Microcracking in multipass weld metal of alloy 690 Part 1 – Microcracking susceptibility in reheated weld metal

Abstract

Microcracking behaviour in the gas tungsten arc multipass weld metal of alloy 690 was investigated. The majority of microcracks occurred within about 300 μm from the fusion line of the subsequent weld bead and propagated along the solidification boundaries in the multipass weld metal. The morphology of the crack surface indicated the characteristic texture of ductility dip cracking. The microcracking susceptibility of the reheated weld metal was evaluated via the spot Varestraint test using three different filler metals having varying contents of impurity elements such as P and S. Microcracking occurring in the spot Varestraint tests consisted predominantly of ductility dip cracking, with a small amount of liquation cracking. The ductility dip cracking temperature range was about 1350–1600 K in the weld metal FF1, and narrowed in the order of weld metals FF1>FF3>FF5. The ductility dip cracking susceptibility was reduced with decreasing contents of impurity elements in the filler metal. It was concluded that the amount of (P + S) in the filler metal should be reduced as much as possible (to about 30 ppm in total) to suppress microcracking in the multipass weldment.     

Investigation of boundaries and structures in dissimilar metal welds

Abstract

Cracking, or disbonding, along the fusion boundary in dissimilar metal welds has been a persistent problem, particularly in applications where austenitic alloys are clad on to structural steels for corrosion protection. Many failures in dissimilar metal welds occur as a result of cracking along a boundary that runs parallel to the fusion boundary in the adjacent weld metal. A preliminary investigation was undertaken to determine the nature and evolution of boundaries and structure in dissimilar metal welds using a simple ternary system composed of a pure iron substrate and a 70Ni–30Cu (Monel) filler metal. Changes in base metal dilution were found to alter the evolution of boundaries and structures near the fusion boundary dramatically. Optical metallography and electron microanalysis reveal that the resulting weld microstructures and boundaries are similar to those observed in engineering materials used for cladding and corrosion resistant overlay. Transmission electron diffraction analysis revealed orientation relationships between adjacent base metal and weld metal grains at the fusion boundary to be different from the cube on cube relationship normally observed in similar metal welds. A model is proposed describing the evolution of the boundary most susceptible to cracking in dissimilar welds.     

Hydrogen embrittlement of high strength steel weld deposits

Abstract

The risk of hydrogen cracking and embrittlement in high strength (690 MN m^–2 yield stress) steels has been briefly reviewed and an assessment has been made about the risk of increased crack sensitivity associated with an increase in filler metal carbon content from 0·07% to 0·1%. The experimental technique involved reviewing procedure qualification test records for any relationship between composition and weld mechanical properties. It was found that a wide variation in weld metal yield stress may occur during fabrication with 690 MN m^–2 electrodes and that this scatter was sufficient to hide any effect of minor compositional variation. Welds which were deposited on an experimental fabrication using 0·1% carbon electrodes and a wide range of welding procedures have been examined exhaustively by ultrasonic testing and negligible evidence of cracking was found.     

Research on Prediction Method of Liquation Cracking Susceptibility to Magnesium Alloy WeldsEI北大核心CSCD

Magnesium alloys has a wide application prospect due to their good properties, such as high specific strength and specific stiffness, but the susceptibility of liquation cracking is also pretty high. The liquation in partially melted zone of AZ-series magnesium alloys were investigated with circular-patch welding test. The AZ91, AZ31 base alloys were welded with AZ61 and AZ92 filler wires by using the cold metal transter metal inert-gas (CMT-MIG) welding. The results show that, the liquation occurred along the weld edge of AZ91 with the eutectic reaction occurring between gamma(Mg17Al12) phase and Mg-rich phase. The liquation susceptibility of AZ31 was pretty low as gamma(Mg17Al12) was not present in base metal of AZ31. Meanwhile, a new method for predicting liquation cracking based on binary phase diagram was proposed. When the initial solidification temperature of weld is higher and the solidification temperature range of weld is shorter than those of base metal, the liquation crack susceptibility of weld is mostly higher. When the initial solidification temperature of weld is close to or below that of base metal, and the solidification temperature range of weld is close to or longer than that of base metal, the liquation cracking susceptibility of weld is lower. This method worked well on predicting the effect of composition of base metal and filler wires on liquation cracking, and the predicting results are consistent with the experimental results. That is, the liquation cracking susceptibility is higher with AZ91 base metal used than that with AZ31 base metal. And, the liquation cracking susceptibility is lower with AZ92 filler wire than that with AZ61 filler wire.     

