Volume 67, Issue 2

This article intends to draw attention to the problems and some solutions manufacturers face when using tool steels. Remarkable improvements have been made in the performance of this important steel group within the last decade. Enhanced material properties have been achieved by increasing cleanliness, homogeneity and modification of chemistry as well as a better understanding of heat treatment and operating conditions. These advances have led to increased loading as well as larger dimensions of tools and dies. This increase in size in particular implicates significant challenges to steel making, tool making and heat treatment. The material properties of tool steels are discussed exemplary for hot work tools steels along with a description of typical heat treatment parameters. Attention is paid on the very narrow range of treatment parameters that give desirable material properties.

To improve the quality of typical hot work tool steels the tool steel producers did a lot of optimization work on production technology and alloy design since the last years. Thereby the relevance of the heat treatment on the material properties was often neglected. Especially the effect of big dimensions of die casting dies on the quenching rates of hardening processes was discussed on a very small scale [1–4]. The most important material properties are affected by the heat treatment condition. Consequently, the heat treatment has a significant influence on the life time of die casting dies. For this reason a FEM model was adapted to simulate the heat treatment processes especially for tool steel dies for die casting processes. The main target of heat treatment simulation is to determine at least the temperature evolution in every point of the material as a function of cooling characteristics. Additional results of this simulation are the microstructure, residual stresses and distortion of the dies after the heat treatment procedure. The model includes physical properties, phase transformation and transformation plasticity. Cooling curves can be determined depending on the quenching situation and the quantity of phases like martensite, bainite and retained austenite. A correlation between cooling condition and toughness of the material was developed and compared with measured results.

Adequate selection of quenching media decreases the risk of tensile stresses, and also of cracking and workpiece distortion. High-performance quenching oils must have very high oxidation and thermal resistance and low sludge formation, must be non-staining and have a high flash point and acceptable heat-transfer characteristics. The quenching media contain a base oil and different types of additives according to application requirements. Mineral oils, synthetic and natural oils, used separately or in combination, may be used as base oils. Mineral base oils are widely used because of their advantages with respect to stability in comparison to natural oils, or lower prices in comparison to synthetic oils. There are numerous compounds available for use but, besides their functional properties, the environmental and safety requirements have to be considered in the selection of components. Based on that, barium additives must be replaced with less harmful compounds. Petroleum derivatives should be replaced with renewable base stocks that are biodegradable, which is a general trend in lubricant development. By changing the composition of quenching oils, base oils and additives, heat transfer characteristics are also changed. In this study, the results of investigation into physical and chemical properties and also cooling characteristics of new quenching oils with different compositions are presented. Cooling curves for different compositions have been evaluated according to the ISO 9950 Standard.

It has been suggested that when tool steels are exposed to temperatures around the boiling point of liquid nitrogen (−196°C) for extended periods some properties of the steel, particularly wear, are improved. Dissenters claim that the improvements are due to statistical variation and cannot be reproduced consistently. The most commonly reported experimental results are for AISI D2 and AISI M2. This review examines the available evidence and suggests a way forward for the debate. It has also been suggested that the colder the sub-zero treatment temperature after quenching, the greater the amount of the retained austenite that existed at room temperature that is converted to martensite. The implication being that the true martensite finish temperature (M f ) is almost always close to or below absolute zero. Others suggest that M f is in the range −80 °C to −120 °C for most steels. Some of the available evidence is reviewed particularly with respect to AISI D2 and AISI M2 and the conclusion drawn that transformation continues until all the austenite is eliminated or absolute zero is reached.

An intensive research work was carried out in the frame of a RFCS (Research Found for Coal and Steel) project, to investigate the influence of different deep cryogenic treatments (DCT) on hardness, strength, toughness and wear resistance of AISI M2. Short and a long classical DCT, providing the soaking at temperature close to the boiling point of liquid nitrogen (−196 °C) for 6 h and 20 h, respectively, were carried out prior to and after tempering. Furthermore, a third short DC route, providing temperature cycling between RT and low temperature was also considered. Care was taken to avoid stabilization of retained austenite or self tempering due to wait at room temperature prior to DCT and/or tempering. All treatments were calibrated to get 840 HV 10, in order to compare the properties of steel with the same reference hardness. DCT does not allow the complete transformation of retained austenite in the investigated high speed steel, due to the stabilizing effect of alloying elements. Tempering is necessary to completely transform this phase and to allow proper secondary hardening. If carried out after quenching, DCT shifts the secondary hardness peak to lower temperature, evidencing the need to adjust the tempering parameters to avoid overtempering. The microstructure didn't show any significant influence of DCT in terms of carbides distribution, due to the conditioning of martensite at low temperature. The same can be also concluded for the other properties (toughness, tensile strength and wear resistance), which are practically the same for samples having the same hardness.

