Present quality level of large heat resistant rotor forgings made of 1 % CrMoV steels – Part 1

The mechanical and toughness properties of steam turbine rotors of different production methods and different heat treatments are described and compared. Air and oil quenched rotors have similar properties in respect to creep behaviour while the toughness level of the oil quenched rotors is better. Rotors manufactured since about 1975 have a better level of all properties than older ones which is due to higher degree of purity, lower level of trace elements and a today better controlled temperature process during the whole manufacturing.     

Creep and microstructure evolution of foreign and domestic 10Cr rotor steel

In order to understand the property features of domestic 10Cr steel used for turbine rotor in ultra- supercritical unit, the research was conducted on domestic and foreign 10Cr steel for comparison. Firstly, the development of rotor steel and research situation of foreign and domestic 10Cr steel were introduced. The creep and microstructure evolution was studied on foreign and domestic 10Cr rotor steel for comparison. The results indicate that creep rupture strength at 595?? of the foreign 10Cr steel is higher than that of domestic 10Cr steel. The foreign and domestic 10Cr rotor steels have similar microstructure evolution features. With increasing time, different phases including M23C6?? MX?? Laves propagate, accumulate and grow subsequently. The Nb- rich phase with large size is found in domestic 10Cr steel and thought to have negative influence on properties.     

Microstructure,creep properties,and rejuvenation of service-exposed alloy 713C turbine blades

A study was carried out on the microstructure and creep properties of aero engine first-stage turbine blades made from Alloy 713C nickel-base superalloy. Results are reported for new blades, blades in two service-exposed conditions, and service-exposed blades subjected to one of three rejuvenation treatments: a recoating heat treatment, a hot isostatic pressing (HIP) + recoating heat treatment, and a HIP + controlled cooling + recoating heat treatment. The blade microstructure undergoes significant change during service, and this leads to a loss in creep properties exhibited by specimens machined from the blade airfoils. Good correlations were observed between the rupture time and the amount of blade airfoil untwist and between the minimum creep rate and the amount of untwist. The recoating heat treatment and the HIP + controlled cooling + recoating treatment were moderately successful in restoring the microstructure and creep properties of the service-exposed blades. In comparison, the HIP + recoating treatment was very successful in rejuvenating creep properties but only for blades having a chemical composition with a lower propensity to form σ phase. For the blades with an unfavorable composition, σ phase was found to form preferentially near the grain boundaries during creep testing, and this had a detrimental effect on the creep properties. Nonetheless, the degree of rejuvenation for these blades was always at least as good as that obtained through the recoating heat treatment alone. Formerly National Aeronautical Establishment     

Room temperature creep of high strength steels

The room-temperature creep behaviour of three high strength steels has been investigated. Several parameters such as creep stress, loading rate, stress history and heat treatment has been altered and their influence on the low temperature creep has been reported. The primary creep in all three alloys agreed well with the logarithmic creep law and the creep mechanism has been identified as pure dislocation creep. Higher stresses and high loading rates led to increased creep strains and strain rates. Reloading after a period of creep resulted in significantly decreased creep strains and no recovery of the time dependent deformation could be detected. The yield strength of the materials per se had no influence on the room temperature creep whereas the same material with decreased 0.2% offset strength showed significantly reduced time dependent deformation. The possible interaction between primary creep and stress corrosion cracking has been discussed.     

Microstructure behaviour of steel 15 CrMoV 5 10 under creep conditions

Investigations of the influence of heat treatment (cooling rate from austenitizing temperature, tempering conditions) on the mechanical properties at room temperature as well as the structural parameters, which influence the creep resistant properties - bainite content, type, quantity, size and distribution of carbides, alloying content of the matrix - were made on the heat resistant pipe steel 15 CrMoV 5 10. The structures, produced by optimized heat treatment conditions, are characterized by a high bainite content as well as finely dispersed VCN- and Mo₂C-precipitates and small grain boundary carbides. During the creep process more Mo₂C-carbides precipitate above all at the dislocations inhibiting their motion. This process is accelerated by creep deformation and essentially contributes to the long-term creep resistance of the steel 15 CrMoV 5 10.     

Effect of weld heat input and creep strain on the elevated temperature and crack growth properties of austenitic steels

A potential material class for use at 600°C and more, e.g. for steam turbines with improved thermal efficiency, are austenitic steels. Using these steels with welded joints, it is to be considered that, by superposition of weld residual stresses and service stresses, extensive creep strains – and in the worst case crack formation – can occur locally. To assess the influence of these effects on service behaviour, different material states of CrNi-steels and Incoloy 800 were investigated with respect to strength, ductility and, especially, to crack and creep crack growth in the temperature range around 600°C. It is shown that creep embrittlement, not microstructural changes as effected by weld heat input, causes heat affected zone (HAZ)-reheat cracking. Creep embrittlement can be avoided by special design and fabrication rules.     

