Studies on the role of microstructure on performance of a high strength armour steel

This investigation describes and analyses the experimental results pertinent to the ballistic performance of two apparently identical low alloy high strength steel plates against deformable lead projectiles at a velocity about 840 m/s. All the tests are carried out at normal impact angle, i.e. zero obliquity. One plate stopped all projectiles fired at it. However, the other plate failed to stop the projectiles at some locations. Both the plates were subjected to detailed analysis using standard metallurgical techniques to identify the cause of failure in one plate. The experimental results presented include the variation in the microstructure, hardness and retained austenite of the two target plates. The study concludes that the failure is caused by the decrease in resistance of the plate possibly due to higher retained austenite and coarser martensitic structure.     

Improved wear resistance of steels by cryotreatment: the current state of understanding

An important factor to enhance the efficiency and productivity of advanced manufacturing processes is to improve the wear resistance (WR) of tool steels. Cryotreatment in recent years has established itself as a relatively inexpensive additional step in conventional heat treatment schedule that can substantially improve the WR of tool steels. This report highlights the state of understanding of this emerging process with an emphasis on illustrating the underlying mechanisms responsible for the enhancement of WR of tool steels. Cryotreatment almost completely removes retained austenite and induces favourable precipitation of refined secondary carbides with improved distribution. These benefits, if harnessed suitably, are capable of providing significant improvement in both productivity and product quality leading to an efficient economy of manufacturing.     

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240,360,and 400 ℃.The results show that martensite,bainite,and retained austenite (RA) are the main microstructural phases.The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix,and the transformation of ultrafine bainite into coarse lath bainite and granular bainite,respectively.Thick filmy RA was distributed between bainite laths.The polygonal martensiteaustenite islands and blocky RA formed along the grain boundaries.The average carbon concentration in the matrix decreased with the temperature increase,while the impact toughness initially increased and then dropped with temperature.The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps.The microcracks disappeared in the RA on the prior austenite grain boundaries.On the other side,the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns.The polygonal martensite-austenite islands or blocky RA constrained the microcracks.After austempered at 400 ℃,the brittle fracture was dominant,showing river patterns,and the microcracks propagated through the granular bainite without any resistance.     

Stabilization of retained austenite by the two-step intercritical heat treatment and its effect on the toughness of a low alloyed steel

Fine film-like stable retained austenite was obtained in a Fe–0.08C–0.5Si–2.4Mn–0.5Ni in weight percent (wt.%) steel by the two-step intercritical heat treatment. The first step of intercritical annealing creates a mixed microstructure of preliminary alloy-enriched martensite and lean alloyed intercritical ferrite, which is called as “reverted structure” and “un-reverted structure”, respectively. The second step of intercritical tempering is beneficial for producing film-like stable reverted austenite along the reverted structure. The stabilization of retained austenite was studied by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dilatometry and X-ray diffraction (XRD) analysis. The two-step austenite reverted transformation associated with intercritical partition of C, Mn and Ni is believed to be the underlying basis for stabilization of retained austenite during the two-step intercritical heat treatment. Stable retained austenite is not only beneficial for high ductility, but also for low temperature toughness by restricting brittle fracture. With 10% (volume fraction) of retained austenite in the steel, high low temperature toughness with average Charpy impact energy of 65 J at −80 °C was obtained.     

New analytical approximate solution of the generalised temperature integral for kinetic reactions

Simple closed-form expression of the generalised temperature integral in the basic equation to describe kinetic reactions for solid materials in linear heating process is always suitable for use in determining parameters. Many developed solutions only can give high accuracies on the general conditions. A new analytical approximate solution was deduced in this work. The deviations of this solution from the true value are fully analysed. This solution takes advantage in broader application conditions than other known solutions. The application of the new analytical approximate solution in austenite kinetic reaction in this work reinforces that austenite reaction rate is feasible to be given a priori.     

