Materials Science Forum Vol. 941

Abstract: It is well known that austempered spheroidal graphite cast iron shows good tensile and elongation. And, retained austenite include in the austempered spheroidal graphite cast iron structure was known to effect for the mechanical properties when TRIP was occurred. On the other hand, fineness and homogeneity of the structure are considered to influence as a factor to improve mechanical property. In this study, the mechanical properties of spherical graphite cast iron was evaluated that it treatmented fineness prior-structure and austenitized at (α + γ) range followed by isothermal treatment at bainitic transformation temperature range. Compared the heat treatment of this study and typical austempering treatment, the mechanical properties with good balance of strength and ductility were obtained in the heat treatment of this study. But, retained austenite that it determined by x-ray diffraction was slightly changed before and after the tensile test. Therfore, it was considered that not TRIP but complex phase effect on mechanical properties.

Abstract: Influence of chemical composition (C, Mn and Nb) and soaking temperature on the evolution of austenite grain size from a cold-rolled microstructure was studied on several Advanced High Strength Steels. A wide range of soaking temperatures was used to perform the heat treatments. Characterization of prior austenite grain size from the annealed samples using optical and confocal microscopes, Scanning Electron Microscope and Electron Back-Scattered Diffraction. Comparison of different methods was done to validate the methodology and the results were quite satisfactory. Concomitant effects of Manganese, Niobium, Carbon and of soaking temperature on the prior austenite grain size were analyzed and discussed. Important effect of Mn and Nb was underlined.

Abstract: Advanced High Strength Steel (AHSS) developments have largely focused on automotive applications using metallurgical approaches to develop retained austenite-containing microstructures in a variety of new steels, using the transformation-induced plasticity (TRIP) effect to achieve better combinations of strength and ductility. These efforts have been extended in recent studies to explore the potential to improve wear resistance, using metastable retained austenite to enhance wear resistance for earth-moving and other applications. This paper provides selected highlights of the authors’ efforts to develop wear resistant steels using AHSS processing approaches. Some attractive product/process development opportunities are identified, and it appears that martensite-austenite microstructures produced using “quenching and partitioning” exhibit increased wear resistance.

Abstract: Low carbon lath martensitic microstructures are used in various steel products requiring high strength and toughness. These microstructures are conventionally produced through re-austenitizing and quenching followed by low or high temperature tempering. It is also possible to produce lath martensite through direct quenching immediately following thermomechanical processing. In this study, deformation below the austenite recrystallization temperature before quenching to form martensite was simulated through laboratory scale Gleeble processing of a 0.2 weight percent carbon ASTM A514 steel microalloyed with up to 0.21 weight percent niobium. Thermomechanical processing generally increases the dislocation density of the as-quenched martensite, which is sensitive to the austenite grain size before thermomechanical processing. The hardness of the thermomechanically-processed steels is generally greater than steels austenitized at comparable temperatures without deformation; this hardness difference is attributed to the increase in dislocation density and increased lath misorientation in the thermomechanically-processed conditions. The hardness is generally independent of prior austenite grain size for the thermomechanically processed conditions in contrast to conventionally austenitized and quenched conditions, which have a Hall-Petch correlation with austenite grain size. The strength increase of the thermomechanically processed conditions compared to the conventionally austenitized and quenched conditions is maintained after tempering. However, there is a larger drop in strength for small prior austenite grain sizes for both conventionally austenitized and quenched and thermomechanically processed steels. Overall, the strength of these lath martensitic steels can be directly related to dislocation density through a Taylor hardening model.

Abstract: Spheroidal Graphite Irons (SGIs) are ductile cast irons with toughness and ductility comparable to those of carbon steel. In particular, high silicon Solution Strengthened Ferritic (SSF) SGIs are developed to provide higher strength with excellent ductility suitable for structural applications. The main characteristics of these materials lie in the graphite particles inclusions whose morphology and count greatly influence the mechanical properties and more specifically the fatigue crack initiation and propagation behaviour of the SGI components. In this work, SGIs specimens have been subjected to various thermomechanical treatments in order to analyse the influence of these treatments on the microstructure of the material. Observations of degenerated forms of graphite particles alongside the spheroidal nodules in the microstructure were then used as a basis for correlation with damage mechanisms at the microscale. In static tensile testing, it was observed that the matrix-nodule interface decohesion and plastic deformation of the ferrite matrix were the dominant damage mechanisms. In separately performed fatigue crack initiation and fatigue crack propagation tests, it was confirmed that the graphite particle shape played a decisive role in crack initiation and propagation. The results of the microstructural characterization have been implemented in a computational model for further study of the influence of the microstructure on the fatigue behaviour of these materials.

