Hardening and softening mechanisms of pearlitic steel wire under torsion

The mechanical behaviors and microstructure evolution of pearlitic steel wires under monotonic shear deformation have been investigated by a torsion test and a number of electron microscopy techniques including scanning electron microscopy (SEM) and transmission electron microscopy (TEM), with an aim to reveal the softening and hardening mechanisms of a randomly oriented pearlitic structure during a monotonic stain path. Significantly different from the remarkable strain hardening in cold wire drawing, the strain hardening rate during torsion drops to zero quickly after a short hardening stage. The microstructure observations indicate that the inter-lamellar spacing (ILS) decreases and the dislocations accumulate with strain, which leads to hardening of the material. Meanwhile, when the strain is larger than 0.154, the enhancement of dynamic recovery, shear bands (SBs) and cementite fragmentations results in the softening and balances the strain hardening. A microstructure based analytical flow stress model with considering the influence of ILS on the mean free path of dislocations and the softening caused by SBs and cementite fragmentations, has been established and the predicted flow shear curve meets well with the measured curve in the torsion test.     

Non-monotonic radial distribution of tensile yielding strength in cold-drawn pearlitic wire

It is commonly accepted that the surface layer in the cold-drawn pearlitic steel wire has a higher strength than the centre. In the present work, via testing the cold-drawn wire after removing the surface layer by an electrochemical method, it is discovered that the tensile yielding strength distributes non-monotonically from the surface to the centre. A valley is observed at the sub-surface. It is found that the maximum accumulative strain during drawing occurs at the sub-surface which leads to more hardening. The presence of residual stress after drawing reduces the tensile yielding stress. With increasing thickness of the removed layer, the tensile yielding stress increases monotonically. Finally, the only reason for the non-monotonic distribution of the tensile yielding stress is the strain path which the material at the sub-surface experiences in drawing and subsequent tension. It actually belongs to a reverse and a cross loading, which result in softening.     

Influences of simple strain path changes on mechanical behaviours of pearlitic steel wire

The mechanical behaviours of the pearlitic steel wires under simple strain path changes (SSPCs) are investigated, including monotonic, cross and reversal loadings. Both the cross and reversal loadings lead to an apparent softening. Especially during the whole reversal loading, the stress is much lower than before unloading. And it is a function of the pre-strain. Additionally, it is found that the maximum strain increases significantly in reversal loading. The microstructures under different SSPCs are investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), with an aim to reveal the micro-mechanisms of the SSPC effects on the mechanical behaviours.     

Medium carbon fully pearlitic steel: microstructures and properties after wire drawing

In the current work, fully pearlitic microstructure was initially developed in a medium carbon steel by isothermal holding at different temperatures. Subsequently, these samples were drawn in laboratory scale wire-drawing machine without any intermediate heat treatment. Microstructures and mechanical properties of as-heat-treated and heat-treated plus drawn samples were characterised. The results were also compared with some commercially produced steel. It was found that the new steel can offer a very good wire drawing behaviour. The steel possesses a good combination of strength and ductility along with a high-value torsion, bending and reverse bending properties after drawn condition as well.     

Study on dislocation slips in ferrite and deformation of cementite in cold drawn pearlitic steel wires from medium to high strain

Abstract

The deformation of cementite was studied via optical microscopy and SEM in the longitudinal sections of pearlitic steel wires from medium to high strain. The cementite shows good deformability, the angle between the deformation direction of cementite and drawing direction decreases with increasing strain, and finally the deformation directions of cementite turn to the drawing axis at high strains. The deformation of the cementite is strongly related to plastic deformation in the ferrite, with coarse slip steps, S-bands and cracks across cementite observed parallel to either {110}_α-Fe or {112}_α-Fe plane traces determined by the largest Schmid factors.     

Strain aging of pearlitic steel wire during post­drawing heat treatments

Abstract

The thermal treatment involved in galvanising affects the mechanical properties of cold drawn pearlitic steels. There is a trough in the ductility at short heating times, which recovers after further heating, but only at the expense of strength. The objective of this work was to gain an understanding of the underlying processes that lead to these changes in properties. A 0.85 wt-%C, vanadium microalloyed, fully pearlitic steel was studied. Samples were heat treated for up to 90 s in a salt bath at 500°C to simulate the galvanising process. The effects of the post­drawing heat treatment on the microstructure and carbon atom distribution were investigated using differential scanning calorimetry, SEM, TEM, and atom probe field ion microscopy. Carbide fragmentation occurs during the heat treatment, representing the onset of the spheroidisation process. The carbon concentration in the ferrite matrix remains low in all heat treatment conditions. However, localised regions of ferrite containing approximately 2.5 at.-%C are observed, indicating the presence of Cottrell atmospheres around dislocation lines. Planar regions enriched in carbon are also seen, which demonstrate segregation to ferrite sub­boundaries. A simple model of the carbon redistribution process is proposed.     

