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.     

TP304H与12Cr1 MoV锅炉管异种钢的焊接

利用舍夫勒组织图对韶关发电厂SA2 13TP30 4H与 12Cr1MoV异种钢接头可能出现的焊接组织进行了分析 ,制订出了合理的焊接工艺 ,并对锅炉上的SA2 13TP30 4H与 12Cr1MoV钢焊口进行了施焊 ,经 17个月的运行 ,10 4个焊口未出现爆漏现象     

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.     

Creep deformation and failure of E911/E911 and P92/P92 similar weld-joints

This paper deals with characterisation of microstructure and creep behaviour of similar weld-joints of advanced 9% Cr ferritic steels, namely E911 and P92. The microstructures of the investigated weld-joints exhibit significant variability in different weld-joint regions such as weld metal (WM), heat-affected zone (HAZ), and base metal (BM). The cross-weld creep tests were carried out at 625 °C with initial applied stresses of 100 and 120 MPa. Both weld-joints ruptured by the “type IV cracking failure mode” in their fine-grained heat-affected zones (FG-HAZ). The creep fracture location with the smallest precipitation density corresponds well with its smallest measured cross-weld hardness. The welds of P92 steel exhibit better creep resistance than those of E911 steel. Whereas the microstructure of P92 weld after creep still contains laths, the microstructure of E911 weld is clearly recrystallized. The creep stress exponents are 14.5 and 8 for E911 and P92 weld-joints, respectively. These n-values indicate the “power-law creep” with dislocation-controlled deformation mechanism for both investigated weld-joints.     

Limits of the oxidation resistance of several heat-resistant steels under isothermal and cyclic oxidation as well as under creep in air at 650°C

In order to guarantee the oxidation resistance of Cr-steels the Cr content in the alloy must be above a critical limit. Recently developed 9Cr steels are close to that limit and as ongoing oxidation leads to Cr subsurface zone depletion the question arises as to how the oxidation behaviour is affected by the decrease in Cr concentration with oxidation time. Four ferritic heat-resistant commercial steels containing 9–12% Cr and the austenitics AISI 304 and Alloy 800 were investigated at 650°C in air to determine their oxidation behaviour and the course of Cr-depletion in the metal subsurface zone for times up to 3000 hours. In addition to isothermal tests, thermal cycling tests and creep tests were also performed. Surprisingly large differences in oxidation behaviour were found between the two 9Cr steels. Furthermore, of the two steels designated as 12Cr steel, one was even worse than the 9Cr steels while the other one was best. Thermal cycling improved the oxidation behaviour of the steels which was worse under isothermal conditions by almost two orders of magnitude. The oxidation behaviour as a function of time very much reflected the amount of Cr in the metal subsurface zone. The breakaway effects observed could be correlated with a drop in the Cr content in the subsurface zone below a critical value which had been determined by model calculations. The tendency observed under isothermal conditions is enhanced by superimposed creep deformation. It is concluded from the results that growth stresses in the oxide scales combined with the actual Cr-concentration in the metal subsurface zone play a major role in oxidation resistance.     

Stress Corrosion Cracking and Blue Brittleness of Rotor Steels in High Temperature Aqueous Solutions

The tensile behaviors of two bolt steels 20Cr12NiMoWV and 25Cr2MoV, and rotor steel 30Cr2Ni4MoV used in a nuclear power plant were studied at different temperatures up to 280 degreesC in air and aqueous solution. The results showed that blue brittleness occurred near 230 degreesC and 180 degreesC for steels 30Cr2Ni4MoV and 25Cr2MoV, respectively. No blue brittleness was found for the steel containing higher Cr content. Susceptibility to stress corrosion cracking.(SCC) for the steels in aqueous solution at the test temperatures was very low except at the blue brittleness temperature. At the blue brittleness temperatures, however, the susceptibility to SCC was very high.     

Mechanistic approach for prediction of creep deformation,damage and rupture life of different Cr–Mo ferritic steels

Abstract

A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.     

Creep rupture behaviour of circumferentially welded mod. 9Cr–1Mo steel pipe subject to internal pressure and axial load

The multi-axial creep strength of circumferential welds in power piping, including failure mode and failure life against a wide range of stress ratios and stress levels, was newly examined in this paper. The creep rupture behaviour of modified 9Cr–1Mo steel (9Cr–1Mo–VNb steel; ASME P91) pipe with a circumferential weld subject to combined internal pressure and axial load was experimentally investigated at 650 °C. The test results, with several kinds of stress ratios of macroscopic axial and hoop stress can be summarised as follows. Along with the increased stress ratio (axial/hoop), both failure location and failure mode changed from a base metal failure caused by hoop stress to an FGHAZ (fine-grained heat affected zone) failure due to axial stress. The stress ratio where the failure mode changed was ‘0.8’. Strength reduction in FGHAZ failure by axial stress should be considered in the structural design of circumferential welds. A series of FEM creep analysis was carried out to discuss the relation of the failure mode to both the local stress distribution and the damage. The failure mode variation along with the increase in the stress ratio is also discussed based on simplified ductile creep failure analysis.     

