Metallurgical analysis of a failed maraging steel shear screw used in the band separation system of a satellite launch vehicle

Maraging steels have excellent combination of strength and toughness and are extensively used for a variety of aerospace applications. In one such critical application, this steel was used to fabricate shear screws of a stage separation system in a satellite launch vehicle. During assembly preparations, one of the shear screws which connected the separation band and band end block has failed at the first thread. Microstructural analysis revealed that the crack originated from the root of the thread and propagated in an intergranular mode. The failure is attributed to combined effect of stress and corrosion leading to stress corrosion cracking.

Effect of Rare Earth on Void Band of Diffusion Layer and Properties of Aluminized Steel

The effects of the addition of rare earth （RE） elements on the void band in the diffusion layer, and the re sistances to both oxidation and spalling of aluminized steel were investigated through high temperature oxidation and spalling tests. The results showed that RE had significant effects on the void band in the diffusion layer and the properties of aluminized steel. After diffusion treatment, a considerable number of the voids between the middle layer and transitional layer of pure aluminized coating, aggregated into wavy-line-shaped void bands parallel to the outer surface. For the RE added aluminized coating, only a few voids aggregated into intermittent block shapes. During high temperature oxidation at 800 ℃ for 200 h, the wavy void band of pure aluminized coating aggregated further into a linear crack parallel to the outer surface, and the internal oxidation occurred within them; the open cracks perpendicular to the surface penetrated through the diffusion layer. For the RE added aluminized coating, only a few voids aggregated into intermittent meniscus shapes. During cyclic spalling tests, the peeling, spallation, and pulver ulent cracking occurred along the void band in the diffusion layer of pure aluminized coating, but only a little spallation occurred in the diffusion layer of the RE-added aluminized coating, in which cracks perpendicular to the surface were much smaller than those of pure aluminized coating and did not penetrate through the diffusion layer. It is evident that RE addition can restrain the formation and aggregation of voids and subsequently improve the resistances to oxidation and spalling. The mechanism of the RE effect on the void band in the diffusion layer is also discussed.

Effect of Cr/Mn segregation on pearlite–martensite banded structure of high carbon bearing steel

The effect of Cr/Mn segregation on the abnormal banded structure of high carbon bearing steel was studied by reheating and hot rolling.With the use of an optical microscope, scanning electron microscope, transmission electron microscope, and electron probe microanalyzer, the segregation characteristics of alloying elements in cast billet and their relationship with hot-rolled plate banded structure were revealed.The formation causes of an abnormal banded structure and the elimination methods were analyzed.Results indicate the serious positive segregation of C, Cr, and Mn alloy elements in the billet.Even distribution of Cr/Mn elements could not be achieved after 10 h of heat preservation at 1200℃, and the spacing of the element aggregation area increased, but the segregation index of alloy elements decreased.Obvious alloying element segregation characteristics are present in the banded structure of the hot-rolled plate.This distinct white band is composed of martensitic phases.The formation of this abnormal pearlite–martensite banded structure is due to the interaction between the undercooled austenite transformation behavior of hot-rolled metal and the segregation of its alloying elements.Under the air cooling after rolling, controlling the segregation index of alloy elements can reduce or eliminate the abnormal banded structure.

Influence of band microstructure on carbide precipitation behavior and toughness of 1 GPa-grade ultra-heavy gauge low-alloy steel

This study investigated the influence of band microstructure induced by centerline segregation on carbide precipitation behavior and toughness in an 80 mm-thick 1 GPa low-carbon low-alloy steel plate.The quarter-thickness(1/4t)and half-thickness(1/2t)regions of the plate exhibited similar ductility and toughness after quenching.After tempering,the 1/4t region exhibited~50%and~25%enhancements in both the total elongation and low-temperature toughness at-40°C,respectively,without a decrease in yield strength,whereas the toughness of the 1/2t region decreased by~46%.After quenching,both the 1/4t and 1/2t regions exhibited lower bainite and lath martensite concentrations,but only the 1/2t region exhibited microstructure bands.Moreover,the tempered 1/4t region featured uniformly dispersed short rod-like M_(23)C_(6)carbides,and spherical MC precipitates with diameters of~20–100 nm and<20 nm,respectively.The uniformly dispersed nanosized M_(23)C_(6)carbides and MC precipitates contributed to the balance of high strength and high toughness.The band microstructure of the tempered 1/2t region featured a high density of large needle-like M3C carbides.The length and width of the large M3C carbides were~200–500 nm and~20–50 nm,respectively.Fractography analysis revealed that the high density of large carbides led to delamination cleavage fracture,which significantly deteriorated toughness.

