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Models describing the constitutive flow behaviour of a metallic material are desired for appropriate process design and realization of defect-free components. In this study, constitutive equations based on the hyperbolic-sinusoidal Arrhenius-type model have been developed to define the hot deformation characteristics of a CoCrFeMnNi high-entropy alloy. The experimental true stress-true strain data were generated over a wide temperature (1023-1423 K) and strain rates (10 −3-10 s −1) ranges. The impact of strain rate and temperature on deformation behaviour was further characterized through a temperature compensated strain rate parameter, i.e. Zener-Hollomon parameter. Additionally, a mathematical relation was employed to express the influence of various material constants on true-strain ranging from 0.2 to 0.75. Typical third order polynomial relations were found to be appropriate to fit the true-strain dependency of these material constants. The accuracy of the developed constitutive equations was evaluated by using the average absolute relative error (AARE) and correlation coefficient (R); the obtained values were 7.63% and 0.9858, respectively, suggesting reasonable predictions. These results demonstrate that the developed constitutive equations can predict the flow stress behaviour of the alloy with a good accuracy over a wide range of temperature and strain rate conditions and for large strains.

The tempering of re-austenized, quenched and tempered (RAQT) martensitic steels is an extensively studied and well understood field of metallurgy. However, a similar understanding of the effect of tempering on direct-quenched (DQ) high-strength steels has been lacking. Now, for the first time, the effect of tempering in the range of 250-650 °C on the strength, toughness, bendability, microstructure, crystallography and dislocation density of a DQ steel is reported. In the case of tempering at 570 °C, the effects of having a RAQ or DQ starting condition are compared. For the composition and thermal cycles studied, it was found that a peak tempering temperature in the range of 570-600 °C resulted in a DQT steel with an optimal balance of strength, bendability and toughness, i.e. a yield strength greater than 960 MPa, a minimum usable bending radius of 2 times the sheet thickness and T28J of-50 to-75 °C depending on the test direction. Crystallographic texture, dislocation density and the distribution of carbides are important factors affecting the bendability of DQT strip. Tempering had no effect on texture, but strongly influenced the size and distribution of carbides thereby resulting in differences in bendability and impact toughness transition temperature.

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Samples were prepared by different methods, for determination by ultraviolethisi ble (UVNIS) spectrophotometry, of chromium and titanium in silica-based catalysts prepared by saturating gas-solid reactions of titanium tetrachloride and tris(acety1acetone)chromium. Chromium was determined in basic solution as chromate and titanium in acidic solution as its peroxy complex. Chromium(iii) was oxidized to chromium(v1) either in concentrated sodium hydroxide solution with hydrogen peroxide, or by heating in air at 400 "C. The latter method enabled the chromium to be leached with de-ionized water without decomposition of the silica. Depending on the reaction temperature used in the catalyst preparation, titanium was released either by decomposing the silica in concentrated sodium hydroxide solution or by leaching with dilute sulfuric acid. Comparison with chromium determinations by atomic absorption spectrometry, and titanium determinations by instrumental neutron activation analysis, demonstrated the quantitative nature of the determinations by UVNIS spectrophotometry and confirmed the efficiency of concentrated sodium hydroxide solution in bringing titanium and chromium into solution. Because the exact history of the preparation of the catalyst was known, the determinations could be simplified and the chromium and titanium species on silica be characterized by means of different sample preparations.

The direct quenching of low-carbon steels after thermomechanical processing on hot strip mills is able to produce both strong and tough coiled plate without the need for subsequent tempering. The process is energy and time efficient with relatively low emissions when compared to conventional reheating, quenching and tempering. For some applications, however, it is desirable to combine direct quenching with tempering, and, bearing in mind the form of the semi-finished product, it is of interest to study the effect of tempering whole coils in a bell furnace. Here, the effects of boron, carbon, titanium, vanadium and tempering temperature on the microstructure, crystallography and mechanical properties of direct-quenched steels has been studied with the aid of simulated bell furnace heating and cooling cycles. All steels contained (in wt.%) 0.2Si-1Mn-1Cr-0.65Mo-0.03Al, while there were two levels of C (0.095 / 0.140), V (0 / 0.08), Ti (0 / 0.025) and B (0 / 0.0015). Tempering was performed with peak temperatures at 180 and 570 °C. The paper reveals several possible alloying and processing routes to strong and tough low-C steel. Carbon controls the strength and toughness, while titanium and boron affects the grain size of coarsest grains (d 90% ), Vanadium has a strong effect on strength retention during tempering at 570 °C: an addition of 0.08 wt.% vanadium increases yield strength by 70 MPa and ultimate tensile strength by 100 MPa. The removal of boron from the steel is shown to have a huge impact not only on the microstructure but also on the impact toughness.

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