Phase evolution in P92 and E911 weld metals during ageing

Phase evolution in the weld metals of P92 and E911 steels weld joints were studied during ageing at 625 °C for up to 9000 h. The phases: ferrite + M₂₃C₆ + MX + Laves found by means of analytical TEM in the annealed states agree with the results of the thermodynamic calculation of equilibrium phases. The cross-weld hardness values, HV10, after 1000, 3000, and 9000 h ageing overlap each other and are approximately 15 units below that of the post-weld heat-treated (PWHT) state. Charpy impact energy with the notch at the centerline of the weld metal was measured. Its values decrease after ageing from approximately 60–80 to 12 J compared to the PWHT state. In 9–12%Cr steel with W this phenomenon can be explained by a priori heterogeneity in the weld metal, its large former austenite grain size, the precipitation and growth of M₂₃C₆ and Laves phase particles on grain and packet boundaries.     

Creep failure model of a tempered martensitic stainless steel integrating multiple deformation and damage mechanisms

A new model considering both deformation and damage evolution under multiple viscoplastic mechanisms is used to represent high temperature creep deformation and damage of a martensitic stainless steel in a wide range of load levels. First, an experimental database is built to characterise both creep flow and damage behaviour using tests on various kinds of specimens. The parameters of the model are fitted to the results and to literature data for long term creep exposure. An attempt is made to use the model to predict creep time to failure up to 10⁵ h.     

P92钢表层纳米组织的热稳定性

本文通过表面纳米化处理(SMAT)在P92钢表层中形成纳米组织结构,研究了回火温度对表层纳米组织演化行为和析出行为的影响.结果表明:淬火态和回火态P92钢组织经过SMAT处理后沿深度方向依次是纳米层、剧烈变形层、最终过渡到正常组织.随后分别研究了淬火和回火态SMAT试样经不同温度回火后微观组织的再结晶及长大行为.经SMAT处理的铁素体纳米晶粒在550℃时仍能保持较好的纳米结构,甚至高达650℃时表层晶粒仍为纳米晶,当温度超过760℃时表层组织发生显著的再结晶和晶粒长大现象.纳米晶界能抑制淬火态P92钢在较低温度回火时M23C6碳化物的析出.纳米组织提高了高温回火过程中合金元素在铁素体中的扩散速率,加速了M23C6碳化物的长大过程.不同温度回火过程中SMAT纳米层中析出行为的变化将在本文中详细描述.     

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.     

基于Norton-Bailey模型的P92钢初期蠕变过程分析

以Norton-Bailey时间硬化模型和Kachanov损伤模型为基础,提出一种能够全面描述蠕变全寿命期蠕变变形发展的本构模型。与Norton-Bailey模型相比,该模型可以更准确地反映蠕变第一阶段的变形累积,以Kachanov损伤演化方程描述蠕变第三阶段的变化;推导模型第一阶段和第三阶段的参数确定方法;根据实验数据,确定P92钢的模型参数。将该模型嵌入到在ANSYS接口程序中,并用于高温合金钢P92的蠕变寿命计算,结果表明模型与实验数据基本吻合。     

Hot deformation behaviour and constitutive modelling of P92 heat resistant steel

Abstract

Hot compression tests are conducted in the present paper to investigate hot deformation behaviour of the newly developed heat resistant steel P92, which is used to fabricate main steam pipes for ultra supercritical power plants. Stress–strain curves at elevated temperatures and different strain rates are obtained. It is found that dynamic recrystallisation happens only when the temperature is above 1100°C and strain rate is below 0·1 s^?1. Otherwise, dynamic recovery is the main softening mechanism. Constitutive modelling with the hyperbolic sine, including an Arrhenius term, is used to predict peak and saturated stresses. Material constants for this model are determined. Results show that the model can be used to predict peak and saturated stresses. However, the model would fail in predicting flow stress with respect to strain; thus, a model containing nine independent parameters and the complete form of Spittel equation are utilised to predict flow stress curves softened by dynamic recrystallisation and dynamic recovery respectively since there are no unified equations. The predicted stress–strain curves are in good agreement with experimental results, which confirmed that the models developed in the present paper are effective and accurate for P92 steel.     

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.     

多耦合拘束效应对P92钢蠕变裂纹扩展行为的影响

基于断裂力学理论,针对不同面内拘束效应下P92钢高温蠕变裂纹扩展(Creep crack growth,CCG)进行了数据模拟与分析。研究发现不同的拘束效应对CCG行为有着各异的影响,主要分析了试样几何形状、尺寸以及初始裂纹深度等对CCG行为的影响,并且着重研究比较这3种耦合拘束效应对CCG行为的影响程度。基于多种耦合拘束对CCG行为的影响程度做了横向比较,研究发现:试样几何形状对CCG行为的影响程度要大于几何尺寸,试样尺寸和初始裂纹深度对CCG行为的影响程度相似,试样形状比初始裂纹深度的影响程度更大。     

不同拘束条件下P92钢高温蠕变裂纹扩展速率的有限元模拟

基于蠕变损伤力学,仿真了蠕变裂纹扩展过程。用ABAQUS软件模拟研究了试样几何形状和宽度不同的两种面内拘束条件对P92钢在650℃时蠕变裂纹扩展(CCG)速率的影响,并进行了试验验证。仿真结果表明:试样几何形状和宽度对CCG速率的影响与载荷水平(C*)有关;在低载荷水平区内,C(T)试样的拘束水平大于SEN(T)试样,并且对应的CCG速率较大;在中载荷水平区,随着试样宽度的增大,裂纹尖端拘束和CCG速率增大;在高载荷水平区,CCG速率基本不受面内拘束(试样几何形状、宽度)条件的影响;在条件相同的情况下,试样宽度拘束水平大于试样几何形状。试验结果表明,试验与有限元分析(FEA)的蠕变裂纹扩展结果相符合。     

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.     

