D. Mendeleev University of Chemical Technology of Russia (Moscow, Russia):

V. A. Tsevkova, Mag. Eng., Dept. of Innovation Materials and Corrosion Protection, e-mail: vikunya_n1996@mail.ru
Yu. M. Averina, Cand. Eng., Associate Prof., Dept. of Innovation Materials and Corrosion Protection, e-mail: AverinaJM@mail.ru
E. N. Kuzin, Cand. Eng., Associate Prof., Dept. of Industrial Ecology, e-mail: e.n.kuzin@mail.ru

Bauman Moscow State Technical University (Moscow, Russia):
V. S. Boldyrev, Cand. Eng., Associate Prof., Dept. of Chemistry, e-mail: boldyrev.v.s@bmstu.ru

The paper considers the quality problems of bolted joints made of steel. A heat treatment process has been developed for bolts made of austenitic-martensitic steel grade 07XI6H6 (07Kh16N6) that have passed the hardening stage at a temperature of 1000 °C to obtain a metal structure with a given grain size (5 points). Nine samples were prepared for the study. The data on the dependence of the grain size of austenitic-martensitic steel grade 07XI6H6 (07Kh16N6) on the conditions of the heat treatment process before hardening are presented. In all samples of microsections, the microstructure of steel was investigated. The grain size was determined under a microscope at a magnification of 100 times, comparing the visible grains with the reference provided by the standard scale of grain size. Based on this, it was concluded that only one mode gives a grain of 5 points. It is recommended to switch to the use of steel with a small amount of molybdenum, which significantly increases hardenability, grinds natural metal grain, and favorably affects the operational characteristics of steel in order to obtain finer grain after the recommended heat treatment in bolts of austenitic-martensitic steel grade 07XI6H6 (07Kh16N6). In some cases, depending on the task, it is possible to use two more modes of heat treatment of the previous hardening, giving grains of 4 and 5 points. The proposed heat treatment process will allow the use of ultrasonic testing to assess the degree of wear and failure of the bolts.
The work was carried out within the framework of funding the D. Mendeleev University of Chemical Technology according to No. Kh-2020-011 project.

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