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.
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 aims to evaluate the effect of pre-coating of forged parts on decarburization in the die forging process. The studies consisted of three stages. In the first instance, different coatings ...were tested under laboratory conditions by heating steel samples to the temperature of 1200 °C for over five minutes to model the preheating conditions of the induction. Next, testing continued in a commercial forging stand where we tested the effects of different coatings on the rods decarburization during the induction heating process, usually performed before forging. Once completed testing, the measurements and observations of the decarbonized layer were made. The third stage involved analysis of the decarburization of the forged parts after forging. The forged parts were made using precoating of pre-forging elements; pieces cut off a metal rod. Based on tests results, the possibility of using this solution in the technique of industrial hot forging was evaluated. The results of laboratory tests have confirmed that lubrication of metal pieces is sufficient, as well as proved it to be effective in reducing decarburization of the surface layer. Research works conducted in an induction heater showed differences in decarburization depending on a substance and concentration of lubricants that were used. These differences become more apparent when observing the surface layer of the forged parts. Results indicate that decarburization may be reduced to a minimum when we use Bonderite product in a concentration of 66% and 50%. Another lubricant, Berulit 913, may also be used. However, due to burning graphite in high temperatures, reduction of decarburization goes only as far as half of the thickness of the decarbonized layer. Condursal has no significant effect; nevertheless, it protects over the induction heating stage.
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.
The global production of die forgings is an important branch of the motor industry for obvious reasons, resulting from the very good mechanical properties of the forged products. The expectations of ...the recipients, beside the implementation of the forging process, include also a range of supplementary procedures, such as finishing treatment including shot blasting, thermal treatment, and machining, in order to ensure the proper quality of the provided semi-product or the ready detail for the assembly line. Especially important in the aspect of the operational properties of the products is the thermal treatment of the forgings, which can be implemented in many variants, depending on the expected results. Unfortunately, a treatment of this type, realized separately after the forging process, is very time and energy-consuming; additionally, it significantly raises the production costs due to the increased energy consumption resulting from the necessity of repeated heating of the forgings for such thermal treatment. The article reviews the most frequently applied (separately, after the forging process) thermal treatments for die forgings together with the devices/lines assigned for them, as well as presents an alternative (thermoplastic) method of forging production with the use of the forging heat. The paper also presents a prototype semi-industrial controlled cooling line developed by the authors, which allows the development of the assumed heat treatment of forgings directly after forging with the use of forging heat, together with sample results of conducted tests.
The article performs an analysis of a hot die forging process of producing an elongated forging ended with a joggle in a double system realized on a crank press Masey 1300 t, in open dies, in 3 ...operations. The thermomechanical model of the forging process considering the changes in the grain size and the forging material recrystallization was elaborated with the use of the calculation packet Qform 7. In the first place, an in-depth analysis of the currently realized forging technology was made, with a special consideration of the temperature changes in the tools as well as in the formed forging. Next, numerical modelling of the process was carried out, as a result of which the following were obtained: correct filling of the tool impressions by the deformed material, the temperature distributions for the forging and the tools, the plastic deformation distributions (considering the thermally activated phenomena), the changes in the grain size and the forging force courses. The results obtained from FEM enable a thorough analysis of the forging process, including: the effect of the deformation time and temperature on the grain size in the forging material, which was confirmed by the microstructure examination results.
This study presents the effect of high current pulses on the structural and mechanical behavior of the 6016 aluminium alloy in three different states of hardening: naturally aged, super saturated, ...and annealed. The 6016 aluminium alloy was used for the first time in terms of electrically-assisted forming. The influence of the application of different current parameters on the material behavior was conducted. The study of electrically-assisted tensile tests showed that the application of current pulses results in a distinct response of the material, depending on the hardening state. Although in a hardened state, the mechanical properties and plasticity are deteriorated, in the solution treated state, they are improved. For the changes of the material properties is responsible the interaction of the flowing current with the precipitates and the aging process. The new parameters were proposed to describe the distinctions in the material properties between the different states of hardening of the aluminium alloy during the electrically-assisted tension. The material examination was conducted using light and scanning electron microscopy, using also electron backscattered diffraction methods. The application of, for example, the grain orientation spread parameter demonstrated the presence of recrystallized grains, in electrically-assisted specimens.