Abstract: In this work, two samples of service grade Esshete 1250 stainless steel, as-received and aged, were characterised to determine the microstructural differences between the parent material and weld in terms of the grain structures and the phases present using XRD, EBSD, SEM and EDS. There was no difference found in the grain structure, but the phases present in the aged weld showed that sigma phase developed during aging.

Abstract: Metals with a body-centered cubic structure such as iron exhibit a ductile-brittle transition which results in a brittleness below a particular temperature. This temperature depends on many factors such as strain rate, size and geometry of investigated samples or the purity of the material. Another important parameter influencing fracture mechanism is grain size. It is known that grain refinement can be an efficient way to change fracture mechanism.The goal of this study was to investigate fracture mechanisms of ultrafine-grained iron processed by hydrostatic extrusion (HE). Materials subjected to various total strain levels were tested. The average grain size of the HE-processed iron was below 350 nm. The mechanical tests were carried out at various temperatures, ranging from room temperature to liquid nitrogen temperature. It was found that the fracture mechanism depends on a density of dislocation and the loading direction. It was found that materials with the dislocation density above a certain critical value break in a ductile manner even at impact tensile test in liquid nitrogen. However, bending tests of miniature beams have shown that ductile fracture occurs only when the crack propagates along the radial direction of the extruded material, whereas, on the direction parallel to the axial direction, cleavage fracture was observed. A theoretical model explaining this phenomenon was proposed. This model is based on the Rice model and it considers the competition between two phenomena - dislocation slip in the stress field of the crack front and Griffith cleavage.

Abstract: In recent years, the efficient grain size refinement in austenitic stainless steels by the martensitic reversion process and the mechanical properties achieved in a laboratory-scale have been investigated extensively. In order to demonstrate the feasibility of this processing in an industrial-scale, a commercial 18Cr-7Ni-0.15N Type 301LN steel was cold rolled to various relative low thickness reductions (32–56%) to obtain 70–95% deformation induced martensite and subsequently annealed in an industrial-scale pilot induction line at the peak temperatures of 660–820 °C. Some sheets were subsequently cold rolled 10–20% to compare the mechanical properties with those of the commercial strengthened grades. Results showed that the induction annealing at around 700 °C can produce reversed structures with much enhanced tensile and fatigue strengths compared to those of the commercial steel. The stability of the grain-refined austenite is lower than that in the commercial steel, but still cold rolling strengthening remains ineffective.

Abstract: The reheat furnace process step has a profound effect on the TMCP performance, final hot rolled steel quality and mechanical property consistency during the production of hot rolled steels. The uniformity of heating applied across the entire width and length of the slab or billet is critical in the achievement of customer properties regardless of the chemistry. The resultant ferrite grain size in the final hot rolled product is significantly governed by the initial prior austenite grain size. Numerous reheat furnace process metallurgy and combustion parameters in actual operation affect mill productivity, microstructure, austenite grain size, scrap rate and diverts. This reheating step in the steelmaking process often receives low priority in the evaluation of product quality and mechanical property performance, especially the toughness through the plate thickness. Heat transfer conditions of radiation, convection and conduction affect furnace heating efficiency. In laboratory studies, the furnace heating step is typically quite uniform resulting in a homogeneous and fine prior austenite grain size. During production, it is much more difficult to control the uniformity of heating and heat transfer consistency along the entire length and through the thickness of the work piece. The furnace conditions are correlated to product quality via furnace process variables such as the air to gas ratio, furnace burner condition, furnace pressure, energy efficiency, adiabatic flame temperature (AFT) and furnace refractory condition. Operational practice recommendations are presented to minimize inhomogeneous heating which results in inferior product quality, hot rolling model anomalies and toughness variations in the through-thickness-direction.

Abstract: Fatigue tests of ultrafine-grained copper processed by equal channel angular pressing were conducted on the round-bar specimens with a small artificial-defect. The fatigue crack initiated from the defect at an early fatigue stage. After the crack initiation, the crack grew with a 45° inclination to the loading axis at stress amplitudes above 180 MPa. At the stress less than 160 MPa, however, the crack grew perpendicular to the loading axis. The physical background of deferent crack path directions between high-and low-stresses was discussed from the viewpoint of a morphological feature of damaged traces along the crack path.