In this paper, the effect of the heating rate on the process of tempering and the obtained hardness of the DIN 100 V1 (AISI W2) carbon tool steel has been investigated and determined. Steel specimens in two initial states were investigated: after quenching and after quenching and deep-cryogenic cooling. The specimens were submitted to tempering in a dilatometer, varying the heating rates, temperature of tempering between 200°C and 690°C and duration of keeping the specimens at that temperature. A common effect of these factors on the temperature of the start of particular stages in tempering and the obtained properties was determined using dilatometry. Differential dilatation analysis was carried out, together with hardness tests and XRD (X-ray diffraction) analysis.

Kurzfassung Laserautragschweißen und Laserlegieren sind beides Technologien zur Oberlächenmodiikation von Werkzeugen und werden hauptsächlich zur lokalen Verbesserung des Widerstands gegen Verschleiß und/oder Korrosion eingesetzt. Während es sich beim Laserautragschweißen um eine mittlerweile weit verbreitete Technologie handelt, zu der es auch Konkurrenztechnologien gibt (z. B. PTA-Schweißen, Flammspritzen), ist das Laserlegieren eine einzigartige Technologie mit zurzeit nur einer realisierten industriellen Anwendung, jedoch hohem zuküntigen Potenzial. In diesem Artikel werden die Grundlagen, Unterschiede und Vorteile der beiden Technologien beschrieben sowie ausgewählte Anwendungen präsentiert. Ähnliche Technologien, welche nicht den Laser als Energiequelle verwenden, sind nicht Gegenstand dieses Artikels.

Conventional aluminising procedures are mostly conducted at temperatures well above 900 °C, and can last for hours. When applied to hot-work tool steels, aluminising has to be done at notably lower temperatures in order to prevent grain growth and carbide formation, and to improve creep resistance. The kinetics of aluminium coating formation on hot-work tool steels was studied in the temperature range of 550–610 °C. The pack Al content was varied from 5–15 wt.-% and aluminising time from 1–9 hours. The halide activator AlCl 3 was applied. A series of statistically designed experiments were conducted to determine how key process factors influence the aluminide coating formation. A Box-Behnken experimental design was used to evaluate three process factors at three levels. The microstructures of coated samples were analysed by a scanning electron microscope (SEM). Glow Discharge Optical Spectroscopy (GDOS) and energy-dispersive X-ray spectroscopy (EDX) were employed to investigate element distributions in the coating layer

High temperature friction and wear behaviour of a hot work tool steel sliding against AA6082 alloy were investigated by means of a customary test configuration, which provides the conformal contact between a die block and an AA-disc. Uninterrupted tests in air were carried out. The friction behaviour is characterized by limited fluctuations just after the beginning of test, finally evidencing a transition to a more unstable contact regime after progressive transfer of Al to the die surface. The tendency towards Al pick up was evaluated by measuring the mass increase of die samples and their capability in delaying the formation of hard Al oxide particles, which are very detrimental for the surface finishing of the extrudate. The efficacy of different surface treatments, i.e. plasma nitriding and low pressure carburizing, was studied. Furthermore, samples with different level of roughness, corresponding to conventional machining (0.7–0.8 μm) and mirror polishing (0.02 μm), respectively, were produced. Finally the influence of hardness was also evaluated by considering two different hardness of the base steel (56 HRC and 44 HRC). In general, surface hardening improves the load bearing capability of the die surface, causing an extended permanence in the stable friction regime. Plasma nitrided samples (1125 HV 0.1) evidenced a better behavior than case hardened (670 HV 0.1) and base steel (56 HRC, 620 HV 0.1), respectively. Overtempered samples (44 HRC, 500 HV 0.1) confirm this trend. In view of the experimental error, a lower surface roughness did not improve the tribological properties.

Kurzfassung Ermüdung von Beschichtungen und Oberflächen ist ein häufiger Schädigungsmechanismus von Werkzeugen und Maschinenelementen, da in diesen Kontakten weder plastische Deformation noch Dreikörperverschleiß auftreten. In diesen Fällen führt die Ermüdung oftmals zum frühzeitigen Versagen der Komponente, Verlust der Funktionalität und somit zu einer reduzierten Standzeit. In dieser Arbeit wurde eine Methode entwickelt, die es ermöglicht, das zyklische Ermüdungsverhalten von Oberflächen zu charakterisieren. Die vorliegende Arbeit zeigt erste Charakterisierungsergebnisse einer auf Titan basierenden Hartstoffbeschichtung im tribologischen Kontakt mit einem typischen Umformschmierstoff. Um den zyklischen Tribokontakt nachzustellen, wurde eine Hartmetallkugel mithilfe eines “Scratch-Testers” zyklisch über eine geschmierte und beschichtete Probe bewegt. Nach einer definierten Anzahl von Lastzyklen wurde die Verschleißspur mit sowohl einem optischem als auch mit dem Rasterelektronenmikroskop (REM) untersucht und die Schadensausbildung von Rissen und Delamination bestimmt. Für die Abschätzung der Pressung wurde das Hertz‘sche Kontaktdruckmodell verwendet. Es zeigte sich, dass Schichtsysteme, die für höhere Belastungen optimal sind, nicht für geringere Belastungen geeignet sind und umgekehrt. Für eine optimale Auslegung eines wechselseitig belasteten Systems ist somit ein Oberflächenkonzept zu wählen, welches in dem entsprechenden Lastbereich optimale zyklische Eigenschaften aufweist

