Forging Tools Research Articles

DESIGN IMPROVEMENT OF THE STEP-WEDGE DIES AND EXPERIMENTAL STUDY OF THE FORGING PROCESS OF 45 STEEL BILLETS

In this paper, a forging tool of a new design is proposed, which allows to implement shear and alternating strain in the entire volume of the deformed billet with a smaller change in its initial dimensions. To confirm the effectiveness of the proposed new forging tool in obtaining billets and forgings of the plates and slabs type of the required quality, a laboratory experiment was conducted. During the laboratory experiment, the forging of billets made of the steel 45 was carried out in such forging tools as step-wedge dies of two different variants (proposed and previously known) and in step dies. Analysis of the shape change of billets and the evolution of the microstructure of steel after two deformation cycles showed that the use of the step-wedge dies with a wedge protrusion on the upper die and a wedge cavity on the lower die is more promising for forging billets and forgings of the plates and slabs type with a given level of mechanical properties, since it allows to obtain a fine-grained structure with a smaller change in the initial dimensions of the billet.

Nitriding of hardfaced layers as a method of improving wear resistance of hot forging tools

The article deals with novel hybrid layers combining hardfacing and nitriding to improve forging tool durability. It includes a study of the nitriding capabilities of hardfacings made of typical materials used to repair key groups of hot forging tools. Tests were conducted on samples and on tools in forging processes. Tests on samples included hardfacing, nitriding, microstructure, microhardness, XRF phase analysis and abrasive wear tests. Experimental tests on forging tools included operational tests and comprehensive wear analysis by scanning, macroscopic and microscopic observations by light and scanning microscopy, and analysis of wear mechanisms. Tests on the samples confirmed the positive effect of nitriding on performance properties at room and elevated temperatures. They revealed the reduced susceptibility to nitriding of some welds and the variable composition of nitrides present on the surface of the samples, which has a key effect on the observed wear mechanisms in the ball-on-disc test. Operational tests showed the effect of improved durability, which is greatest for the DO*15 (Fe–Cr–Mo–W) + nitriding layer which is due to the increased hardness of the weld itself and the increased susceptibility to nitriding.

Investigation of Forging Tools on the Basis of Subjective Assessment of Tool Life

Forging tools must be able to withstand very strong mechanical, thermal, tribological, and chemical stresses. The extent to which a tool can withstand these stresses depends on the material used and its pre-treatment as well as the heat and surface treatment, i.e. the load capacity. The ratio of stress to load capacity determines how high the tool life of a forging tool is. This paper deals with the variations in the tool life of forging tools using the example of a specific industrial stage sequence and production conditions. Due to a large number of influencing variables that have an effect on the tool during the entire tool life history, the focus of this work is placed on influencing variables of the forming process. Based on real production parameters of a forging company, which are recorded during a period for the investigation, the process data are analyzed about an influence on the tool life. The investigation focuses on four influencing variables, namely the subjective assessment of the end of the tool life, the interaction between the forming stages, production interruptions, and the cooling and lubrication of the forming tools. For the parameters that are not yet recorded during the trials, promising available measurement methods are identified and tested under laboratory conditions. One example of this is the recording of the actual spray quantities that are sprayed onto the tool surface before the forming process. The results of the investigations show that the tool life fluctuations can be reduced by about 16% and as a consequence, the average tool life can be increased by about 13%.

A Temperature-Dependent Viscoplasticity Model for the Hot Work Steel X38CrMoV5-3, Including Thermal and Cyclic Softening under Thermomechanical Fatigue Loading.

In this paper, a temperature-dependent viscoplasticity model is presented that describes thermal and cyclic softening of the hot work steel X38CrMoV5-3 under thermomechanical fatigue loading. The model describes the softening state of the material by evolution equations, the material properties of which can be determined on the basis of a defined experimental program. A kinetic model is employed to capture the effect of coarsening carbides and a new isotropic cyclic softening model is developed that takes history effects during thermomechanical loadings into account. The temperature-dependent material properties of the viscoplasticity model are determined on the basis of experimental data measured in isothermal and thermomechanical fatigue tests for the material X38CrMoV5-3 in the temperature range between 20 and 650 ∘C. The comparison of the model and an existing model for isotropic softening shows an improved description of the softening behavior under thermomechanical fatigue loading. A good overall description of the experimental data is possible with the presented viscoplasticity model, so that it is suited for the assessment of operating loads of hot forging tools.

