The synthetic isothermal martensitic transformation in high carbon Fe-Ni-C steel

For the analysing of the results from earlier hydrostatic isothermal tests a new athermal test series was conducted. Suitable amounts of martensite were brought into the specimens by quenching. From these specimens the amount of martensite, number of martensitic plates per unit area of random section and hardness were measured. Using the measured data, the effects of the mechanical surroundings around and inside the specimen were taken up for consideration. It seems that the mechanical surroundings inside the specimen have an important effect on martensitic transformation. It also seems that there are temperature dependent components in the transformation. It may be that the temperature dependence of nucleation needed plastic deformation and the temperature dependence of relaxations.     

新型高碳马氏体不锈钢的组织与性能

高碳马氏体不锈钢因其高硬度、优异的耐磨性以及适中的耐蚀性,被广泛应用于刀剪行业。主要通过金相,扫描电镜,硬度、冲击韧性、耐磨性能、耐蚀性能以及抗菌性能检测等方法,对新型刀具用高碳马氏体不锈钢6Cr16MoVRE进行微观组织表征与性能研究,并与5Cr15MoV和9Cr18MoV钢对比。研究发现,6Cr16MoVRE的碳化物尺寸细小且分布均匀。相比于另2种材料,6Cr16MoVRE具有高硬度与最佳冲击韧性,良好的耐磨性,优异的耐蚀性。此外,Ag的添加使6Cr16MoVRE具有极好的抗菌性。新型6Cr16MoVRE性能优异,为国内生产高档刀具提供了材料保障,有利于中国刀剪行业转型升级。     

高碳高铬马氏体不锈钢铸锭开裂原因

基于金相分析、热力学计算、高温拉伸试验和高温共聚焦原位观察等一系列方法,对高碳高铬马氏体不锈钢8Cr17Mo2铸锭表面裂纹产生原因进行了研究。结果表明,铸锭的心部存在一定面积的中心疏松,成为裂纹源头;铸锭浇铸完毕后的冷却时间较短,心部余温仍处于塑性比较低的范围,在钢锭脱离锭模的过程中,在外部产生的机械应力作用下容易在中心疏松处萌生裂纹;在中温退火后空冷过程中仍然会发生马氏体相变,由于应力作用促进萌生的裂纹扩展,并延伸至铸锭表面,形成裂纹。     

Isothermal martensitic transformation in Fe-Ni and Fe-Ni-C alloys at subzero temperatures

A survey of experimental work on the isothermal martensitic transformation in the presence of prior martensite is given in Fe-Ni and Fe-Ni-C alloys having subzero Ms (Mb) temperatures. Low temperature dilatometric measurements are correlated with internal friction measurements in the 5 to 200 K temperature range. This study will show that this transformation can be discussed in terms ofC-curve behavior as in Fe-Ni-Mn alloys. Nevertheless, in Fe-Ni and Fe-Ni-C alloys internal stresses created by the austenite-to-martensite transformation during cooling play an important part in the development of the isothermal transformation. Furthermore, internal friction is shown to be proportional to expansivity thus indicating that the internal friction technique can be applied successfully for the study of phase transformations.     

Optimizing fracture toughness and abrasion resistance in white cast irons

A series of twelve Cr-Mo white irons varying in carbide volume from 7 to 45 pct were tested for dynamic fracture toughness and wet sand abrasion resistance. Carbon content was varied from 1.4 to 3.9 pct. Two matrix microstructures were employed, and the compositions (copper and chromium content) were varied to assure constant matrix compositions. Chromium was varied from 11.6 to 25.7 pct. In addition, one composition of white iron was subjected to thirty different heat treatments to define the effect of matrix microstructure on dynamic fracture toughness and abrasion resistance. It was shown that for the abrasive wear system used, a carbide volume of about 30 pct represented an optimum quantity, above which abrasion resistance decreased. Martensitic irons provided consistently better abrasion resistance than austenitic irons. Dynamic fracture toughness decreased with carbide volume, as expected. Higher toughness values were obtained with predominantly austenitic matrix microstructures than with predominantly martensitic matrix microstructures. Considering both abrasion resistance and fracture toughness, it was shown that heat treated irons could provide an optimal combination of these properties. Formerly Visiting Research Metallurgist, Climax Molybdenum Co. Research Laboratory.     

