David Aišman

Structure of miniature components from steel produced by forming in semi-solid state

Abstract To obtain new unconventional structures with specific mechanical and physical properties is possible not only by the development of new types of materials but also by treatment of conventional materials using unconventional innovative technological procedures. One of these technologies is the forming in semi-solid state involving rapid solidification of miniature components from steels. Production of such components is complicated by a number of technical problems. To explain phenomena of the process and structure development, the production of miniature components from the tool steel X210Cr12 difficult to form was experimentally tested. The structure of this originally ledeburite steel consisted of 95 % of metastable austenite after the treatment. Metastable austenite was located particularly in globular and polygonal grains while the remaining interspaces were filled by lamellar network. The detected high stability of extremely high fraction of metastable austenite was tested under different conditions of thermal exposition and mechanical loading.

Mini-thixoforming of a Steel Produced by Powder Metallurgy

Semi-solid processing is complicated by various inherent technical problems. However, once these problems are solved, thixoforming allows intricately shaped components to be manufactured very effectively – often with microstructures that cannot be produced by any other techniques. The recently introduced mini-thixoforming method is an example of such a novel technique for semi-solid processing of steel. The wall thicknesses of resulting parts are about 1 mm. Microstructures of semi-solid-processed steels typically consist of a high proportion of globular particles of metastable austenite embedded in a carbide network, the latter being much harder and more brittle. This paper illustrates that mini-thixoforming allows inverting that microstructural configuration. As an experimental material, powder steel with increased content of vanadium and chromium was used. The post-thixoforming microstructure consisted of a dispersion of carbides and high-vanadium and high-chromium eutectic in an austenitic matrix. Applying optimised processing parameters, complex-shaped parts could be produced. According to the high hardness of resulting microstructural components, the new materials are likely to exhibit extraordinary strength and wear resistance.

Mini-Thixoforming of High-Alloyed CPM REX 121 Steel

Thixo-forming is an unconventional semi-solid forming process, by which complex-shaped products can be manufactured using a single forming operation. It can even be applied to difficult-to-form materials, including those which are impossible to process by conventional methods. Today, commercial semi-solid processing is used for low-melting materials, primarily aluminium and magnesium alloys. Due to its technological complexity, thixo-forming of high-melting alloys is still under development. For this reason, the present experimental programme was focused on the tool steel CPM REX 121 with a melting point above 1200 °C produced by powder metallurgy. The total content of alloying elements in this steel is 37.5 %. Owing to the high levels of alloying elements, namely Co, Mo, W, V and Cr, this material cannot be formed by conventional methods. The purpose of the present experiment was to explore its potential for forming in semi-solid state and to find suitable processing parameters. Experimental forming took place in a mini-thixoforming die, a tool specially-developed for this thixo-forming variant intended for producing very small parts. The resulting microstructures were examined by means of optical and electron microscopy. It was found that semi-solid processing leads to the development of microstructure with austenitic grains, martensite, chromium and V-W-Mo complex carbides and also a eutectic formed by partial melting of carbides.

Micro-Compression Test of Thixoformed Austenite

Thixoforming is an alternative forming method, by which intricate and complex-shaped products can be manufactured using a single production step. This technology allows a material’s microstructure to be altered profoundly. Typical microstructure of steels processed in this manner consists of quasi-polyhedral austenite grains embedded in a ledeburite-carbide network. This type of microstructure was produced by processing the experimental material in this study: the X210Cr12 steel. Since austenite is a metastable component depending on oversaturation with a number of elements, its thermal and mechanical stability needs to be known. This information is required for further modification and enhancment efforts. In previous experiments, the thermal stability was tested by thermal exposure. In the present work, the behaviour of austenite was explored under mechanical load at room temperature in a micro-compression test. A single block of austenitic material was used for making a test specimen with the dimensions of 2.4×2.2×4.9 µm. Its mechanical properties were measured and deformation stability was investigated using compressive deformation.

Semi-solid processing of high-chromium tool steel to obtain microstructures without carbide network

Treatment of high-alloy tool steels that involves transition to the semi-solid state can transform the sharp-edged primary carbides which usually form during solidification. These carbides severely impair toughness and are virtually impossible to eliminate by conventional treatment routes. Upon classical semi-solid processing which dissolves these carbides, the resulting microstructure consists of polyhedral and super-saturated austenite embedded in lamellar austenite-carbide network. This type of microstructure reflects in the mechanical properties, predominantly in material behaviour under tensile loading. Such a network, however, can be removed by appropriate thermomechanical treatment. In the present experiment, various procedures involving heating to the semi-solid state were tested on X210Cr12 tool steel. The feedstock was heated to the temperature range of 1220 – 1280 °C. The heating was followed by procedures involving either water quenching to the forming temperature, room temperature or temperature from the range from 500 °C to 1000 °C followed by reheating to the forming temperature. It was found that the development of the lamellar network strongly depends on the temperature of heating to semi-solid state. Thermomechanical treatment produced microstructures in which the matrix consisted of a mixture of polyhedral austenite grains and the M-A constituent. In addition, the initial lamellar eutectic network was partially or even completely melted and substituted with a mixture of very fine recrystallized austenite grains and precipitates of chromium carbides. Some fine M7C3 carbides were present in the austenitic-martensitic matrix as well. When appropriate processing parameters were chosen, very good mechanical properties were obtained, among them a hardness of 860 HV10.