309S耐热不锈钢蠕变行为及寿命预测

以工业化生产的309S奥氏体耐热不锈钢为研究对象,在不同温度和应力条件下进行单轴拉伸蠕变实验.结果表明:700℃/100 MPa蠕变断裂时间最长为480.13 h,蠕变曲线有明显的3个阶段;800℃/100 MPa蠕变断裂时间最短为4.61 h,蠕变曲线只有第3阶段,直至断裂.扫描电子显微镜(SEM)观察分析表明,蠕变...     

A planar model study of creep in metal matrix composites with misaligned short fibres

The effect of fibre misalignment on the creep behaviour of metal matrix composites is modelled, including hardening behaviour (stage 1), dynamic recovery and steady state creep (stage 2) of the matrix material, using an internal variable constitutive model for the creep behavvour of the metal matrix. Numerical plane strain results in terms of average properties and detailed local deformation behaviour up to large strains are needed to show effects of fibre misalignment on the development of inelastic strains and the resulting over-all creep resistance of the material. The creep resistance for the composite is markedly reduced by the fibre misalignment and the time needed to reach an approximate steady state is elongated due to the strain induced rotation of the short fibres in the matrix.     

The impact of welding on the creep properties of advanced 9–12% Cr steels

The term “long-term creep properties” for heat resistant steels is mainly reflected by the 100.000 hour creep rupture strength at elevated testing temperature. Often, results of high stress, short-term creep tests are extrapolated to this 100.000 hour target value. Results of long-term creep tests are rather rare because of high testing costs and the time consuming testing procedure. Especially, long-running crossweld creep tests have not been performed in a sufficient extent so far, although, the heat-affected zone of crosswelds of ferritic chromium steels is known as a possible weak point. The long-term creep properties of crosswelds is linked to microstructure of the heat-affected zone of 9–12% chromium steels. The formation of heat-affected zone microstructures is studied by dilatometry, in-situ X-ray diffraction using synchrotron radiation, optical microscopy as well as most advanced electron microscopic methods. Results of crossweld creep tests up to duration of 40,000 hours are directly linked to the heat-affected zone microstructure at the location of fracture. The most predominant failure mechanism at lower stress levels and long term duration is Type IV cracking in the fine-grained heat-affected zone region of crosswelds. The failure mechanism is discussed in detail in this paper.     

Review of research developments of 12Cr1MoVG heat resistant steel in China

In this paper,we review the evolution of 12CrlMoV steel standards at home and abroad,analyze the effects of various elements and determine their optimal contents in steel.The influence of heat treatment and microstructure on the creep strength of 12Cr1MoV steel is investigated.Statistical results from conventional mechanical properties,ductile-brittle transition temperature,high temperature oxidation resistance,aging,instant high temperature properties,and creep are introduced.The results show that the chemical composition and heat treatment process of 12CrlMoVG steel identified in the GB 5310-2008 standard is appropriate,resulting in a steel with higher creep strength and good comprehensive properties.     

Short time creep behaviour of Invar steel

During processing of so‐called true flat shadow/masks made of Invar steel for large size TV screens the mask is stretched onto a solid frame. Mask and frame are submitted to a final heat treatment (blackening treatment) in order to enhance the emissivity of the material. Elevated temperatures and pretension makes the mask material prone to plastic straining, resulting in disutility of the unit for the application. From a technological point of view, the material behaviour under these conditions has been defined as short time creep behaviour. The mechanisms causing creep in this special case are not fully understood. Focus of the present work was the examination of the effect of cube twins on the short time creep behaviour of the investigated Invar steel by texture analysis using EBSD technology. This investigation was conducted under four different material conditions covering a whole set of production steps during shadowmask processing. Detailed information about the change of the volume fractions and the orientation of characteristic texture components, most of all the cube twins, could be gained. The observations revealed that during the creep test two types of cube twins make an orientation change, thus plastic deformation takes place, causing relaxation of the material. The observation of cube twin rotation during the creep test is partly confirmed by the simulation of the respective Taylor factors.     

The effect of high-temperature oxidation on the creep behavior of a superalloy (NIMONIC-105)

Enhanced creep damage occurring in a NIMONIC 105 superalloy at 750 °C due to prior heat treatment at 1150 °C was investigated. The medium of heat treatment and creep testing was air. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) examination indicated that the heat treatment resulted in a surface region consisting of oxide casing, γ′ depleted zone, and grain boundary oxides. Rule of mixture-based mathematical analyses and experiments revealed that this region (referred to here after as the oxygen-affected zone (OAZ)) extended much deeper than that perceived by the SEM and EPMA observations. The formation of OAZ reduced the creep ductility drastically while its influence on the minimum creep rate was low. The probable mechanisms affecting the secondary and tertiary regimes are discussed. Prolonged heat treatments did not damage the creep properties any more significantly than the shorter heat treatments, indicating that all the important structural variations occurred during the initial stages of exposure.     

