| 1. |
- Hallberg, Håkan, et al.
(författare)
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Model Describing Material-Dependent Deformation Behavior in High-Velocity Metal Forming Processes
- 2009
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Ingår i: Journal of Engineering Mechanics - ASCE. - 1943-7889 .- 0733-9399. ; 135:4, s. 345-357
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Tidskriftsartikel (refereegranskat)abstract
- A constitutive model for rate-dependent and thermomechanically coupled plasticity at finite strains is presented. The plasticity model is based on a J(2) model and rate-dependent behavior is included by use of a Perzyna-type formulation. Adiabatic heating effects are handled in a consistent way and not, as is a common assumption, through a constant conversion of the internal work rate into rate of heating. The conversion factor is instead derived from thermodynamic considerations. The stored energy is assumed to be a function of a single internal variable which differs from the effective plastic strain. This allows a thermodynamically consistent formulation to be obtained which, as shown, can be calibrated by use of simple procedures. Choosing 100Cr6 steel in two differently heat treated conditions as prototype material, experimental tests are performed, enabling the model to be calibrated. Significant differences in deformation behavior are noted as the differently heat treated specimens are compared. In addition, the local stress-updating procedure is reduced to a single scalar equation, permitting a very efficient numerical implementation of the model. The constitutive formulation proposed was employed in an explicit finite element solver, illustrative simulations of a high-velocity metal forming process being performed to demonstrate the capabilities of the model and certain characteristic traits of the materials that were studied.
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| 2. |
- Hosseini, Seyed, 1981, et al.
(författare)
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A Methodology for Temperature Correction When Using Two-Color Pyrometers : Compensation for Surface Topography and Material
- 2014
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Ingår i: Experimental mechanics. - : Springer Science and Business Media LLC. - 0014-4851 .- 1741-2765. ; 54:3, s. 369-377
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Tidskriftsartikel (refereegranskat)abstract
- In this investigation, the applicability of the two-color pyrometer technique for temperature measurements in dry hard turning of AISI 52100 steel was studied, where both machined surfaces as well as cutting tools were considered. The impacts of differing hard turned surface topography on the two-color pyrometer readings was studied by conducting temperature measurements on reference samples created using cutting tools with different degrees of tool flank wear. In order to conduct measurements in a controlled environment, a specially designed furnace was developed in which the samples were heated step-wise up to 1,000 °C in a protective atmosphere. At each testing temperature, the temperatures measured by the two-color pyrometer were compared with temperatures recorded by thermocouples. For all materials and surfaces as studied here, the two-color pyrometer generally recorded significantly lower temperatures than the thermocouples; for the hard turned surfaces, depending on the surface topography, the temperatures were as much as 20 % lower and for the CBN cutting tools, 13 % lower. To be able to use the two-color pyrometer technique for temperature measurements in hard turning of AISI 52100 steel, a linear approximation function was determined resulting in three unique equations, one for each of the studied materials and surfaces. By using the developed approximation function, the measured cutting temperatures can be adjusted to compensate for differing materials or surface topographies for comparable machining conditions. Even though the proposed equations are unique for the hard turning conditions as studied here, the proposed methodology can be applied to determine the temperature compensation required for other surface topographies, as well as other materials. © 2013 Society for Experimental Mechanics.
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| 3. |
- Hosseini, Seyed, 1981, et al.
