Non-linear elastic behaviour of the roller bearing steel SAE 52100 during cyclic loading

When subjected to uniaxial symmetrical push-pull tests, the hardened roller bearing steel SAE 52100 (100Cr6) exhibits unusual sickle-shaped hysteresis loops. This observation can be attributed to the fact that the elastic modulus is stress-dependent as a consequence of non-linear elastic effects. Elastic unloading experiments performed in tension and in compression during a cycle permitted the stress dependence of the elastic modulus to be determined quantitatively. The experimental observations can be described in good approximation by a quadratic extension of Hooke's law in accord with an anharmonic lattice potential. Applying this concept, the originally bent hysteresis loops can be converted into hyseteresis loops of the usual shape in the representation of stress vs plastic strain. A comparison of the results with measurements on α-iron whiskers reported in the literature indicated that the departures from linear elasticity observed in high-strength steels are in the same order of magnitude as in the case of pure iron.     

High-temperature deformation behaviour and microstructural evolution of low carbon steel based on processing map

The high-temperature deformation behaviours of low carbon steel QD08 were investigated by hot compression tests over temperatures from 1000 to 1200°C and strain rates from 0.1 to 10 s^?1. The processing map was obtained by superimposition of the power dissipation and the instability maps and the regions having the lowest strain rate sensitivity added for more clarification of low and high workability regions. The results show that the security domain mainly of hot deformation with a higher powder dissipation factor and maintain a smooth variation, by the metallographic observations, the grain refinement by DRX under the stable deformation conditions. On the basis of processing map and microstructure evolution, the optimal deformation processing parameters are the hot deformation temperature range from 1070 to 1100°C, and strain rates range from 5 to 10 s^?1.     

钒微合金化低碳钢高温变形动态再结晶

利用热模拟压缩试验测定了不同钒含量的钒微合金化低碳钢在900～1000℃温度区间和0.1～1s-1变形速率范围内的真应力-真应变曲线.对曲线的分析表明:随钢中钒含量的增加,低碳钢的动态再结晶开始时间延长,变形奥氏体的动态再结晶名义激活能提高.实验钢薄膜试样的TEM观察表明,钢中的微量钒以固溶态存在于奥氏体中,微量的固溶钒对奥氏体动态再结晶起到抑制作用.     

齿轮钢SAE8620H高温变形行为及热加工图

通过高温压缩试验研究齿轮钢SAE8620H在950~1100℃、应变速率0.01~10 s^-1条件下的高温变形行为.该合金钢的流动应力符合稳态流变特征,流变应力随变形温度升高以及应变速率降低而减小,其本构方程可以采用双曲正弦方程来描述.基于峰值应力、应变速率和温度相关数据推导出SAE8620H高温变形激活能Q=280359.9 J·mol^-1.根据变形量40%和60%下应力构建该齿轮钢的热加工图,通过热加工图中耗散值及流变失稳区确定其热变形工艺参数范围.SAE8620H钢在在变形程度较小时宜选取低的应变速率进行成形,而在变形程度大时则要选取低温低应变速率或者高温高应变速率.     

