Ductility-dip Cracking Research Articles

690合金と316Lステンレス鋼異材多層盛溶接部のミクロ割れ発生抑制に対するREM添加効果

The microcracking susceptibility in dissimilar multipass welds of alloy 690 to type 316L stainless steel was investigated by the Varestraint test and the multipass welding test using the four different stainless steels varying the amounts of impurity elements such as P and S, and using the nine different filler metals of alloy 690 varying the REM(La) content. In order to simulate the dissimilar weld metals, stainless steels were gas tungsten arc (GTA) welded using alloy 690 filler metals with varying the dilution ratio. Microcracks occurring in the reheated weld metals were classified into ductility dip, liquation and solidification cracks. The ductility dip cracking susceptibility decreased with increasing the La content in the weld metal, while the liquation and solidification cracking susceptibilities increased contrarily when the La content in the weld metal exceeded ∼0.02 mass%. The relation among the microcracking susceptibility, the (P+S) and La contents in every weld pass of the multipass welding was investigated. Ductility dip cracks occurred in the compositional range (atomic ratio) of La/(P+S)<0.13 and solidification/liquation cracks occurred in that of La/(P+S)>0.55, while any cracks did not occur at La/(P+S) being between 0.13-0.55. The ductility dip cracking susceptibility could be improved by adding La due to the scavenging of the impurity elements. The excessive La addition to the weld metal resulted in the solidification/liquation cracking alternatively attributed to the formation of Ni-La intermetallic compounds with low eutectic point. The optimal filler metal for the dissimilar multipass welding of alloy 690 to stainless steel was selected as La/(P+S) would be 0.13-0.55 for every weld pass.

REM添加690合金によるレーザクラッド溶接部のミクロ割れ抑制効果の検証

Effect of REM addition to alloy 690 filler metal on microcracking prevention was verified in laser clad welding. Laser clad welding on alloy 132 weld metal or type 316L stainless steel was conducted using the five different filler metals of alloy 690 varying the La content. Ductility-dip crack occurred in laser clad welding when La-free alloy 690 filler metal was applied. Solidification and liquation cracks occurred contrarily in the laser cladding weld metal when the 0.07mass%La containing filler metal was applied. In case of laser clad welding on alloy 132 weld metal and type 316L stainless steel, the ductility-dip cracking susceptibility decreased, and solidification/liquation cracking susceptibilities increased with increasing the La content in the weld metal. The relation among the microcracking susceptibility, the (P+S) and La contents in every weld pass of the laser clad welding was investigated. Ductility-dip cracks occurred in the compositional range (atomic ratio) of La/(P+S) 0.99(on alloy 132 weld metal), >0.90 (on type 316L stainless steel), while any cracks did not occur at La/(P+S) being between 0.21-0.99 (on alloy 132 weld metal) 0.10-0.90 (on type 316L stainless steel). Laser clad welding test on type 316L stainless steel using alloy 690 filler metal containing the optimum La content verified that any microcracks did not occurred in the laser clad welding metal.

Contribution of precipitate on migrated grain boundaries to ductility-dip cracking in Alloy 625 weld joints

We investigated the crack properties in Alloy 625 weld metals and their characteristics using experimentally designed filler wires fabricated by varying the niobium and manganese contents in the flux with the shield metal arc welding (SMAW) process. The fast diffusivity of niobium on the migrated grain boundary (MGB) under strong restraint tensile stress, which was induced by the hardened matrix in weld metal containing high niobium and manganese, accelerated the growth of niobium carbide (NbC) in multipass deposits. Coalescence of microvoids along with incoherent NbC and further propagation induced ductility-dip cracking (DDC) on MGB.

The effect of sigma phases formation depending on Cr/Ni equivalent ratio in AISI 316L weldments

The crack properties and toughness characteristics in AISI 316L weld metals were investigated in different chemical composition ranges of Cr and Ni contents. Three new filler wires were designed for the flux-cored arc welding (FCAW) process, and the Cr/Ni equivalent ratio and solidification mode of each weld metal was controlled. The microstructure revealed ductility-dip cracking (DDC) generated in the specimens containing <3 wt% of delta-ferrite ( δ-ferrite). Sigma ( σ) phase precipitation occurred in the weld metals heat-treated at temperatures ranging from 650 °C to 950 °C. The amount of σ in the δ-ferrite region increased significantly with increasing heat treatment time and temperature. At the highest Cr/Ni equivalent ratio, more of the σ phase of the weld metal transformed faster after heat treatment, which deteriorated the toughness. Meanwhile, less σ phase precipitation and DDC occurrence in the specimen containing <3 wt% of δ-ferrite did not affect the toughness reduction.