Occurrence of Dynamic Shear Bands in AISI 4340 Steel under Impact Loads

In this study, occurrence of adiabatic shear bands in AISI 4340 steel under high velocity impact loads is investigated using finite element analysis and experimental tests. The cylindrical steel specimen subjected to impact load was divided into different sections separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower at the section where the adiabatic shear bands are expected to initialize. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for formation of transformed band in the alloy. The experimental results also show that cracks were initiated and propagated along transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by strain-rates. The simulation results obtained were compared with experimental results obtained for the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

Effect of hot-band annealing process on microstructure,texture,and r-value of final sheets of ferritic stainless steel

In this paper,the effect of different annealing processes on the microstructure,texture,and formability of ferritic stainless steel was studied in detail.The results showed that the grain size in the final sheet was larger and the recrystallization texture was more uniform after the final recrystallization annealing of hot-rolled steel with continuous annealing than that without annealing or with batch annealing.In addition,the final sheet had a higher average r-value and the lowest planar anisotropy.

Separation occurring during the drop weight tear test of thick-walled X80 pipeline steels

A separation phenomenon occurring during the drop weight tear test of commercial thick-walled API （American Petroleum Institute） X80 strip steel was investigated in this work. Microstructural analysis showed that the band structure of bainite elongated along the rolling direction works as the initiation sites of separation. The propagation of separation can be promoted not only by the occurrence of the band structure of martensite/austenite constituent, prior austenite grain boundaries, and elongated bainite, but also by fine acicular ferrite and bainite. Wide separation formed in the former case, while the narrow one appeared in the latter case. acicular ferrite in thick-walled X80 pipeline steel in order Some methods were proposed to obtain fine and homogeneous to minimize the occurrence of separation.

Adiabatic shear localization evolution for steel based on the Johnson-Cook model and gradient-dependent plasticity

Gradient-dependent plasticity is introduced into the phenomenological Johnson-Cook model to study the effects of strainhardening, strain rate sensitivity, thermal-softening, and microstructure. The microstructural effect （interactions and interplay among microstructures） due to heterogeneity of texture plays an important role in the process of development or evolution of an adiabatic shear band with a certain thickness depending on the grain diameter. The distributed plastic shear strain and deformation in the shear band are derived and depend on the critical plastic shear strain corresponding to the peak flow shear stress, the coordinate or position, the internal length parameter, and the average plastic shear strain or the flow shear stress. The critical plastic shear strain, the distributed plastic shear strain, and deformation in the shear band are numerically predicted for a kind of steel deformed at a constant shear strain rate. Beyond the peak shear stress, the local plastic shear strain in the shear band is highly nonuniform and the local plastic shear deformation in the band is highly nonlinear. Shear localization is more apparent with the increase of the average plastic shear strain. The calculated distributions of the local plastic shear strain and deformation agree with the previous numerical and experimental results.

Adiabatic Shear Localization for Steels Based on Johnson-Cook Model and Second-and Fourth-Order Gradient Plasticity Models

To consider the effects of the interactions and interplay among microstructures, gradient-dependent models of second- and fourth-order are included in the widely used phenomenological Johnson-Cook model where the effects of strain-hardening, strain rate sensitivity, and thermal-softening are successfully described. The various parameters for 1006 steel, 4340 steel and S-7 tool steel are assigned. The distributions and evolutions of the local plastic shear strain and deformation in adiabatic shear band （ASB） are predicted. The calculated results of the second- and fourth- order gradient plasticity models are compared. S-7 tool steel possesses the steepest profile of local plastic shear strain in ASB, whereas 1006 steel has the least profile. The peak local plastic shear strain in ASB for S-7 tool steel is slightly higher than that for 4340 steel and is higher than that for 1006 steel. The extent of the nonlinear distribution of the local plastic shear deformation in ASB is more apparent for the S-7 tool steel, whereas it is the least apparent for 1006 steel. In fourth-order gradient plasticity model, the profile of the local plastic shear strain in the middle of ASB has a pronounced plateau whose width decreases with increasing average plastic shear strain, leading to a shrink of the portion of linear distribution of the profile of the local plastic shear deformation. When compared with the sec- ond-order gradient plasticity model, the fourth-order gradient plasticity model shows a lower peak local plastic shear strain in ASB and a higher magnitude of plastic shear deformation at the top or base of ASB, which is due to wider ASB. The present numerical results of the second- and fourth-order gradient plasticity models are consistent with the previous numerical and experimental results at least qualitatively.

