Key Engineering Materials Vols. 622-623

Abstract: A stress analysis for cast compound rolls is presented. Stress is divided into components as residual stress, stress implied by load, stress implied by torque, Hertz’ian contact stress and thermal stress. Furthermore, heat transfer is considered. Residual stress is measured and a 3D representation is given. All further stress components are calculated by FEM calculations. The data allow a complete description of the roll behavior in operation.

Abstract: In the present work cyclic torsion test was used to simulate hot plate rolling process in order to study the effect of strain reversal on non-recrystallisation temperature using unalloyed and microalloyed austenite model alloys. It was found that the amount of strain reversal directly influences both static recrystallisation and strain-induced precipitation process significantly delaying their kinetics. The proper assessment of the interactions between strain reversal and microstructure evolution plays a crucial role during hot rolling process - as continuous changes in the deformation mode (strain reversal) affect the level of redundant strain (in the areas near the surface of the stock) and lead to strain inhomogeneity across the plate thickness. This complex strain path introduces microstructural inhomogeneity and makes its predictions very difficult. Proper understanding of the effects of strain reversal on microstructure evolution in the austenite will help to optimise the hot rolling process.

Abstract: Cu-Nb nanolayered material was produced through an accumulative roll bonding (ARB) technique. Using this technique, two different rolling schedules were employed to produce a normal and transverse rolled material. This resulted in specimens with differing microstructures within the 135nm thick nanolayers and interface structures between the layers. The dynamic response of these bulk Cu-Nb nanocomposites was then investigated under planar shock loading. It was observed in dynamically fractured specimens that the characteristics of ductile failure features formed on the fracture surface after dynamic loading were dependent upon the processing route of the nanocomposite. Specifically, grain shape differences due to dissimilar rolling passes are linked with differences in the failure response, particularly kinetics of fracture. In addition, incipient failure immediately below the primary fracture surface was also observed. Numerous nanovoids were nucleated and aligned linearly in the middle of Cu layers within the shocked Cu-Nb nanocomposites. These observations indicate relative stability of Cu-Nb interfaces produced by the ARB methods utilized in this study under dynamic loading conditions.

Abstract: High temperature tensile tests have been conducted on double-notched bars (DNBs) of varying notch acuity ratios (a/R), achieving a range of triaxiality, to evaluate the effect of stress state on edge cracking in hot rolling of free cutting steel. The conditions of the tensile tests are selected to physically simulate hot rolling condition of as-cast material to wrought structure. All experiments were conducted using a Gleeble 3800 thermo-mechanical testing system. The aim here is to develop a qualitative relationship between the activated damage mechanisms with respect to different stress states (arising from the sample geometry). Double-notch bars have been selected for this investigation as they enable the optical examination of the pre-failure mechanisms, captured in the unbroken notched area. The damage features have been observed at the notch areas and different damage mechanisms have been identified for different stress state deforming at different temperatures.

Abstract: A direct punching of inclined ultra-high strength steel sheets having low ductility was carried out to improve the quality of the sheared edge. In the direct punching of an inclined sheet, the contact between the sheet and punch became gradual because of touch from the bottom edge of the punch, and thus the sheared portion tended to bend in the latter half of punching. As the strength of the sheet increased, defects such as burr and secondary burnished surface were occurred around the sheared edge due to the low ductility. The range of direct punching for sheared edge without defects was shown for the high strength steel sheets. To prevent the occurrence of defects, a punch having inclined bottom was used. In this punch, the contact with the sheet was changed, the uniform contact with the inclined bottom was given. In the punch having the inclined bottom, the secondary burnished surface in the edge of 980 MPa grade steel sheet was prevented.

Abstract: In the production process of sheet metal parts, oil is widely used as lubricant, not only in sheet metal forming but also in shearing and blanking. Due to environment, health and cost reasons, the absence of lubricants is an aim for future production as it has initiated for machining in the last years. For lubricant-free shearing, it has to be known if there is an influence on the process itself when using oil or not. To find this out, experiments are carried out with a small testing device installed in a tensile testing machine and a blanking tool installed in a servo press. With the small device it is possible to make a piercing process with a circle punch of 16 mm diameter. The blanking tool produces a larger cut part with different holes and open cuts. Without lubricant, there is no difference in the maximum shearing force for the small device while the stripping force is higher and the cut edge zones differs slightly. Using oil or not has a small effect on the force using the blanking tool.

Abstract: In this study, WS₂-dispersed Al composite material was fabricated by Compression Shearing Method at Room Temperature, using various WS₂ content ratios. The mechanical and friction properties of the WS₂-dispersed Al composites were measured. As a result, the density measurements showed that the compacted WS₂-dispersed aluminum composite had a relative density of 95 to 99%. Tensile strength of WS₂-dispersed Al has 200 MPa. The friction coefficient of Al/0.5vol.%WS₂ was 0.14, a reduction of 83%, in comparison with the 1.0 friction coefficient of the pure Al matrix material. The addition of WS₂ to the matrix systems used reduced the friction coefficient. Therefore, WS₂-dispersed Al composite material is useful for maintenance-free material of slide member.

Abstract: We investigated punching properties (crack in punched surface and hole expansion ratio) of high tensile strength steel sheets with and without center segregation. High strength steel sheets were heat-treated to reduce center segregation. Tensile strength, shear surface ratio, depth of rollover and burr height were measured on heat-treated steel sheets to confirm the effect of heat-treatment on strength. The EPMA analysis showed that the center segregation of Mn was reduced by the diffusion during heat-treatment. Crack-formation frequency and hole expansion ratio were also measured. As a result, the center segregation of Mn in high tensile strength steel sheets decreased by the heat-treatment (600 ^oC for 100 h) with maintaining the tensile strength, the depth of rollover and the burr height. The crack-formation frequency of the steel sheets decreased through heat-treatments.

Abstract: Shaving is one of the precision shearing methods used to remove the rollover and fractured surface. The rollover and fractured surface formed in the punching stage are removed by shaving. It is expected that the rollover will be reduced when half punching with a negative clearance is carried out in the punching stage. In this study, the influence of the punch penetration depth in the punching stage on the cut edge after shaving was investigated. The results of this study clarified the followings: When the punch penetration depth in the punching stage with negative clearance is large, the sheared surface obtained by shaving after half punching with negative clearance is larger than that obtained by conventional punching. The sheared surface obtained by shaving after half punching with negative clearance increased with increasing the punch penetration depth in the punching stage with a negative clearance. The rollover produced by shaving after half punching with negative clearance is smaller than that produced by conventional shaving. In addition, the chip produced by shaving after half punching is connected to the sheet material under the punch. From the observed deformation of the chip produced in the shaving stage after half punching with negative clearance, the mechanism of shaving after half punching is considered to be cutting followed by shearing.

Abstract: Flatness of blanked parts is important as well as the cut edge quality and often becomes worse when blanking the low stiffness parts such as narrow shim rings. In this study, the affecting factors on the flatness of shim rings in conventional blanking were investigated through the experiments varying the ring widths and materials. Flatness was evaluated with the deflection angle. It was found that the shim ring with a smaller width and the material having a larger n value exhibited larger deflection angles. Deformation during blanking was analyzed by FE simulation and the experimental results were explained based on the strain distributions.