Physical properties of selected brazing filler metals

Abstract

A suitable selection of the filler metal is vital for producing satisfactory brazed joints. The wettability of brazing alloys with base metals depends on physical properties such as surface tension, density, melting point, and viscosity. Thermal conductivity and electrical resistivity are also important since the filler metal is frequently required to have similar values to those of the base metal. In the present paper, the physical properties of liquid alloys relevant to brazing have been evaluated. Six different filler metal systems were analysed, comprising alloys based on Ag, Al, Au, Cu, Ni, and Ti. Results show that the viscosity values for most binary brazing filler alloys are of the order of 2–8 mPa s, with Cu and Al alloys exhibiting the lowest viscosities. The surface tensions of brazing alloys vary from 800 to 1800 mN m^-1, with the lowest surface tension values corresponding to the Ag and Al alloys. Thermal conductivity and electrical resistivity values fall in the range 10–200 W m^-1 K^-1 and 17–300 μΩ cm, respectively.     

Sulphur-induced ductility-dip cracking in low carbon steel weld metal

Abstract

Two types of hot cracks in low carbon steel weld metal, namely, solidification cracks and ductility-dip cracks have been verified using hot cracking tests and hot ductility tests. The formation temperature ranges of the two types of hot cracks are 1480–1450°C, and 1100–850°C, respectively. The surface morphology of ductility-dip cracks is intergranular fracture with dimple zones and smooth facets containing voids which are associated with (Mn, Fe)S inclusions. This type of cracking can be prevented by increasing the manganese content in the weld metal.     

Ultrasonic hardness measurements

Abstract

When austenitic high-alloy steel weld metal sustains single-phase solidification generally described as A-mode solidification, this is well-known to result in heightened solidification cracking susceptibility.^1–3 To reduce the solidification cracking susceptibility of austenitic stainless steel, it is known to be effective to undertake component modification such as to obtain a solidification mode called the AF mode or FA mode involving the ä phase being crystallized or retained.^{1, 2} To obtain a complete γ solidification mode in the case of high Nibase alloys, such as Fe–36%Ni alloy, however, it is necessary to arrange for high Cr addition in order to achieve component modification facilitating AF or FA mode solidification such as affects austenitic stainless steel. The result of such addition, however, is that impairment of base metal properties also self-evidently cause heavy loss of hot workability in a way that makes this approach difficult to describe as effective.     

Hot-cracking in Aluminium Alloys 7050 and 7010—a Comparative Study

Abstract

Using a recently developed hot cracking test, the cracking susceptibilities of the high-strength aluminium alloys 7010 and 7050 were investigated. The solidification conditions in the test are similar to those encountered in the ingot shell zone during semi-continuous direct chill casting. The influence of grain structure on cracking susceptibility was investigated. Al-Ti and Al-Ti-B grain refining master alloys were added to the melt at various levels in order to obtain grain structures ranging from columnar-dendritic to equiaxed-cellular. Without grain- refiner additions, columnar structures formed which had high cracking susceptibilities; 7010 being marginally less susceptible than 7050. With high grain-refiner additions, equiaxed-cellular grains formed in both alloys, again leading to high cracking susceptibility. Equiaxed-dendritic grains formed at moderate addition levels of grain refiner and were found to be more resistant to hot cracking than either of the other morphologies.     