Effects of asymmetric rolling process on ridging resistance of ultra-purified 17%Cr ferritic stainless steel

In ferritic stainless steels, a significant non-uniform recrystallization orientation and a substantial texture gradient usually occur, which can degrade the ridging resistance of the final sheets. To improve the homogeneity of the recrystallization orientation and reduce the texture gradient in ultra-purified 17%Cr ferritic stainless steel, in this work, we performed conventional and asymmetric rolling processes and conducted macro and micro-texture analyses to investigate texture evolution under different cold-rolling conditions. In the conventional rolling specimens, we observed that the deformation was not uniform in the thickness direction, whereas there was homogeneous shear deformation in the asymmetric rolling specimens as well as the formation of uniform recrystallized grains and random orientation grains in the final annealing sheets. As such, the ridging resistance of the final sheets was significantly improved by employing the asymmetric rolling process. This result indicates with certainty that the texture gradient and orientation inhomogeneity can be attributed to non-uniform deformation, whereas the uniform orientation gradient in the thickness direction is explained by the increased number of shear bands obtained in the asymmetric rolling process.

Characteristics of Strain-Induced Ferrite in Low Carbon Steel

The strain-induced ferrite formed under different conditions was observed with SEM and optical microscope.The nucleation sites of strain-induced ferrite include grain boundary,grain inside,deformed band and annealing twin boundary.The shapes of the ferrite accordingly are equiaxed irregular polygonal,strip-shaped and acicular.

GROWTH PROCESS OF LOW-TEMPERATURE PLASMA-NITRIDING LAYER ON AUSTENITIC STAINLESS STEEL

The growth process of low-temperaturs plasma-nitriding layer was investigated by scan-ning electron microscopy (SEM) and X-ray diffraction (XRD). The layer is composedof expanded fcc phase (γN), whose lattice parameter of the layer increases with processtime resulting from increasing the nitrogen content. The layer hardness increases grad-ually with nitrogen content. The high slip band density on the layer surface observedin situ by SEM shows that the surface yield occurs when supersaturated nitrogen con-tent in the layer attains to some value, which is also responsible for the increase inlayer hardness.

Microstructure of stainless steel weld in double-sided arc welding

Double-sided arc welding with a single power source can effectively increase the weld penetration, diminish distortion, improve welding speed and save energy. Compared to conventional arc welding processes, double-sided arc welding can generate a penetrating electromaguetic field to help to form fine dendritic microstrueture in the weld due to the symmetry of heating. Type 1Cr1SNi9Ti aastenitic stainless steel was bead-on-plate welded with double-sided arc welding and conventional plasma arc welding processes, respectively, and microstructure in the weld, heat-affected zone and base metal were examined. After analyzing the black carbon-enriched band in the weld during plasma arc welding with electron probe microanalyzer （ EPMA ） and X-ray diffraction （XRD） technology, it was found that the black band was shaped from the aggregation of ferrite in the fasion boundary. Hardness measurement showed that this black band does not apparently affect the microhardncss distribution in the weld.

Deformation-induced Microstructures of High-Mn Austenite Steel

Deformation-induced microstructures of high-Mn austenite steel was investigated by metallography,X-ray diffraction and SEM.The ε-martensite and slip-bands are deformation-in- duced on the{111} planes,and appear as thin straight laths with 60～80° alignment difference be- tween them.It was found that ε-martensite and slip bands are kinked at fcc twin boundaries with the kinked angle 35～40°.The bands of equilateral triangle in the microstructure of tensile deformation are presented.

冷轧压下率对高强无取向电工钢变形组织和磁性能的影响

借助场发射扫描电镜(SEM+EBSD)、快速升温管式炉和交流磁性能测量仪研究高强无取向电工钢冷轧变形组织和磁性能随冷轧压下率的变化规律。结果表明:冷轧高强无取向电工钢中形成含有大量亚结构的粗糙条带和变形量较小的光滑条带。粗糙条带具有γ取向,主要为{111}〈110〉,光滑条带具有{112}〈110〉和{001}〈110〉取向。粗糙条带中存在大量宽度约为2~3μm的剪切带,与轧向呈20°~35°夹角,且随压下率增大,数量逐渐增多。剪切带与基体间具有一定取向差,随压下率增大,剪切带逐渐从{111}〈110〉取向转向{223}〈110〉取向,与基体间取向差逐渐增大。退火后Goss和{111}面织构增多,{001}面织构减少;轧向磁感应强度增大,横向磁感应强度减小,各向异性显著,铁损下降。

临界淬火态热轧中锰钢单轴拉伸过程中的塑性失稳行为

结合数字图像相关法(digital image correlation,DIC)、红外热成像(infrared thermal imaging,IRT)、电子背散射衍射(electron backscatter diffraction,EBSD)和显微硬度分布,重点分析临界淬火态中锰钢在单轴拉伸过程中的塑性失稳行为。结果表明:实验钢在单轴拉伸过程中应变诱导马氏体转变主要发生在拉伸变形前期,存在明显的PLC(Portevin-Le Chatelier)带形成与传播,伴随着热量波动,经历了从A型到B型带再到A型PLC带的转变;PLC带的形成原因是增殖位错与间隙溶质原子的相互作用,即动态应变时效(dynamic strain aging,DSA),前期位错会优先向低密度位错区域运动即沿轴向单向传播(A型PLC带),中期位错的增殖和开始交互作用的影响致使形成跳跃性(B型)PLC带,后期位错密度的增大和交互作用的增强,再次致使A型PLC带的形成。

