Effect of Laves phase on the toughness of P92 weld metals

In this investigation, Laves phase was found to gradually precipitate from the tempered martensitic matrix of the two P92 weld metals (WMs) during the aging treatment. The particle size of Laves phase in the two WMs was ranged from 0.18 to 0.24 μm and was larger than the M₂₃C₆ particle size (ranged from 0.09 to 0.13 μm). The dislocation density of the two WMs was stable during aging. The hardness of the two WMs was stable during aging treatment, as the loss of solid solution strengthening was continuously compensated by the precipitation strengthening of Laves phase. The impact toughness of the two WMs declined gradually as the extending of aging time. The crack initiation zone and shear lip zone of the fracture surface were ductile fracture mode which consumed more fracture energy during impacting. The generation of Laves phase narrowed the crack initiation zone and shear lip zone and contributed to the decline of ductile fracture area percentage of aged WMs. The smaller ductile fracture area percentage contributed to the toughness reduction of aged WMs.     

Creep resistance of similar and dissimilar weld joints of P91 steel

Abstract

Two experimental weld joints, a similar weld joint of 9Cr–1Mo steel and a dissimilar weld joint of 9Cr–1Mo and 2.25Cr–1Mo steels, were fabricated by the TIG+E method and post-weld heating was applied. Creep testing was carried out at temperatures ranging from 525 to 625°C in the stress range 40–240 MPa. Creep rupture strength was evaluated using the Larson–Miller parameter. Extended metallography including transmission electron microscopy was performed and critical zones were indicated where fractures were concentrated during the creep exposure. At high temperatures rupture of the dissimilar weldment occurred in the heat affected zone (HAZ) of the weld metal while rupture of the similar weldment was located in the HAZ of the parent material. The processes of recovery seem to be the main causes of decrease in creep rupture strength of both weld joints in comparison to the parent materials.     

Numerical analysis of residual stresses in welds of similar or dissimilar steel weldments under superimposed tensile loads

This paper presented the characteristics of residual stresses in welds of similar or dissimilar steel weldments by carrying out three-dimensional (3-D) thermal elastic–plastic finite element (FE) analyses. Moreover, residual stress behavior in welds of the weldments under superimposed tensile loads was further investigated. The materials used in this investigation were SM400, SM490, SM520 and SM570, widely used structural steels in welded structure. Results show that the maximum longitudinal residual stresses in welds of the similar steel weldments increase with increasing yield stress of the steel welded (SM400 < SM490 < SM520 < SM570). When tensile loads are superimposed to edges of the weldments, the maximum longitudinal residual stresses are increased to a higher value. In case of the dissimilar steel weldments, the difference between the longitudinal residual stresses in welds increases with increasing yield stress of the steel welded together with SM400 (SM490 < SM520 < SM570). When tensile loads are superimposed to edges of the weldments, same pattern is noted for the longitudinal stresses in welds and the difference is almost the same as that between the longitudinal residual stresses in welds.     

Influence of Aging treatment on the microstructure and mechanical properties of T92/Super 304H dissimilar metal welds

Influence of aging treatment on the microstructure and mechanical properties of T92/Super 304H dissimilar steel joints was investigated. The microstructure of T92/Super 304H dissimilar steel joints was characterized using optical microscopy, scanning electron microscopy and energy dispersive spectrometer. The results show that the tensile strength of dissimilar metal welds (DMWs) after 10,000 h aging treatment met the ASME T92 and Super 304H standards. Rupture positions were located in the T92 base metal because of the precipitates formed along the sub grain and prior grain boundaries. The tensile strength of DMWs initially increased with time up to 4000 h, then decreased between 4000 to 6000 h, and finally came to almost a constant value from 8000 to 10,000 h exposure. The decrease in the tensile strength resulted from the nucleation and growth of Laves phases at the sub-grains and prior austenitic grain boundaries. The low absorption of impact energy in the weld metal was related to the coarse grains and its grain orientation.     

