Carbides in high-speed steels containing silicon

The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M₂C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undis-solved M₆C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M₆C carbides. The amount of MC and M₂C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.     

Structure-property relationships in bainitic steels

Bainitic microstructures can be produced in a variety of steels either as a result of a deliberate attempt to achieve a particular combination of strength and toughness or in response to welding during fabrication. In addition, such microstructures can offer advantages in terms of their resistance to creep or fatigue deformation or susceptibility to hydrogen embrittlement. The relationships among chemical composition, processing, microstructure, and the mechanical properties will be reviewed. Particular emphasis will be placed on recent advances in alloy design. These developments rely on an improved understanding of the mechanisms of bainitic transformation, and the relevance of recent research in this area to the design of new alloy systems will be discussed. Bainitic structures which arise during welding can have a significant and sometimes detrimental effect on the fracture toughness of the welded joint. The fracture toughness of bainitic microstructures in so-called “local brittle zones” in the heat-affected zone and in weld metals and the importance of controlling the bainitic morphology will be considered and the transformation mechanisms discussed. In summary, the aim of this review will be to indicate the prospects for improved microstructural control of structure-property relationships in steels containing a significant proportion of bainite. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

低碳贝氏体钢中的时效析出行为

测定了Cu-Nb-Cr-Mo系低碳贝氏体钢的时效硬度变化行为,在Gleeble-1500热模拟试验机上模拟了各钢种的蠕变行为,利用金相显微镜、TEM研究了不同钢种的微观组织与沉淀析出.结果表明,含铜钢在不同温度时效时会发生明显的时效硬化效应,在不同温度的蠕变曲线上会出现平台,平台现象是由于蠕变过程中出现了第二相析出,平台出现意味着开始了第二相析出,平台结束则析出过程结束.     

Grain structure of bainitic and martensitic steels

In contrast to austenitic and ferritic microstructures, in the case of bainite and martensite the identification of that structural unit representing a grain is less straightforward. There is a general agreement in the literature that the γ → α-transformation follows the Kurdjumow-Sachs relationship (KSR). For the complex microstructures resulting, however, the investigations include practically exclusively lath-type structures. These relatively simple structures may be described as follows. Within the original austenite grain there are several lath packets with only a few variants of the KSR occuring within one bainite or martensite packet. Opinions diverge with regard to the relative orientations within a packet and thus the type of grain boundaries occurring. Most authors, however, agree that a micro-structural unit – such as the grain in the ferrite structure – which determines both the yield strength and toughness properties, does not exist in lath-type structures, and that the yield strength mainly depends on the lath dimensions. The toughness properties, especially the transition temperature, are governed by the packet size, and possibly by the width of the co-variant packet.     

硅含量对低碳贝氏体相变动力学和性能的影响

摘要：设计了3种不同Si含量的低碳贝氏体钢,进行了不同温度下等温淬火热处理热模拟实验、X-射线衍射和拉伸实验等,研究Si含量对贝氏体相变动力学和贝氏体钢性能的影响。结果表明,随Si含量增加,贝氏体相变量降低,动力学方程参数b减小,n增大。另外,随贝氏体相变温度增加,参数b减小,n增大。而且在较低相变温度下,因Si含量不同造成参数b和n的值的差异更大,说明低温下Si含量对贝氏体相变动力学的影响更大。此外,Si含量越低,因相变温度不同造成贝氏体相变孕育期和完成时间的差异逐渐增大,表明随Si含量的增加,Si对贝氏体相变动力学的影响降低。最后,随着Si含量的增加,试样抗拉强度和伸长率均逐渐增加。研究结果为优化低碳贝氏体钢成分设计、调控其力学性能提供了参考。     

我国中低碳贝氏体钢的发展

综述了中，低碳贝氏体钢在我国的发展，展现了中，低碳贝氏体钢广泛的应用前景。     

含铜取向硅钢磁性能波动成因分析

含铜取向硅钢是一种成本低、成品率高的新型CGO取向硅钢.其制备工艺明显区别于主流的低温加热渗氮高磁感（HiB）取向硅钢,其产品的磁性能波动范围显著高于低温渗氮钢.本文对实际生产中收集到的一些含铜CGO钢成品板中的组织与磁性能进行研究分析,尝试建立不同组织和磁性能相互间的对应关系,并对磁性能波动现象进行分析.分析结果表明,含铜CGO钢成品板组织与抑制剂有明确的对应关系,而晶粒尺寸与Goss晶粒取向度并不完全呈对应关系.同时对热轧板中的异常组织进行了深入研究,认为热轧板表层脱碳区和中心层粗大形变晶粒的存在,直接影响了抑制剂的分布,导致最终成品板中组织和磁性能的波动.     

Martensite and retained austenite in hot-rolled,low-carbon bainitic steels

The microstructure and occurrence of a microconstituent, consisting of martensite and retained austenite in hot-rolled plates of low-carbon bainitic steels was studied by electron microscopy and microprobe analysis. The results of the studies showed that the formation of the martensite-austenite constituent is controlled by the composition of the steel and by the cooling rate of the plates following hot-rolling. The mechanisms involved in the formation of the martensiteaustenite constituent are discussed.     

