Materials Science Forum Vol. 879

Abstract: The machinability of an experimental medium-carbon steel with a composition designed to promote rapid graphitisation during a high temperature anneal has been studied. The goal has been to explore alternative routes to a competitive free-cutting composition enabling less expensive steelmaking, manufacturing and recycling. Three starting microstructures prior to annealing have been considered; martensite, bainite and ferrite/pearlite. The microstructures and graphite dispersions formed have been characterised by optical and electron microscopy and the performance of the steel during machining compared with commercial free-cutting steel grades. A bench-top drill rig and metallographic techniques were used to evaluate relative machinability parameters, including surface roughness, tool wear and chip morphology. Thus it proved possible to rank the experimental steel graphitised from the three starting microstructural conditions and also against the commercial free-cutting steels.

Abstract: Dual-phase steels are the most important AHSS grades for automotive applications. Microalloying elements such as Ti, Nb or B are widely used to improve the strength of dual-phase steels. Thus, understanding the influence of these elements on the microstructure and mechanical properties of dual-phase steels along the processing route is critical for the development of new steel grades. In this work, different microalloying elements were investigated, separately or in combination. The influences of the different elements on the microstructure and mechanical properties of dual-phase steels in the hot-rolled condition and after annealing of cold-rolled material. Dilatometer measurements were performed to investigate phase transformation during a typical continuous annealing treatment of dual-phase steel after cold rolling. It was shown that, for example, Ti has a strong influence on the mechanical properties of hot-rolled material while its influence on annealed materials after cold rolling was relatively small. Conversely, B had a strong influence on cold-rolled materials but an insignificant influence on hot-rolled materials.

Abstract: The present article is dealing with the conditioning of martensitic steel plates for armoring troops carriers. The steel alloy was processed into ingots. The ingots were hot flat rolled to 80 mm, followed by rolling to 6 mm. Specimens for detection of the critical transformation temperatures were taken. The Ac₁, Ac₃, M_s and M_f temperatures were detected as 716, 835, 356 and 218 °C respectively. The 80mm microstructure contains plate martensite colonies, while the 6mm plates reveal complete plate martensitic structure. Both cases contain embedded carbides. Heat treatment cycles were proposed for enhancing the ballistic resistance. A treatment was consisting of austentizing at 910 °C for 20 min., followed by water quenching. This process revealed fully fine lath martensitic packets. Tempering was done on the quenched as well as on rolled plates. Tempering at 350°C, causes dissociation of some martensite, and encourages migration of the entrapped carbon atoms forming carbide aggregates embedded between martensite plates packets. Tempering at 250°C for 20 min., relieves quenching induced stresses and reveals a lath martensitic structure with a small fraction of rosette like carbides. The process enhances elongation from 3% to 11.35%. The microstructure of as-rolled tempered plates at 300 ^oC for 20-30 minutes contains a thin ferrite layer on the boundaries surrounding the martensite packets, and few fractions of cotton wool shape carbides between the martensite packets. The treatment cycle improves the mechanical properties. Both treatments, at 250 and 300 ^oC, show successful ballistic resistance plates against shooting by 3 bullets.

Abstract: The present work is dealing with a physical simulation of thermo-mechanical processing of ferritic-bainitic dual phase (FBDP) steel alloy containing 0.1% C, 0.3% Si, 0.9% Mn and 0.7% Cr. The microstructure changes and allotropic transformations during thermo-mechanical simulation are investigated. A series of heating – cooling cycles to detect the critical and allotropic transformation temperature by dilatation were carried out on the thermo-mechanical simulator (Gleeble 3500). On the other hand, five – consecutive hits were used during the physical simulation of hot rolling process. Two hits were representing the roughening stage followed by three ones representing finish rolling. Holding at 500°C for 5, 7, 10, 12 and 15 min. after last hit has been applied and then followed by air cooling. Dilation curves appear that Ac₁= 766 °C, while Ac₃ was detected as 883 °C. Baintic allotropic transformation temperatures were clearly noticed as 618 °C for Bs and 542 °C for Bf. The recrystallization temperature was also detected as 1035 °C. Holding for 5-7 min. at 500 °C was concluded as the optimum for creation a bainite volume fraction. Rough hot deformation a higher temperature above the recrystallization temperature is essential, where no strain hardening and possibility for achieving high strains without excessive loads. Finishing deformation at temperature lower than Tr would create fine bainitic structure. The flow curve of the steel ensures continuous strain hardening. The strain hardening rate (σ_f/ε) was directly proportional to temperature difference from pass to pass.