Wear Mechanisms of the Forging Tool Used in Pre-Forming in a Double Forging System of Truck Parts

Tool life in plastic forming processes is a problem of the utmost importance as it significantly affects the cost of production. Hot forging with hammers and mechanical presses is an example of the technological process in which the load on tools is extremely high and, consequently, the lifetime of tools is short. Considering, additionally, that this applies to large-scale production, from an economic point of view, the key issue will be to extend the tool life, make an accurate prediction of the number of parts that can be forged before the replacement of dies is necessary, and develop a system for quick tool changeover. Initially, however, it is necessary to understand the causes of excessive tool wear, which may lie in phenomena occurring at the level of microstructure. The aim of this article was to outline an example of the coexistence of multiple wear mechanisms in hot forging dies. For the modified chemical composition, the microstructure examinations were performed in selected areas of the tool. The research has revealed the causes of cracks in tools and some irregularities in the preparation of tools for production process.

An Experimental Analysis of the High-Cycle Fatigue Fracture of H13 Hot Forging Tool Steels.

In this study, the axial fatigue behaviour of hot forging tool steels at room temperature was investigated. Fatigue tests were performed on two steels within the same H13 specification. The fatigue tests were carried out in the high-cycle fatigue domain under normal conditions. These tests were also performed on specimens in contact with a corrosive medium, applying stress values that led to the high-cycle fatigue domain under normal conditions for the sake of comparison. Both materials showed similar fatigue strengths when they were tested under normal conditions. In contrast, corrosion fatigue lives were much lower than in normal tests and differed significantly between the two steels. Crack initiation was triggered by microstructural and surface defects in the normal tests, whereas the formation of corrosion pits caused crack initiation in the corrosion fatigue tests. Moreover, a fracture surface analysis revealed dissimilar crack propagation areas between both steels, which suggested that both steels had different fracture toughness. These results were in line with the differences observed between the carbide and grain sizes of both of the material microstructures.

Hardness Assessment Considering Nitrided Layers Based on Tempering Tests for Numerical Wear Prediction for Forging Processes.

The nitriding of forging tools is an industrially established standard used to increase the hardness of the tool surface layer and reduce wear. However, this modification of the tool surface layer, as well as the microstructural changes that occur during this operation due to the thermo-mechanical load, cannot be considered during wear calculations with the widely used Archard wear model in the context of FE simulations. Based on previous work, this study further develops two tempering tests for the investigation of the hardness evolution of two nitride profiles based on H11 tool steel. Here, significant tempering effects could be observed depending on temperature, mechanical stress superposition and time. The results are used for setting up a new material model that is implemented in an existing numerical wear model. The validation is carried out in two laboratory forging test series. The evaluation shows that the hardness development in terms of tempering effects of a nitrided forging tool can be numerically predicted, especially for high forging cycles. However, due to the unexpected occurrence of adhesion effects, only limited applicability of the wear prediction then carried out is achieved.

Load-Adapted Surface Modifications to Increase Lifetime of Forging Dies

Diffusion treatments offer possibilities to enhance the performance and the service lifetime of hot forging tools. In combination with coating after nitriding, the surface layer hardness can be further increased. Within the scope of this study, a surface layer hardness above 2,000 HV0.005 was determined for borided or DLC (diamond-like carbon) coated surface layers. An increased surface layer hardness improves the abrasive wear resistance of forging dies. Furthermore, the plastic deformation of thermally softened forging die areas can be reduced. Beside these desirable effects, the ductility of diffusion treated or coated near surface layers is reduced and thermomechanical cracks are promoted. Therefore, additional approaches were developed to improve the thermomechanical crack behaviour of forging dies. Patterned plasmanitriding by the use of coverages to prevent areas from nitrogen diffusion, new combination processes of plasmanitrocarburizing (PNC) followed by plasmanitriding (PN) and the innovative boriding were investigated on different abstraction levels. A system of several testing rigs was set up to enable the abstraction of the thermal shock conditions in different stages. The patterned nitriding, boriding and combination plasma process (PN + PNC) were evaluated in a series of industrial field tests to derive recommendations for suitable tool treatments.