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.     

The 3-dimensional structure of macrosegregation in continuously cast high-carbon steel

Three-dimensional segregation maps were obtained by serially sectioning rolled billets of high carbon steel. The sections were macroetched to reveal phosphorus segregation then digitised and combined using a computer. Volume rendering software was used to create images that represented views of continuously cast blooms with unsegregated steel transparent and macrosegregation opaque. The images showed the overall V-structure of macrosegregation and channel like morphology of individual macrosegregates. Samples with a fully columnar structure and a mixed columnar-equiaxed structure were compared. A large equiaxed zone disperses the macrosegregation over a wider area and increases the angle and length of channels. The observed structures suggest that macrosegregation forms through a combination of internal hot tearing and erosion of solid by flowing segregated liquid.     

Study of the austempering transformation kinetics in compacted graphite cast irons

In this work, the phases involved in the transformation austenite→ferrite+high-carbon austenite in cast irons were assessed by means of Mössbauer spectroscopy and hardness measurements for samples austempered at five different temperatures between 573 and 673 K with two Mn contents. The C content in the high-carbon austenite was found to have a dependence of t ^0.40±0.05 on the austempering time t, which evidences that diffusion of carbon has taken place. The kinetic parameter k determined using the Johnson-Mehl-Avrami equation has a maximum of 3.9×10^?3 (s^?1) at ≈623 K and corroborates that Mn slows the transformation rate.     

Solidification and solid-state transformation mechanisms in Si alloyed high-chromium white cast irons

Chromium white cast irons are widely used in environments where severe abrasion resistance is a dominant requirement. To improve the wear resistance of these commercially important irons, the United States Bureau of Mines and CSIRO Australia are studying their solidification and solid-state transformation kinetics. A ternary Fe-Cr-C iron with 17.8 wt pct (pct) Cr and 3.0 pct C was compared with commercially available irons of similar Cr and C contents with Si contents between 1.6 and 2.2 pct. The irons were solidified and cooled at rates of 0.03 and 0.17 K · s^-1 to 873 K. Differential thermal analysis (DTA) showed that Si depresses the eutectic reaction temperature and suggests that is has no effect upon the volume of eutectic carbides formed during solidification. Microprobe analysis revealed that austenite dendrites within the Si alloyed irons cooled at 0.03 and 0.17 K·s^-1 had C and Cr contents that were lower than those of dendrites within the ternary alloy cooled at the same cooling rate and a Si alloyed iron that was water quenched from the eutectic temperature. These lower values were shown by image analysis to be the result of both solid-state growth (coarsening) of the eutectic carbides and some secondary carbide formation. Hardness measurements in the as-cast condition and after soaking in liquid nitrogen suggest an increase in the martensite start temperature as the Si content was increased. It is concluded that Si’s effect on increasing the size and volume fraction of eutectic carbides and increasing the matrix hardness should lead to improved wear resistance over regular high-chromium white cast irons.     

Experiments on directional martensitic transformation in steel

This paper describes some preliminary experiments on the feasibility of producing steels with an aligned martensitic microstructure. The parent austenite is initially textured, following which the martensite habit plane “activity” is regulated by transforming under strain. An Fe-20 pct Ni-5 pct Mn alloy exhibiting a lowM _s temperature and a lath mar-tensite morphology was found suitable for producing aligned martensite. On heavy defor-mation this alloy developed a typical (123) [412] fcc texture. Subsequent annealing then produced a strong (001) [100] cube texture. Nearly 80 pct of the martensite formed was aligned when such a cube-textured specimen was strained 2.5 pet at 45 deg to the rolling direction followed by cooling under strain in liquid nitrogen to produce martensite. M. Khobaib and R. Quattrone, Formerly with the Construc-tion Engineering Research Laboratory, U.S. Army Corps of Engineers, Champaign, Illinois     

