Improvement of gigacycle fatigue properties by modified ausforming in 1600 and 2000 MPA-class low-alloy steels

Fatigue tests were conducted for a modified ausformed and tempered JIS-SCM440 low-alloy (denoted as AF) steel with tensile strengths of 1600 and 2000 MPa, respectively, and were compared with normally quenched and tempered (QT) steel. The 1600 MPa-class modified ausformed steel (AF1600) developed no fish-eye fractures and a fatigue limit of 920 MPa, whereas normally quenched steel (QT1600) developed fish-eye fractures and showed a lower fatigue limit of 840 MPa. The 2000 MPa-class modified ausformed steel (AF2000) achieved a high fatigue limit of 1010 MPa, but developed fish-eye fractures. The fatigue limit of normally quenched steel (QT2000) was 840 MPa. Results of fatigue tests at 20 kHz up to 10⁹ and 10¹⁰ cycles agreed with the fatigue limits for QT1600 and QT2000, respectively. The fracture sites of fish-eye fractures in QT1600 were Al₂O₃ and TiN inclusions, and those in AF2000 and QT2000 were Al₂O₃ and TiN inclusions and internal facets. Fracture surfaces of QT2000 that broke at over 10⁷ cycles had large, clearly identifiable optically dark areas (ODAs). The AF2000 showed no fractures over 10⁷ cycles, and the authors could not conclude whether the fracture surface of the specimens broken at around 5 × 10⁶ cycles had ODAs. The fact that long-life fatigue failure over 10⁷ cycles did not occur in AF2000 showed that ODAs did not form in this steel, since ODAs developed in proportion to cycle number and became large and clearly identifiable at over 10⁷ cycles. Their incidence was quantitatively demonstrated by modified Stress amplitude vs Number of cycles to failure (S-N) diagram analysis. The modified S-N curve for AF2000, in which the specimens were assumed to have no ODAs, showed good agreement with that for QT2000, in which ODAs were assumed to be formed, especially at over 10⁷ cycles.     

Strength and fatigue properties in medium carbon duplex steels

The effect of microstructure on strength and fatigue properties has been investigated in two medium carbon alloy steels (BS 817M40 and BS 835M30) by developing dual-phase, ferritic-martensitic microstructures. Hardness-strength relationships and fatigue resistance at comparatively high strength levels were investigated by producing various microstructures. Conventional quenching and tempering, intercritical annealing and step quenching were used to vary the proportion, morphology and distribution of the ferrite and martensite phases. The results of the present study show that both hardness and strength increase with increasing proportion of martensite and/or hardness of the second phase. The relationship between hardness or strength and martensite percent is not in good agreement with a simple “law of mixtures” but is compatible with a more rapid strength increase at high martensite contents. The dual phase microstructures from the present study show superior near threshold ΔK_TH values than normal tempered martensite. The results also show a high degree of correlation between Paris equation m values and fracture toughness K_IC, showing that for high m values K_IC is low and vice versa. The present experiments show that although crack initiation resistance in dual-phase steels is excellent crack propagation rates are higher than in quenched and tempered microstructures for a given ΔK.     

Thermomechanical treatment to improve the fatigue behaviour of phosphorus containing high strength spring steels

As a simulation of the manufacturing of parabolic leaf springs a conventional heat treatment (CHT) and two variants of thermomechanical treatment (TMT) were applied on a group of CrV high strength steels, without and with Nb‐microalloying. In a part of the steels the bulk concentration of phosphorus was increased above 0.02 %. The aim of the study was to determine fatigue properties for various tempering temperatures ?_temp, especially of those steels contaminated with P. In a series of pretests the process parameters of CHT and TMT were optimized. After CHT a fatigue strength σ_fat of between 750 and 825 MPa was established over a wide range of ?_temp, rather independent of P content. But for the highest strength values (R_m ≥ 2100 MPa), produced at ?_temp ≤ 280 °C, TMT performs the best fatigue properties (σ_fat ≈ 900 MPa) superior to those after CHT. This study reveals that the advantageous effects of TMT can be reproduced even with impurity contaminated industrial grades.     

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.     

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.     

