Вплив режимів нітроцементації в пастах з нагрівом струмами високої частоти на структуру та механічні властивості сталі 25ХГТ

Abstract

В роботі було проведено дослідження умов протікання процесів формування покриттів при нітроцементації в пастах з нагрівом струмами високої частоти на фазовий і хімічний склад, структуру і властивості сталі 25ХГТ та порівняння даного процесу з традиційною газовою нітроцементацією. Нітроцементація в пастах з нагрівом струмами високої частоти є одним із прогресивних і економічних методів хіміко-термічної обробки. Даний процес дозволяє: істотно зменшити деформації під час нагріву і охолоджування, що досягається за рахунок жорсткості холодної серцевини; усунути окислення і зневуглецювання; зменшити витрати енергії на нагрів: шар, що нагрівається, складає невелику частину від маси деталі; використовувати більш дешеві марки сталі; забезпечити властивості осердя незалежно від властивостей твердого шару. Нітроцементація в пастах з нагрівом струмами високої частоти: підвищує мікротвердість та поліпшує механічні властивості сталі 25ХГТ особливо контактну міцність та також зносостійкість, порівняно з газовою нітроцементацією, оскільки забезпечує отримання структури з дрібнодисперсним мартенситом; скорочує процес проведення ХТО, економить електроенергію, час та кошти.

