Electron-beam rapid quenching of an ultra high-strength alloy steel

Rapid quenching using electron beams has been used to treat the surface layers of an ultra high-strength alloy steel. The microstructure of the treated surface layers has been investigated by optical, scanning and electron microscopy and by X-ray diffractometry. The microstructure produced by conventional solid state quenching of the same steel has also been examined for comparison. This microstructural study shows that the rapid quenching process leads to a high degree of grain refinement and an increase in solid solubility which, in turn, increases the amount of retained austenite. The lowering ofM _s temperature due to the high cooling rate and the increased solid solubility favour the formation of twinned martensite. These interlinked phenomena have increased the microhardness of the rapidly quenched layer considerably, with respect to that of the solid state quenched steel.     

A microstructural study of a melt-spun ultra high-strength alloy steel

Melt spinning in a controlled atmosphere has been used to produce ribbons of an ultra high-strength alloy steel. The microstructure of these ribbons has been investigated using both optical and transmission electron microscopy. Comparison of the microstructure with that produced by conventional solid-state quenching shows that the martensitic structure is refined in the melt-spun ribbon and the formation of alloy carbides suppressed. These factors lead to a considerable increase in hardness of the meltspun ribbon with respect to that of the solid-state quenched material. The occurrence of interlath austenite is discussed in terms of the crystallography of the martensite transformation.     

Effect of shear cutting on ductility of a dual phase steel

Both an experimental investigation with an especially designed setup and a mechanical FE analysis of the cutting process showed that for the laboratory dual phase steel investigated, cutting involves positive stress triaxiality and ductile fracture mainly due to void nucleation and coalescence at ferrite-martensite interfaces. Tensile tests on as-cut strip specimens showed a large reduction in ductility due to the presence of damage on the edges of the strips. Tensile tests on strip specimens containing short precracks and mechanical analysis showed that the cutting affected area behaves as a precrack during subsequent mechanical testing.     

Effects of stirrup,steel fiber,and beam size on shear behavior of high-strength concrete beams

This study investigates the effectiveness of steel fibers and minimum amount of stirrups on the shear response of various sized reinforced high-strength concrete (HSC) beams. For this, six large reinforced HSC beams with a shear span-to-depth ratio (a/d) of 3.2 were manufactured. Three of them contained 0.75% (by volume) steel fibers without stirrups as per ACI Committee 318, while the rest were reinforced with the minimum amount of stirrups without fibers. Test results indicate that, with increasing beam size, significantly lower shear strength was obtained for steel fiber-reinforced high-strength concrete (SFR-HSC) beams without stirrups, than for the plain HSC beams with stirrups. The inclusion of steel fibers effectively limited crack propagation, produced more diffused initial flexural cracks, and led to higher post-cracking stiffness, compared to plain HSC. On the other hand, the use of minimum stirrups gave better shear cracking behaviors than that of steel fibers, and effectively mitigated the size effect on shear strength. Therefore, a large decrease in shear strength, with an increase in the beam size, was only obtained for SFR-HSC beams without stirrups. A shear strength decrease of 129% was obtained by increasing the effective depth from 181 mm to 887 mm. The shear strengths of reinforced steel fiber-reinforced concrete beams were not accurately predicted by most previous prediction models. Therefore, a new shear strength formula, based on a larger dataset, that considers the size effect, is required.     

正交切削高强度钢绝热剪切带组织和硬度研究

为了研究切削速度和工件硬度对高强度钢锯齿形切屑内绝热剪切带显微组织和硬度的影响,利用光学显微分析、SEM和TEM以及硬度测量等方法观察和测量了不同切削速度下正交切削两种回火硬度的30CrNi3MoV钢形成的锯齿形切屑中绝热剪切带的微观组织和显微硬度的变化过程.结果表明:低速下形成以组织剧烈拉长为特征的形变带,高速下形成以组织严重细化为特征的转变带;工件硬度的提高有利于形成转变带;增加切削速度和工件硬度对转变带硬度影响很小,但会显著提高形变带硬度.     

Evaluation of shear flow in composite wind turbine blades

The present work addresses the evaluation of the shear flow as an extension of the Jourawski’s formula. This idea is developed here for the case of a multi-celled composite thin-walled section. Firstly, the explicit formulation of the shear flow due to shear forces and torsion is derived, noting the simplificative hypothesis adopted. Then, the implemented model is verified by means of a benchmark problem with a known analytical solution. Finally, this model is utilized to evaluate the shear flow on an actual blade configuration, comparing the results obtained with those of a Finite Element model of the same blade, with a similar discretization.     

Axial crushing of thin-walled high-strength steel sections

Quasi-static and dynamic axial crushing tests were performed on thin-walled square tubes and spot-welded top-hat sections made of high-strength steel grade DP800. The dynamic tests were conducted at velocities up to 15 m/s with an impacting mass of 600 kg in order to assess the crush behaviour, the deformation force and the energy absorption. Typical collapse modes developed in the sections and the associated energy absorbing characteristics were examined and compared with previous studies on high-strength steel. A significant difference was observed between the quasi-static and the dynamic crushing tests in terms of the deformation force and impact energy absorption. As this difference is attributed to strain-rate and inertia effects, material tensile tests at elevated strain rates have been carried out. A comparison is made with analytical methods and the response was under-predicted. In addition, numerical simulations of the axial crushing of the thin-walled sections were performed and comparisons with the experimental results were satisfactory. The validated numerical model was used to study the energy absorption capacity of thin-walled sections with variations in the yield strength, sheet thickness, flange width and spot-weld spacing. Structural effectiveness differences have been captured through simulations between spot-welded top-hat sections made of mild steel and high-strength steel.     

