{"title":"High Technologies in Materials Science: High-Temperature Through-Thickness Nitriding of Heat-Resistant Steel","authors":"L. G. Petrova","doi":"10.1134/S0036029525700582","DOIUrl":null,"url":null,"abstract":"<p><b>Abstract</b>—The importance of this work is caused by more severe operating conditions of high-temperature products made of heat-resistant sheet alloys, which include, in particular, austenitic chromium–nickel steels. When steel parts are loaded in an oxidizing atmosphere and aggressive media, they must have a high strength, hardness, and heat resistance along with resistance to electrochemical and gas corrosion. These properties are increased by volume and surface hardening methods, which include nitriding. The application of traditional furnace gas nitriding technologies to chromium–nickel steels encounters problems, namely, a low rate of nitrogen saturation, which significantly increases the process time, and the formation of chromium nitrides, which negatively affects corrosion and heat resistance. New nitriding technologies for high-alloyed chromium-containing steels are being developed in the field of intensifying the saturation process and regulating the phase composition of a nitrided layer to minimize the formation of chromium nitrides. The aim of this work is to determine rational technological versions and conditions of high-temperature gas nitriding of austenitic steel in order to increase the strength characteristics at room and elevated temperatures while maintaining its heat resistance. Thermodynamic modeling of the phase composition with the CALPHAD method has shown that the main measures to minimize the precipitation of chromium nitrides at a nitrided surface are an increase in the titanium concentration in steel and a decrease in the activity of the saturating gas atmosphere, which is achieved by diluting nitrogen with an inert gas. Experimental studies are carried out on 1.5-mm-thick sheet samples of Kh18N10T austenitic steel with a standard (0.5% Ti) and increased (1.0% Ti) titanium content. Experiments are performed in a laboratory facility for high-temperature nitriding (900–1200°C), and pure nitrogen and nitrogen and argon mixtures are used as saturating media. Two-stage processes consisting of nitriding in nitrogen followed by annealing in argon are also studied. Metallographic analysis has shown that, at the same nitriding temperature, the amount of chromium nitrides decreases in experimental steel with increasing titanium content and nitrogen dilution with argon decreases the temperature of chromium nitride precipitation. When studying the saturation kinetics, we determine the through nitriding time of a sheet sample under various saturation conditions and calculate the denitriding annealing time using the determined chromium nitride zone thicknesses. The precipitation hardening of internal nitriding zones with titanium nitrides is found to increase the strength characteristics of steels both at room and at elevated temperatures compared to the characteristics of 08Kh18N10T base steel after typical heat treatment, and the maximum hardening effect is achieved upon through nitriding of steel with 1.0% Ti. The versions recommended for through nitriding of a 1.5-mm-thick experimental steel sheet are as follows: <i>t</i><sub>nit</sub> = 1050°C, N<sub>2</sub>, 16 h; <i>t</i><sub>nit</sub> = 1100°C, 50% N<sub>2</sub> + 50% Ar, 22 h; and <i>t</i><sub>nit</sub> = 1100°C, N<sub>2</sub>, 5 h + <i>t</i><sub>ann</sub> = 1200°C, Ar, 9 h. The ultimate tensile strength of nitrided steel increases by 45–50% at room temperature and by 40–65% at 800°C depending on the process conditions. Through nitriding makes it possible to increase the operating temperature of steels by 100–150°C while ensuring the same long-term strength. The heat resistance at 900°C is retained at the level of non-nitrided steel after two-stage processes, which ensure the maximum removal of chromium nitrides from the surface at the annealing stage.</p>","PeriodicalId":769,"journal":{"name":"Russian Metallurgy (Metally)","volume":"2024 12","pages":"1965 - 1973"},"PeriodicalIF":0.4000,"publicationDate":"2025-05-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Russian Metallurgy (Metally)","FirstCategoryId":"5","ListUrlMain":"https://link.springer.com/article/10.1134/S0036029525700582","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q4","JCRName":"METALLURGY & METALLURGICAL ENGINEERING","Score":null,"Total":0}
