Ductility-dip Cracking Susceptibility Research Articles

Correlation of imposed mechanical energy with ductility-dip cracking in a highly restrained weld of Alloy 52

Ductility-dip cracking (DDC) is a complex weldability issue affecting industries using high-chromium nickel-based filler metals required for superior resistance to stress corrosion cracking, most notably in the nuclear power generation industry. DDC is a solid-state cracking mechanism which occurs in the temperature range of 0.5–0.8TM and is primarily attributed to grain boundary sliding. Above 0.8TM, recrystallization inhibits initiation of DDC and has been observed to arrest propagation. A computational model of thermo-mechanical behavior in a narrow groove multipass weld of DDC-susceptible filler metal was developed in past research and revisited here to calculate and analyze the mechanical energy imposed during the welding process. It was found that areas in the weld with high imposed mechanical energy (IME) in the recrystallization temperature range coincided with regions devoid of cracking but rich in evidence of recrystallization. Weld areas with low IME in the recrystallization temperature range but with higher IME in lower temperature ranges coincided with regions where DDC is observed. This agrees with past research suggesting recrystallization acts to prevent cracking. This paper presents an integrated computational materials engineering (ICME) approach that includes the IME to better predict and quantify DDC susceptibility in multipass welds.

Ductility-dip cracking susceptibility in dissimilar weld metals of alloy 690 filler metal and low alloy steel

The effect of dilution ratio and chemical composition on the ductility-dip cracking (DDC) susceptibility of dissimilar overlay welds of Alloy 690 filler wire and low alloy steel (LAS) was investigated. We measured the hot ductility curve using a hot tensile test coupled with an in-situ measurement system. The minimum critical strain of Alloy 690 was 4.0% at 943 °C. The minimum strain and the temperature decreased with increasing LAS content from 3.2% (901 °C) to 2.4% (842 °C) and 2.2% (802 °C) at LAS contents of 25%, 50%, and 75%, respectively. The formation of tortuous grains resulting from microconstituents containing niobium improved the DDC susceptibility. With increasing dilution of LAS, the niobium content decreased, resulting in a reduction in the tortuosity ratio at the cracked grain boundaries. In addition, the grain boundaries embrittled with increased dilution ratio owing to an increased phosphorous and sulfur content. Therefore, at high LAS content, the combination of low tortuosity ratio of the grain boundaries and high phosphorous and sulfur content increases DDC susceptibility.

Reduction of ductility-dip cracking susceptibility by ultrasonic-assisted GTAW

The ability to reduce ductility-dip cracking (DDC) susceptibility in nickel filler metals is one of the keys to their reliable applications in nuclear industry. Here, it is demonstrated that improvement in DDC susceptibility can be attained for nickel filler metal 52M (FM–52M) through the superimposition of ultrasonic field during gas tungsten arc welding (GTAW). A concept of Detrimental Grain Boundary Length (DGBL) has been proposed to elucidate DDC features in two kinds of samples: without ultrasound and with 20kHz ultrasonic-assisted GTAW. Electron backscattered diffraction, thermo-mechanical simulation tests, scanning electron microscopy and optical microscopy were utilised to quantitatively analyse the microstructure and the cracking features in FM–52M. The results reveal that the external ultrasonic field can refine the grain of FM–52M welds, and can further decrease the DGBL from 2890μm to 1700μm. This decrement of ∼41.2% in DGBL arises from the violent stirring induced by ultrasonic cavitation. The suppressed DGBL, under ultrasonic circumstance, shortens the potential path for crack propagation, and improves the DDC resistance in FM–52M.

Effect of local texture and precipitation on the ductility dip cracking of ERNiCrFe-7A Ni-based overlay

The multi-pass ERNiCrFe-7A overlay depositions, produced by cold wire feed (CWF) or hot wire feed (HWF) gas tungsten arc welding (GTAW) process with typical parameters, were employed to investigate the effects of different micro-characteristics on ductility-dip cracking (DDC) susceptibility. The results show that, the HWF-produced deposit exhibited the texture with larger quantity of 〈001〉 oriented grains in the pass region (away from the fusion line), while the CWF-produced deposit was weakly textured with increased crystallographic misorientation for the higher heat input per unit volume. Crystal misorientation also increased in the interpass region (near the fusion line) of both the overlays. Results of strain-to-fracture (STF) tests indicated that the overall DDC resistance in the deposit CWF was lower than that in the deposit HWF. And the interpass region exhibited higher DDC susceptibility than the pass region. The intergranular cracking is found to be triggered by the strain accumulation between differently oriented grains. Meanwhile, both the over 30° misorientation angle and melting of Cr-rich M23C6 carbides reduced the grain boundary (GB) strength and promoted the cracking. Therefore, the DDC resistance would be increased by decreasing the GBs those have misorientation angles over 30°, as well as the carbides distributed on the GBs.

