The modification of surface of barrels by gas nitriding

Abstract

Steels which are used for manufacturing of highly stressed parts of military equipments such as barrels of small arms weapons must be modified by heat treatment due to achieve required mechanical properties. Moreover, other important properties of functional parts, especially a friction coefficient and surface parameters of bore of barrels, must be optimized due to obtain the best accuracy in target and to increase the life time of barrel [1, 2]. The technology chromium plating, very widespread in the past, has been gradually replacing by more ecological methods. This article is devoted to find a technology as a replacement of traditional chromium plating. Experiments are focused on using of optimized gas nitriding processes for surface treatment. As a result, a higher surface hardness with low influence on the dimensional accuracy is expected. Gas nitriding process was applied to steels 42CrMo4 and 32CrMoV12-10 used for barrel manufacturing. The most important parts of barrel were evaluated, especially surface parameters as a morphology then chemical composition and naturaly a microstructure of the material. The analysis of surface morphology was performed by scanning electron microscopy; chemical composition was evaluated by GDOES method and mechanical properties by using microhardness methods. The measurements of depth of diffusion layer were performed by destructive and non-destructive methods. The measurements showed the influences of chemical composition of alloying elements in core of material after chemical-heat treatment process on depth of diffusion and influence of technology on development of porosity. All fundamental mechanical properties significant for requiring function of barrel were documented [1, 2]. Nevertheless, the main task was the description of the porosity development in compound layer after gas nitriding and the increasing of surface hardness and the depth of diffusion layer according to chemical composition. Nitriding process was applied for increasing of surface hardness of material in depth and improving of mechanical properties. Mechanical properties of tested material were significantly increased.

Comparison of physical, mechanical and chemical properties of barrel steel 32CrMoV12-10 and nitriding steel 31CrMoV9 after nitriding in gas

“…The graph in Figure 4 shows the drop in hardness of the nitride layers of 31CrMoV9 and 32CrMoV12-10 steels. The steeper gradient for the hardness of the nitride layer of 32CrMoV12-10 steel between 0.135 mm and 0.185 mm is probably due to the presence of nickel, which increases the toughness of the diffusion layer at the expense of the hardness [1,5]. The fracture toughness test was carried out according to ISO 28079, which consists of measuring the cracks in the surface layer after the insertion of a quadrilateral diamond needle with a peak angle of 136° under a load of 50 kg.…”

“…50 HV was added to the observed hardness in the cores of the samples and these new hardness values were plotted in the graph in Figure 3. The intersections of thes hardness profile curves were run perpendicularly to determine the depth of the nitride layers in the nitrided samples [1,5]. The graph (Figure depth of about 0.29 mm for 32CrMoV12 sections of the metallographic cores of the steel samples with structure, where the coarser grained structure of 31CrMoV9 steel is visible.…”

Effects of Boriding, Nitrocarburizing, Nitriding and Post-oxidation Treatments on The Tribological Properties of DIN 32CrMoV12-10 Steel

“…Although there are many studies on the surface treatment of tool steels in the literature, there are limited studies on nitriding, nitrocarburizing, boron plating and chrome plating of DIN 32CrMoV12-10 steel (Pokorný et al, 2011;Pokorny, Dobrocky, & Joska, 2020).…”

Modification of Surface Structure by Diffusion Processes

“…On the top of the surface a white layer composed of ε -(Fe 2-3 N) or ε and γ´ -(Fe 4 N) is created (7,8). The second area is called a diffusion layer (9). It consists of a dispersion of nitrides and carbides in the origin microstructure.…”