Effects of solution treatment temperature on grain growth and mechanical properties of high strength 18%Ni cobalt free maraging steel

Abstract

The effects of solution treatment (ST) temperature (1073–1473 K) on the prior austenite grain size, microstructure, and mechanical properties of a 2000 MPa grade 18%Ni Co free maraging steel have been investigated. The results show that prior austenite grain size normally increases with increase of ST temperature. Strength and ductility in the solution treated condition are independent of both ST temperature and prior austenite grain size due to constant martensite lath spacing and dislocation tangles. In the solution treated + aged condition, the relationship between yield strength and prior austenite grain size follows the Hall- Petch equation, and ductility improves until the ST temperature used is >1373 K. Accordingly, the fracture mode transforms from intergranular to transgranular at a critical prior austenite grain size of ~ 150 μ m, because of severe segregation of Ni₃(Mo,Ti) and reverted austenite at prior austenite grain boundaries and martensite lath boundaries. The variation of Charpy V notch impact energy with increase of ST temperature in both the solution treated and solution treated + aged conditions is similar to that of the tensile ductility. The fracture toughness K_IC, however, increases with increase of ST temperature. No thermal embrittlement resulted from the Ti(C,N,S) inclusion segregation at prior austenite grain boundaries and martensite lath boundaries in the high temperature solution treatment.     

Tensile and work hardening properties of low carbon dual phase strip steels at high strain rates

Abstract

As a result of their unique combination of strength and ductility dual phase steels play an important role in reducing weight in automobile components and improving crashworthiness. The purpose of this paper is to quantify the crash performance of dual phase steels, as defined by the influence of low and high strain deformation rates (0·001 s^-1 and 100 s^-1 respectively), on the tensile and work hardening properties of a range of commercial dual phase products. The objective is to establish whether dual phase steels maintain their desirable mechanical property characteristics of low yield strength, high tensile strength and high work hardening rates during plastic deformation under the application of a high strain rate loading. The results confirmed that the yield/proof strength and tensile strength increased with increasing volume fraction of second phase constituents and increasing strain rate. In particular, a dual phase steel with a microstructure consisting of a significant volume fraction (>10–15%) of additional second phase material (bainite) is shown to display superior energy absorption properties. However, this is accompanied by poor ductility and work hardening characteristics.     

Laser polishing and laser form correction of fused silica optics

With laser polishing, the roughness of ground glass surfaces can be significantly reduced. With process speeds up to 1 cm²/s independent of the processed surface shape, the resulting roughness is already sufficient for illumination optics. To further reduce the roughness, the polishing process itself will be optimized and moreover combined with a subsequent laser based form correction. Within this paper, results of laser polishing as well as laser form correction and combination of the two processes are presented and discussed.     

Studies on process optimisation of multistage atomisation

Abstract

In order to understand more about the multistage atomisation process, process experiments and orthogonal optimisation of multistage atomisation were carried out for pure tin to make clear the effect of every main process parameter and the optimum conditions. The orthogonal experiments showed that the disk rotating speed and superheat temperature have the largest effects on the mean particle size and that spraying height affects the powder shape coefficient remarkably. The delivery tube position has a pronounced effect on the result and rate of atomisation through changing the pressure characteristics of the nozzle tip and is also an important process factor, while the effect of gas pressure on atomisation result is relatively small. The optimum process conditions for multistage atomisation of pure tin are as follows: superheat 350 K; spraying height 85 mm; gas pressure 0·8 MPa; disk rotating speed 4000 rev min^-1; and protrusion height of delivery tube 2–3 mm.     

Microstructural alterations through heat treatment and its influence on wear response of a silicon containing zinc based alloy under different test conditions

Abstract

Observations pertaining to the influence of microstructural alterations brought about through heat treatment on the sliding wear behaviour of a zinc based alloy comprising of silicon have been analysed in this study. The effects of sliding conditions such as pressure and speed on the wear response of the alloy in as cast and heat treated conditions have also been investigated. The as cast alloy revealed dendritic structure consisting of primary α, eutectoid α + η, and ? phase. Silicon was present in the alloy microstructure as discrete particles. Heat treatment caused breaking of the dendritic structure and more homogeneous distribution of various microconstituents without affecting the morphology and mode of distribution of the silicon particles. The heat treated alloy attained superior wear response as compared with the as cast one especially under more severe wear conditions. Wear rate versus pressure plots revealed two slopes wherein the slope was low at low pressures and increased considerably beyond a critical pressure. The critical pressure decreased with speed while it was more for the heat treated alloy. The wear behaviour of the specimens deteriorated with pressure and speed. High wear rates were supplemented with severe surface/subsurface damage and coarse debris formation and vice versa. Changing microstructural features of the regions at different depths below the wear surface were attributed to the changing degree of deformation they experienced during wear. The wear behaviour of the specimens has been explained in terms of specific characteristics of various phases such as lubricating and load carrying capability, thermal stability and cracking tendency. Typical characteristics of worn surfaces/subsurface regions and debris further supplemented the specific wear behaviour of the alloy in different test/material conditions.     

