Papers by Keyword: Work-Hardening

Abstract: The behavior of work hardening by cold rolling and tensile deformation was investigated in an ultralow carbon and carbon bearing martensitic steels, and then the effect of carbon on the work hardening behavior was discussed in terms of the change in dislocation density and the microstructure development during deformation. In the ultralow carbon 18%Ni steel (20ppmC), the hardness is almost constant irrespective of the reduction ratio. On the other hand, the carbon bearing 18%Ni steel (890ppmC) exhibits marked work hardening. The dislocation density of these specimens was confirmed to be never increased by cold rolling. It was also found that 10% cold rolling gives no significant influence on the morphology of martensite packet and block structure. TEM images of the 10% cold-rolled steels revealed that the martensite laths in the ultralow carbon steel are partially vanished, while those in the carbon bearing steel are stably remained. These results indicate that the solute carbon retards the movement of dislocations, which results in the high work hardening rate through the formation of fine dislocation substructure within laths.

Abstract: Permanent mold and semi-liquid die A356 cast alloys were used to examine the roles of microstructures and aging conditions on fatigue crack growth. HIP treatment to the A356 alloy generates substructure like dendritic arm boundaries as well as reduction of pores, which improves fracture elongation and fatigue fracture toughness. The similar substructure occur at primary α-Al and inter Si particles of semi-liquid die cast, too. Fracture elongation of HIPed permanent mold cast is comparable to that semi-liquid die cast, the fatigue crack growth is faster than in semi-liquid die cast. Plastic hardening occurs around fatigue crack flank, which decreases fatigue crack growth rate, and such effect appears highly in under aged alloy.

Abstract: It is well known that during sliding friction the properties of rubbing surfaces are changed owing to transformations caused by applied loads, friction and wear processes. In the last years many research works were devoted to reveal the properties of the surfaces transformed during the friction and wear at the Budapest University of Technology and Economics in the Department of Machine Design. During these investigations the conditions of developing beneficial transformed surface layers and their operating boundaries were determined by experiments on metal sliding pairs in laboratory. The conditions of evolving large-scale adhesion junctions, scuffing and seizure were investigated on heavy-loaded surfaces at slow sliding velocity. During the experiments hard steel balls slid on dry or lubricated metal disks. Coefficient of friction and wear tracks were measured and structures and damages of sliding surfaces were evaluated using profilometry, SEM and normal microscope photographs, and micro-hardness measurement.

Abstract: Three kinds of materials, pure aluminum (Al1050), carbon steel (S25C) and eutectoid steel (NHH) with V-notch are used in this study to compare the effect of roller working on the fatigue properties among the test materials. As the result, all of the fatigue strengths are improved after roller working and the optimum deformation values exist for every test materials. Fiberized microstructure at the notch bottom, work hardening and compressive residual stress caused by roller working are considered as the main reasons for the fatigue strength improvement. The highest fatigue strength improvement of Al1050, S25C and NHH was 30%, 118% and 155%, respectively. The optimum fatigue improvement of NHH is the highest and it is much higher than that of Al1050, and the optimum deformation value of Al1050 is much lower than that of the other materials. Moreover, the roller working effect is much remarkable for the materials with better hardenability.

Abstract: This paper is aimed to investigate the effect of roller working on the torsional fatigue properties of a typical low carbon steel referring by the resutls of FEM analysis. Three types of specimens had been prepared with plastic deformation value of 0, 0.5 and 1.0mm, respectively. The main results obtained in this study are as follows: (1) The torsional fatigue strength of roller-worked specimen with 1.0mm plastic deformation is increased more than twice of that of non-roller
worked specimens. (2) Surface hardness of roller worked specimen is much higher than that of the non-roller worked one, and compressed and elongated structure is formed at the notch bottom of the specimen. (3) Crack length along the axial direction of roller worked specimens is longer than that of the non-roller worked specimen, and plural cracks initiated and propagated in the roller worked specimens. (4) The residual stress distribution at the specimen’s notch bottom was analysed using FEM analysis and it is verified the same tendency as the experimental results.

Abstract: The work hardening of alloys hardened by precipitate heat treatments depends on the distribution of the precipitate sizes and the solute level left in the metal matrix. A mean field theory for precipitation is first applied for the ageing and subsequently it is coupled to a work hardening model to study the stress-strain responses of age hardened conditions of AA6xxx alloys. The predictions are compared to mechanical experiments and to TEM characterisations.

Abstract: An experimental AA5182 sheet was cold rolled 80%, and tensile specimens removed with orientations 0, 45 and 90 degrees to the rolling direction. Room temperature monotonic tensile tests were performed on the specimens in different recovered states obtained by isothermal annealing at 230°C from 0.1 to 10h. The tests were instrumented to measure instantaneous plastic strain ratio, and unloaded just after incipient necking, but before failure if possible. With annealing the flow curves are characterized by lower strains to the onset of jerky flow, the reappearance of yield point elongation, decrease in work hardening, and increasing ductility. The recovery in substructure was described using a constitutive parameter proportional to the mean slip distance.

Abstract: The precipitation-hardened aluminium wrought alloy AA6110-T6 (Al-Mg-Si-Cu) was mechanically surface treated (deep rolled) at room temperature. The cyclic deformation behavior and s/n-curves of deep rolled AA6110-T6 have been investigated by stress-controlled fatigue tests at room and elevated temperatures up to 250°C and compared to the polished condition as a reference. The effect of deep rolling on fatigue lifetime under high-loading and/or elevatedtemperature conditions will be discussed. The stability of near-surface residual stresses as well as work-hardening states (FWHM-values) was investigated by X-ray diffraction methods. Residual stress- and FWHM-depth-profiles before and after fatigue tests at elevated temperature are presented. It was found that the investigated AA6110-T6 aluminium alloy shows cyclic softening during stress controlled fatigue tests at room and elevated temperatures. Below a certain stress amplitude at a given temperature, deep rolling can enhance the fatigue lifetime of AA6110-T6 as compared to the untreated state through cyclically stable near-surface work hardening as indicated by stable FWHM values. From the s/n data of deep rolled and polished AA6110-T6, an effective boundary line for the deep rolling treatment in a stress amplitude-temperature diagram can be established.

Abstract: Multiple forging (MF) can be used to attain large plastic strains in bulk alloys by successive forging along three orthogonal directions to retain the initial sample shape. An original multiple forging technique enabling 3-D cross forging at constant temperature up to 500°C has been applied to two Al alloys (Al-1%Mn and Al-3%Mg-Sc,Zr). Their rheology, texture and microstructure evolution are compared with those obtained in plane strain compression (PSC). The results are interpreted in terms of slip activity behaviour during both deformation modes. They can also be correlated with the contributions of free dislocations and sub-boundaries.