An investigation on ductility-dip cracking in the base metal heat-affected zone of wrought nickel base alloys—part I: metallurgical effects and cracking mechanism

Ductility-dip cracking (DDC) in the heat-affected zone (HAZ) of NiCr15Fe-type alloys was studied using the programmable-deformation-crack (PVR) and the strain-to-fracture (STF) test. This paper concentrates on the cracking morphology and the effect of metallurgical factors for DDC susceptibility. The obtained results are discussed in the context of the underlying cracking mechanism. Evidence was found that DDC in the heat-affected base metal occurs due to a relative grain boundary movement, which induces high strain buildup at grain boundary triple points, subsequent cavity formation, and intergranular crack propagation. Base metal grain size and intergranular carbide precipitation are shown to have a significant effect on DDC susceptibility and are discussed in the context of the observed grain boundary sliding mechanism.

New measurement technique of ductility curve for ductility-dip cracking susceptibility in Alloy 690 welds

The coupling of a hot tensile test with a novel in situ observation technique using a high-speed camera was investigated as a high-accuracy quantitative evaluation method for ductility-dip cracking (DDC) susceptibility. Several types of Alloy 690 filler wire were tested in this study owing to its susceptibility to DDC. The developed test method was used to directly measure the critical strain for DDC and high temperature ductility curves with a gauge length of 0.5mm. Minimum critical strains of 1.3%, 4.0%, and 3.9% were obtained for ERNiCrFe-7, ERNiCrFe-13, and ERNiCrFe-15, respectively. The DDC susceptibilities of ERNiCrFe-13 and ERNiCrFe-15 were nearly the same and quite low compared with that of ERNiCrFe-7. This was likely caused by the tortuosity of the grain boundaries arising from the niobium content of around 2.5% in the former samples. Besides, ERNiCrFe-13 and ERNiCrFe-15 indicated higher minimum critical strains even though these specimens include higher content of sulfur and phosphorus than ERNiCrFe-7. Thus, containing niobium must be more effective to improve the susceptibility compared to sulfur and phosphorous in the alloy system.

An investigation of ductility-dip cracking in the base metal heat-affected zone of wrought nickel base alloys—part II: correlation of PVR and STF results

Ductility-dip cracking (DDC) in the base metal heat-affected zone (HAZ) of NiCr15Fe-type alloys was studied using the programmable-deformation-crack (PVR) and the strain-to-fracture (STF) test. This paper concentrates on the correlation of the different cracking criteria used in both tests for DDC susceptibility assessment. Full temperature-strain curves were developed for three base metal NiCr15Fe-alloy variants using the STF test. The results were compared to a previous material ranking obtained by PVR testing. To determine a local cracking criterion for base metal DDC in the PVR test, a finite element analysis (FEM) was conducted on the thermomechanical aspects of crack initiation during PVR testing. The local temperature and strain conditions associated with DDC in the PVR base metal HAZ were observed and discussed with regard to the comparability and transferability of the cracking criteria used in the PVR and STF test.

バレストレイン試験による690合金レーザ多層盛溶接金属の延性低下割れ感受性評価

Ductility-dip cracking susceptibility in the laser multipass weld metal of alloy 690 was quantitatively evaluated using different filler metals varying the contents of P and S. In order to synthesize a ductility-dip crack in the laser multipass weld metal, the cross-bead longitudinal-Varestraint test with laser welding was applied under the various welding speed. The ductility-dip temperature range (DTR) was increased with an increase in P and S contents in the weld metal, and S was more effective to enhancing the DTR compared with P. Furthermore, the DTR was decreased with an increase in the welding speed. A numerical simulation of grain boundary segregation of P and S revealed that the segregated concentrations of P and S at grain boundaries were increased with an increase in the P and S contents in the weld metal, and that those were slightly decreased with an increase in the welding speed due to the rapid heating and cooling rates. It follows that the changes in ductility-dip cracking susceptibility with the filler metal composition and welding speed were consistent with the grain boundary segregation behaviours of P and S during laser multipass welding process. According to the regression analysis of the DTR, ductility-dip cracking in the laser multipass weld metal would be inhibited by reducing the segregated concentration at grain boundary (P+2.42S) to less than approx. 8at%.

