沉淀硬化不锈钢弹簧低周疲劳性能研究

研究了Ｃｒ１２Ｍｎ５Ｎｉ４Ｍｏ３Ａｌ沉淀硬化不锈钢弹簧低周疲劳性能和疲劳断口，指出在５２０℃时效时，由δ铁素体析出的大片状Ｊ相和α＇相诱发疲劳裂纹是降低疲劳寿命的主要因素；害３００℃时效处理可大幅度提高疲劳寿命。     

硼对刃具钢组织与性能的影响

为研制开发适合水淬的新型刃具钢,采用显微组织观察、力学性能测试、热处理试验和磨粒磨损试验等技术手段,对比研究了含硼中碳钢与不含硼的65Mn钢热轧后与热处理后的显微组织和应用性能.结果表明:热轧态中碳含硼钢中存在较多铁素体相,强硬性更低,韧塑性更好;中碳含硼钢水淬回火后组织为回火马氏体,硬度可达50 HRC以上,高于油淬回火的65Mn钢,韧塑性和耐磨性也明显好于65Mn钢.中碳含硼钢完全适合水淬工艺,减少了工业污染,符合环保要求.     

Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 10⁷ cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of K_t = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 10⁶ cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.     

Metallurgical investigation of a prematurely failed roller bearing used in the support and tilting system of a steel making converter used in an integrated steel plant

An extensive metallurgical investigation was carried out on samples of a failed roller bearing from the support and tilting system of a basic oxygen furnace (BOF) converter used in the steel melting shop of an integrated steel plant. The converter bearing was fabricated from low-carbon, carburizing grade steel and had failed in service within a year of fitting to a repaired shaft. Microscopic observations of both the broken roller and inner-race samples revealed subsurface cracking and preponderance of brittle oxide and other macroinclusions. Electron probe microanalysis (EPMA) studies confirmed that the brittle oxides that formed stringers were alumina, and the other macroinclusions were complex silicates. Both the alumina and silicate inclusions were deleterious to contact-fatigue properties. Microstructurally, the carburized regions of the broken roller and of inner-race samples contained high-carbon tempered martensite. Microhardness measurements revealed that although the core hardness of the roller and the inner-race samples were similar, the surface hardness of the roller was approximately 8.5 HRC units harder than that of the inner-race. Scanning electron microscope (SEM) observations of the roller fracture surface revealed striations indicative of fatigue, and energy-dispersive spectrometric (EDS) analyses corroborated a high incidence of silicate inclusions at crack sites. The study suggests that the failure of the bearing occurred because the hardness difference between the roller bearing and the inner-race surfaces resulted in wear of the inner-race. The wear led to shaft misalignment and play during service. The misalignment, coupled with the presence of inclusions, caused fatigue failure of the roller bearing.     

316L不锈钢焊接接头高温低周期疲劳显微结构变化和断裂特征

本研究对316L奥氏体不锈钢母材和焊缝分别进行高温低周疲劳试验,对试样的微观结构和裂纹扩展形貌进行观察,分析母材和焊缝在高温低周疲劳循环应力响应下的位错结构和损伤机制。结果表明,316L奥氏体不锈钢母材在试验过程中由于位错增殖和位错湮灭导致发生循环硬化和循环稳定,在焊缝中由于位错湮灭导致发生循环软化。母材和焊缝在连续低周疲劳试验中裂纹主要以穿晶方式扩展,焊接接头处孔洞的连接是最终导致焊接接头疲劳断裂的主要机制。     

Effect of initial microstructure on surface relief and fatigue crack initiation in AISI 410 ferritic‐martensitic steel

This work is focused on the effect of the initial tempered‐lath microstructure on the surface relief and nucleation of microstructurally short fatigue cracks developed during low‐cycle fatigue tests of the ferritic‐martensitic AISI 410 steel. Transmission and scanning electron microscopy as well as electron back‐scattered diffraction were used to study the surface‐damage evolution in smooth, cylindrical, notched specimens. Results from the electron back‐scattered diffraction analysis broaden the understanding of the processes of nucleation of microstructurally short fatigue cracks in the initial tempered‐lath microstructure of AISI 410 ferritic‐martensitic steels. Results prove that during fatigue, microcracks nucleate mainly at high‐angle boundaries represented by block subunit interfaces formed in this tempered microstructure. Besides, the progress of fatigue cycling causes the reorientation of the {112} <111> systems to a direction more favourable to slip, giving rise to the formation of extrusions within the blocks and consequently the formation of microcracks.     

A new low-carbon microalloyed steel wire in drilling rope

SAE 1065 steel wire ropes are prone to individual wire fracture due to the martensite structure with high carbon and high microhardness formed on the wire surface because of friction. A new low-carbon microalloyed steel wire composed of lath bainite with fine carbides dispersed in bainitic ferrite and lath martensite is developed in this paper. For the developed steel wire with a diameter of 1.0?mm, the tensile strength and low-cycle fatigue resistance are higher than those of commercial SAE 1065 wire. It is shown that the individual wire fracture caused by as-quenched martensite formation and severe plastic deformation can be avoided in the developed steel wire. Also, the design process and associated manufacturing process are discussed in detail.     

Low cycle fatigue life enhancement of 316 L stainless steel by nitrogen alloying

Tests carried out at room temperature on 316 L stainless steels with different nitrogen contents show that nitrogen improves the low cycle fatigue resistance of the materials. However, saturation occurs when nitrogen content is above 0.12 weight per cent. The microstructural aspect is also studied; the deformation is more difficult and more planar when nitrogen is present. Moreover, nitrogen delays the formation of cells. A single relation, derived from the Manson-Coffin formula, describes the low cycle fatigue behaviour of these steels by taking into account plastic strain range and nitrogen content.     

