Key Engineering Materials Vol. 716

Abstract: In this paper, an analytical buckling model is established to predict the flange wrinkling behavior of deep drawn cylindrical cups of aluminium alloy sheet in warm forming conditions using macro-textured blankholders for the first time. A continuum damage mechanism (CDM) based material model was utilized to reflect the visco-plastic feature of material at elevated temperatures. Forming speed and temperature effects were investigated, and texture ratio and draw ratio effects were also discussed. The developed analytical buckling model was validated by finite element simulations. The increase of forming temperature and forming speed is prone to cause wrinkling for AA5754, but the effects are not as significant as the texture geometry and draw ratio. The analytical model presented in this paper can be used as a design guide to determine tool texture geometry necessary to avoid wrinkling defects in the warm forming processes of aluminium alloy.

Abstract: At present, the discussion about pores/ voids in large ingots is still actual. Researchers investigated the closing behavior during the open die forging process. Mostly, the approaches take the account into void closure with artificial pores. But artificial pores do not reflect the real closing behavior. This is caused in the surface/ structure of inner voids. During drilling and heating, the voids got a smooth surface and have contact to the atmosphere. Therefore, the material oxidizes on surface and the closing behavior is not similar to the real process. Real pores show a fractal surface with dendrites. And the inner of voids contains a vaccum. In the framework of a new approach, for void closure cast ingots with pores were generated and the structure and the closing behavior were investigated.The final goal is to find out a global closing function “Z”. This closing function should improve the understanding of void behavior and in the future the industrial process. The function depends on different parameters, like yield strength of material (P_kf), the pore size (A_Eq), the pore structure (P_struct) and process parameters (P_proc), such as tool geometry or bite ratio. Finally, there is a parameter P_disturb. This parameter works against the final closure and is important for the understanding of the process, because it represents the influence of dendrites. Furthermore the closing behavior is not comprehensible without the consideration of recrystallization.

Abstract: The current trend in temper rolling is to make a small reduction to steel strip in order to achieve higher strength with good formability and toughness. In addition, very high strength steels can be cold rolled twice with very small reductions. This causes problems in setup values for cold rolling. Rolling models are usually overestimating roll flattening in the case of small reductions.In temper rolling thickness reduction is small (0.5 – 3%) and the elastic deformation of the work roll should be taken into account [3]. However, standard circular arc roll gap models (e.g. Bland Ford Ellis combined with Hitchcock model) fail to predict the roll flattening and thus the rolling force [4]. In this work, finite element method has been used to define a simplified model for work roll flattening and contact length. Model describes the effect of reduction, strength of steel strip and roll radius.

Abstract: In this work a simulation of the innovation process "helical rolling – pressing" in the software package Simufact.Formingwas performed. The result is several models of this process. To analyze the feasibility of the process various key parameters that have a significant impact on the implementation processwere carried out. As a result, the optimal values of friction coefficients in the rolls and in the matrix, the angle of intersection of channels in the matrix and the distance of matrix from the deformation zone of the rollswere obtained.

Abstract: Axis pins, shafts and other rod-like parts are some of the main components of units and machines. They are manufactured from steel rods ranging from 7 to 60 mm in diameter after single-pass drawing. Unalloyed carbon steel grades are generally used to produce these items. TRIP steels application is advantageous in terms of achieving new properties when manufacturing steel rods. The initial billet size, the need to take account a transformation in TRIP steel microstructure and a great number of analysed technological conditions make it challenging to apply new materials into rod drawing process. The research aim is to explore the stress-strain state during rod drawing of steel TRIP700. Modified multiscale computer simulation method has been applied. The simulation method takes into account transformation of retained austenite into martensite during plastic deformation. Decreasing of the computational resource intensity and calculation time has been achieved by application of concept of Statistically Similar Representative Volume Element (SSRVE). Comparative analysis of rod drawing micromodels with and without the TRIP-effect simulation has been performed. The analysis showed that a values of equivalent strains in the deformation zone of a TRIP-ignored micromodel was three-four times lower than in a TRIP-factored micromodel. The analysis of simulation results has revealed that, due to a wide contact area with adjacent grains and interaction between microstructure elements, more intensive martensitic transformation occurred within larger grains of retained austenite. The micromodel shown that position and orientation of grains in the deformed TRIP steel microstructure are some of the factors that predetermine transformation of retained austenite. On the basis of simulation results recommendations on preparation of rod drawing conditions have been developed. Developed technological conditions provides ability to obtain: high-plasticity characteristics and high potential strain-hardening capability during exploitation of a future part; maximum strengthening throughout the cross-section of the rod after drawing; high strain-hardening of the surface layer only. The using of SSRVE concept reduced a number of elements within the micromodel in 20 times, while it lowered the calculation time in 16 times.

Abstract: A constitutive equation of flow stress based on the Norton-Hoff equation has been developed for the high chromium and nitrogen containing super duplex stainless steel, ZERON® 100 (UNS S32760). This was then used to model, using the finite element method, the strain distribution within a uniaxial compression test under typical two-phase forging conditions. Predictions from the model were used to correlate deformation history to microstructure morphology. The microstructure consisted of austenite, γ islands (both primary and secondary) within a ferrite, δ matrix that contained chromium nitride precipitates. For deformation temperatures of 1050°C and 1120°C, the small secondary austenite was equiaxed, whilst at 1280°C the secondary austenite exhibited a Widmanstätten morphology. The highest level of nitride precipitation occurred at the highest deformation temperatures, i.e. highest volume fraction of ferrite, independent of strain rate. This suggests that nitride precipitation appears to be driven to a greater extent by thermal factors than deformation substructure.