Low temperature aluminisation of alloy steels by pack cementation process

Abstract

The pack aluminisation process is normally applied at temperatures >973 K at which the mechanical properties of alloy steels would degrade. Thus, the present study was undertaken to apply this process to aluminising the alloy steels at temperatures <973 K in order to increase their high temperature oxidation resistance while maintaining their microstructure and hence mechanical strength and creep resistance. A type of commercial alloy steel P92 (9Cr–1Mo) was used for the present study. Pack powder mixtures consisting of Al, AlCl₃ (anhydrous) or NH₄Cl and Al₂O₃ were used to carry out the process. The aluminising temperature was varied from 773 to 973 K, pack Al content from 1 to 30 wt-% and aluminising time from 1 to 16 h to investigate their effects on the coating growth kinetics in the AlCl₃ activated packs. It was observed that all the coatings formed in the AlCl₃ activated packs were of a single layer structure with Fe₂Al₅ as the main coating phase. It was established that the interrelationship between the thickness h (in μm) of this coating layer and aluminising temperature T (in K), time t (in h) and pack Al content W (in wt-%) can be described by h=83005·9W^1/2t^1/2e^?73330/(RT). In the NH₄Cl activated packs, it was found that coating formation and dissolution took place simultaneously at 923 K and stable growth of a coating layer was only possible when the pack Al content was sufficiently high. However, the coatings formed in these packs had highly uneven regions.     

Model for prediction of processing window for austempered ductile iron

Abstract

A computer model has been developed to predict the processing window (austempering window) for austempered ductile iron (ADI). The model is a modification of an existing model for the isothermal decomposition of austenite in bainitic steels. It was calibrated using experimental data from the literature. In order to validate the model, the processing window corresponding to a ductile iron of composition 3.41%C, 2.46%Si, 0.36%Mn, 0.18%Mo, and 0.25%Cu is predicted and compared to experimental data. Computer assisted image analysis was used to investigate the volume fraction of martensite at the lower boundary of the processing window. X-ray diffraction was used to calculate the normalised volume fraction of austenite at the upper boundary of the processing window. The results show that the model successfully predicts the processing window corresponding to the iron investigated in this study.     

Oxidation behaviour of three-dimensional woven C/SiC composites

Abstract

The oxidation behaviour of a three-dimensional woven C/SiC composite protected with an SiC seal coating and with an SiC coating combined with an SiO₂–B₂O₃ glassy coating have been respectively investigated through an experimental approach based on mass and flexural strength changes. Three main temperature domains exist for C/SiC composites protected with an SiC seal coating. At low temperatures (<700°C), the mechanisms of reaction between carbon and oxygen control the oxidation kinetics. At an intermediate temperatures (between 700 and 1100°C), the oxidation kinetics are controlled by gas phase diffusion through a network of microcracks in the SiC matrix and coating. At high temperatures (>1100°C), the oxidation kinetics are controlled by oxygen diffusion through the SiO₂ scale formed on the SiC coating. Composites of C/SiC with an SiC/(SiO₂–B₂O₃) coating exhibit better oxidation resistance. The filling of the pores and the microcracks and the flow of the glassy coating at higher temperatures result in a global decrease of mass loss in the composites. By researching the relationship between the residual flexural strength and the mass variation in different temperature ranges, it is shown that the change in the residual flexural strength is dominated by the degradation of carbon phase.     

Effect of microstructural variables on tensile behaviour of A356 cast aluminium alloy

Abstract

The effects of microstructural variables, including secondary dendrite arm spacing (SDAS), the size of primary α phase, the aspect ratio of eutectic Si particle and the thickness of eutectic wall structure, on tensile behaviour of A356 cast aluminium alloy, were quantitatively identified using linear regression analysis method. For systematic microstructural control of A356 specimen, directional solidification method was used with different solidification rates of 5, 25, 50 and 100 μm s^?1 respectively. The linear regression analysis suggests that each microstructural variable affects tensile strength and tensile elongation of A356 cast aluminium alloy in a similar fashion. The change in tensile behaviour with varying microstructural variables in A356 cast aluminium alloy is discussed based on fractographic and micrographic observations.     

