Casting of High Alloy Steels in the Mushy State

In order to broaden the field of application for the innovative thixocasting process, much research is dedicated to the thixocasting of high melting point alloys. The wide property range of modern high alloy steels combined with the productive semi‐solid die casting process opens up new fields of application. The Foundry Institute of the Aachen University has therefore been concentrating on the research of the possibilities and limits of high pressure die casting of high alloy steels in the semi‐solid state. This paper gives an overview of the current work dedicated to thixocasting of steel alloys by a high pressure die casting machine at the Foundry‐Institute of the Aachen University of Technology. In order to understand and describe the material properties in the semi‐solid state, basic test specimens have been investigated. Weak points of tool preheating as well as directional solidification of the produced parts can be controlled by numerical simulation of the temperature distribution inside the dies. In consideration of the outstanding flow properties of semi‐solid steels more complex geometries with accurately defined applications are now being investigated. Extensive metallographical analyses of the pre‐material, the reheated billets and the produced parts have been done to evaluate the viability of the process. The mechanical properties of the specimens outline the outstanding potential of the thixocasting process.     

DTA‐Measurements to Determine the Thixoformability of Steels

Semi‐solid metallurgy (SSM), also known as “thixoforming” or “thixoprocessing”, is of special interest as a new potential manufacturing technology for components in the automobile, machine and electronic industries. The aim of this technology is to produce complex shapes which cannot be produced with conventional processing methods. An important process step of semi‐solid processing (SSP) is the reheating and isothermal holding of the billet within the solid‐liquid range in order to obtain the required fraction liquid content and the desired globular microstructure. Aside from the investigation of billet heating and the development of a suitable tool design, the development and evaluation of adequate microstructures over a wide temperature area is very important. The focus of this paper is to determine the semi‐solid area of different steels through Differential Thermal Analysis (DTA) measurements. To determine a process window for handling the alloys in the semi‐solid state, the DTA‐results can be combined with microstructure parameters. Subsequent quenching experiments show the development of the microstructure parameters (e.g. grain size, phase distribution, volume fraction, shape factor, matrix character, contiguity, and particle density of the primary solid and liquid phases). A comparison of the slopes of the determined solid‐liquid areas for different steels show the width of the melting or freezing intervals to evaluate the possible process windows. DTA‐experiments performed at different heating rates show the influence of faster heating and cooling rates on the solidus‐liquidus interval. To evaluate the suitability for the thixoforming processes, this paper describes, and then compares, the semi‐solid intervals of different steel grades, which have been investigated in the Department of Ferrous Metallurgy at the RWTH Aachen University. The tool steel HS 6‐5‐3 and the cold work tool steel X210CrW12 have a wide semi‐solid area, which can be explained due to the dissolution of different carbides. In contrast to this, the steels C45, 42CrMo4, 16MnCr5, 34CrNiMo4, 100Cr6, X220CrVMo13‐4 and the Alloy 33 show a much smaller semi‐solid area.     

Advanced Processing and Properties of Thixotropic Formed Components Based on Partially Melted Magnesium

Contrary to the manufacture of aluminium components in the semi‐solid, thixotropic state the production of magnesium based components by semi‐solid techniques is still uncommon. For this reason, the advantage of this production method is analysed with regard to the commercial magnesium alloy AZ80. The objective of this research is semi‐solid‐casting (SSC) of AZ80 for the production of a light weight component in near‐net‐shape quality and with advanced properties. Using extruded primary feedstock material, the behaviour and the advantages are investigated. Billets with a weight of up to 2 kg are heated up into a semi‐solid state. To avoid any risk of self‐ignition of the material an automatic, temperature controlled induction heating system is used. To achieve an optimum homogeneous grain structure the induction heating power is varied making use of a process control system based on power‐time‐curves. The heated billets are transported in the soft semi‐solid condition from the induction heating system to a die casting machine to produce components with wall thickness’ between 2 and 10 mm. After forming of the components, the influence of heat treatment on the grain structure and especially on the mechanical properties is determined to provide parts with optimised characteristics. To compare the properties of the special globular grain and microstructure, the results of various static and dynamic tests are analysed. It is found that components can be manufactured with a magnesium alloy in a thixotropic state in near‐net‐shape quality, with low porosity and with excellent mechanical properties like elongation of up to 15%.     