Effect of Cr/Ni equivalent ratio on ductility-dip cracking in AISI 316L weld metals

The ductility-dip cracking (DDC) susceptibility of AISI 316L stainless steel weld metals was examined. Modified flux cored arc welding (FCAW) filler wires were fabricated with various chromium and nickel contents. The solidification mode and delta-ferrite content were determined from the chromium and nickel equivalent ratios (Cr eq/Ni eq). Ductility-dip cracking occurred through a grain boundary sliding mechanism in the reheated region of the weld metal in the ferrite at cell and dendrite boundaries (AF mode), and the primary course of DDC formation was associated with the straight migrated grain boundary (MGB) morphology. No DDC was observed in the tortuous MGB due to the pinning effect of delta-ferrite in the continuous network of vermicular type of ferrite (FA mode)/acicular ferrite and continuous austenite network (F mode) weld metals. The DDC at the triple point or the intersection of the MGB showed a creep-like morphology. Severe localized and thermal plastic deformation was observed through the formation of micro-voids when grain boundary sliding was generated in the ductility-dip temperature range under strong restraint conditions.

Grain Boundary Sliding Phenomenon and Its Effect on High Temperature Ductility of Ni-Base Alloys

The structural materials phase transformations and failure mechanisms have been under scrutiny for many years. However, the advent of new and more powerful techniques is always making possible to address unsolved problems. Nowadays, the implementation of sophisticated in-situ electron microscopy tests is providing new insights in several fields of chemistry, physics, and materials science by allowing direct observation of a wide variety of phenomena at submicron and even atomic scale. These experiments may involve controlled temperature and atmospheres, mechanical loading, magnetic and/or electric field among other conditions that are imposed to the sample while its response or evolution is studied. An in-situ high temperature deformation experiment was developed and adapted within the vacuum chamber of a scanning electron microscope (SEM). This setup was used to study the grain boundary sliding (GBS) mechanism and its effect on the high temperature cracking phenomenon known as ductility-dip cracking (DDC). The Ni-base filler metals AWS A5.14, ERNiCrFe-7 and ERNiCr-3, which correspond to alloys 690 and 600, respectively, were studied within the temperature range between 700 and 1000 °C. Analysis of the recorded digital videos that registered the high temperature deformation made possible differentiating and quantifying, with submicron resolution, two different components of GBS. The designated pure-GBS and deformation-GBS components were described and quantified. In addition, the GBS relationship with the material high temperature ductility and the DDC failure mechanism was established.

690合金多層盛溶接金属のミクロ割れ発生挙動および機構

Microcracking behaviours in the multipass welds of alloy 690 were investigated. The effect of impurity elements such as P and S on ductility-dip cracking susceptibility in the reheated weld metal was evaluated by the spot-Varestraint test using different filler metals varying the contents of P and S. The ductility-dip cracking susceptibility was reduced with a decrease in the content of impurity elements in the filler metal, and the amount of (P+1.2S) in the weld metal should be limited to 30ppm in order to prevent the microcracking in the multipass weld metal. Numerical calculation of microsegregation and molecular orbital analysis have suggested that ductility-dip cracking was attributed to the grain boundary embrittlement due to the grain boundary segregation of P and S. FEM analysis of thermal strain predicted that ductility-dip cracks would occur during multipass welding when the plastic strain-temperature curve intersected the ductility-dip temperature range (DTR). The effect of the addition of rare earth metals (REM) to the weld metal on the microcracking susceptibility was examined by using the La or Ce containing filler metals. The ductility-dip cracking susceptibility could be significantly improved by adding 0.01-0.025mass%REM to the weld metal. Microstructural analyses revealed that the ductility-dip cracking susceptibility decreased as a result of lowering the grain boundary segregation P and S due to the scavenging effect of REM. The multipass welding test confirmed that microcracks in the multipass welds of alloy 690 were completely prevented by using the filler metals containing approx. 0.03mass%REM.

