The structure, phase composition, and residual stresses of diffusion boride layers formed by thermal-chemical treatment on the die steel surface

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 The structure, phase composition, and residual stresses of diffusion boride layers formed by thermal-chemical treatment on the die steel surface Undrakh Mishigdorzhiyn 1, 2, a, * , Nikolay Ulakhanov 2, 1, b , Aleksandr Tikhonov 3, 2, c , Pavel Gulyashinov 4, 2, d 1 Institute of Physical Material Science of the Siberian Branch of the RAS, 6 Sakhyanovoy str., Ulan-Ude, 670047, Russian Federation 2 East Siberia State University of Technology and Management, 40V Kluchevskaya str, Ulan-Ude, 670013, Russian Federation 3 Irkutsk National Research Technical University, 83 Lermontov str., Irkutsk, 664074, Russian Federation 4 Baikal Institute of Nature Management Siberian branch of the Russian Academy of sciences, 6 Sakhyanovoy str., Ulan-Ude, 670047 Russian Federation a https://orcid.org/0000-0002-7863-9045, undrakh@ipms.bscnet.ru, b https://orcid.org/0000-0002-0635-4577, nulahanov@mail.ru , c https://orcid.org/0000-0002-4917-9916, tihonovalex90@mail.ru , d https://orcid.org/0000-0001-6776-9314, gulpasha@mail.ru. Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science. 2021 vol. 23 no. 2 pp. 147–162 ISSN: 1994-6309 (print) / 2541-819X (online) DOI: 10.17212/1994-6309-2021-23.2-147-162 Obrabotka metallov - Metal Working and Material Science Journal homepage: http://journals.nstu.ru/obrabotka_metallov ARTICLE INFO Article history : Received: 18 март 2021 Revised: 05 April 2021 Accepted: 17 April 2021 Available online: 15 June 2021 Keywords : Thermal-chemical treatment (TCT) Surface layer quality Boriding Residual stresses Structural state Phase composition Die steel Funding The research was supported by a grant from the Russian Science Foundation (project 19-79-10163). Acknowledgements The authors are grateful to Svetlana Mantstova for translation the English- language section of the paper. ABSTRACT Introduction . Control and management of technological residual stresses (TRS) are among the most critical mechanical engineering technology tasks. Boriding can provide high physical and mechanical properties of machine parts and tools with minimal impact on the stress state in the surface layers. The purpose of this work is to determine the temperature modes of diffusion boriding, contributing to a favorable distribution of TRS in the surface layer of die steel 3Kh2V8F. The paper considers the results of studies on the TRS determination by the experimental method on the UDION-2 installation in diffusion layers on the studied steel surface. Boriding was carried out in containers with a powder mixture of boron carbide and sodium fluoride as an activator at a temperature of 950 °C and 1050 °C for 2 hours. The obtained samples of steels with a diffusion layer were examined using an optical microscope and a scanning electron microscope (SEM); determined the layers’ microhardness, elemental, and phase composition. The experiments resulted in the following findings: as the boriding temperature rose from 950 °C to 1050 °C, the diffusion layer’s thickness increased from 20 to 105 μm. The low-temperature mode of thermal-chemical treatment (TCT) led to the formation of iron boride Fe 2 B with a maximum boron content of 6 % and a microhardness up to 1250 HV. A high-temperature mode resulted in FeB formation with a top boron content of 11 % and a microhardness up to 1880 HV. Results and Discussions . It is found that boriding at 950 °C led to a more favorable distribution of compression TRS in the diffusion layer. However, significant TRS fluctuations in the diffusion layer and the adjacent (transitional) zone could affect the operational properties after TCT at a given temperature. An increase in the TCT temperature led to tensile TRS’s appearance in the layer’s upper zone at a depth of up to 50 μm from the surface. Despite tensile stresses on the diffusion layer surface after high-temperature TCT, the distribution of TCT is smoother than low-temperature boriding. For citation: Mishigdorzhiyn U.L., Ulakhanov N.S., Tikhonov A.G., Gulyashinov P.A. The structure, phase composition, and residual stresses of diffusion boride layers formed by thermal-chemical treatment on the die steel surface. Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science , 2021, vol. 23, no. 2, pp. 147–162. DOI: 10.17212/1994-6309-2021-23.2-147-162. (In Russian). ______ * Corresponding author Mishigdorzhiyn Undrakh L., Ph.D. (Engineering), Head of the laboratory Institute of Physical Material Science of the Siberian Branch of the RAS 6 Sakhyanovoy str., 670047, Ulan-Ude, Russian Federation Tel.: 8 (3012) 43-48-70, e-mail: undrakh@ipms.bscnet.ru