Ni-based Weld Metal Research Articles

Preliminary investigation on underwater wet welding of Inconel 625 alloy: microstructure, mechanical properties and corrosion resistance

This paper aimed to investigate the feasibility of underwater wet welding for Inconel 625 alloy using Ni-based filler. The microstructure, elemental distribution, and properties of underwater wet welded Inconel 625 alloy joints were analyzed. The grain boundary type and grain growth orientation across the fusion boundary were particularly investigated. A continuous and smooth weld with no visible defects was obtained. The nickel based welds was fully austenite structure with extensive grain boundary migration. The heat-affected zone experienced obvious grain growth. A compositional transition zone with a width of 50 μm was observed across the fusion boundary. Apparent epitaxial growth across the fusion boundaries was found and the grain boundaries roughly perpendicular to the fusion boundaries are was mainly high angle grain boundaries. The density of low Σ CSL grain boundary in Inconel 625 base metal was higher than that in Ni-based weld metal. The ultimate tensile strength of underwater wet welded joints reached up to 83% that of Inconel 625 alloy. The impact toughness of HAZ was deteriorated because of the segregation of carbide and Laves phase in grain boundary. The corrosion resistance of Ni-based welds maintained at a high level based on the corrosion current density, although it was inferior to that of Inconel 625 alloy. The expected research results could enrich the underwater welding/repair technology of Ni-based alloy structures.

Low cycle fatigue properties of a 9–12% Cr martensitic steel welded joint with Ni-based weld metal based on a local strain approach

The effect of mechanical heterogeneity on the low cycle fatigue (LCF) properties of a 9–12% Cr martensitic steel welded joint with Ni-based weld metal are studied using smooth and hourglass specimens based on a local strain approach. The results indicate that the smooth specimens of the welded joint fracture at the softened weld metal (WM) due to the strain concentration. Moreover, the strain–life curves of the smooth specimens of the welded joint are close to the strain–life curves of the WM smooth specimens after the local strain approach considering the mechanical heterogeneity of welded joints is adopted. The LCF properties of the inter-critically heat-affected zone (ICHAZ) are obtained through hourglass specimens based on the local strain approach. The LCF properties of the base metal (BM), WM, and ICHAZ in the welded joint are compared to investigate the relationships of the LCF properties of the welded joint and different microzones.

Fatigue crack growth mechanism of Ni-based weld metal in a 9% Ni steel joint

This research focused on the impact of temperature and driving force ΔK on the fatigue crack growth (FCG) mechanism of C-276 Ni-based weld metal in a 9% Ni steel joint. On one hand, the FCG resistance increased due to higher yield strength at 77 K than at room temperature (RT), and the decrease of FCG rate can be well explained by Weertman's model. On the other hand, critical ΔK values (37.5 MPa m1/2 at 77 K and 30.2 MPa m1/2 at RT) existed in the process of FCG, accompanied by the transition of FCG mechanism at both 77 K and RT. In detail, the fatigue crack mainly propagated in a quasi-cleavage mode at a lower ΔK level at 77 K, in a single-slip mode at a higher ΔK level at 77 K or a lower ΔK level at RT, and in a multi-slip mode at a higher ΔK level at RT. Furthermore, the dislocation-precipitate interaction was enhanced and even provided favourable locations for FCG at a higher ΔK level at RT, leading crack to propagate via linking after the formation of micro-cracks and micro-voids near precipitates.

Stress corrosion crack initiation testing with tapered specimens in high-temperature water – results of a collaborative research project

ABSTRACT The applicability of an accelerated test technique using tapered tensile specimens for investigating the stress corrosion crack (SCC) initiation behaviour of structural materials in high-temperature water was assessed in the framework of a European collaborative research project (MICRIN – MItigation of CRack INitiation). The main advantage of using a tapered geometry is, that in a single test a stress gradient is obtained through the gauge length, and therefore a stress threshold for SCC initiation can be determined in a reasonable timeframe. This method was used to investigate two different materials that were known to be susceptible to SCC in light water reactor environment: a high-Si stainless steel and a Ni-base weld metal (Alloy 182). The results of the international test programme confirmed that the tapered specimen test methodology could be used to identify a SCC initiation stress threshold, albeit that significant scatter was present in the data.