轴承钢滚动接触疲劳亚表面夹杂处损伤分析

轴承钢在滚动接触疲劳(RCF)中失效的主要原因之一是亚表面白蚀区(WEA)的形成.本文中从塑性应变累积引起剪切局域化新的角度对WEA进行了研究.通过耦合晶体塑性和相场损伤理论建立了损伤演化本构模型,研究了非金属夹杂处塑性应变累积和损伤演化.研究表明,接触疲劳载荷引起的塑性应变局域化导致了剪切带的形成.剪切带的形貌、取向和应变与WEA的一致,表明WEA实际上是应变局域化的剪切带.晶体取向对WEA损伤的形成和发展有很大的影响,WEA仅在择优的晶体取向下形成.与软夹杂周围的剪切带和损伤演化不同,硬夹杂处的剪切带与夹杂相切,形成的4条剪切变形带将夹杂“包围”.剪切带内部处于高应变和低应力的状态,带中心处应变达到最大,随带宽两侧急剧减小,而中心处应力却最小,几乎为零,沿带宽两侧增大,这说明裂纹在剪切带内萌生和扩展.该结论阐明了裂纹和WEA形成的关系,即裂纹是在WEA形成过程中因应变不协调产生,而非裂纹先产生,裂纹上下表面相互摩擦导致WEA形成.

微合金钢等温过程中带状组织演变规律研究

采用热模拟试验机、金相显微镜和扫描电镜研究了微合金钢在等温过程中的带状组织演变规律,并对其形成机理进行了分析。结果表明:试样在650~700℃等温时,带状组织为铁素体/马氏体。在550~600℃等温时,带状组织为针状铁素体/马氏体,且随着等温温度的下降,针状铁素体的体积分数不断升高。Mn的带状偏析是导致该等温期间不完全转变现象的根本原因。当等温温度降到450℃时,组织全部为针状铁素体,带状组织基本消失。

晶体结构对316不锈钢腐蚀影响的第一性原理研究

316不锈钢的耐腐蚀问题是研究者关注的热点之一。基于密度泛函理论第一性原理,运用Materials Studio软件中CASTEP模块,对316不锈钢的晶体(奥氏体、铁素体、渗碳体和Fe合金)进行总能量和电子结构模拟计算,考察晶胞总能量、能带结构及电子态密度对316不锈钢腐蚀性能的影响。晶胞总能量计算结果表明,渗碳体和Fe合金的晶胞总能量最小,其次是奥氏体,最后为铁素体。说明,316不锈钢中渗碳体和Fe合金的性质最稳定、抗腐蚀性能最好,而铁素体的抗腐蚀性最差。随着316不锈钢中Cr含量增加,晶胞总能量逐渐减小,各相态晶体结构的稳定性逐渐变强。说明,当受到腐蚀介质侵蚀时,增加Cr的含量可提高316不锈钢的稳定性和耐腐蚀性。能带图和电子态密度结果表明,渗碳体和Fe合金的能带宽度小而密集且局域性强,而铁素体能带宽度大且非局域性强。说明,渗碳体和Fe合金的性质更稳定、抗腐蚀性能更好,而奥氏体次之,铁素体抗腐蚀性最差。随着316不锈钢中Cr含量增加,含Cr奥氏体的最大峰值对应的能量越低,区域分布的核外电子数越少。核外电子分布较少,不易失去电子,也不易发生氧化还原反应,结构越稳定。在发生腐蚀过程中,含Cr高的奥氏体结构更稳定,耐腐蚀性能更好,这一结论符合贫铬理论或晶间区偏析杂质理论。

基于BSCWEformer的退火炉内分组式辊速序列预测

退火炉内带钢的长度受到温度、张力等因素的影响而变化,导致辊的转速改变以及焊缝位置的不确定,从而威胁生产安全.为了准确预测辊的转速以计算焊缝的实时位置,本文提出基于带状稀疏柯西自注意力的BSCWEformer (banded sparse Cauchy weight enhanced Transformer)模型.模型采用带状稀疏的、使用基于相对位置计算的柯西分布权重值增强的自注意力结构,在提高相邻输入序列的重要性的同时,将自注意力的复杂度由二次方降低为线性.通过实际生产数据进行实验,并与LogSparse Transformer、Transformer、RNMT+等模型进行对比,得出本文所提出的BSCWEformer模型在退火炉内分组式辊速序列预测任务上具有较高的预测精度.