This study refers to an analysis of the dies used in the first operation of producing a valve forging from chromium-nickel steel NC3015. The analyzed process of manufacturing forgings of exhaust valves is realized in the co-extrusion technology, followed by forging in closed dies. This type of technology is difficult to master, mainly due to the increased adhesion of the charge material to the tool substrate as well as the complex conditions of the tools’ operations, which are caused by the cyclic thermo-mechanical loads and also the hard tribological conditions. The average durability of tools made from tool steel WLV (1.2365), subjected to thermal treatment and nitriding, equals about 1000 forgings. In order to perform an in-depth analysis, a complex analysis of the presently realized technology was conducted in combination with multi-variant numerical simulations. The obtained results showed numerous cracks on the tools, especially in the cross-section reduction area, as well as sticking of the forging material, which, with insufficient control of the tribological conditions, can cause premature wear of the dies. In order to increase the durability of forging dies, alternative materials made of hot work tool steels were used: QRO90 Supreme, W360, and Unimax. The preliminary tests showed that the best results were obtained for QRO90, as the average durability for the tools made of this steel equaled about 1200 forgings (with an increase in both the minimal and maximal values), with reference to the 1000 forgings for the material applied so far.

The study constitutes an analysis of the durability of dies used in the first operation of producing valve forgings from chromium–nickel steel (NCF 3015) for motor trucks. The average durability of the dies (subjected to standard thermal treatment and nitriding) in this operation equals about 800 forgings. To perform an in-depth analysis of the effect of the nitrided layer thickness (0.1 mm and 0.2 mm) and the tool material (W360 and QRO90) on the possibilities of increasing the die durability, complex studies were carried out, which included: a macroscopic analysis combined with 3D scanning, microstructural examinations using a scanning microscope and a metallographic microscope, as well as hardness measurements. A minimum of three tools were tested for different variants, and for each of them, one representative die was selected for detailed examinations. The research showed the presence of abrasive wear, thermo-mechanical fatigue and traces of adhesive wear as well as plastic deformation on the surface of the working impressions. Also observed was the effect of the extruded material sticking to the tools (high friction and the presence of intermetallic phases in the extruded material) and the forging being blocked in the smallest section of the die, which is a critical factor causing a production shutdown and the necessity of tool replacement. The highest mean durability equalling 2600 forgings was obtained for the dies with a lower carbon content and a higher content of vanadium and the nitrided layer thickness at the level of 0.2 mm. The lowest mean durability (after one forging item) was recorded for the dies made of steel with a higher carbon content and a higher chromium content, forming less stable compound carbides and the thickness layer at the level of 0.1 mm.

The article performs an analysis of the durability of punches applied in the process of producing a valve forging from chromium-nickel steel. A forging of this type is made in two operations: coextrusion of a long shank, followed by finishing forging in closed dies of the valve head. The product obtained in this way (after other additional finishing procedures) constitutes the key element of the combustion engine (resistant to high pressures and temperatures) in motor trucks. Unfortunately, a significant problem in this production process is a relatively low durability of the forging tools, especially the punch used in the second forging operation. The key element at this stage, deciding about the punch’s further operation, is the area of the so-called “calotte”. The short-term life of the tools results from very hard performance conditions present during the forging process (periodical high mechanical and thermal loads, long path of friction). The latter cause intensive abrasive wear as well as high adhesion of the forging material to the tool surface. Based on the performed studies, including the following: technology analysis, numerical modelling, macro analyses combined with 3D scanning of tool sections as well as microstructural tests and hardness measurements, it was established that it is crucial to properly select the process parameters (charge and tool temperature, tribological conditions), as even slight changes introduced into them significantly affect the operation time of the forging tools. Mastering and proper implementation of the analyzed forging technology requires numerous further studies and tests, which will enable its perfection and thus increase the durability of the tools as well as the quality of the produced items.

This study refers to an analysis of the durability of forging tools applied in the second operation of producing a valve forging from the chromium–nickel steel, NC3015. Due to the extreme working conditions of the tools, caused by cyclic thermo-mechanical loads, the average durability of tools made from tool steel WLV (1.2365) equals about 1500 forgings. An in-depth, complex analysis was performed on the technology, using macroscopic tests combined with a measurement of the wear/allowance on the tool working surface through 3D scanning; microstructural tests by means of light microscopy; observations of the changes taking place on the working surface with a scanning electron microscope; microhardness measurements; and multi-variant numerical simulations. It was established that the key issue is the proper selection of the process technological parameters, such as the input material and tool temperature, friction, lubrication, tribological parameters, type of tool material, or punch design, because even small changes made to them significantly affect the service life of forging punches. Therefore, to increase the durability of the forging dies, alternative materials made of W360, as well as two high speed steels, S600 and S705, were applied. However, the implemented punch materials did not bring the assumed effect of increased durability, as the highest average durability of steel W60 equaled only 1500 forgings, whereas in the case of the tool steels, this was below 900 forgings. For this reason, at the further stage, punches with sintered carbide inserts were introduced, which made it possible to significantly improve the durability up to the level of as many as 20,000 forgings, which, at the same time, points to a promising direction of further studies on the use of materials and solutions of this type.

This paper focused on determining the increased tendency of cracking after the die forging process of high nickel and chromium steel. The increase in carbon content in austenitic nickel–chromium steel promoted the tendency of valve forgings to forging intergranular crack on the valve head. Attention was paid to issues related to the chemical composition of the material to be considered when hot forming nickel–chromium steel components. Optical and scanning electron microscopies were used to examine the microstructure and fracture features of the samples removed from a fractured valve head. The embrittlement was due to microcavity formation at grain boundaries. Creep theory at grain boundaries was used to explain crack formation. The tensile behavior was interpreted from the evolution of the microstructure during deformation and referred to intermediate brittleness to explain the effect of carbon. It was found that the increased carbon content of the nickel–chromium steel and the strong undercooling observed at the edges of the valve head are factors that promote a reduction in grain boundary cohesion and enhance intermediate temperature embrittlement. Finally, it was found that the formation of a heterogeneous structure manifested by the presence of grain boundary M₂₃C₆-type carbides in the austenitic matrix was most likely related to the occurring brittleness.

This paper provides a detailed analysis of the operation of representative forging tools (in the context of using various surface engineering techniques) used in the process of the hot forging of nickel–chromium steel elements. The influence of the microstructure and hardness of the material on the durability of the tools is also discussed, which is important for understanding the mechanisms of their wear. The research showed that the standard tools used in the process (only after nitriding) as a reference point worked for the shortest period, making an average of about 1400 forgings. In turn, the tools coated with the CrAlSiN coating allowed for the production of the largest number of forgings, reaching 2400 pieces, with uniform wear. In comparison, the tools with the CrAlBN coating made 1900 forgings. Three-dimensional scanning analysis showed that CrAlSiN- and CrAlBN-coated tools have lower volumetric wear, around 41–43 mm³, compared to 59 mm³ for nitrided tools. For a better comparison of tool life, the authors proposed the Z-factor, as the material loss to the number of forgings. The CrAlSiN coating showed the lowest material loss, despite a slightly higher Z-factor value compared to the CrAlBN coating. The use of hybrid coatings such as CrAlSiN and CrAlBN significantly reduces tool wear while increasing service life compared to tools that are nitrided only.