(författare)
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Atomic-scale investigation of carbon atom migration in surface induced white layers in high-carbon medium chromium (AISI 52100) bearing steel
- 2017
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 130, s. 155-163
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure and chemical composition of white layers (WLs) formed during hard turning of AISI 52100 steel were studied using atom probe tomography (APT) and transmission electron microscopy (TEM). APT analyses revealed a major difference in the re-distribution of the carbon (C) atoms between WLs formed above and below the Ac1 temperature, i.e. T-WL and M-WL, respectively. In T-WL, the C-atoms segregate to grain boundaries (GBs) forming interconnected or isolated C-rich clusters, ∼5 nm, with a concentration of 9.8 ± 0.3 at.%C. Apart from the GB segregation, in M-WLs, large C-rich regions were found with 24.8 ± 0.4 at.%C. Owing to the chemical composition (stoichiometry) and element partitioning of such regions, they were assigned as θ-carbides (cementite). The APT results reveal that the original θ-carbides remain un-dissolved in the M-WLs, but might be plastically deformed due to the excessive strain that exists in hard machining process. The obtained results are in good agreement with the temperatures that are reached during formation of M-WLs. The isolated nano-sized C-clusters were assigned as off-stoichiometric carbides whereas the interconnected C-rich clusters were attributed to Cottrell atmospheres, evident by the linear shape of the C-enrichment as observed in the APT reconstructions. The C-contents in the nano-sized martensitic and ferritic grains were estimated to 0.50 ± 0.06 at.%C and ∼0.46 ± 0.02 at.%C, respectively. The C-content in the ferritic grains, beyond the C-solubility limit in ferrite (
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| 4. |
- Hosseini, Seyed, 1981, et al.
(författare)
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Characterization of the Surface Integrity induced by Hard Turning of Bainitic and Martensitic AISI 52100 Steel
- 2012
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Ingår i: Procedia CIRP. - : Elsevier BV. - 2212-8271. ; 1:1, s. 494 - 499
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Konferensbidrag (refereegranskat)abstract
- Depending on the process parameters and the tool condition, hard turned surfaces can consist of a “white” and a “dark” etching layer having other mechanical properties compared to the bulk material. X-ray diffraction measurements revealed that tensileresidual stresses accompanied with higher volume fraction of retained austenite are present in the thermally induced white layer. While compressive residual stresses and decreased retained austenite content was found in the plastically created white layer. The surface temperature was estimated to be ~1200 C during white layer formation by hard turning.
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| 5. |
- Hosseini, Seyed, 1981, et al.
(författare)
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Cutting temperatures during hard turning : Measurements and effects on white layer formation in AISI 52100
- 2014
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Ingår i: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136 .- 1873-4774. ; 214:6, s. 1293-1300
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Tidskriftsartikel (refereegranskat)abstract
- This paper concerns the temperature evolution during white layer formation induced by hard turning of martensitic and bainitic hardened AISI 52100 steel, as well as the effects of cutting temperatures and surface cooling rates on the microstructure and properties of the induced white layers. The cutting temperatures were measured using a high speed two-colour pyrometer, equipped with an optical fibre allowing for temperature measurements at the cutting edge. Depending on the machining conditions, white layers were shown to have formed both above and well below the parent austenitic transformation temperature, Ac1, of about 750 C. Thus at least two different mechanisms, phase transformation above the Ac1 (thermally) and severe plastic deformation below the Ac1 (mechanically), have been active during white layer formation. In the case of the predominantly thermally induced white layers, the cutting temperatures were above 900 C, while for the predominantly mechanically induced white layers the cutting temperatures were approximately 550 C. The surface cooling rates during hard turning were shown to be as high as 104-105 C/s for cutting speeds between 30 and 260 m/min independent of whether the studied microstructure was martensitic or bainitic. Adding the results from the cutting temperature measurements to previous results on the retained austenite contents and residual stresses of the white layers, it can be summarised that thermally induced white layers contain significantly higher amounts of retained austenite compared to the unaffected material and display high tensile residual stresses. On the contrary, in the case of white layers formed mainly due to severe plastic deformation, no retained austenite could be measured and the surface and subsurface residual stresses were compressive. © 2014 Elsevier B.V.
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| 6. |
- Hosseini, Seyed, 1981, et al.
