This paper presents investigations of the structural changes and flow stress of advanced high-strength steels (AHSS) subject to deformation at high strain rates, taking into account the ...transformation of retained austenite into martensite. TRIP690 and DP600 steel sheets were selected for the investigations. Steel HC380 was selected as the reference material.
The investigations have shown that besides the thermodynamical stability of retained austenite, also the temperature (the higher, the higher the strain rate) generated due to plastic deformations influences the shape of the stress-strain curve. This is linked with the actual changes in temperature during deformation and in the ultimate structure of the investigated steels.
Analyses of the specimens carried out using an SE detector and the EBSD method showed that the tempering process was initiated in the microstructure of the DP600 steel, owing to the heat released during the deformation of the specimens. Because of the delayed transformation of austenite into martensite, the tempering of the martensite was not observed during the deformation of steel TRIP690.
This article is devoted to the issues of thermal softening of materials in the surface layer of forging tools. The research covers numerical modeling of the forging process, laboratory tests of ...tempering of nitrided layers, and the analysis of tempering of the surface layer of tools in the actual forging process. Numerical modeling was supported by measuring the temperature inside the tools with a thermocouple inserted into the tool to measure the temperature as close to the surface as possible. The modeling results confirmed the possibility of tempering the die material. The results of laboratory tests made it possible to determine the influence of temperature on tempering at different surface layer depths. Numerical analysis and measurement of surface layer microhardness of tools revealed the destructive effect of temperature during forging on the tempering of the nitrided layer and on the material layers located deeper below the nitrided layer. The results have shown that in the hot forging processes carried out in accordance with the adopted technology, the surface layer of working tools is overheated locally to a temperature above 600 °C and tempering occurs. Moreover, overheating effects are visible, because the surface layer is tempered to a depth of 0.3 mm. Finally, such tempering processes lead to a decrease in the die hardness, which causes accelerated wear because of the abrasion and plastic deformation. The nitriding does not protect against the tempering phenomenon, but only delays the material softening process, because tempering occurs in the nitrided layer and in the layers deeper under the nitrided layer. Below the nitrided layer, tempering occurs relatively quickly and a soft layer is formed with a hardness below 400 HV.
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 performs an overview and evaluation of the currently applied methods of preheating forging tools used in hot die forging processes performed on presses and presents the development and ...tests of new, more effective methods. Based on the conducted complex analysis of the temperature changes taking place on and directly beneath the surface of forging dies during their preliminary heating, an analysis was performed of one of the most commonly used methods of tool heating,
i.e.
, using a charge material and a waste material. Next, the study proposes the introduction of changes and improvements of this method to achieve repeatability and stability of the obtained temperatures. To that end, detailed guidelines concerning the manner and time of the heating process were elaborated. The following step was the development of a new, more effective heating method. The first draft involved a new concept of heating using heaters localized in the die’s housing or heaters in the shape of forgings placed between the dies. Both concepts of heating were numerically modeled, which made it possible to assess their effectiveness. The last step of the research was the development of a heating method using induction heaters. This method was the most effective and brought the best results, which was confirmed during the modeling of the heating process and in the tests conducted on the prototype test stand. The obtained results confirm the effectiveness of almost all the methods, especially the method of heating by means of a charge material, supported by new guidelines, the method of heating with the use of forging-shaped heaters and the method of induction heating. Only in the case of heating with heaters mounted in the tool housing was the assumed working temperature not achieved. Some differences were observed mainly related to efficiency, that is, the heating speed in respect to the assumed temperature. The performed tests showed that the best method is induction heating as it is over twice as fast. Therefore, it is usually recommended for the process of heating and the additional heating realized in the work center of the press on the mounted tools, where the time aspect is especially important.
This study examines the possibilities of applying numerical and physical modeling to the elaboration of technology and design of tools used in the hot forging of needle rails for railroad turnouts. ...First, a numerical model of a three-stage process for forging a needle from lead was built in order to develop a proper geometry of the tools' working impressions for physical modeling. Based on preliminary results of the force parameters, a decision was made to verify the numerical modeling at 1:4 scale due to forging force values as well as agreement of the numerical and physical modeling results, which was confirmed by the similar courses of forging forces and a comparison of the 3D scan image of the forged lead rail with the CAD model obtained from FEM. The final stage of our research was modeling an industrial forging process in order to determine the preliminary assumptions of this newly developed method of precision forging using a hydraulic press as well as preparing tools to reforge a needle rail from the target material, i.e., 350HT steel with a 60E1A6 profile to the 60E1 profile used in railroad turnouts.
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.
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 comparison of the dynamic mechanical behavior and microstructure of two medium manganese sheet steels (3Mn–Al and 5Mn–Al) alloyed with aluminum is aimed. Mechanical properties under dynamic ...tensile loads are determined by means of rotary hammer dynamic tests at strain rates of 250 and 1000 s
−1
and analyzed together with the results of static tensile test. It is found that the results are significantly affected by the variations in Mn content in the range from 3 to 5 wt%. In both steels, the tensile strength increases with increasing strain rate, but the variation in the strain rate range has a moderate effect on mechanical behavior. The highest ultimate tensile strength of 1475 MPa is measured in the 5Mn–Al steel, whereas the 3Mn–Al steel is characterized by better total elongation due to a larger fraction of retained austenite and more pronounced transformation‐induced plasticity effect. The results show that the mechanical properties of 3Mn steel are more strain rate sensitive than those of 5Mn steel. The microstructural features are characterized qualitatively and quantitatively by X‐ray diffraction, scanning electron microscopy, and electron back‐scattered diffraction techniques.
The work concerns the possibility of producing B-pillars from the 7075 aluminum alloy. First, extensive material tests were carried out. The as-delivered sheets (T6 temper) were heated up to 500 °C ...and solution heat-treated to dissolve the phases in the aluminum matrix. Then, the foot of the B-pillar was stamped from water-cooled and conductively-cooled blanks. When the initial process parameters were established, stamping tests of B-pillars were carried out. The strength and hardness of the manufactured elements were tested. The selection of process parameters allowed to obtain a product with an appropriate strength, similar to the strength of the T6 temper.
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