Direct or indirect: Influence of type of retained austenite decomposition during tempering on the toughness of a hot-work tool steel
Introduction
The classical heat treatment of tool steels consists of two procedures called hardening and tempering. Hardening means austenitisation for a specific time followed by rapid quenching, which leads to a martensitic microstructure. Subsequent multi-step tempering leads to the relaxation of the stressed martensite, the formation of secondary carbides and the decomposition of retained austenite [1]. The more-step strategy enables the relaxation of untempered martensite newly formed from untransformed retained austenite during cooling after the first tempering step [2]. In order to gain required material properties, predominantly hardness and toughness, hardening as well as tempering parameters have to be chosen from a narrow range. This is hardly achievable when tool dimensions become large and results in insufficient toughness behaviour.
When no grain-boundary carbides are formed during quenching from austenitisation temperature, as it is the case for the investigated hot-work tool steel, austenite retained after quenching has been found to be responsible for undesired toughness behaviour [3]. Ultra-thin carbon enriched retained austenite films between martensitic laths lead to carbide formation at lath boundaries during tempering [4] which reduces toughness. The amount of retained austenite after quenching strongly depends on the quenching rate and goes hand in hand with a thickness increase of the interlath films [5], hence, the potential for interfacial carbides increases. Regarding large tool dimensions, the amount of retained austenite after quenching is not adjustable in any order by the manufacturer for a given alloy composition. Therefore, to manipulate toughness properties, retained austenite decomposition during tempering could be manipulated.
Decomposition of retained austenite in steels is generally proposed to be a transformation into ferrite and cementite [6] or the formation of bainite [7]. Van Genderen et al. [8] divided the retained austenite decomposition of a FeC model alloy into two successive ways. A preceding ferrite formation occurs before the final transformation into ferrite and cementite takes place. Saha Podder and Bhadeshia [9] showed by means of dilatometer tests that during heating at 450 °C of a bainitic steel ferrite formation does not occur within 1 h of tempering. Previously published work on the investigated hot-work tool steel X38CrMoV5-1 shows that heating to 610 °C already leads to alloy carbide formation from the retained austenite though no reaction is visible from the dilatometer curves [4]. This carbide formation progresses during tempering. Similar to that, Kulmburg et al. [2] proposed for a high speed steel that carbides form during tempering from the retained austenite which transforms into martensite during cooling. No transformations take place during heating to tempering temperature. All these findings demonstrate that the behaviour of retained austenite during tempering significantly varies for different steel grades and lead to the assumption that individual heat treatments for individual steel grades might improve material properties.
However, traditional multi-step tempering is somehow manifested in steel industry. The intention of the present work is the evaluation of different heat treatment strategies with special regard laid on the retained austenite decomposition. Therefore, the standard tempering treatment has been compared with respect to toughness to a treatment where the retained austenite has been transformed into martensite instead of a direct decomposition into ferrite and cementite. Additionally, the effect of retained austenite decomposition manipulation has been determined with respect to different cooling rates during hardening.
Section snippets
Experimental
Dilatometric experiments have been performed in a dilatometer Dil 805A from Bähr Thermoanalyse GmbH. Therefore, samples of 15 mm in length and 5 mm in diameter have been produced. The material has been austenitised at 1020 °C with a dwell time of 30 min. Subsequent quenching has been conducted with quenching rates λ=0.6, 6 and 12, which correspond to linear quenching rates of 5 K/s, 0.5 K/s and 0.25 K/s, respectively. Then, the samples have been tempered at 610 °C two times for 2 h simulating the
Material
The nominal composition of the alloy investigated is given in Table 1. The initial microstructure after hardening consists of a martensitic matrix with nanometric interlath retained austenite films [5]. The volume fraction of retained austenite is <3% [3], 10% [5] and 17% [3] corresponding to the quenching rates λ=0.6, 6 and 12, respectively.
Dilatometer
Tempering at 610 °C of the differently quenched samples leads to a dilatation as given in Fig. 1a. The diagram shows the dilatation corresponding to the section from reaching 610 °C to the end of 2 h dwell time. The relative expansion during tempering qualitatively shows similar behaviour for the samples with quenching rates λ=0.6, 6 and 12. In order to compare the behaviour of the different samples, the corresponding curves have been translated in a way that at tempering start point the curves
Discussion
The demand for larger tools and enhanced material properties by the tool manufacturing industry forces the tool steel producers to improve their materials and processes. The heat treatment of large blocks does not allow for desired cooling rates during hardening, therefore, modified heat treatments can be the key for improved material properties. As the decomposition of retained austenite during tempering might negatively influence the toughness behaviour of this hot-work tool steel, a
Conclusions
Two different tempering treatments have been applied on the common hot-work tool steel X38CrMoV5-1 in order to investigate the influence of the type of retained austenite decomposition with respect to the toughness behaviour. Following conclusions can be drawn:
- 1.
The direct retained austenite decomposition into ferrite and cementite could be eliminated by reducing the dwell time during the first tempering step. Instead of that, the majority of the retained austenite has been transformed into
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