抗蠕变耐热镁合金的发展现状

讨论了镁舍金抗高温蠕变机理和提高镁合金抗高温蠕变的途径,重点介绍了Mg-Al系、Mg-Zn系和Mg-RE系耐热铸造镁合金的研究现状;对高性能耐热镁合金的发展趋势提出了一些见解,认为应该进一步优化合金元素,以解决铸造镁舍金在耐高温、抗蠕变等方面存在的问题。     

New approach to predict the long-term creep behaviour and evolution of precipitate back-stress of 9–12% chromium steels

Modern advanced 9–12 % Cr steels are complex alloys with excellent creep strength even at high temperatures up to 620°C. The mechanical properties of these steels are significantly influenced by the presence and stability of various precipitate populations. Numerous secondary phases grow, coarsen and, sometimes, dissolve again during heat treatment and service, which leads to a varying obstacle effect of these precipitates on dislocation movement. In this work, the experimentally observed creep rupture strength of an modified 9–12% Cr steel developed in the European COST Group is compared to the calculated maximal obstacle effect (Orowan stress) caused by the precipitates present in these steels for different heat treatment conditions. It is shown that the differences in creep rupture strength caused by different heat treatments disappear after long time service. This observation is discussed on the basis of the calculated evolution of the precipitate microstructure. The concept of boosting long-term creep rupture strength by maximizing the initial creep strength with optimum quality heat treatment parameters for precipitation strengthening is critically assessed.     

Contraction of high strength Invar steel during creep test

Modern large size Cathode Ray Tubes are equipped with so called “true flat” shadowmasks made of Invar steel. The mask is stretched onto a solid frame and both are submitted to a final heat treatment (blackening treatment). Elevated temperatures and pretension make the mask material prone to creep, resulting in disutility of the unit for the application. In order to reduce creep elongation of the mask material to a minimum, Mo added high strength Invar steels have been considered to provide the required specifications. Depending on prior processing this type of Invar steel shows an inexplicable contraction during the creep test. Even though this effect can be perfectly used to fulfil the creep requirements, the mechanisms involved were not understood. Focus of the present work was the examination of the effect of precipitations on the “negative creep” behaviour of the investigated Invar steel using carbon extraction replicae, transmission electron microscopy as well as SAED. Information about the chemical composition, morphology, size and number of the precipitations in the different states could be gained. The observations revealed that during the creep test, depending on the prior annealing temperature, the chemical composition of the precipitates changed. The Nb content decreased while simultaneously the Mo content increased. Due to the volume difference caused by Mo in solid solution and in precipitated form respectively, the precipitation of Mo during the creep test is supposed to cause the observed sample contraction. The results can be confirmed by calculating the effect of Mo on the distortion of the FeNi lattice.     

Elevated-temperature low-cycle fatigue behavior of different heats of type 304 stainless steel

The low-cycle fatigue results of three heats of Type 304 stainless steel have been ob-tained at 593°C under selected cyclic-loading conditions. The results are compared with those generated for a reference heat of steel for which extensive low-cycle fatigue data are available. Observation of the microstructures of specimens in the pretest condition after a given heat treatment and examination of fatigue fracture surfaces were con-ducted by means of optical and scanning electron microscopy and X-ray analysis. The three heats of stainless steel, which exhibit different microstructural features, show approximately the same continuous-cycling low-cycle fatigue behavior as that of the re-ference heat. However, the three materials show improved fatigue strength during tensile hold-time conditions where significant creep occurs. The fatigue properties determined in the present study for the different heats of steel are consistent with the observed mi-crostructural features. Finally, the creep-fatigue properties of the heats as well as the microstructural observations are discussed in terms of a damage-rate approach re-cently developed by the authors.     

Effect of microstructure on creep and creep crack growth behaviour of titanium alloy

Creep and creep crack growth behaviour of a near α titanium alloy has been investigated at 600°C which is affected by primary α content. The alloy was heat treated at different temperatures so as to obtain different levels of equiaxed primary α in the range from 5 to 24 %. Constant load creep tests were carried out at 600°C in the stress range 250 to 400 MPa till rupture of the specimens. Creep crack growth tests were carried out at 600°C. Creep data reveals with increase in primary α content leads to creep weakening. On similar lines maximum creep crack growth resistance is associated with the alloy with lowest primary α content. Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary β / transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary β / transformed β (matrix) interfaces in the interior of the specimen.     

The Effect of Secondary Gamma-Prime on the Primary Creep Behavior of Single-Crystal Nickel-Base Superalloys

Some second-, third-, and fourth-generation single-crystal Ni-base superalloys (i.e., Re-containing alloys) have demonstrated the propensity for excessive primary creep at intermediate temperatures. This behavior has been attributed to the presence of secondary gamma-prime precipitates in the gamma channels as well as on the Re content of the alloys. This investigation examined creep behavior for a common first-generation alloy, PWA 1480, a common second-generation alloy, PWA 1484, as well as a modified first-generation alloy, PWA 1480, with 3 wt pct rhenium added. In addition, two different aging heat treatments were given to each alloy to either precipitate or prevent the formation of fine (nanometer-scale) secondary gamma-prime in the gamma channels. The intermediate creep properties and tensile properties of the alloys were determined for both conditions. The microstructures of these samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD), and then the role of the fine-scale microstructure and the alloy composition on the primary creep deformation was determined.