真空渗碳处理38CrMoAl渗氮钢的微观组织及氢脆敏感性分析

为了满足汽车轻量化对材料性能的要求,选择齿轮用38CrMoAl渗氮钢作为测试材料,以电化学充氢方法与慢应变拉伸相结合的方式测试了真空渗碳(Vacuum Carburizing,VC)试样的氢脆敏感性,同时与淬火+低温回火态(Quenching+ Low-Tempering,QL)处理试样进行了比较.研究结果表明:QL试样中形成了均匀形态的低碳回火马氏体;VC试样渗碳层中以高碳回火马氏体为主,存在少量的残余奥氏体.VC试样在表面部位的最大硬度达到了732 HV;QL试样在450 HV附近.对QL试样进行充氢后表现出了与初始试样接近的拉伸强度,而断后伸长率显著减小,计算相对断后伸长率差异得到QL试样氢脆敏感性指数(HEI)为54.3％.对38CrMoAl渗氮钢进行渗碳处理后会引起其氢脆敏感性的快速提高.对QL试样充氢处理后试样产生了更明显的脆性断裂,表现出了与高强度马氏体钢氢致断裂相近的特性;充氢后VC试样在断裂后形成了许多韧窝状的结构,呈现韧性断裂的特性.     

热处理对特种设备用高强钢激光–电弧复合焊接头氢脆断裂行为的影响研究

目的 提高800 MPa级特种设备用低碳贝氏体高强钢激光–电弧复合焊接头的抗氢脆性能。方法 采用预充氢后慢应变速率拉伸试样的方法,定量评估焊态、焊后直接高温回火和焊后调质3种状态下800MPa级低碳贝氏体高强钢激光–电弧复合焊接头的氢脆敏感性,结合扫描电镜下的初始微观组织和断裂特征,讨论抗氢脆性能的改善机理。结果 焊后调质处理有效消除了焊接热循环形成的马氏体组织,使接头各区域的微观组织趋于一致,接头的抗氢脆性能较焊态和直接焊后高温回火态的显著提高,断裂特征也从沿晶和穿晶的混合断裂转变为穿晶解理断裂。结论 焊后调质处理可以有效提高800MPa级低碳贝氏体高强钢激光–电弧复合焊接头的抗氢脆性能。     

不同环境下TiAl基金属间化合物的力学行为

研究了室温、473K和573K充氢前后Ti-49Al合金在不同应变速率下在真空、空气以及流动的氢气中的拉伸行为.结果表明,在5.6×10-6～2×10-4s-1应变速率范围内,Ti-49Al合金的力学行为没有明显的变化,但温度的影响很大.充氢样品中的内氢通过降低原子键合力来引起氢脆,而未充氢样品在流动的氢气中拉伸时,主要是由于氢引起了局部塑性变形而导致样品的脆性断裂.     

铁素体-粒状贝氏体型双相钢冷变形后的氢脆断裂行为

本实验采用阴极电解预充氢、慢应变速率拉伸的方法,研究了_(05)Si_2铁素体-粒状贝氏体双相钢经70%冷拔形变后的氢脆敏感性及断裂行为,并与_(05)Si_2铁素体-马氏体型。70%冷拔形变双相钢进行比较。发现冷拔铁素体-粒状贝氏体型双相钢的氢脆敏感性高于冷拔相同程度的铁素体-马氏体型双相钢,但由于原来的塑性较好,在充氢条件下仍有较好的塑性。铁素体-粒状贝氏体型冷拔双相钢在预充氢条件下拉伸时,微孔或裂纹在铁素体-粒状贝氏体相界面上形核,并沿着与外力约呈45°方向优先向粒状贝氏体-侧扩展。     

