Fatigue Properties of Heat-Treated 30MSV6 Vanadium Microalloyed Steel

In the present study, 30MSV6 microalloyed steel was heat treated under different conditions, and the relation between its microstructure and mechanical properties was investigated. Scanning electron microscopy and transmission electron microscopy were used to characterize the microstructure of the heat-treated steel, and the effect of microstructure on tensile strength and fatigue behavior was determined. Microstructural analysis indicated that precipitates were formed at different sites such as grain boundaries and sub-grain boundaries. Furthermore, microstructural studies accompanied by the evaluation of mechanical properties revealed that the optimal heat treatment cycle of 30MSV6 microalloyed steel involved austenitization at 1223 K for 1 h and cooling in air to room temperature, followed by aging at 873 K for 1.5 h. The optimal heat treatment cycle resulted in significant improvement in the fatigue strength, tensile strength, and ductility because of the development of a uniform distribution of fine precipitates in a refined microstructure. The fatigue limit under optimum conditions (~384 MPa) was greater than that under other conditions (~321 and 312 MPa).     

Enhanced Strength in Cu-Ag-Zr Alloy by Combination of Cold Working and Aging

In this investigation, the effect of different degree of cold rolling and post-aging treatment on the microstructure and mechanical properties of a Cu-3wt.%Ag-0.5wt.%Zr alloy was studied by means of hardness measurement, tensile tests, optical and electron microscopy. The alloy was subjected to cold rolling up to 80% followed by aging in the temperature range of 400-500 °C. The yield strength, ultimate tensile strength and hardness were found to increase as degree of cold rolling increased, but at the expense of ductility. Aging of cold rolled samples in the studied temperature range has resulted in different combinations of strength and ductility. However, aging of cold rolled samples at 400 °C for 1 h has resulted in a combination of high strength and moderate ductility. A yield strength and ultimate tensile strength of 511 and 560 MPa, respectively with a ductility of 12% were achieved for 80% cold rolled and aged (400 °C for 1 h) sample. The high strength achieved after 80% cold rolling and aging is mainly attributed to precipitation of fine silver precipitates.     

Effect of surface rolling on fatigue behavior of a pearlitic ductile cast iron

Surface rolling is a mechanical treatment usually used in parts fabricated with steel and ductile cast iron, specifically in stress concentration regions, to improve fatigue properties. This process hardens and introduces compressive residual stresses to the surface of the material through the application of controlled strains, thus provoking a reduction of resulting tensile stress at its surface under cyclic loading. This work deals with the effect of surface rolling on high cycle fatigue behavior of a pearlitic ductile cast iron used in crankshafts by the automotive industry. Rotating bending fatigue tests were performed in both smooth and notched specimens, the latter either with or without a surface rolling treatment. Compressive residual stresses and heavy plastic deformation imposed on the surface grains due to cold work made difficult the nucleation and propagation of the crack at the rolled surface of the notch. As a consequence, surface-rolled notch testpieces presented a higher endurance limit (478 MPa) than both smooth (299 MPa) and notched (166 MPa) testpieces did. The surface rolling apparatus developed for this work proved to be very efficient and simple, providing good control of parameters involved in the process (i.e., rolling load, frequency, and number of revolutions).     

加磷高强IF钢的最优冷轧压下率确定

为了确定加磷高强IF钢的最优冷轧压下率,以工业生产的加磷高强IF钢热轧钢板为试验材料,在实验室进行了冷轧试验和盐浴退火试验,研究了冷轧压下率对试验钢显微组织和力学性能的影响。结果表明：在试验条件下,试验钢冷轧压下率为50%～80%,退火温度为820～850℃时,再结晶完成;随着冷轧压下率的增加,晶粒变得细小均匀;冷轧压下率为50%～80%,退火温度为850℃时,屈服强度为160 MPa左右,抗拉强度为345 MPa左右,延伸率为35.0%左右,塑性应变比r值和应变硬化指数n值都较高,r值为1.5左右,n值为0.30左右。最终确定工业生产中最优冷轧压下率为60%～70%。     

