Low-cycle fatigue properties of microalloyed medium carbon precipitation hardening steels in comparison to quenched and tempered steels

Monotonic and cyclic stress strain curves and strain fatigue-life curves of a normalized carbon steel Cf 53 N, two quenched and tempered steels Ck 45 QT, 34 CrMoS 4 QT and three microalloyed medium-carbon precipitation-hardening steels 27 MnSiVS 6 + Ti BY, 38 MnSiVS 5 BY and 44 MnSiVS 6 + Ti BY, have been evaluated. Similar strain hardening was observed in the monotonic tensile tests whereas different hardening or softening was found under cyclic loading conditions. QT steels reveal pronounced cyclic softening over the entire strain range investigated, the ferritic pearlitic steels show only a slight decrease in the cyclic proof stress and cyclic hardening at larger strains. Strain fatigue-life curves result in a common scatterband of all steels investigated with the microalloyed steels 27 MnSiVS 6 + Ti BY and 44 MnSiVS 6 + Ti BY lying at the upper limit. Crack initiation probability of the microalloyed medium-carbon precipitation-hardening steels in the low-cycle fatigue range is equivalent or lower than for the normalized carbon steel and the QT-steels.     

Lowcycle fatigue behavior on duplex stainless steels

Stainless steels are used predominantly for their corrosion resistance in moderate to highly aggressive environments. For construction purposes, engineers normally select carbon steel due to low cost, long experience, applicable design rules and a large variety of strength classes. However, different stainless steel types can also provide a very wide range of mechanical properties and they have the advantage of not needing surface protection. Duplex Stainless Steels (DSSs) in particular, are austeno-ferritic steels with twice the mechanical strength of conventional austenitic and ferritic stainless steels and have a potential use in construction. In the early 1980’s, a ‘second generation’ of duplex steels was introduced with better weldability mainly through nitrogen alloying. The most common duplex grade today is the UNS S32205/S31803, which is used in a great number of applications in a wide variety of product forms. This grade was the basis for the development of a ‘third generation’ of duplex steels. These higher alloys are called super-duplex stainless steels and identified as UNS S32750/S32760. The cyclic hardening-softening response, the cyclic stress-strain curve and the microstructure evolution of a high nitrogen duplex stainless steel S32750 have been evaluated and the results compared with reference to low and medium nitrogen duplex stainless steels, S32205 and S32900 grades, respectively. The beneficial effects of nitrogen on the cyclic properties of most modern alloys have been analyzed in terms of the flow stress components, i.e. the back and the friction stress. A phenomenological model is proposed to explain the influence of nitrogen atoms on the cyclic behavior of these steels.     

Spheroidization cycles for medium carbon steels

An investigation has been made of spheroidization of medium carbon steels used in the bolt industry. Two process cycles were considered. One was the intercritical cycle, widely used in industry, in which the steel was heated above the lower critical, A1, temperature for approximately 2 hours; then cooled below it; and held for various periods to allow the austenite to transform and carbides to spheroidize. The other process was a subcritical cycle, which involved heating to below the A1 for various times. Wire samples of two steels were studied: AISI 1541, which is high in manganese and considered difficult to spheroidize, and AISI 4037, which is considered easier to spheroidize and is used extensively in industrial applications. Both cycles produced similar drops in hardness. However, 1 hour of the subcritical cycle yielded greater ductility than 32 hours of the intercritical process, as measured by tensile tests. Results of a new flare test designed to evaluate formability also indicated much faster spheroidization in the subcritical cycle. The level of spheroidization was defined in this study to be the percentage of carbide particles with aspect ratios less than 3. In 30 minutes, the subcritical cycle produced the same percentage of particles with an aspect ratio of less than 3 as produced by the intercritical cycle in 32 hours. The fast spheroidization in the subcritical process is attributed to the fine pearlite generated by the current practice of rapid cooling off the hot mill. This advantage is lost in the intercritical process as the original pearlite is dissolved above the A1 temperature.     

Retained austenite and tempered martensite embrittlement in medium carbon steels

Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Somein situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M₃C filM_s upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe₃C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The sit11Ation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which o°Curs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can o°Cur in the same steels, depending on the tempering conditions.     

