T92/HR3C异种钢焊接接头蠕变断裂研究

针对采用手工氩弧焊GTAW打底,手工电弧焊SMAW焊接的T92/HR3C异种钢接头,在USC条件下对接头进行蠕变断裂加速试验,并分析蠕变断口微观形貌,研究结果表明:不同载荷条件下的蠕变断裂的位置不同;T92/HR3C异种钢焊接接头蠕变断裂属于韧窝聚集型韧性断裂;T92/HR3C异种钢焊接接头在大容量超超临界机组(USC)条件下服役是完全安全可靠的。     

Creep fatigue crack development in dissimilar metal welded joints between steels and nickel based alloy

Abstract

Creep and strain controlled cyclic/hold creep fatigue tests have been performed at temperatures in the range of 550–575°C on specimens extracted from dissimilar metal welded (DMW) joints between two classes of steel and a nickel based alloy. The details and results of the tests are described. While crack development in the cyclic/hold creep fatigue test specimens tends to be creep dominated, the microstructural paths followed in the steels in the vicinity of their heat affected zones are not identical to those observed in creep rupture testpieces taken from the same DMW joint. In pure creep tests, cracking may occur adjacent to the fusion line and/or in the fine grain heat affected zone (FGHAZ), with rupture location being dependent on temperature stress and microstructural condition. In contrast, creep dominated creep fatigue cracking typically occurs in the intercritical heat affected zone/FGHAZ or the overtempered parent material on the steel side of such weldments, depending on the composition of the joint.     

Creep properties and microstructural evolution of austenitic TEMPALOY steels

The steam parameters in the new high efficiency fossil fuel power plants are continuously increasing, requiring new advanced materials with enhanced creep strength able to operate on the most severe temperature and pressure conditions. For super-heater and re-heater applications, TEMPALOY AA-1 steel, an evolution of 18Cr10NiNbTi alloy, has been developed through the addition of 3%Cu and B, significantly enhancing the creep resistance, while offering typical corrosion properties of 18%Cr steels. This paper describes Tenaris’ tubular products in the field of austenitic grades for applications in Ultra Super Critical power plants: the production route and the main microstructural and mechanical properties of TEMPALOY AA-1 and TEMPALOY A-3 steels, including the effect of shot blasting on steam-oxidation resistance, their creep–rupture properties and their microstructural evolution during temperature exposure are presented.     

The fracture toughness behaviour of austenitic steels and weld metal including the effects of thermal ageing and irradiation

Stainless steel components used in nuclear power plant must be capable of maintaining reasonable mechanical properties after thermal ageing and irradiation damage has accumulated over the lifetime of the system. This study examines the fracture toughness behaviour of wrought and welded Type 316 material in long-term thermally aged and irradiated conditions. The results indicate that whilst some potentially detrimental microstructural changes have occurred during ageing, the degradation in mechanical properties is not large. In wrought material some comparisons are made between the toughness of Type 316 grade and recent results obtained on modified Type 316LN grade materials. The effects of welding processes on oxygen and inclusion content have been quantified in MMA and TIG welds, and the results have been used to explain the higher toughness of TIG-welded material. Comparisons of fracture toughness after irradiation have also been made between arc welds in Type 316 and other grades.     

HR3C钢内压蠕变试验后显微组织与力学性能试验研究

研究了HR3C钢管试样在不同内压蠕变试验时间下显微组织和力学性能的变化,分析了HR3C钢析出相对力学性能的影响.结果表明:内压蠕变试验后,HR3C钢管试样晶内和晶界上均析出弥散分布的NbCrN相,此外M23C6碳化物在晶界形成连续链状分布,并随着试验时间的延长发生聚集和粗化,M23C6碳化物在晶内析出弥散分布的方形颗粒,在孪晶界以长板状析出;弥散分布的NbCrN相和M23C6碳化物以及晶界连续分布的M23 C6碳化物产生弥散强化和晶界强化,导致内压蠕变试验后HR3C钢的强度和硬度均明显增大,随着试验时间的延长,不断聚集的M23C6碳化物弱化了晶界强化效果,使得HR3C钢抗拉强度略有下降;内压蠕变试验后,晶界连续分布的M23C6碳化物导致HR3C钢室温下冲击吸收功和断后伸长率明显减小.     

Mechanical behavior of sorbitic and austenitic steels in strongly hydrogenating environments: Relevant use prospects

Premature fracture tests were carried out on two different types of steel: AISI 9840 of sorbitic structure and AISI 304 stainless of austenitic structure. Quite a number of tests were made, varying both the pre-load and the discharge density of hydrogen. Examinations at SEM of the fracture surface were carried out. In this context many behavior differences in the alterations of the mechanical characteristics in the two materials tested were shown. It has been established through these tests that AISI 304 is the ideal material to be put in contact with hydrogen, provided the value of σ_0.2 in its walls is not exceeded.     

