Secondary hardening and fracture behavior in alloy steels containing Mo, W, and Cr

In 4Mo, 6W, 2Mo3W, 2Mo2Cr, and 3W2Cr alloy steels, which cointain alloying elements, such as Mo, W and Cr, contributing to the secondary hardening by forming M₂C type carbide, the secondary hardening and fracture behavior were studied. Molybdenum had a strong effect on secondary hardening, while W had a very weak effect on it but delayed the overaging. The MoW steel exhibited both moderately strong hardening and considerable resistance to overaging. On the other hand, the secondary hardening effect was diminished by the Cr addition, because the cementite of M₃C type was stabilized at higher temperatures and the formation of M₂C carbides was thus inhibited. Although the Cr addition had no merit in the secondary hardening itself, it eliminated the secondary hardening embrittlement (SHE). This was observed as a severe intergranular embrittlement due to the impurity segregation for the Mo and MoW steels and as a decrease in upper shelf energy for W steel, even in the overaged condition.     

Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo and W

The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic steels containing both Mo and W were investigated. The secondary hardening response and properties of these steels are dependent on the composition and distribution of the carbides formed during aging (tempering) of the martensite, as modified by alloying additions and austenitizing treatments. The precipitates responsible for secondary hardening are M₂C carbides formed during the dissolution of the cementite (M₃C). The Mo-W steel showed moderately strong secondary hardening and delayed overaging due to the combined effects of Mo and W. The addition of Cr removed secondary hardening by the stabilization of cementite, which inhibited the formation of M₂C carbides. The elements Co and Ni, particularly in combination, strongly increased secondary hardening. Additions of Ni promoted the dissolution of cementite and provided carbon for the formation of M₂C carbide, while Co increased the nucleation rate of M₂C carbide. Fracture behavior is interpreted in terms of the presence of impurities and coarse cementite at the grain boundaries and the variation in matrix strength associated with the formation of M₂C carbides. For the Mo-W-Cr-Co-Ni steel, the double-austenitizing at the relatively low temperatures of 899 to 816 °C accelerated the aging kinetics because the ratio of Cr/(Mo + W) increased in the matrix due to the presence of undissolved carbides containing considerably larger concentrations of (Mo + W). The undissolved carbides reduced the impact toughness for aging temperatures up to 510 °C, prior to the large decrease in hardness that occurred on aging at higher temperatures.     

Heat treatment behavior and tensile properties of Cr−W steels

Ferritic steels containing Cr, W, and V are being developed for fusion reactor applications. These steels would be alternatives to the Cr−Mo steels that are being considered for structural components. Eight experimental steels were produced. Chromium concentrations of 2.25, 5, 9, and 12 pct were used. Steels with these chromium compositions and with 2 pct W and 0.25 pct V were produced. To determine the effect of tungsten and vanadium, 2.25Cr steels were also produced with 2 pct W and no vanadium, and with 0.25 pct V and zero and 1 pct W. A 9Cr steel containing 2 pct W, 0.25 pct V, and 0.07 pct Ta was also studied. For all alloys, carbon was maintained at 0.1 pct. Tempering behavior of the steels was similar to that of the Cr−Mo steels being considered. Tensile studies indicated that the 2.25Cr−2W−0.25V and 9Cr−2W−0.25V−0.07Ta steels had the highest strengths with properties similar to those of the 9Cr−1MoVNb and 12Cr−1MoVW steels, which are the strongest of the Cr−Mo steels of interest.     

时效温度对2.25Cr1Mo钢断裂行为的影响

通过系列冲击试验,结合晶界偏聚理论,研究时效温度对经980.℃淬火和3个不同的温度时效后的2.25Cr1Mo钢的韧脆转变温度的影响.由系列冲击试验结果知,2.25Cr1Mo钢的韧脆转变温度随时效温度的降低而升高.据平衡晶界偏聚理论,说明时效温度越低,磷的平衡偏聚量越大,而对应试样的韧脆转变温度越高,断裂形态由解理方式转变为沿晶方式.     