High temperature low cycle fatigue behaviour of P92 tungsten added 9Cr steel

Modified 9Cr-1Mo steel is being used extensively as structural material for steam generator components of liquid metal cooled fast breeder reactor and fossil fired power plants. The selection of this material is primarily based on a good combination of mechanical properties, high thermal conductivity, low thermal expansion coefficient and good resistance to stress corrosion cracking in water-steam and sodium environment systems compared to austenitic stainless steels. A further improvement of the thermal efficiency of modern steam turbines can be achieved by increasing the steam operating temperature. It has been found that the substitution of W for Mo enhances the high temperature tensile strength, fracture toughness and creep strength. This had led to the development of P92 Steel. The components of the steam generators are often subjected to repeated thermal stresses as a result of temperature gradients that occur on heating and cooling during start-ups and shut-downs or during temperature transients. This leads to low cycle fatigue (LCF) damage in the component. However, the data on LCF properties of the W added 9Cr steel (designated as P92 steel) is scarce and this paper aims in understanding the high temperature low cycle fatigue behaviour of P92 Steel. Total axial strain controlled low cycle fatigue tests have been conducted in air in the temperature range 823 and 873 K to ascertain the influence of temperature on the LCF behaviour of the steel. A constant ramp rate of 3 × 10^?3 s^?1 was employed for all the tests. The alloy exhibited a continuous softening before the final load drop that occurred due to the propagation of macro fatigue cracks. The softening rate is more for the tests conducted at higher strain amplitudes compared to that of lower strain amplitudes. Fatigue life decreased with increasing temperature and strain amplitude. In this paper the observed LCF results have been explained based on the detailed metallography and fractography conducted on the failed samples.     

Q235低碳钢高温变形过程的动态组织演化分析

通过Gleeble 2000上的热模拟压缩实验,分析了Q235低碳钢在不同热加工参数下的动态组织演化特征.结果表明:应变速率和温度对Q235钢的奥氏体形变特征影响强烈.在相同变形温度下,应变速率的提高可以明显推迟动态再结晶的发生:应变速率较低时,降低温度同样可以延迟动态再结晶的发生.利用定量金相技术及线性、非线性拟合算法,建立了 Q235钢热变形过程的唯像本构关系及组织演化动力学模型,并将其应用于Autoforge3.1有限元软件平台.压缩过程有限元模拟分析表明,分别采用Arrhenius双曲正弦方程描述Q235钢的唯像本构关系及Yada模型表征Q235钢变形过程的平均晶粒尺寸,可以满足预测精度,与实际变形过程基本吻合.     

The effect of frequency on the cyclic strain and low cycle fatigue behavior of cast Udimet 500 at elevated temperature

An experimental investigation was undertaken on cast Udimet 500 to study principally the effect of frequency on low cycle fatigue at temperatures from 730° to 900°C, but mostly at 815°C (1500°F). Total strain range and stress range vs fatigue life relationships were determined which included the frequency following the form of author's recent work. The stress range was found to obey a power law relationship involving the fatigue life and frequency, such that at a specific stress range, a ten-fold decrease in frequency corresponded to a 3.7-fold decrease in life. While stress results were well-ordered with frequency, the scatter in the cyclic strain data was severe. A random crystallographic orientation of the few grains in the cross-section led to an anisotropy in the material which produced as much as a three-fold variation in diametral strain around the circumference. Results were compared to those for directionally solidified Mar-M200. In load-controlled applications cast U-500 gave better life, while in strain-controlled situations, the directionally solidified material excelled. ForN _f>1000 cycles, the difference in life was related to elastic modulus. Evidence for cyclic strain aging is given, the peak effect occurring at 790°C. This temperature corresponding to that where the well-documented tensile ductility minimum occurs.     

The phenomenon of strain avalanches in cyclic deformation

By studying cyclic deformation of both mono- and polycrystalline copper with a fast response recorder, we have discovered an interesting substructure in hysteresis loops. This effect is caused by strain avalanches, which occur in loop patches, persistent slip bands and cellular dislocation structures. The behavior of the strain avalanches changes as life is expended and offers a means of measuring the current state of fatigue damage. They are especially pronounced when cracks are propagating. We believe that strain avalanches provide a useful tool for understanding cyclic deformation and fracture and for a novel method of nondestructive testing.     