Cumulative strain theory for simulation and development of wire drawing

The problem of deformability of metal without fracture is of great importance. In drawing practice it is required that the wire of the finished size should have specific mechanical and technological properties. In choosing the diameter of the initial rolled wire it is necessary to know not only initial parameters of plasticity and strength but also the character of their change in drawing. In the absence of a generalized parameter of metal plasticity it is not possible to make a critical examination of wire‐drawing routes in terms of parameters of plasticity. In the present work an attempt is made to provide a mathematical apparatus based on a phenomenological model of fracture in order to compute the plasticity reserve of hot‐rolled rod when it is subjected to both drawing and to control tests.     

基于数值模拟的冷拉钢丝表面精度Ra的优化

为研究拉拔参数对冷拉钢丝表面精度Ra的影响,采用有限元软件MARC建立了钢丝表面微观形态的分析模型,对钢丝拉拔过程进行了模拟仿真,利用正交试验设计法研究了主要拉拔参数(拉拔速度v、过渡圆弧半径R、模具定径带长度L、模具入模角α、摩擦系数μ)对钢丝成形表面精度Ra的影响,并对模拟结果进行了极差分析和方差分析.结果表明:冷拉钢丝在一定断面缩减率的条件下Ra最佳的拉拔参数组合为v=30 mm/s、R=1 mm、L=3 mm、α=11°、μ=0.4);各因素对Ra的影响程度次序为B(R)C(L)E(μ)A(v)D(α);随v和L的增大Ra先减少后增大,α、R及μ对Ra的影响并无明显规律.在此基础上对优化方案进行模拟仿真,发现钢丝表面粗糙度有了显著改善,这表明正交试验设计对冷拉钢丝表面精度优化的有效性.     

铁素体管线钢的分层裂纹及其对断裂的影响

通过对针状铁素体管线钢缺口根部三维应力状态的有限元分析和不同形式的断裂实验,研究了管线钢分层裂纹产生的条件及其对断裂性能的影响.结果表明裂纹或缺口根部的三维应力状态是产生分层裂纹的必要条件,材料的强度分布影响分层裂纹的形式和方向.分层裂纹均为主裂纹扩展前材料中的弱界面在垂直该弱界面的拉应力作用下产生的,其数量和方向受裂纹端部三维应力场和材料的强度分布状态控制.分层裂纹面上的应力为零,分层裂纹有一定的间距.在断裂过程中产生的分层裂纹使裂纹或缺口根部的构形发生改变,从而对裂尖的应力状态和材料的断裂性能产生巨大的影响.穿透裂纹体的分层裂纹使其有效厚度减小,表面裂纹体的分层裂纹与裂纹扩展方向垂直.在断裂过程中产生分层裂纹需要消耗更多的能量、降低裂端三维应力约束、有效厚度降低或裂尖钝化.这些因素均使断裂扩展更加困难,而使材料韧性得到提高.     

The effect of niobium on the microstructure of ferritic stainless steel

The effect of different amounts of Nb and of homogenization on the ferritic stainless steels containing 17–18 wt.% Cr was investigated with scanning electron microscopy (SEM), optical microscopy, energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and differential thermal analysis (DTA). It was observed that M₂₃C₆, NbC and sigma phase formed in these steels. In addition, the formation of Nb₂C was observed in the sample containing 3.0 wt.% Nb. While the amount of Nb increased from 0.5 to 3.0 wt.% Nb, the microhardness of the matrix and the amount of M₂₃C₆ decreased and the toughness of the samples increased. After homogenization, the increase in the toughness of the samples containing 1.5–3.0 wt.%Nb was considerable and impressive.     

The effect of moisture on buckle delamination of thin inorganic layers on a polymer substrate

Moisture-induced buckle delamination of thin inorganic layers on a polymer substrate was studied. Moisture has been found to have a significant effect on the failure mode. Experimentally, an increase in the buckle width, height and the total buckle delamination length with time and humidity was observed. Moreover, a transition from straight to telephone-cord buckle pattern was taken place in a humid environment. Applying only a uniaxial compressive strain on the thin layers did not result in the transition from straight to telephone-cord. For a compliant substrate the transition from straight to telephone-cord buckle occurred at significantly higher ratio of residual strain over critical buckling strain than for a rigid substrate. A simple model for buckling was applied. Using the energy release rate, the interfacial toughness was investigated as a function of relative humidity.     

The microstructure and mechanical properties of drawn and aged pearlitic steel wires

Cold-drawn pearlitic steel wires exhibit the highest strength amongst steel products for commercially use. The microstructure and the mechanical properties of wires are influenced by the drawing and post-drawing aging process. The transition of lamellar structure, e.g. cementite fragmentation, dissolution and re-precipitation, ferrite refinement, by drawing and recovery and recrystallisation by aging are summarised. In addition, the corresponding changes in mechanical properties, e.g. yield strength, delamination, hydrogen embrittlement and fatigue are summarised.     