Study on fracture toughness of 617 Ni-based alloy welded joint under different elevated temperatures

Abstract

The Effects of Electrochemical Hydrogen Charging on Room-Temperature Tensile Properties of T92/TP316H Dissimilar Weldments in Quenched-and-Tempered and Thermally-Aged Conditions

The influence of isothermal aging at 620 °C in combination with subsequent electrochemical hydrogen charging at room-temperature was studied on quenched-and-tempered T92/TP316H martensitic/austenitic weldments in terms of their room-temperature tensile properties and fracture behavior. Hydrogen charging of the weldments did not significantly affect their strength properties; however, it resulted in considerable deterioration of their plastic properties along with significant impact on their fracture characteristics and failure localization. The hydrogen embrittlement plays a dominant role in degradation of the plastic properties of the weldments already in their initial material state, i.e., before thermal aging. After thermal aging and subsequent hydrogen charging, mutual superposition of thermal and hydrogen embrittlement phenomena had led to clearly observable effects on the welds deformation and fracture processes. The measure of hydrogen embrittlement was clearly lowered for thermally aged material state, since the contribution of thermal embrittlement to overall degradation of the weldments has dominated. The majority of failures of the weldments after hydrogen charging occurred in the vicinity of T92 BM/Ni weld metal (WM) fusion zone; mostly along the Type-II boundary in Ni-based weld metal. Thus, regardless of aging exposure, the most critical failure regions of the investigated weldments after hydrogen charging and tensile straining at room temperature are the T92 BM/Ni WM fusion boundary and Type-II boundary acting like preferential microstructural sites for hydrogen embrittling effects accumulation.

Microstructure and mechanical properties of dissimilar welds between 16Mn and 304L in underwater wet welding

ABSTRACTDissimilar joint between 304L austenitic stainless steel and low-alloy steel 16Mn was underwater wet welded using self-shielded nickel-based tubular wire. Microstructure, mechanical properties and corrosion behaviour of dissimilar welded joints were discussed. Ni-based weld metal was fully austenitic with well-developed columnar sub-grains. Type II boundary existed between Ni-based weld metal and ferritic base metal in underwater welds similar to that in air welds. Major alloying elements distributed non-uniformly across the austenitic weld metal/16Mn interface. Maximum hardness values in wet welding appeared in a coarse-grained heat-affected zone at the 16Mn side, which possessed very low impact toughness. Underwater Ni-based welded joints fractured at Ni-based weld metal under tensile test. Ni-based weld metal had favourable corrosion resistance similar to 304L base metal.

Ageing Effects on Room-Temperature Tensile Properties and Fracture Behavior of Quenched and Tempered T92/TP316H Dissimilar Welded Joints with Ni-Based Weld Metal

The present work is focused on the investigation of isothermal ageing effects on room-temperature tensile properties and the failure of quenched and tempered martensitic/austenitic weldments between T92 and TP316H heat-resistant steels. The dissimilar weldments were produced by gas tungsten arc welding technique using a Ni-based Thermanit Nicro 82 filler metal. The welded joints were subjected to unconventional post-welding heat treatment consisting of the welds solutionizing (1060 °C/30 min), followed by their water quenching and final stabilization tempering (760 °C/60 min). The treatment was completed by spontaneous air cooling within a tempering furnace. The welds in their initial quenched and tempered condition were subsequently aged at 620 °C for up to 2500 h. Apart from room-temperature tensile tests performed for all the welds material states, additional cross-weld hardness measurements were carried out on longitudinal sections of broken tensile specimens. The applied thermal exposure resulted in recognizable deterioration of plastic properties, whereas their effects on strength properties were rather small. The welds tensile straining and fracture evolution exhibited competitive behavior between the austenitic TP316H region and Ni-based weld metal. The observed failure locations showed significant hardness peaks due to intensive, necking-related strain hardening effects occurred during the tensile tests.

