The role of grain boundary migration in reheat cracking

Prior austenite grain boundary (PAGB) reheat cavitation has previously been shown to be controlled by a fine dispersion of incoherent PAGB particles and by the concentration of the overall deformation into deformation zones adjacent to the PAGB. A series of low-alloy baintic CrMoV steels have been investigated to establish the microstructural basis for PAGB deformation zones. It is shown that the fundamental cause of reheat cavitation in low alloy bainites is stress induced Grain Boundary Migration, which initiates at temperature immediately upon application of a load. The migrating boundary sweeps out a volume of material that is both particle and dislocation-free, relative to the secondary hardened grain interiors. This creates a mechanically weak zone at the PAGB. The overall strain during deformation is subsequently concentrated into these regions, producing extensive plastic deformation leading to failure by cavity growth and crack propagation within the PAGB deformation zone. The presence of large, stable carbides in high chromium CrMoV steels prevents PAGB migration and thus eliminates the PAGB deformation zones. Also matrix carbide coarsening in the high chromium steels leads to an increase in matrix recovery rates, which further permits more uniform deformation of the material.     

Grain boundary void nucleation in astroloy produced by room temperature deformation and anneal

Dyson and co-workers have shown that the creep life of a nickel base superalloy can be greatly shortened if the material is strained at room temperature before the creep test is carried out. They found that a prestrain followed by a short annealing time produces small grain boundary cavities, and it is the presence of these prenucleated voids which so seriously degrades service life at elevated temperatures. The present work explores the relationship between microstructure and prestrain void nucleation. Samples of the nickel base superalloy astroloy were given various heat treatments which led to significantly different microstructures. It was found that voids resulting from a prestrain-anneal treatment form preferentially at the ends of carbides on grain boundaries oriented roughly parallel to the prestrain tensile axis or rolling direction. Void spacing in the various microstructures is proportional to (but larger than) carbide spacing. The growth of these cavities during annealing is attributed to the presence of tensile residual stresses arising from the difference in deformability between grain boundary regions and the relatively soft matrix. Formerly Postdoctoral Fellow with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL Formerly Postdoctoral Fellow with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL     

Novel hafnium containing steels for power generation

Abstract

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for grain boundary cavities and microcracks. The formation of M₂₃C₆ carbides as grain boundary precipitates can also lead to grain boundary chromium depleted zones which are susceptible to corrosive attack. Such precipitates are the causing loss of creep life in the later stages of creep because of their very high coarsening rate. Through Monte Carlo based grain boundary precipitation kinetics models, combined with continuum creep damage modelling it is predicted that improvements in creep behaviour of power plant steels can be achieved by increasing the proportion of MX type particles. Studies of a Hf containing steel have produced improvements in both creep and corrosion properties of 9%Cr steels. Hf has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study its effect on precipitation. Two new types of precipitates are formed, Hf carbide, (an MX type precipitate) and a Cr–V rich nitride, with the formula M₂N. The Hf carbide particles were identified using convergent beam diffraction techniques, and micro-analysis. The nanosized particles are present in much higher volume fractions when compared to VN volume fractions in conventional power plant ferritic steels. Furthermore it is confirmed that the Hf causes the removal of M₂₃C₆ grain boundary precipitates. This has led to an increased concentration of Cr within the matrix, reduced chromium depleted zones at grain boundaries, and increased resistance to intergranular corrosion cracking.     

Combined matrix/boundary precipitation strengthening in creep of Fe-15 Cr-25 Ni alloys

High temperature creep behavior of carbide precipitation strengthened Fe-15 Cr-25 Ni alloys with different carbon content have been investigated. Grain boundary carbides obstruct dislocation annihilation at the grain boundary and, therefore, increase the dislocation density near the grain boundary. This gives rise to formation of a hard grain boundary region and significantly increase creep resistance of the alloy. The grain boundary precipitation strengthening and combined matrix/boundary strengthening are modeled following the concept of hard-soft composite structure, and a unified creep equation is derived by taking account of back stress from intergranular carbide particles, “boundary obstacle stress”. The models and analysis show that grain boundary precipitation strengthening is predominant for soft matrix but decreases with the increase of matrix strength, indicating the existence of coupled matrix/boundary strengthening.     