Effect of heat treatment on tensile properties of friction stir welded joints of 2219-T6 aluminium alloy

Abstract

The effect of post-weld heat treatment (PWHT) on the tensile properties of friction stir welded (FSW) joints of 2219-T6 aluminium alloy was investigated. The PWHT was carried out at aging temperature of 165°C for 18 h. The mechanical properties of the joints were evaluated using tensile tests. The experimental results indicate that the PWHT significantly influences the tensile properties of the FSW joints. After the heat treatment, the tensile strength of the joints increases and the elongation at fracture of the joints decreases. The maximum tensile strength of the joints is equivalent to 89% of that of the base material. The fracture location characteristics of the heat treated joints are similar to those of the as welded joints. The defect free joints fracture in the heat affected zone on the retreating side and the joints with a void defect fracture in the weld zone on the advancing side. All of the experimental results can be explained by the hardness profiles and welding defects in the joints.     

Effects of quench aging treatment on microstructure and tensile properties of thixoformed ZA27 alloy

Abstract

The effects of quench aging heat treatment on microstructure and tensile properties of thixoformed ZA27 alloy were investigated. The results indicated that the microstructure of the alloy became into polygonal β phase particles after solid solution treatment at 350°C for 48 h. The β particles then decomposed into α and η phases which would coarsen during the subsequent aging at 150°C. It was observed that the Zn concentration in the β phase near the polygonal boundaries was higher than that within the β particles. As a consequence, both the decomposition speed of the former β phase and the subsequent coarsening speed were faster than those of the latter β phase. Thus, α and η phases near the boundaries were always coarser than those within the particles during aging. Owing to the coarsening, the ultimate tensile strength continuously decreased with increasing aging time. The percentage elongation increased up to 10 h, but decreased with aging time owing to bad deformation accommodation and low bonding strength between particles. Cracks initiated from some defects (e.g. inclusions and porosities) during tensile test, and the path for the cracks to propagation changed with the aging time.     

Review: Interaction of precipitation with recrystallisation and phase transformation in low alloy steels

Abstract

The processes of precipitation, restoration and phase transformation can interact in complex ways during thermomechanical processing of microalloyed steels to profoundly alter their structures and properties. Precipitation in austenite during hot deformation can strongly modify the kinetics of recovery and recrystallisation, subsequently affecting the nucleation and growth of ferrite during cooling. For steels containing strong carbide/nitride formers, interphase precipitation (IP) can occur in ferrite at the austenite/ferrite interface, conferring significant coherency strengthening. Much of what is known about this phenomenon is attributable to the impressive research efforts of Robert Honeycombe and his colleagues at Cambridge.     

Evaluating fatigue performance of down gauged high strength steel suspension arm

Abstract

The use of high strength steels (HSS) in automotive components is steadily increasing as automotive designers use modern steel grades to improve structural performance, reduce vehicle weight, and enhance crash performance. Weight reduction can be achieved by substituting mild steel with a thinner gauge HSS, however it must be ensured that no deterioration in fatigue performance occurs. Fatigue studies have been carried out to determine the effects various welding processes, gauge, and material strength can have on the fatigue performance of an automotive suspensionarm.Test methodology has also been investigated and the merits of both uniaxial constant amplitude and multiaxial simulation testing have been studied. Results have shown the fatigue performance of welded components to be independent of the strength of the parent material for the steel grades studied. Also, little correlation was found between the fatigue performance of simple welded samples under uniaxial, constant amplitude loading and complex components under biaxial in service loading, road load data. This highlights the care required when estimating component in service performance from small, simplified samples. The work also highlights the need for testing components under in service conditions if optimum use of materials, design, and manufacturing methods is to be achieved.     

Influence of alloying elements on hot corrosion of superalloys and coatings: necessity of smart coatings for gas turbine engines

Abstract

Modern gas turbine engines require high performance materials and coatings to ensure high efficiency. The selection of high performance materials and coatings depends on the nature and concentration of alloying elements. The composition of materials and coatings, in particular, plays a major role in enhancing the life of gas turbine engines by exhibiting good resistance to oxidation and hot corrosion, which are major problems in gas turbine engines. The performances of several superalloys containing different alloying elements and MCrAlY type coatings containing a variety of major and minor alloying elements are described in detail. The effect of major and trace elements on the life of superalloys and coatings in the presence of pure Na₂SO₄, NaCl and vanadium containing environments is detailed. The relevant reaction mechanisms leading to the failure of superalloys and coatings are discussed. The major factors involved when selecting alloying elements for the preparation of superalloys to manufacture components intended for use under hot corrosion conditions and the selection of appropriate coatings are suggested. Finally, the necessity of innovation of 'smart coatings' to combat both oxidation and hot corrosion is discussed.     

