Mechanisms of Oxidation Degradation of Cr12 Roller Steel during Thermal Fatigue Tests

Degradation by the penetration of oxidation into the Cr12 roller steel is evaluated during thermal fatigue tests in the laboratory in the temperature range of 500–700 °C. A qualitative assessment is carried out with regard to the thermal load, the microstructure and the test temperature. The results show that the specific properties of the microstructure with respect to thermal stress and temperature have a significant influence on the oxidation behavior as well as on the crack propagation mode and crack growth. The conditions that lead to an increase in the oxidation rate and thus to premature and sudden local chipping of the roll surface layer are analyzed and explained.</jats:p

Testing of thermal fatigue resistance of tools and rolls for hot working

In the present contribution two tests for thermal fatigue testing, which have been developed in our group, are presented. First test has provided internal cooling system of sample, while second has external cooling. For both tests heating and cooling of samples are computer guided that enables very reliable results of testing. The first test is more appropriate for testing the base material, i.e. roll cast irons, roll steels, tool steels. The second test is more appropriate for experiments that are aimed for selection of appropriate tool surface treatment, i.e. laser cladding, nitriding, coating, etc., and to compare and to achieve improved resistance against thermal fatigue of produced surface layers

Thermal fatigue degradation progress in SiMo ductile cast iron under oxidation conditions

Thermal fatigue tests were conducted on a high Si low Mo ductile cast iron (DCI) under oxidation conditions at 600 °C. The study aimed to understand the evolution of surface degradation, focusing on complex composed graphite nodules, pearlite islets, and their impact on crack initiation and growth in relation to oxidation processes, etc. Degenerated graphite nodules, that include ferrite particles, and areas of increased graphite nodule density, along with pearlite islets, were examined for their oxidation behavior. At lower thermal cycles numbers cracks primarily initiated of at the graphite-matrix interface and grew faster in the case of successively arranged graphite nodules and their higher local number density. Graphite degradation involved debonding between graphite and matrix, followed by complex oxidation processes. Degenerated nodules exhibited higher oxidation rates due to the presence of ferrite which acts as oxidation pathways. Areas with increased graphite density and larger sizes facilitated accelerated oxidation via crack formation. Additionally, pearlite degradation started with cracked cementite lamellae, followed by oxidation. Strategies to enhance thermal fatigue resistance included reducing graphite nodule diameter, ensuring their uniform distribution, preventing of formation of degenerated nodules and eliminating porosity. These findings improve understanding of thermal fatigue behavior in DCI that has proven to be even more complex than previously thought. The findings will guide the development of advanced materials for demanding applications

Quantifying complex influences of chemical composition and soaking conditions for increasing the hot workability of M2 high-speed steel by using the alternative approach

The conditions for increasing the hot workability and extending the temperature range for the safe hot working of M2 high-speed steel (HSS) were studied and revealed. This was enabled by combination of two approaches, i.e. results obtained by an analysis of so individual as well as spatial influences of chemical elements on the hot workability using a conditional average estimator neural networks in combination with the results obtained from hot-compression tests that revealed the appropriate soaking conditions. The Latin Hypercube Sampling technique was used to model the uncertainty of the collected data used in the analysis. The obtained results reveal new, surprisingly complex, typically spatial and (highly) non-linear relationships between the chemical elements and the hot workability of M2 HSS, i.e. common mutual influence of carbon, carbide-forming elements as well as elements, i.e. Si, Mn and Co, which indirectly influence the formation of carbides. Further also new allowed upper limits for contents of some harmful elements like S, P, Al, Sb, Cu, Sn, As, Ni, etc. at which transition from higher to lower workability takes place were revealed. Finally, by applying a specially developed procedure for hot-compression tests the appropriate soaking time and temperature were assessed. New findings explain and considerably improve the intrinsic hot workability and extend the temperature range for safe hot working at its upper and lower limits

Contour maps for simultaneous increase in yield strength and elongation of hot extruded aluminum alloy 6082