(författare)
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Formation mechanisms of white layers induced by hard turning of AISI 52100 steel
- 2015
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 89, s. 258-267
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Tidskriftsartikel (refereegranskat)abstract
- This paper concerns the formation mechanisms of white layers (WLs) induced by machining of through-hardened (martensitic and bainitic) AISI 52100 steel. The microstructures of different types of WLs were investigated using transmission electron microscopy; those that had been predominantly mechanically induced (M-WL) and those that had been predominantly thermally induced (T-WL). Independent of the process parameters and the starting microstructure, the WLs consisted of a randomly oriented nano- and submicron-sized microstructure with an average grain size in the order of several tens of nanometres. The M-WLs were characterised as bcc-(α) ferrite and orthorhombic-(θ) cementite where the initial martensite/bainite platelets had been reoriented along the shear direction and broken-down into elongated sub-grains through dynamic recovery. The T-WLs were shown to consist of fcc-(γ) austenite, bcc-(α) martensite, and orthorhombic-(θ) cementite. Here the elongated sub-structure was found to coexist with equiaxed grains, meaning that the formation was initiated by dynamic recovery, which advanced to dynamic recrystallisation at the increased temperatures caused by the higher cutting speeds. Although times and temperatures are considered to be insufficient to dissolve the secondary carbides, carbide refinement was observed independently on the type of WL. The carbide refinement was controlled by (i) deformation and fragmentation of the carbides via dislocation movement along the {1 1 0} and {1 0 0}, (ii) precipitation of nano-sized carbides and (iii) diffusion-based carbide refinement by carbon depletion via dislocations in the nano-sized grains and grain boundaries, which act as high diffusivity paths.
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| 7. |
- Ryttberg, Kristina, 1977, et al.
(författare)
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Electron microscopy of white-etching band generated by high-velocity parting-off of 100CrMn6 steel
- 2008
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 480:1-2, s. 489-495
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Tidskriftsartikel (refereegranskat)abstract
- High-velocity parting-off has been applied to 80 mm bars of pearlitic 100CrMn6, resulting in shear localisation and white-etching bands in a severely deformed region below the fracture surface. Electron microscopy showed that going from the bulk material towards the fracture surface the grains become elongated and refined. The region below the fracture surface can be divided into three subzones: 50–100 μm below the surface grains are elongated, cementite lamellae are distorted, break up and the lamellar spacing decreases.
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| 8. |
- Ryttberg, Kristina, 1977
(författare)
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Fracture behaviour and related microstructures of tool steels parted-off at high rates of strain
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increasing steel prices and environmental aspects have put forward the demand to reduce material consumption in manufacturing industry and near-net-shape manufacturing techniques have thus become increasingly important. High-velocity parting-off, or adiabatic cutting, has shown to result in very low material waste as well as narrow dimensional tolerances of parted-off samples. However, for optimisation of the process improved knowledge is required regarding fracture characteristics and deformation mechanisms associated with the process. For evaluation of samples cut by commercially available technology, incorporating impact velocities of 5-10 m/s, a hydraulic high-velocity pressing machine with a parting-off tool was used. In addition, for evaluation of the possible influence of a higher impact velocity on the shear localisation, a non-conventional method was developed. In that case velocities up to 285 m/s could be employed. By measurement of velocities an estimation of fracture energies could be made. Microstructure evolution during deformation and fracture was characterised using optical microscopy combined with scanning and transmission electron microscopy.It has been concluded that for impact velocities of 5-285 m/s, the parting-off is, within the full range of velocities, initiated through shearing resulting in ductile shear fracture. However, depending on sample size and velocity the fracture mechanisms active in some parts of the fracture are ductile tensile indicating a triaxial stress state. Microstructural studies showed that the depth of the severely deformed region below the fracture surface is more dependent on the size of the sample than on the microstructure, resulting in much smaller deformed regions for smaller samples. The severe deformation results in cracks: For small samples only microcracks initiated on precipitates such as carbides and MnS inclusions, but for larger samples inter- and intragranular cracking also occurred. Electron microscopy of the severely deformed region below the fracture surface has shown three different types of structures: Right below the fracture surface a white-etching band (appears white in optical microscopy) consisting of nanocrystalline equiaxed grains was occasionally found. The grain size within this region was between 50 and 150 nm. The region below the white-etching band consisted of a mixture of equiaxed grains and highly elongated subgrains. The third region, located below region two, is composed of highly elongated subgrains. In the case of speroidise-annealed 100Cr6 parted-off at an impact velocity of 225 m/s the elongated subgrains were shown to have a mutual orientation, adjacent subgrains having (110) type of planes parallel. This could be an indication of formation of white etching bands being a mechanically rate controlled process involving dynamic recovery. In the case of parting-off pearlitic 100CrMn6 steel it has been shown that the lamellar spacing of the cementite decreases and cementite lamellae aligned in a direction perpendicular to the generated fracture surface are severely distorted during the extreme deformation, resulting in breaking up of the lamellar structure. The white-etching band consists of small finely distributed carbides in a ferrite matrix.