Evaluation of the Heat Treatment Response of a Multiphase Hot-Rolled Steel Processed by Controlled Rolling and Accelerated Cooling

The tensile properties of hot-rolled multiphase steel after heat treatment were analyzed on a laboratory scale. Subcritical treatments applied to the hot-rolled strip revealed an increase in the yield stress and elongation with increasing temperature. Normalizing of the strip at 920°C notably improved the ductile response, while both the yield stress and the anisotropy of every property evaluated at 0°, 45°, and 90° to the rolling direction in the rolling plane were significantly reduced.     

Residual stress fields in surface-treated silicon carbide for space industry—comparison of biaxial and triaxial analysis using different X-ray methods

Any mechanical surface treatment and machining leaves ‘footprints’ in the form of residual stress fields in the surface region of technical parts or components, which are detectable by X-ray diffraction. In the present paper, we applied different X-ray methods to investigate the residual stress state in the near-surface zone of sintered silicon carbide after mechanical surface processing. Using the sin² ψ-based ‘universal plot’ method, we found steep gradients for the in-plane components σ₁₁ and σ₂₂ in the form of high compressive stresses at the surface, which change into tensile stresses within a few microns. To gain information on the triaxial residual stress state, we applied the scattering vector method, which is based on strain depth profiling by sample rotation around the diffraction vector. For the in-plane stresses, we observed gradients similar to those obtained by the ‘universal plot’ method, but they were shifted on the absolute scale towards tensile stress. We explain this difference by ‘pseudo-macroscopic’ tensile residual stress fields σ₃₃, which act normal to the surface and therefore pretend higher in-plane compressive stresses σ_ii (i = 1, 2), if they are not regarded in the evaluation procedure.     

Influence of feed rate on surface integrity and fatigue performance of machined surfaces

This study investigates the influence of the feed rate on the surface integrity and fatigue performance of machined surfaces. The results demonstrate that a higher feed rate increases crack initiation life and crack propagation life. A higher feed rate induces more compressive residual stresses and a more softened layer. The feed rate influences crack initiation life up to 45% and crack propagation life up to 149%. Consequently, the feed rate affects fatigue life up to 132%. The fatigue tests substantiate that the feed rate influences fatigue life significantly and that the effect increases significantly if the loading is reduced.     

An improved back computation procedure for the parting-out step of a destructive method for measuring residual stresses in pipes

One method for measuring through the thickness residual stress distributions in welded pipes involves a destructive, three step laboratory procedure consisting of parting out, splitting, and layer removal operations. In the parting out step, a coupon of material is removed from the wall of the pipe while strain gauges on the inner and outer surfaces are monitored. In back computing the through the thickness residual stresses from the parting-out strain gauge data, it is generally assumed that stress changes through the thickness of the coupon vary as a straight line. If the circumferential dimension of the coupon is large compared to the thickness, this assumption is valid. However, in the case of a pipe, the circumferential dimension is usually limited to reduce curvature effects during the subsequent splitting and layer removal steps. If this dimension is too small, the assumption of straight line stress changes through the thickness can lead to serious inaccuracies. In this paper, the validity of the straight line assumption is examined over a range of conditions. To handle cases where the straight line assumption is not valid, a modified back computation procedure for the analysis of parting-out strain gauge data is developed and presented with numerical results.     

Finite element method and experimental investigation on the residual stress fields and fatigue performance of cold expansion hole

Cold expansion is an efficient way to improve the fatigue life of an open hole. The residual stress fields of cold expansion holes are vital for key components designing, manufacturing and fatigue properties assessment. In this paper, three finite element models have been established to study the residual stress fields of cold expansion hole, experiments were carried out to measure the residual stress of cold expansion hole and verify simulation results. Three groups of specimens with different cold expansion levels are examined by fatigue test. The fracture surfaces of specimens are observed by scanning electron microscope. The finite element method (FEM) results show, with interference values develop, the maximum values of circumferential residual compressive/tensile stresses increase in “infinite” and “finite” domain, and a higher positive stress values are obtained at the boundary of “finite” domain. The effects of the friction between the mandrel and the hole’s surface and two cold expansion techniques on the distribution of residual stress is local, which only affects the radial residual stress around the maximum value and the circumferential residual stress near the hole’s edge. Crack always initiates near entrance face and the crack propagation speed along transverse direction is faster than that along axial direction.     

