Morphology of Upper and Lower Bainite with 0.7 Mass Pct C

There has been an on-going discussion on the difference in formation mechanisms of upper and lower bainite. Various suggestions have been supported by reference to observed morphologies and illustrated with idealized sketches of morphologies. In order to obtain a better basis for discussions about the difference in mechanism, the morphology of bainite in an Fe-C alloy with 0.7 mass pct carbon was now studied in some detail from 823 K to 548 K (550 °C to 275 °C) at temperature intervals of 50 K or less. The work focused on bainite seen to start from a grain boundary in the plane of polish and showing an advancing tip in the remaining austenite. The results indicate that there is no essential difference with temperature regarding the ferritic skeleton of feathery bainite. The second stage of bainite formation, which involves the formation of both ferrite and cementite, was regarded as a eutectoid transformation and the resulting morphologies were analyzed in terms of two modes, degenerate and cooperative eutectoid transformation. There was no sharp difference between upper and lower bainite. Ways to define the difference were discussed.     

Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel

The sequence of eutectoid microstructures, obtained by lowering the temperature of the isothermal transformation, was studied in synthetic steel with 4.12 mass pct Cr 0.88 mass pct C. The results were compared with observations on plain carbon steels with 1.65 and 1.67 mass pct C. In both cases, the main features can be explained as an effect of a lowered temperature on the increasing supersaturation of cementite in austenite and an even stronger effect on that of ferrite. One distinction was a continuous change in the pearlite structure toward a more acicular structure. This structure is named acicular pearlite.     

Influence of Cobalt on Bainite Formation Kinetics in 1 Pct C Steel

The effect of cobalt on bainite kinetics formation in a 1C-1.5Si wt pct steel is investigated. Two laboratory casts were manufactured with no or 2.5Co wt pct. Bainite transformation kinetics at 493 K, 523 K, and 573 K (220 °C, 250 °C, and 300 °C) were measured using dilatometry. Careful control of the alloy composition, in particular with respect to carbon content, allowed unambiguous identification of the expected accelerating effect of Co. This effect was quantified and compared to that of other possible alloying additions. It is shown that Co has an acceleration effect of around 18 to 29 pct (per wt pct added) for bainite formation between 220 °C and 300 °C. Comparison with published data indicates that this influence is orders of magnitude smaller than that achieved through reduction of C, Mn, or Cr. The influence on hardness is quantified and shown to be significant, and possible origins for hardening are discussed.     

Bainite Growth Behavior at Early Stage of Transformation in a High Carbon Silicon-Containing Steel

 The growth behavior at the early stage of bainitic transformation was investigated using optical microscopy, X-ray diffraction analysis and transmission electronic microscopy. The bainite was obtained by isothermal transformation at 200 ℃ only for a short time in a high carbon silicon-containing steel after austenitization at 200 ℃ only for 20 min. Transmission electronic microscopy shows that the bainite appears in the form of plates with a width of about 30 nm, and that the interface of the bainite leading tip is wedge shaped. X-ray diffraction analysis reveals that the bainite plates consist of single ferrite phase, with absence of carbides. The results confirm the occurrence of the moiré which suggests the existence of austenite grain boundaries at the bainite leading tip. Both the lateral growth and longitudinal growth of bainite have weak ability to traverse the lattice-distortion strain fields and austenite grain boundary. The austenite grain boundary impedes the longitudinal growth of the bainite plate, i. e. , the growth of bainite plate stops at the austenite grain boundary. The longitudinal growth of bainite associated with the features of shear mechanism can not completely be in accordance with that of martensitic transformation.     