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| 9. |
- Ryttberg, Kristina, 1977, et al.
(författare)
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High strain rate deformation induced by high-velocity forming of 100Cr6 steel
- 2008
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Ingår i: proceedings of The 9th International Conference on Technology of Plasticity (ICTP), Gyeongju, Korea, September 7-11, 2008. - 9788957081525 ; , s. On CD or www.ictp2008.org-
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Konferensbidrag (refereegranskat)abstract
- This paper concerns the development of the near-net-shape manufacturing technique of high-velocity forming. The microstructural response to the high strain-rate deformation was evaluated for 100Cr6 steel in two differently heat treated conditions: spheroidise annealed (hardness 250 HV) and quenched and tempered (330 HV). A high-velocity pressing machine was used to conduct the forming tests on cylindrical specimens. By applying different impact velocities on samples with varying sizes deformation mechanisms could be related to strain and strain rate. Extreme localization of deformation resulting in white etching bands was seen in quenched and tempered samples but not in spheroidise annealed samples. Furthermore, the strain and strain rates were shown to affect the tendency for extreme localization. No white etching bands were found in samples strained to less than 1. Appearance of white etching bands could be seen in the macroscopic shape of the samples where the more homogeneously deformed samples showed a typical barrel shape while the strongly localized samples were slightly more bell shaped. It can be concluded that for high-velocity forming to be applicable to 100Cr6 steel a careful choice of the combination strain/strain rate needs to be made.
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| 10. |
- Ryttberg, Kristina, 1977
(författare)
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Microstructural changes in high strength steels exposed to large deformation and high strain rates
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increasing steel prices and environmental aspects have put forward the demand to reduce material consumption and energy usage in manufacturing industry and near-net-shape manufacturing techniques have thus become increasingly important. High-velocity parting-off,high-velocity forming and cold ring rolling are three such manufacturing methods that exhibit great potentials in terms of material waste reduction. However, all three processes involve large degrees of deformation that is not homogeneously distributed in the samples and the scientific knowledge regarding deformation mechanisms active in these processes is low. In order to allow for process optimisations a thorough understanding of associated deformation behaviour and microstructural changes is needed. Three steels have been used in this work: two bearing steels and one carbon steel.Through studies employing high-velocity parting-off incorporating impact velocities of 5-285 m/s, the fracture mechanisms active during material separation was shown to be a mix of ductile shear and ductile tensile fracture and to some extent failure by adiabatic shear banding. High-velocity forming tests were then conducted to evaluate parameters controlling the strain localisation and initiation of adiabatic shear bands (ASBs). Strain and strain rate were shown to be important for strain localisation. However, most important was shown to be the microstructure coupled to hardness where quenched and tempered samples developed ASBs while spheroidise annealed samples did not. By using electron microscopy the microstructure in the ASB regions generated by parting-off and forming were compared and shown to be composed of three types of structures: Within the ASB nanocrystalline equiaxed grains with a size of 50-150 nm were found, while adjacent to the ASB the microstructure consisted of a mixture of equiaxed grains and highly elongated subgrains. Outside this region only highly elongated subgrains were found. The elongated subgrains were shown to have a mutual orientation, adjacent subgrains having {110} type of planes parallel. This could indicate that formation of ASBs is a mechanically rate controlled process. In addition it was shown that smaller carbides were dissoluted while larger spheroid carbides remained and possibly also facilitated the refinement of microstructure in the formation of ASBs.Cold ring rolling tests were done to further investigate the effect of large deformations on microstructure and texture. The plastic deformation was shown to be most severe near theinner diameter of the rings decreasing towards the area of the outer diameter. By employing electron back scatter diffraction the ring rolling process was shown to result in a combined fibre texture where was parallel to the rolling direction and was parallel to the ring radial direction in the centre of the ring, indicating more or less plain strain in this region. Below the inner diameter the texture was of the {110} type indicating contribution of shear.
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