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。     

连铸与模铸高铁车轴钢的高周疲劳破坏行为

连铸车轴钢能否达到模铸车轴钢的性能水平是其能否应用的一个关键。对此,采用旋转弯曲疲劳试验及疲劳裂纹扩展速率试验对比研究了连铸与模铸工艺生产的高铁车轴钢的高周疲劳破坏行为。结果表明,工业试制的连铸车轴钢的强度和疲劳极限均低于模铸车轴钢,且前者的疲劳裂纹扩展速率略高于后者。疲劳断口分析表明,疲劳断裂大部分起源于试样表面基体。微观组织分析表明,尽管两者的微观组织均为高温回火马氏体,但连铸车轴钢中原奥氏体晶粒尺寸及碳化物均略大于模铸车轴钢。金相评级法及夹杂物极值统计法的结果均表明,连铸车轴钢中的夹杂物尺寸明显大于模铸车轴钢。因此,为了以连铸工艺取代模铸工艺,还需要进一步优化连铸车轴钢的成分、冶金生产和热处理等工艺,以获得优良的冶金质量和组织性能。     

Parameters affecting sag resistance in spring steels

Recent trends toward reducing the weight of automobile suspension springs have led to the development of a number of microalloyed steels and a variety of processing treatments which have claimed to improve the sag resistance of springs while increasing their operating stresses. More often than not, however, the subtle effects of varying levels of hardness and prior austenite grain size, as well as small but significant differences in critical elements such as Si, are over-looked in comparing these new steels with the conventional grades. Hysteresis loops measured in tension (related to the Bauschinger effect) were used to determine the relaxation behavior of a number of microalloyed and standard grade spring steels. The effects of hardness level, austenitizing temperature, prior austenite grain size, and warm prestressing on the Bauschinger hysteresis loops were also established. Silicon was found to be the most important factor influencing the size of the hysteresis loops; the greater the Si content of the steel (up to 2.2 pct), the larger the loops at a given strength level and the greater the expected relaxation resistance of the spring. The standard AISI 9261 steel containing 2.2 pct Si showed the same Bauschinger loops as the microalloyed grades containing Nb and V at the same hardness of 50 HRC.     

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.     

51CrV4弹簧钢相变规律研究

利用Gleeble-3800热模拟机研究51CrV4弹簧钢过冷奥氏体连续转变规律,采用热膨胀法测定其相变临界点,同时测定51CrV4钢过冷奥氏体在不同冷却速度下连续转变时的膨胀曲线,绘制其静态连续冷却转变（CCT）曲线.结合金相-显微硬度法,分析不同冷却速度对51CrV4钢组织性能的影响.结果表明：冷却速度为0.5℃/s 时,冷却转变的产物为铁素体和珠光体;当冷速增加,达到1℃/s 后,贝氏体开始生成;马氏体转变冷速区间为2~30℃/s;当冷却速度超过12℃/s后,冷却产物只有马氏体.     

Developing vibration-damping spring steels to reduce factory noise

This article reports on the results of a study undertaken to make use of the acoustic properties of standard spring steels 65, 60G, and U7 and four steels developed by the authors: steels ZIP-1, ZIP-2, ZIP-3, and ZIP-4. Comparison of the characteristics of these steels showed that steel ZIP-3 is characterized by less sound radiation during impact, which ensures a reduction in noise at work stations in factories.     

Corrosion fatigue in nitrocarburized quenched and tempered steels

In order to investigate the fatigue strength and fracture mechanism of salt bath nitrocarburized steels, specimens of the steels SAE 4135 and SAE 4140, in a quenched and tempered state, and additionally in a salt bath nitrocarburized and oxidizing cooled state as well as in a polished (after the oxidizing cooling) and renewed oxidized state, were subjected to comparative rotating bending fatigue tests in inert oil and 5 pct NaCl solution. In addition, some of the quenched and tempered specimens of SAE 4135 material were provided with an approximately 50-μm-thick electroless Ni-P layer, in order to compare corrosion fatigue behavior between the Ni-P layer and the nitride layers. Long-life corrosion fatigue tests of SAE 4135 material were carried out under small stresses in the long-life range up to 10⁸ cycles with a test frequency of 100 Hz. Fatigue tests of SAE 4140 material were carried out in the range of finite life (low-cycle range) with a test frequency of 13 Hz. The results show that the 5 pct NaCl environment drastically reduced fatigue life, but nitrocarburizing plus oxidation treatment was found to improve the corrosion fatigue life over that of untreated and Ni-P coated specimens. The beneficial effect of nitrocarburizing followed by oxidation treatment on cor-rosion fatigue life results from the protection rendered by the compound layer by means of a well-sealed oxide layer, whereby the pores present in the compound layer fill up with oxides. The role of inclusions in initiating fatigue cracks was investigated. It was found that under corrosion fatigue conditions, the fatigue cracks started at cavities along the interfaces of MnS inclusions and matrix in the case of quenched and tempered specimens. The nitrocarburized specimens, however, showed a superposition of pitting corrosion and corrosion fatigue in which pores and nonmetallic inclusions in the compound layer play a predominant role concerning the formation of pits in the substrate.     

