High strength low-alloyed (HSLA) Cr-Mn-Si steels belong to a group of steels that can reach their full mechanical properties after quenching and tempering. Those properties depend both on the temperature and time of tempering. Knowing the tempering parameters, it is possible to reach the desired properties of the treated steel. Some results on investigating the Hollomon-Jaffe equation (in parametric form) application for tempering of HSLA steel, are shown in this paper. The experiments were performed in real production conditions, using a standard material. The quenching was performed at 870 °C, the heating period was always 30 min, with subsequent cooling into the oil bath. The tempering was carried out in temperature range from 480 to 680 °C, while tempering time varied from 15 min to 24 h. The degree of tempering is referred through the hardness values changing. The experimental results have shown a pretty well agreement to tempering parameters, included in Hollomon--Jaffe equation, for this kind of HSLA steel.Quenched steels, from a group of heat-treatable steels, always have high values of both hardness and strength, but pretty low values of impact properties. The residual stresses are also on a high level. So, any kind of tempering has to be done -depending of the steel type. However, the tempering [1-4] is provided in a wide temperature range (even up to Ac 1 point), and then a wide diapason of structures will appear: from the untransformed martensite (with differrent amounts of retained austenite) to the structure, which contains carbide globules into ferrite matrix.Although the tempering temperature shows a dominant influence on the structure changes, the tempering time also has a strong influence but is usually less investigated. One more reason for investigating the tempering time is a fully diffusion character of all processes which are involved in tempering processes [1,4]. In relation with the processes mentioned, many efforts are still made to find out one proper parameter, which will include both the temperature and time of tempering. Only in that way, i.e., on the equivalency of a simultaneous influence of tempering temperature

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Article Highlights• Defined technological scheme of the calcium production from Serbian limestones • The influence of operating parameters on CaCO 3 calcination was examined • The dissociation is completed in 15 min at 1200 °C • The satisfactory rate for CaO aluminothermic reduction is accomplished at 1200 °C in 2 h • The content of hardly evaporated metals in obtained Ca is very low AbstractWe present experimental investigations that define both the technological scheme of calcium production from limestone by aluminothermic process and the basic operating parameters of the particular technological phases. The limestone with high content of Mg, Na and K was used in the study. X-ray analysis reveals that the samples contain mainly calcite with small amount of dolomite. At first, the effects of temperature, time and granulometry on the calcium carbonate calcination were examined. The dissociation process was completed in 10-15 min at 1200 °C, and the dissociation rate increased with decreasing particle size down to 5 mm. Afterwards, the aluminothermic reduction process of calcium oxide was investigated. At a temperature of 1200 °C and vacuum of at least 3 kPa, the reduction process completed within 2 h. The chemical composition of calcium oxide and calcium showed increased content of magnesium oxide and alkaline oxides (especially sodium).Pure calcium is a bright silvery-white metal, extremely soft and ductile. The metal oxidizes rapidly in the presence of moisture or in a dry air at temperature above 300 °C [1,2]. Calcium reacts readily with water, forming hydrated lime (calcium hydroxide) and hydrogen. It melts at 845 °C, boils at 1420 °C, and can be purified by distillation in an inert atmosphere or in a vacuum.Owing to its chemical reactivity with oxygen, calcium never occurs naturally in the free state although the compounds of the element are widely distributed among geological materials. The main calcium-bearing minerals are: the three carbonates -calcite, ara-

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