HYDROGEN EMBRITLEMENT OF COLD DRAWN FERRITE-GRANULAR BAINITE DUAL-PHASE STEEL

本实验采用阴极电解预充氢、慢应变速率拉伸的方法,研究了_(05)Si_2铁素体-粒状贝氏体双相钢经70%冷拔形变后的氢脆敏感性及断裂行为,并与_(05)Si_2铁素体-马氏体型。70%冷拔形变双相钢进行比较。发现冷拔铁素体-粒状贝氏体型双相钢的氢脆敏感性高于冷拔相同程度的铁素体-马氏体型双相钢,但由于原来的塑性较好,在充氢条件下仍有较好的塑性。铁素体-粒状贝氏体型冷拔双相钢在预充氢条件下拉伸时,微孔或裂纹在铁素体-粒状贝氏体相界面上形核,并沿着与外力约呈45°方向优先向粒状贝氏体-侧扩展。     

LC4高强铝合金的慢应变速率拉伸试验

采用慢应变速率拉伸 (SSRT)技术测试了LC4铝合金在空气和质量分数为 3.5 %的NaCl溶液中的应力腐蚀断裂 (SCC)行为 .研究了应变速率对铝合金SCC行为的影响和氢在LC4高强铝合金应力腐蚀断裂过程中的作用 .试验结果表明 ,LC4合金具有SCC敏感性 ,在潮湿空气中发生应力腐蚀断裂 ,而在干燥空气中不发生应力腐蚀断裂 .对于长横取向的LC4铝合金试样 ,在应变速率为 1.331× 10 6s 1时 ,其SCC敏感性比应变速率为 6 .6 5 5× 10 6s 1时的敏感性大 .在潮湿空气和阳极极化条件下 ,铝合金的应力腐蚀断裂机理是以阳极溶解为主 ,氢几乎不起作用 .在预渗氢或阴极极化条件下 ,氢脆起主要作用 ,预渗氢时间延长可加速LC4合金的应力腐蚀断裂 .     

2091合金的显微组织与氢脆敏感性

用动态充氢慢拉伸试验研究了氢对2091，合金力学性能的影响，用离子探针分析氢在合金中的行为，用透射电镜研究了合金显微组织与氢脆敏感性的关系，结果表明，充氢使合金抗拉强度，延伸率和断裂寿命明显降低，氢易于在晶界处偏聚和富集造成晶界弱化，晶界沉淀相种类，形态和分布是影响合金氢脆敏感性的主要因素。     

显微组织对贝氏体钢筋氢脆敏感性的影响

通过电化学充氢、慢拉伸实验并结合XRD、SEM、TEM和EBSD等显微组织表征方法,研究了显微组织对两种不同强度级别贝氏体钢筋氢脆敏感性的影响。结果表明:PSB1080钢筋强度高,但氢脆敏感性却低于PSB830钢;PSB830钢筋的组织分布不均匀,马氏体块尺寸差异较大,马氏体中高密度的位错为可逆氢陷阱,充氢之后氢分布不均匀,在拉伸的过程中,氢原子随位错迁移,扩散富集至裂纹尖端,裂纹在脆性大的马氏体和强度低的铁素体中扩展迅速,氢脆敏感性大。PSB1080钢筋板条间的残留奥氏体为不可逆氢陷阱,阻碍了氢原子的扩散富集,此外其组织的均匀性使钢中氢的分布也相对均匀,氢脆敏感性小。亚微米、纳米级的残留奥氏体同时具有良好的机械稳定性和化学稳定性,缓解了应力集中,阻碍了裂纹的扩展。     

外加电位条件下X100/X80管线钢的应力腐蚀行为研究

以X100、X80管线钢为研究对象，通过在外加电位条件下的慢应变拉伸速率试验(SSRT),获取管线钢材料在空气中和不同外加电位下的慢拉伸应力腐蚀的应力-应变曲线，分析其应力腐蚀敏感性可知：外加电位对X100和X80管线钢在3.5%NaCl中性溶液中的SCC敏感性和腐蚀开裂机理有显著影响。相同应力腐蚀条件下，X100管线钢的SCC敏感性相对于X80管线钢更低。结合断口微观形貌和极化曲线快、慢扫测试分析X100/X80耐腐蚀性能的特征和差异，可以得出X100和X80管线钢材料在不同外加电位条件下的应力腐蚀机理类型：当外加电位高于-395 mV时，金属处于活化溶解状态；当外加电位置于-395～-462 mV(X80钢)或-395～-504 mV(X100钢)时，机理为膜破裂-阳极溶解(AD)和氢致开裂(HIC)型；如果外加电位进一步降低，机理表现为氢致开裂型。     