Effects of Pack Rolling Temperature on Microstructure and Mechanical Properties of Powder Metallurgical Ti-45Al-7Nb-0.3W Alloy

Powder metallurgical Ti-45Al-7Nb-0.3W (at.%) alloys were pack rolled at temperatures of 1240°C, 1255°C, 1270°C, and 1285°C. The microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy. The tensile properties were tested at room temperature and 800°C. After rolling, the sheets exhibited duplex microstructures with refined grains. The tensile test results showed the sheet rolled at 1270°C displayed excellent room temperature tensile properties with an ultimate tensile strength (UTS) of 782 MPa and an elongation of 1.95%. When tested at 800°C, all sheets showed UTS of over 600 MPa and elongations of around 50%. The dislocation movements and mechanical twinning played important roles at the initial stage of rolling deformation. However, during the subsequent deformation process, the deformation mechanism should mainly be the result of dynamic recrystallization.     

Microstructural changes in weld metal during isothermal process. Study of reheat cracking in flux‐cored arc welding stainless steel weld metal (5th report)

The soiling of the slag, spatter and the fume, etc., which come into contact with the steel sheet surface with welding, is cleaned making use of steel sphere shot material of large particle diameter, high projection pressure with strong peening processing (below, called strong peening cleaning). In this research, the cleaning state of the soiling with welding and improvement of fatigue strength of the hot galvanized welded joint was inspected, when the surface of a SM490A welded joint was cleaned with strong peening cleaning.

The following experimental results were obtained:

1. The fatigue limit of smooth base metal which received strong peening cleaning at about 320 MPa was remarkably high in comparison with smooth base metal at about 245 MPa.

2. The fatigue limit of a welded joint which received strong peening cleaning at about 300 MPa was remarkably high in comparison with a welded joint at about 170 MPa.

3. The strong peening cleaning was highly efficient and the cleaning state was satisfactory.

4. The cause of the remarkable rise of the fatigue limit (300 MPa) of the welded joint which received strong peening cleaning was because the fatigue limit (about 170 MPa, 57%) of the welded joint was improved (about 130 MPa, 43%) with peening cleaning. It was considered that improvement effects were: a rise (about 68 MPa, 23%) of hardness of the weld toe; relief (about 43 MPa, 14%) of stress concentration; increase (about 136 MPa, 45%) of compressive residual stress; and the decrease (about ? 96 MPa, ? 32%) by increase of surface roughness.

5. The fatigue strength of the hot galvanized welded joint decreased remarkably. This was thought to be due to the decrease (about HV40) of hardness of the surface, the decrease (about 188 MPa) of the compressive residual stress and the influence of many factors which accompanied hot galvanizing.

     

淬火后的逆相变退火温度对5%Mn冷轧中锰钢组织与性能的影响

对5%Mn冷轧中锰钢进行930 ℃×20 min淬火后再进行660、665、675、685 ℃保温30 min的逆相变退火处理,并用光学显微镜、扫描电镜、X射线衍射仪等研究退火温度对中锰钢组织和力学性能的影响。结果表明:5%Mn冷轧中锰钢经过高温淬火和逆相变退火后的组织为超细晶铁素体、板条马氏体和奥氏体。随着逆相变退火温度由660 ℃增加至685 ℃,奥氏体含量先增加后降低并在665 ℃逆相变退火后达到最大值,抗拉强度持续增加,屈服强度先升高后降低并在675 ℃退火时达到最大,伸长率先升高后降低并在665 ℃时达到最大值。综合来看,5%Mn中锰钢冷轧板经过930 ℃×20 min淬火和665 ℃×30 min逆相变退火后的综合力学性能最佳,此时奥氏体体积分数为24.24%,抗拉强度为980 MPa,伸长率为23.68%,强塑积达到了23.21GPa·%。     

退火温度对镀锌780MPa级双相钢组织性能的影响

研究了不同退火温度对780 MPa级冷轧双相钢组织性能的影响。试验表明:试验钢热轧态组织为铁素体、贝氏体和少量的珠光体,经过冷轧后形成纤维状组织,退火后组织为铁素体和岛状马氏体。对不同退火温度和速度下带钢组织性能进行了分析,带钢在820℃退火、保温100s后,可以获得双相组织且抗拉强度大于780 MPa。     

