Correlation between microstructure and creep performance of martensitic/austenitic transition weldment in dependence of its post-weld heat treatment

This paper deals with the influence of post-weld heat treatment (PWHT) of T92/TP316H martensitic/austenitic transition weldment on the resulting microstructure and creep characteristics. Experimental weldments were fabricated by gas tungsten arc welding using a nickel-based weld metal (Ni WM). After the welding, two individual series of produced weldments were heat-treated according to two different PWHT procedures. The first “conventional PWHT” was carried out via subcritical tempering (i.e. bellow Ac₁ temperature of T92 steel), whereas the other one, the so-called “full PWHT” consisted of a complete reaustenitization of the weldments followed by water-quenching and final tempering. The use of “conventional PWHT” preserved microstructural gradient of T92 steel heat-affected zone (HAZ), consisting of its typical coarse-grained and fine-grained subregions with tempered martensitic and recrystallized ferritic–carbidic microstructures respectively. In contrast, the “full PWHT” led to the complete elimination of the original HAZ via transformation processes involved, i.e. the reaustenitization and back on-cooling martensite formation. The observed microstructural changes depending on the initial PWHT conditions were further manifested by corresponding differences in the weldments’ creep performance and their failure mode. The weldments in “conventional PWHT” state ruptured after long-term creep tests by premature “type IV failure” within their recrystallized intercritical HAZs. On the contrary, the long-term creep behavior of the weldments processed by “full PWHT” was characterized by their remarkable creep life extension but also by the occurrence of unfavorable “decohesion failure” along T92/Ni WM interface.     

Weld repair of Grade 91 piping and components in power generation applications,creep performance of repair welds

Creep strength-enhanced ferritic steels, such as Grade 91, are the preferred material for much of the high-energy boiler tubing and piping components used in modern power generating plants. Weld repair techniques that achieve the necessary performance without the need for high-temperature post weld heat treatment (PWHT) offer particular benefits for Grade 91 steel. These benefits arise since there are many examples of poor heat treatment control which have resulted in component microstructures with below the minimum properties expected by design codes. Furthermore, even a controlled PWHT at temperatures at around 760 °C will further temper the base material. This is significant because excessive base metal tempering is one suggested criterion requiring component replacement. Successful demonstration of controlled welding techniques linked to minimal or no PWHT would alleviate these problems. This article presents results from a major project which is aimed at considering options for designing a ‘well-engineered’ repair. In this project, the creep performance of candidate repair methods was evaluated using large, feature test-type specimens containing the entire weldment including both fusion lines and heat-affected zones. The results show that the cross-weld life in Grade 91 steels does not appear to be a function of whether or not the welding procedures include PWHT. The results offer the potential to qualify ‘cold’ weld repairs in these steels.     

New 12% Cr-steel for tubes and pipes in power plants with steam temperatures up to 650°C

Abstract

Nowadays, intense efforts are made to increase efficiency and thereby minimize harmful emissions of power plants. This can be achieved by increasing operating pressure and temperature to ultrasupercritical conditions. Presently martensitic 9% Cr-steels, e.g. P91, E911 and P92 are used for power plants with advanced steam parameters. Whilst these materials have the highest creep rupture strength values of ferritic steels, their oxidation resistance is lower than 12% Cr-steels, such as X20CrMoV12-1. With increasing steam temperature (target: 650°C) the lifetime of components made of 9% Cr-steel becomes limited not only by creep but also by oxidation. The present paper reports a new 12Cr martensitic steel developed by Vallourec & Mannesmann, which is designed for use at temperatures up to 650°C. It is the outcome of a normative research project of the Belgian Welding Institute in collaboration with Laborelec and with industrial partners (Carnoy Industrial Piping, Cockerill Mechanical Industries, Fabricom, Stork Mec, Böhler Thyssen Welding Germany, Vallourec & Mannesmann Tubes, AIB-Vinçotte, VCL, Tractebel and WTCM). Base metal properties (creep strength, toughness, reheat cracking susceptibility, oxidation behavior…), welding and high temperature behavior of the new 12% Cr-steel, and welds are addressed.     

