Vol. 135 (2019) ACTA PHYSICA POLONICA A No. 2 Proceedings of XIX International Scientific Conference “New Technologies and Achievements in Metallurgy, Material Engineering, Production Engineering and Physics”, Częstochowa, Poland, June 7–8, 2018 Surface Morphology Analysis of Martensitic Stainless Steel after Different Treatments M. Gwoździk a, * , S. Kulesza b , M. Bramowicz c and Z. Bałaga a a Częstochowa University of Technology, Faculty of Production Engineering and Materials Technology, Institute of Materials Engineering, al. Armii Krajowej 19, 42-200 Częstochowa, Poland b University of Warmia and Mazury in Olsztyn, Faculty of Mathematics and Computer Science, Department of Relativistic Physics, Olsztyn, Poland c University of Warmia and Mazury in Olsztyn, Faculty of Technical Sciences, Department of Materials Technology and Machines, Olsztyn, Poland The paper presents research on fractal analysis of martensitic stainless steel used for the manufacturing of surgical instruments. Fractal analysis was performed on samples subjected to sequential processes: heat treatment, surface treatment, and sterilization. According to scanning electron microscopy images, the surface treatments end up in highly anisotropic surface texture composed of a bunch of straight, narrow ridges. Closer examination reveals that the ridges are actually chains of nearly identical spherical grains. DOI: 10.12693/APhysPolA.135.157 PACS/topics: X39Cr13, SEM, fractal analysis, surface treatment 1. Introduction Material engineering is a field of science used on a large scale [1–7], especially surface engineering [8–20]. Increas- ingly, attempts are made to modify the surface layer by various methods, and in particular nitriding is widely used [9, 10, 12–20]. In paper [8], the spray-formed Al alloy was selected for testing. Specimens were subject to surface treatment in the form of plasma nitriding with subsequent electron beam remelting. Researchers have shown that the AlN layer fitted perfectly to the surface of the electron beam remelting layer. Moreover, the met- allurgical bonding was very well improved by the elec- tron beam remelting process due to the elimination of the pores caused by nitriding. In paper [10] modifica- tion were made in Al alloy. On this alloy, the layer was prepared through the deposition of Ti film by magnetron sputtering ion plating system. After this, the plasma nitriding of titanium was used to coated on. The re- searchers have shown that the preparation of multiphase layer can remarkably improve the surface hardness and the wear rate for the multiphase layer decreases. In turn, Li et al. [9] conducted tests on martensitic steel, where the steel was treated by active screen plasma ni- triding. In the paper it was shown that the anodic ac- tive screen plasma nitriding technique was found to be able to significantly improve the corrosion properties, hardness and wear resistance of AISI 420 martensitic stainless steel. * corresponding author; e-mail: gwozdzik.monika@wip.pcz.pl 2. Material and methods X39Cr13 martensitic stainless steel was selected for testing. The material was delivered in the form of soft an- nealed sheet (1 mm thick). Specimens were subjected to heat treatment (hardening — H) and surface treatment in the form of plasma nitriding (hardening + nitriding — HN). The hardening was carried out at T = 1050 ◦ C for time t = 1200 s. The nitriding was carried out at T = 460 ◦ C and pressure p = 145 Pa for time t = 20 h. Reactive atmosphere contained 25% of molecular nitro- gen diluted with molecular hydrogen. Part of specimens was sterilized (hardening + nitriding + sterilization — HNS). The sterilization by steam was carried out in an autoclave at T = 134 ◦ C with pressure p =0.21 MPa for t =0.5 h in four cycles. Determination of characteristics of surface height vari- ability of steel samples relies on processing SEM im- ages in order to extract underlying statistical dependen- cies and non-random patterns in data series [21, 22]. Appropriate numerical routine involves multistep opera- tions and begins with calculations of the autocorrelation map R: R m,n = 1 2S 2 q N−n  k=1 N−m  l=1 (z (x + m, y + n) z (x, y)), (1) where (m, n) establishes discrete shift between original image and its lagged copy, S q — root-mean-square sur- face roughness, and N — number of scan steps along each direction. Directional inhomogeneity in surface ge- ometry can be expressed in terms of anisotropy ratio S tr , defined as the ratio of extreme lateral correlations [23]: S tr = L a1 L a2 , (2) (157)