Hot cracking behaviour and susceptibility of extra high purity type 310 stainless steels

Recent progress in the refining technology has enabled the production of highly pure commercial stainless steels. The hot cracking behaviour of these stainless steels was investigated with respect to type 310 stainless steel. For comparison, four types of 310 stainless steels with various amounts of minor and impurity elements such as C, P and S were used. The purity of type 310 stainless steels used was enhanced in the order of type 310<type 310S<type 310ULC<type 310EHP steels. The hot cracking susceptibility was evaluated by the transverse Varestraint test. Two types of hot cracks occurred in these steels by Varestraint test; solidification and ductility-dip cracks. The solidification cracking susceptibility was significantly reduced as the amount of C, P and S decreased, and that in type 310EHP steel reached a level so low that solidification cracking did not occur in practical welding. On the other hand, the ductility-dip cracking susceptibility adversely increased as the purity of the steels was enhanced. However, the ductility-dip cracking susceptibility of type 310EHP steel was sufficiently low as not to yield ductility-dip cracking in practical multipass welding. The experimental and numerical analyses on the solidification brittle temperature range have revealed that the reduced solidification cracking susceptibility upon decreasing the amount of C, P and S in stainless steel can be attributed to the reduced BTR due to the suppression of minor and impurity elements, such as C, P and S, in the finally solidified liquid film between the dendrite.

Hot cracking in laser braze joint of high strength stainless steel using gold brazing filler metal – prevention of hot cracking by controlling thermal cycle

The hot cracking behaviour in the diode laser braze joint of 13Cr–4Ni stainless steel using Au–18Ni, Ag–10Pd and Ag–21Cu–25Pd filler metals has been investigated. The types of joint investigated were the T fillet joint and the L fillet joint which simulated the second braze bead in the T fillet joint. Tandem beam brazing was also carried out in order to prevent the hot cracking by post-heating treatment with a trailing beam. A centreline crack, characterised as a ductility dip crack, occurred in the second braze bead of the T fillet joint using Au–Ni filler metal. On the other hand, no cracks occurred in either the first or the second braze bead in T fillet braze joints using Ag–Pd and Ag–Cu–Pd filler metals. The hot cracking susceptibility of the Au–Ni braze metal was evaluated by the spot Varestraint test. Most of the cracks observed in the spot Varestraint test specimen were also characterised as ductility dip cracks, and the susceptibility to such cracking increased with increasing the augmented strain. The ductility dip temperature range (DTR) was estimated from the crack position and length to be 1000–1250 K at strains over 0·4%. Numerical analyses of the thermal stress and strain revealed that the plastic strain–temperature curve intersected the DTR in the single beam brazing, but did not intersect the DTR with post-heating during the laser brazing. The effect of post-heating on the crack prevention was verified by tandem beam brazing of L fillet joint with a trailing beam. No cracks occurred in the braze bead made with a trailing beam at laser powers of 200–300 W. The authors concluded that hot cracking in the Au–Ni braze metal could be successfully prevented by controlling the thermal cycle during the laser brazing process.

Pitting corrosion behavior on crack property in AISI 304L weld metals with varying Cr/Ni equivalent ratio

The pitting corrosion behavior on welded joints of AISI 304L austenitic stainless steel was investigated with the flux-cored arc welding (FCAW) process. The dependence of pitting corrosion susceptibility on the microstructure and constituents was compared in weld metals by using three newly designed filler wires. Mechanical examination demonstrated that the tensile and yield strengths were increased with increasing equivalent weight ratio of chromium/nickel (Cr eq/Ni eq). Ductility-dip cracking (DDC) was observed in the fully austenitic structure in the deposited metal, and it detrimentally affected the pitting resistance. Potentiodynamic anodic polarization results revealed that the passive film breakdown was initiated in the crack opening, and that the concentration of chloride ions and corrosion products induced severe damage near to the DDC.

The dependence of crack properties on the Cr/Ni equivalent ratio in AISI 304L austenitic stainless steel weld metals

The crack properties and mechanical characteristics in AISI 304L stainless steel weld metals were evaluated by differentiating solidification mode using three newly designed filler wires with flux-cored arc welding (FCAW) process. Microstructure examination showed that the delta ferrite content in the deposited metals increased with an increasing equivalent ratio of chromium/nickel. Solidification cracking was observed in the first layer of deposited metal in the fully-austenite structure and its fracture surface was coated by a film of low-melting eutectics. Ductility-dip cracking (DDC) was propagated along the migrated grain boundary (MGB). The fracture surface of the DDC was shaped along the sharp ridgeline, and the preferential angle for the formation of DDC was 45° to the applied load.