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.     

Development of transition metal joint for steam generator circuit of prototype fast breeder reactor

Abstract

A transition metal joint between type 304 stainless steel and 2·25Cr–1Mo steel, with Alloy 800 as the transition piece, is being developed for application in the steam generator circuit of the 500 MW prototype fast breeder reactor. As part of this programme, the hot cracking susceptibility of Inconel 82/182 and of 16–8–2 welding consumables were compared and the microstructure and mechanical properties of butt welds between type 304 stainless steel and Alloy 800, welded by the two consumables, were studied to select the appropriate welding consumables for this joint. It is recommended that the 16–8–2 consumable should be used for welding this joint because of its lower microfissuring tendency and reduced mismatch in the coefficient of thermal expansion across the joint, although this would mean a slight adverse effect on the elevated temperature mechanical properties. Further, to select the optimum post-weld heat treatment (PWHT) of the joint between Alloy 800 and 2·25Cr–1Mo steel, welded with Inconel 82/182 welding consumables, the effect of PWHT on the microstructure and mechanical properties was studied. Decreasing the PWHT temperature was found to improve the mechanical properties and the microstructural condition of this joint.

MST/842     

Comparison of creep behaviour of 2.25Cr–1Mo/9Cr–1Mo dissimilar weld joint with its base and weld metals

Abstract

Evaluation of the creep behaviour of 2.25Cr–1Mo and 9Cr–1Mo ferritic steel base metals, 9Cr–1Mo steel weld metal, and 2.25Cr–1Mo/9Cr–1Mo ferritic–ferritic dissimilar weld joints has been carried out at 823 K in the stress range 100–260 MPa. The weld joint was fabricated by shielded metal arc welding using basic coated 9Cr–1Mo electrodes. Investigations of the microstructure and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenitic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft decarburised zone in 2.25Cr–1Mo steel base metal and a black etched hard carburised zone in 9Cr–1Mo steel weld metal around the weld fusion line developed. Hardness troughs also developed in the intercritical HAZ regions of both the steels. The width of the carburised and decarburised zones and hardness differences of these zones were found to increase with creep exposure. The 9Cr–1Mo steel weld metal showed higher creep strength compared to both the base metals. The 9Cr–1Mo steel base metal exhibited better creep resistance than the 2.25Cr–1Mo steel base metal at lower applied stresses. The dissimilar joint revealed lower creep rupture strength than both the base metals and weld metal. The creep strain was found to concentrate in the decarburised zone of 2.25Cr–1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the intercritical HAZ of 2.25Cr–1Mo steel even though this region showed higher hardness than the decarburised zone. Extensive creep cavitation and cracks were observed in the decarburised zone.     

Microstructure and mechanical behavior in dissimilar 13Cr/2205 stainless steel welded pipes

This work aims to investigate the microstructure and the mechanical behavior of dissimilar 13Cr Supermartensitic/2205 Duplex stainless steel welded pipes. A wide variety of microstructures resulting from both solidification and solid state transformation is induced by the fusion welding process across the weld joint. The tensile tests show that the deformation process of the dissimilar weld joint is mainly controlled by the two base materials: the duplex steel at the beginning of the deformation and the supermartensitic one at its end. This is confirmed by the micro-tensile tests showing the overmatching effect of the weld metal. The fatigue tests conducted on dissimilar welded specimens led us to conclude that the weld metal is considered as a weak link of the weld joint in the high cycle fatigue regime. This is supported by its lower fatigue limit compared to the two base materials that exhibit a similar fatigue behavior.     