甲醛车间外管线裂纹泄露机理分析

通过对甲醛生产车间室外主工艺管线工作环境考察,管线弯管处的裂纹部位进行观察,并通过对裂纹处成分、组织和裂纹形貌进行分析,得出奥氏体不锈铜管线预制产生加工硬化和马氏体相变,焊接产生马氏体相变和焊接残余应力,以及管线所在外部环境造成管表面氯离子富集是造成管线裂纹的根本因素,得出了管线弯管裂纹泄露失效的原因;并提出了治理和防范管线弯管处容易产生裂纹泄露失效的具体办法。     

Influence of the Preheat Treatment on the Microstructure and Properties of X37CrMoV5-1 Steel

The technological properties of the hot-work tool steels depend on their microstructural quality induced by the production process and the heat treatment. One of the frequently applied methods to improve microstructure is preheat treatment. The treatment of carbonitriding X37CrMoV5-1 steel has been investigated in fluidized bed furnaces. Thickness and phase composition have been conducted using optical microscopy Neophot 32 and scanning microscopy, JEOL 5400, after etching in nital.     

Effects of residual stress and shape of web plate on the fatigue life of railway wheels

Railway wheels have been one of the most critical components in a railway vehicle. Fatigue design of railway wheel is one of the most important factors. Damages on the wheel can be divided into three types, such as the contact fatigue of the tread, the thermal fatigue of the rim due to braking and the mechanical fatigue of the web plate. The railway wheel has the initial residual stress formed during the manufacturing process, and this residual stress changes due to the thermal stress induced by braking. In this study, we evaluated residual stress of web plate by heat treatment due to the manufacturing process and changes of residual stress by braking using finite element analysis. The cyclic stress history for fatigue analysis is determined by applying finite element method. The fatigue strength evaluations of the web plate are performed to investigate the effect of the residual stress.     

Comparison of techniques for residual stress determination in a bead on plate titanium sample

Residual stress determination has been carried out on production welds to provide engineering data.
A parting out technique has been used in which a strain gauged through thickness slug of material is removed from the weld area.
The centre hole technique has been used with the blind hole in the weld material.
There were differences between the results from the two techniques and a fully penetrant bead on plate fatigue sample with the bead running axially was used in order to compare the two techniques in more detail.
After parting out, the layer removal technique was carried out in order to provide a relatively detailed through thickness residual stress distribution in order to explain the differences between the results from the centre hole and parting out techniques.     

The effects of forging pressure and temperature field on residual stresses in linear friction welded Ti6Al4V joints

Linear friction welding (LFW), as a solid state joining process, has been developed to manufacture and repair blisks in aeroengines. The residual stresses after welding may greatly influence the performance of the welded components. In this paper, the distribution of residual stresses in Ti6Al4V joints after LFW was investigated with numerical simulations. The effects of applied forging pressure and temperature field at the end of the oscillating stages on the residual stresses within the joints were investigated. The results show that, the residual tensile stresses at the welded interface in the y-direction are the largest, while the largest compressive stresses being present at the flash root in the z-direction. Furthermore, the forging pressure and temperature field at the end of the oscillating stages strongly affect the magnitude of the residual stresses. The larger forging pressure produced lower residual stresses in the weld plane in all three directions (x-, y-, and z-directions). Larger variance, σ, which decides the Gaussian distribution of the temperature field, also yields lower residual stresses. There is good agreement between simulation results and experimental data.     