多尺度实验测试评价高强钢氢脆的研究进展

氢脆是高强钢中普遍存在的现象,也是其研发过程中必须攻克的难题。为了深入理解高强钢的氢脆与其缺陷之间的关系,发展了许多测试评价方法,如宏观尺度的慢应变速率拉伸、线性增加应力、恒载荷拉伸这类力学实验以及检测氢含量的热脱附光谱法和电化学氢渗透法,根据高强钢的塑性损失、最大断裂应力、断裂时间、应力强度因子、氢的俘获能和扩散速率等参数直接进行氢脆敏感性的评价。但宏观尺度的实验无法深入地研究高强钢发生氢脆的机理,通过介观、微观尺度的实验和表征手段,如压痕法、纳米压痕法、微悬臂梁弯曲实验、原子探针技术、氢微印技术、扫描开尔文探针显微镜等,从局部测试高强钢性能变化和准确检测氢被俘获的位置,能够在解释氢脆机理和认识氢与高强钢中缺陷之间相互作用的问题上提供更加准确的依据。本文介绍、对比了上述这些实验方法并调研了多尺度实验测试评价高强钢氢脆的研究进展,总结了高强钢氢脆研究现状和主流的测试评价方法,为深入探索高强钢氢脆提供了思路。     

0Cr18Ni5和AF1410高强度钢的腐蚀行为研究

采用失重法和XRD法研究了0Cr18Ni5钢和AF1410钢在中性盐雾环境中的耐蚀性能.结果发现0Cr18Ni5钢由于高含量的Cr元素而只发生了轻微的点蚀,而AF1410钢则发生了严重的全面腐蚀,腐蚀产物主要由FeOOH和Fe3O4组成.采用恒载荷应力腐蚀拉伸和剩余力学性能测试法,并结合SEM断口形貌分析,研究了两种高强钢在3.5% NaCl(质量分数)水溶液中应力腐蚀行为.结果表明,0Cr18Ni5钢和AF1410钢在该环境中都表现出应力腐蚀敏感性.最后,结合实验结果提出了一种简易快速评价材料抗应力腐蚀性能的方法.     

Thermo-mechanical behavior of the Al–Si alloy coated hot stamping boron steel

Hot tensile tests of boron steels with and without an Al–Si coating were performed using a Gleeble 3500 test system, at temperatures of 700–850 °C and strain rates of 0.01–1/s. The phase and microstructure of the coating in as-coated and press-hardened conditions were observed under scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis and X-ray diffraction (XRD). Experimental results indicate that the Al–Si coating gave an unignorable influence on the thermo-mechanical properties of the boron steels. The ultimate tensile strength (UTS) of the Al–Si coated boron steel was almost equal to that of the uncoated under the lower strain rate at the same deformation temperature. At a higher strain rate, the UTS value appeared to be lower than that of the uncoated. Moreover, the UTS difference increased with the decreasing deformation temperature. The ductility of the Al–Si coated steel was lower than that of the uncoated under the described test conditions. Following the tensile tests, extensive cracks were visible in the Al–Si coating layer. SEM observation showed that microcracks and voids appeared after austenization, which may act as nucleation sites for the cracks. The cracks first propagated in the direction perpendicular to the coating/substrate interface and were identified as Type I cracks. The propagation was hindered by the substrate when these cracks reached the coating/substrate interface. This occurred because the interfacial bonding strength between the coating and the substrate was lower than the substrate strength. Following this initial failure, the cracks turned to propagate paralleled to the coating/substrate interface. In addition with the shear stress resulting from the substrate yielding, Type II cracks formed. Eventually, the cracked coatings were accompanied by interface decohesion from the substrate. The width and density of the cracks were found to increase with the decreasing deformation temperature and rising stain rate.     