Electron beam welding characteristics of high strength aluminium alloys for express train applications

Abstract

The basic characteristics of electron beam (EB) welding of high strength aluminium alloys for express train applications were evaluated. The aluminium alloys tested were non-heat treatable A5083–O, heat treatable A6N01–T6, and A7N01–T6. Principal welding process parameters, such as accelerating voltage, beam current, welding speed, and chamber pressure were investigated. The dimensions and microstructures of welds were evaluated using optical light microscopy and SEM (EDAX). In addition, variation in weldability (in terms of formation of cracking and porosity) owing to process parameters was evaluated. Electron beam welds showed discontinuities such as cracks, cold shuts, porosities, and spikes; the tendency to form weld discontinuities was strongly dependent on the EB process parameters and chemistry. Although the three aluminium alloys were welded using the same conditions, alloying elements had an important effect on the dimensions of the weld and thus on weldability. Alloy A6N01 showed a lower depth than the other alloys and variation in the weld depth was found to be sensitive to the vaporisation tendency of the alloying elements. Silicon, which is a major element of A6N01, is more difficult to vaporise than other elements (such as aluminium, magnesium, and zinc). The degree of cracking in the EB fusion zone appeared to be affected mainly by aspect ratio (depth/width), such that as aspect ratio increased the cracking tendency also increased. The alloying element itself may also affect the hot cracking resistance, but its role is considered to be an indirect effect, such that the relatively higher vaporisation pressures for zinc and magnesium give deeper weld penetration and thus result in a greater tendency towards cracking.     

Influence of scandium on weldability of 7010 aluminium alloy

Abstract

The commercial 7000 series aluminium alloys are based on medium strength Al–Zn–Mg and high strength Al–Zn–Mg–Cu systems. The medium strength alloys are weldable, whereas the high strength alloys are non-weldable. This is because the amount of copper present in these alloys gives rise to hot cracking during solidification of welds. As a result, the high strength Al–Zn–Mg– Cu base alloys are not used for applications where joining of components by welding is an essential step. In the present study, using a combination of qualitative Houldcroft test and quantitative Varestraint test, it is shown that a small addition of scandium to the commercial 7010 alloy reduces the hot cracking susceptibility during solidification of welds produced by the gas tungsten arc welding process. The improvement in weldability is found to be the result of the considerable grain refinement in the weld structure following the scandium addition. The results of microhardness and tensile tests are further described within the context of the present work to demonstrate that the 7010+Sc welds also exhibit a combination of improved strength and ductility.     

Search for filler metal for welding of ferrous alloys to titanium

Abstract

The use of a filler metal to facilitate the gas tungsten arc (GTA) welding of ferrous alloys to titanium alloys has been investigated. Semi-empirical rules have been applied to identify alloying elements that would resolve the important problems of brittle intermetallic formation and weld cracking. Vanadium was found appropriate. The GTA welds between a low carbon steel and Ti–15V–3Cr–3Sn–3Al made with a vanadium filler wire resisted cracking better than comparable autogenous welds. However, the presence of a hard, brittle eutectic microconstituent along the ferrous side of the fusion boundary drastically limited the gain in weldability. As anticipated, analysis of GTA welds produced with vanadium filler wire suggested the presence of a ternary (Fe,Ti,V) single phase. Although cracking was reduced with vanadium, the practical benefit of a vanadium filler wire for GTA welding is small because the weld metals remain very hard and brittle.     

Control of weld composition when welding high strength aluminium alloy using the tandem process

Abstract

A new cost effective process for generating different weld element compositions has been examined. Utilising tandem welding technology, different series aluminium filler wires were mixed in a single weld pool with the result that the composition of the principal alloy elements, copper and magnesium were accurately controlled. Thermodynamic modelling was then used to predict an optimum weld bead composition for eliminating solidification cracking when welding Al2024. In order to validate the predicted target composition, the tandem process was used to control the composition of the weld bead. The presented results show that using this system to deposit a controlled ternary composition weld, solidification cracking was eliminated when welding highly constrained test pieces. In contrast, cracking was evident when using commercially available binary filler wires under the same conditions.     