Microstructure,Tensile and Fatigue Properties of Al–5 wt.%Mg Alloy Manufactured by Twin Roll Strip Casting

This study investigated the microstructure, tensile and fatigue properties of Al–5 wt.%Mg alloy manufactured by twin roll strip casting. Strips cast as a fabricated (F) specimen and a specimen heat treated (O) at 400 °C/5 h were produced and compared. In the F specimen, microstructural observation discovered clustered precipitates in the center area, while in the O specimen precipitates were relatively more evenly distributed. Al, Al₆(Mn, Fe), Mg₂Al₃ and Mg₂Si phases were observed. However, most of the Mg₂Al₃ phase in the heat-treated O specimen was dissolved. A room temperature tensile test measured yield strength of 177.7 MPa, ultimate tensile strength of 286.1 MPa and elongation of 11.1% in the F specimen and 167.7 MPa (YS), 301.5 MPa (UTS) and 24.6% (EL) in the O specimen. A high cycle fatigue test measured a fatigue limit of 145 MPa in the F specimen and 165 MPa in the O specimen, and the O specimen achieved greater fatigue properties in all fatigue stress conditions. The tensile and fatigue fracture surfaces of the above-mentioned specimens were observed, and this study attempted to investigate the tensile and fatigue deformation behavior of strip cast Al–5 wt.%Mg based on the findings.     

Thermal Stability Study of Ultrafine Grained 304L Stainless Steel Produced by Martensitic Process

An ultrafine grain 304L stainless steel with average grain size of about 650 nm was produced by martensitic process. 10 mm as-received sheets were 80% cold rolled in the temperature of ?15 °C and then annealed at 700 °C for 300 min to obtain ultrafine grained microstructure. The results showed that the ultrafine grained 304L steel has yield strength of 720 MPa, tensile strength of about 920 MPa, and total elongation of 47% which is about twice that of coarse grain structure. The effect of annealing temperature (750-900 °C) on the grain growth kinetics was modeled by isothermal kinetics equation which resulted in the grain growth exponent (n) and activation energy for grain growth of 4.8 and 455 KJ/mol, respectively. This activation energy was also compared with those for other austenitic steels to better understanding of the nature of grain growth and atoms mobility during annealing. It was found that activation energy for grain growth is about twice higher than self-diffusion activation energy of austenite that is related to the Zener pinning effects of the second phase particles.     

Fatigue behavior of a Zr-based bulk metallic glass under uniaxial tension–tension and three-point bending loading mode

The fatigue behavior of as-cast Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass was systematically investigated under uniaxial tension–tension and three-point bending loading modes. To obtain the fatigue stress-life (S–N) diagram, stress-controlled experiments were conducted using a computer-controlled material test system electrohydraulic testing machine at 25 Hz with a 0.1 R ratio in air at room temperature. The fatigue endurance limit (630 MPa) in stress range terms under cyclic tensile load was almost three times higher than that under the three-point bending condition (225 MPa). Both fatigue resistances were similar at higher stress level. The fatigue fracture morphologies associated with the S–N curve indicated that the defects have little to no influence on the crack initiation and the fatigue life in low cycle fatigue range. However, at lower stress level, the most detrimental factor was the number of defects that resulted in fatigue endurance limit discrepancy.     

550 MPa级冷轧滑轨用钢的开发

针对我国高强度重型滑轨用冷轧板卷需进口的现状，采用低碳Nb微合金化钢和碳素结构钢两种化学成分设计，在工业冷轧生产线试制了滑轨用550 MPa冷轧钢卷。对试制钢卷的力学性能、微观组织和使用情况等进行了对比分析，结果表明，碳素结构钢的化学成分设计优于低碳Nb微合金化钢，其抗拉强度达550 MPa、屈服强度达380 MPa、伸长率达27.3%，满足客户要求且成本低，完全可以替代进口产品。     