Fatigue Crack Growth Behavior of a New Type of 10% Cr Martensitic Steel Welded Joints with Ni-Based Weld Metal

In the present work, the fatigue crack growth (FCG) behavior of a new type of 10% Cr martensitic steel welded joints with Ni-based weld metal was comparatively studied for different regions including base metal (BM), heat-affected zone (HAZ) and weld metal (WM). FCG results indicated that the tempered lath martensite BM has a higher fatigue crack growth resistance than the tempered granular martensite HAZ that without a typical lath structure. In comparison, the austenitic WM has the highest fatigue crack growth threshold. Meanwhile, due to the microstructural and chemical compositional differences between BM and WM, a clear interface existed in the welded joints. At the region of the interface, the microstructures were physically connected and an element transition layer was formed. Although the starter notch was positioned at the region of interface, the fatigue crack gradually deviated from the interface and ultimately propagated along the inter-critically heat-affected zone. The difference in microstructure is considered as the primary factor that resulted in the different fatigue crack growth behaviors of the welded joints. In addition, the continuous microstructure connection and composition transition at the interface contributed to the good fatigue resistance at this region.

高温クリープ環境下におけるNi基溶接金属の延性低下

高温クリープ環境下におけるNi基溶接金属の延性低下

Low-cycle fatigue behaviors of a new type of 10% Cr martensitic steel and welded joint with Ni-based weld metal

In the present work, Ni-based filler metal was used to weld a new type of 10% Cr martensitic steel. Due to the microstructure and chemical composition difference between martensitic steel and the Ni-based weld metal, a clear interface existed in the dissimilar welded joint. At the region of the interface, the microstructure was physically connected and an element transition layer was formed. Low-cycle fatigue (LCF) results showed that the welded joint was not fractured at the interface. Meanwhile, the martensitic steel and Ni-based weld metal exhibited cyclic softening and cyclic hardening behaviors, respectively. For martensitic steel, the width of the laths increased and the dislocation density decreased after the fatigue test, whereas in the fatigue-tested Ni-based weld metal, the dislocation density increased. The continuous connection and composition transition at the region of interface, combined with the high ductility and cyclic hardening behavior of Ni-based weld metal, is beneficial for the LCF properties of the welded joint.

Precipitation of Niobium Boride Phases at the Base Metal/Weld Metal Interface in Dissimilar Weld Joints

In this work, the analysis of failure mechanism in the heat affected zone is described in dissimilar weld joints between advanced martensitic steel T92 and Ni-base weld metal. The joints were treated with two different post-weld heat treatments and tested. For the creep, tensile, and Charpy impact tests, the samples with interfacially located notch were used. Moreover long term aging at 625 °C was applied before the tensile and notch toughness tests. Decohesion fractures ran along carbides at the T92 BM/WM interfaces in case of the modified PWHT, whereas type IV cracking was the prevailing failure mechanism after the classical PWHT in the creep test. In the notch tensile and Charpy impact tests, with the notch at T92 base metal/weld metal interface, fractures ran along the interface with a hard phase on the fracture surface along with the ductile dimple and brittle quasi-cleavage fracture. The phase identified as niobium boride (either NbB and/or Nb3B2) was produced during welding at the end of the solidification process. It was found in the welds regardless of the post-weld heat treatment and long-term aging.

Effects of aging temperature on microstructural evolution at dissimilar metal weld interfaces

From the earlier study which characterized the region of a fusion boundary between a low-alloy steel (LAS) and a Ni-based weld metal of as-welded and aged samples at 450°C for a 30-y-equivalent time, it was observed in the microstructure that the aging treatment induced the formation and growth of Cr precipitates in the fusion boundary region because of the thermodynamic driving force. Now, this research extends the text matrix and continues the previous study by compiling all the test data, with an additional aging heat treatment conducted at 400°C for 15- and 30-y-equivalent times (6450 and 12,911h, respectively). The results for the extended test matrix primarily represent the common features of and disparities in the effects of thermal aging on the aged samples at two different heat-treatment temperatures (400 and 450°C). Although no difference was expected between the samples, because the heat treatment conditions simulate thermal aging effects during the same service time of 30y, the sample aged at 450°C exhibited slightly more severe effects of thermal aging than the sample aged at 400°C. Nevertheless, the trends for these effects are similar and the simulation of thermal aging effects for a light-water reactor appears to be reliable. However, according to a simulation of the same degree of thermal aging effects, it appears that the activation energy for Cr diffusion should be larger than the numerical value used in this study.