Investigations on the high temperature behaviour of welded martensitic joints

As in Germany in the year 2015 the next generation of fossil fired power plant with steam parameters up to 700 °C will be erected, intensive R&D work focused on materials capable of operating in that high temperature regime is ongoing. Modern nickel-based alloys offer the possibility to be used for components for the highest temperatures and pressures in such power stations. Nevertheless, martensitic heat resistant 9-12% chromium steels will be widely used for the majority of the components subjected to “lower” temperatures up to 650 °C maximum, as they are much cheaper than nickel-base alloys. By welding these martensitic components, the heat affected zone (HAZ) has to be considered as a location of premature failure due to the change in the material’s microstructure i.e. size and number of precipitates, dislocation density, etc. Thus fully loaded weldments are of specific interest with regard to their possible optimization as well as their inspection during service. To be able to develop optimization strategies to increase the lifetime of welded martensitic components, the better knowledge of the HAZ’s dimension and extent in addition with the understanding of the development of the time dependent complex stress and strain states in this area are mandatory. In this paper, the results of an national funded project [1] focused on the optimization of creep loaded welds in pipes made of 9-11% Cr-steels will be reported. The project was aimed on the evaluation of the influence of the mismatch (in terms of different creep deformation of weld and base metal respectively) on the formation of the local stress situation in the weldment. In the frame of this work, extensive numerical and experimental investigations on component like specimens and lab specimens but also on measuring the temperature fields during the welding process lead to a comprehensive picture of the HAZ. Data of creep tests of thermally simulated HAZ material at various peak temperatures ranging from 780 °C to 1300 °C, of the weld metals but also of the base material formed the basis for a realistic simulation of the weldment’s behaviour. Finally the component tests under long term creep loading situations have been used for the validation of the numerical simulation.     

Correlation between microstructure evolution and cryogenic fracture toughness in aging ITER-grade 316LN weldments

In the present work, the correlation between microstructural evolution and fracture toughness in 316LN joints welded by Tungsten Inert Gas (TIG) was investigated. The effect of post-weld heat treatment (PWHT) on the microstructure and toughness was characterized. The welding process can significantly change the equiaxed grains of base metal to cells and dendrites, while the PWHT can increase the dendrite size, mitigate the texutre intensity, reduce the dislocation density, and slightly weaken the ultimate tensile strength of the joints. Fracture toughness tests reveal that the strain-induced martensitic transformation at cryogenic temperatures can remarkably deteriorate the fracture toughness. Due to the microstructural evolution during PWHT, the J-integral values at 77 K and 4.2 K decrese to 85% and 54% of those in the as-welded conditions, respectively. The fracture morphology of the as-welded joint shows a characterization of ductile fracture, while the PWHT joint features a mixture of ductile and brittle fracture.     

Microstructural characterization of weld joints of 9Cr reduced activation ferritic martensitic steel fabricated by different joining methods

This paper presents a detailed electron microscopy study on the microstructure of various regions of weldment fabricated by three welding methods namely tungsten inert gas welding, electron beam welding and laser beam welding in an indigenously developed 9Cr reduced activation ferritic/martensitic steel. Electron back scatter diffraction studies showed a random micro-texture in all the three welds. Microstructural changes during thermal exposures were studied and corroborated with hardness and optimized conditions for the post weld heat treatment have been identified for this steel. Hollomon–Jaffe parameter has been used to estimate the extent of tempering. The activation energy for the tempering process has been evaluated and found to be corresponding to interstitial diffusion of carbon in ferrite matrix. The type and microchemistry of secondary phases in different regions of the weldment have been identified by analytical transmission electron microscopy.     

Creep behaviour of P91/GTR-2CM/12Cr1MoV dissimilar joint predicted by the modified Theta method

Creep behaviour of P91, 12Cr1MoV steels and the P91/12Cr1MoV dissimilar joint were investigated at 823 K. Results show that the creep strength of P91 is much higher than 12Cr1MoV and than the dissimilar joint. The stress dependence of minimum creep rate and rupture life for both steels and the dissimilar joint obeyed Norton’s power law. The values of stress exponent are similar for 12Cr1MoV steel and the dissimilar joint in high stress region, indicating similar creep mechanism. However, the creep behaviour at 140 MPa for the dissimilar joint showed deviation from the other higher stresses, indicating different creep mechanism as the stress is decreasing. Microstructure showed that creep ruptured on the 12Cr1MoV side of the dissimilar joint as conducted in the high stress region, whereas ruptured on the carbon decarburized zone as conducted in the low stress region. Fracture location changed from 12Cr1MoV base metal to inter-critical heat affected zone as the creep time going. A modified theta equation was proposed to predict the creep behaviour, and the random errors from fitting to experimental creep data were smaller than obtained from the traditional theta method. The predicted creep behaviour showed good agreement with experimental ones.     