基于C(t)积分及参数Ac的P92钢高温蠕变裂纹扩展研究

对于在高温环境下工作的构件,蠕变裂纹扩展是一种主要的失效机制,而裂纹尖端的拘束水平对蠕变裂纹扩展率有很大的影响。通过数值仿真与相关试验数据对比的方法,对裂纹扩展尖端的应力应变率场表征参量C(t)积分进行了相关研究,并基于参数Ac研究了P92材料裂纹尖端的拘束水平对蠕变裂纹扩展的影响。研究结果表明,C(t)积分值随裂纹扩展急剧减小,其数值及变化与积分路径到裂纹尖端的距离相关性很强,并且与拘束水平有一定的关系;拘束水平影响蠕变裂纹扩展率,拘束越大,裂纹扩展速率越快;参数Ac可以有效表征裂纹尖端拘束水平,其在寿命预测方面的应用有待进一步研究,同时在含裂纹的高温工作构件寿命评估方面有重大的意义。     

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.     

Creep failure simulations of 316H at 550 °C: Part I – A method and validation

This paper proposes a method to simulate creep failure using finite element damage analysis. The creep damage model is based on the creep ductility exhaustion concept, and incremental damage is defined by the ratio of incremental creep strain and multi-axial creep ductility. A simple linear damage summation rule is applied and, when accumulated damage becomes unity, element stresses are reduced to zero to simulate progressive crack growth. For validation, simulated results are compared with experimental data for a compact tension specimen of 316H at 550 °C. Effects of the mesh size and scatter in uniaxial ductility are also investigated.     

An assessment of mechanical properties of P92 steel weld joint and simulated heat affected zones by ball indentation technique

Mechanical properties of P92 steel weld joint fabricated by shielded metal arc welding were evaluated using ball indentation (BI) technique. Microstructure of the P92 weld joint consisted of the weld metal, coarse grain region, fine grain region, intercritical region and base metal. The individual microstructural regions of the heat affected zones (HAZs) were separately prepared by heat treating the steel at particular temperatures. Ball indentation and uniaxial tensile tests were carried out across the weld joint and on the simulated HAZ microstructures at temperatures of 300 K (27 °C) and 623 K (350 °C). The tensile strengths gradually decreased from weld metal to the base metal with trough in the intercritical region (ICR) of the joint and simulated at 1173 K (900 °C) steel. The formation of coarser M₂₃C₆ precipitates and sub-grain formation with reduced dislocation density led to soften the ICR/simulated at 1173 K (900 °C) steel than the other regions of the joint. The variation of mechanical properties across the joint was comparable with variation of hardness and microstructural constituents across the joint.     

Creep deformation of Alloys 617 and 276 at 750–950 °C

Alloys 617 and 276 were subjected to time-dependent deformation at elevated temperatures under sustained loading of different magnitudes. The results indicate that Alloy 617 did not exhibit strains exceeding 1 percent (%) in 1000 h at 750, 850 and 950 °C when loaded to 10% of its yield strength (YS) values at these temperatures. However, this alloy was not capable of sustaining higher stresses (0.25YS and 0.35YS) for 1000 h at 850 and 950 °C without excessive deformation. Interestingly, Alloy 617 showed insignificant steady-state creep rate at 750 °C irrespective of the applied stress levels. Alloy 276 almost met the maximum creep deformation criterion when tested at 51 MPa–750 °C. Severe creep deformation of both alloys at 950 °C could be attributed to the dissolution of carbides and intermetallic phases remaining after solution annealing or precipitated during quenching.     

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.     

Effect of calcium addition on the microstructure and compressive deformation behaviour of 7178 aluminium alloy

Aluminium 7178 alloys containing 1% calcium are used to study the effect of calcium addition on their microstructure and compressive deformation behaviour. The compressive deformation behaviour of aluminium alloy containing 1% calcium is studied at varying strain rates (10⁻²–10/s). The material is prepared using stir casting technique. The yield stress, flow stress and elastic limit are measured from the true stress–strain graph. The strain rate sensitivity and strain-hardening exponent was also determined for each material at different strain rates. Its microstructural characterization reveals that Ca particles act as grain refiners for primary base alloy and helps in improving the strength of the virgin alloy. An empirical relationship has been proposed to predict the flow curve of the alloys as a function of strain and strain rate.     

Investigating the effect of displacement rate on deformation and failure mechanisms in bonded elastomers

Deformation and failure mechanisms in bi-material specimens were investigated under three different loading speeds (0.0254 cm/min, 0.254 cm/min and 2.54 cm/min). Two different viscoelastic materials were used to generate bi-material specimens. The experimental data were analyzed and the effect of applied displacement rate on strain distributions and interfacial failure mechanisms in the bonded specimens are discussed. Close inspection of the specimens showed that at failure initiation widely different deformation rates occurred at roughly the same global strain levels and were concentrated in the interphase regions near the bondlines.