Abstract: The microstructure (crystal grain and the center segregation of Mn) around the voids and cracks in punching process was investigated using Hot-rolled 780 MPa-grade high tensile strength steel sheets. Steel sheets without center segregation were prepared by grinding only one side, while ones with center segregation were prepared by grinding both side. Punching tests were conducted with these two kind of steel sheets. Crack length and number of cracks on the punched surface were measured and counted by an optical microscope. Steel sheets with center segregation had more cracks in total and longer average crack length than that without center segregation. The Mn mapping and the crystal orientation mapping around the crack obtained through EPMA and EBSD showed that the range larger than 70 µm in crack length tend to cause intergranular fracture along Mn segregated area. Also in the range between 10 µm and 70 µm in crack length, cracks forming both along and away from Mn segregated area were observed. Concentration of Mn around the grain boundary of the center segregation area measured by TEM/EDS revealed that grain boundary contains high Mn concentration. The calculation on distribution of equivalent stress just before the onset of crack for steel sheets with and without center segregation using finite element model showed that equivalent stress concentrates at both edges of the punch and die and the center segregation part. Next, interrupted punching tests were conducted with two kind of steel sheets. The observation around voids through SEM, EPMA, and EBSD showed that voids initiate at the ferrite-Ti precipitate interface. From these results, following tendencies were found, within and near Mn center segregated area, voids initiate at the ferrite-Ti precipitate interface, and crack propagates along Mn center segregation. Moreover, Mn segregates at the grain boundary, and Mn weakens grain boundary cohesion which leads to an intergranular fracture. However, without Mn center segregated area, voids initiate at the ferrite-Ti precipitate interface, and crack propagates easily into the ferrite matrix by cleavage which leads to cause a transgranular fracture.

Abstract: This work is concerned with the development of a new process map for wire drawing of pearlitic steel considering damage evolution. In this study, a ductile damage is defined as a porosity or void volume fraction and the porosity evolution model proposed by Lee and Dawson is adopted. Dilatational plastic deformation due to growth of micro voids is also considered. Correspondingly, an Eulerian finite element analysis coupled with damage evolution model is utilized in order to reflect the effects of dilatational plasticity due to growth of micro voids. Also, the accumulated damage in wire drawing could be evaluated. Finite element simulation for wire drawing of pearlitic steel are performed for various process conditions such as a half die angle and an area reduction ratio. Especially, the effects of process parameters on the deformation characteristic as well as damage evolution in wire drawing are carefully examined. Finally, a new process map is presented in terms of a half die angle and an area reduction ratio, which can identify the successful process conditions for wire drawing of pearlitic steel. Thus, it would be expected that this process map will help an engineer for the design of wire drawing of pearlitic steel.

Abstract: Metallurgy and heavy engineering construction, which are considered the most energy-intensive industries, place great focus on complex shaped thick-gage plate metal items of equipment with wall thickness exceeding 40 mm and diameter/width of up to 4000 mm. Such items of equipment include large machine parts manufactured by means of hot plate stamping, such as bodies of rotation (for example, the segment of the radial surface of converter shell, the bottom part of degassing unit, etc.) utilized as pressure-operated devices, vessels, tanks and other facilities by metallurgical, petrochemical, oil and gas, and nuclear industries. Presently known manufacturing methods of such items of equipment, for instance, stamping methods, have a number of technological problems. In the present paper, creation and development of the theory and technology of manufacturing different large bodies based on the combination of plate rolling and stamping processes, as well as the combination of asymmetric rolling and plastic bending processes, are proposed. The goal was to develop a cost-effective technology of producing large-size bodies of rotation in the conditions of a thick-plate mill. The rolling of the package forms the basis of the first stage of the new process. The package consists of the upper (punch) base, the lower (matrix) base, and the blank plate situated between them. The economic benefit from installing the developed technology based only on the combination of asymmetric rolling and plastic bending processes was more than 1 million dollars. Casings on two converters were produced and installed in the oxygen-converter plant.

Abstract: Abrasive wear and low cycle fatigue (LCF) have the greatest influence on the durability of dies. This paper presents a new criterion for estimation of the LCF of forging dies. The deformation model of Manson-Coffin is the classic model for the calculation of LCF. The pulsating cycle of loading dies and the phenomenon of cyclic thermal softening doesn’t provide implementation of the full Manson-Coffin’s model for the analysis of the tool life. At the same time the majority of researchers do not take into account the plastic strain component for estimation the fatigue durability of the dies and use only elastic component of the Manson-Coffin’s law or model of Basquin that is based on the analysis of the stress cycle. The present work uses the strain-kinetic criterion to analyze the durability of the dies that allows taking into account the elastic-plastic strain components and thermos-cyclic softening. During loading in bulk forging the first cycle is to be with plastic deformation while all remaining cycles would have to remain within the elastic limit due to metal hardening. Moreover it has been shown that cyclic softening effect may also be observed in the thermomechanical fatigue. This approach has been implemented for LCF simulation of the die in hot forging in FE program QForm by introducing a special subroutine. The comparison of results of the die fatigue failure simulation has shown good correspondence with practice.