Semi-solid Processing Of Steel Research Articles

Steels with High Temperature Carbides - New Possibilities for Semi-Solid State Processing

Semi-solid processing of steels is typically studied using high-alloy steels with higher carbon levels, as those offer a long freezing range which is favourable for conducting the process. The drawback to their application is their microstructure which typically consists of austenite grains embedded in ledeburitic network. This type of microstructure typically fails in brittle manner by fracturing along the interface of the hard network and ductile austenite grains. This is why a way was sought to altering or even inverting the configuration of the microstructure. Eventually, suitable steel chemistries were found which allow the inverted microstructure to be obtained. With regard to the high content of alloy additions, these steels have to be made by powder metallurgy methods. Five different steels of this kind were selected for the experimental programme. All contained high amounts of alloying elements and a large fraction of carbides. Their carbon content was taken into account as well, ranging from 0.55 to 3.4 %. Differences between the steels consisted in the levels of major alloying elements, namely chromium, vanadium, molybdenum, tungsten and cobalt. After suitable process parameters were found, semi-solid processing was used to prepare demonstration products. The transition through semi-solid state transformed the ferritic matrix to austenitic-martensitic one, in which the high-stability carbides were retained. The resulting microstructures were of unconventional nature where carbide particles were embedded in tough metal matrix. Their configuration was thus inverted in contrast to the ones typically obtained by semi-solid processing of tool steels.

Microstructural Evolution of Semi-Solid Type 304 Stainless Steel Deformed Severely by ECAP

Thixoforming or semi-solid metal forming offers many advantages in comparison with casting and conventional forging. However, due to the high-melting temperature and related difficulties, there is relatively a few experimental data on the semi-solid processing of steels. Therefore, this study is subjected to study the microstructure evolution during partial remelting of 304 stainless steel which is priorely deformed in solid state by equal channel angular pressing (ECAP) at room temperature. ECAP is a promising technique to induce a great homogeneous strain in bulk metals known as a new method for strain induced melt activation (SIMA) process for preparing thixoforming ingot. Simultaneously, the effect of number of passes during ECAP was also discussed in terms of the microstructure change. The results showed that the microstructure was well refined with formation of fine twins and partial martensite along with many fine equiaxed grains which in turn lead to fine and spheroidal semi-solid slurry. It can be confirmed that ECAP followed by reheating was an effective method to produce semi-solid ingot for thixoforming.

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.

Thermal Cycling of Yttria-Stabilized Zirconia-Coated Hot Work Tool Steel

Thermal fatigue performance of yttria-stabilized zirconia-coated hot work tool steel was investigated under conditions encountered by thixoforming dies in semi-solid processing of steels. The thermal barrier effect of the YSZ coating was evident from the relatively smaller and more uniform temperature gradients set up across the section of the sample, which in turn, had a direct impact on the magnitude of thermal stresses generated at the surface of the tool steel. In spite of some thermal expansion mismatch with the substrate tool steel and occasional discontinuous thermally grown oxides along the splat boundaries and between the BC and TC, the YSZ coating survived thermal cycling under steel thixoforming conditions for over 12,000 cycles with no evidence of debonding and spallation. This is a nearly ten-fold increase in thermal fatigue life with respect to the uncoated counterpart. An increasingly dense YSZ layer and the toughening linked with the tetragonal to monoclinic ZrO2 phase transformation are credited for the remarkable performance of the YSZ-coating. It is fair to conclude that the YSZ coating offers adequate protection for the underlying tool steel, which without YSZ coating, suffers severe oxidation after only several hundred cycles and extensive cracking after 1000 cycles.

Response to thermal cycling of plasma nitrided hot work tool steel at elevated temperatures

Thermal fatigue performance of plasma nitrided hot work tool steel was investigated under conditions encountered by thixoforging dies in semi-solid processing of steels. Plasma nitriding does not offer any improvement in the thermal fatigue performance of hot work tool steels at elevated temperatures, due essentially to poor resistance to oxidation and to temper softening. Fe 3O 4 and Fe 2O 3 scales produced on the nitrided surface fail to sustain the thermal stresses produced by thermal cycling. They spall off, generating fresh surfaces for further oxidation. This sequence leads to substantial material loss and impairs the integrity of the surface beyond a quality level that would be tolerated in steel thixoforming. The surface hardening provided by plasma nitriding is also completely erased. The tempered martensitic structure is replaced by fine, equiaxed ferritic grains implying a dynamic recrystallization process during thermal cycling.