A comparative study of ductile-brittle transition behavior and fractography of P91 and P92 steel

The modified 9Cr-1Mo (P91) and 9Cr- 0.5Mo- 1.8W (P92) steel used in fast breeder reactor is exposed to irradiation during service which severely affects the dynamic fracture resistance by increasing the ductile to brittle transition temperature (DBTT). Thus, even at room temperature, the steel can become brittle and prone to cracking. In the present investigation, to elucidate the influence of low temperature on the DBTT, Charpy toughness test was performed on creep strength enhanced ferritic P91 and P92 steel. Lower DBTT was observed for the P92 steel as compared to P91 steel. To find the mode of fracture, the fractured Charpy toughness specimens were investigated using a field electron scanning electron microscope (FESEM). The fracture surface revealed the brittle mode of fracture at a lower temperature for both the steels while the mixed mode of fracture was noticed at room temperature and above.     

Mechanical properties characterisation of dissimilar joint of high-temperature materials using Thermo-Calc Classic (TCC) diagram analysis

Weldments were fabricated between Incoloy 800HT and P91 steel using ERNiCr-3, and ER505 filler materials by GTAW process. The ERNiCr-3 weld has to interface columnar and equiaxed dendrites whereas the ER505 weld has cellular dendrites. The ER505 weld / P91 steel base interface shows δ ferrite. Unmixed zone (UMZ) on Incoloy 800HT side and Type I and Type II zones were observed on P91 steel side of weldments. It was observed that with an increase in heat input, there was an increase in tensile strength of ERNiCr-3 weldments whereas there was a decrease in tensile strength of ER505 weldments. The impact energy of ERNiCr-3 welds was 96 Joules whereas ER505 welds had the impact energy of 178 Joules. Based on microstructure and mechanical properties, ER505 was found to be the ideal filler for joining of Incoloy 800HT to P91 steel than ERNiCr-3 filler.     

Microstructures and mechanical properties in dissimilar AZ91/AZ31 spot welds

The stir zone microstructures and mechanical properties of dissimilar AZ91/AZ31 friction stir spot welds made using different tool designs and tool rotational speed settings are investigated. Intermingled AZ91 and AZ31 lamellae are formed in the stir zones of dissimilar spot welds made using threaded, three-flat/0.7 mm/threaded and three-flat/no-thread tools and tool rotational speeds ranging from 1500 to 3000 rpm. The intermingled lamellae have chemical compositions, which are similar to those of the upper and lower sheets in the dissimilar sandwich. The flats on the rotating tool facilitate the downward transfer of upper and lower sheet materials in the location close to the pin periphery and therefore intermingled AZ91 and AZ31 lamellae are formed in the stir zones of dissimilar spot welds produced using a three-flat tool without a thread.The distance (Y) from the tip of the hook region to the keyhole periphery has a dominant influence on the mechanical properties of dissimilar AZ91/AZ31 spot welds, since the hook regions are curved inwards towards the axis of the rotating tool. The highest failure load properties and largest Y-values are found in dissimilar spot welds made using threaded and three-flat/0.7 mm/threaded tools and tool rotational speeds from 1500 to 3000 rpm. Dissimilar spot welds made using a rotational speed of 1000 rpm have the smallest Y-values and the lowest failure load properties.     

Fracture toughness of laser welds in ship steel

Shipbuilders are showing increasing interest in laser welding as a means of reducing fabrication costs. The nature of the weld zone is very different to that in an arc weld. Careful research is needed to establish the safety of laser welded structures against a range of risk scenarios. This report deals with fracture toughness and defect tolerance. The laser weld is shown to exhibit very unusual behaviour. The lowest toughness is obtained when a crack is located at the heat affected zone (HAZ), but there is no apparent microstructural embrittlement at this location. It is proposed that the effect arises from an elevation of crack tip tensile stresses induced by the strength overmatch of the adjacent weld zone.     

Softened zone formation and joint strength properties in dissimilar friction welds

The mechanical properties of dissimilar MMC/AISI 304 stainless steel friction welds with and without silver interlayers were examined. The notch tensile strengths of MMC/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds increased when high friction pressures were applied during the joining operation. The higher notch tensile strengths of dissimilar MMC/AISI and MMC/Ag/AISI 304 stainless steel friction welds resulted from the formation of narrow softened zones in MMC material immediately adjacent to the bondline. The influence of softened zone width and hardness (yield strength) on the notch tensile strengths of dissimilar welds was analysed using finite element modelling (FEM). FEM in combination with the assumption of a ductile failure criterion was used to calculate the notch tensile strengths of dissimilar joints. The key assumption in this work is that dissimilar weld failure wholly depended on the characteristics (mechanical properties and dimensions) of the softened zone formed in MMC material immediately adjacent to the bondline. The modelling results produced based on this assumption closely correspond with the actual notch tensile strengths of dissimilar MMC/Ag/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds.     