In this paper, the Conditional Average Estimator artificial neural network (CAE ANN) was used to analyze the influence of chemical composition in conjunction with selected process parameters on the yield strength and elongation of an extruded 6082 aluminum alloy (AA6082) profile. Analysis focused on the optimization of mechanical properties as a function of casting temperature, casting speed, addition rate of alloy wire, ram speed, extrusion ratio, and number of extrusion strands on one side, and different contents of chemical elements, i.e., Si, Mn, Mg, and Fe, on the other side. The obtained results revealed very complex non-linear relationships between all of these parameters. Using the proposed approach, it was possible to identify the combinations of chemical composition and process parameters as well as their values for a simultaneous increase of yield strength and elongation of extruded profiles. These results are a contribution of the presented study in comparison with published research results of similar studies in this field. Application of the proposed approach, either in the research and/or in industrial aluminum production, suggests a further increase in the relevant mechanical properties

Early Spalling Analysis of Large Particles in High-Cr Steel during Thermal Fatigue: Relevant Mechanisms

The aim of this study was to investigate the surface deterioration of high-Cr roll steel caused by the spalling of larger particles during thermal fatigue. The mechanisms of surface deterioration due to spalling of larger particles are discussed. Using a laboratory thermal fatigue test that replicates hot rolling conditions, samples were tested cyclically (up to 4500 times) at maximum cycle temperatures of 500, 600 and 700 °C, followed by water cooling. Specimens with surface deterioration were selected for analysis, revealing important influencing parameters, i.e., the combination of test temperatures, chemical composition, thermal stress and microstructural properties, leading to oxidation-assisted crack growth in different directions and consequent surface deterioration due to early spalling of larger particles. Here, we describe the mechanisms of crack propagation, especially in the lateral direction, and their relation to the subsequent spalling of larger particles, which depend on the influence of the local chemical composition on the microstructural constituents, as well as their distribution and properties. The results obtained in this study can be used in the development of roll steel microstructures with improved resistance to the identified mechanisms of surface degradation

Influence of cooling mode in relation to casting and extrusion parameters on mechanical properties of AA6082

In this study, conditional average estimator neural networks (CAE NNs) were used for an analysis of the common influences of the cooling mode in relation to the ram speed, extrusion ratio, casting speed and casting temperature on the yield strength and the elongation of an extruded profile made from aluminium alloy (AA)6082. The obtained results from the analysis revealed very complex relationships between these parameters. In order to maximise the values for the yield strength and the elongation, the values for the ram speed, extrusion ratio, casting speed and casting temperature should be optimised in relation to the mode of cooling

A NiTi alloy weft knitted fabric for smart firefighting clothing

A prototype of a new shape-memory nitinol knitted fabric intended for use as an active thermal insulating interlining in firefighting protective clothing was developed in the study presented in this paper. Weft knitted fabrics were made from commercially available cold-worked nickel titanium alloy monofils. Knits were made on a manual knitting machine from a monofil measuring 0.1 mm in diameter, while a hand-made knit was prepared from a monofil measuring 0.2 mm in diameter. Nitinol fabrics were annealed at 500 °C to achieve an austenite transition temperature of 75 °C. A special constructed mould made of a steel frame and aluminium domes measuring 30 and 20 mm in height was used to give the nitinol fabrics a new temporary shape. A two-way, shape-memory effect of the nitinol fabrics was achieved using a 15-cycle training process. The achieved shape-memory effect was tested in a heated chamber at 100 °C, where bulges measuring 12–25 mm in height occurred. NiTi knits made from finer monofil were the most successful shape-memory knits. They were machine knitted and achieved sufficiently high bulges, measuring 18 or 12 mm, that facilitated large enough air gaps for effective thermal protection. A smart textile system was prepared by inserting the trained nitinol fabric into a pocket made from two textile fabric layers sewn together. When it was exposed to environmental temperatures of 75 °C and higher, it instantly changed its form from a two-dimensional shape to a three-dimensional shape, while increasing the air gap in the pocket. A quilted fabric made from such a smart textile system could be used in firefighting protective clothing to locally improve thermal insulation and protect the human skin from overheating or burns

Asymmetric rolling process

Asymmetric rolling is a novel technique used to control both the texture and the grain refinement of metallic materials. The aim of asymmetric rolling is to apply a large shear strain uniformly through the thickness of the plate, by maintaining a high degree of friction between the sheet and the rolls. It can be used to improve the formability of material. One of the advantages of asymmetrical rolling is that the rolling force and torque can be decreased. The methods used for the asymmetric rolling are single roll drive, different work roll speeds, different work roll diameters or different lubricated work roll surfaces