Microcracking susceptibility in dissimilar multipass welds of Ni-base alloy 690 and low-alloy steel

Microcracking susceptibility in the dissimilar multipass weld metal of alloy 690 and low-alloy steel A533B was evaluated, and the effect of dilution on hot cracking (ductility-dip and liquation cracking) behaviour was investigated. In order to simulate the dissimilar multipass weld metal of alloy 690 to A533B steel, the A533B plate was welded under various dilution ratios using alloy 690 filler metal with different contents of P and S. Several weld metals, which had different alloy compositions at the fixed (P+S) content, were manufactured, and then ductility-dip and liquation cracking susceptibilities of the reheated weld metals were evaluated by the spot-Varestraint test. Ductility-dip cracking susceptibility heightened as the dilution ratio was increased even when the amounts of P and S were fixed. The increased dilution ratio (contamination of Fe into the weld metal) should reduce the tortuous character of the grain boundary (GB) due to inhibiting the constitutional supercooling (the instability of the solidification boundary), as well as enhance the GB embrittlement due to promoting the GB segregation of P and S. Furthermore, the liquation cracking susceptibility slightly heightened with an increase in the dilution ratio at the fixed (P+S) content. The increased liquation cracking susceptibility would be attributed to the enhancement of solidification segregation of P and S with increasing the dilution ratio.

Investigation on the Microstructure and Ductility-Dip Cracking Susceptibility of the Butt Weld Welded with ENiCrFe-7 Nickel-Base Alloy-Covered Electrodes

The weld metal of the ENiCrFe-7 nickel-based alloy-covered electrodes was investigated in terms of the microstructure, the grain boundary precipitation, and the ductility-dip cracking (DDC) susceptibility. Besides the dendritic gamma-Ni(Cr,Fe) phase, several types of precipitates dispersed on the austenitic matrix were observed, which were determined to be the Nb-rich MC-type carbides with “Chinese script” morphology and size of approximately 3 to 10 µm, the Mn-rich MO-type oxides with size of approximately 1 to 2 µm, and the spherical Al/Ti-rich oxides with size of less than 1 µm. The discontinuous Cr-rich M23C6-type carbides predominantly precipitate on the grain boundaries, which tend to coarsen during reheating but begin to dissolve above approximately 1273 K (1000 °C). The threshold strain for DDC at each temperature tested shows a certain degree of correlation with the grain boundary carbides. The DDC susceptibility increases sharply as the carbides coarsen in the temperature range of 973 K to 1223 K (700 °C to 950 °C). The growth and dissolution of the carbides during the welding heat cycles deteriorate the grain boundaries and increase the DDC susceptibility. The weld metal exhibits the minimum threshold strain of approximately 2.0 pct at 1323 K (1050 °C) and the DTR less than 873 K (600 °C), suggesting that the ENiCrFe-7—covered electrode has less DDC susceptibility than the ERNiCrFe-7 bare electrode but is comparable with the ERNiCrFe-7A.

Ductility dip cracking mechanism of Ni–Cr–Fe alloy based on grain boundary energy

Inconel 690 and the companion filler metal 52M (FM-52M) have susceptibility of ductility dip cracking (DDC) in welding procedure. According to test results, DDC was not only preferential to propagate on grain boundaries vertical to loading direction but also related to grain boundary structure. To reveal the relation of grain boundary angle and cracking, a three-dimensional polycrystalline model was constructed to simulate the formation of DDC. In this work, the initiation and propagation of DDC were studied based on extended Read–Shockley formula. The results indicate that grain boundaries with approximate 45° disorientation are most prone to cracking, and the simulated DDC morphology shows agreement with experimental results. The study provides an extra method to predict DDC propagation and helps to evaluate DDC susceptibility in grain level.

Influences of phosphorus and sulphur on ductility dip crackingsusceptibility in multipass weld metal of alloy 690

The influence of P and S on ductility dip cracking susceptibility in thereheated weld metal of alloy 690 was evaluated by the spot Varestraint testusing different alloy 690 filler metals, while varying the contents of P andS. The ductility dip cracking susceptibility was reduced with a decrease inthe content of P and S in the filler metal; the amount of (P+1·2S)in the weld metal should be limited to 30 ppm in order to prevent microcrackingin the multipass weld metal. A numerical simulation of cosegregation behaviourof P and S revealed that both elements were segregated at the grain boundaryin the ductility dip temperature range during multipass welding. A molecularorbital analysis has suggested that ductility dip cracking can be attributedto grain boundary embrittlement due to grain boundary segregation of P andS.