Inclusion-controlled fatigue properties of 1800 MPA-class spring steels

Fatigue tests were conducted on a series of 1800 MPa-class spring steels whose fatigue properties were inclusion controlled. The fatigue tests were conducted on billets and on hot-rolled bars, taking into account the elongation of the oxide-type composite inclusions that were deformed during hot rolling, i.e., controlled inclusions. Anisotropy of the fatigue properties due to the slender shape of the elongated inclusions was also discussed. Fatigue tests were then conducted for both the rolling direction (RD) and transverse direction (TD) in the case of the billets. The fatigue test results in the RD showed a slight difference between billets and bars. The inclusions that were deformed during hot rolling were sufficiently elongated, even for the billet specimens, and differences in the effective inclusion sizes between the billets and their hot-rolled bars for the RD were small. However, there were marked differences in fatigue strength between the RD and TD in the billet specimens: the fatigue strength was almost half in the TD due to the presence of fish-eye fractures originating in large and slender MnS inclusions. In these fatigue tests, the two types of deformable inclusions revealed remarkably different effects on fatigue strength: the deformable oxide-type inclusions never caused fish-eye fracture, although the MnS inclusions found in the billets were extremely detrimental to fatigue strength when stress was applied in the TD.     

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。     

New post forging treatment of medium carbon microalloyed steels

In drop forging of parts for the transport industry the classical quench and tempering (QT) of alloyed steels is nowadays substituted by direct continuous cooling (CC) of microalloyed steels with elimination of quench cracking and expensive straightening and stress relieving cycles. Nevertheless, there are some limitations on strength and toughness achievable by this technique. On two commercial medium carbon (0.3%C) steels microalloyed with vanadium or vanadium and titanium, modified forging parameters and a new two-step cooling (TSC) strategy combined with an additional annealing were applied. Some increase in manufacturing costs, when compared to CC, can be justified by a significant increase in strength and ductility. The improvements attainable through such a modified treatment give evidence of the large potential of multi-phase ferrite-containing microstructures as a substitute of a tempered martensite.     

The influence of a duplex microstructure in steels on fatigue crack growth in the near-threshold region

The influence of a duplex microstructure on fatigue crack growth in the near threshold region has been studied for AISI types 1018, 1045, and 10B35, as well as 2.25 Cr-1 Mo steels. For a duplex microstructure which consists of a continuous martensitic phase encapsulating ferrite, an increase in threshold level and yield strength in both the AISI 1018 and 2.25 Cr-1 Mo steels was observed. However, with increase in carbon content and consequent decrease in the volume of ferrite, the threshold levels were not as significantly affected, although the yield strengths were higher. Y. MATSUO, formerly Graduate Student, University of Connecticut     

Corrosion fatigue of some carbon steels in main aqueous environments in Kuwait

Corrosion fatigue of carbon steel 1018 and 4140 was studied in three types of water in Kuwait, potable, brackish and sea water using a rotary fatigue bending machine with a corrosion chamber attached. Results have shown that fatigue strength of both steels is reduced much more in potable water than in brackish water, with maximum reduction occurring in seawater. The low alkalinity and chemical instability of the potable water in Kuwait is a factor that influences the corrosion fatigue of such carbon steels.     

Improvement of fatigue properties of PM steels by shot peening

Abstract

Shot peening was used for improving the fatigue properties of Fe–2Cu–0·5C PM steel. The steel is generally used for production of high performance sintered parts such as small connecting rods for cooling system compressors. To distinguish the effects of each alloying element, Fe, Fe–2Cu, and Fe–0·5C were also investigated. Optimum shot peening intensities are: 25A for pure iron and Fe–2Cu, 30A for Fe–0·5C, and 35A for Fe–2Cu–0·5C. For these intensities, improvements of the fatigue strengths of the materials investigated are as follows: 31% for pure iron, 48% for Fe–0·5C, 46% for Fe–2Cu, and 38% for Fe–2Cu– 0·5C. From the experimental results it could be concluded that both of the alloying additions, carbon and copper, are contributing to the fatigue strength improvements by shot peening.     