Study and prevention of cracking during weld-repair of heat-resistant cast steels

Heat-resistant cast steels are highly sensitive to cracking as they are weld-repaired because of their very low ductility. To prevent weld-repair cracking of three different heat-resistant cast steels used for the manufacturing of superplastic forming (SPF) dies, the effect of various welding parameters, such as the choice of the filler material, the number of weld passes and the pre-heating temperature has been investigated. The choice of an appropriate filler metal and the pre-heating to 400 °C of the material prior to welding drastically lower the propensity to cracking, but remain unable to eliminate cracks entirely. To further reduce weld-repair cracking and hopefully prevent it completely, a buttering technique has been developed. Buttering of the base metal surface with nickel alloys before weld-repair has been shown to prevent cracking of the base metal, but results in some hot-cracking of the buttering layer itself. On the other hand, buttering with Ni–Fe alloys, less sensitive to hot-cracking, results in crack-free weld-repairs.     

Modelling hot cracking in 6061 aluminium alloy weld metal with microstructure based criterion

Abstract

Hot cracking in welding is a complex phenomenon due to coupling between process, metallurgy and mechanical loading. A methodology based on process simulation, simple microstructural prediction and a pressure model along columnar grains is developed in order to integrate all factors that influence hot cracking. The model is based on some developments of Rappaz, Drezet and Gremaud and takes into account the influences of grain morphology, mechanical and welding thermal loading, on hot cracking. The model based on the microstructure behaviour is able to predict crack onset location in columnar grains on 6061 aluminium alloy.     

Mitigation of hot cracking of alloy 52M overlay on cast stainless steel CF8A

Abstract

Effects of ER308L buffer layer and welding parameter slope down time on the hot cracking susceptibility of alloy 52M overlay on CF8A base metal were studied. The results indicated that Si segregation was a critical factor affecting Alloy 52M hot cracking. Applying ER308L buffer layers between CF8A and Alloy 52M can reduce the dilution of Alloy 52M weld beads and minimise the contribution of Si from CF8A into Alloy 52M, thereby alleviating Si segregation and the hot cracking susceptibility of Alloy 52M. When the Si content in the grain boundary region was lower than 0·81 wt-%, the hot cracking in the weld bead could be mitigated completely. In many cases, crater cracks occurred in the end crater of the weld bead. Increasing the number of ER308L buffer layers and extending the slope down time could reduce crater crack susceptibility. However, the Si content in the grain boundary region should be controlled to be lower than 0·63 wt-% to prevent crater cracking.     

Metallographic and OIM study of weld cracking in GTA weld build-up of polycrystalline,directionally solidified and single crystal Ni based superalloys

Abstract

The repair of gas turbine components is of importance both commercially and scientifically to ensure cost effective repair schemes that will extend the lives of hot end components such as blades and stators. The present communication reports the results of a metallographic and orientation imaging microscopy study of weld cracking observed in the gas tungsten arc repair welds of a polycrystalline (IN738LC), a directionally solidified (Rene 80) and a proprietary single crystal (SX) alloy. The three alloys were welded with low, intermediate and high strength weld fillers, using a weld build-up approach rather than a conventional weld repair of a through thickness crack. This procedure would be applicable for example to worn area on the tips of turbine blades. Inhomogeneous initial microstructures and those from solidification processes led to extensive heat affected zone microfissuring in the IN738LC alloy, associated with MC carbide liquation, liquation of gamma prime (γ′), segregation of boron and strain effects from precipitation of γ′ in both single and double pass welds. As observed previously in a V shaped weld preparation, the extent of microfissuring in alloy IN738LC increased substantially from the use of the low and intermediate strength weld fillers, to extensive heat affected zone microfissuring by using the high strength IN738 filler. In the directionally solidified Rene 80 welds, due to the reduction in grain boundary area per unit volume, only minor heat affected zone cracking was observed, while the SX alloy did not crack at all. The absence of any cracks in the SX alloy welds despite the presence of stray grains in the fusion zone appears to be related to reduced stress levels in the welds due to the choice of welding technique and the welding parameters.     