Microstructure model for the heat-affected zone of X80 linepipe steel

An important aspect of the integrity of oil and gas pipelines is the heat-affected zone (HAZ) of girth welds where the microstructure of the as-hot rolled steel is altered with potentially adverse effects on the HAZ properties. Therefore, it is critical to evaluate the HAZ microstructure for different welding scenarios. Here, an integrated microstructure evolution model is proposed and applied to the HAZ of an X80 linepipe steel. The model considers dissolution of Nb-rich precipitates, austenite grain growth and austenite decomposition into ferrite and bainite. Microstructure maps showing the fraction of transformation products as a function of distance from the fusion line are obtained and used to compare the effect of different welding procedures on the HAZ microstructure.     

Influence of Ti/TiN bilayered and multilayered films on the axial fatigue performance of Ti46Al8Nb alloy

The effects of Ti/TiN bi- and multilayered films on the fatigue performance of the Ti46Al8Nb alloy were investigated. Ti/TiN films with a total thickness of 1 μm were deposited on the Ti46Al8Nb alloy substrate by the hollow cathode deposition method. The samples were examined with various analytical techniques including nanoindentation, scratch test, stripping layer substrate curvature test and scanning electron microscopy. The results show that multilayered Ti/TiN films can enhance the fatigue strength of the Ti46Al8Nb alloy, whereas bilayered films have no obvious effect. Compared with the bilayer, the multilayer exhibits higher hardness, higher residual compressive stress and higher adhesion strength to the substrate. It is also demonstrated that the multilayer is responsible for retarding fatigue crack growth. All the superior properties make the hard Ti/TiN multilayer to be an effective protection coating for the enhanced fatigue strength of the brittle substrate.     

Influences of deposition rate and oxygen partial pressure on residual stress and microstructure of YSZ thin films

Yttria-Stabilized Zirconia (YSZ) thin films were deposited on borosilicate crown glass substrates using electron beam evaporation technique and controlling technological parameters: deposition rate and oxygen partial pressure. Spectrophotometry, optical interferometry and X-ray diffraction were used to investigate how the thin film optical properties, residual stresses, and structure depend on these parameters. The results showed that the deposition rate had a significant influence on the increase of the refractive index of YSZ thin films while the oxygen partial pressure had less influence on it. In all samples, the tensile stress increased with the increasing of deposition rate and the decreasing of oxygen partial pressure. Meanwhile, all deposited films were poly-crystallizations, while crystallite size and preferential orientation of YSZ thin films changed as a function of deposition rate and oxygen partial pressure. The variations of the optical spectra and residual stress corresponded to the evolution of the film structures induced by the deposition parameters.     

Environment and time dependent effects on the fatigue response of an advanced nickel based superalloy

The fatigue behaviour of the nickel based superalloy RR1000 is characterised using double edge notch specimens incorporating shot peening. Evaluations were conducted at two test temperatures, 300 °C and 650 °C, employing baseline and dwell waveforms. The effects of air and vacuum environments plus prior exposure at 650 °C were also assessed. It is demonstrated that surface oxidation does not control performance at the test conditions of interest. Rather, the modification to stabilized peak and mean stresses resulting from either thermal relaxation of peened stresses or a time dependent shake down of stress under mechanical loading governs ultimate behaviour.     

Feasibility study of polyurethane shape-memory polymer actuators for pressure bandage application

The feasibility of laboratory-synthesized polyurethane-based shape-memory polymer (SMPU) actuators has been investigated for possible application in medical pressure bandages where gradient pressure is required between the ankle and the knee for treatment of leg ulcers. In this study, using heat as the stimulant, SMPU strip actuators have been subjected to gradual and cyclic stresses; their recovery force, reproducibility and reusability have been monitored with respect to changes in temperature and circumference of a model leg, and the stress relaxation at various temperatures has been investigated. The findings suggest that SMPU actuators can be used for the development of the next generation of pressure bandages.