变形对亚稳态奥氏体不锈钢低温敏化的影响

采用扫描电镜(SEM)、X射线衍射分析(XRD)及透射电镜(TEM)研究了不同形变量下的亚稳态不锈钢(AISI 304)经过低温敏化处理(380 ℃、240 h的真空热处理)后组织的变化。通过双循环电化学动电位活化技术(DL-EPR)对敏化态的试样进行敏化度(DOS)测试,并用电化学阻抗谱表征了不同试样的耐腐蚀性能。结果表明,随着变形量的增加,低温敏化试样的敏化度递增,材料的耐腐蚀性能下降,当变形量达到40%时,敏化度达到71%。经过DL-EPR测试后,试样奥氏体晶粒和大部分晶界完好,马氏体区域是主要腐蚀区域,腐蚀模式为穿晶腐蚀。马氏体板条和基体中的滑移带是碳化物容易形核的位置。     

低碳钢的高温力学性能

双辊铸轧薄带钢在金属刚刚凝固的同时就承受一定量的加工变形 ,若变形量控制不当 ,极易产生内部裂纹和表面裂纹 .为此 ,研究实验用钢在高温下的变形和力学性能具有重要意义。本文利用Gleeble 1 5 0 0热模拟试验机 ,采用加热法和凝固法两种加热变形制度 ,研究了实验用低碳钢的热塑性及强度 ,测定了该钢种的零塑性温度 (ZDT)和零强度温度 (ZST) ,分析了其裂纹敏感性及断口组织。结果表明 ,凝固法所测结果更符合实际 ;实验钢的高温脆性温度范围为 1 3 5 0℃至熔点 ,其ZDT和ZST分别为 1 40 0℃和 1 45 0℃     

变形温度对ULCB钢动态再结晶的影响

取得800 MPa级和900 MPa级ULCB钢,在1100~850℃进行单道次变形的热模拟试验,变形量为40%,应变速率为2 s-1。将应力-应变变化特征和显微组织观察相结合,分析研究变形温度对ULCB钢奥氏体动态再结晶的影响规律。结果表明,温度低于950℃时以形变硬化和动态回复为主,奥氏体形变再结晶主要发生在1000℃以上的高温变形中;奥氏体再结晶百分数随变形温度升高而增加,在1050℃变形后奥氏体再结晶百分数约40%,在1100℃变形后则发生完全再结晶。     

16MnR 钢的高应变超低周疲劳性能

从压力容器的实际工况和抗震安全性设计出发,研究了压力容器用钢16MnR 钢的高应变超低周疲劳性能。采用横向应变控制方法,在 MTS809拉扭复合疲劳试验机上测定了试验钢应变疲劳过程中的循环应力响应特征及循环应力与应变的关系等超低周疲劳行为;通过拟合 Basquin 公式和 Coffin-Manson 公式,获得了试验钢的高应变超低周疲劳寿命预测公式。断口扫描发现,超低周疲劳下,多处裂纹萌生于试件表面,然后裂纹共同向内扩展,直到最后快速断裂。     

石墨化碳素钢室温压缩过程中的不均匀变形行为

将0. 46%含碳量(质量分数) 的石墨化碳素钢在万能材料试验机上进行室温压缩变形, 试验钢表现出良好的压缩变形性能.根据载荷-位移曲线的变化特点, 试验钢的压缩变形过程以位移7. 0 mm (对应相对压下量为58. 3%) 为节点分为两个阶段: 在位移≤7. 0 mm的压缩阶段, 载荷呈线性增加, 压缩试样的鼓度值逐渐增加而达到一个极大值(14. 6%), 压缩试样中心位置的维氏硬度增幅最大, 为38. 1 HV, 至位移7. 0 mm时试样端面径向伸长率的增幅为34%;而在位移 > 7. 0 mm的压缩阶段, 载荷呈指数增加, 压缩试样的鼓度值从极大值开始逐渐减小, 至位移为10. 72 mm时(相对压下量为89. 3%), 试样端面的径向伸长率相比于位移7. 0 mm时增加了83. 1%, 压缩试样的中心位置的维氏硬度增幅最小, 为32. 7 HV.上述试验数据表明, 在位移≤7. 0 mm的压缩过程中, 压缩试样内的三个不均匀变形区的位置与传统压缩模型一致, 但是当压缩变形进入位移 > 7. 0 mm的压缩过程中, 试样中心位置已不再是传统压缩模中变形程度最大的变形区了, 即在这个阶段试样中的3个不均匀变形区的变形程度发生了改变.正因这种不均匀变形区变形程度的改变导致了变形过程中载荷的急剧增加和鼓度值的减低.另外, 在压缩变形过程中, 三个不均匀变形区中石墨粒子的微观变形量总是高于铁素体基体, 其原因之一可以归结为石墨粒子中层与层之间容易于滑动的结果.      