Substructure and strengthening mechanisms in 2.25 Cr-1 Mo steel at elevated temperatures

The substructures of thermally aged, creep deformed and fatigued 2.25 Cr-1 Mo steel have been studied using optical and transmission electron microscopy. In agreement with earlier work, the substructure of the proeutectoid ferrite was found to be very stable when exposed to thermal aging or creep deformation. This stability is explained based on the tendency of molybdenum atoms to form pairs in the ferrite matrix. Nucleation and growth of additional carbide particles during creep testing was not observed. The results of these creep tests and those of Klueh have been interpreted on the basis of Mo pair stability and the affinity between molybdenum and carbon. Fatigue tests at 866 K, however, did produce a fine Mo_u2C precipitate which contributed to secondary cyclic hardening in tests lasting longer than 200 h. The alloy was found to undergo early cyclic hardening followed by abrupt softening within the first tens of cycles.     

Assessment of creep and rupture behavior of 2.25Cr-1Mo steel — A strain based approach and its limitation

Strain based approach for reliable assessment of creep behavior is employed for creep data generated for different microstructural conditions as well as obtained from literature for 2.25Cr-1Mo steel. The influence of thermal ageing and pre-strain on the shape of creep curves has been studied. Analysis of data revealed that irrespective of initial microstructures, thermal ageing increases the tendency to soften but not the pre-strain. Softening due to carbide coarsening thus appears to be the dominant mechanism of creep. Based on this, a strain based approach for creep life assessment was developed. A fairly accurate prediction of creep life up to 5% creep is achieved using eight materials constants extracted from the creep curves of steel having similar initial microstructure. The limitation and reliability of the approach used to assess creep behavior of service-exposed steels have been discussed.     

Mechanisms of creep crack growth in 1 wt% antimony-copper: Implications for fracture parameters

A copper alloy with 1% (by weight) antimony was used as a model material and tested at 400°C to study the mechanisms of creep crack growth. At this temperature, the creep deformation in this material was dominated by secondary and tertiary creep. The expression for estimating C_t (a crack tip parameter for creep conditions) in a compact type specimen used for testing was modified to include tertiary creep deformation. Extensive damage characterization was conducted on tested creep crack growth specimens using optical metallography and scanning electron microscopy. The following observations were derived from the test results. The creep crack growth rates correlated with C_t only when intense cavitation damage was restricted to a region approximately 1.3 mm in size near the crack tip and no crack branching occurred. It was observed that the average diameter, areal density, and percent of grain boundary area cavitated decreased as function of distance from the crack tip. From these results it is argued that simultaneous nucleation and growth of cavities occur on grain boundary facets during the creep crack growth process. Percent grain boundary area cavitated is proposed as the most reasonable measure of creep damage. The critical amount of damage for crack extension appears to depend on the magnitude of the C_t parameter.     

不同氧化条件下 Cr35 Ni45 Nb合金组织特征及其与持久寿命的关系

以Cr35Ni45Nb合金为研究对象,进行不同气氛条件下高温时效及高温持久试验,并结合扫描电镜及定量电子探针对样品表面及横截面的观察分析,系统研究不同氧化条件下Cr35Ni45Nb合金组织演变与持久寿命的关系.结果表明:随着处理时间的延长,管材边缘及内部组织均逐渐发生变化.氧化序列体现为先是在样品边界不同地点形成不连续的氧化膜,随后氧化膜连成一体形成连续氧化膜,同时贫化区也逐渐形成;由于温度波动,氧化膜破裂脱落,而在氧分压较高的时效环境中,氧化膜得不到及时修复,使得贫化区晶界中出现内氧化.此外,在空气中氧化与在真空充氩气石英管中持久试验结果表明,较薄的连续氧化膜有助于提高合金的蠕变性能,而随着高温时效时间的延长,边界贫化区逐渐形成,氧化膜破裂及内氧化的发生,使得合金的蠕变性能逐渐弱化.      