A review of the development and application of microalloyed medium carbon steels

Microalloyed medium-carbon steels with ferrite-pearlite microstructure were developed in the FRG in early 1972, with the primary aim of saving the cost of heat treatment. A steel with roughly 0.47% C, 0.75% Mn, 0.060% S and 0.1 % V was first used for crankshafts in cars manufactured by one of the largest European automobile companies. The effect of microalloying elements such as vanadium and niobium (niobium instead of columbium is used in this paper) in these steels and their dependence on the cooling rate from drop-forging temperatures is reviewed. Although niobium is more effective than vanadium, it leads to problems while manufacturing these steels with ~0.47% C, due to the high solution temperature of the niobium precipitates, so that preference has been given to vanadium. Further development work carried out to improve the ductility of these steels is reported. Steel compositions, which could make these steels applicable for various automobile and other engineering components, are presented.     

双相不锈钢超塑性变形机理

从材料的晶体结构出发,研究了双相不锈钢超塑性变形的机理.利用背散射电子衍射花样分析系统(EBSD),获得了双相不锈钢变形过程中的ODF图、极图和取向与转轴分布等晶体取向分布规律.结合透射电镜对微观组织的观察结果进行了综合分析.研究表明,双相不锈钢超塑性变形的机理为形变诱导析出和动态再结晶、晶界滑移以及变形中的晶粒转动.     

Pearlite in ultrahigh carbon steels: Heat treatments and mechanical properties

Two ultrahigh carbon steel (UHCS) alloys containing 1.5 and 1.8 wt pct carbon, respectively, were studied. These materials were processed into fully spheroidized microstructures and were then given heat treatments to form pearlite. The mechanical properties of the heat-treated materials were evaluated by tension tests at room temperature. Use of the hypereutectoid austenite-cementite to pearlite transformation enabled achievement of pearlitic microstructures with various interlamellar spacings. The yield strengths of the pearlitic steels are found to correlate with a predictive relation based on interlamellar spacing and pearlite colony size. Decreasing the pearlite interlamellar spacing increases the yield strength and the ultimate strength and decreases the tensile ductility. It is shown that solid solution alloying strongly influences the strength of pearlitic steels.     

中碳珠光体型高速车轮钢的韧化机理

 韧性是影响高速车轮运行安全的关键性能指标。为了阐明中碳珠光体型高速车轮钢的韧化机理,对夹杂物改性和组织韧化两方面进行了深入研究。研究结果表明,硫化物包裹氧化物的夹杂物改性提高了车轮钢的韧性。车轮钢中的氧化物夹杂容易在夹杂物与基体界面处产生裂纹,并向周围基体扩展;当氧化物被硫化物包裹后,裂纹仅在夹杂物本身产生,保护了周围基体。在w（Mn）＝0.75%的成分体系下,当硫的质量分数提高到0.006%及以上时,硫化物在固相线温度以上析出,可以实现对氧化物的较好包裹,改善车轮钢的韧性;硫化物在车轮热加工过程中会发生回溶与再析出,破坏复合夹杂物的包裹效果,提高硫质量分数或降低热加工温度,可以提高复合夹杂物的热稳定性。奥氏体晶粒尺寸和先共析铁素体体积分数是车轮钢组织韧化的关键控制因素。细化奥氏体晶粒尺寸、提高铁素体体积分数,断口中解理面尺寸减小,韧性撕裂区增多。     

控轧控冷中碳非调质钢的高周疲劳性能

 采用中碳非调质钢制造的轴类等零件常承受交变载荷，因而对钢材疲劳性能具有高的要求。为了评估控轧控冷工艺生产的非调质钢棒材的疲劳性能，利用旋转弯曲疲劳试验机研究了一种常用的钒微合金化中碳非调质钢38MnVS及对比钢38MnS的高周疲劳性能。结果表明，与38MnS钢相比，38MnVS钢中铁素体体积分数增加，组织明显细化；相分析表明约有54%的钒处于M（C，N）相中，且尺寸小于10 nm的M（C，N）粒子质量分数为32%，这些细小粒子的析出强化增量约为116 MPa。38MnVS钢的疲劳极限较38MnS钢提高了62 MPa，提高幅度约为18%；疲劳极限比从38MnS钢的0.43提高到38MnVS钢的0.48。M（C，N）相的析出强化及组织细化是38MnVS钢较38MnS钢具有优异疲劳性能的主要原因，但其疲劳性能仍低于锻态非调质钢。根据试验结果及文献数据，给出了预测铁素体＋珠光体型非调质钢疲劳极限的简便方法。     