Thixocasting Steel Hand Tools using Al2O3‐coated Steel and Molybdenum Dies

Forging is state‐of‐the‐art for producing hand tools on an industrial scale. Due to high demands on the stiffness and the fracture toughness, high‐strength forging steels are used to provide cavity‐free components with high mechanical load capacity. Moreover, forging is a cost‐effective mass production process but, in spite of all its advantages, it has its limitations, e.g. in the freedom of designs. However, because of the extreme thermal loading (particularly with regard to permanent moulds) and the frequently unavoidable casting defects, hand tools are not cast. By means of thixocasting steel, technical difficulties can be reduced and new options are provided which allow the manufacturing of components with much higher complexity than that using forging. Through near‐net shape production, manufacturing steps and costs can be reduced. Furthermore, steels, which are difficult to forge but nonetheless have high potential for specific applications (such as high strength or corrosion resistant steels), can also be processed. In cooperation with industrial partners, X39CrMo17 stainless steel size 17 combination spanners were thixocast. Forming dies were designed and optimized by simulation, the hot forming X38CrMoV5 tool steel as well as the molybdenum alloy TZM were selected as the tool alloys. The dies were treated by a plasma nitriding process and subsequently coated with crystalline Al₂O₃ protective coatings by plasma‐enhanced chemical vapor deposition (PECVD). During the experiments, combination spanners were successfully cast in the semi‐solid state. Cast parts were heat‐treated to enhance the components' toughness, which was subsequently measured by Charpy impact and tensile tests.     

Surface Oxidation of Steel Powder

Powder metallurgical production of fully dense steel such as high‐speed steel, tool steel and stainless steel is of large industrial importance. The process route is as follows: melting – inert gas atomization – encapsulation – hot isostatic pressing and subsequently often also hot forming. Final products comprise near‐net‐shape components, billets and semi‐finished parts e.g. rods. The mechanical and functional properties of the PM steels are often superior to those of conventional manufactured steels over casting + forging with the same chemical composition. The properties of the PM‐steels are highly dependant on their oxide contents. There are three types of oxides in PM‐steels: exogenous and endogenous slag inclusions and oxides originating from surface oxidation of the atomized particles. The negative effect from the inclusions on the properties of the PM‐steels is dependant on their origin. The exogenous inclusions are normally highly detrimental while the endogenous inclusions are harmless. Surface oxides are harmful when present in larger amounts. A special experimental technique was developed to study surface oxidation of powders. Large emphasis was also focused on sampling to enable that the oxygen determination was representative of the bulk powder. The results thus obtained gave most important information on when and how the atomized powder was oxidized during the process. Furthermore, a new methodology was developed based on the experimental results. It is now used world‐wide for quality control of atomized steel powders. This method enables the distinction between exogenous + endogenous inclusions and surface oxides.     

Theoretical and Experimental Damage Prediction in Cold and Semi‐Hot Bulk Forming of Ductile Steels

Bulk forging is among the most important manufacturing methods in metal forming, due to its wide applicability from some ounces to several tons of steel in a high diversity of shapes and forming conditions. Economical constraints demand for further optimisation and cost‐effective production. This requires the application of suitable finite elements simulation software, in order to support the already digitalised construction processes. Ductile damage is one of the most severe problems to arise during the production sequences, not only in cold but also in semi‐hot forging operations. Mathematical approaches exist for the modelling and simulation of ductile fracture in steel. In this paper some widespread used damage models are introduced and discussed. Their damage prediction quality has been verified by experiments, the tensile test and the collar specimen upsetting with several different steels under cold and semi‐hot forging conditions. The methods for the experimental fracture detection are introduced as well. In cold forging the passive ultrasonic testing with integrated statistical filtering algorithms is used. As this method is not applicable to semi‐hot forging experiments, optical fracture detection by means of a high‐speed camera is used instead. A very interesting material behaviour of the steels tested has been identified in the semi‐hot upsetting of collar specimen. For every steel a distinct temperature crossover interval exists, in which the forging process abruptly changes from damaged to undamaged state. This interval amounts to some degrees Celsius only for each of the seven materials investigated. Among the damage models proposed, the Model of Effective Stresses by Lemaitre is chosen for the application to a cold and a semi‐hot forging operation. These industrial processes of an axle end (cold) and a journal bearing (semi‐hot) are susceptible to damage for reasons to be discussed in this paper. It will be shown that the internal fracture of the axle end (chevrons) and the surface fissures of the journal bearing can be predicted with high accuracy. Moreover, the application of the damage model in the finite element software MSC.SuperForm 2004 offers a promising approach for process optimisation. Several possibilities could be tested for their suitability of reducing the calculated damage: geometry variation of the forming tools, process annealing, different materials. The use of damage models in finite element simulation can be regarded as a further step towards an optimal process design.     