Effect of carbide precipitation on the creep behavior of Alloy 800HT in the Temperature Range 700 ° to 900 °

The creep behavior of alloy 800HT was studied at 700 °, 800 °, and 900 ° under stresses ranging from 30 to 170 MPa. Samples that were tested in the as-quenched condition after solution treatment exhibited longer creep life than those that were overaged before testing. This difference in creep life was found to increase at lower creep stresses at a given temperature. This phenomenon is attributed to the precipitation of M₂₃C₆carbides during the early stages of creep, which strengthen the material by exerting threshold stresses on moving dislocations and thereby reducing the creep rate. A model is developed to describe the influence of carbide precipitation during creep on the behavior of the material under different creep temperatures and stresses. Comparison with the experimental results shows that the model gives accurate predictions of the creep behavior of the material in the range of stresses and temperatures used in the present study. In addition to its predictive value, the model is useful in understanding the factors that affect the creep behavior of materials when precipitation of hard phases is taking place during creep. The strengthening effect of particle precipitation during creep, as represented by the value of the threshold stress, is shown to be a complex function of the supersaturation of the matrix, the applied creep stress, and the test temperature.     

焊后热处理对07MnCrMoVR钢热影响粗晶区-20℃冲击韧性的影响

采用光学显微镜、扫描显微镜、透射电镜和冲击韧性试验机对07MnCrMoVR钢热影响粗晶区在460～660℃ 2h焊后热处理工艺下的组织性能进行了分析。结果表明,随着焊后热处理温度的升高,焊缝热影响粗晶区-20℃冲击韧性呈现先降低再升高的现象。焊后热处理钢在580℃和620℃出现再热裂纹倾向,冲击试样为脆性断口,解理断裂,沿着晶界出现了微裂纹,主要是因为碳化物沿着晶界析出并长大弱化了晶界的结合能,导致低温冲击韧性出现降低。≥620℃焊后热处理,07MnCrMoVR钢出现再结晶的现象,位错消失,铁素体晶粒合并长大使其低温冲击韧性又重新升高。该钢最优焊后热处理为460～500℃ 2 h。     

碳化物对热连轧403Nb钢位错组态及蠕变行为的影响

对热连轧403Nb钢进行了蠕变试验,并对其蠕变前后的组织进行了研究,结果表明:403Nb钢在轧制过程中发生了动态回复及动态再结晶,组成相中,Cr23C6主要为二次碳化物,而NbC则主要源于一次碳化物。在连轧过程中,一次碳化物通过阻碍位错滑移制约动态回复,其分布影响回复后的组织结构。蠕变期间,晶内碳化物阻碍位错运动,晶界碳化物"钉扎"晶界,阻碍晶界滑动,由此提高钢的蠕变抗力。     

The effect of phosphorus on the microstructure and creep properties of 214%Cr1%Mo steel

Controlled additions of the impurity element phosphorus have been made to a base melt of $\text{2}\text{1}\text{4\%}\text{Cr1\%Mo}$ steel, heat treated to produce microstructures typical of those encountered in both service components and the heat affected zones of weldments. Using anodic dissolution to extract carbide phases and subsequent analysis by X-ray diffraction it is shown that the effect of phosphorus is to enhance the development of M₆C carbide precipitates in the initial microstructures and/or during the subsequent high temperature exposure of creep testing. This reaction is at the expense of two of the principal microstructural features on which the creep strength depends, namely the fine dispersion of molybdenum carbide (Mo₂C) precipitates and molybdenum in solid solution. As a consequence, creep rates increase and times to failure decrease with increasing phosphorus content and the desirability of reducing the phosphorus concentration to a minimum is emphasised. By studying steels of the same nominal composition but with different deformation characteristics by virtue of the varying impurity levels, a rationalisation of creep behaviour in terms of the above strengthening mechanisms has been possible. The implications for life assessment procedures are discussed.     

