Thermodynamically Based Prediction of the Martensite Start Temperature for Commercial Steels

A thermodynamic method for predicting the martensite start temperature of commercial steels is developed. It is based mainly on information on M _s from binary Fe-X systems obtained from experiments with very rapid cooling, and M _s values for lath and plate martensite are treated separately. Comparison with the experimental M _s of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of M _s from commercial steels.     

Chi-Carbide in Tempered High Carbon Martensite

Martensite in an Fe-1.22C alloy was tempered at 523, 573, and 623 K and examined by transmission electron microscopy (TEM) and Mössbauer effect spectroscopy (MES) to identify the morphology and type of carbide formed at the beginning of the third stage of tempering. Carbides formed in three morphologies: on twins within the martensite plates, in the matrix of twin-free areas of the martensite plates, and along the interfaces of the martensite plates. Chi-carbide (χ), as identified by selected area diffraction (SAD), was associated with each carbide morphology in specimens tempered at 573 K. Cementite (θ) together with chi-carbide was observed in specimens tempered at 623 K. Small amounts (about 2 pct) of retained austenite were observed by MES of specimens tempered at 523 K. The transformation of the 25 pct retained austenite in as-quenched specimens was related to the χ-carbide formed at the martensite plate interfaces during tempering. The MES results also show the presence of χ-carbide in the specimen tempered at 523 K and yields parameters indicative of a mixture of χ and θ carbides for the specimens tempered at 573 K and 623 K. MES measurements of the magnetic transition temperatures of the carbides show diffuse transitions but suggest thatχ is the dominant carbide in the tempering temperature range examined.     

High Strength High Carbon Low Alloy Pearlite-Ferrite-Tempered Martensite Steels

High strength multiphase steels have been developed consisting of combination of pearlite, tempered martensite and small amount of ferrite, by suitable heat treatment of a high carbon low alloy rail steel (0.7 % C). The desired microstructure has been obtained by holding fully homogenized steel in pearlitic range for small durations followed by water quenching and subsequent tempering at 773 K for 18 h. Variation in mechanical properties has been studied with the change in volume fraction of different phases. Yield strength, ultimate tensile strength and elongation are observed to be in the range of 500–1,000 MPa, 900–1,185 MPa and up to 16.8 %, respectively. Continuous and discontinuous yielding along with substantial work hardening has been explained as a function of tempered martensite content.     

Engineered Coatings for Ni Alloys in High Temperature Reactors

Alloy 617 is a candidate material for the intermediate heat exchanger of the He-cooled very high temperature reactor. At target temperatures ≥1223 K (950 °C), low level impurities in the gas stream may cause carburization, decarburization, and/or oxidation of 617 with deleterious effects on its mechanical properties. The chromia scale formed naturally by 617 does not provide adequate protection in the expected environment. Alpha alumina offers a greater potential as an effective diffusion barrier with superior stability, but it requires modification of the alloy surface. This work explores two approaches to surface modification based on aluminizing, either alone or in combination with FeCrAlY cladding, followed by pre-oxidation to form alpha alumina. Both approaches yield coatings with promising diffusional stability on alloy 617. Initial corrosion studies in impure He environments reveal that the alpha alumina is stable and protects the underlying substrate in both carburizing and decarburizing environments.     

Apparent Morphologies and Nature of Packet Martensite in High Carbon Steels

The apparent morphologies of packet martensite in eight high carbon steels were researched by using optical microscope, scanning electron microscope, and transmission electron microscope. It was found that the apparent morphologies, substructures, and habit plane of packet martensite in high carbon steels are entirely different from that in low carbon steels; the substructures of packet martensite in high carbon steels possess fully twinned structure, while the substructures of individual coarse martensite plates in these steels bear both fully and partially twinned structures. The formation reason for apparent morphologies, substructures and two habit planes （i. e, { 111 }, and { 225}r） of high carbon martensite were discussed in detail.     