国内外热作模具钢的研究进展

介绍了国内外传统和新型热锻、热挤压和压铸用热作模具,以及特殊用途热作模具的钢种和特点;热作模具钢热处理和表面处理技术的研究进展。提高模具钢纯净度、均匀和细化组织,用计算机模拟技术开发热作模具钢新钢种是目前提高热作模具钢寿命的重要方法。     

The Influence of Isothermal Semi‐solid Processing on Microstructure,Hardness and Young's Modulus of a Hypoeutectic Chromium Steel

Steel billets of the hypoeutectic chromium steel X210CrW12 (material number 1.2436, AISI D6) are reheated into the semi‐solid state and isothermally extruded using a ceramic extrusion tool at low process forces. Extruded bars show good microstructural as well as chemical homogeneity. Nanoindenter measurements of untreated and extruded samples prove that hardness and Young's modulus are increased after semi‐solid extrusion. Hardness values are retained after annealing, while Young's modulus drops back to the initial value of untreated X210CrW12. Results are consistent with metallurgical investigations on phase formation and mechanical properties of X210CrW12 processed in the semi‐solid state. Thus, the viability of isothermal processing and the underlying tool concept is demonstrated in terms of work piece quality. The suitability of X210CrW12 for semi‐solid processing and the potential improvement of mechanical properties are validated.     

Microstructural investigations in the semi‐solid state of the steel X210CrW12

Thixoforming is an emerging young technology to produce complex structural parts and near net shape components. Thixoforming stands for the forming of materials in the semi‐solid state. One precondition for the thixoformability of materials is the minimum temperature range for the solidus‐liquidus interval and the globulitic formation of the solid phase during the thixoforming process. Besides this other parameters like shape factor, contiguity, matrix character, melting interval, and phase distribution are important process parameters. Aluminium and magnesium alloys are the objectives of numerous investigations, but research activities concerning the thixoformability of steel alloys have been commenced recently. This article provides metallographic information on the relevant parameters of the steel X210CrW12, taking into account the microstructural evolution and the establishment of a parameter field for forming this material in the semi‐solid state.     

Analysis of Microstructure and Mechanical Properties of Different Hot Stamped B‐bearing Steels

Hot stamping is a technique to produce ultra high strength automobile components. The common material used in hot stamping process is coated and/or uncoated 22MnB5 boron alloyed steel. Ferritic‐pearlitic microstructure in as‐delivered sheets is transformed to fully lath martensitic after hot stamping. In the present research, hot stamping under water or nitrogen cooling media was investigated using different boron alloyed steel grades. Microstructural analyses, linear and surface hardness profiling as well as tensile tests of hot stamped samples were performed. Various microstructures of fully bainitic and/or fully martensitic were produced. The resulting microstructures provided yield strengths of 650–1370 MPa and tensile strengths of 850–2000 MPa. There is an optimum carbon equivalent content for which the highest formability index value, UTS × A₂₅, is achieved. Using a nitrogen cooled punch resulted in higher yield strength without significant changes in ultimate tensile strength. It is concluded that a wide range of B‐bearing steels having an extended carbon equivalent range with an acceptable formability index value can be used by increasing the cooling rate in the die assembly.     

Ferritic Fine Structure Characteristics in Hot Die Forged Powder Steels. I. Effect of Heating Temperature during Die Forging on Ferritic Fine Structure in Specimens of Powder Technical Grade Iron and Carbon Steels

We have studied the effect of the composition and heating temperature during die forging on the microstructure and fine structure of ferrite in powder iron and powder carbon steels. From the structural characteristics obtained for different planes of the blank, we used harmonic analysis of the x-ray line shape to establish the nonuniformity of deformation over the volume. We have established the heating temperature during die forging of powder technical grade iron and carbon steels, for which the deformation is close to uniform over the die forging volume: 1050 °C for technical grade iron and low-carbon steel, 1100 °C for carbon steel. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 108–119, May–June, 2005.     

变形铝合金7050半固态锻造成形过程组织演变特征与力学性能研究

主要研究了变形铝合金7050在半固态成形过程中的组织演变及成形零件的热处理和力学性能。研究结果表明：1）采用低过热法能够细化组织,得到比较均匀的球团化的枝晶组织;在二次重熔过程中,α组织逐渐圆整化,温度越接近最终的半固态温度,组织的圆整性越好,并且锻件中的组织大小比较均匀。2）制备合金坯料的合理低过热浇注温度为623℃,合理的半固态锻造温度为610℃;3）合金热处理以后,其抗拉强度比一般锻造法提高了24％。     

Effect of melting and casting parameters on the hot ductility behavior of Nb-bearing beams,billets and slabs

The production of defect free continuously cast microalloy Nb-bearing steel slabs, blooms and billets have increased in importance for both the producer and the end user. The minimization of surface and internal defects has a substantial impact on steel producing operating costs, internal and external cost of quality and delivery performance. A keen and thorough understanding of the hot ductility behavior of various steel grades is essential to successfully melt and cast high quality microalloy steels. This paper describes current steelmaking and casting considerations and recommendations to successfully cast high quality niobium-bearing slabs, billets and blooms. This study also integrates the process metallurgy and the physical metallurgy to better understand the hot ductility mechanisms and process metallurgy control practices in use today for the high quality production of Nb-bearing steels.     

