Development of an ultra-high-strength low-alloy NiSiCrCoMo steel

An ultra-high-strength low-alloy NiSiCrCoMo steel has been developed. The development work is part of a major programme at the Defence Metallurgical Research Laboratory in the field of ultra-high-strength, high-fracture-toughness steels. In this context we undertook investigations to understand the effect of solute additions on the fracture behaviour of Armco iron and Fe-C alloys. We investigated Fe-Ni, Fe-Co, Fe-Si, Fe-Mo, Fe-C-Ni and Fe-C-Co alloys for mechanical behaviour. The report by Garrison (1986) on a Fe-C-Ni-Si-Cr alloy was an important pointer to a low-alloy, ultra-high-strength steel with high fracture toughness. The material we have now arrived at is a Fe-C-Ni-Si-Cr-Co-Mo steel with tensile, impact and fracture toughness properties matching those of maraging steel 250 grade in tonnage scale melts.     

Soft magnetic structural alloys for elevated-temperature applications

Metallurgical efforts to develop a soft magnetic material suitable for application in the rotor of a generator or motor in advanced aerospace electric systems are reviewed. Commercial materials which have been considered include AISI 4340 steel, H-11 steel, Nivco alloy, and 15- and 18-percent Ni maraging steels. Developments described have led to several new materials with combination of good mechanical and magnetic properties at elevated temperature. Such materials include an improved maraging steel a precipitation hardenable cobalt-base alloy, a carbide strengthened Co-W alloy, dispersion-strengthened soft magnetic alloys, and unidirectionally solidified Co-Nb-Fe eutectic alloys.     

Role of pulsed current on metallurgical and mechanical properties of dissimilar metal gas tungsten arc welding of maraging steel to low alloy steel

This research work encompasses the investigations carried out on the mechanical and metallurgical properties of maraging steel and AISI 4340 aeronautical steel weldments. The materials were joined by continuous current gas tungsten arc welding (CCGTA) and pulse current (PCGTA) gas tungsten arc welding processes using ErNiCrMo-3 filler wire. Cross sectional macrostructures confirmed proper deposition of the fillers and lack of discontinuities. Optical microscopy studies revealed that at the maraging steel–weld interface, martensite in distorted and block forms prevailed in CCGTA and PCGTA weldments whereas tempered martensite was predominant at the low alloy–weld interfaces of both the welds. Scanning electron microscopy (SEM) with energy dispersive analysis of X-rays (EDAX) analysis apparently showed less elemental migration in PCGTA weldments as compared to the other. Results of X-ray diffraction analysis recorded possible phase formations in various zones of the weldments. Microhardness profiles in either weld zones followed a constant trend whereas it showed a downtrend in the heat affected zones (HAZ) of the maraging steel and very high hardness profiles were observed in the low alloy steel side. Tensile studies on various factors and impact testing showed that PCGTA weldments outperformed the continuous ones in terms of strength, ductility and toughness. Fractograph analysis depicted the nature of failures of tensile and impact tested specimens. Comparison analyses involving influence and nature of pulsed current welds over continuous ones were done to determine the possibility of implementing these joining processes in aerospace applications. Weldments fabricated using PCGTA technique proved to be superior to the other, resulting in exceptional mechanical properties.     

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.     

New developments in carbon and alloy steels

Recent developments in carbon and alloy steels outlined in this paper include those pertaining to low-carbon mild steels, high-strength low-alloy (HSLA) steels, dual-phase steels, low-carbon bainitic steels, ultrahigh strength steels and ferritic stainless steels. The factors that improve the cold-forming characteristics of low carbon sheets and strips are outlined. The physical metallurgy principles governing the ferrite grain refinement in HSLA steels are discussed, pointing out how it can be achieved by the controlled rolling process. The importance of sulphide shape control in imparting the necessary through-thickness ductility in HSLA steels is discussed and the various methods available for inclusion shape control are outlined. Improved formability coupled with adequate strength characterizes the dualphase steels. Among the ultrahigh strength steels, two recent developments,viz. TRIP steels and maraging steels are outlined. It is pointed out how the improved formability of ferritic stainless steels is making them compete with the more expensive austenitic stainless steels. The scope for future developments in these steels is discussed at the end.     