Grain growth in lr- 0.3 Pct W alloys

The fuel cladding material for radioisotope heat sources operating above 1300°C is currently an Ir-0.3 pct W alloy. The ductility of this material under high-temperature impact accident conditions is reduced by large grain sizes. Knowledge of grain size as a function of time and temperature is required to determine the effects of high-temperature exposure during fabrication, assembly, ground testing, and operation of the heat source. The average grain diameter of three different heats of Ir-0.3 pct W is reported as a function of time at 1583 K (1310°C), 1688 K (1415°C), and 1808 K (1535°C) up to 2000 h. Growth rates were measured both perpendicular and parallel to the sheet surface in longitudinal cross sections of sheet samples. In samples with Th, Al, Fe, Ni, and Rh dopant additions, the grain structure was elongated and grain growth was retarded. Grain size was measured by the line in-tercept method and the data were described by the grain growth equation,d ² -d ₀ ² =kt. An activation energy for grain growth was determined for each heat, in both the perpendicular and parallel directions, and was found to be consistent with the expected activation energy for self-diffusion in the alloy. Abnormal grain growth was not observed within the time and temperature ranges of this experiment. This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

Wear mechanisms in hybrid composites of Graphite-20 Pct SiC in A356 Aluminum Alloy (Al-7 Pct Si-0.3 Pct Mg)

The wear behavior of A356 aluminum alloy (Al-7 Pct Si-0.3 Pct Mg) matrix composites reinforced with 20 vol Pct SiC particles and 3 or 10 vol Pct graphite was investigated. These hybrid composites represent the merging of two philosophies in tribological material design: soft-particle lubrication by graphite and hard-particle reinforcement by carbide particles. The wear tests were performed using a block-on-ring (SAE 52100 steel) wear machine under dry sliding conditions within a load range of 1 to 441 N. The microstructural and compositional changes that took place during wear were characterized using scanning electron microscopy (SEM), Auger electron spectroscopy (AES), energy-dispersive X-ray spectroscopy (EDXA), and X-ray diffractometry (XRD). The wear resistance of 3 Pct graphite-20 Pct SiC-A356 hybrid composite was comparable to 20 Pct SiC-A356 without graphite at low and medium loads. At loads below 20N, both hybrid and 20 Pct SiC-A356 composites without graphite demonstrated wear rates up to 10 times lower than the unreinforced A356 alloy due to the load-carrying capacity of SiC particles. The wear resistance of 3 Pct graphite 20 Pct SiC-A356 was 1 to 2 times higher than 10 Pct graphite-containing hybrid composites at high loads. However, graphite addition reduced the counterface wear. The unreinforced A356 and 20 Pct SiC-A356 showed a transition from mild to severe wear at 95 N and 225 N, respectively. Hybrid composites with 3 Pct and 10 Pct graphite did not show such a transition over the entire load range, indicating that graphite improved the seizure resistance of the composites. Tribolayers, mainly consisting of a compacted mixture of graphite, iron oxides, and aluminum, were generated on the surfaces of the hybrid composites. In the hybrid composites, the elimination of the severe wear (and hence the improvement in seizure resistance) was attributed to the reduction in friction-induced surface heating due to the presence of graphite- and iron-oxide-containing tribolayers.     

预变形上贝氏体组织对GCr15轴承钢球化退火质量的影响

对GCr15轴承钢的预变形上贝氏体组织形态以及不同形变量的预变形上贝氏体组织对球化退火质量的影响进行了分析,并提出了最佳的球化退火工艺。     

Variant Selection in Grain Boundary Nucleation of Upper Bainite

The crystallography of bainitic ferrite nucleated at austenite grain boundaries was studied in an Fe-9Ni-0.15C (mass pct) alloy. The relationship between bainitic ferrite orientations (variants) and grain boundary characters, i.e., misorientation and boundary orientation, was examined by electron backscatter diffraction analysis in scanning electron microscopy and serial sectioning observation. Bainitic ferrite holds nearly the Kurdjumov–Sachs (K-S) orientation relationship with respect to the austenite grain into which it grows. At the beginning of transformation, the variants of bainitic ferrite are severely restricted by the following two rules, both advantageous in terms of interfacial energy: (1) smaller misorientation from the K-S relationship with respect to the opposite austenite grain and (2) elimination of the larger grain boundary area by the nucleation of bainitic ferrite. As the transformation proceeds, variant selection establishing plastic accommodation of transformation strain to a larger extent becomes important. Those kinds of variant selection result in formation of coarse blocks for small undercooling. This article is based on a presentation given in the symposium entitled “Solid-State Nucleation and Critical Nuclei during First Order Diffusional Phase Transformations,” which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati, Ohio under the auspices of the TMS/ASMI Phase Transformations Committee.