550 MPa级冷轧滑轨用钢的开发

针对我国高强度重型滑轨用冷轧板卷需进口的现状，采用低碳Nb微合金化钢和碳素结构钢两种化学成分设计，在工业冷轧生产线试制了滑轨用550 MPa冷轧钢卷。对试制钢卷的力学性能、微观组织和使用情况等进行了对比分析，结果表明，碳素结构钢的化学成分设计优于低碳Nb微合金化钢，其抗拉强度达550 MPa、屈服强度达380 MPa、伸长率达27.3%，满足客户要求且成本低，完全可以替代进口产品。     

1 200 MPa级冷轧先进高强钢轧制稳定性的分析及控制

针对1 200 MPa级冷轧先进高强钢轧制不稳定问题,对热轧原料组织性能均匀性、冷轧压缩比、冷连轧机组轧制策略等进行了分析。结果表明,热轧工序投入边部加热器,采用分段冷却等手段,可有效降低热轧原料头尾部组织性能差异,保证通卷性能均匀,进而保证通卷轧制过程稳定;通过优化冷连轧机组压缩比,可有效降低材料本身的加工硬化强度,进而避免连轧机组后面机架的轧制超负荷情况;通过优化冷连轧机组轧制策略,可保证轧制过程中各机架均匀变形,避免出现轧制力差异较大的情况,进而保证轧制过程稳定。采用上述措施,1 200 MPa级冷轧先进高强钢轧制力控制在约15 000 kN,厚度精度控制在±0.06 mm以内,可保证该级别高强钢的稳定轧制。     

Study on static and fatigue strength of structural adhesive-bonded joints of steel sheets for automotive application

The static and fatigue strength of crush durable structural adhesive-bonded lap joints of steel sheets for automobiles was evaluated by tensile shear tests. The steel sheets used in this study were uncoated and galvannealed (GA) with tensile strength ranging from 270 MPa-grade to 980 MPa-grade and the thickness ranging from 0.7 to 1.8 mm. Also, the effects of the adhesive types were evaluated. The results are as follows: In the static tensile shear tests, when the steel sheets deformed during the tensile test, the tensile shear strength increased with the increase in the base metal properties, such as the yield strength and thickness; however, when the base metal properties were sufficiently high not to undergo plastic deformation, the tensile shear strength exhibited a constant value. On the other hand, the effect of base metal properties on the fatigue joint strength was relatively small. The static joint strength of the GA steel joints was slightly lower than that of the uncoated steel sheets; however, the fatigue strength of the GA steel joints was higher than that of the uncoated steel sheets. The coating failure of the GA was affected by the type of adhesive, base metal properties and type of test. Choosing the proper adhesive can reduce the failure of the GA coating, and the high strength steel showed fewer coating failures than the mild steel.     

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应用AG-IS 100kN岛津电子万能试验机测试超低碳钢冷轧薄板的拉伸性能,分析了试验温度(-40～200℃)、轧制方向和试样厚度(0.27～0.35mm)对其拉伸性能的影响.研究结果表明:抗拉强度随着温度的升高先降低而又略有增加,在200℃时材料出现屈服平台,其强度为135 MPa.伸长率A值随着温度的升高先增加后降低,而后又有所增加.纵向试样的抗拉强度和伸长率值较横向试样的高5％～15％.0.27 mm厚度试样的抗拉强度最高,伸长率最小,0.3mm厚度试样抗拉强度最小、伸长率最大.     

Development of Precipitation Strengthened Steel with Adequate Stretch Flangeability

An experimental steel of the composition (in wt.%) 0.04C-0.81Mn-0.38Si-0.15Ti-0.01S-0.013P-0.043Al was hot rolled into 4 mm plates at three different temperatures of 1100, 1000, and 900 °C. The yield strengths of these plates were in the range of 434-484 MPa while the ultimate tensile strength varied from 508 to 586 MPa. Elongation values ranged from 13.0 to 17.8%. Hole expansion ratios (λ) varied from 23 to 30.7%. In particular, the plate rolled at 1000 °C showed a yield strength of 484 MPa, an ultimate tensile strength of 586 MPa, a total elongation of ~15%, and a hole expansion ratio of ~23%. Transmission electron microscopy showed the presence of fine precipitates of titanium carbosulfide (~10-50 nm). Therefore, maximum precipitation strengthening was obtained in the plate that was hot rolled into a thickness of 4 mm at 1000 °C.     