High-temperature tensile and creep deformation of cross-weld specimens of weld joint between T92 martensitic and Super304H austenitic steels

The high-temperature mechanical behavior of cross-weld specimens prepared from a dissimilar weld joint between T92 martensitic and Super304H austenitic heat-resistant steels incorporating Ni-based weld metal was evaluated at temperatures up to 650°C. For both high temperature tensile and creep tests, failure took place in T92 due to its faster degradation with temperature increase. The heat-affected zone of T92 played a critical role during creep deformation, resulting in type IV failure under the long-term creep condition. For the creep specimens, the location of failure shifted from the base metal region to the fine-grained heat-affected zone as the creep duration time increased from the short-term to the long-term condition. The massive precipitation of Laves phase on the grain boundaries of the fine-grained heat-affected zone during creep deformation was observed and found to be responsible for the accelerated void formation in the area leading to the premature failure.

The Influence of Thermal Exposure and Hydrogen Charging on the Notch Tensile Properties and Fracture Behaviour of Dissimilar T91/TP316H Weldments

Abstract The effects of ageing and hydrogen charging on the notch tensile properties and fracture behaviour of individual heat-affected zones (HAZ) and Ni-based weld metal (Ni WM) of T91/TP316H weldments were investigated. After the post-weld heat treatment at 750 °C for 1 h the weldments were annealed at 600 °C for 1000 h and 5000 h, respectively. All heat-treated states were studied in condition without as well as with hydrogen charging. Thermal expositions led to additional precipitation and microstructure coarsening but their influence on tensile strength was insignificant. In contrast, remarkable plasticity decrease and the fracture mode transition from ductile dimple tearing to transgranular cleavage were observed. The combined effects of thermal exposure and hydrogen charging were more complex. Whereas the regions of Ni WM and TP316H HAZ did not show any significant change in strength, the hydrogen effect caused the strength increase in T91 HAZ. Although the hydrogen embrittling effects were clearly manifested by decreasing plasticity, their significance was getting smaller with increasing annealing duration. The fracture behaviour of thermally exposed and hydrogen charged regions exhibited mixed fracture modes including transgranular cleavage, intergranular dimple fracture and intergranular decohesion.

Microstructure and creep characteristics of dissimilar T91/TP316H martensitic/austenitic welded joint with Ni-based weld metal

This paper deals with characterization of microstructure and creep behavior of dissimilar weldment between the tempered martensitic steel T91 and the non-stabilized austenitic steel TP316H with Ni-based weld metal (Ni WM). Microstructure analyses were performed using light microscopy, scanning and transmission electron microscopy and energy-dispersive X-ray spectroscopy. The martensitic part of the welded joint exhibited a wide heat-affected zone (HAZ) with typical microstructural gradient from its coarse-grained to the fine-grained/intercritical region. In contrast, the HAZ of austenitic steel was limited to only a narrow region with coarsened polygonal grains. The microstructure of Ni WM was found to be very heterogeneous with respect to the size, morphology and distribution of grain boundaries and MC-type precipitates as a result of strong weld metal dilution effects and fast non-equilibrium solidification. Cross-weld creep tests were carried out in a temperature range from 600 to 650 °C at applied stresses from 60 to 140 MPa. The obtained values of apparent stress exponents and creep activation energies indicate thermally activated dislocation glide to be the governing creep deformation mechanism within the range of used testing conditions. The creep samples ruptured in the T91 intercritical HAZ region by the “type IV cracking” failure mode and the creep fracture mechanism was identified to be the intergranular dimple tearing by microvoid coalescence at grain boundaries. The TEM observations revealed pronounced microstructural differences between the critical HAZ region and the T91 base material before as well as after the creep exposure. ► Phase transformations affect the microstructures of T91 and TP316H HAZ regions. ► High weld metal dilution results in heterogeneous microstructure with MC carbides. ► Creep behavior of the studied weldment is controlled by its martensitic T91 part. ► The lowest hardness of T91 ICHAZ region corresponds with its lowest creep strength.