钢液凝固与冷却过程及固体钢加热过程钢中非金属夹杂物成分动力学转变的几个概念和特征曲线

利用钢中非金属夹杂物成分变化的集成模型,介绍了夹杂物成分随时间和冷却速率的变化,提出了夹杂物成分转变分数的概念,然后介绍了夹杂物成分转变的等温转变曲线（TTT）、连续冷却转变曲线（CCT）和等径转变曲线（TDT）的概念及应用。该集成模型考虑了钢液流动、传热、凝固和元素偏析,也考虑了钢与夹杂物反应的热力学和动力学。然后以管线钢、重轨钢和轴承钢为例,进一步分析讨论了钢液凝固与冷却过程中的冷却速率、固体钢加热过程中的加热温度和加热时间、钢成分以及夹杂物尺寸等参数对夹杂物成分转变的影响。这些概念和特征曲线能够直观展示在钢液凝固冷却过程及固体钢加热过程钢中非金属夹杂物的成分转变,将钢中夹杂物的控制方略从钢液拓展到固体钢中。      

FEM Analysis of Void Closure Behaviour during Open Die Forging of Rectangular Billets

Finite element analysis of open die forging to make rectangular billets has been performed in this study. Three‐dimensional rigid‐plastic finite element method (FEM) was used to analyse the effects of process variables, forging pass design and die configurations on the void closure phenomena. The major objective was to control the internal deformation for better structural homogeneity and centreline consolidation of the rectangular billet. The effect of die width ratio, die feed rate, die shape, and number of passes on the void closure ratio has been examined. Although it is difficult to optimize process parameters in industrial environments, favourable process conditions are recommended to achieve better product quality.     

Effects of Pre‐Processing on Thixoformability of Steel Grade 100Cr6

Thixoforming is a new manufacturing technology which has been widely investigated for aluminium alloys. The thixoformability of steel and especially the impact of different pre‐processing is subject of this paper. For the thixoforming process it is of special interest to obtain the required fraction liquid content between 20 and 40%, a globulitic microstructure and low process temperatures. Therefore, steel primary material from three different processing routes was compared. The primary materials are rolled bars, laboratory cast billets and laboratory cast billets with liquid core reduction. The melting behaviour of the different materials was investigated by using Differential Thermal Analysis (DTA). Furthermore the materials were reheated into the solid‐liquid range and quenched to monitor the development of the microstructure. No significant differences between the three different pre‐processing routes could be determined with regard to thixoforming. Thus it seems possible to start thixoforming with as‐cast primary material.     

Thermal Fatigue Testing of Tool Materials for Thixoforging of Steels

Semisolid processing, already a well established manufacturing route for the production of intricate, thin‐walled aluminium and magnesium parts with mechanical properties as good as forged grades, faces a major challenge in the case of steels. The tool materials must withstand complex load profiles and relatively higher forming temperatures for thousands of forming cycles for industrial application to be attractive. Since the forming pressures are much lower than those encountered in conventional forging, the principle die failure mechanism in steel thixoforging is expected to be thermal fatigue. Hence, suitable materials able to withstand the steel thixoforming environment for an economically acceptable life, can be best identified with a thermal fatigue test. Such a test is described in the present work. A novel CrNiCo and a nickel‐base superalloy, reported to exhibit superior thermal fatigue resistance in demanding tooling applications, was tested under thermal fatigue conditions encountered in the thixoforming of steels.     

The structure and properties of forged composite high-speed steel: Titanium carbide

The structure of a powder composite (carbide steel) based on high-speed steel with dispersed inclusions of titanium carbide and obtained by forging powdered billets is investigated. The elemental and chemical composition of the material is determined. The physicomechanical properties of the forged carbide steels obtained are compared with those obtained by other methods.     

高强抗震钢筋中铌的碳、氮化物析出热力学研究

摘要：连铸过程中铌的氮化物、碳化物和碳氮化物在奥氏体晶界的析出对铸坯的质量产生严重的影响。分析了500MPa级高强度抗震钢筋(HRB500E)中铌的氮化物、碳化物和碳氮化物在液相、凝固过程以及奥氏体相等不同阶段的析出热力学，计算了不同温度下NbN和NbC的平衡/实际浓度积，得到NbN和NbC在微合金钢连铸过程中的析出规律。计算结果表明：在HRB500E钢中，NbN、NbC在钢液成分均质状态和凝固过程中难以析出；在奥氏体相中，随着温度的降低，NbC、NbN及NbC0.85N0.15具备析出热力学条件，析出温度分别为1377、1229 和1378K，析出顺序依次为：NbC0.85N0.15、NbC、NbN。