石墨化碳素钢室温压缩过程中的不均匀变形行为

将0. 46%含碳量(质量分数)的石墨化碳素钢在万能材料试验机上进行室温压缩变形,试验钢表现出良好的压缩变形性能.根据载荷-位移曲线的变化特点,试验钢的压缩变形过程以位移7. 0 mm(对应相对压下量为58. 3%)为节点分为两个阶段:在位移≤7. 0 mm的压缩阶段,载荷呈线性增加,压缩试样的鼓度值逐渐增加而达到一个极大值(14. 6%),压缩试样中心位置的维氏硬度增幅最大,为38. 1 HV,至位移7. 0 mm时试样端面径向伸长率的增幅为34%;而在位移> 7. 0 mm的压缩阶段,载荷呈指数增加,压缩试样的鼓度值从极大值开始逐渐减小,至位移为10. 72 mm时(相对压下量为89. 3%),试样端面的径向伸长率相比于位移7. 0 mm时增加了83. 1%,压缩试样的中心位置的维氏硬度增幅最小,为32. 7 HV.上述试验数据表明,在位移≤7. 0 mm的压缩过程中,压缩试样内的三个不均匀变形区的位置与传统压缩模型一致,但是当压缩变形进入位移> 7. 0 mm的压缩过程中,试样中心位置已不再是传统压缩模中变形程度最大的变形区了,即在这个阶段试样中的3个不均匀变形区的变形程度发生了改变.正因这种不均匀变形区变形程度的改变导致了变形过程中载荷的急剧增加和鼓度值的减低.另外,在压缩变形过程中,三个不均匀变形区中石墨粒子的微观变形量总是高于铁素体基体,其原因之一可以归结为石墨粒子中层与层之间容易于滑动的结果.     

受热不均匀对低碳钢SAE1010板材组织及成形性能的影响

为了研究退火过程受热不均匀对板材组织及成形性能的影响规律,对热轧态SAE1010低碳钢板进行冷轧及连续退火,分析热轧态、冷硬态及退火态钢板边部和芯部的显微组织,分别对退火态钢板边部和芯部试样进行拉伸和折弯试验。结果表明,热轧态试样少量岛状珠光体分布在铁素体晶界处,边部组织晶粒尺寸小于芯部,珠光体中部分片层渗碳体退化为球状。冷硬态板材组织沿轧制方向呈现明显的晶粒破碎特征,退火态边部组织存在较多的大晶粒和粒状渗碳体团,芯部组织较为均匀细小。退火态板材芯部材料伸长率高于边部而强度低于边部,经180°折弯后芯部钢板无开裂而边部出现裂纹。退火温度均匀性对于SAE1010低碳钢板组织和性能具有重要的影响。     

Microstructural effects on fatigue crack growth in a low carbon steel

A study of the influence of microstructure on fatigue crack growth in an AISI 1018 steel has been carried out. Two distinctly different duplex microstructures were investigated. In one microstructure ferrite encapsulated islands of martensite; in the other martensite encapsulated islands of ferrite. The latter structure resulted in a significant increase in threshold level (18 MPa√mvs 8 MPa√m) together with an increase in yield strength. Fractographic analysis was used to investigate the influence of microstructure on the mode of fatigue crack growth. Formerly at the University of Connecticut