Microstructures and mechanical properties of creep resistant steel for application at elevated temperatures

The subject of this paper is the microstructural and mechanical characterisation of regions of the heat-affected zone (HAZ) in steels containing 9–12% Cr that are used for operation at elevated temperatures. Tests were performed on regions in the HAZ, which was created by physical simulation using a thermal welding simulator. Half of the simulated samples (SSs) were tested at room temperature (RT) and at an operating temperature (OT) of 600 °C immediately after simulation/welding, while the rest of the simulated samples were tested at RT and at the OT after heat treatment following the welding, i.e., post-weld heat treatment (PWHT). In addition to the results from mechanical testing, the results from microstructural analysis using light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are also presented. The manner in which PWHT contributes to the creep resistance of the HAZ in P91 steel is demonstrated.     

Microstructures and mechanical properties of welded joints of novel 3Cr pipeline steel using an inhouse and two commercial welding wires

The welded joints of the novel 3Cr pipeline steel were fabricated via the gas tungsten arc welding (GTAW) technique using an inhouse welding wire labeled as R01 and two kinds of commercial wires (H08Cr3MoMnA and TGS-2CML). Microhardness, impact toughness and tensile properties of the joints were measured, and microstructure characteristics were observed by scanning electron microscopy (SEM). The results show that under selected welding procedure, the joints of R01 can achieve quite good mechanical properties without preheating and post weld heat treatment (PWHT). After thermal refining, elongation (15.2%) doubled and met the DNV-OS-F101 standard. For low carbon or super low carbon pipeline steels such as 3Cr steel, the revised formula with the carbon applicable coefficient (A(c)) was quite good for predicting the maximum hardness in heat affected zone (HAZ). Compared with these two selected commercial wires, the inhouse welding wire R01 can provide the highest cost-performance ratio.     

船用吊杆0Cr16Ni5Mo不锈钢的焊接

概述了与0Cr16Ni5Mo马氏体不锈钢配套的焊接材料、焊后热处理制度以及适用的焊接工艺。试验选定采用M831A焊条,双U型坡口双面对称焊,焊后进行950℃/2h油淬+500℃/10h回火,得到的焊接接头性能完全达到指标要求。     

20Cr钢与B级钢焊接接头的组织和性能

采用低温冲击试验、拉伸试验、硬度试验以及金相分析等方法对ER50-6焊丝气体保护焊焊接20Cr钢与B级钢焊接接头的显微组织和性能进行了研究.结果表明,用ERS0-6焊丝气体保护焊焊接20Cr钢和B级钢可获得性能良好的焊接接头,其接头的强度不低于B级钢.焊缝组织为网状分布的先共析铁素体及少量无碳贝氏体;20Cr钢侧近缝区有无碳贝氏体及少量板条马氏体;在B级钢侧热影响区的块状铁素体基体上有少量粒状贝氏体.     

Stress corrosion cracking and hydrogen embrittlement cracking of welded weathering steel and carbon steel in a simulated acid rain environment