Dilution and microsegregation in dissimilar metal welds between super austenitic stainless steel and nickel base alloys

Abstract

Super austenitic stainless steels are often welded using high Mo, Ni base filler metals to maintain the corrosion resistance of the weld. An important aspect of this processing is the weld metal dilution level, which will control the composition and resultant corrosion resistance of the weld. In addition, the distribution of alloying elements within the weld will also significantly affect the corrosion resistance. Dissimilar metal welds between a super austenitic stainless steel (AL-6XN) and two Ni base alloys (IN625 and IN622) were characterised with respect to their dilution levels and microsegregation patterns. Single pass welds were produced over the entire dilution range using the gas tungsten arc welding process. Microstructural characterisation of the welds was conducted using light optical microscopy, scanning electron microscopy, and quantitative image analysis. Bulk and local chemical compositions were obtained through electron probe microanalysis. The quantitative chemical information was used to determine the partition coefficients k of the elements in each dissimilar weld. The dilution level was found to decrease as the ratio of volumetric filler metal feedrate to net arc power increased. Reasons for this behaviour are discussed in terms of the distribution of power required to melt the filler metal and base metal. In addition, the segregation potential of Mo and Nb was observed to increase (i.e. their k values decreased) as the Fe content of the weld increased. This effect is attributed to the decreased solubility of Mo and Nb in austenite with increasing Fe additions. Since the Fe content of the weld is controlled by dilution, which in turn is controlled by the welding parameters, the welding parameters have an indirect influence on the segregation potential of Mo and Nb. The results of the present work provide practical insight for corrosion control of welds in super austenitic stainless steels.     

Hot cracking of welds on heat treatable aluminium alloys

Abstract

The Spot Varestraint test was used to evaluate the hot cracking susceptibility of several aluminium alloys namely 6061-T6, 6061-T6 (H), 7075-T6, 7075-T6 (H). The effects of augment strain, the number of thermal cycles and cold working (rolling) on the cracking susceptibility were investigated, and the total crack length was used to evaluate the hot cracking susceptibility. The results indicate that the number of thermal cycles is irrelevant to the hot cracking susceptibility in the weld fusion zone, but does affect this susceptibility in the heat affected zone (HAZ). More thermal cycles correspond to larger hot cracks in the HAZ, especially in the weld metal HAZ. The hot cracking susceptibility of materials increased with augment strain in both the fusion zone and the HAZ. Cold working of the materials can reduce their hot cracking susceptibility. The hot cracking susceptibility of 7075-T6 aluminium alloys is higher than that of 6061-T6. There was significant Cu segregation in the HAZ of 7075-T6 aluminium alloy, resulting in a higher susceptibility to hot cracking in this zone.     

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.     

Liquation cracking in partial penetration aluminium welds: assessing tendencies to liquate,crack and backfill

Abstract

Aluminium alloys are susceptible to liquation cracking in the partially melted zone (PMZ), where grain boundary liquation occurs during welding. Alloys 2024, 6061 and 7075 were gas-metal arc welded with fillers 1100 and 4043, and liquation cracking near the weld root was examined. Curves of temperature (T) versus solid fraction (f_S), based on the Scheil equation for Al-Cu-Mg-Mn-Si-Zn alloys, were calculated for the solidifying PMZ and weld metal at the fusion boundary. Judging from the freezing temperature range and the liquid fraction, these curves suggested that liquation decreases in the order of 7075, 2024 and 6061, consistent with experiments. They also suggested that 1100 increases the weld metal f_S, thus promoting liquation cracking (by strengthening the solidifying and contracting weld metal that pulls the PMZ) and discouraging backfilling (by reducing the interdendritic liquid), while 4043 does the opposite except during PMZ terminal solidification in 7075 and 2024. These are consistent with experiments.