The fracture toughness of Fe-20%Co-15%Cr-5%Mo maraging alloy

The paper describes how the $\text{K}{}_{\mathrm{IC}}$ fracture toughness of a high-purity Fe-20%Co-15%Cr-5%Mo maraging alloy is influenced by other mechanical properties and metallurgical factors. Particular consideration is given to the influence of residual oxygen content and to the strain-hardening characteristics of the material. The Krafft process-zone concept and the Hahm-Rosenfield relationship are validated.Although the heat-treatment parameters have not been optimised, the results show this stainless alloy to be capable of developing a high tensile strength, combined with good fracture toughness and the manner in which certain features of the microstructure influence these properties has been discussed.     

Mossbauer spectroscopic study of heat-treated and control-cooled Fe3Al alloys

The Fe–Al system forms a model object of transition metal-sp element system. Fe–Al alloys containing 21–31 at% Al are known to exhibit several attractive physical properties which render them candidate materials for structural and magnetic applications. These magnetic and physical properties can be varied by altering the composition and the processing routes. Keeping this in mind, Mossbauer spectra of five Fe–24 at% Al and Fe–25 at% Al alloy samples processed via different routes have been studied. The analysis of the Mossbauer parameters, distribution of hyperfine magnetic fields, P(H) and H-values were used to identify various Fe-atom configurations (nearest neighbour or next nearest neighbour) associated with the phases formed by different processing routes. The results obtained from the present study indicate that the average hyperfine field H depends on the average rate of cooling followed during the precipitation of these phases.     

Thermodynamic Functions of Undercooled Fe–P Melts

The approaches to calculating the thermodynamic properties of undercooled melts are critically evaluated. It is shown by the example of the eutectic Fe–P alloy that the rough estimates relying on various models of melt structure, primarily on the hole theory, provide inadequate results. Based on the concept of associated solutions and the thermodynamic functions of the constituent components found by solving the inverse problem of chemical thermodynamics, a procedure is proposed for analyzing the thermodynamic behavior of undercooled melts, in particular, near the glass transition. The thermodynamic quantities calculated by this procedure coincide with experimental data to within the measurement accuracy. The procedure is used to calculate the thermodynamic properties and viscosity of Fe–P melts between the glass-transition temperature (650 K) and 1873 K. The results are in perfect agreement with experimental data.     

A Comparative Study on the Production of a Hat Profile by Roll Forming and Stamping

Lightweight design using high-strength aluminum alloys has gained importance due to the continuing need for weight reduction and increasing crash safety requirements in the automotive industry. There are various manufacturing processes available for processing high-strength aluminum alloys. Herein, the production of high-strength aluminum parts by roll forming and stamping based on the example of an AA7075-T6 hat profile is compared. Roll forming represents a continuous manufacturing process, while stamping is a discontinuous process. Different process routes (T6, W-Temper and O) for roll forming as well as for stamping (T6, W-Temper, O and hot forming) are in focus of the investigation. Fundamental differences of the forming processes and the tempering condition are observed and criteria for the choice of the manufacturing process and process route are presented. The temperature-supported process routes improve the poor cold formability of AA7075 alloy and thus enhance the process window. Potential is offered for both manufacturing processes by applying tailored properties achieved through targeted quenching.     

Nonlinear free vibration analysis of shape memory alloy embedded laminated composite shell panel

High specific strength and stiffness are characteristics desired for aircraft and launch vehicle domains to enhance the payload gain and performance. The mechanical properties of the composites can be further tailored by embedding structural components, such as shape memory alloys, into the passive composite structure. The present study is primarily focused on the nonlinear free vibration analysis of spherical and cylindrical composite shell panels embedded with shape memory alloy fibers. The nonlinear finite element governing equations based on the higher-order shear deformation plate theory and principle of virtual work with nonlinear von-Karman strain displacement relations are employed for the analysis. The temperature-dependent material properties of shape memory alloy are considered in the formulation. A nine-noded isoperimetric element is accounted for synthesizing the element for the finite formulation. The Young's modulus and the recovery stress vary with temperature and higher nonlinearity. The incremental method is used to generate the inputs for the temperature-dependent nonlinear properties of materials. The temperature change is divided by many small temperature increments. The temperature-dependent material properties are assumed constant during the small increment. The mechanics of shape memory alloy in substrate are presented and the governing equation of laminated composite with shape memory alloy is obtained and implemented in the MATLAB 7.8 program.     

