Influence of the different variants of the surface treatment on the durability of forging dies made of Unimax steel

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 [13‐15] 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].

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 [17‐19].

There are many methods of improving the durability of forging tools [1, 4]. Procedures aiming at an improvement of a tool’s durability are usually performed in three different directions. The first one takes into account the technological and construction aspects of the forging process [20]. Here, the most crucial is the optimization of the construction of dies and impressions. As the finishing impression has to reflect the ready forging and it cannot be changed without consulting the recipient, the modification usually refers only to the roughing passes and auxiliary impressions [21]. Through the proper selection of shape, the aim is to synchronize the wear of the roughing passes and finishing impressions, especially when they are in the same die [22]. This is also supported by modern production supervision systems, which stabilize the process and eliminate errors [23], (https://​www.​thomasnet.​com/​articles/​custom-manufacturing-fabricating/​forging-parts-supplies/​). The second direction concerns modification of the material used to make the tools [24]. The steels used to produce die impressions for presses have to exhibit high strength, hardness, and abrasive resistance, at the same time characterizing in proper ductility at the working temperature [25, 26]. The above-mentioned properties can be obtained through the proper selection of the steel’s chemical composition. The small content of carbon (0.30 ÷ 0.55%) ensures ductility, whereas alloy elements such as tungsten, molybdenum, and vanadium maintain high hardness at elevated temperature [27, 28]. From this reason, the new hot work steels are used and developed, like Supreme, Thermodur, and Unimax steel. For example, the Unimax steel has been found to be more wear-resistant than conventional tool steels such as H11 or H13. The publication presents a comparison showing that tools made of Unimax have up to 250% longer durability [29]. In addition, in other works, the resistance of this steel to abrasive wear was analyzed [30, 31], which is much higher compared with other steels. The application of Unimax steel as a material for hot forging tools [32] can also be pointed out, also as an excellent substrate for nitriding [33] and as a substrate for hybrid layers combining nitriding and PVD coatings [34].

In order to obtain the assumed mechanical properties, it is necessary to perform the appropriate thermal treatment, which usually consists of quenching and, performed twice, high tempering [34]. The third direction refers to modern surface engineering techniques [35]. Among the most popular ones, we can name welding techniques (pad welding), thermo-chemical treatment (nitriding, etc.), beam techniques, and mechanical methods (shot peening) as well as hybrid layers and techniques [36, 37]. There are no clear criteria for the selection of a particular durability improvement method, as each case should be analyzed individually with respect to the conditions present during the forging process [3]. Also, oftentimes, certain methods are combined in order to obtain the optimal solution or the effect of synergy, which makes it possible to increase the durability of forging tools. An important criterion is also the availability of solutions, of which some require access to a big and costly industrial infrastructure (nitriding furnaces, coating application apparatus). The selected improvement manner has to be justified economically, and it should be analyzed whether the gain resulting from an increase of tool life will be higher than the sum of the expenses (material, machining, thermal and thermo-chemical treatment) [38].

In the investigations, an attempt was made at increasing the hardness of the tools used so far (upper die inserts applied in the roughing operation) through the use of another tool material—steel with the commercial name Unimax. Also, for this material, two surface treatment variants were applied—in the form of ion nitriding (for nitrides α) and gas nitriding together with the application of a PVD-Alvin coating—which were compared with the results for an insert not subjected to surface treatment. The properties of the newly applied material make it possible for the Unimax steel to find its application for tools assigned for hot and semi-hot operations, which require both high hardness and ductility. Alvin coating is a nanocomposite CrAlSiN coating with high chromium content and high resistance to oxidation and adhesion to the shaped material of the tool (https://​shm-cz.​cz/​pl/​powloki-pvd/​). The chemical compositions of both steels have been compared in Table 1. It can be inferred from the comparison that the Unimax steel contains more molybdenum (Mo) and slightly more carbon, with a similar share of the other alloy additions. The recommended quenching and tempering is constituted by the following: quenching at 1025 °C as well as, performed twice, tempering at minimum 525 °C, which guarantees hardness at the level of 56–58 HRC. Table 2 presents the denotations as well as the applied thermo-chemical treatment variants together with the number of performed cycles.

The prepared tools underwent performance tests (three tools for each variant), where, by means of each of the three variants, the maximal number of forgings fulfilling the required criteria was made. Next, the tools, after their operation, were subjected to an in-depth analysis in order to compare and evaluate the effectiveness of the durability improvement using another material.

The utilitarian aim of the study was to determine the effect of the applied tool material—Unimax—(also, after different treatment variants with the use of surface engineering techniques), in respect of the 1.2344 steel tool for hot operations used so far, on the increase of the hardness of forging tools. The analysis of the industrial forging process supported by temperature measurements as well as numerical modeling enabling the determination of many difficult, or even impossible to establish by other methods, parameters made it possible to point to the key process parameters, which decides about the wear. The first macroscopic observations showed that the best resistance was exhibited by the inserts for variants 2 and 3, although it should be noted that only the introduction of the new tool material itself caused an over twofold increase of hardness. The geometrical analysis of the wear provided valuable information on the degree of wear, and also enabled its quantitative evaluation and a comparison of both tools. It can be inferred from the analysis that the tools made of the Unimax steel without an additional thermo-chemical treatment wear off much faster and in a very irregular manner, as opposed to the tools made of the same steel but after nitriding or a hybrid layer application, for which the tool exploitation proceeds in a stable way. Also, the wear of the insert made of the Unimax steel was more uniform, as opposed to the inserts made of the standard steel 1.2343 examined so far, for which traces of local spalling and damage have been observed, which are also confirmed by the observations performed by means of a scanning microscope. The wear observed during macroscopic analysis was then subjected to careful observation in scanning and optical microscopy. The result of this analysis is to observe the effects of wear on the surface and in the surface layer. This wear was caused by mechanisms that result from the adopted surface treatment. The tool made of Unimax steel was mainly subjected to abrasive wear, which was manifested through abrasive furrows forming along the direction of flow of the material on the surface of the tool. These furrows are deep (up to 1 mm, which was measured in Fig. 6a) and smoothed at the edges (Fig. 11). They are located on the rounded edges near the matrix bridge (Fig. 8). Their presence is undesirable and causes the tool to be withdrawn from use. The nitrided tool underwent other wear mechanisms, among which cracking due to thermal and mechanical fatigue predominated. Observations of the microstructure show small defects in the form of chipping and cracks that are shallow and densely arranged. Wear occurred locally by cracking and chipping fragments from the surface, followed by further abrasive wear in a place weakened by cracks and chipping (Fig. 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 chipping, 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.