铜/钢异种材料等离子弧焊接头显微组织分析

为实现铜/钢异种材料的优质连接,研究了铜/钢焊接接头的显微组织特点及其组成,试验采用LHM-200等离子弧焊系统对紫铜(T2)与不锈钢(304)异种材料进行焊接,然后通过光学金相显微镜及能谱分析仪,观察分析接头显微组织,研究其接头组织结构及组成.研究结果表明:铜/钢异种材料的等离子弧焊焊缝在铜一侧为平直的,而在钢一侧则为半χ型,在铜侧界面形成漩涡状;在焊接接头的形成过程中铜元素的迁移和扩散主要依靠钢液的流动所带动;焊接接头显微组织主要为奥氏体组织,弥散分布着块状和粒状的铜和钢的固溶体组织,和黑色鱼骨状的α相铁素体组织;焊缝区组织主要是以α、γ富铁相和ε富铜相固溶体组织的形式存在.通过实现发现最佳的焊接参数为:保护气流量与离子气流量分别为0.25 L/min和0.75 L/min,焊接电流为65 A,焊接速度为4 mm/s,焊接后的接头抗拉强度能达到174 MPa.     

Prediction of residual stresses in welds of similar and dissimilar steel weldments

The fabrication of structural member using dissimilar steels renders steel structures lighter and more economical. However, it always involves welding process and produces different residual stresses in welds as compared with welding of similar steels. This paper presents the characteristics of residual stresses in welds of similar and dissimilar steel weldments by carrying out three-dimensional (3-D) thermal elastic-plastic finite element (FE) analysis. The materials used in this investigation were SM400, SM490, SM520 and SM570, widely used structural steels in welded structure. Results show that the maximum longitudinal residual stresses in welds of the similar steel weldments increase with increasing yield stress of the steel welded (SM400 < SM490 < SM520 < SM570). In case of the dissimilar steel weldments, the difference between the longitudinal residual stresses in welds increases with increasing yield stress of the steel welded together with SM400 (SM490 < SM520 < SM570).     

Estimation of embrittlement during aging of AISI 316 stainless steel TIG welds

Weldments of AISI grade 316 stainless steel, having a ferrite content of 4–6% and a variety of nitrogen concentrations were prepared using a modified element implant technique. Charpy impact specimens prepared from these weldments were subjected to a variety of aging treatments. Impact toughness decreases with aging time at all aging temperatures. Nitrogen is found to be beneficial to toughness. An empirical relation connecting the aging temperature, aging time and nitrogen content with toughness has been developed which can be used to estimate the time for embrittlement.     

Embrittlement of austenitic stainless steel welds

To prevent hot-cracking, austenitic stainless steel welds generally contain a small percent of delta ferrite. Although ferrite has been found to effectively prevent hot-cracking, it can lead to embrittlement of welds when exposed to elevated temperatures. The aging behavior of type-308 stainless steel weld has been examined over a range of temperatures 400–850C for times up to 10,000 hr. Upon aging, and depending on the temperature range, the unstable ferrite may undergo a variety of solid state transformations. These phase changes affect creep-rupture and Charpy impact properties.     

Influence of precipitation on dislocation substructure and creep properties of P91 steel weld joints

Abstract

Two trial weld joints were prepared using the GTAW and SMAW methods and they underwent creep testing at temperatures between 525 and 625°C. The longest time to rupture was 45,811 h. Two main processes occurred during creep exposures: recovery and precipitation of secondary phases. Slight coarsenings of the M₂₃C₆ carbide, precipitation of Laves phase and Z-phase were observed after long tests at high temperatures. Some differences in microstructure and creep failure were found in the individual zones of weldments. After long exposure at temperatures up to 600°C, fractures occurred in the fine-grain heat-affected zone as a result of a low density of fine vanadium nitride and a high density of coarse particles at grain and subgrain boundaries. At 625°C, growth of Laves phase caused a softening of the ferritic matrix and crack propagation in the weld metal.     