Abstract: The evolution of phase composition, microstructure and hardness in 316L austenitic stainless steel processed by high-pressure torsion (HPT) was studied up to 20 turns. It was revealed that simultaneous grain refinement and phase transformation occur during HPT-processing. The γ-austenite in the initial material transformed gradually to ɛ-and α’-martensites due to deformation. After 20 turns of HPT the main phase was α’-martensite. The initial grain size of ~42 μm was refined to ~48 nm while the dislocation density increased to ~143 × 10¹⁴ m^-2 in the α’-martensite phase at the disk periphery processed by 20 turns. The microstructure and hardness along the disk radius became more homogeneous with increasing numbers of turns. An approximately homogeneous hardness distribution with a saturation value of ~6140 MPa was achieved in 20 turns.

Abstract: In respect of weld quality, weld strength and the heat affected zone (HAZ) size, linear friction welding (LFW) has significant advantages compared to flash-butt and resistance welding. The production of chain links is realized by one c-shaped part in reciprocating motion being moved towards a stationary one under applied forces. The friction at the contact faces causes heat and thus, the material is plastically deformed. Due to relative movement excessive material is pressed out of the joint area leading to a characteristic weld burr. Relative motion is stopped once the defined burn-off is reached and by applying the forge force the chain is welded. In this paper, welding parameters for three different cross sections are investigated. Based on successful welding trials on 30CrNiMo8 steel chains with 26 mm diameter, parameters are adapted for smaller cross sections. On 48 steel specimens with 7, 10 and 15 mm diameter systematic welding trials are performed. The influence of the welding parameters frequency, amplitude, forge force and burn-off on welding strength and welding time are investigated with and without application of heat treatment. This test series was evaluated using DoE.

Abstract: Many crystals nucleate on the mold surface when the molten alloy is poured in a mold cavity. Because the crystallographic orientations of these crystals are random, the solidified structure near the mold surface is very complex. The ghost lines, which are sometimes thick and the angle between them is not 90 degrees, are often observed in this region. However, if the crystallographic structure of this alloy is cubic, such as bcc or fcc, the ghost lines are very regular. In order to understand the geometry of ghost lines, Al-20 mass%Cu alloys were unidirectionally solidified with constant growth velocity. The solidified structures on the obliquely crossed section were observed. The ghost lines were quite regular and parallel to each other in a solidification grain. The angles and the ratio of the width of ghost lines were measured and crystallographic orientations were estimated using these parameters, based on the solid analytical geometry. EBSD analysis were also performed on the area, where the ghost lines were characterized, and the precise crystallographic orientations were decided. The comparison between both analytical values indicated that the differences between them are within 10 degrees and it can be safely concluded that the estimation for crystallographic orientation using ghost lines agreed well with the EBSD analysis.

Abstract: In the present work, the crystal structure and microstructure of martensite in Ni-Mn-In alloys and the variant rearrangement behavior under the external mechanical loading were investigated. Results show that the martensite has an incommensurate 6M modulated structure (I2/m (α0γ)00) with the modulation wave vector q = 0.3437c*. Microstructure of martensite is in plate shape and self-organized with four orientation variants that are alternately distributed and related in either type-I, or type-II or compound twin relation. The variant interfaces are in coincidence with their corresponding twinning plane and then should be considered coherent. Under the uniaxial compression, the loading located in the common positive Schmid factor zone of the three types of detwinning systems might be favorable to obtain the single variant state. This study is expected to offer a fundamental information of crystal structure, microstructures and variant rearrangement behaviors in Ni-Mn-In alloys, so as to understand the underlying mechanisms of their multifunctional magneto-responsive properties.

Abstract: AA2024 aluminium alloy is used in aeronautics for various applications. However, the aging treatment (“T4 temper”) at ambient temperature used to bring it to optimal mechanical properties causes several problems. The main one is its duration (5 days) which brings the need for storage of the parts. Moreover temperature control during such a long period is difficult.The aim of this work was to develop an alternative treatment leading to optimized mechanical properties in a shorter time. For this, we investigated the kinetics of the aging process and observed the apparition and disappearance of “Portevin-Le Chatelier (PLC) effect” in AA2024 aluminium alloy.The knowledge of the time at which PLC effect is observed is important in terms of process because shaping may be carried out before the end of the aging process.

Abstract: Mg-Zn-Y alloys containing a thermally stable icosahedral quasicrystal phase (I-phase) will have wide application future on condition that primary α-Mg dendrite and the I-phase can be refined during the casting process. In this research, the microstructure and mechanical properties of the rheo-squeeze casting (RSC) Mg-6Zn-1.4Y alloys have been investigated. The Mg alloy melt was exposed to ultrasonic vibration (USV) with different acoustic power densities from 0 W/mL to 9 W/mL, and then the slurry was formed by squeeze casting. The results show that good semi-solid slurry with fine and spherical α-Mg particles could be obtained with the acoustic power density of 6 W/mL, and the average grain size and shape factor of primary α-Mg were 32 μm and 0.76, respectively. Meanwhile the coarse eutectic I-phase (Mg₃Zn₆Y) was refined obviously and dispersed uniformly. Compared with the samples without USV, the tensile strength and elongation of the RSC casting samples with 6 W/mL acoustic power density were elevated by 10.6% and 55.5%, respectively.