Green Forging of Titanium and Titanium Alloys by Using the Carbon Supersaturated SKD11 Dies

The carbon-supersaturated SKD11 punch was proposed as a green, or, a galling-free, long-life and low energy-consuming forging tool of pure titanium and β-titanium alloy that works with low friction and less work hardening and without galling. The reduction in thickness was increased up to 50% to investigate the friction process on the contact interface and the work-hardening behavior. The nitrogen-supersaturated SKD11 punch was utilized as a reference tool for this forging experiment. Three-dimensional finite element analysis was employed to derive the regression curve between the contact interface width and the friction coefficient. The friction coefficient was estimated in forging the pure titanium wires by using the regression curves. The work-hardening process was analyzed by the hardness mapping on the cross-section of forged wires. The SEM-EDX analysis on the contact interface proved that no adhesion of fresh metallic titanium and titanium oxide debris was seen on the interface between the carbon-supersaturated SKD11 punch and the titanium work. In particular, the work hardening is suppressed without shear localization in forging the β-titanium. Finally, the uniform carbon layer was derived from the supersaturated carbon solute from the punch matrix and wrought as a friction film on the contact interface to reduce the friction and the work hardening as well as suppress the chemical galling. This in situ carbon lubrication must be essential in green forging to highly qualify the titanium and titanium alloy products and to prolong the punch-and-die lives in practical operation.

Simulation of CuE Copper Alloy in a Closed-Die Multi-Axial Forging Tool

Abstract Two-way multi-axial forging was performed on a newly designed closed-die forging tool. The tool was operated on an MTS 810 material testing system. The connected computer recorded force and crosshead displacement as a function of time during operation. The sample material of the four-step forging experiment was CuE copper alloy. The plastic deformation was 0.8 per step, thus the rate of cumulative equivalent plastic strain was 3.2 by the end of the process. The speed of movement of the active tools during the whole test was 2 mm/min. Finite element simulation was performed with QForm3D software to investigate the force conditions of the process. The necessary flow curve was determined by Watts-Ford test. The force-displacement curves of the physical simulation were compared with the results of the finite element modeling.

Use of elastomers as pressure media for fatigue testing of cold forging tools

Cold forging has high potential as a sustainable production technology. With net-shape production, only the material needed for the produced part is used. By not heating up the workpiece, energy consumption is also low. However, this leads to high forming forces and consequently critical stresses in the tools. An accurate prediction of tool life is therefore necessary; both to find measures for its increase and to plan tool changes with minimal machine downtime. Currently there is little data about the fatigue properties of cold forging tool materials. Existing tests to determine this data strongly simplify the tools’ stress state to a uniaxial load. Therefore, a new fatigue test is presented in this paper, which incorporates the actual load conditions of cyclically swelling inner pressures. For this purpose, a high-strength elastomer is used as pressure medium. Its low compressibility and high elasticity allow for cyclic testing with pressures up to at least 2000 MPa. This load causes wear on the elastomer, as it is pressed into the gap between the punch used to apply the load and the fatigue specimen. Therefore, the gap between punch and fatigue specimen should be as low as possible to ensure stable test conditions. Within the scope of the research, fatigue failure already occurred in one specimen. This shows that the compression of elastomers is able to generate very high loads comparable to the cold forging of steel.

Local prestressing of cold forging tools by reinforcements with adapted interference

With contact pressures of more than 3000 MPa cold forging dies belong to the highest loaded components in mechanical engineering. Especially in non-symmetrical dies, this leads to a complex stress state with locally varying stresses. Cyclically occurring tensile stress concentrations cause crack initiation, which is why fatigue is the most important failure mechanism for complex cold forging tools. To reduce tensile stresses and counteract fatigue failure, dies are conventionally prestressed by inserting them into a reinforcement ring with a uniformly distributed interference. However, the compressive prestress is often not enough to compensate the high local stresses occurring in processes for non-symmetrical parts. Therefore, this paper focuses on using a non-uniformly distributed interference to locally increase prestresses in critical areas. To achieve this, gaps are inserted between die and reinforcement, inducing a bending stress. A numerical model is used to analyse how to design the gaps depending on the interference fit and how effective the use of gaps is for different part geometries with an increasing number of functional elements. Results show, that gap angles between 40° and 60° are suitable for locally improving the stress state. For high interferences, too large gap angles increase tensile stresses occurring in other areas of the dies, possibly impeding tool life. Since the adapted interferences work based on a bending effect, gaps are effective for up to four uniformly distributed critical areas. For more local elements, the lever distance is too low to apply an effective bending stress and the stress state does not improve.