Fe-Mn-Si-Al系和Fe-Mn-C系TWIP钢加工硬化行为

研究了在不同应变量下Fe-Mn-Si-Al系和Fe-Mn-C系孪晶诱导塑性(TWIP)钢的力学性能以及微观组织,分析了TWIP效应在两种不同系列TWIP钢中发挥的作用,阐明了TWIP钢的强化机制.两种系列的TWIP钢都具有高加工硬化能力,但层错能较低的Fe-Mn-C系TWIP钢加工硬化能力更强.两种系列的TWIP钢加工硬化表现为多加工硬化指数行为,这是由多种强化机理在不同阶段起主导作用的结果.微观组织形态与加工硬化强度之间存在着较强的关联性.位错的增殖和形变孪晶的产生对两个系列TWIP钢硬化曲线形态有着明显的影响.在高应变阶段,Fe-Mn-C系TWIP钢大量的第一位向形变孪晶T1和第二位向形变孪晶T2,以及附着在孪晶界旁的高密度位错区域是造成其具有高加工硬化能力的原因,而Fe-Mn-Si-Al系TWIP钢细密的第一位向形变条纹和孪晶片层间的位错是其高加工硬化原因,且其微观组织更为均匀细致.     

合金元素Ni、Mo、W及熔渗铜对烧结钢耐磨性的影响

通过测定加入Ni、Mo、W等合金元素的烧结钢的磨损量，研究和探讨合金元素对烧结钢磨损特性的影响及烧结钢的磨损机制。研究表明，在烧结态下，复合加入Ni-Mo的烧结钢的耐磨性最好，而在热处理态下较烧结态的耐磨性有很明显的提高，且加入Mo-W的烧结钢的耐磨性最好。烧结钢的密损机制均是由粘着磨损、磨粒磨损和疲劳磨损共同作用的。且以磨粒磨损机制为主。由于游离铜的粘着倾向大。熔渗铜对材料的耐磨性不利。     

Comparison of Mn,Cr and Mo alloyed sintered steels prepared from elemental powders

Abstract

Molybdenum, chromium and manganese offer considerable potential as alloy elements in sintered steels, especially for PM precision parts used, for example, in automotive engines and transmissions. This holds in particular for recycling and health/safety aspects. Within this work, the influence of these elements as admixed alloy metals on the sintering behaviour and the properties of structural PM steels is discussed. The considerable differences in the homogenisation behaviour during sintering are described as well as the respective advantages and drawbacks. It is shown that for high density PM steels, Cr and Mo are better suited while for conventionally produced mass products Mn is attractive, its affinity to oxygen being less of a problem today than commonly assumed and since Mn addition promotes swelling, this element might be a replacement for Cu. For Mn, admixing is the more attractive route, for Cr, in contrast, prealloying offers advantages, while for Mo both alloying techniques are feasible.     

Plasma nitriding of Fe–Cr–Mo sintered steels

Abstract

Plasma nitriding was applied to Fe–Cr–Mo and Fe–Cr–Mo–C alloys with different Cr content and microstructure. Chromium allows a hard and controlled diffusion layer to be obtained, almost independent of the microstructure. Interesting results in terms of wear resistance and impact resistance were obtained by nitriding the as sintered carbon alloyed material. Analogous results were obtained by conventional quenching and stress relieving. The two treatments may constitute alternative solutions for the production of sintered components subjected to contact Hertzian stresses or fatigue.     