Material behaviour and plant experience of P91/P92 components

In several lignite-fired power plants of the ‘Vattenfall Generation AG’ in Germany components of P91/P92 material are used in long-time operation. About this experience in operation of selected components will be reported. In this context own experimental results of a research project in the damage evolution will be presented. The project ‘Damage development III’ was edited together with the MPA Stuttgart and was supported by Vattenfall and AVIF. The aim of the project was to improve the knowledge about the process of creep damage by experimental tests and additional numerical calculations. An instruction was given for planning, implementation and analysis of recurrent investigations on components consisting of 9% Cr steels which are subjected to high operation loading. Finally, the damage phenomena are presented by two case studies, a damage in a pipe bend due to faulty heat treatment and the creep-crack assessment of a lack of side-wall fusion in a reheater weld by fracture mechanics.     

Weldability in dissimilar welds between Type 310 austenitic stainless steel and Alloy 657

Microstructural evolution and solidification cracking susceptibility of dissimilar metal welds between Type 310 austenitic stainless steel and Inconel 657, a nickel-based alloy, were studied using a combination of electron microscopy analysis and Varestraint testing techniques. In addition, the effect of filler metal chemistry on the fusion zone composition, microstructure, and resultant weldability was investigated. The good cracking resistance of welds prepared with Inconel A was due to a small amount of secondary phase (NbC) and narrow solidification temperature range. The relatively poor cracking resistance of welds prepared with Inconel 82 and Type 310 stainless steel (310 SS) was a result of a wide solidification temperature range and an increase in the amount of secondary phases. Consequently, it is concluded that for the joint between Inconel 657 and 310 SS, filler material of Inconel A offers the best weldability.     

Effect of material inhomogeneity on fracture modes in aluminium–steel resistance spot welds

Resistance spot welding (RSW) is attractive for joining dissimilar materials, especially, aluminium to steel in automotive body. The direct joining of aluminium to steel forms an intermetallic compound (IMC) layer at their interface that dominates mechanical behaviour of the joint. A new formula was developed that considers material inhomogeneities such as the different mechanical properties in the weld such as base metal, heat affected zone (HAZ) and the weld nugget to accurately calculate the minimum weld nugget diameter required to enable pull‐out fracture. The shear strengths of weld regions such as the HAZ and IMC were directly measured and used as inputs to this new formula. The new formula was validated using experimental measurements from six combinations of aluminium–steel welds in comparison with analogous aluminium–aluminium welds. The new derivation was able to accurately predict fracture modes for all material combinations.     

Galvanising and galvannealing behaviour of B added TiNb IF high strength steel

Abstract

Solid solution strengthened steels stabilised with Ti and Nb are known to have poor galvanising and galvannealing properties. When these steels are alloyed with boron to prevent cold work embrittlement, the selective oxidation processes are influenced by the presence of B. The continuous galvannealing process has therefore been simulated in a laboratory in order to study the effect of B on the fundamental surface and interface processes that control the wetting of the surface by liquid Zn, the inhibition layer formation, and the Fe-Zn reaction.     

Fatigue performance of butt welds between cast steel joint and steel tubular members

Cast steel joints have gained increasing popularity for use in engineering. The fatigue performance of butt welds between cast steel joint and steel tubular members, however, is not yet well characterized. A series of fatigue tests were conducted on a new type of welding detail, cast steel joint with sloped end and integrated backing ring (Type C), for butt welds between cast steel joint and steel tubular members. Fatigue failure mechanism and S–N curves obtained from the tests were compared with those of Type A connection with backing ring and Type B cast steel joint with integrated nose obtained in literature. Fatigue behaviours of all three welding details were successfully predicted by local strain approach, the results of which are consistent with test results, especially for Types A and B welding details. According to the results of tests and analysis, fatigue cracking of butt welds between cast steel joint and steel tubular members always initiates from the bottom of the weld root, and the fatigue performance of Type C welding detail is better than those of Types A and B because of its lower stress concentration level. A simplified fatigue design equation was then proposed based on the results of local strain approach, in which the fatigue notch factor was adopted in fatigue behaviour evaluation. The accuracy of this simplified fatigue design equation was verified for Type C welding details with various geometry configurations.