Research on martensitic transformation of 304 stainless steel

采用光学显微镜、扫描电镜、磁性测量和力学分析等手段,研究了304奥氏体不锈钢形变过程中发生开裂的原因,研究发现开裂是由于形变过程中发生了马氏体相变,导致塑性变差引起的,开裂处的磁性率达到了22.9%。发生马氏体转变的原因是由于w（Ni）低于标准下限,其组织稳定性下降,在形变过程中诱发了马氏体转变。     

304不锈钢马氏体组织转变探讨

采用光学显微镜、扫描电镜、磁性测量和力学分析等手段,研究了304奥氏体不锈钢形变过程中发生开裂的原因,研究发现开裂是由于形变过程中发生了马氏体相变,导致塑性变差引起的,开裂处的磁性率达到了22.9%。发生马氏体转变的原因是由于w（Ni）低于标准下限,其组织稳定性下降,在形变过程中诱发了马氏体转变。     

1500MPa级冷轧马氏体钢板的弯曲性能和韧性研究

冷轧马氏体钢板以其高强度特点,在汽车结构件制造中具有较大的减重潜力,逐步得到越来越多的应用。马氏体钢板延伸率较低,随着钢板强度提高,塑性进一步下降,能否满足汽车零部件制造对成形性和安全保护的要求,值得关注。以1 500 MPa级别的冷轧马氏体钢为研究对象,测试了其90°弯曲性能和在不同温度的韧性及断裂方式,并根据试验结果对1 500 MPa级马氏体钢板应用的条件进行了分析,希望能够对该类型的超高强钢板在汽车制造方面的应用有一定的提示作用。     

The influence of heat treatment on the high-stress abrasion resistance and fracture toughness of alloy white cast irons

The influence of a range of austenitizing and subcritical (tempering) heat treatments on the high-stress abrasion resistance and fracture toughness of four commercially significant grades of alloy white cast iron was investigated. Complementing an earlier study^[1] on the influence of a more limited range of heat treatments on the gouging abrasion performance of the same alloys, the results showed that the effect of austenitizing temperature on high-stress abrasion pin test weight loss differed for each alloy. With increasing austenitizing temperature, these results ranged from a substantial improvement in wear performance and retention of hardness through to vir-tually no change in wear performance and substantial falls in hardness. Fracture toughness, however, increased markedly in all alloys with increasing austenitizing temperature. Tempering treatments in the range 400 °C to 600 °C, following hardening at the austenitizing temperature used commonly in industrial practice for each alloy, produced significant changes in both hard-ness and wear performance, but negligible changes in fracture toughness. Most importantly, the data showed that selection of the correct temperature for subcritical heat treatment to reduce the retained austenite content for applications involving repeated impact loading is critical if abrasion resistance is not to suffer.     

Stress-induced γ→ α martensitic transformation in two carbon stainless steels. Application to trip steels

The influence of the temperature θαof a prestraining of austenite above M_don the subsequent stress-induced γ→ α’ transformation in the(M _s, M_d) range is examined in two carbon stainless steels. It is shown that the yield stress, which is controlled by the transformation, increases with θαat given testing temperature and amount of prestraining. This behavior is related to the influence of θαon the nature and arrangement of the defects present in austenite after the prestraining: planar defects(i.e., stacking faults, twins, e platelets) predominate if θαis close to M_dwhereas undissociated dislocation cells are only to be observed if θif higher. This is consistent with the strong increase of the intrinsic stacking fault energy of the austenite, as inferred from measurements using the node method on a hot stage microscope. In addition, the ability of plane defects to propagate under stress is shown to be lower after a prestraining at higher θα, which is attributed to a segregation of impurity atoms on dislocations. It is concluded that the nucleation stress of the γ→ α’ transformation is the stress necessary to allow planar defects to propagate in the prestrained austenite. This work is part of a thesis prepared at the Centre des Matériaux de l’Ecole des Mines, Corbeil, France, and submitted at the University of Nancy, June 1972.