Materiálový A Metalurgický Výzkum (Czechia)

In December 2017 there was a crash of footbridge in Prague -Troja. This failure led to an increase in interest in current status of prestressed concrete stress-ribbon bridges in the Czech Republic and maximum effort to prevent another accident. During next years were identified and subsequently closed other footbridges with proven structural failure of bearing parts. The presented paper describes material analysis of prestressed concrete construction performed on one of observed footbridge. Material analyses confirmed the good material condition except of corrosion attack concentrated in the middle part of the footbridge and especially in the connections between neighbouring segments. Performed analyses revealed fatal decrease of diameter of most strands in the critical middle part of the bridge caused by corrosion enhanced by the maintenance by salting during winter and, most probably, also by two floods in the near past. Main purpose of experimental research was to identify weak spots in construction which contributes to development of corrosion of bearing cables in prestressed concrete construction.

The paper deals with the influence of physical-metallurgical factors on resistance of the X52 and X70 steels in accordance with API 5L to sulphide stress cracking. The resistance against this kind of damage is relatively clearly claimed by usually used approach in this field by the tensile strength of steel, its hardness level, respectively. However, the experimental results had shown that the microstructural parameters are also the significant factors, which affect the resistance of steels to sulphide stress cracking. It was found that the quenching and tempering can significantly increase the resistance to sulphide cracking as in the case of hydrogen induced cracking. It would be appropriate to re-evaluate the material selection process that recommends to use the steels not exceeding approved strength limit in a case of the sulfane environments where the risk of the sulphide stress cracking exists.

The paper is focused on the evaluation of the course of Cr2O3 reduction from slag to alloyed steel under laboratory conditions. The experiments were aimed at the evaluation of increase in the chromium content in the melt together with the studying the behaviour of Cr2O3 and the mechanism of reduction by means of three reducing agents. Anthracite, ferrosilicon and mixtures of anthracite and ferrosilicon belong among the selected reducing agents. The experimental melts were focused on the proposal of a theoretical calculation of the consumption of selected reducing agents, study of reduction under laboratory conditions at application of alloyed steel with content of chromium of 12.16 wt. %, temperatures of 1600 °C and 1650 °C, together with the change of amount of reducing agents and reduction time. The results indicated in the paper constitute basic information on the possibilities of Cr2O3 reduction from slag; they will be used for verification of results in the pilot plant and operation experiments which will simulate operating conditions in the electric arc furnace.