Contribution to process digitisation of cold forging processes using additive manufactured tools

Cold forging tools become increasingly complex and require enhanced functionality, especially in the context of digitisation. Conventional subtractive manufacturing processes often reach their limits when the geometric complexity of the workpiece increases, hence additive manufacturing processes have become increasingly important in the last decades. Additive manufacturing processes have already been used in many fields of manufacturing technology to produce tool components with promising results, but the potentials of additive manufacturing processes have not yet been applied to cold forging tools. Therefore, the Institute for Metal Forming Technology (IFU) of the University of Stuttgart has developed an additive manufactured cold extrusion tool with integrated functional features. As functional features in the additive manufactured extrusion tool, a close contour glass fiber sensor for temperature measurement, a cooling system and a lubrication system for the controlled injection of minimal lubricant amounts during the forming process were integrated. Due to the integrated functional features, structural degradation appears in the tool, therefore the structural-mechanical tool properties were analyzed numerically with the FE-Software DEFORM 3D™ in this report as well. Furthermore, the additive manufactured cold extrusion tool was experimentally evaluated in sequentially executed extrusion operations. Thereby the integrated functional features were used and gathered data were recorded. As a result of the experimental forming tests, near-contour temperature measurements in the extrusion tool with and without the use of the integrated cooling system as well as the modification of the maximum punch forces by an inline lubricant application were obtained. In addition, the experimentally determined temperature fields in the extrusion die are validated with numerically calculated results.

Improvement of forging die life by failure mechanism analysis**

Abstract This study analyses the causes of rapid failure at the early stages of hot forging tools. Tool life generally refers to the number of forgings that meet the required geometry, properties and quality produced by a tool. Die failure in hot forging processes are caused by failure modes such as plastic deformation, wear and thermomechanical fatigue. This study focuses on plastic deformation failure. The practical experience was carried out in the upset forge fastener head, having a workpiece material of Ck45 alloy steel and tool made of 56 NiCrMoV7 tool material, which was forged at 950–1150°C, under an 800-ton, horizontal mechanical press. The finite element (FE) software MSC Simufact Forming 16.0 was used to simulate the process. The results showed plastic deformation occurring in the upper and lower dies because the temperature increase progressively reduced the hardness, creating softening effects during the forging process. The optimal forging temperature to form the fastener head was determined to be lower at 1000°C, saving time and cost. The FE method can be efficiently used to enhance the formation and to predict diet life.

Analysis of stress pins for the local prestressing of cold forging tools

The trend towards lightweight design leads to an increasing demand for sophisticated part geometries with high functional integration. In order to use the advantages of cold forging regarding the time- and resource-efficient production of high-quality parts, high local stresses causing fatigue failure in geometrically complex tools have to be controlled. The objective of this manuscript is to analyse the use of stress pins for a local influence on the stress state within forging, especially in non-circular symmetrical cold forging dies. For this purpose, a closed-die forging process for elliptical parts is designed and analysed regarding the die stresses. Distinct areas with local compressive and tensile stresses occur in the process. To counteract the tensile stresses critical for fatigue failure, the effect of stress pins pressed into the die creating a local prestress is analysed. Around the pins, compressive radial and tensile tangential stresses occur. While large pin diameters, interferences and close positioning to the tensile area lead to an increasing prestressing effect, too high values of these parameters cause a detrimental superposition of tensile process and pin stresses. If used correctly, there is high potential to improve the stress state and tool life especially for locally stressed complex tools.

The metal forging tools of the Late Bronze Age population of the forest-steppe Tobol River region (experimental-traceological analysis)

The paper reports on the results of experimental-traceological study of stone tools used for metal forging by the Late Bronze Age population of the Middle Tobol River region (Western Siberia). The chronological span of the study, according to the radiocarbon dating, extends from the 17th to 9th centuries BC. This paper aims to substan-tiate and expand the existing knowledgebase on the metalwork production with the aid of experiments in forging copper and bronze. The research materials include about 60 stone tools from the Late Bronze Age sites and 23 experimental tools. The trace evidence analysis and microphotography of the signs of use-wear have been performed using a continuous-zoom microscope MC-2 ZOOM with 10х to 40х magnification and a Canon EOS-1100-D camera. The experimental study involved cold and hot forging of copper and bronze items with different tin content and their surface smoothening. As a result, the efficiency of the stone tools in molding has been con-firmed. The signs of use-wear of the tools have been recorded. Distinctive use-wear features of hammers for cold and hot forging associated with the tool kinematics have been identified. It has been concluded that the wear signs on the hammers used for incomplete hot and hot forging were identical. It appeared that the smooth working area of some flatters was the result of preliminary abrasive treatment. Smoothening and drawing proved efficient in processing of copper items and low-alloyed bronzes. This treatment was carried out on the hot metal. It has been ascertained that the kinematics of processing of the copper and bronze items was the same. The signs of usage of the tools employed for shaping bronze moldings with different tin content differ from those on the tools used on copper by more extensive chipping, which is due to a higher hardness of tin-copper alloys. Prototype multi-functional tools used in different operations, viz., forging and drawing, have been identified. Their specific is the working area with a combination of several groups of wear marks overlapping each other. Therefore, we managed to produce a series of the experimental tools used in copper and bronze forging. This allowed us to elaborate the functional identification and technology of some archaeological instruments from the Late Bronze Age sites of the forest-steppe Tobol River region.