Using of material-technological modelling for designing production of closed die forgings

Production of forgings is a complex and demanding process which consists of a number of forging operations and, in many cases, includes post-forge heat treatment. An optimized manufacturing line is a prerequisite for obtaining prime-quality products which in turn are essential to profitable operation of a forging company. Problems may, however, arise from modifications to the manufacturing route due to changing customer needs. As a result, the production may have to be suspended temporarily to enable changeover and optimization. Using material-technological modelling, the required modifications can be tested and optimized under laboratory conditions outside the plant without disrupting the production. Thanks to material-technological modelling, the process parameters can be varied rapidly in response to changes in market requirements. Outcomes of the modelling runs include optimum parameters for the forging parts manufacturing route, values of mechanical properties, and results of microstructure analysis. This article describes the use of material-technological modelling for exploring the impact of the amount of deformation and the rate of cooling of a particular forged part from the finish-forging temperature on its microstructure and related mechanical properties.

Development of Unconventional Processing of Steels in Semi-Solid State

This paper describes selected capabilities of unconventional processing of steels in semi-solid state under various process conditions and with the use of various steel chemistries for obtaining unusual structures formed by rapid solidification in combination with other procedures. This investigation involves the use of severe plastic deformation techniques (SPD) and in-situ observation of the transformation of microstructure from solid state to semi-solid state at temperatures above 1200°C using a high-temperature microscope.

Semi-solid processing and its as yet unexplored potential

Thanks to the available advanced control technology, manufacturing processes which have been described in the past but their hidden potential remained untapped are now continuing to be developed. As a result, even conventional materials which have been around for years can be manipulated to obtain unusual microstructures with specific mechanical and physical properties. Semi-solid processing belongs to the above-described group: it had been studied in the past but, due to complicated process control, it gradually lost its appeal. However, advanced techniques of temperature field control enable engineers to control this complex process accurately. One of the innovative methods of semi-solid processing is mini- thixoforming. As it focuses on very small-size products, it offers very steep heating curves and extremely high solidification and cooling rates, when compared to conventional thixoforming. The capabilities of this process were tested on X210Cr12 ledeburitic tool steel. After the optimum processing conditions were found, additional materials were tried, ranging from low- carbon microalloyed steels through medium-alloy steels to high-alloy tool steels. The microstructure evolution upon the mini-thixoforming process is an issue of its own. The final microstructure of X210Cr12 consisted of more than 90% of austenite and chromium carbides. Semi-solid processing of a steel with a high vanadium content led to a microstructure comprising MA matrix and globular vanadium carbides. In a low-alloy steel, martensitic microstructure was obtained.

Steel - a Classic Material with a Large Potential for the Future

Steel is a traditional material which has been used by mankind for more than five thousand years. We may therefore be tempted to believe that we know practically everything about steel and its forms and variants which offer an extraordinary and broad range of properties. It is this diversity of properties which makes steel such a popular and widely used material. And yet, in recent years new opportunities have emerged for processing steel by unconventional techniques and producing novel, as yet unknown or unusual microstructures. This paper describes several examples of how microstructure evolution can be modified and how new and unconventional processing routes can be developed. These examples present several results of projects carried out in recent years by FORTECH Research Centre of Forming Technology and the University of West Bohemia in collaboration with their research partners.

Development of microstructure in high-alloy steel K390 using semi-solid forming

Semi-solid processing of light alloys, namely aluminium and magnesium alloys, is a widely known and well-established process. By contrast, processing of powder steels which have high levels of alloying elements is a rather new subject of research. Thixoforming of high-alloy steels entails a number of technical difficulties. If these are overcome, the method can offer a variety of benefits. First of all, the final product shape and the desired mechanical properties can be obtained using a single forming operation. Semi-solid forming can produce unusual powder steel microstructures unattainable by any other route. Generally, the microstructures, which are normally found in thixoformed steels, consist of large fractions of globular or polygonal particles of metastable austenite embedded in a carbide network. An example is the X210Cr12 steel which is often used for semi-solid processing experiments. A disadvantage of the normal microstructure configuration is the brittleness of the carbide network, in which cracks initiate and propagate, causing low energy fractures. However, there is a newly-developed mini-thixoforming route which produces microstructures with an inverted configuration. Here, the material chosen for this purpose was K390 steel, in which the content of alloying elements is up to 24%. Its microstructure which was obtained by mini- thixoforming did not contain polyhedral austenite grains but hard carbides embedded in a ductile austenitic matrix. This provided the material with improved toughness. The spaces between the austenite grains were filled with a eutectic in which chromium, molybdenum and cobalt were distributed uniformly. After the processing parameters were optimized, complexshaped demonstration products were manufactured by this route. These products showed an extraordinary compressive strength and high wear resistance, thanks to the hardness of their microstructure constituents, predominantly the carbides.