Fatigue performance and microanalysis of heat treated 2205 duplex stainless steel

Abstract

The reduced corrosion fatigue performance of a heat treated grade of duplex SAF 2205 stainless steel has been quantified using Wöhler rotating bending fatigue machines. The reduction in fatigue performance has been related to elemental profiles around intergranular and intragranular precipitates and grain boundary segregation measured using EDAX analysis and scanning transmission electron microscopy.     

Effect of vapour phase hydrogen peroxide,as a decontaminant for civil aviation applications,on microstructure,tensile properties and corrosion resistance of 2024 and 7075 age hardenable aluminium alloys and 304 austenitic stainless steel

Abstract

A response to the chemical or biological contamination of aircraft requires the use of a suitable decontaminant. Among possible chemical decontaminants, vapour phase hydrogen peroxide appears to be a likely candidate in terms of a combination of efficacy, low environmental impact and potential for materials compatibility. The present paper examines the effect of hydrogen peroxide, both in the vapour phase and as a liquid concentrate on two common structural materials used in aviation, namely 2024 and 7075 age hardenable aluminium alloys and on 304 austenitic stainless steel, the latter as employed in galley and lavatory surfaces. The present paper characterises both the effects of hydrogen peroxide on the microstructure of the materials and the impact that decontamination has on the tensile properties and corrosion resistance of these materials. Microstructural effects are both relatively small in magnitude and confined to a region immediately beside the exposed surface. No systematic effect is found on either the tensile properties or the post-exposure corrosion resistance of the three alloys examined. These observations are encouraging in terms of the use of vapour phase hydrogen peroxide for decontamination applications.     

Microstructure,hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel

Abstract

The present investigation reports on a study that has been taken up to develop an understanding of the electron beam welding characteristics of similar and dissimilar combination of maraging steel and high strength low alloy steel, which are in the hardened condition, i.e. maraging steel, in a solution that was in treated and aged condition, whereas high strength low alloy steel in a quenched and tempered condition before welding. The joint characterisation studies include microstructural examination, microhardness survey across the weldment and measurement of residual stresses. Maraging steel weld metal is under compressive stress rather than tensile stress as observed in low alloy steel welds because the martensite transformation occurs at a relatively low temperature. It has been observed that, in dissimilar metal welds, tensile stress is observed at the fusion boundary of low alloy steel and weld metal, whereas compressive stress is obtained at the location between weld and maraging steel fusion boundary. Dissimilar weldment contains a soft region beside the interface on maraging steel side because of the diffusion of manganese from low alloy steel towards maraging steel. The observed residual stresses, hardness distribution across the similar and dissimilar metal welds are correlated with the observed microstructures.     

Laser polishing for topography management of accelerator cavity surfaces

Improved energy efficiency and reduced cost are greatly desired for advanced particle accelerators. Progress toward these goals can be made by atomically‐smoothing the interior surface of the niobium superconducting radiofrequency (SRF) accelerator cavities at the heart of these machines. Laser polishing offers a green alternative to the present aggressive chemical processes. We found parameters suitable for polishing niobium in all surface conditions that are expected for cavity production. Careful measurement of the resulting surface chemistry revealed a modest thinning of the surface oxide layer, but no contamination.     

熔石英元件抛光加工亚表面缺陷的检测

亚表面缺陷的准确检测是进行亚表面损伤研究的前提和基础,对保证光学元件加工质量至关重要.基于HF酸化学蚀刻法对熔石英元件抛光加工产生的亚表面水解层、缺陷层深度和亚表面损伤形貌进行了定量检测,并利用X射线荧光光谱法研究了熔石英抛光试件杂质元素的种类和元素含量沿深度分布规律,提出了熔石英元件抛光加工亚表面损伤深度的判定方法.研究表明:由于水解层和亚表面缺陷层的存在,熔石英抛光试件的蚀刻速率随着时间的增加呈现递减的趋势,且在蚀刻的初始阶段蚀刻速率下降尤为明显;当蚀刻深度超过某一特定值后,全部或部分覆盖在水解层以下的缺陷层将会被完全蚀刻去除,蚀刻速率基本保持不变;另外,熔石英抛光试件存在多种形式的表面及亚表面缺陷,在不同蚀刻深度,亚表面损伤形貌、划痕的宽度和深度也存在一定的差异.