Materials Science Forum Vol. 941

Abstract: When 9Cr-1Mo steel is exposed to CO₂-rich advance gas-cooled reactor (AGR) gases it forms a duplex oxide which consists of an outer Fe rich layer and an inner Cr rich spinel which provides oxidation resistance allowing the steel to resist the corrosive atmosphere of the plant. The oxide scale develops, growing both into the substrate and outwards from the initial metal/air interface. The spinel develops porosity through the coalescence of Fe vacancies which over time alters the properties of the oxide and potentially allows a transport network to form within the oxide. The porosity of the duplex oxide was measured using scanning electron microscopy of oxides on 9Cr steel samples oxidised in a CO₂ atmosphere. Results show that samples which have suffered breakaway oxidation show larger oxide scales with alternating Fe/Cr bands whereas samples which have yet to suffer from breakaway show higher peak porosity values but thinner oxide scales. Furthermore the samples which are currently under protective oxidation show a high max porosity peak in comparison to those which have suffered breakaway.

Abstract: Sigma formation in duplex stainless steels is known to be detrimental to the mechanical properties of the material and limits the alloys to low temperature applications. In this paper, the surface damage caused by grinding on different grit sizes or polishing, is assessed using EBSD. The surface finish is then related to the depth of sigma formation in the surface region with a quantification of how the surface finish effects the sigma nucleation and growth. Finally, the effect of surface finish and sigma formation on the oxidation characteristics of the material is considered.

Abstract: The simulation of industrial rolling processes has been shown to be an important method to optimize rolling parameters, reduce production costs and improve product quality. Previous works have shown the value of hot rolling simulation by means of torsion tests where the mean-flow-stress (MFS) can be successfully predicted. In the present work, three rolling schedules are simulated by hot torsion tests and compared. It is important to note this methodology provides the flexibility to test different ideas without the risk of downtime or damage to plant equipment that could result from an unsuccessful industrial trial. The simulation analysis considered the production steps from reheating through the final accelerated cooling as well as the final product microstructures. The study provides important information to the production of various steel grades such as pipeline, shipbuilding, structural and other high-end products.

Abstract: Welding of very high strength quenched and tempered steel, R_p0.2 ≥ 900 MPa, put high demands on choice of welding process, consumables and also on the heat input. Such steels commonly have a quite narrow tolerance box for a suitable weld heat input to control the cooling time, Δt_8/5, pertaining to minimize the risk for generating weld defects such as undercut, a too soft weld heat affected zone, delayed cracking etc. When welding very high strength steel is the risk for cold cracking in the weld metal not negligible due to the high alloy content of such high strength filler wires used. Furthermore, filler metal wires having equal strength levels to very high strength steel are hard to find which, together with the above described drawbacks, favour use of under-matching filler wires.

Abstract: The steels consisting of multi-phase structure show an advantage for their high strength and good formability. The ductile to brittle transition with decreasing temperature has been investigated by Charpy impact test with sub-size specimen for the ferrite + pearlite structure steel sheet. The transition curve of the absorbed energy showed a two-step transition behavior, and the “middle shelf” appeared clearly in the curve. The cleavage-like fracture with few dimples appeared on the fracture surface of the specimens at the middle shelf, and the plastic strain was detected just below the fracture surface. This result suggested that the fracture at the middle shelf propagates with the quasi-cleavage fracture accompanied with plastic deformation. Although the traces of fracture surface corresponded to (001), (011), and (112) bcc-iron planes, the (001) cleavage plane was not dominant for the fracture propagation path at the middle shelf.

Abstract: The behavior of a 13% chromium steel subjected to hot deformation has been studied by performing hot compression tests in the temperature range of 850 to 12000C and at strain rates from 0.01 to 10 s^-1. The uniaxial hot compression tests were performed on a Gleeble thermo-mechanical simulator. The best function that fits the peak stress for the material and its relation to the Zener-Hollomon parameter (Z) is derived. The average activation energy of this alloy in the entire test domain was found to be about 557 [kJmol^-1] and the dynamic recrystallization (DRX) kinetics was studied to find the fraction DRX during deformation.