低铬合金化TiAl的高温氧化

用热重分析法（TGA）研究了Ti-48Al-(0～4）Cr合金在各种条件下100h内的高温氧化行为,氧化增重△M随TiAl中铬元素添加量的增大而先升后降。Ti-48Al-Cr在各种条件下均表现为耐氧化性最差,原因在于适量铬固溶于TiAl基体中,有助于氧化时钛离子的扩散;铬含量进一步增高时,将有可能引入第三相Ti2CrAl(β2）,易形成较具保护性的Cr2O3,从而抑制氧化反应速度。一般地,低铬合金化后TiAl的耐氧化性下降,但氧化膜粘附性、致密性均有明显提高。TiAl表面氧化物呈分层结构,主相为TiO2与Al2O3。     

Martensite Transformation in Ni-Mn-Ga Ferromagnetic Shape-Memory Alloys

The crystal structure of Ni-Mn-Ga ferromagnetic shape-memory alloys is extremely sensitive to composition. Several martensitic structures including tetragonal (five-layered), orthorhombic (seven-layered), and nonmodulated tetragonal have been observed. Temperature-dependent X-ray diffraction measurements and calorimetry have revealed markedly different transformation behavior in the tetragonal and orthorhombic materials. The orthorhombic material shows a much larger difference between the martensite start and finish temperatures as compared to tetragonal martensite. The difference in transformation character can be explained from a thermodynamic standpoint by including the difference in the strain energy contribution for the two different martensite phases. This article is based on a presentation made in the symposium entitled “Phase Transformations in Magnetic Materials," which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS-MPMD and ASMI-MSCTS Phase Transformations Committee.     

Development of a New Ferrous Aluminosilicate Refractory Material for Investment Casting of Aluminum Alloys

Investment casting is a time-consuming, labour intensive process, which produces complex, high value-added components for a variety of specialised industries. Current environmental and economic pressures have resulted in a need for the industry to improve current casting quality, reduce manufacturing costs and explore new markets for the process. Alumino-silicate based refractories are commonly used as both filler and stucco materials for ceramic shell production. A new ceramic material, norite, is now being produced based on ferrous aluminosilicate chemistry, having many potential advantages when used for the production of shell molds for casting aluminum alloy. This paper details the results of a direct comparison made between the properties of a ceramic shell system produced with norite refractories and a typical standard refractory shell system commonly used in casting industry. A range of mechanical and physical properties of the systems was measured, and a full-scale industrial casting trial was also carried out. The unique properties of the norite shell system make it a promising alternative for casting aluminum based alloys in the investment foundry.     

New Space Morphology and Habit Plane of Low Carbon Martensite

GKraussetal[1]haveobservedthespacemor phologyofmartensiteinas quenchedlowcarbon steelsat750timesmagnificationunderopticalmi croscopeusingserial sectioningexperiment.Speci menswith11μmand18μminthicknesswerere movedbyelectro polishing.Itwasfoundthatth…     

Impulse Excitation Internal Friction Study of Retained Austenite in Ferrous Martensite

Internal friction (IF) measurements were conducted using ferrous lath martensite specimens with different retained austenite fractions. While a distinctive Snoek-Kê-Köster (SKK) peak was detected in the IF spectra, the peak shape did not conform to the peak shape observed for fully martensitic steel. Peak analysis suggests that the peak shape is related to the overlap of the SKK peak with a Finkelstein–Rosin peak, additional Debye-type peak related to the retained austenite.     