The failure mechanisms of hot forging dies Gronostajski, Zbigniew; Kaszuba, Marcin; Polak, Sławomir ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
03/2016, Letnik:
657
Journal Article
Recenzirano
This paper describes the phenomena taking place on the surface of the dies used for hot forging. Because of this paper’s limited space only changes in the tool surface layer during the forging of a ...gear wheel, as most representative, are presented. Similar changes were observed in the case of the other two investigated closed die forging processes, i.e. the forging of a cover and a yoke, respectively. The research was aided by FEM, which supplied a lot of information about the forging conditions. The most intensive wear of the tools occurs in the place of their longest contact with the material being forged, regardless of the number of produced forgings. The research has shown that the one of the most adversely factor affecting the investigated forging process is thermomechanical fatigue which results in fine cracks quickly developing into a network of cracks extending over the entire tool/forged material contact surface. Also the abrasive wear of the investigated die is high due to the intensive flow of the material in the presence of abrasive oxide particles and tools bits created by thermomechanical fatigue. An attempt to model the abrasive wear using the Archard model is presented.
Electrically assisted forming brings many advantages to the forming processes of metal alloys through microstructure modification by a flowing current. However, the influence of strain rate on these ...changes and the real value of the stress-drop are not yet specified. In this study, the effect of strain rate on flow stress, material plasticity and, microstructure was analysed for the electrically assisted tension of the 5754-H111 aluminium alloy. Three different strain rates (0.0025, 0.01, and 0.04 s
−1
) were applied under a pulsed electric current tension. The study showed that at decreasing strain rate, an increase in engineering strain is observed. This effect was explained by the periodic strain-hardening and annealing of the sample under a pulsed electric current. The study also showed the possibility of determining the real value of stress-drop, which occurred when a pulsed current was applied during tension. Finally, applying the current pulses led to a meaningful increase in the material plasticity. Transmission electron microscopy and electron backscatter diffraction were used to identify the microstructural changes. It was shown applying pulsed current allowed a change of the dislocation pattern and its annihilation, resulting from the dynamic recovery process.
The work concerns research related to the assessment of the influence of the impact of the steel grade used for forging punches in terms of the durability of tools used in the precision forging ...process in closed dies of a chrome-nickel steel valve forging. The tests were carried out on post-service punches, showed microstructural differences in the intensity of carbide precipitations banging. Two of them were made of a materials showing carbide banding (W360 after ESR; 1.2365), which achieved average durability at the level of: 2400 and 1200 forgings. The other two were without clear carbide banding (Unimax after ESR; 1.2344). They also had much lower durability at the level of: 700 and 200 forgings. Tests of post-service punches were carried out in order to assess the mechanism of their destruction. They were supplemented with hardness measurements. The dominating mechanism of their wear was the loss of properties of the surface layer. The decrease in hardness and plastic deformation of the material resulted in increase of the punches wear rate. As part of laboratory tests, scratch-tests and abrasion wear tests on a “ball on disc” tribotester at elevated temperatures (above 400 °C, i.e. contact temperature: tool-forging) were carried out. The obtained test results showed that punches made of materials with distinct banding, compared to materials with evenly spaced carbide clusters, are characterized by better resistance to the destructive mechanisms prevailing in hot forging processes.
Display omitted
•The surface plastic deformation and triboxidation affect the wear of the tool material.•Finely dispersed carbide bands parallel to the punch axis can have a positive effect on tool life.•The formation of an oxide film on the wear surface can increase the material wear resistance.•The tribo-oxide layer on the surface of the tool preventstempering of its surface.
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