Effect of Hydrogen on Mechanical Properties of 23Co14Ni12Cr3Mo Ultrahigh Strength Steel

In order to evaluate the effect of hydrogen on mechanical properties of 23Co14Ni12Cr3Mo ultrahigh strength steel, the specimens were electrochemically hydrogen charged for different times. The tensile property, fatigue fracture behavior, fatigue crack growth (FCG) behavior, and threshold stress intensity (ΔK _th) of the samples were studied. The fracture morphology was characterized by scanning electron microscopy. It was shown that tensile strength decreases from 2300 to 2000 MPa, critical fatigue stress from 577 to 482 MPa, and ΔK _th from 27.4 to 14.3 MPam^0.5 with the increasing hydrogen contents from 0.0001 to 0.0008 wt.%. Hydrogen enhances the FCG rate from 2.4 × 10^?3 to 3.6 × 10^?3 mm/cycle at ΔK = 80 MPam^0.5 in the hydrogen-charging range. Microscopic observation showed that the tensile fracture is a combination of overload microvoids and some intergranular regions for 0 h, and isolated areas of transgranular (TG) fracture are observed with brittle cleavage for 24-72 h. The fatigue fracture is ductile for the uncharged specimens, while the hydrogen-charged specimens show mainly brittle TG fracture. These results suggest that hydrogen degrades the fracture behavior of 23Co14Ni12Cr3Mo ultrahigh strength steel.     

快速凝固Cu-5Ag-0.5Zr-0.4Cr-0.35Nb合金的组织与力学性能

采用快速凝固方法制备了Cu-5Ag-0.5Zr及Cu-5Ag-0.5Zr-0.4Cr-0.35Nb(wt%)合金粉末,采用热等静压将粉末压制成坯料,随后进行热锻、冷轧处理。测试了合金在室温及高温(500 ℃)下的力学性能,并分析了合金的显微组织及断口形貌。结果表明,冷轧态合金具有更优异的室温拉伸性能,冷轧态Cu-Ag-Zr合金抗拉强度为739.3 MPa,伸长率7.1%,这与铜基体中密集的Cu₄AgZr颗粒及纳米级Ag颗粒有关。除Cu₄AgZr颗粒及Ag颗粒外,Cr、Nb元素的添加还生成高温稳定的Cr₂Nb颗粒,同时提高了合金的室温和500 ℃拉伸强度。冷轧态Cu-Ag-Zr-Cr-Nb合金的室温极限抗拉强度和伸长率分别为799.1 MPa与5.3%。因为热锻态合金晶粒尺寸粗大,Ag颗粒尺寸细小,相比冷轧态合金拥有更好的抗高温弱化性能。热锻态Cu-Ag-Zr-Cr-Nb和Cu-Ag-Zr合金的500 ℃抗拉强度分别为186.8和129.2 MPa,而冷轧态Cu-Ag-Zr-Cr-Nb和Cu-Ag-Zr合金在500 ℃抗拉强度分别仅为113.1和95.4 MPa。     

CO_2激光焊接600MPa DP钢接头组织与性能

采用CO2激光对抗拉强度为600MPa,厚度1.4mm的DP钢进行焊接.研究焊接速度对焊缝外观和截面成形的影响、接头的组织特点、硬度、强度和成形能力.结果表明,激光功率相同,焊接速度较低时焊缝易产生气孔,焊接速度较高时易发生飞溅;焊接速度对焊缝熔深及熔宽也有影响.焊缝区组织主要由马氏体构成,从焊缝、焊接热影响区到母材,组织中马氏体含量下降,接头的最高硬度出现在焊缝或热影响区.在平行于焊缝方向,焊接接头的抗拉强度高于母材,垂直于焊缝方向,接头的抗拉强度与母材相当.由于焊缝出现马氏体组织,接头的塑性和韧性降低,板材的冲压成形能力下降.