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超低碳贝氏体钢生产概述

超低碳贝氏体钢（ULCB）被国际上称为21世纪的新一代钢铁材料，由于ULCB具有相对较低的生产成本，并具有高强度、高韧性、理想的屈强比及优良的焊接性能等，近年来，国内外各钢铁公司都致力于ULCB的开发研制。介绍了ULCB中合金元素的作用机理，并对国内相关厂家生产ULCB的工艺控制及产品性能作了分析。     

Isothermal martensite transformation in a 1.80 Wt Pct C steel

The linear products formed isothermally at 373 K in a 1.80 wt pct C steel (Ms = 346 K) were examined by means of transmission electron microscopy. They were first reported as “black line products” by Greninger and Troiano. The isothermal product was of a thin plate with about 0.5 μm width, and it contained {112}_b transformation twins and revealed a habit plane of {3 15 10}_f. The orientation relationship between austenite and product was close to the Nishiyama relationship. These crystallographic data were in good agreement with those calculated by the phenomenological theory of martensitic transformation. Consequently, the product was determined not to be lower bainite, but isothermal martensite. The black color of isothermal martensites resulted from the fact that they were easily etched by the precipitates of epsilon carbide formed during the isothermal holding.     

An induction heating process with coil design and solutions avoiding coarsening phenomena of Al-6 Pct Si-3 Pct Cu-0.3 Pct Mg alloy for thixoforming

During the thixoforming process, it is very important to obtain a uniform temperature, which affects the uniformity of the solid fraction, throughout the billet. Consequently, a heating method that can provide a suitable temperature profile throughout the billet must be chosen. Other parameters that must be considered include heating time (in order to minimize the total processing time), the level of control, and temperature consistency. An optimal design of the induction coil has been identified that best meets these criteria. In a previous study, the theoretical optimal coil design was verified through the FEM simulation of the induction heating process by using a general purpose finite element analysis code, ANSYS. So, in this study, the suitability of the coil design was also demonstrated by conducting induction-heating experiments. The optimal reheating conditions to apply the thixoforming (thixoforging and semisolid die casting) process were investigated by varying the reheating time, the holding time, the reheating temperatures, the capacity of the induction heating system, and the size of the adiabatic material. The final holding time was observed to be the most important factor in obtaining a fine globular microstructure and to prevent coarsening in the three-step reheating process.     

Formation mechanism of bainitic ferrite in an Fe-2 Pct Si-0.6 Pct C alloy

The bainite transformation at 723 K in an Fe-2 pct Si-0.6 pct C alloy (mass pct) was investigated with transmission electron microscopy (TEM) and quantitative metallography to clarify the growth mechanism of the ferritic component of bainite. In early stages of transformation, the bainitic ferrite was carbide free. The laths of bainitic ferrite within a packet were parallel to one another and separated by carbon-enriched retained austenite. The average carbon concentration of the bainitic ferrite was estimated to be 0.19 mass pct at the lowest, indicating that the ferrite was highly supersaturated with respect to carbon. The laths did not thicken during the subsequent isothermal holding, although they were in contact with austenite of which the average carbon concentration was lower than the paraequilibrium value. In the later stage of transformation, large carbide plates formed in the austenite between the laths, resulting in the decrease in the carbon concentration of the austenite. Subsequently, the ferrite with a variant different from the initially formed ferrite in the packet was decomposed for the completion of transformation. The present results indicate that the bainitic ferrite develops by a displacive mechanism rather than a diffusional mechanism. Formerly Graduate Student, Kyoto University, Kyoto 606-01, Japan This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

包钢贝氏体钢冶炼中氢的控制研究

文章介绍了钢中氢对钢质量的影响以及对贝氏体钢的危害,并通过生产实际试验数据,研究包钢150 t转炉冶炼贝氏体钢时,钢中氢的来源极其过程w[H],并提出了有效控制贝氏体钢中w[H]的措施及效果。