure on weld properties in A-TIG welding of titanium alloy

1　INTRODUCTIONTheactivatingTIGwelding (A TIG)wasfirstlydevelopedbyPatonWeldingInstitute ,Ukraine ,inthe 196 0s[1,2 ] .Itisnotuntiltheendof 1990sthatA TIGweldinghasbeenwidelyresearchedinEuropeandUS ,forexample ,TWIandEWI[3] .Thefluxes ,whichweredevelopedbyEWI ,havebeenappliedintheUSNavyshipbuildingindustry[4 ] .Theprincipleofthistechniqueisthatathincoatingofactivatingfluxiscoatedonthesurfaceofthebasemetalbeforeweld ing .ComparedwiththeconventionalTIGwelding ,the penetrationca…     

Mechanism of effect of rare earth oxides on high temperature corrosion resistance of weld metal

Through the addition of rare earth oxides into the coming of electrodes, the high temperature oxidation and sulphidation resistance of weld metal were studied. The transfer mechanism of rare earth oxides from coating to the welding pool and the physicochemical reaction during the process were analyzed. With the application of scanning electron microscope and energy spectrometer, the mechanism of weld metal corrosion in air and sulphur environment were studied. The result shows that the addition of rare earth oxides can improve the high temperature resistance of weld metal, especially in sulphidation environment. The electrodes in which La2O3 is added have better performance than the electrodes in which CeO2 is added. The existence form and distribution characteristic of rare earth are defined. It is believed that the marked effect of rare earth oxides in sulphur environment is related to the corrosion mechanism.     

New methods of predicting dissimilar steel weld metal microstructures by Schaeffler Diagram

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Calculation of AF transformation kinetics in HSLA steel weld

It is of great importance for obtaining the perfect welding properties to control the acicular ferrite (AF)transformation behavior reasonably in steel weld. AF continuous transformation kinetics in the HSLA steel weld was calculated and modeled based on the direct growth on the inclusions inert interface. The simulation results are coincident with the experimental value well.     

Prediction and verification of melting front in GTA weld pool of full-penetration

A thermal conduction model is applied to speed up the numerical analysis of the temperature distribution and the weld pool geometry of full penetration in gas tungsten arc （GTA） welding. With considering both top and bottom flee surface deformation of full-penetrated weld pool, three-dimensional weld pool with melting front and solidification front is predicted. Welding experiments are conducted to measure the melting front curves at the top surface and the longitudinal section of the weld. It shows that the predicted and measured results are in good agreement.     

Closed-loop control of weld penetration in keyhole plasma arc welding

To improve the penetrating ability and the welding quality of keyhole plasma arc welding, a novel penetration closed-loop control system was established. In the system, welding current and plasma gas flow rate were selected as adjusting variables. The wavelet method was used to detect penetration status from welding arc voltage in real-time. The control strategy of one-keyhole-per-pulse was adapted to fulfill stable and high quality welding process. Experimental results show that the developed system can apparently increase the penetrating force of plasma arc and keyhole plasma arc welding is realized successfully in stainless steel with 10 mm in thickness. Moreover,the disturbances of gradual change and break change from 3mm to 6mm in thickness are come over due to the good response property of the developed system.     

Vision-based detection of weld pool width in TIG welding of copper-clad aluminum cable

In order to realize automatic control of the width of weld pool, a visual sensor system for the width of weld pool detection is developed. By initiative arc light, the image of copper plate weld pool is taken back of the torch through the process of weakening and filtering arc light. In order to decrease the time of processing video signals, analog circuit is applied in the processing where video signals is magnified, trimmed and processed into binary on the datum of dynamic average value, therefore the waveform of video signals of weld pool is obtained. The method that is used for detecting the width of weld pool is established. Results show that the vision sensing method for real-time detecting weld pool width to copper-clad aluminum wire TIG welding is feasible. The response cycle of this system is no more than 50 ms, and the testing precision is less than 0. 1 mm.     

Visual sensing of weld pool in variable polarity TIG welding of aluminium alloy

1 Introduction Real-time measuring of the size of welding pool is the precondition to weld shape control[1?4]. Among the researched sensing methods, the passive visual image sensing has been proved to be a feasible and promising method due to its abundan…     

Investigation on mechanism of reducing diffusible hydrogen in weld using microelements

0IlltroductionRareearthshavethefunctionofreducinghydrogenduringweldingmetallurgy,andhavenotableeflbctonimprovingthecrackingresistanceinweldedjoint.RareelementTealsocangreatlydecreasetheditTusiblehydrogeninweldbead,andimprovethecoldcrackingresistancepropertyinweldedjoint.Inthisthesis,theinfluenceofmicroelementsyttrium(Y)andtclluriunl(Te)ontheditTusiblehydrogeninweldbeadandonthecoldcracldngsusceptibilityofxveldedjointhasbeeninvestigated,andthemechanismofmicroelementshasbeenanalyzed.lTestmethod…     

New methods of predicting dissimilar steel weld metal microstructures by Schaeffler Diagram(Part 2)

On the base of the methods of predicting weld metal microstructures of pearlitic dissimilar steel welded joints using austenitic type filler materials by Schaeffler Diagram, the other new methods of predicting and expressing weld metal microstructures of two kinds of dissimilar steel welded joints (pearlite/pearlite and austenite/pearlite) using austenitic filler materials by Schaeffler Diagram are suggested. Those new methods resolve some difficult problems which the microstructure kinds in two heterogeneous mixture zones of weld metal neighbouring two kinds of welded base metals are difficult to be accurately ascertained and the fluctuations of weld metal microstructures across fusion line are difficult to be conveniently expressed according to the traditional predicting method. The new predicting methods are more concise and practical.     

Vision-based detection of MAG weld pool

Weld pool contains significant information about the welding process. The weld pool images of MAG welding are detected by LaserStrobe system. An algorithm for extracting weld pool edge is proposed according to the characteristics of MAG weld pool images. The maximum weld pool length and width are calculated. The measurement data can be used to verify the results of welding process simulation and to provide a good foundation for automatic control of MAG welding process.     

Investigation on mechanism of reducing diffusible hydrogen in weld using microelements

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