Predictability of Charpy impact toughness in thermomechanically control rolled (TMCR) microalloyed steels

Abstract

A number of empirical equations, based primarily on average metallographic grain size and carbide thickness, exist to predict the Charpy impact transition temperature in steels. Some of these commonly used equations, successful for normalised steels, have been shown in this paper to be inadequate for predicting the transition temperature for thermomechanically control rolled (TMCR) microalloyed steels. Thermomechanical control rolling can produce clusters of small grains with low angle grain boundaries, i.e., the steel shows mesotexture. The cleavage facet size in TMCR steels has been found to be significantly larger than the optical grain size and it was also observed that individual cleavage facets can be comprised of multiple grains. In contrast, it was observed for a heat treated steel that the facet size matches the optical grain size and that individual facets consist of single grains. It is concluded that in TMCR steels, the average microstructural unit experienced by the crack front is larger than the optical grain size because mesotexture causes groups of closely orientated grains to be treated as single 'effective' grains. This paper shows that the 50% and 27 J impact transition temperatures can be predicted for TMCR steels using the existing equations if mesotexture and grain boundary carbide size are taken into account.     

The sintering of creep-induced cavities in a low alloy ferritic steel (1Cr1Mo0.75V)

Sintering heat treatments at temperatures in the range 1173 to 1273 K have been undertaken in an atmospheric pressure of argon gas to remove grain boundary cavities within a lCrlMo0.75V ferritic steel (Durehete 1055) following creep rupture after service operation at 838 K. High sensitivity density measurements and optical metallography have been used to monitor the progressive sintering of cavities. The results are discussed in relation to existing models which describe the sintering of cavities by grain boundary diffusion.     

Improving the creep properties of 9Cr-3W-3Co-NbV steels and their weld joints by the addition of boron

New ferritic steels with a controlled addition of boron have been developed recently for ultrasuper-critical fossil power plants. These steels possess excellent creep resistance compared to conventional steels like P91, P92, P122, etc., and this has been attributed to the delay in coarsening of the carbides during creep owing to partial replacement of carbon by boron in these carbides. However, the susceptibility of the weld joints of the boron-containing ferritic steels to type IV cracking, which significantly brings down the rupture life of the weld joints, has not been investigated so far. In the present work, the creep properties of recently developed 9Cr-3W-3Co-NbV steels with boron contents varying from 47 to 180 ppm and of their weld joints have been studied. Creep tests were carried out at 923 K in the stress range of 140 to 80 MPa. Specimens were examined for particle coarsening using field-emission scanning electron microscopy, and the boron content in the precipitates was estimated using field-emission auger electron spectroscopy (FE-AES). The grain size of the parent metal and the heat-affected zone (HAZ) were estimated using electron backscattered pattern (EBSP) imaging. Results showed that the creep properties of the steels with 90 and 130 ppm boron and of their weld joints are superior to those of the P92 steels and its weld joints. Further, no weld joints exhibited type IV cracking. No significant coarsening of the carbides was observed, not only in the parent metal but also in the HAZ of the steels with ≥90 ppm of boron. In addition to the delay in carbide coarsening, the large prior-austenite grain size of the parent metal and the absence of a conventional fine-grained HAZ (FGHAZ) in the weld joints also seem to have a beneficial effect on improving the creep properties of these steels and their weld joints.     

