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 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.

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.

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.

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.

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.

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.

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.

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。

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。

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.

轻质中锰钢局部变形带的研究现状

轻质中锰钢是第三代先进高强钢的代表,在变形过程中由于C、Mn、Al等合金元素的存在其可动位错经历被固溶原子和C-Mn原子团钉扎和脱扎的过程,从而在应力-应变曲线上呈现出吕德斯(Lüders)带和Portrvin-Le Chatelier(PLC)带这两种最典型的局部变形带,导致材料表面质量降低,对后续加工使用不利。目前,消除Lüders带和PLC带引起的表面缺陷的常用方法是在成形前对材料施加小的预应变或者调控材料的组织。然而这种方法只适用于经历小Lüders带应变和PLC效应的材料,因此寻求一种更为有效的延迟或消除Lüders带与PLC带的方法显得极其重要。本文从发生机理入手,讨论预应变、退火温度、退火时间等工艺和合金元素对上述两种局部变形带的影响,进而从晶粒尺寸、组织形貌、位错密度等微观角度总结减弱或加剧二者的具体机制,并着重解释不同成分与热处理工艺对C-Mn原子团形成、移动速率和与位错交互时间的影响以及抑制局部变形带的形成与拓展的作用。随后根据已有研究提出的不同解决方案,结合上述宏观与微观影响因素,展望了未来该领域研究的发展。

Q460C中厚钢板折弯裂纹形成原因

某厂生产厚度为20 mm的Q460C钢板在折弯过程中出现裂纹。采用宏观观察、化学成分分析、金相检验、扫描电镜及能谱分析等方法分析裂纹产生的原因。结果表明:试样折弯裂纹产生的主要原因为炼钢过程中钢液中P、S元素含量过高,且连铸过程存在钢包下渣的情况,导致铸坯中产生P、S元素偏析,铸坯内部存在大量非金属夹杂物;随着轧制过程温度的不断降低,钢板内部形成严重的带状组织,最终导致钢板在折弯过程中发生开裂。

水泥选粉机40CrMoA钢轴断裂原因

某水泥选粉机40CrMoA钢轴在服役10 d后出现断裂事故,采用宏观观察、化学成分分析、力学性能测试、金相检验等方法对其断裂原因进行分析。结果表明:钢轴显微组织为回火索氏体+条状或块状铁素体,组织中存在硫化物等非金属夹杂物和带状组织,夹杂物和带状组织处容易产生应力集中;裂纹在缺陷处萌生并扩展,最终导致钢轴断裂。

改善55钢零件高频感应淬火金相组织的措施

通过分析55钢制造减速器零件的高频感应淬火组织,并分别对零件的基体组织、感应加热功率、加热时间及淬火液浓度等影响淬火组织的各种因素进行试验验证,最后提出了改善55钢感应淬火金相组织的有效措施。

在线加速冷却对热轧C-Mn无缝钢管组织及性能影响

文章介绍了包钢钢管公司460生产线上一种C-Mn热轧无缝钢管在线加速冷却工艺试验。试验结果表明,在线加速冷却工艺对试验钢组织、性能影响显著,在线控冷能够抑制两相区先共析铁素体的产生,细化了基体晶粒,晶粒度从非控冷状态下的6.5级提高到了8级,改善了钢的带状组织。钢管力学性能及冲击性能有明显提高,屈服强度达到300~345 MPa,抗拉强度达到485~545 MPa,纵向室温冲击功平均值达到102 J,纵向0℃冲击功平均值达到86 J,纵向-20℃冲击功平均值达到60 J。

连铸轻压下对焊瓶钢成分及组织的影响

以HP295焊瓶钢为研究对象,针对焊瓶钢连铸生产过程中出现的铸坯中心偏析、轧后产品局部区域组织异常等问题进行了生产跟踪试验,发现轧后产品的夹杂物及带状组织与铸坯中心偏析位置存在对应关系。阐明了铸坯中心偏析问题如何遗传影响轧后产品的组织及夹杂物分布。通过调整压下量、压下速率及压下位置节点等措施,设计了全新的焊瓶钢连铸轻压下工艺模型;并依据工艺对比试验,结合低倍分析、原位分析、金相分析等检测手段,发现应用新轻压下工艺方案后,连铸坯中心偏析、中心疏松、三角区裂纹均不超过0.5级,Mn、S元素的偏聚情况均得到了显著减轻,相应的卷板产品组织带状及夹杂物也得到了明显改善。最终确认了合理的连铸轻压下工艺能够有效改善HP295焊瓶钢铸坯中心成分偏析,减轻轧后产品的带状组织及夹杂物偏聚问题。同时优化后的轻压下工艺已通过数字化模型方案,推广应用于焊瓶类系列钢种,并实现了批量稳定生产。