Influence of the processing route on the microstructure of aluminum alloy A356 for thixoforming

Raw materials for thixoforming processes can be obtained by various routes. An analysis was made of the influence of five different processing routes on the production of thixotropic A356 alloy: 1) direct casting in a water-cooled mold; 2) direct casting as in (1), plus enhancement by electromagnetic stirring and grain refining; 3) as in (1), but enhanced by mechanical vibration; 4) as in (1), followed by one ECAP pass (Equal Channel Angular Pressing); and 5) using a commercial product as comparison. The alloy was characterized by both conventional and color microstructure for a comprehensive understanding of its structure prior to and after heating to 580 °C, which is the ideal processing temperature for this specific raw material in the semi-solid state. B&W (black and white) and polarized light color metallography of the microstructure were employed to better characterize these structures. Among the five tested routes, the samples prepared by the ECAP route presented the most suitable characteristics for thixoforming, showing smaller grain sizes (measured by polarized light color metallography), smaller globule size and greater sphericity (both measured by B&W metallography) in the semi-solid state. Interconnections in the residual solid fraction in the semi-solid state are usually visible only by polarized light color metallography. However, the use of both color and B&W metallography enabled the best morphological structures for thixoforming to be clearly identified, confirming the promising potential of the ECAP technique in processing raw materials for thixoforming.     

Evaluation of giga-cycle fatigue properties of some maraging steels by intermittent ultrasonic fatigue testing

ABSTRACT In evaluating the giga-cycle fatigue strength of some high strength steels, information on the size distribution of nonmetallic inclusions contained in the material is indispensable. To save time and effort of obtaining such data concerning the inclusions, a convenient dissolution method to evaluate the maximum inclusion size is proposed, in place of a conventional method of measuring the inclusion sizes on many cross-sectional areas. Meanwhile, to save time-consuming work of obtaining giga-cycle fatigue properties of some metallic materials, an intermittent ultrasonic fatigue testing method has also been developed. In the present paper, these two newly developed methods were successfully combined to assess the long life fatigue properties of maraging steels as a function of inclusion size.     

Thermal stresses in aluminium alloy die casting dies

The aim of this research is to analyze the influence of Aluminium Alloy die casting parameters, die material, and die geometry on in-service tool life. An innovative immersion testing apparatus is developed, at which Aluminium Alloy die casting is simulated. It enables controlled thermal fatigue cycling. Special specimens with different edge geometry and specimens with maraging steel welds deposited by Gas Tungsten Arc (GTA) welding are prepared. They are subjected to cyclic heating in bath of molten Aluminium Alloy 226 and cooling in bath of water-based lubricant. The specimens are continuously internally cooled with cold water. The microstructure, hardness profile, and the surface cracks developed are periodically analyzed after completion of a particular number of cycles. Temperature transients at different locations of the specimens are measured and used in calibration of finite element model (FEM). The computation of transient stresses is performed by developed FEM. The influence of immersion test parameters, material, specimen edge geometry, and thickness of maraging steel surfacing welds on thermal stresses is studied. To improve thermal fatigue testing efficiency, a specimen of particular geometry and immersion test parameters are developed based on finite element analysis. The results showed significant differences in produced thermal stresses for analyzed materials, test parameters, and edge geometries. Maraging steel is found to be superior material for die casting dies, due to generation of lower stresses.     

基于微观结构的高强铝合金应力集中系数研究

为了更好地理解铝合金材料的微观力学性能,基于MATLAB编写了Voronoi算法的微观结构模拟程序,并将程序导入ABAQUS有限元软件建立铝合金晶粒模型.推导出六结点内聚线单元模型的界面单元刚度矩阵,利用内聚力模型的内聚力-位移关系描述铝合金晶粒界面间的粘着力(法向力)和摩擦力(切向力),建立了微观晶粒结构的有限元模型.研究结果表明:单个夹杂粒子随着弹性模量的增加应力集中系数先减小再增加;相对于单个夹杂粒子,两个夹杂粒子的应力集中会增加,当d/r接近2时应力集中系数明显增加,当d/r值处在6左右时应力集中系数基本恢复到单夹杂粒子时的大小.夹杂粒子的形状、数量及分布状态对结构微观应力集中均有影响.     