Plastically elastically dominant fatigue interaction in 316L stainless steel and 6061‐T6 aluminium alloy

Abstract Extensive studies involving multilevel loading have been performed to study the interaction effects of High–Low and Low–High loading sequences on various metals. ^{1 - 10} High–Low sequences generally yield ‘damage’ sums less than unity while ‘damage’ sums for Low–High sequences are typically > 1. It can be appreciated that the mechanisms governing fatigue behaviour under elastically dominant conditions differ from those observed under predominantly plastic conditions. This paper presents results on the interaction between plastically dominant fatigue (PDF) and elastically dominant fatigue (EDF) in 316L stainless steel and 6061‐T6 aluminium alloy. In addition, overstraining effects coupled with PDF and EDF interaction in 316L stainless steel are also reported.     

Dislocation configurations through austenite grain misorientations

In the present investigation, many strain controlled low cycle fatigue experiments of metastable austenitic stainless steel were performed at various total strain amplitudes under ambient temperature where the strain rate was kept constant. Dislocation cell developed due to strain cycling was measured through extensive analytical transmission electron microscopic investigation and the deformed austenite grains’ misorientation was measured through electron back scattered diffraction experiments. A strong connection has been established with the dislocation substructures’ configurations, the deformed austenite grains’ misorientation and the extents of induced phase transformation occurs while cyclic plastic deformation of metastable austenite at various total strain amplitudes. It has been investigated that with the increase in strain amplitude, dislocation cells are getting more uniform. It has also been found that with the increase in strain amplitude, dislocation cell size decreases drastically towards the higher strain amplitudes.     

Basic studies leading to the development of an ultrahigh strength,high fracture toughness low-alloy steel

Ultrahigh strength steels have been used increasingly in recent years for critical aircraft and aerospace structural applications. In such applications, though materials performance is of prime consideration, cost and availability makes the low-alloy steels an attractive option. This paper describes the development of an ultrahigh strength NiSiCrCoMo low-alloy steel, supported by significant findings obtained from the basic studies that were aimed at understanding how solute additions influence fracture resistance of iron, with and without the presence of carbon. The results of the basic studies, in combination with the work of Garrison (1986) on a NiSiCr steel, have profitably been employed in the development of a NiSiCrCoMo low-alloy steel possessing a strength-toughness combination quite comparable to the highly alloyed 250-grade maraging steel. Reproducibility of attractive strength and toughness properties has been established in tonnage scale melts. This steel, in the softened condition, has good formability and machinability. Weld parameters have also been established. The NiSiCrCoMo low-alloy steel thus meets the requirements of performance and cost rendering it an attractive option for advanced structural applications.     

Development of an ultra-high-strength low-alloy NiSiCrCoMo steel

An ultra-high-strength low-alloy NiSiCrCoMo steel has been developed. The development work is part of a major programme at the Defence Metallurgical Research Laboratory in the field of ultra-high-strength, high-fracture-toughness steels. In this context we undertook investigations to understand the effect of solute additions on the fracture behaviour of Armco iron and Fe-C alloys. We investigated Fe-Ni, Fe-Co, Fe-Si, Fe-Mo, Fe-C-Ni and Fe-C-Co alloys for mechanical behaviour. The report by Garrison (1986) on a Fe-C-Ni-Si-Cr alloy was an important pointer to a low-alloy, ultra-high-strength steel with high fracture toughness. The material we have now arrived at is a Fe-C-Ni-Si-Cr-Co-Mo steel with tensile, impact and fracture toughness properties matching those of maraging steel 250 grade in tonnage scale melts.     

Carbide-free tempered martensite in Fe-C steels

1.IntroductionQuenched Fe-C martensite is very strong but brittle due to su-persaturated carbon atoms located at octahedron sites[1].In order to improve the ductility,tempering is necessary.With increasing tempering temperature,the saturated carbon can be sequentially arranged as clusters,segregation into defects like dislocations and interfaces,transition carbides,and cementite[2].As a result,the strength is reduced and the ductility is improved predominantly due to the decreased amount of solid solution carbon[3].     

Morphologies and characteristics of deformation induced martensite during low cycle fatigue behaviour of austenitic stainless steel

Quantification, formation and nucleation micro-mechanisms of deformation induced martensite during low cycle fatigue behaviour of austenitic stainless steel have been investigated at various strain amplitudes tested at ambient temperature. The evolutionary deformation induced martensite has been quantified through magnetic measurement technique. It has been found that as strain amplitude increases, the volume fraction of deformation induced martensite increases. Extensive analytical transmission electron microscopy studies showed more than one nucleation site for martensitic transformation and the transformation micro-mechanisms have been observed to be: γ (fcc) → ? (hcp), γ (fcc) → α′ (bcc), γ (fcc) → deformation twins → α′ (bcc) and γ (fcc) → ? (hcp) → α′ (bcc).     

双面双弧焊对高强钢焊接效率和质量影响分析

对高强钢10CrNi3MoV钢厚板进行了双面双弧焊接试验研究。结果表明,焊接长2 m、厚50 mm的钢板,双面双弧焊与传统单弧MAG焊相比,生产效率提高1.4倍,焊接接头力学性能满足指标要求;双面双TIG打底的热影响区金相组织为板条状马氏体,焊缝区为板条马氏体+针状铁素体。     

发动机涡轮盘的应变疲劳行为

研究了发动机涡轮盘材料在３０００ｈ应力时效后和未经长时时效状态下的低周疲劳行为，测定了循环稳定和单调拉伸应力－应变曲线，给出了应变－寿命关系。讨论了该材料的循环硬化（软化）特征。