Modelling flow behaviour of quenched boron steel with different microconstituent volume fractions

Hot stamped component of boron steel with tailored mechanical properties has multiphase microstructures. It is very important to establish a constitutive model related to multiphase microstructures for predicting the mechanical properties in the final component. Boron steel quenching experiments under different tool temperatures were performed with self-made experimental apparatus to achieve quenched specimens with different microconstituent volume fractions. Colour tint etching and image processing techniques were used to quantify the volume fractions of microconstituents. Tensile tests were also conducted to obtain stress–strain curves of the component with different volume fractions of microconstituents. The constitutive model of flow behaviour for quenched boron steel related to strain, strain rate and the volume fraction of quenched microconstituents was established by modified Field–Backofen.     

高Cl~-环境对M152和17-4PH高强钢应力腐蚀开裂行为的影响

利用中性盐雾实验、慢应变速率拉伸实验研究M152和17-4PH高强钢在高Cl~-环境中的应力腐蚀行为和机理。结果表明:M152和17-4PH钢在高Cl~-环境中均有一定的应力腐蚀开裂(SCC)敏感性,且随着前期中性盐雾时间的延长,其伸长率逐渐降低,SCC敏感性逐渐升高;通过扫描电子显微镜对试样的断口和侧边裂纹进行观察比较发现,中性盐雾时间的延长会使M152和17-4PH高强钢的断裂机制由韧性断裂向脆性断裂转变。分析得出M152和17-4PH钢在高Cl~-环境中发生SCC是阳极溶解(AD)和氢脆(HE)的协同作用,Cl~-会加速AD过程。经过不同时间中性盐雾后17-4PH钢的SCC敏感性均比M152钢要高,HE作用也越明显。高Cl~-环境中,17-4PH高强钢相对M152钢更易发生SCC。     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

Hydrogen embrittlement property of a 1700-MPa-class ultrahigh-strength tempered martensitic steel

The hydrogen embrittlement property of a prototype 1700-MPa-class ultrahigh-strength steel (NIMS17) containing hydrogen traps was evaluated using a slow strain rate test (SSRT) after cathodic hydrogen precharging, cyclic corrosion test (CCT) and atmospheric exposure. The hydrogen content in a fractured specimen was measured after SSRT by thermal desorption spectroscopy (TDS). The relationship between fracture stress and hydrogen content for the hydrogen-precharged specimens showed that the fracture stress of NIMS17 steel was higher, at a given hydrogen content, than that of conventional AISI 4135 steels with tensile strengths of 1300 and 1500 MPa. This suggests better resistance of NIMS17 steel to hydrogen embrittlement. However, hydrogen uptake to NIMS17 steel under CCT and atmospheric exposure decreased the fracture stress. This is because of the stronger hydrogen uptake to the steel containing hydrogen traps than to the AISI 4135 steels. Although NIMS17 steel has a higher strength level than AISI 4135 steel with a tensile strength of 1500 MPa, the decrease in fracture stress is similar between these steels.     

Stress Corrosion Cracking and Blue Brittleness of Rotor Steels in High Temperature Aqueous Solutions

The tensile behaviors of two bolt steels 20Cr12NiMoWV and 25Cr2MoV, and rotor steel 30Cr2Ni4MoV used in a nuclear power plant were studied at different temperatures up to 280 degreesC in air and aqueous solution. The results showed that blue brittleness occurred near 230 degreesC and 180 degreesC for steels 30Cr2Ni4MoV and 25Cr2MoV, respectively. No blue brittleness was found for the steel containing higher Cr content. Susceptibility to stress corrosion cracking.(SCC) for the steels in aqueous solution at the test temperatures was very low except at the blue brittleness temperature. At the blue brittleness temperatures, however, the susceptibility to SCC was very high.     

