1 Introduction

The durability of forging tools is an important as well as difficult issue in both the scientific and economical aspect, in the case of plants producing forgings [1, 2]. This results from the significant share of the tool costs in the total costs calculated based on the production of one single forging. That is why, often, the durability itself is described by the number of forgings which has properly produced geometry and quality wise by the given tool. At present, it is estimated that the cost of a forging is on average about 8–15% of the cost of the tools, whereas it can reach even 30% in the case of small production series, and in extreme cases, for complicated shapes of steel forgings or for austenitic forgings, it can be as high as 50–60% [3, 4] and also lubrication [5]. A majority of these costs is covered by the dies or die inserts. The remaining instrumentation elements, such as mounts and flattening plates, undergo wear to a much lesser degree. In turn, trimming dies and piercing dies, which wear quite quickly, can be easily regenerated through pad welding and regeneration of only the cutting edges, which significantly lowers their share in the total cost of the forging [6].

The wear of forging tools used in hot forging processes is complicated and difficult to analyze, as it is affected by many phenomena, which often occur simultaneously [7]. Among the most important ones, we can name thermo-mechanical fatigue, plastic deformation, abrasive wear, and oxidation (Fig. 1). All these factors simultaneously contribute to the wear of forging tools, while the share of the effect of the particular factors is different depending on the type of the tool (its size, shape, and production manner) as well as on the operation conditions (type of the forging aggregate, pressures, process temperature, etc.) [8]. In the case of die inserts whose hardness is significantly higher (the hardness of tool steel after thermal treatment reaches even 58–60 HRC, i.e., 650–700 HV), the plastic deformation does not take place very quickly [6]. Due to high hardness, numerous cracks are formed on the working surface, which is described as the effect of the mechanism of thermal and mechanical fatigue [9]. These cracks form a characteristic network of a regular or irregular shape in the surface, visible in Fig. 1a. Usually, during further operation, these cracks become deeper and wider as a result of the accumulating oxides, which work as “wedges” [7]. The deepened cracks gradually transform into grooves, which are usually formed parallel to the direction of the material flow. The surface of the die near the groove undergoes local deepening, and the edges of the cracks become smooth (Fig. 1c). On the surfaces, one can also observe adhesive wear that is oxidation and tearing-off of fragments from the die surface (Fig. 1d). The share of particular mechanisms in the total wear of the given tool can vary during the operation [7, 10]. As a rule, at the beginning, a network of fatigue cracks appears, and next, abrasive wear, material tempering, and local plastic deformation simultaneously occurs [11, 12]. Depending on the forging process and its conditions, one can also observe a secondary fatigue crack network originating from both thermal fatigue [13,14,15] and, intensifying the degradation process, mechanical fatigue [16] as a result of high forming forces. All this causes rapid and accelerated wear as well as tearing-off of increasingly bigger particles of the material, in which case, abrasive wear is not necessarily the dominant phenomenon [3].

Fig. 1
figure 1

The most common degradation mechanisms of forging tools. a Thermo-mechanical fatigue. b Plastic deformation. c Abrasive wear. d Oxidation

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Often, the biggest share in the wear of tools used in hot forging processes is also attributed to abrasive wear, while, in fact, other mechanisms are dominant, and abrasive wear is only the final and most easily measurable effect. The intensity of the occurrence of the particular mechanisms is significantly affected by the forging conditions, and more precisely the lubrication. In the case of lubricated and cooled tools, thermo-mechanical fatigue is mostly dominant, whereas for non-lubricated and cooled tools, abrasive wear and plastic deformation dominate [4]. The wear of the forging tools as well as the remaining instrumentation causes a change in the geometry of the product, and any surface flaws of the tools (cracks, defects) are reflected in the forged product, affecting its quality [9). Similar wear but to a lesser extent was observed with the tool with Alvin coating. The presence of the coating led to long-term wear resistance. However, in the areas with the highest load indicated in the FEM analysis (Fig. 4), damage was caused by cracking and chip**, which led to the removal of both the coating and the nitrided layer (Fig. 10).

The obtained results suggest that despite the higher price of the Unimax steel (about 6 euro per 1 kg), in the case of the analyzed die inserts (1 kg/ 3euro), its application is economically justified. We need to manufacture 2.5 die inserts in relation to 1 die insert made of Unimax steel to achieve similar durability of forging tools. Also, taking into account the costs of the mechanical treatment of the tools and their thermal treatment as well as the costs of the shutdown of the production line resulting from the necessity of replacing the worn tools, the benefits coming from the use of a better tool material are much greater.

It should be pointed out that the results presented in the study were obtained for three variants with three repetitions, which creates the necessity of performing further tests in order to standardize the durability improvement process by way of applying an alternative material together with an additional thermo-chemical treatment as well as to determine the stability and cost-effectiveness of the introduced changes depending on the production scale.