Observation of hydrogen distribution in high-strength steel

The diffusible hydrogen in Cr-Mo steels are observed with autoradiography technique. Specimens with the diffusible hydrogen are prepared by an electrochemical cathodic charging method and those without the diffusible hydrogen by annealing at 373 K after charging hydrogen. TEM autoradiographs suggests, by the developed silver grains, that the hydrogen trapping sites are the grain boundary and internal interface of ferrite-cementite and ferrite-lath structure. After keeping the sample at 373 K, the silver grains disappeared. Most of hydrogen trapping sites release almost all the hydrogen at 373 K. It is clear that these sites of high-strength steels supplies the diffusible hydrogen. Hydrogen absorption characteristics of quench hardening tempering Cr-Mo steels have been evaluated by thermal desorption spectrometry (TDS). From tritium electron microscopic autoradiography and TDS analysis, the lower temperature (360 K–370 K) peaks show the diffusing hydrogen which is released a few days. The diffusible hydrogen from trapping sites such as the internal interface of ferrite-cementite or ferrite-lath structure are distinguished to the diffusing hydrogen.     

Effect of steel fibres on mechanical properties of high-strength concrete

Steel fibre reinforced concrete (SFRC) became in the recent decades a very popular and attractive material in structural engineering because of its good mechanical performance. The most important advantages are hindrance of macrocracks’ development, delay in microcracks’ propagation to macroscopic level and the improved ductility after microcracks’ formation. SFRC is also tough and demonstrates high residual strengths after appearing of the first crack. This paper deals with a role of steel fibres having different configuration in combination with steel bar reinforcement. It reports on results of an experimental research program that was focused on the influence of steel fibre types and amounts on flexural tensile strength, fracture behaviour and workability of steel bar reinforced high-strength concrete beams. In the frame of the research different bar reinforcements (2∅6 mm and 2∅12 mm) and three types of fibres’ configurations (two straight with end hooks with different ultimate tensile strength and one corrugated) were used. Three different fibre contents were applied. Experiments show that for all selected fibre contents a more ductile behaviour and higher load levels in the post-cracking range were obtained. The study forms a basis for selection of suitable fibre types and contents for their most efficient combination with regular steel bar reinforcement.     

Effect of microstructures on deformation behaviour of high-strength low-alloy steel

The role played by microstructural constituents of high-strength low-alloy (HSLA) steel in controlling the deformation processes has been studied. The steel was solution treated and water quenched followed by ageing at various temperatures. Microstructural characterization has been carried out by using scanning electron microscope and transmission electron microscope. Tensile tests were conducted as per ASTM standard at constant displacement rate. The conditions under which microvoid coalescence was suspended in spite of a constant resident population of void initiating carbide and carbo-nitride particles have been explained. The major role played by the coherency of Cu precipitates in controlling dislocations movement; and hence, plastic flow is thought to be responsible for the effects observed.     

Effect of tempering treatment on hydrogen-induced cracking in high-strength steel

The fracture characteristics of high-strength steel ASTM A-490 under a hydrogen environment were investigated, with special emphasis placed on changes in fracture characteristics due to a tempering treatment at temperatures from 200 to 400 °C. A mechanical test was performed on cathodically charged specimens subjected to a constant load. Experimental analyses show that tempering treatment in the range from 200 to 400 °C does not alter the essential nature of delayed fracture due to crack growth. However, the role of intergranular (IG) cracking becomes prominent in the subcritical crack growth period with an increase in the tempering temperature to 400 °C. This development of IG cracks in the subcritical crack growth period is uniquely dependent on the tempering treatment performed in the tempering range from 250 to 400 °C. Furthermore, an increase in the fraction of the IG facet in the subcritical crack growth area is dependent on the increase in the stress intensity at the crack tip in those specimens tempered at 300 and 400 °C.     

The influence of hammer peening on fatigue in high-strength steel

An investigation of the influence of hammer peening on fatigue crack initiation and propagation in high-strength steel has been performed using edge notch specimens. The crack initiation time was found to decrease after peening; however, the fatigue crack growth stage remained unchanged or decreased depending upon the peening parameters. The results have been used to develop an analytical model to predict the fatigue crack growth rate in peened materials, based upon the compressive residual stress effect.     

基于高强度钢的冷挤压内螺纹抗疲劳机理研究

根据高强度钢内螺纹的成形过程,从微观的角度研究表层残余应力、表面粗糙度以及表面显微硬度与硬化层等零件状态方面的因素对内螺纹疲劳性能的影响。结果表明,冷挤压内螺纹表层金属具有细长纤维状结构,呈流线型沿牙面法相分布,晶粒被破碎和细化,位错密度显著增加,出现加工硬化现象,使得金属的强度与硬度增加。螺纹表层形成的残余压应力场,能够消除应力集中的影响,减小疲劳缺口敏感,延长裂纹萌生期,减慢或抑制裂纹的扩展。挤压螺纹的表面没有明显的加工痕迹,表面平整性较好,可有效地减小螺纹表面粗糙度,减少产生应力集中,有助于提高螺纹的疲劳性能。