拉深比对304不锈钢圆筒件残余应力的影响

304不锈钢圆筒拉深件在其口部易发生纵向开裂,外壁残余拉应力过大是造成其纵向开裂的根本原因之一.本文结合有限元法分析不同拉深比所得圆筒拉深件的残余应力,用纳米压痕试验研究了拉深比对304不锈钢圆筒拉深件筒壁残余应力的影响.结果表明:304不锈钢圆筒拉深件外壁的残余拉应力从筒底到口部先增大后减小,最大残余应力出现在筒壁中部约60%筒壁高度处;纳米压痕测得拉深比为1.43、1.54、1.67和1.82拉深圆筒件筒壁的最大残余应力分别为391.87、745.30、793.74和1 013.1 MPa;最大残余应力随拉深比的增大而增大.与其他文献对比分析,此研究结果是正确可靠的.     

Loading sequence effect on fatigue life of Type 316 stainless steel

The change in fatigue life due to variable cyclic loading was investigated experimentally in order to consider the loading sequence effect in fatigue damage assessment for a component design, and the reason for the change was discussed. Strain-controlled fatigue tests, that is, two-step, surface removal two-step, repeating two-step and periodical overload tests were conducted using Type 316 stainless steel specimen in a room temperature laboratory environment. The high-low loading amplitude sequence for the two-step test, and the repeating two-step and periodical overload tests showed a shorter fatigue life than that predicted by the linear damage accumulation rule. On the other hand, the low–high loading amplitude sequence for the two-step test exhibited a longer fatigue life. The reduction in the fatigue life was mainly attributed to the change in effective strain amplitude. The fatigue life reduction due to the loading sequence effect could be assessed conservatively by determining the allowable number of cycles for effective stress amplitude. Namely, by assuming the crack mouth was fully opened in the assessment, predicted fatigue life became shorter than the experimental results. It was concluded that the margin of 1.3–2.3 should be considered in the design fatigue curve in order to take account of the reduction in fatigue life due to the loading sequence effect.     

Effect of stress profile on microstructure evolution of cold-drawn commercially pure aluminum wire analyzed by finite element simulation

The evolution of microstructure in the drawing process of commercially pure aluminum wire (CPAW) does not only depend on the nature of materials, but also on the stress profile. In this study, the effect of stress profile on the texture evolution of the CPAW was systematically investigated by combining the numerical simulation and the microstructure observation. The results show that the tensile stress at the wire center promotes the formation of <111> texture, whereas the shear stress nearby the rim makes little contribution to the texture formation. Therefore, the <111> texture at the wire center is stronger than that in the surface layer, which also results in a higher microhardness at the center of the CPAW under axial loading.     

Crystallographic texture evolution and martensite transformation in the strain hardening process of a ferrite-based low density steel

The strain-induced martensite transformation is of great importance in the strain hardening process of ferrite based low-density steel.Based on the microstructure analysis,the texture evolution and martensite transformation behavior in the strain hardening process were studied.The results show that martensite transformation accompanied by TWIP effect and high density dislocations maintains the con-tinuous hardening stage.As the strain increases,the texture of retained austenite evolves towards the F orientation{111}〈112〉,which is not conducive to martensite transformation.After the strain of 5％,the number of austenite grains with high Schmid factor orientations is gradually increased,and then signif-icantly reduced when the strain is over 10％due to the occurrence of martensitic transformation,which results in a high martensitic transformation rate.However,the unfavorable orientation and the reduced grain size of austenite slow down the martensite transformation at the final hardening stage.Moreover,because of the coordination deformation of austenite grains,strain preferentially spreads between adja-cent austenite grains.Consequently,the martensite transformation rate in strain hardening process is dependent on the orientation and grain size evolution of austenite,leading to a differential contribution to each strain hardening stage.     

The effect of microstructure on hardness and toughness of low carbon welded steel using inert gas welding

The heat affected zone (HAZ), has a great influence on the properties of welded joints since it can alter the microstructure and residual stresses. In this paper, the effect of welding parameters and heat input on the HAZ and grain growth has been investigated. The role of grain size on hardness and toughness of low carbon steel has also been studied.     

制备参数对不锈钢丝网催化剂结构和性能的影响

采用阳极氧化工艺在不锈钢丝网载体上自生长一层氧化膜,再在其上负载活性组分铈、铂和钯,制备了金属丝网催化剂.研究了阳极氧化的电流密度和电解液种类等因素对阳极氧化膜形成的影响,以及还原时间和焙烧温度对催化剂氧化活性的影响.结果表明,改变制备参数对氧化膜结构有较大影响,进而影响到催化剂的活性.采用10%H2SO4电解液,控制电流密度为1.0 A时得到的氧化膜载体浸渍活性组分后,具有良好的催化活性和稳定性.     

The effect of duplex stainless steel microstructure on its cavitation morphology in seawater

An ultrasonically induced cavitation facility was used to study the effect of a cast duplex stainless steel (DSS) microstructure on its corrosion behavior in seawater. Under cavitation conditions, small cavities initiated in the ferritic matrix and at the ferrite–austenite boundaries. With the progress of cavitation, the attack concentrated in the austenitic phase and then spread to the ferritic phase and was associated with cleavage-like facets, ductile tearing, river patterns and crystallographic steps at later stages. Cross-sections of specimens revealed microcracks initiating from the ferritic matrix at the bottom of cavities. Crack propagation into the matrix was impeded by the austenitic islands.