Comparative analysis of TIG welding distortions between austenitic and duplex stainless steels by FEM

In this study, thermal stress analyses were performed in the tungsten inert gas (TIG) welding process of two different stainless steel specimens in order to compare their distortion mode and magnitude. The growing presence of non-conventional stainless steel species like duplex family generates uncertainty about how their material properties could be affected under the welding process. To develop suitable welding numerical models, authors must consider the welding process parameters, geometrical constraints, material non-linearities and all physical phenomena involved in welding, both thermal and structural. In this sense, four different premises are taken into account. Firstly, all finite elements corresponding to the deposition welding are deactivated and, next, they are reactivated according to the torch’s movement to simulate mass addition from the filler metal into the weld pool. Secondly, the movement of the TIG torch was modelled in a discontinuous way assuming a constant welding speed. Thirdly, the arc heat input was applied to the weld zone using volumetric heat flux distribution functions. Fourthly, the evolution of the structural response has been tackled through a stepwise non-linear coupled analysis. The numerical simulations are validated by means of full-scale experimental welding tests on stainless steel plates. Finally, the results and conclusions of this research work are exposed.     

3D modelling of a multi pass dissimilar tube welding and post weld heat treatment of nickel based alloy and chromium steel

A dissimilar tube welding is performed between the nickel based Alloy617 and creep resistant steel VM12 using the former as the weld material. SYSWELD welding software is used to model the thermal and mechanical analysis. A readily available thermal history is used to calibrate the heat source input for the thermal analysis to generate the adequate thermal cycle by fitting the welding velocity, heat intensity factor of the GOLDAK heat source and the length of molten zone. The transient temperature field is then incorporated as the input for the mechanical analysis to obtain the residual stresses in which the phase transformation of the materials during welding is taken into account. Subsequently, the weld materials are characterized by using the Norton’s creep law to determine the Norton parameters based on relaxation experiments. The residual stresses generated after the multi pass welding by SYSWELD is transferred into ABAQUS as the initial condition for the post weld heat treatment (PWHT) simulation. The simulations show that the residual stresses reduce in magnitude but still present even after PWHT.     

奥氏体钢SP2215和马氏体钢X19CrMoNbVN11−1在高温高压超临界CO2环境下的腐蚀行为研究

超临界CO₂布雷顿循环具有循环效率高、体积小等优点,实际应用中循环系统部件面临因焊接及叶片自身旋转等拉应力的存在而加重腐蚀的问题,因此探明部件材料腐蚀行为是其在工程领域中应用的前提条件之一。搭建了超临界CO₂布雷顿循环实验台,模拟实际应用工况,并添加四点应力装置为样件施加载荷。研究了奥氏体钢SP2215和马氏体钢X19CrMoNbVN11−1(简称X19)在20 MPa/550 ℃的CO₂环境中未加载和加载应力下的腐蚀行为（900 h）。利用X射线衍射、X射线光电子能谱、扫描电镜及X射线能谱对两种材料腐蚀产物的形貌和成分进行了分析,并对材料的腐蚀行为进行了对比,结果发现： SP2215样件的氧化物主要为Cr₂O₃ 和Fe₃O₄,但应力加载样件表面Fe₃O₄信号更强烈。应力加载加剧了样件的腐蚀,增加了氧化膜厚度,使样件表面发生了严重的氧化层脱落。未加载的X19 表面出现了单质碳,生成的氧化物为Fe₃O₄与FeCr₂O₄,且可能存在分层现象。SP2215抗腐蚀能力优于X19,实验中并未出现明显的渗碳腐蚀现象。研究可为超临界CO₂布雷顿循环工程应用选材提供参考。     

On the physical differences between tensile testing of type 304 and 316 austenitic stainless steels with internal hydrogen and in external hydrogen

Seventeen metastable austenitic stainless steels (type 304 and 316 alloys) were tested in tension both with internal hydrogen and in external hydrogen. Hydrogen-assisted fracture in both environments is a competition between hydrogen-affected ductile overload and hydrogen-assisted crack propagation. In general, hydrogen localizes the fracture process, which results in crack propagation of particularly susceptible materials at an apparent engineering stress that is less than the tensile strength of the material. Hydrogen-assisted crack propagation in this class of alloys becomes more prevalent at lower nickel content and lower temperature. In addition, for the tests in this study, external hydrogen reduces tensile ductility more than internal hydrogen. External hydrogen promotes crack initiation and propagation at the surface, while with internal hydrogen surface cracking is largely absent, thus preempting hydrogen-assisted crack propagation from the surface. This is not a general result, however, because the reduction of ductility with internal and external hydrogen depends on the specifics of the testing conditions that are compared (e.g., hydrogen gas pressure); in addition, internal hydrogen can promote the formation of internal cracks, which can propagate similar to surface cracks.     