A CrNiCo Alloy as a Potential Tool Material in Semi‐solid Processing of Steels

Semisolid processing of aluminium and magnesium alloys has matured to become a well established manufacturing route for the production of intricate, thin-walled parts with mechanical properties as good as forged grades. However, this innovative forming technology faces a major challenge in the case of steels. The tool materials must withstand the complex load profile and relatively higher forming temperatures which promote chemical interaction with steel slurries. Thixoforming tools ought to last thousands of forming cycles for industrial application to be attractive. Hot work tool steel dies proved to be entirely inadequate when thixoforming steels. In spite of extensive research on tool materials for the semisolid processing of steels, there is yet no material to fulfil this critical role. The present work was undertaken to explore the potential of a novel CrNiCo alloy as the tooling material in semisolid processing of steel.

PVD coated hot work tool steels for tooling applications in semi-solid processing of steels

Semi-solid processing of metals combines the advantages of forging and casting for the shaping of metallic components. Having already matured into an industrial practice for Al and Mg alloys, this innovative forming technology could upgrade the market and provide lightweighting for forged steel parts. However, with process temperatures above 1250°C, the surface-to-interior temperature differentials in steel thixoforming dies are much larger than with Al and Mg. Combined with the erosive wear caused by abrasion and impact of the already solid particles in the slurries and high temperature oxidation of the die surface, these cyclic thermal stresses confer very specific requirements on tool materials. Hot work tool steel dies proved to be entirely inadequate when thixoforming steels, due basically to the limited temper resistance of the commercially available grades. Physical Vapour Deposition (PVD) hard coatings, shown to have beneficial impact on the lifetime of pressure die casting tools, could offer a solution. X32CrMoV33 hot work tool steel widely used in the manufacture of conventional forging dies was coated with PVD AlTiN and AlTiON in the present work. The performance of the coated samples was subsequently tested under thermal fatigue conditions encountered in the thixoforming of steels.

A review of the semi-solid processing of steel

Steel is a particularly challenging material to semi-solid process because of the high temperatures involved and the potential for surface oxidation. Here the experience of semi-solid processing will be reviewed and the current situation in relation to commercial application assessed. The review will include discussion of the range of potential steel materials which are amenable to thixoforming including thermodynamic prediction of suitable alloy compositions.

Testing of a novel CrNiCo alloy for tooling applications in semi-solid processing of steels

Semisolid processing which has become 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 the complex load profile and relatively higher forming temperatures which promote chemical interaction with steel slurries. Thixoforming tools ought to last thousands of forming cycles for industrial application to be attractive. Hot work tool steel dies proved to be entirely inadequate when thixoforming steels. In spite of extensive research on tool materials for the semisolid processing of steels, there is yet no material to fulfil this critical role. The present work was undertaken to explore the potential of a novel CrNiCo alloy as the tooling material in semisolid processing of steel.

The Use of CrNiCo-Based Superalloy as Die Material in Semi-Solid Processing of Steels

Semi-solid processing of aluminium and magnesium alloys has matured to become a well established manufacturing route for the production of intricate, thin-walled parts with mechanical properties as good as forged grades. This innovative forming technology, however, still remains as a major challenge in the case of steels. One major limitation in the thixoforming of steels has been the lack of suitable tool materials which could withstand the complex load profile and significantly increased forming temperatures which promote chemical interaction between forming dies and liquid steel. For industrial application to be attractive, the lifetime of the thixoforming tools has to be sufficient to ensure the cost effectiveness of this technology. Forming experiments using conventional hot working tool steel as die material have shown massive wear and plastic deformation. Several alternatives including copper alloys, titanium-zirconium-molybdenum based high temperature application alloy TZM, graphite and ceramics with and without suitable ceramic coatings have been tested with limited success. An attempt was made in the present work to investigate the applicability of CrNiCo-based superalloy dies in small scale forming tests in an effort to contribute to the development of semi-solid processing of steels.