Mechanical Approach for Prediction of Microcracking in Multipass Weld Metal of Ni-Base Alloy 690

The occurrence of microcracks, especially ductility-dip crack in multipass weld metal during GTAW and laser overlay welding processes of Ni-base alloy 690 was predicted by the mechanical approach. The stress/strain analysis in multipass welds was conducted using the thermo elasto-plastic finite element method. The brittle temperature range for ductility-dip cracking (DTR) of the reheated weld metal was determined by the Varestraint test. Plastic strain in the weld metal accumulated with applying the weld thermal cycle in multipass welding. The plastic strain-temperature curve in the La free weld metal did not cross the DTR in the cooling stage of GTAW process, however, it crossed the DTR in the cooling stage of reheating process by subsequent welding. On the other hand, the plastic strain-temperature curves of any weld passes in the La added weld metal did not cross the DTR. Ductility-dip cracks occurred in the La free weld metal except for the final layer, however, any ductility-dip cracks did not occur in the La added weld metal during multipass welding. It could be understood that ductility-dip crack would occur during not only single-pass welding but also multipass welding when plastic strain intersected the DTR.

Investigation of Structure and Crack Formation in Welded Joints of Single Crystal Ni-Base Alloys

Conditions of initiation of hot cracks in welding of single crystal heat-resistant alloys with a 56% content of the γ’-phase were studied. Conditions for providing a polycrystalline structure in HAZ and part of the weld metal were artificially created in one region of a welded joint with the single crystal structure. Welding of the single crystal alloy was performed by the electron beam method in vacuum. Cracks in a low-temperature ductility dip range (ductility dip cracking) were found to form in HAZ and weld of a welded joint with the polycrystalline structure. The nature of cracking of the single crystal weld is related to formation of low-melting point eutectoid interlayers, as well as to exhausting of the safety factor for ductility. Cracks in the single crystal weld along the strain grain boundaries are formed by a similar mechanism.

Microcracking susceptibility in dissimilar multipass welds of alloy 690 to type 316L stainless steel using La added filler metals

The microcracking susceptibility in dissimilar multipass welds of alloy 690 to type 316L stainless steel was investigated by the Varestraint test and the multipass welding test using the four different stainless steels varying the amounts of impurity elements such as P and S, and using the nine different filler metals of alloy 690 varying the La content. In order to simulate the dissimilar weld metals, stainless steels were gas tungsten arc (GTA) welded using alloy 690 filler metals with varying the dilution ratio. Microcracks occurring in the reheated weld metals were classified into ductility dip, liquation and solidification cracks. The ductility dip cracking susceptibility decreased with increasing the La content in the weld metal, while the liquation and solidification cracking susceptibilities increased contrarily when the La content in the weld metal exceeded ∼0·03 mass-%. The relation among the microcracking susceptibility, the (P + S) and La contents in every weld pass of the multipass welding was investigated. Ductility dip cracks occurred in the compositional range (atomic ratio) of La/(P + S)<0·17 and solidification/liquation cracks occurred in that of La/(P + S)>0·76, while any cracks did not occur at La/(P + S) being between 0·17–0·76. The ductility dip cracking susceptibility could be improved by adding La due to the scavenging of the impurity elements. The excessive La addition to the weld metal resulted in the solidification/liquation cracking alternatively attributed to the formation of Ni–La intermetallic compounds with low eutectic point. The optimal filler metal for the dissimilar multipass welding of alloy 690 to stainless steel was selected as La/(P + S) would be 0·17–0·76 for every weld pass. Microcracking could be completely prevented in the dissimilar multipass weld metal of alloy 690 to type 316L stainless steel by using two kinds of filler metals containing 0·03 mass-%La for the weld passes adjacent to the stainless steel base metal and containing 0·01 mass-%La for other weld passes.

Further Investigations of Ductility-Dip Cracking in High Chromium, Ni-Base Filler Metals

The ductility-dip cracking (DDC) susceptibility of high-chromium, nickel-base filler metals was evaluated using the strain-to-fracture (STF) test technique. These filler metals were of the Ni-30Cr type and included INCONEL® Filler Metals 52 and 52M supplied by Special Metals Welding Products Company, and Sanicro 68HP® and Sanicro 69HP® supplied by Sandvik AB. In addition, two experimental Ni-30Cr filler metals were evaluated which contained variations in other alloy additions including Nb above 1 wt % and Mo up to 4 wt %. A wide range in DDC susceptibility was observed with these filler metals, including large heat-to-heat variations in filler metals with similar compositions. The experimental filler metals were found to be remarkably resistant to DDC. Cracking susceptibility is primarily associated with the type and form of precipitate that forms along the weld metal migrated grain boundaries. The formation of Nb-rich, M(C, N) at the end of solidification has the most profound effect on DDC, since these precipitates are most effective in pinning the boundaries. The formation of M23C6 carbides during weld cooling or subsequent reheating can also affect DDC susceptibility in these filler metals.