High-Temperature Sulfidation of Mo-Nb Alloys

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Numerical Analysis of Distortion due to Inhomogeneous Distribution of Martensite Start Temperature within SAE 52100 Bearing Rings

This paper reports about numerical investigations regarding the spatial distribution of martensite start temperature (M_s) within bearing rings made out of SAE 52100 (100Cr6). Out‐of‐roundness values due to inhomogeneous M_s distribution are calculated by means of FE simulations. In a first step the distribution of M_s is modelled with simple trigonometric functions with different wavelengths and amplitudes of M_s. In addition, more complex distributions of M_s are investigated by means of superposition of different trigonometric functions. Simulations with the commercial FE simulation program SYSWELD^® yield dependencies of out‐of‐roundness values of bearing rings on wavelength and amplitude of M_s. The numerical study is supplemented by experimental investigations concerning the distribution of M_s. Typical scatter‐bands of M_s within a work piece were found to be ± 10 K. Concerning this scatter‐band, different possible distributions of M_s are analysed by Fourier transformation. With the resulting trigonometric functions the out‐of‐roundness values are calculated and compared with experimental data.     

Cu-Cr合金高温氧化行为分析

该文用热分析天平，结合金相、X射线衍射(XRD)、扫描电镜(SEM)和能谱(EXD)研究了Cu—Cr合金在不同温度下的氧化行为。结果表明，Cu—Cr合金在700℃-900℃氧化符合抛物线规律，其最外层氧化膜为CuO，内层为Cu2O和Cr2O3，铬有助于提高合金的抗氧化能力。     

Nb-Ti-Al高温铌合金氧化行为研究

研究了Nb-35Ti-6Al、Nb-15Ti—llAl以及Nb-30Ti-15Al三组合金于900℃和1000℃在空气中的氧化行为，建立了Nb-Ti—A1合金高温氧化动力学模型。研究表明，元素Ti和Al的加入能有效改善合金的抗氧化性能，合金中占相的存在降低了氧的溶解度。同时抑制氧的扩散，因而两相合金Nb-15Ti-llAl和Nb-30Ti-15Al（β＋δ相）抗氧化性能优于单相合金Nb-35Ti-6Al（β相）。     

钼镧合金和TZM合金的高温性能

研究了钼镧合金和TZM合金在1000～1800℃的高温性能和相应的组织.结果表明小于1400℃的情况下,钼镧合金有较高的强度和塑性的综合性能,当温度大于等于1400℃时,其抗拉强度明显降低,同时塑性也有明显的下降.而随着测试温度的提高,TZM合金的抗拉强度降低,但是其塑性升高,这一点和钼镧合金恰恰相反.同时,不管是强度还是塑性,TZM合金较之相同温度的钼镧合金有明显的优势.组织观察表明这两种钼合金在1100℃开始再结晶,一直延续到1550℃,并且其再结晶晶粒都呈现拉长的组织,这明显不同于纯钼再结晶状态下的等轴晶粒.     

高温下几种铜合金流动应力的研究

采用Gleeble-1500热模拟实验机，测定了4种铜合金材料在高温热压缩状态下的流动应力，分析了应变速率、变形程度及变形温度对流动应力的影响规律。通过分析，提出了拟合精度较高的流动应力数学模型，并给出了回归系数值。     

硫氧化环境中高温合金的蠕变—腐蚀交互作用

论述了蠕变—腐蚀交互作用的存在和研究的必要。分别就环境对蠕变变形和蠕变断裂的作用,应力与蠕变对腐蚀的作用的宏观现象及微观机制进行了归纳和评述。     

新型中锰马氏体高强度钢的耐磨性能

 利用销盘式磨料磨损试验和三体冲击磨料磨损试验研究碳质量分数分别为0.12%和0.19%新型中锰马氏体高强度钢的磨损行为,并与Hardox450钢和21C钢进行耐磨性能对比。用磨损失重量表征耐磨性能,利用LOM、SEM和XRD等设备研究材料磨料磨损机制。结果表明,新型中锰钢耐磨性能与Hardox450钢及21C钢相当。在销盘式磨料磨损试验和三体冲击磨料磨损试验中,马氏体高强钢的耐磨性能与材料的硬度呈线性关系,硬度越高,材料耐磨性越好。由于锰的添加,新型中锰钢的优点不仅在于具有锰的固溶强化特性提高耐磨性能,还在于该钢的淬透性几乎与该钢的冷却速度无关,因而该钢具有大规模工业生产的潜力。