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.     

Carbon diffusion in dissimilar joints between P91 and 12Cr1MoV steels welded by different consumables at high temperature

Three consumables: 9CrMoV(N), R31, and GTR-2CM are considered in welding P91 and 12Cr1MoV together. The stable phases and carbon activities of the involved materials were calculated by THERMO-CALC. Carbon diffusion after aging for varying times at 700°C was simulated by dispersed multiphase model in DICTRA. The usage of different weld metals causes different carbon concentration profiles near weld interfaces both on the P91 and 12Cr1MoV sides, driven by their carbon activity difference. Mole fractions of carbides varying with distance were investigated, showing that the formation of carbon-depleted zone (CDZ) and carbon-enriched zone (CEZ) is related to the dissolution and precipitation of M₂₃C₆ carbides. To validate the simulation results, the microstructure and hardness near the interface between P91 base and GTR-2CM weld metal were studied. Good agreements on the widths of CDZ and CEZ between modelled and experimental results were obtained.     

Mechanism of intergranular reheating crack in butt-welded joint of 12Cr1MoV tube served for 40,000 hours

Cracking failure of butt-welded joint of 12Cr1MoV tube was comprehensively studied. Results show that both of initiation and propagation of the primary crack were circumferentially intergranular in the coarse-grained heat affect zone (CGHAZ). Many isolated and intergranular micro-cracks and cavities were observed near the primary crack．Neither oxide or corrosion products were observed in the isolated cavities or micro-cracks. According to the microstructure, location, propagation mode and morphology of the crack,the primary crack in butt-welded joint is concluded to be the intergranular reheating cracking (IRC). The crack failure is mainly due to poor welding quality, characterised by high residual stress and coarsened grain size. Mechanisms on the IRC based on previous laboratory research were studied on the failed tube sample, and results showed that the IRC is accumulation of high-stress induced creep damage, such as cavities along prior austenite grain boundaries (PAGBs). Neither segregation of alloys elements nor trace impurities were detected.     

Microstructural characterization of oxide scales formed on steels P91 and P92

Abstract

In this paper, an in-depth study of steam oxidation of two 9Cr ferritic-martensitic steels (P91 and P92), for advanced power plant, has been carried out. The steels investigated were exposed to a 100% flowing steam environment at 650°C for 1,000 – 3,000 hours. Metallographic analysis showed a multilayered scale was formed on both substrates consisting of an outer Fe-rich phase and inner Fe – Cr spinel, of varying oxide stoichiometry, as well as Cr –Mn-rich bands. The results of scale thickness measurements showed that the oxide formed on the P92 steel was thicker compared to that on the P91 steel. Crystallographic analysis showed that the P91 steel exhibited a martensite to ferrite transformation adjacent to scale – substrate interface which was not observed on the P92 steel. The outer oxide layer on both substrates exhibited a region of equiaxed grains followed by a region of columnar crystal growth. However, on the P91 steel the longer exposure time (3,000 hours) resulted in the outer layer having a region with coarsened equiaxed magnetite grains. The roles of alloying elements (Cr, Mo, W and Mn) were investigated to provide a better understanding of the oxide growth behaviour.     

Creep behavior and in-depth microstructural characterization of dissimilar joints

Abstract

The 700 °C power plants currently under development will utilize Ni-base alloys such as alloy 617 for components to be operated at temperatures >650 °C. Due to economic reasons for components or parts of components which are subjected to temperatures <650 °C, 2% Cr or 9–12% Cr steels is used, depending on the required mechanical properties. This makes the dissimilar joining of Ni-base alloys and Cr steels a necessity in these plants. Experimental investigations show that these joints have to be identified as weak points with regard to damage development under creep and creep-fatigue loading. The present investigation focuses on welds between the alloy 617 and 2% Cr steel. Under creep load the fracture occurs near the fusion line between the 2% Cr steel base metal and alloy 617 weld metal. To explain the reasons for this fracture location, the microstructure of this fusion line was investigated using TEM and FIB techniques after welding and after creep loading. The TEM investigations have shown a small zone in the weld metal near the fusion line exhibiting chromium depletion and clearly reduced amounts of chromium carbides, leading to a weakening of this zone.     