Abstract: Using a novel TMR-DQP processing route, two ultrahigh-strength steels have been developed with yield strengths up to 1100 MPa combined with good uniform and total elongations and low-temperature impact toughness. Processing involved thermomechanically controlled rolling including significant reductions below the recrystallization stop temperature (RST), subsequent direct quenching to desired quench stop temperatures between M_s and M_f and finally partitioning of carbon from the supersaturated martensite to the untransformed austenite in a furnace at the quench stop temperature. Samples were cooled slowly in the furnace over 50 hours to simulate the cooling of coiled strips on industrial hot strip mills. The approach used was to utilize a suitable 0.3C steel composition based on high silicon and/or aluminium contents. Detailed metallographic studies using LOM, FESEM-EBSD, TEM and XRD showed that the desired martensite-austenite microstructures were achieved. The advantage of strained austenite in respect of refinement of martensite packets/blocks was clearly evident. Austenite was finely divided between martensite laths and only an insignificant amount of austenite existed as pools. The fine lath martensite structure with narrow interlath retained austenite films enabled the achievement of excellent combinations of mechanical properties. Promising results in respect of microstructures and mechanical properties indicate that there are possibilities for developing tough ductile structural and abrasion-resistant steels through the TMR-DQP route.

Abstract: We examined the effects of tempering process and alloying elements on the microstucture, tensile properties, bendability and impact property of direct quenched (DQ), and re-austenitizing and quenched (RQ) low-carbon martensitic steels. For this purpose, four low carbon martensitic steels (Fe-0.07C-1.8Mn-Cr-Nb-Ti-B) were selected. We have investigated the effects of tempering temperature and alloying elements of chromium (Cr), titanium (Ti) and niobium (Nb) on mechanical properties and microstructures. Mechanical properties and microstructures were analyzed as well using tensile test, V-bending test, charpy V-notched impact test and electron microscopy for DQ, DQ and tempered (DQ-T), RQ and RQ and tempered (RQ-T) low-carbon martensitic steels. It has been found that the as-quenched microstructures of the DQ and RQ specimens were fully martensitic structure. Prior austenite grain size and effective grain size after quenching were larger in the case of RQ steel. In both cases, tempering made the needle-shaped carbides. It is shown that the strength decreased when the tempering temperature increased. The strengths of the DQ and DQ-T steels were 30~50MPa higher than those of the RQ and RQ-T steels. Despite the higher strength of the DQ and DQ-T states, both had similar impact properties with the RQ and RQ-T states. However, the impact properties of the Nb added RQ and RQ-T steels with fine martensite morphology exhibited higher than those of DQ and DQ-T steels.

Abstract: Abaqus software is used to simulate the deformation behavior of typical steel materials during the shearing process, and to analyze the displacement change of the materials during the shearing process, and to study the main causes of the steel plate shear cracks. The results show that the mechanical properties of the steel, especially the uniform elongation of the steel plate, are important factors affecting the shear crack. At the same time, the room temperature tensile curves of several typical steels are measured. The corresponding relationship between the curve characteristics and the shear cracks is in agreement with the simulation results. Keywords: shear crack, simulation, mechanical properties, uniform elongation

Abstract: The effects of hot deformation conditions in the stable austenite state on the transformation kinetics and morphology of bainite were examined using dilatometry, electron back scatter diffraction (EBSD) and X-ray diffraction (XRD) measurements in a carbide-free bainitic steel with a composition of Fe-0.34C-2Mn-1.5Si-1Cr (in wt.%). Both uniaxial tensile and compression tests were applied to study the effect of the deformation mode. The temperature, strain and strain rate of deformation were varied in the ranges of 820-1000 °C, 0.1-0.6 and 0.001-0.1/s respectively. It has been revealed that hot tensile deformation retards the austenite transformation to lower bainite. The overall transformation kinetics slows down and the final attained amount of bainite decreases after completion of the isothermal transformation at 350 °C. However, hot compression deformation accelerates the bainite transformation, increasing both the bainite transformation rate and the final amount of bainite formed. The total amount of bainite increases with decreasing the strain rate irrespective of the mode of deformation. The effect of the deformation temperature and strain on the bainite transformation is in a complicated manner depending on the deformation mode.