The methodology of controlling the surface texture of dies by means of Hybrid-shot peening was examined for the improvement of the die life cycle applied to high strength steel sheets. Hybrid-shot is double-shot peening, where the first treatment is performed using large particles, and the second one is performed with finer shot particles. The specimens of tool steels for dies treated by Hybrid-shot showed a high resistance against galling which was evaluated by the galling generation load of the draw bead test by using an oil lubricant, compared with the single shot specimens. This result suggested that the surface texture influenced the galling property. For a quantitative analysis of surface texture of die material, Fast Fourier Transformation (FFT) was used. Findings revealed that the ratio of the spectrum amplitude between high and low frequencies calculated by FFT corresponded to the galling generation load of draw bead tests. On the other hand, the conventional index of surface roughness, Rz, had no relationship to the galling generation load. Furthermore, the effect of solid lubricant, that is, the powder produced by scallop shell, on the galling property was studied. The addition of fine scallop shell powder in the oil lubricant increased the galling generation load.

Kurzfassung In den letzten Jahren haben chromhaltige pulvermetallurgische Stähle auf der Basis vorlegierter Pulver wie Astaloy CrL (Eisen + 1,5% Chrom + 0,2% Molybdän) und Astaloy CrM (Eisen + 3% Chrom + 0,5% Molybdän) zunehmend Interesse gefunden. Die Gründe hierfür sind einerseits die niedrigeren Kosten für Chrom verglichen mit traditionellen Legierungselementen in der Pulvermetallurgie, wie Molybdän, Nickel und Kupfer, andererseits die mittlerweile in der Industrie verfügbaren deutlich verbesserten Sinteröfen, die eine sauerstofffreie Atmosphäre und hohe Temperaturen ermöglichen. Bei der Verwendung entsprechender Sinter- und Wärmebehandlungsbedingungen erzielen diese chromhaltigen Stähle hervorragende mechanische Eigenschaften bei relativ niedrigen Materialkosten. Allerdings ist für diese Materialien eine konventionelle Gasaukohlung mit anschließender Ölabschreckung nicht möglich, da die Kohlungsgase immer signiikante Mengen an sauerstohaltigen Verbindungen enthalten, welche in die ofenen Poren der Sinterteile eindringen können und Oxidation der Teile, nicht nur der Oberläche, sondern auch des Inneren, bewirken, was zu einer deutlichen Versprödung führen würde. Auch kann das Härteöl die Poren kontaminieren, was die Notwendigkeit für teure Waschoperationen der Teile bedingt und damit zu erhöhten Kosten führt. In der hier vorliegenden Arbeit wurde als Alternative für diese oxidationsempindlichen, chromhaltigen Materialien die Niederdruckaukohlung kombiniert mit einer Hochdruckgasabschreckung untersucht. Es konnte gezeigt werden, dass diese Wärmebehandlung eine sehr gute Methode für die Randschichthärtung chromhaltiger Sinterstähle ist.

The microstructure of a 17wt%Cr-3wt%C white cast iron with relatively high Mo addition (7 wt.-% Mo) has been investigated by X-ray diffraction, light microscopy, scanning electron microscopy and transmission electron microscopy together with energy dispersive X-ray spectroscopy. Mo dissolves partly in the matrix and partly in the typical Cr-rich M 7 C 3 eutectic carbide and also leads to microstructural modification by forming a fine Mo-rich lamellar structure at interdendritic regions with a volume fraction of 11 %. The Mo-rich phase in this lamellar structure may also be a carbide of the M 6 C type and is believed to form at the later stage of solidification by a subsequent eutectic reaction. Destabilization heat treatment at 1000°C for 4 hours followed by air hardening results in precipitation of M 7 C 3 and M 23 C 6 secondary carbides, transformation of dendritic and eutectic austenite to martensite with some austenite being retained, and modification of local chemical composition, importantly an increase in the Mo and C content in the matrix. This causes a significant improvement in hardness, corrosion and dry wear properties. Tempering at 450°C for 4 hours leads to more temper-carbide precipitation and formation of ferrite, possibly by decomposition of retained austenite. However, an improvement in properties was not observed.