Evaluation of Acoustic Anisotropy to Image Defects in Weld Metal by Ultrasonic Phased Array

An ultrasonic phased array (PA) does not work well in some weld metals owing to the strong acoustic anisotropy. However, the analysis of anisotropy for the compensation of the shift of the defect image in the PA has not been performed. In this study, we compared the results of imaging a slit in a Ni-based weld metal, commercially available as Inconel alloy 600, in an experiment and in simulations using the analytical solution of group velocity or the finite-difference time-domain (FDTD) method. As a result, we succeeded in reproducing the shift and broadening of the defect image obtained in an experiment by the simulations.

异材焊接件A508/52M/316L在高温水环境中的应力腐蚀破裂

The stress corrosion cracking(SCC) behavior of advanced dissimilar metal weld A508/52M/316L in simulated primary water environments of pressurized water reactor(PWR) at 290℃was investigated by means of slow strain rate testing(SSRT).The tests were performed at various applied electrode potentials which correspond to the electrochemical conditions of the weld in various water environments,from low potentials with ideal water chemistry to high potentials with oxygen -contaminated water chemistry.The weld exhibits complicated microstructure and chemical composition distributions,especially,significant changes appear around the A508/52M interface and the 52M/316L interface.For tensile specimens in SSRT,sharp notches were machined at important and typical places,i.e.,at the two interfaces and in the bulk parts of the low alloy steel,Ni base weld metal and stainless steel of the weld.Results showed that the specimens always failed in bulk zone of the Ni base weld metal with ductile appearances when tested in the potential range from -780 mV to—300 mV(vs SHE).When electrode potential was raised into the range from -200 mV to +200 mV which corresponds to oxygen-contaminated water chemistry,the weld exhibits significant SCC. The area around the A508/52M interface is the weakest place,transgranular stress corrosion cracking (TGSCC) happened both along the interface and in A508 heat affected zone(HAZ),intergranular stress corrosion cracking(IGSCC) occurred in the Ni base weld metal close to the interface.The cracking mechanism and the engineering practical significance were discussed.

OS2507 Ni基合金溶接金属/低合金鋼境界部における材料組織学的特徴と高温水中SCC感受性(OS25-2 応力腐食割れ・配管,OS-25 供用エネルギー機器の経年変化と健全性評価)

OS2507 Ni基合金溶接金属/低合金鋼境界部における材料組織学的特徴と高温水中SCC感受性(OS25-2 応力腐食割れ・配管,OS-25 供用エネルギー機器の経年変化と健全性評価)

Residual Stress Evaluation of Ni Based Weld Metal Using Neutron Diffraction Method

Residual stress generated in Type-316 austenitic stainless steel butt-weld jointed by Inconel-182 was measured using a neutron diffraction method and compared with values calculated using FEM analysis. The measured values of Type-316 austenitic stainless steel as base material agreed well with the calculated ones. The diffraction had high intensity and a sharp profile in the base metal. However, it was difficult to measure the residual stress at the weld metal due to very weak diffraction intensities. This phenomenon was caused by the texture in the weld material generated during the weld procedure. As a result, this texture induced an inaccurate evaluation of the residual stress. Procedures for residual stress evaluation to solve this textured material problem are discussed in this paper. As a method for stress evaluation, the measured strains obtained from a different diffraction plane with strong intensity were modified with the ratio of the individual elastic constant. The values of residual stress obtained using this method were almost the same as those of the standard method using Hooke’s law. Also, these residual stress values agreed roughly with those from the FEM analysis. This evaluation method is effective for measured samples with a strong texture like Ni-based weld metal.