Study on ductility dip cracking susceptibility in Filler Metal 82 during welding

In this paper, Ductility Dip Cracking (DDC) susceptibility in Inconel600 companion Filler Metal 82 (FM82) under different stress states is investigated. Inconel600 is a Ni-Cr-Fe alloy with excellent resistance to general corrosion, localized corrosion, and stress corrosion, which has been widely used in nuclear power plants. However, the companion FM82 has been shown to be susceptible to DDC in welding process. To resolve the problem, this work is mainly focused on evaluating DDC susceptibility in FM82 in welding process. First of all, Strain to Fracture (STF) test is used to achieve the DDC criterion under simple stress state, and the formation mechanism of DDC was explained. Real welding is a process with complex stress state. Later, to get the DDC susceptibility under complex stress state, models about multi-pass welding were built up by means of finite element method. According to numerical simulation results, relationship of deformation and temperature history is achieved. Moreover, susceptible locations and moments could be determined associated with STF results. The simulation results fairly agree with welding experiment from another research.

無粒界腐食型25Cr-35Ni系超高純度ステンレス鋼の高温割れ感受性

The hot cracking susceptibility of type 25Cr-35Ni extra high-purity stainless steels was evaluated by the transverse-Varestraint test with varying the contents of Cr as well as P and S. Two types of hot cracks occurred in these steels by Varestraint test; solidification and ductility-dip cracks. The solidification cracking susceptibility was reduced as the amount of Cr in steels increased, and 25Cr-35Ni stainless steel was negligibly susceptible to solidification cracking. On the other hand, the ductility-dip cracking susceptibility adversely increased with an increase in Cr content as well as P and S contents in steels. There was a good linear relationship between the compositional parameter of (P+1.22S) and the DTR, as well as between (P+1.19S) and the BTR in 25Cr-35Ni stainless steel welds. Accordingly, the quantitative influence of S to an increase in the ductility-dip as well as solidification cracking susceptibility was approx. 1.2 times as large as that of P. The amount of P+1.22S in steels should be limited to approx. 90ppm in order to obtain the sufficiently low ductility-dip cracking susceptibility in 25Cr-35Ni stainless steel welds. Numerical simulation of segregation behaviours of P and S revealed that they were segregated at the grain boundary in the ductility-dip temperature range during welding. Molecular orbital analysis suggested that ductility-dip cracking was attributed to the grain boundary embrittlement due to the grain boundary segregation of P and S. In order to further inhibit the ductility-dip cracking in 25Cr-35Ni extra high-purity stainless steel welds, the effect of La addition on hot cracking susceptibility was investigated. The solidification and ductility-dip cracking susceptibilities could be improved by adding 20-70ppmLa to the extra high-purity steel containing P and S of 6-8ppm. The ductility-dip cracking susceptibility was decreased as a result of the desegregation of P and S to grain boundaries due to the scavenging effect of La.

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.

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.

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.

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.

改良型ＨＰ系耐熱鋳鋼の経年使用材の補修溶接割れ ３ 補修溶接部における高温割れ発生機構

Mechanism of hot cracking, especially ductility-dip cracking in HAZ during repair welding was investigated using modified HP-type heat-resistant cast alloys. The change in hot ductility was evaluated with the reduction of area at 773 K by the Gleeble test. The hot ductility of each alloy was almost at the level of 20% in as-cast situation, while decreased with aging and fell down below about 8% after service exposure or long term aging at elevated temperature. Microscopic observation revealed that the brittle fracture occurred in service exposed alloys, and that cracks initiated and propagated preferentially through microconstituents. Hot ductility was decreased remarkably with increasing the amount of microconstituents on dendritic boundary, and with proceeding the phase transformation of microconstituents as NbC → η phase or M23C6→G phase. It was deduced that the hot ductility would be deteriorated by the stress concentration in microconstituents and/or the plastic constraint on dendritic grain due to the formation of continuous network of microconstituents surrounding the dendritic boundary. It follows that ductility-dip cracking susceptibility during repair welding was enhanced by the reduction of hot ductility.

改良型ＨＰ系耐熱鋳鋼の経年使用材の補修溶接割れ １ 小型スポットバレストレイン試験による高温割れ感受性評価

The hot cracking susceptibilities of HP-modified heat-resisting cast alloys exposed at the elevated temperature for the long term were evaluated by the miniature spot-Varestraint testing. The as-cast, aging and/or used (service exposed) specimens of seven types of the HP-modified alloys were employed for the spot-Varestraint testing. Ductility-dip cracking as well as liquation cracking was occurred in HAZ of each specimen after the spot-Varestraint testing. The ductility-dip cracking susceptibilities of alloys except for HP43AZ much increased in aged and used conditions compared with as-cast condition, while the liquation cracking susceptibilities did not change so much during long term aging. The ductility-dip cracking susceptibility of HP43AZ containing zirconium was remarkably restrained even in used condition. It was deduced that the improvement in the ductility-dip cracking susceptibility of HP43AZ was attributed to reducing the occupied ratio of microconstituents on cellular dendrite boundary by the zirconium addition which possessed refinement effect of dendritic cell.