Increasing the quality and durability of industrial products is one of the most urgent problems today. Improving the quality of metal and its mechanical properties is the main way to increase the durability of parts and one of the main sources of saving steels and alloys. To solve this problem, a technologist (metallurgical engineer, mechanical engineer, technological engineer) must possess the basics of improving the quality and durability of products due to the rational choice of materials and methods of their strengthening through thermal, chemical-thermal, thermocyclic, thermomechanical processing when achieving high technical - economic efficiency. Chemical-thermal treatment is used as the final operation to give the metal or alloy such a complex of mechanical, physical and chemical properties that ensures the necessary operational characteristics of the product. The wide application of chemical and thermal treatment in mechanical engineering is due to the possibility of effectively improving the quality of the reliability of machines and the mechanisms included in them, increasing the service life and improving technological characteristics, which will lead to an improvement in the quality of work on such machines. In many cases, coatings on steels and hard alloys, obtained by single-component saturation during chemical-thermal treatment, cannot meet the requirements of modern practice. Therefore, today works on saturation with several elements are carried out using different methods: sequentially or simultaneously. Thus, it is of undeniable interest to determine the relationships between the phase, chemical composition of the coating, its structure, microhardness, crack resistance, adhesion of the coating to the base material, and wear resistance. The obtained results will allow you to correctly approach the choice of a rational type of coating and its application technology. In this master's thesis, in addition to standard methods of heat treatment, one of the progressive methods of СТT is investigated - nitrocementation in pastes using heating with high-frequency currents (HFC). This determined the relevance of the subject of the conducted research. The object of research is nitrocementing layers obtained in pastes heated by high-frequency currents on 25ХГТ steel. The subject of the research is the composition, structure, mechanical properties and characteristics of diffuse coatings based on nitrogen together with carbon after nitrocementation on 25ХГТ steel. The purpose of the work is to study the conditions of the process of coating formation during nitrocementation in pastes heated by high-frequency currents on the phase and chemical composition, structure and properties of 25ХГТ steel and to compare this process with traditional gas nitrocementation. During the period of completion of the master's qualification work, the following tasks must be completed: 1. To analyze literary sources regarding nitrocementation processes as a modern method of strengthening the surface of materials. 2. To develop the technology of nitrocementation in pastes heated by high-frequency currents for 25ХГТ steel. 3. To conduct a study of the effect of nitrocementation in pastes heated by high-frequency currents on the structure and properties of 25ХГТ steel. 4. To determine the phase composition and surface microstructure of 25ХГТ steel after nitrocementation in pastes heated by high-frequency currents. 5. To investigate the microhardness of 25ХГТ steel after nitrocementation in pastes heated by high-frequency currents. 6. To investigate the mechanical properties of the diffusion layers and the surface of 25ХГТ steel after nitrocementation in pastes heated by high-frequency currents. 7. To make a comparative analysis of the proposed process of nitrocementation in pastes heated by high-frequency currents with gas nitrocementation for 25ХГТ steel and issue recommendations for the use of nitrocementation in pastes heated by high-frequency currents. Research methods. 1. The main theoretical results of the work were obtained using modern research methods, such as theoretical analysis of the state of the problem, using available data from literary sources, as well as scientific substantiation of ways to solve the problem, theoretical and numerical studies of the effect of nitrocementation in pastes heated by high-frequency currents on the structure and properties of 25ХГТ steel. 2. Experimental research methods include: obtaining samples for research; preparation of microsands; structure detection; methods of microscopic research - macrostructures (macroanalysis) and microstructures (microanalysis) on the MIM-7 microscope; study and evaluation of the microstructure of steel 25ХГТ; determination of microhardness - device ПMT-3; tests on wear resistance during friction, other studies were carried out according to standard methods. In this work, we compare two processes of thermal and chemical-thermal treatment - gas nitrocementation and nitrocementation in pastes heated by high-frequency currents. Nitrocementation in pastes heated by high-frequency currents is one of the progressive and economical methods of chemical and thermal treatment. This process allows: to significantly reduce deformations during heating and cooling, which is achieved due to the rigidity of the cold core; eliminate oxidation and decarbonization; reduce energy consumption for heating: the heated layer is a small part of the mass of the part; use cheaper grades of steel; ensure the properties of the core regardless of the properties of the solid layer. Scientific novelty. Nitrocementation in pastes heated by high-frequency currents: - increases the microhardness and improves the mechanical properties of 25ХГТ steel, especially contact strength and wear resistance, compared to gas nitrocementing, as it provides a structure with finely dispersed martensite, - shortens the process of carrying out СТТ, saves electricity, time and money. The scientific novelty of the obtained results: 1. In the master's qualification work, a study of the conditions of the processes of coating formation during nitrocementation in pastes heated by high-frequency currents was carried out on the phase and chemical composition, structure and properties of 25ХГТ steel and a comparison of this process with traditional gas nitrocementation. 2. Nitrocementation in pastes heated by high-frequency currents is one of the progressive and economical methods of chemical and thermal processing. This process allows: to significantly reduce deformations during heating and cooling; eliminate oxidation and decarbonization; reduce energy consumption for heating: the heated layer is a small part of the mass of the part; use cheaper grades of steel; ensure the properties of the core regardless of the properties of the solid layer. 3. Unlike gas nitrocementation, nitrocementation in pastes heated by high-frequency currents is an efficient process. This is confirmed by the value of the microhardness of the nitrocemented layer. During gas nitrocementation, the microhardness of the surface is Hμ 690 kgf/mm2, and during nitrocementation in pastes heated by high-frequency currents – Hμ 780 kgf/mm2. The obtained results indicate that the nitrocemented layer after the proposed surface treatment has greater hardness, and therefore greater wear resistance, which is a very important indicator for products made of 25ХГТ steel. 4. Based on the obtained results, the optimal composition of the paste was determined: yellow blood salt 20 %; coal 70 %; barium carbonate 10 %; ethyl silicates - 5-10 %, with a paste thickness of 0.5 mm and a holding time of 1.5 minutes. A layer with a depth of 0.5 mm was obtained. 5. Nitrocementation in pastes heated by high-frequency currents is a short-term process. Therefore, heating even to a very high temperature does not lead to grain growth in steel. 6. Nitrocementation in pastes heated by high-frequency currents: increases microhardness and improves the mechanical properties of 25ХГТ steel, especially contact strength and wear resistance, compared to gas nitrocementation, as it provides a structure with finely dispersed martensite; shortens the process of carrying out СТТ, saves electricity, time and money. 7. It was established that the wear resistance of the layer after nitrocementation with heating by high-frequency currents is 20-30 % higher than that of the nitrocemented layer obtained by gas nitrocementation. Wear of the layer for steel 25ХГТ after nitrocementing in pastes with microwave heating is 25 % less than that of the nitrocemented layer.