CCT Curves of low carbon Mn-Si steels and development of water cooled bainitic steels

CCT curves of Mn-Si steels with different manganese contents or carbon contents were determined. The results show that the transformation range of bainite can be separated from that of ferrite when the manganese content approaches a certain content,and the incubation period of ferrite increases more significantly than that of bainite transformation with the increase of carbon content in Mn-Si steels. Furthermore,water-cooled bainitic steels without adding expensive alloying element were developed. Granular bainite was obtained when a bar with diameter of 300mm was cooled by water,and a mixed microstructure of granular bainite and martensite was obtained in water-cooled plate with thickness of 40mm. The developed water-cooled bainitic steels containing no expensive alloying element showed a good combination of strength and toughness. The tensile strength,yield strength,and toughness (AKU at -20℃) of bar with diameter of 300mm after water cooling were higher than 850MPa,620MPa,and 65J,respectively,and those of plate with thickness of 40mm after water cooling were higher than 1000MPa,800MPa,and 50J,respectively.     

Mechanism of bainitic transformation in compacted graphite cast irons

Samples of unalloyed compacted graphite cast iron (CGI) have been thermally treated to obtain a bainitic structure. Heat treatments consisting of various holding times at two different austenitizing temperatures and two different austempering temperatures have been carried out followed by metallographic observations of the resultant structures by scanning electron microscopy (SEM). The formation of bainite is discussed in terms of the temperature difference (undercooling) between the austenitizing temperature and austempering temperature, and the subsequent development of the phases present to bainite is related to the carbon concentration gradient caused by compacted graphite acting as a sink for carbon. This favors the final stages of the transformation. A hypothesis for the bainitic transformation mechanism in CGI's is thus proposed.     

Laws of the deformation-induced structural transformation in bainitic steel

The evolution of the defect and carbide subsystems of steel with a bainitic structure during deformation by compression is quantitatively analyzed by transmission electron diffraction microscopy and X-ray diffraction. The strain dependences of the parameters of a dislocation substructure and a carbide phase are determined, and the possible causes of stages in their changes are discussed.     

Isothermal studies of bainitic and martensitic transformations in some low alloy steels

Ohne Zusammenfassung     

Prediction of transformation textures in steels

Both discrete and continuous mathematical formalisms are employed to simulate texture evolution during the γ-to-α transformation in steels. Five f.c.c. NiCo alloys with different stacking fault energies (SFE's) were previously cold rolled to four reductions (40, 70, 90 and 95%) and their textures characterized by the orientation distribution function (ODF) method. The corresponding b.c.c. transformation textures are calculated from these experimental textures according to three different orientation relationships. The ODF's derived from the Bain relation are much sharper than the ones deduced from the Kurdjumov-Sachs (KS) or the Nishiyama-Wassermann (NW) relations, although the general trends of the three families of textures are similar. Ferrite textures determined on controlled rolled steels, heavily deformed in the unrecrystallized γ region, agree reasonably well with the b.c.c. textures calculated, according to the KS relationship, from the NiCo alloy with similar SFE. The two major ferrite components, namely the {332} 〈113〉 and {113} 〈110〉, are shown to originate from the three main orientations of cold rolled f.c.c. material, i.e. the {112} 〈111〉 (Cu), {110} 〈112〉 (Bs) and {123} 〈634〉 (S). Such ferrite formation from heavily deformed austenite follows the KS relationship without any variant selection. By contrast, the texture of martensite produced from deformed austenite involves significant amounts of selection.     

Effect of ausforming on isothermal bainitic transformation and microstructure

Deformation affects the microstructure and morphology of the parent austenite, which affects the subsequent bainite transformation. The effects of ausforming on bainite transformation and microstructure were investigated by means of thermal simulation experiment, TEM and SEM etc. Different deformation temperatures and deformation strains were designed. The amount of bainitic transformation during isothermal holding and the volume fraction of retained austenite at room temperature were analyzed. The results show that the isothermal bainitic transformation is promoted by the deformation at 300?? and 400??. Moreover, the lower deformation temperature leads to larger amount of bainite. In addition, the volume fraction of retained austenite increases with the increase of the deformation strain, and more retained austenite can be obtained by decrease the deformation temperature. It indicates that deformation at lower temperature contributes to the mechanical stabilization of austenite.     

The influence of warm rolling on the fracture toughness of bainitic steels

The effect of warm rolling on upper bainitic structures is to promote the development of a spherodized carbide structure and a clearly defined dislocation substructure. An investigation of the mechanical properties of as transformed and warm rolled structures in SAE 4340 steel revealed that the microstructural changes result in an increase in the strain to fracture and in fracture toughness for the plate thickness studied in this work. These factors are related using a simple model based on the critical strain to fracture. The detailed microstructural features are incorporated by examining the strain to cause void nucleation and growth at carbide particles in terms of their size shape and volume fraction.     

Bainite transformation temperatures in high-silicon steels

The bainite transformation temperatures of eight high-silicon steels were determined metallographically. The bainite start (B _s ) temperatures, which define the highest temperature at which bainite can form, all lay below the T ₀ loci, where ferrite and austenite of the same chemical compositions have identical free energy. The established method of calculating B _s temperatures gave reasonable agreement with the experimental results. Careful study of the isothermally reacted samples revealed that Widmanstätten ferrite and bainite could both be observed, even at the beginning of the transformation, at around the B _s temperature. On the other hand, the lower bainite start (LB _s ) temperatures of these steels were found to be very close to the martensite start (M _s ) temperatures. Silicon is considered to be responsible for depressing the LB _s temperature by retarding the formation of cementite. The coformation of upper and lower bainite near the LB _s temperature is also confirmed. The results indicate that the displacive formation mechanism of bainite is sustainable.