Material properties of piezoelectric composites by BEM and homogenization method

Applications of boundary element method (BEM) to piezoelectric composites in conjunction with homogenization approach for determining their effective material properties are discussed in this paper. The composites considered here consist of inclusion and matrix phases. The homogenization model for composites with inhomogeneities is developed and introduced into a BE formulation to provide an effective means for estimating overall material constants of two-phase composites. In this model, a representative volume element (RVE) is used whose volume average stress and strain are calculated by the boundary tractions and displacements of the RVE. Thus BEM is suitable for performing calculations on average stress and strain fields of the composites. Numerical results for a piezoelectric plate with circular inclusions are presented to illustrate the application of the proposed micromechanics––BE formulation.     

无钴马氏体时效钢的研究现状

综述了无钴马氏体时效钢的发展和研究现状,阐述了无钴马氏体时效钢的时效组织结构及强化机制,分析了合金元素在马氏体时效钢中的作用,并提出了无钴马氏体时效钢未来的发展趋势.     

Formation of ZnO/metal composites by rapid oxidation of Zn melts

A novel ZnO-based composite material is fabricated by oxidation of Zn alloy melts, in which outward growth of the reaction products and rapid reaction kinetics are observed. Additions of Na and Bi to the Zn melts are found to be essential to increase the matrix growth rate to a practical level. Optimum reaction rates are observed for Zn-3Na-5Bi alloys oxidized at temperatures between 450 and 550°C in pure oxygen. Processing parameters, such as alloy composition and oxygen activity, also have a profound impact on the resulting microstructure and reaction kinetics. The evidences supported that the continuous outward growth of the matrix is achieved via modification of the ZnO defect structure and partial wetting of the growing oxide by the melts.     

Understanding the Magnetic Shape Memory System Fe–Pd–X by Thin Film Experiments and First Principle Calculations

The magnetic shape memory (MSM) alloy Fe₇₀Pd₃₀ is of particular interest for novel microactuator and sensor applications. This review summarizes the underlying physical and material science concepts for this MSM alloy system. First‐principles calculations of the electronic and crystallographic structure together with combinatorial and epitaxial film studies are presented. By these complementary methods we can address the open key questions of MSM alloys and microsystems: Which are the driving forces for a martensitic transformation and how does this transformation proceed? How is it possible to improve the MSM properties by adding third elements? What is the role of external interfaces and which routes allow the preparation of freestanding epitaxial films?     

Metallurgical aspects of corrosion

Metallurgical properties have strong effects on corrosion. The paper discusses and reviews the work done at CECRI on the metallurgical aspects of corrosion of some industrially important alloys like steel and aluminium alloy weldments, stainless maraging steel and prestressing steel. The corrosion control methods for the above materials are also reviewed.     

The microstructure and mechanical properties of prolonged and lower temperature aged Fe–Ni–Mn–Mo–Ti–Cr maraging steel

In this study, the microstructure and mechanical properties of Fe–Ni–Mn–Mo–Ti–Cr maraging steel at low temperature and prolonged aging condition were investigated. Optical and scanning electron microscopy examinations, tensile and hardness tests were conducted to study the microstructure, aging behavior and mechanical properties of the cold‐rolled steel. The results showed that aging of cold rolled Fe–Ni–Mn–Mo–Ti–Cr maraging steel resulted in the formation of Mo rich and Ti rich Lave phase precipitates. Existence of many dislocation cores due to cold rolling and subsequently, low temperature aging caused to formation of uniform distribution of very fine precipitates. The presence of these precipitates increased the yield and ultimate tensile strengths but couldn't improve the uniform tensile ductility. This alloy showed ultra‐high fracture stress of about 1950 MPa with a negligible tensile elongation (about 2 %) at the peak aged condition. The fractographic studies indicated this alloy shows semi‐brittle fracture in the subsequent aging treatment.