Hydrogen embrittlement property of a 1700-MPa-class ultrahigh-strength tempered martensitic steel

Abstract

The hydrogen embrittlement property of a prototype 1700-MPa-class ultrahigh-strength steel (NIMS17) containing hydrogen traps was evaluated using a slow strain rate test (SSRT) after cathodic hydrogen precharging, cyclic corrosion test (CCT) and atmospheric exposure. The hydrogen content in a fractured specimen was measured after SSRT by thermal desorption spectroscopy (TDS). The relationship between fracture stress and hydrogen content for the hydrogen-precharged specimens showed that the fracture stress of NIMS17 steel was higher, at a given hydrogen content, than that of conventional AISI 4135 steels with tensile strengths of 1300 and 1500 MPa. This suggests better resistance of NIMS17 steel to hydrogen embrittlement. However, hydrogen uptake to NIMS17 steel under CCT and atmospheric exposure decreased the fracture stress. This is because of the stronger hydrogen uptake to the steel containing hydrogen traps than to the AISI 4135 steels. Although NIMS17 steel has a higher strength level than AISI 4135 steel with a tensile strength of 1500 MPa, the decrease in fracture stress is similar between these steels.     

Shear behavior model for steel fiber-reinforced concrete members without transverse reinforcements

Due to the complex shear mechanism of steel fiber-reinforced concrete (SFRC) members, there is lack of comprehensive shear behavior models for SFRC members. The shear behavior model, based on a smeared crack model, requires the tensile stress–strain constitutive equation of SFRC membrane subjected to biaxial stresses. After SFRC panel tests under biaxial stresses were recently conducted, it has been possible to create a more complete smeared crack model for estimating the shear behavior of SFRC members. It is, however, very difficult to conduct such experiments for different types of steel fibers, various amount of steel fibers, different ranges of concrete strengths, etc. Thus, in this study, steel fibers are modeled as average direct tensile contribution elements in a modified smeared crack truss model, considering directionality and distribution of fibers. In this way, only simple bond tests are required to reflect the effects of different characteristics of SFRC. In addition, the shear contribution of steel fibers can be obtained considering the bond failure of steel fibers. The proposed model was compared to the test results of 8 SFRC panels and 80 SFRC beams, and the shear behavior of the SFRC members was well estimated.     

High resistance boron treated steels for railway applications

Abstract

The present study analyses different boron contents (between 10 and 160 ppm) on the structure of a 0·2C–2Mn–1Si (wt-%) steel deformed at a starting temperature of 1050°C in a T. J. Pigott laboratory rolling mill. The steel was made in a laboratory open induction furnace using high purity raw materials and cast into metallic moulds. This experimental steel has proved to have tribological properties, under dry rolling/sliding contact, as good as those for the 0·8% pearlitic steels used in railway applications. Before thermomechanical processing, the steel ingots (70 × 70 × 70 mm) were homogenised at 1100°C for 1·5 h. The thermomechanical treatment was carried out by a reversed multipass process to reach a level of deformation of 60%. Plastic deformation was finished at ～920°C for all the rolled steels and the plates (70 × 150 × 20 mm) were then water quenched and/or air cooled to room temperature. Results show more bainitic structures as boron content increases in the air cooled steel after hot rolling. For the quenched steels, the structure becomes more martensitic as boron content increases. The best combination of mechanical properties was obtained for the air cooled 76 ppm boron containing steel, which had a lower bainitic structure. This steel had the yield strength of 750 MPa, 15% elongation and the hardness of 40 HRC. Materials characterisation was carried out by optical and transmission electron microscopy (TEM). Results are discussed in terms of the boron segregation towards grain boundaries, the effect of boron on the steel hardenability, as well as on the boron carbonitrides (CNB) precipitation.