Effect of high temperature aging on microstructure and mechanical properties of HR3C heat resistant steel

Abstract

Microstructure and mechanical properties of the HR3C austenite heat resistant steel were investigated after artificial aging at 650°C for time up to 3000 h. The results show that as the aging time increased, the room temperature tensile and impact fracture mechanisms of the HR3C steel change from trans- to intergranular fracture. M₂₃C₆ type carbides and MX type carbonitrides continuously precipitate during aging, leading to the change of the mechanical properties and fracture mode of the steel. Moreover, the dissolution of the coherent twins and the transformation from the incoherent twins to the thermodynamically stable austenite subgrains have great effects on the mechanical properties of the aged steel, too. When increasing the aging time to ≧2000 h, the microstructure and mechanical properties of the steel are nearly constant, indicating a good thermal stability of the HR3C steel at elevated temperature.     

Potential suitability of ferritic and austenitic steels as interconnect materials for solid oxide fuel cells operating at 600 °C

The oxidation behavior of a number of commercially available ferritic and austenitic steels was tested in air and in two simulated anode gases of a solid oxide fuel cell (SOFC) to evaluate the potential suitability as construction materials for interconnects in SOFC's operating at 600 °C. During air exposure all studied materials showed excellent oxidation resistance due to formation of a protective, double layered chromia/spinel surface scale even if the steel Cr content was as low as 17%. However, in the anode side gases the presence of water vapour (and possibly CO/CO₂) increased the tendency to form poorly protective Fe-base oxide scales, in combination with internal oxidation of Cr. The occurrence of this adverse effect could be suppressed not only by increased Cr contents of the alloy but also by a small alloy grain size either in the bulk of the material or in the specimen/component surface. The latter can be promoted by cold work e.g. introduced by specimen/component grinding. As high Cr contents may lead to undesired σ-phase formation and defined surface treatments of an interconnect will not be possible in all designs, the relatively low operating temperature of 600 °C, resulting in low Cr diffusivity in the alloy grains, may require the use of a fine grained interconnect material to obtain and sustain protective chromia base surface scale formation during long-term operation.     

HR3C和Super304H不锈钢管子的固溶化热处理试验研究

通过分析HR3C和Super304H不锈钢管子在不同热处理条件下,管子的晶间腐蚀、硬度及机械性能的试验参数,证明HR3C可采用与Super304H相同的固溶化热处理温度进行热处理,并且在管子相对弯曲半径R/D≥31/3时,HR3C和Super304H可免除固溶化热处理.     

Long term creep properties and microstructure of SUPER304H,TP347HFG and HR3C for A-USC boilers

Abstract

SUPER304H (18Cr–9Ni–3Cu–Nb–N; ASME CC2328) and TP347HFG (18Cr–12Ni–Nb; ASME SA213) have been developed for high strength oxidation resistant steel tubes to operate at high steam temperatures and pressures. The longest creep rupture tests performed to date (600°C for 85 426 h for SUPER304H; 700°C for 55 858 h for TP347HFG) showed that the stable strength and microstructure were retained, with very little formation of σ-phase compared with conventional austenitic stainless steels and no other brittle phases. The alloy HR3C (25Cr–20Ni–Nb–N; ASME CC2115) has been developed for the high strength and high corrosion resistant steel tubes used in recent ultrasupercritical (USC) boilers with steam temperatures of ～600°C. The longest creep test conducted to date (700°C, 69 MPa for 88 362 h) confirmed a stable creep strength and microstructure at 600–800°C. Superheater and reheater tubes of these alloys installed in the Eddystone No.1 USC power plant since 1991 have been removed and investigated. Updated long term creep rupture properties of the steels and microstructural changes during service are reported. Three steel tubes have been successfully applied as standard materials for superheater and reheater tubes in newly built USC boilers.     

Investigation on the possibility of using the microshear test as a surveillance method to estimate the mechanical properties and fracture toughness of nuclear pressure vessel steel,A508CL3, and its joints welded by narrow-gap submerged-arc welding

The present paper has investigated the mechanical properties of nuclear pressure vessel steel, A508CL3, and its welded joints by using the microshear test method, and the fracture toughness of A508CL3 steel and its welds has also been estimated. Moreover, a comparison has been carried out between the conventional test, microshear test and fracture mechanics test. In addition, the possibility of using the microshear test on the surveillance program of nuclear pressure vessel embrittlement due to neutron irradiation has also been considered in detail, and the results indicate that the microshear test can be used successfully to estimate the degradation of mechanical properties both for A508CL3 steel and its welded joints. It has been found that the lower the microshear toughness, the smaller the Charpy V-notch (CVN) impact toughness and fracture toughness, as well as the tearing modulus. Finally, the results show that the microshear test method may be developed as a supplemental test method or standard of ASTM E185 and E636.