Jaroslav Sojka

Examination of hydrogen interaction in carbon steel by means of quantitative microstructural and fracture descriptions

The relations between the quantitative microstructural characteristics and the resistance of carbon steels to hydrogen-induced cracking (HIC) were studied for plates used in the oil and refinery industry. The width of the pearlitic bands and the degree of banding were considered if the testing of the resistance to HIC was performed in accordance with the NACE TM 0284 standard. The role of the degree of banding was important while that of the width of pearlitic bands was negligible. Additional hydrogen embrittlement testing of tensile specimens oriented in longitudinal and through-thickness directions revealed that hydrogen strongly increased the anisotropy of mechanical properties. These changes could be correlated with the geometric characteristics of nonmetallic inclusions (MnS) and the pearlitic bands in different metallographic sections. A quantitative description of fracture surfaces has been made by means of a profilometric method.

High temperature oxidation effects on surface roughness of Ti–4Al–2V

Abstract The excellent properties of titanium and its alloys make them a perfect choice for many applications. In this paper, the effects of thermal oxidation condition on the surface morphology and surface roughness have been studied. The results showed that the average roughness at temperatures 450 and 600°C did not change significantly. Despite the constant average roughness, skewness and kurtosis parameters have been changed considerably during the thermal oxidation. The results suggest that the valleys are preferred sites for nucleation and growth of oxides. This mechanism changes the surface morphology from a scratched surface to a bumpy one.

Long-term changes in electromotive force and microstructure of Rh-Pt thermocouples

Abstract This paper examines the decrease in the electromotive force (e.m.f.) and attendant microstructural changes in Rh-Pt thermocouples during their long-term exposure to temperatures in the interval 450–1000 °C. The decline in e.m.f. is ascribed chiefly to the formation of rhodium oxides in the surface layers of the thermocouple and consequent non-homogeneity of the rhodium concentration in the matrix in these layers. The activation energy for the e.m.f. reduction indicates that the mechanism governing this process is diffusion of oxygen from the atmosphere at locations where the periodic character of the lattice is disturbed. Reconditioning by annealing raises the e.m.f. of a thermocouple, the probable mechanism behind this process being decomposition of rhodium oxides and replenishment of the rhodium concentration in the solid solution of the surface layer.

Influence of Finish-Rolling Conditions on Microstructure and Mechanical Properties of Low-Alloy Mn-Ni-Cr-Mo Steel Grade

A physical simulation of the thermomechanical processing of the Mn-Ni-Cr-Mo low-alloy steel was performed in the laboratory rolling mill Tandem in the Institute of Modelling and Control of Forming Processes at VŠB – Technical University of Ostrava. The task was to determine the influence of the finish rolling temperature on the structural and mechanical properties of the rolled products. After different modes of rolling and slow cooling in the furnace, the final structure of the tested samples was in all cases composed of ferrite, bainite and islands of martensite. The finish rolling temperature markedly influenced a part by volume of the individual phases as well as the structure homogeneity. The results of the tensile tests at room temperature indicated that the studied steel did not show any pronounced dependence of the yield stress on the finish rolling temperature in the investigated range of values (750 – 1000 °C). On the other hand, the closely corresponding dependences of the ultimate tensile stress and elongation exhibited a considerable and very complex course, which can be explained mainly by the martensite fraction originating during the last stage of the final air cooling from temperature 600 °C.

Hydrogen Diffusion Characteristics in the TRIP 800 C-Mn-Si Steel

The presented paper is devoted to the study of hydrogen diffusion characteristics in the C-Mn-Si TRIP 800 steel. The steel was tested in three different states: in as-received state after hot and cold rolling and subsequent heat treatment; and furthermore after 5% and 10% tensile deformation. Hydrogen diffusion characteristics were studied by means of electrochemical permeation method. Two build up transients corresponding to lower and higher charging current densities as well as a decay transient were recorded during experiment. The lowest values of hydrogen diffusion coefficient (from 1 to 3.5.10-7 cm2.s-1) were observed during the first build up transient; the value of 3.5.10-7 cm2.s-1 corresponded to 10% tensile deformation. During the 2nd build up transient corresponding to the higher charging current density, hydrogen diffusion coefficients increased markedly reflecting thus the fact that hydrogen trapping was less pronounced. For decay transients hydrogen diffusion coefficients were situated between values obtained for the 1st and 2nd build up transients. In all studied states, a rather high sub-surface hydrogen concentration was observed during the 1st build up transient rising to 12.6 ppm of hydrogen in as-received state. The obtained results are explained taking into account steel microstructure and hydrogen trapping.

Defects and Particles in Laboratory Hot Rolled Steel of C-Mn-Cr-Nb Type

Susceptibility to cracking of the as-cast C-Mn-Cr-Nb steel was studied by laboratory rolling. The variable parameters were the heating temperature (1150 - 1340 °C) as well as the rolling temperature (950 - 1150 °C). Final microstructure of the free-cooled samples was constituted by bainite, pearlite and ferrite with different morphology and various contribution. Deformation temperature below 1000 °C yielded in the incomplete recrystallization of austenite. Surface cracks originated preferentially on the austenite grains boundaries. Size of the present particles (inclusions and precipitates) varied from 101 nm to 101 μm. SEM and EDS analysis revealed that the inclusions ware mostly of the MnS type. TEM analysis confirmed that the grain boundaries were not enriched by any particles. In addition to the Fe3C particles, the discoid niobium carbide particles with approximately 40 nm diameter and 10 nm thickness were detected. These small particles were not connected by any notable pinning of dislocations.

Influence of Cooling Rate on Final Properties of Laboratory Rolled Products

The objective of investigation was to determine the influence of chosen cooling rates after finish rolling on final microstructural and mechanical properties of the laboratory rolled products from steel 42CrMo4. Metallographic analysis showed that microstructure of rolled products, which were after finish rolling cooled down in accelerated manner by water sprays, was composed mostly by hardening phases and by smaller amount of ferrite. Microstructure of the rolled product cooled down in furnace in decelerated manner was formed by pearlitic blocks with minority occurrence of ferrite. Laboratory rolled products cooled down by more complicated modes, which consisted of combination of their cooling by water sprays with subsequent annealing in furnace, showed different phase morphology and distinct band structure. Results of tensile test have proved that the applied modes of cooling of rolled products by accelerated manner had only very small influence on their final mechanical properties. Considerably cooling the rolled product by decelerated manner in furnace resulted in drop yield strength, but on the other hand in an increase of ductility. More complicated modes of cooling the rolled products, which comprised also their annealing, showed the possibilities of significant influencing the strength, and particularly the plastic properties of the steel 42CrMo4.