蠕变/疲劳共同作用下寿命估算方法

通过载荷谱转换，将带保载时间的蠕变／疲劳循环用不带保载时间的纯疲劳循环代替，提出蠕变／疲劳共同作用时的寿命估算方法。对１２Ｃｒ１ＭｏＶ钢母材和焊材的蠕变／疲劳交互作用试验数据的分析结果表明，本方法方便、实用。提出一个表征蠕变／疲劳连续加载时交互作用行为的参数，蠕变／疲劳寿命比。分析认为，材料的蠕变疲劳交互作用行为与该比值的大小腾。材料在蠕变／疲劳共同作用下呈这是呈负交经作用，并非材料固有的特性，还取决于载荷条件。     

Influence of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint

Effect of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint has been investigated. Creep tests on multi-pass double-V cross weld joint flat specimens of the steel having thicknesses in the range 1–17 mm have been carried out at 923 K (650°C) and 50 MPa stress. Creep rupture life of the weld joint was found to increase with thickness and reached a maximum value around 10 mm of thickness followed by decrease with further increase in thickness. Failure in the weld joints occurred in the soft intercritical region of the heat-affected zone (HAZ). Creep strain localisation was observed at the fractured location and was more in the thinner weld joints than in the thicker weld joint. Creep cavitation in the intercritical region of HAZ close to the unaffected base metal was more extensive at the mid-location of the weld pass, where the HAZ width was relatively larger and hardness was lowest. The type IV cavitation in intercritical HAZ was more extensive in thicker joint, whereas creep strain concentration in the intercritical HAZ was more in thinner weld joint. Creep cavitation in the joint was more pronounced at near mid-thickness locations than those beneath the specimen surface. Joints of intermediate thickness possessed higher creep rupture life because of relatively less accumulation of creep deformation coupled with lower creep cavitation in the intercritical region of HAZ.     

Estimation of oxide scale growth and temperature increase of high (9–12%) chromium martensitic steels of superheater tubes

The improved ferritic steels that have very good creep strength such as grades 91 and 92 have been widely used for power plant applications. For example, a chromium–molybdenum–vanadium steel tube (SA213-T91) has been available in the market for two decades. The SA213-T91 material has better high temperature strength and oxidation resistance than those of the more widely used materials such as T11 and T22. In this paper, estimations of the oxide scale growth and temperature increase of 9–12% Cr martensitic steels of superheater tubes are carried out using an incremental procedure, utilizing the finite element simulations and the relation between the Larson Miller Parameter (LMP) and the scale thickness over a period of time. In order to show the superiority of T91 steels over T22 steels, two different steam temperatures are considered in the models for comparisons. The estimations showed as expected that the oxide scale growth and temperature increase of the 9–12% Cr steels were significantly lower compared to those of 2.25% Cr steels.     

Experimental and numerical characterization of low cycle fatigue and creep fatigue behaviour of P92 steel welded joint

Low cycle fatigue (LCF) and creep fatigue interaction (CFI) behaviour of P92 steel welded joint were investigated experimentally and numerically. Strain‐controlled LCF tests at different strain amplitudes and CFI tests at different peak strain holding time were conducted. Evolutions of cyclic stress response, mean stress, and creep strain during cycling were described, in which the influence of strain amplitude and holding time were investigated. A specific heat treatment process was proposed to get the homogenous simulated material of fine grain region and coarse grain region in the heat affected zone. Material parameters of parent material, fine grain heat affected zone, coarse grain heat affected zone, and weld metal in the unified viscoplasticity model were then determined and validated. To predict the LCF and CFI behaviour of welded joint, 3‐dimensional unified viscoplasticity model with a modified isotropic variable was compiled into ABAQUS UMAT. The comparison between the predicted and experimental result under LCF and CFI loadings showed that the simulation results were reasonable and agreed with the experimental data well.     

A model based creep equation for 9Cr–1Mo–0·2V (P91 type) steel

Abstract

The isothermal constant stress creep tests data for a 9Cr–1Mo–0·2V (P91 type) steel were submitted for a phenomenological analysis in order to obtain the relevant creep equation for such steel. Namely, the minimum creep strain rate of P91 type steel cannot be described by the simple Arrhenius type power law constitutive model. The incorporation of the threshold stress concept in the analysis of creep data leads to a modified power law, which satisfactorily describes the creep behaviour of the examined P91 steel. However, the threshold stress is not a good material parameter, as it often varies with temperature and/or applied stress. This adds uncertainty to the extrapolation of the creep rates into ranges where experimental data are not available. Besides the fact that the physical foundation for a threshold stress is questionable from a scientific point of view, this is a serious practical limitation of the modified power law creep equation. The second creep equation proposed in the present paper is the improved stress dependent energy barrier model. The improvement of the standard model is based on two assumptions: first, on the hypothesis that the application of a stress also affects the energy barrier to be overcome when a local region transitions from the initial to the final state, and second, by applying a simple power function of stress instead of a hyperbolic sin function in the model based equation. The obtained value of stress exponent, n=5·5, is too high for entirely climb controlled creep. The apparent activation energy of approximately 510 to 545 kJ mol^?1, which is considerably higher than the activation energy for lattice diffusion, is the stress dependent activation energy of the slowest, dominant rate controlling process of the supposed multiple creep mechanisms.     

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650°C) and 110–100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M₂₃C₆ precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M₂₃C₆ precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint. The enhanced MX precipitation through TMT processing and reduction in coarsening of M₂₃C₆ precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.