金ろうを用いた高強度ステンレス鋼レーザろう付部の高温割れ

Hot cracking behavior in laser braze joint of 13Cr-4Ni stainless steel has been investigated using BAu-4 (Au-18%Ni) filler metal. The diode laser brazing was carried out for the T-fillet joint as well as the L-fillet joint which simulates the second bead in the T-fillet joint. The tandem beam brazing with a post-heating beam was also carried out in order to prevent the hot cracking. The laser powers of main and trailing beams were varied. The center line crack occurred in the second braze bead of the T-fillet joint, and was characterized as ductility-dip crack. Hot cracking susceptibility in Au-Ni braze metal was evaluated by spot-Varestraint test. The ductility-dip cracking temperature range (DTR) was about 1000-1250K. The FEM analysis of thermal stress/strain revealed that the rise curve of plastic strain intersected the DTR in the single beam brazing, however, it would not intersect the DTR by post-heating during laser brazing. The tandem beam brazing confirmed that no cracks occurred in the L-fillet braze bead made with trailing beam at the laser power of 200-300W.

Microcracking in multipass weld metal of alloy 690 Part 3 – Prevention of microcracking in reheated weld metal by addition of La to filler metal

The effect of addition of La to a filler metal on microcracking (ductility dip cracking) in the multipass weld metal of alloy 690 was investigated with the aim of improving its microcracking susceptibility. The susceptibility to ductility dip cracking in the reheated weld metal could be greatly improved by adding 0·01–0·02 wt-%La to the weld metal. Conversely, excessive La addition to the weld metal led to liquation and solidification cracking in the weld metal. Hot ductility of the weld metal at the cracking temperature was greatly improved by adding 0·01–0·02 wt-%La to the weld metal, implying that the ductility dip cracking susceptibility was decreased as a result of the desegregation of impurity elements of P and S to grain boundaries due to the scavenging effect of La. The liquation and solidification cracking resulting from excessive addition of La to the weld metal is attributed to the formation of liquefiable Ni–La intermetallic compound. A multipass welding test confirmed that microcracks in the multipass weldment were completely prevented by using a filler metal containing an addition of 0·01 wt-%La.

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

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.

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

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.

Improving the ductility-dip cracking resistance of Ni-base alloys

Many austenitic alloys undergo a severe ductility drop at intermediate temperatures, which may result on the solid state phenomenon known as ductility-dip cracking (DDC). This intermediate temperature intergranular cracking has been recognized as a grain boundary sliding creep-like phenomenon. Several factors have been identified to control the DDC susceptibility of these austenitic alloys, including grain size, grain boundary orientation to the applied force, impurity segregation to the grain boundaries, grain boundary tortuosity, and intergranular precipitation. This work studies the effect of Nb and Ti additions to Ni-base filler metals 52(ERNiCr-3) and 82(ERNiCrFe-7) on the DDC susceptibility. Strain-to-fracture (STF) test were performed on Ni-base filler metals 52 (0.25%Mn–8.9%Fe–29.1%Cr–0.5%Ti–0.7%Al–Ni) and 82 (2.75%Mn–0.7%Fe–20.1%Cr–2.6%Nb–0.5%Ti–Ni) with different amounts of Nb and Ti additions. Optical and electron microscopy were used to characterize the microstructures and fracture surfaces. Segregation of the added elements to the solidification grain and subgrain boundaries was verified. The Nb addition to FM-52 caused the precipitation of eutectic NbC carbides. The additions of Ti caused the formation of large Ti(C,N) and the additions of Ti and Nb caused the formation of (TiNb)(CN) eutectic-like precipitates in both weld metals. When large amounts of Nb and Ti were added to FM-52, colonies of acicular and blocky precipitates were observed. Solidification cracks formed when large amounts of Nb and/or Ti were added. On the other hand, small Nb and Ti additions caused an important reduction of DDC susceptibility. The Nb- and Ti-rich precipitates pinned the migrated GBs causing an increase in the GB tortuosity. For the temperatures and strains tested, Nb and Ti additions resulted in a general increase in the threshold strain required to initiate solid state cracking during the strain-to-fracture test.