Predicting damage and failure under thermomechanical fatigue in hot forging tools

Finite element (FE) model is developed to examine thermomechanical fatigue behavior of a hot work tool steel. Experimental results for fatigue properties of 9Cr steel were used. An approach, employing a local ductile damage initiation and evolution model, using the hysteresis stress–strain energy concept combined with element removal, has been used to predict the failure. FE analysis of cyclic loading was based on a nonlinear kinematic hardening criterion employing the Chaboche constitutive equations. Parameters of fatigue damage are calculated based on experimental test data at 300 and 620 °C. Using these parameters, simulation of high temperature low cycle fatigue behavior for standard specimens according to test conditions is carried out and the results of LCF simulation were in good agreement with the experimental. Then, a set of thermomechanical fatigue simulations for hot forging die was performed and fatigue crack initiation and growth in forging die are predicted.

Production, Bonding and Application of Metal Matrix Composite Hot Forging Tool Components

Metal matrix composite materials are of high interest for their increased stiffness, strength or wear resistance. Wear resistant composites contain hard ceramic particles to reduce microcutting and grooving of the metal matrix surface. In this paper, a gas atomised hot work tool steel X40CrMoV5-1 (1.2344/AISI H13) was combined with fused tungsten carbide (FTC) particles in order to create forging tools with increased abrasive wear resistance. For that purpose, tool components were manufactured by sinter-forging of stacked powder layers to build up a graded hard phase concentration of up to 10 vol.-%. Subsequently, sinter-forged specimens were combined with basic hot work tool steel components and joined by diffusion bonding to assemble the complete tool. In order to evaluate their performance, the tools were examined in a hot backward can extrusion process of low-alloyed steel. Optical geometry measurements, light microscopy and scanning electron microscopy of the worn tool radii indicated a significant decrease in abrasive wear when using FTC-reinforced tools rather than conventional hardened tool steel.

Comprehensive Review of Methods for Increasing the Durability of Hot Forging Tools

The paper presents a comprehensive review of the most popular methods used for hot forging tools to increase their durability. The main directions of development of methods for increasing durability were indicated and for each of them the most important scientific results and discoveries were described. Only application-related works were considered important, which confirmed the applicability of the proposed solutions. Particular attention was paid to papers published during the last decade that have not been discussed yet. Among the discussed works, the authors’ contribution was also considered important, which was summarized and evaluated in the context of other works. Based on the analysis, it was shown which considered method can be useful in increasing the tool lifetime. A comparison was also made of the methods available in the literature with an indication of their purpose. The aspect of hybrid layers combining several surface engineering techniques simultaneously has been widely discussed. All statements were confirmed experimentally or by reference to appropriate sources in the available literature.

Improved localized fatigue wear resistance of large forging tools using a combination of multiple coupled bionic models

It is important to solve the partial fatigue wear of large forging dies, where the service life can be short. The purpose of this paper is to use laser remelting biomimetic technology to prepare a coupling biomimetic model combination on the mold surface, strengthen the local fatigue wear resistance of the mold, and extend the service life of the mold. Three types of laser energies were used to prepare the coupling biomimetic elements and analyze the microstructure and phase composition of the specific gravity melting zone. The biomimetic units were combined into a variety of coupling biomimetic models (fringe, mesh, and dense array models with different laser energies), and the number of fatigue cracks and the weight of wear loss of the coupling biomimetic model were compared at different temperatures. The mechanism of the coupling bionic model was analyzed, and the coupling bionic surface with a dense row structure was determined to have strong anti-fatigue wear performance. According to the Range Method, the relationship between wear and fatigue temperature and laser energy of the coupled bionic model was determined. Using DEFORM software to simulate the stress, temperature, and wear conditions of the front axle die surface, different coupling biomimetic model combinations were prepared on the abrasive surface. The die wear and cut edge width of the forgings were reduced after practical production, which proved that the laser bionic local strengthening method can effectively prolong the service life of a die.