Sulphur segregation and high-temperature brittle intergranular fracture in alloy steels

High temperature brittle intergranular fracture has recently been identified as a mode of failure in alloy steels. It is associated with the dynamic segregation of sulphur to cracks in hard microstructures stressed at elevated temperatures in a manner analogous to hydrogen embrittlement at ambient temperature. Several models have been proposed to describe the action of sulphur, but insufficient experimental data have been available for their evaluation. The present study characterises sulphur enrichment at cracks and on free surfaces at high temperature in detail using scanning Auger spectroscopy. Both intergranular and transgranular surfaces were studied at pressures of air from 10⁻⁹ to 10⁻³ torr. Two types of sulphur enrichment at cracks were identified; general segregation to crack faces and local enrichment close to crack tips. The source of sulphur was largely that dissolved in the ferrite matrix. Large sulphides, intersecting grain boundaries, made a minor contribution, while small “overheated” intergranular sulphides were inoperative as sulphur sources. The role of stress in encouraging sulphur segregation was confirmed. In addition, an intermediate pressure of air was found to enhance sulphur enrichment, but only at surface oxygen coverages of 15–25 at.%. These observations were generally consistent with the influence of the crack tip stress field on migration of the sulphur solute, described by the “pure drift” model of high temperature brittle intergranular fracture. Refinement of the model, using finite element stress analysis, is included.     

Creep deformation and fracture of a Cr/Mo/V bolting steel containing selected trace-element additions

The article reports the creep behavior, at 565 °C, of 1Cr1Mo0.75V (Ti, B) (Durehete D1055) steel, in each of two grain sizes and doped with individual trace elements such as P, As, and Sn, in comparison to a reference cast of the base material containing 0.08 wt pct Ti. The addition of the trace elements P, As, or Sn (each <0.045 wt pct) appears to produce no significant effect on creep strength or creep crack-growth resistance at 565 °C. The fine-grained material shows low creep strength but notch strengthening, while the coarse-grained material shows higher creep strength and exhibits notch weakening for test times up to 2750 hours. From creep crack-growth tests, it appears that the C* parameter is not appropriate for correlating the creep crack-growth rate under the present test conditions. The parameters K _I or σ _net are found to correlate better, but, from the present data, it is not possible to judge which of these parameters is more appropriate for general use. It is suggested that the presence of Ti in CrMoV steels has an inhibiting effect on trace-element embrittlement.     

Precipitation behavior in ultra-low-carbon steels containing titanium and niobium

This work revealed the basic mechanism for the stabilization of carbon in ultra-low-carbon (ULC) steels that contain moderate S (0.004 to 0.010 wt pct), adequate Ti (0.060 to 0.080), and low Mn (≤0.20). During cooling through the austenitic region to the ferritic, the initially formed sulfide particles (TiS) undergo an in situ transformation into carbosulfides (H-Ti₄C₂S₂) by absorbing C and Ti. The transformation from TiS to H may be considered as a hybrid of shear and diffusion, i.e., faulted Ti₈S₉ (9R)+10[Ti]+9[C] → 4 1/2Ti₄C₂S₂ (H). At low temperature (≤930 °C), the stabilization process continues through epitaxial growth of carbides on H phase, i.e., [M]+x[C]+H → epitaxial MC _x (on H). This mechanism differs from the traditional view of stabilization, where the carbon is removed from solution by the formation of free-standing or independently nucleated H and/or MCN precipitates. While these two forms of carbon stabilization are now well known, this article presents a method of predicting which mechanism of stabilization will be operative in a given steel based on its bulk composition. Implications bearing upon new ULC steel design, considering the role of S, will be discussed.     

Influence of post weld heat treatment on impact toughness properties of 1Cr0.5Mo and 2.25Cr1Mo steels

The influence of post weld heat treatment on hardness and impact toughness properties of weldments of 1Cr0.5Mo and 2.25Cr1Mo steels has been investigated. Post weld heat treatment significantly reduced the hardness and the impact transition temperature and increased the upper shelf energy. The decrease in hardness was essentially larger for 2.25Cr1Mo than for the other steel, probably due to the presence of martensite in the as-welded condition in the former material. The decrease in the impact transition temperature was largest for 1Cr0.5Mo.     

ICP-AES法测定铁镍合金中铌钼钨

ICP-AES法测定铁镍系软磁合金中铌,钼、钨,同时研究了混酸中磷酸及酒石酸存在下对测定元素的干扰.