轴承钢滚动接触疲劳研究进展

对轴承钢的滚动接触疲劳领域的研究重点、研究方法和研究进展进行了综合性的阐述。首先分析了滚动接触条件下材料内赫兹应力的分布以及轴承钢对循环应力的响应,解释了轴承钢发生次表面疲劳的根本原因;接着对轴承钢疲劳测试的方法进行了总结,分析了各种测试方法的优劣势;然后讨论了影响轴承钢滚动接触疲劳寿命的关键因素;最后,对轴承钢次表面诱发的滚动接触疲劳的两大表现形式,即疲劳裂纹生长和显微组织退化,进行了系统性阐释,介绍了重要的疲劳机理并提出了未来的重点研究方向。     

Gigacycle fatigue of ultra high density sintered alloy steels

Abstract

Sintered steel specimens with density levels of up to 7·6 g cm^?3 have been prepared from Cr–Mo and Mo prealloyed powders. The fatigue response has been studied using an ultrasonic resonance testing device that enabled testing up to 10⁹ cycles. It showed that the fatigue endurance strength can be drastically increased by raising the density and that the sintering conditions are effective, though less than the density. The existence of a true fatigue limit was disproved up to 10⁹ cycles for all materials tested, with sintered steels thus being similar to wrought ones. Cr–Mo steels was shown to be superior to Mo alloyed grades due to the markedly finer as sintered microstructure and higher sintering activity. Fatigue crack initiation was found to originate from pores at first at multiple sites, with microstructural orientation being dominant compared to the direction of stress; with progressive loading, some cracks join to form a propagating macrocrack from which the final failure then starts.     

Low-cycle fatigue behavior of Mn/N stainless steels

The low-cycle fatigue behavior of AISI 201 and 202, Mn/N stabilized austenitic stainless steels has been investigated. The fatigue life under controlled strain cycling conditions was determined and correlated with microstructural and fractographic observations. It was determined that both α′ (BCC) and ε (HCP) martensitic phases formed as a result of either monotonie or cyclic deformation in both steel compositions, but the two differed considerably in the amount of martensites formed. AISI 201 steel formed up to 90 vol. pet of α′ and was found to have a shorter fatigue life than AISI 202, which only formed a few tenths of a percent of α′. Comparisons with three other austenitic stainless steels are made.     

Low-cycle fatigue behavior of Mn/N stainless steels

The low-cycle fatigue behavior of AISI 201 and 202, Mn/N stabilized austenitic stainless steels has been investigated. The fatigue life under controlled strain cycling conditions was determined and correlated with microstructural and fractographic observations. It was determined that both α′ (BCC) and ε (HCP) martensitic phases formed as a result of either monotonie or cyclic deformation in both steel compositions, but the two differed considerably in the amount of martensites formed. AISI 201 steel formed up to 90 vol. pet of α′ and was found to have a shorter fatigue life than AISI 202, which only formed a few tenths of a percent of α′. Comparisons with three other austenitic stainless steels are made.

---

---

Formerly a Research Assistant     

不同冶炼方法对扭杆弹簧钢超高周疲劳性能的影响

以一种新型扭杆弹簧用高强钢(代号N1)为研究对象,采用真空自耗(VAR)和电渣重熔(ESR)冶炼方法制备得到2种实验钢(N1-VAR、N1-ESR)。研究了VAR、ESR冶炼方法对实验钢洁净度和超高周疲劳性能的影响。结果表明:采用电渣重熔工艺冶炼的N1-ESR试验钢中氧的质量分数为9×10^-6,而采用真空自耗冶炼工艺的N1-VAR试验钢中氧的质量分数仅5×10^-6,N1-VAR中氧化物夹杂数量和尺寸减小,夹杂周围裂纹萌生驱动力减小,超高周疲劳强度较高,疲劳寿命增加。