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Abstract—The importance of this work is caused by more severe operating conditions of high-temperature products made of heat-resistant sheet alloys, which include, in particular, austenitic chromium–nickel steels. When steel parts are loaded in an oxidizing atmosphere and aggressive media, they must have a high strength, hardness, and heat resistance along with resistance to electrochemical and gas corrosion. These properties are increased by volume and surface hardening methods, which include nitriding. The application of traditional furnace gas nitriding technologies to chromium–nickel steels encounters problems, namely, a low rate of nitrogen saturation, which significantly increases the process time, and the formation of chromium nitrides, which negatively affects corrosion and heat resistance. New nitriding technologies for high-alloyed chromium-containing steels are being developed in the field of intensifying the saturation process and regulating the phase composition of a nitrided layer to minimize the formation of chromium nitrides. The aim of this work is to determine rational technological versions and conditions of high-temperature gas nitriding of austenitic steel in order to increase the strength characteristics at room and elevated temperatures while maintaining its heat resistance. Thermodynamic modeling of the phase composition with the CALPHAD method has shown that the main measures to minimize the precipitation of chromium nitrides at a nitrided surface are an increase in the titanium concentration in steel and a decrease in the activity of the saturating gas atmosphere, which is achieved by diluting nitrogen with an inert gas. Experimental studies are carried out on 1.5-mm-thick sheet samples of Kh18N10T austenitic steel with a standard (0.5% Ti) and increased (1.0% Ti) titanium content. Experiments are performed in a laboratory facility for high-temperature nitriding (900–1200°C), and pure nitrogen and nitrogen and argon mixtures are used as saturating media. Two-stage processes consisting of nitriding in nitrogen followed by annealing in argon are also studied. Metallographic analysis has shown that, at the same nitriding temperature, the amount of chromium nitrides decreases in experimental steel with increasing titanium content and nitrogen dilution with argon decreases the temperature of chromium nitride precipitation. When studying the saturation kinetics, we determine the through nitriding time of a sheet sample under various saturation conditions and calculate the denitriding annealing time using the determined chromium nitride zone thicknesses. The precipitation hardening of internal nitriding zones with titanium nitrides is found to increase the strength characteristics of steels both at room and at elevated temperatures compared to the characteristics of 08Kh18N10T base steel after typical heat treatment, and the maximum hardening effect is achieved upon through nitriding of steel with 1.0% Ti. The versions recommended for through nitriding of a 1.5-mm-thick experimental steel sheet are as follows: tnit = 1050°C, N2, 16 h; tnit = 1100°C, 50% N2 + 50% Ar, 22 h; and tnit = 1100°C, N2, 5 h + tann = 1200°C, Ar, 9 h. The ultimate tensile strength of nitrided steel increases by 45–50% at room temperature and by 40–65% at 800°C depending on the process conditions. Through nitriding makes it possible to increase the operating temperature of steels by 100–150°C while ensuring the same long-term strength. The heat resistance at 900°C is retained at the level of non-nitrided steel after two-stage processes, which ensure the maximum removal of chromium nitrides from the surface at the annealing stage.
期刊介绍：
Russian Metallurgy (Metally) publishes results of original experimental and theoretical research in the form of reviews and regular articles devoted to topical problems of metallurgy, physical metallurgy, and treatment of ferrous, nonferrous, rare, and other metals and alloys, intermetallic compounds, and metallic composite materials. The journal focuses on physicochemical properties of metallurgical materials (ores, slags, matters, and melts of metals and alloys); physicochemical processes (thermodynamics and kinetics of pyrometallurgical, hydrometallurgical, electrochemical, and other processes); theoretical metallurgy; metal forming; thermoplastic and thermochemical treatment; computation and experimental determination of phase diagrams and thermokinetic diagrams; mechanisms and kinetics of phase transitions in metallic materials; relations between the chemical composition, phase and structural states of materials and their physicochemical and service properties; interaction between metallic materials and external media; and effects of radiation on these materials.
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