Effect of Dopants on the Corrosion Behaviour of Zirconia by Steel at High Temperature

Zirconia based material is one of the most interesting materials for semi-solid processing of steel because of its good mechanical properties and high corrosion resistance at high temperatures. In many cases, degradation of zirconia refractories or advanced ceramics is due to unexpected tetragonal-tomonoclinic^transformation accompanied by volume expansion. But only little attention was paid to outdiffusion of stabilising additives. In present paper, zirconia based ceramics with different rare-earth oxides (Y2O3 and CeO2) are used in contact with HS6-5-2 steel at 1330°C in order to obtain a good understanding about the corrosion behaviour of high-strength zirconia tool material, especially the effect of yttria and ceria on the corrosion behaviour of zirconia. The results showed that both Y2O3 and CeO2 diffused out of the zirconia structure and formed new compounds with alloying elements of steel at the zirconia grain boundaries. Both lack of stabiliser and grain growth result in t-m transformation and cracks were formed in zirconia ceramic.

Design, Construction and Performance of Silicon Nitride Tool Parts in Steel Thixoforming

The design and performance of silicon nitride (Si3N4) dies for the semi-solid processing of steels is studied by the example of a punch and a lower swage in upset forging. The observed failure mechanisms and degradation effects are related to short-term effects resulting from mechanical and thermal loads and long-term effects owing to chemical and tribological attack. Results show that well-defined process conditions (cycle time, solidification time in the die) and a ceramic-suitable design are a pre-requisite for reliability and sufficient service life of ceramic dies.

Forming and joining of commercial steel grades in the semi-solid state

The processing of metals in the semi-solid state (thixoforming) promises to enlarge the spectrum of application for various metals. Compared to casting the quality of the final product can be improved, as less defects related to solidification processes occur. Compared to forging processes, the needed forming loads are lower and more complexly shaped parts can be produced net or near-net in a one step process. On the basis of its outstanding mechanical properties that can be adjusted by adequate heat treatments, steel possesses possibilities for light weight constructions realisable by complex shapes. The semi-solid processing of steels is one focal point at the Institute for Metal Forming (IBF). Thixoforming experiments are carried out to investigate the feasibility of different steel grades. The heating of billets into the forming temperature and the forming operation itself takes place within a protective gas atmosphere to avoid the growth of scale on the billet’s surface. To study the applicability of different tool materials the die halves are built up out of segments. Theoretical approaches to predict the suitability of different steel grades for semi-solid processing especially concerning the width of a process window for thixotropic flow behaviour are presented. Furthermore the results of forming experiments regarding chemical homogeneity and mechanical properties of the thixoformed parts are pointed out. Since first forming experiments using conventional hot working tool steel as die material have shown massive wear and plastic deformation, tailored dies have been developed. The supply of suitable die concepts is one major task to ensure the cost effectiveness of this technology. One major potential of the semi-solid technology in the future may be the combination of the processes forming and joining taking advantage of the high flowability of semi-solid materials. This approach will lead to new possibilities for production of functionally integrated parts leading to new component classes and minimising assembly costs. In this paper results of joining functional steel parts (screw) with a steel component during thixoforming in a one step process are presented.

Ductility and toughness in hot-work die steels: the importance of proper test procedures: Norstrom, L.-A. Die Cast. Eng. Mar.–Apr. 1988 33, (2), 42, 44, 46–47

The processing of metals in the semi-solid state (thixoforming) promises to enlarge the spectrum of application for various metals. Compared to casting the quality of the final product can be improved, as less defects related to solidification processes occur. Compared to forging processes, the needed forming loads are lower and more complexly shaped parts can be produced net or near-net in a one step process.On the basis of its outstanding mechanical properties that can be adjusted by adequate heat treatments, steel possesses possibilities for light weight constructions realisable by complex shapes. The semi-solid processing of steels is one focal point at the Institute for Metal Forming (IBF). Thixoforming experiments are carried out to investigate the feasibility of different steel grades. The heating of billets into the forming temperature and the forming operation itself takes place within a protective gas atmosphere to avoid the growth of scale on the billet’s surface. To study the applicability of different tool materials the die halves are built up out of segments.Theoretical approaches to predict the suitability of different steel grades for semi-solid processing especially concerning the width of a process window for thixotropic flow behaviour are presented. Furthermore the results of forming experiments regarding chemical homogeneity and mechanical properties of the thixoformed parts are pointed out. Since first forming experiments using conventional hot working tool steel as die material have shown massive wear and plastic deformation, tailored dies have been developed. The supply of suitable die concepts is one major task to ensure the cost effectiveness of this technology.One major potential of the semi-solid technology in the future may be the combination of the processes forming and joining taking advantage of the high flowability of semi-solid materials. This approach will lead to new possibilities for production of functionally integrated parts leading to new component classes and minimising assembly costs. In this paper results of joining functional steel parts (screw) with a steel component during thixoforming in a one step process are presented.