Kovove Mater. 56 2018 97–103 DOI: 10.4149/km 2018 2 97 97 Inﬂuence of alloying elements on gas nitriding process of high-stressed machine parts of weapons Z. Pokorný*, D. Dobrocký, J. Kadlec, Z. Studený 1 Department of Mechanical Engineering, University of Defence in Brno, Brno, Czech Republic Received 8 February 2018, received in revised form 25 February 2018, accepted 8 March 2018 Abstract The chemical composition of the steel is one of the primary characteristics that inﬂuence the mechanical properties of high-stressed machine parts such as barrels of small arms weapon. Heat treatment is the most important technology to reach requested mechanical properties. By a suitable combination of mechanical and tribological properties, it is possible to im- prove the surface of highly stressed parts as to improve the wear resistance of barrel surface. Thus, surface technology is next step for obtaining new properties as tribological properties or preferable mechanical properties. Due to obtain suitable microstructure and wear resistance the samples of C35, 34Cr4, 37Cr4 and 42CrMo4 were heat-treated with following chemical heat treatment by gas nitriding process, for 6 hours. This paper deals with the inﬂuence of alloying elements of structural steels on surface microhardness and depth of diﬀusion layer. The gas nitriding process caused the creation of a compound layer on the surface of the steel. This layer leads to signiﬁcant improvement in wear resistance. The concentration of alloying elements was analysed by OES methods. The technology of nitriding was applied to annealed, tempered and quenched steels. After chemical heat treatment, new surface morphology was created. The surface layers of the samples were analysed by microhardness method; surface morphology was evaluated by SEM method. Experimental part concerns structural steels with a concentration of alloying elements to 1 wt.%. In experimental part, the inﬂuence of alloying elements on the diﬀusion process in Fe-C system and the microhardness of the surface was proved. K e y w o r d s: gas nitriding, microhardness, chemical composition, diﬀusion, barrel 1. Introduction High-stressed machine parts, such as barrels, are usually made of steels, while wear and erosion have been a limiting factor in barrels performance since their invention [1, 2]. The most frequently used mate- rials for high-stressed parts as barrels or gears are CrMoNi or CrMoV steels [1, 3–5]. Currently, modi- ﬁed chemical heat treatment processes, such as gas nitriding, are widely used to enhance the properties of barrels and gears. Generally, the term gas or plasma nitriding also means a surface treatment of the mate- rial, when the use of the gas or plasma process cause gradual saturation of the surface of components by ni- trogen comprised in the gas under predetermined tem- peratures and pressures [6, 7]. Nitrides occur inside the material as a result of the nitriding process. How- *Corresponding author: tel.: +420 973 442 604; e-mail address: zdenek.pokorny@unob.cz ever, there are some exceptions regarding Cr steels. Due to the diﬀusion process during nitriding, there is an absence of chromium nitrides and carbides in the surface of the nitride layer. The absence of chromium in solid solution by precipitation of chromium nitrides and carbides leads to signiﬁcant decrease of corrosion resistance [8–10]. The process aims to achieve an en- hanced surface hardness, better wear resistance, re- duced friction coeﬃcient, increased fatigue limit and corrosion resistance. The nitrides of iron in the diﬀu- sion layer caused a low increase in surface layer mi- crohardness. The alloying elements as molybdenum, vanadium, aluminium or chromium caused improve- ment of mechanical properties. During the gas nitrid- ing process, mostly two diﬀerent nitride layers are formed. On the surface of the material, the compound layer consists of ε-Fe 2−3 N, and γ  -Fe 4 N phase should