Abstract

The stress corrosion cracking (SCC) and hydrogen embrittlement cracking (HEC) characteristics of welded weathering steel and carbon steel were investigated in aerated acid chloride solution. The electrochemical properties of welded steels were investigated by polarisation and galvanic corrosion tests. Neither weathering steel nor carbon steel showed passive behaviour in this acid chloride solution. The results indicated that weathering steel had better corrosion resistance than carbon steel. Galvanic corrosion between the weldment and the base metal was not observed in the case of weathering steel because the base metal was anodic to the weldment. However, the carbon steel was susceptible to galvanic corrosion because the weldment acts as an anode. Slow strain rate tests (SSRT) were conducted at a constant strain rate of 7.87 × 10⁷ s^-1 at corrosion potential, and at potentiostatically controlled anodic and cathodic potentials, to investigate the SCC and HEC properties in acid chloride solution. The welded weathering steel and carbon steel were susceptible to both anodic dissolution SCC and hydrogen evolution HEC. However, weathering steel showed less susceptibility of SCC and HEC than carbon steel at anodic potential because of Cr and Cu compounds in the rust layer, which retarded anodic dissolution, and at cathodic potential due to the presence of Cr, Cu, and Ni in alloy elements, which inhibit the reduction of hydrogen ions. SEM fractographs of both steels revealed a quasicleavage fracture in the embrittled region at applied anodic and cathodic potentials.     

Enhancement of mechanical properties of low carbon steel joints via graphene addition

Steel base metal laps or welding electrode surfaces were coated using graphene suspensions with various concentrations, and then the steel plates were welded using the shielded metal arc welding process. Microstructural observations showed that the addition of graphene to the weldment significantly refines the microstructure and promotes the formation of fine acicular ferrite. The results of mechanical testing indicated that with lower concentrations of graphene in the weldment, both the strength and ductility improve, but the hardness remains unchanged in comparison to the unreinforced weld metal. However, reinforcing with a higher concentration of graphene gives rise to the significant enhancement of the hardness and strength without deterioration of the ductility.     

Nano-sized precipitates in 11 % Cr ferritic-martensitic steel after thermomechanical treatment

9 %–12 % Cr ferritic/martensitic steels with a good long-term creep strength at temperatures up to 650 °C and higher are being developed in order to increase steam temperature of coal-fired power plants.Thermomechanical treatment can effectively enhance the mechanical properties of high-Cr ferritic/martensitic steels mainly due to plenty of nano-sized precipitates produced by thermomechanical treatment. Nano-sized precipitates in an 11 % Cr ferritic/martensitic steel produced by a thermomechanical treatment, including warm rolling at 650 °C plus tempering at 650 °C for 1 h, were investigated by transmission electron microscopy. The average size of precipitates in the steel after the thermomechanical treatment was determined to be about 30 nm in diameter, which is only one-third of the average size of precipitates in the steel with the normalized and tempered condition. A large number of Cr-rich precipitates having an average diameter of about 25 nm in the steel produced by the thermomechanical treatment were identified as Cr-rich M₂C carbide with a hexagonal crystal structure, rather than M₂₃C₆ or MX phase. The plenty of nano-sized Cr-rich M₂C carbides were dominant phase in the steel after the thermomechanical treatment. The reason why prior precipitate phase formed in the steel during the thermomechanical treatment was Cr-rich M₂C carbide is also discussed.     

40Cr钢焊接区表面高频淬火后的超塑焊接

探讨了４０Ｃｒ钢焊接区表面局部高频淬火后超塑焊接的可行性及影响因素，并对接头组织进行了观察和分析。试验结果表明，焊接区局部高频淬火后的４０Ｃｒ钢在其超塑变形温度及应变速率范围内，经短时超塑压接其接头抗拉强度可达到母材之值。     

Effects of Post-Weld Heat Treatments on the Residual Stress and Mechanical Properties of Laser Beam Welded SAE 4130 Steel Plates

The distribution of the residual stresses in laser beam welded SAE 4130 and the effect of stress relief after various post-weld heat treatments (PWHT) were measured by using the x-ray diffraction method. The mechanical properties and microstructure were also examined. Experimental results show that the tensile residua] stresses increased with the heat input of the laser beam. Most of the residual stresses were relieved by PWHT at 530°C for 2 hours followed by furnace cooling to 50°C. The strength of the as-welded samples decreased in comparison with the base metal. The yield strength of the weldment increased after PWHT at 320°C for 2 hours in comparison with the as-welded sample. The elogation of the laser beam welded samples increased after PWHT in comparison with the as-welded samples.