Mn对2205双相不锈钢耐点蚀性能的影响

双相不锈钢2101生产成本低,性能优异,近年来被逐渐重视。采用“电炉+AOD+模铸”的工艺生产2101双相不锈钢,在AOD精炼过程中,研究了温度和主要成分Cr、C、Si的变化情况,结果显示,AOD炉有很好的脱碳效果,能将C质量分数（w_[C]）由2.5%脱至0.03%以下,在还原期,Si对Cr有很好的还原效果。精炼过程中,最重要的是脱碳,但将碳脱至0.1%以后,所需要的条件变得苛刻。通过热力学计算公式,研究了双相不锈钢2101去碳保铬的影响因素,结果表明,碳铬平衡主要受CO分压和温度的影响,CO分压越低、温度越高越有利于脱碳。在CO分压一定,w_[C]<0.1%时,w_[C]越低,碳铬平衡曲线的斜率越大,脱碳需要的温度越高,脱碳越困难,降低CO分压可进一步脱碳。在P_CO/P₀= 0.4,w_[Cr]=21.5%条件下,为将w_[C]脱至0.03%以下,需要将炉内温度升到1746.1 ℃以上。     

High cycle fatigue behavior of gas-carburized medium carbon Cr-Mo steel

High cycle fatigue properties of gas-carburized 4140 steel were assessed to compare with those of 8620 steel which is widely used as a carburizing steel. Fatigue limit was evaluated associated with microstructure, case depth, and distribution of retained austenite and compressive residual stress near the surface. Test results indicated that the reheat quenching method of 4140 and 8620 steels produced a reduction in grain size, retained austenite level, and compressive residual stress at the surface and an increase in fatigue limit. The fatigue limit of direct-quenched 4140 steel shows substantially lower value than that of direct-quenched 8620 steel due to larger grain size of direct-quenched 4140 steel. However, the fatigue limit of reheat-quenched 4140 steel is greatly improved and is comparable to the reheat-quenched 8620 steel. This is attributed to the larger reduction ratio in grain size and deeper case depth of reheat-quenched 4140 steel as compared to direct-quenched and reheat-quenched 8620 steels.     

特超级双相不锈钢的发展现状及趋势

概述了世界范围内特超级双相不锈钢的发展现状及趋势,重点对特超级双相不锈钢的成分设计思路、组织特点进行了分析,并结合其性能优势介绍了国外在相关领域的应用范例。强度更高、耐蚀性能更优的特超级双相不锈钢正在成为新一代的双相不锈钢材料,其将在海洋工程、石油石化、化工等领域获得广泛的应用。     

Effect of molybdenum on tempered martensite embrittlement of medium carbon Si-Mn steels

This paper reports a study of the effects of molybdenum on tempered martensite embrittlement (TME) of medium carbon Si-Mn steels. The study employed standard U-notch impact tests, scanning electron fractography, transmission electron microscopy, Auger electron spectroscopy, and dilatometric measurements. It is shown that the addition of molybdenum to Si-Mn steels does not eliminate TME, but molybdenum acts to decrease the ductile-brittle transition temperature (DBTT), thus making the impact test temperature for revealing TME lowered. Furthermore, the deferring role of molybdenum on TME is observed. In the molybdenum doped steels the TME embrittlement trough is displaced to a higher tempering temperature. The embrittlement is found to be concurrent with the replacement of ?-carbide by cementite during tempering. In the molybdenum doped steels the displacement of TME to a higher tempering temperature is seen to be associated with the role of molybdenum retarding cementite precipitation.