Creep Performance Modeling of Modified 9Cr-1Mo Steels with Oxidation

In this study, the deformation-mechanism-based true-stress (DMTS) creep model is modified to include oxidation influence on the long-term creep performance of modified 9Cr-1Mo steels. An area-deduction method is introduced to evaluate oxide scale formation on the creep coupons, which is incorporated into the DMTS model formulated based on intragranular dislocation glide (IDG), intragranular dislocation climb (IDC), and grain boundary sliding (GBS) mechanisms, in modifying the true stress. Thus, the modified DMTS model can not only describe the creep curve, but also predict the long-term creep life and failure mode, which is shown to be in good agreement with the creep data generated in the authors’ laboratory as well as by the National Institute for Materials Science (NIMS) of Japan for long-term (> 104 hours) creep life prediction on Grade 91 steels. In particular, the predictability of the model is demonstrated in comparison with the Larson–Miller parameter method. In addition, the modified DMTS model provides quantitative information of mechanism partitioning, insinuating the failure mode via intragranular/intergranular deformation. Therefore, it has advantages over the empirical models in providing physical insights of creep failure, which can be useful to material design for performance optimization.     

High temperature creep damage in steels and superalloys

The nucleation and growth of cavities was examined in steel bicrystals (Fe-3%-Si, X 8 CrNiNb 16 13) and in the ODS superalloy Inconel MA 754 (Inconel MA 754 (78% Ni; 20% Cr; 0.5% Ti; 0.3% Al; 0.6% Y₂O₃). Cavity density distributions were measured on metallographic sections and on cleaved grain boundaries as a function of time, strain, temperature and stress. Nucleation and growth laws were obtained by evaluating the distributions with appropriate models. For the fcc and bcc bicrystals, it was found that cavities nucleated continuously at sulfide and carbide particles during creep. They grew by grain boundary diffusion. But the growth rate was delayed with increasing creep strain due to cavities which nucleated in the surroundings of existing cavities. For the ODS alloy, however, many round cavities preexisted on quasi-boundaries consisting of the aggregate of coarse oxide and carbide particles. They grew initially by diffusion, but with increasing creep time (cavity size), the growth mechanism switched from growth controlled by grain boundary diffusion to growth controlled by power law creep. Implications for life predictions are discussed.     

铣削高锰钢用高性能硬质合金铣刀材料的研制

通过对硬质合金的材料成分、碳化物晶粒度、复杂碳化物相间的交互作用分析,并辅以合适匹配的刀具参数进行分析和研究,优选出固溶体、黏结相和晶粒度合理配比,研制出了新型硬质合金D16刀片材料。结果表明以该材料制成的硬质合金铣刀性能良好,完全可以替代德国维迪亚TN35M铣刀,经试验,其寿命超过进口铣刀一倍,超过国内同类刀片材料(自贡767,YC30S)寿命4~6倍,是一种比较理想的铣削高锰钢用高性能硬质合金铣刀材料。     

Grain boundary sliding in the presence of grain boundary precipitates during transient creep

A constitutive rate equation for grain boundary sliding (GBS), in the presence of grain boundary precipitates, is developed. Langdon’s GBS model is modified by incorporating physically de-fined back stresses opposing dislocation glide and climb and by modifying the grain size de-pendence of creep rate. The rate equation accurately predicts the stress dependence of minimum creep rate and change in activation energy occurring as a result of changing the grain boundary precipitate distribution in complex Ni-base superalloys. The rate equation, along with the math-ematical formulations for internal stresses, is used to derive a transient creep model, where the transient is regarded as the combination of primary and secondary stages of creep in constant load creep tests. The transient creep model predicts that the transient creep strain is dependent on stress and independent of test temperature. It is predicted that a true steady-state creep will only be observed after an infinitely long time. However, tertiary creep mechanisms are expected to intervene and lead to an acceleration in creep rate long before the onset of a true steady state. The model accurately predicts the strain vs time relationships for transient creep in IN738LC Ni-base superalloy, containing different grain boundary carbide distributions, over a range of temperatures.     

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M₂₃C₆ type carbides, having high chromium content, and M₆C type carbides, having high molybdenum content, co-exist on the grain boundaries. M₂₃C₆ type carbides, however, are quantitatively predominant. Furthermore, M₆C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.