metals Article Effect of Active Screen Plasma Nitriding on Mechanical Properties of Spheroidal Graphite Cast Iron Yasuhiro Hoshiyama 1,*, Keisuke Chiba 2 and Tomoki Maruoka 3 1 Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita 564-8680, Japan 2 Graduate School of Science and Engineering, Kansai University, Suita 564-8680, Japan; [email protected] 3 Kyoto Municipal Institute of Industrial Technology and Culture, Kyoto 600-8815, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-6-6368-1121 Abstract: Spheroidal graphite cast iron is a material with a wide range of uses such as in automobile parts. By applying active screen plasma nitriding (ASPN) treatment, the use of spheroidal graphite cast iron is expanded, and it can be expected to be used under special load conditions. In this study, we evaluated the effect of ASPN treatment on the mechanical properties of spheroidal graphite cast iron. With ASPN treatment, a nitride layer was formed on the sample surface and a diffusion layer was formed further inside the nitride layer. The thickness of nitride layer increased as the treatment temperature increased. The hardness was improved by ASPN treatment. The abrasion resistance was improved by ASPN treatment, and longer treatment time resulted in higher abrasion resistance. The fatigue strength was improved by ASPN treatment, and longer treatment time resulted in higher fatigue strength. ASPN treatment also improved the corrosion resistance. Keywords: active screen; plasma nitriding; mechanical property; spheroidal graphite cast iron Citation: Hoshiyama, Y.; Chiba, K.; Maruoka, T. Effect of Active Screen Plasma Nitriding on Mechanical Properties of Spheroidal Graphite 1. Introduction Cast Iron. Metals 2021, 11, 412. Cast iron is used as a raw material for various products, such as industrial and https://doi.org/10.3390/met11030412 mechanical parts. However, depending on the application, treatments aimed at appropriate surface modification are performed to improve the wear resistance and fatigue strength of Academic Editor: Eric Hug cast iron [1–4]. One of the methods used for surface modification is plasma nitriding. In plasma Received: 30 January 2021 nitriding, an external heating device is unnecessary since the material to be treated is Accepted: 2 March 2021 heated by the collision energy of ions. The treatment time is short as it utilizes nitrogen in Published: 3 March 2021 the active plasma state. Moreover, the process is economical as there is less consumption of energy and gas, and there is no burden on the environment because toxic substances are not Publisher’s Note: MDPI stays neutral employed by utilizing abundant resources of nitrogen and hydrogen. In plasma nitriding, with regard to jurisdictional claims in the surface temperature of the material to be treated is increased, and its surface is cleaned published maps and institutional affil- by the sputtering action of a mixed gas of nitrogen and hydrogen. However, since current iations. is directly applied to the material to be treated, there is a possibility of defects in the treated material, such as edge effect, arcing, and hollow cathode discharge. Therefore, a new technique, known as active screen plasma nitriding (ASPN), has been developed [5–18]. In ASPN, the material to be treated is insulated; a mesh-like metal screen is placed around Copyright: © 2021 by the authors. the material and used as the cathode, and the furnace wall is used as the anode. A voltage Licensee MDPI, Basel, Switzerland. is applied between the cathode and anode to allow nitrogen to diffuse into the material This article is an open access article to be treated. In other words, because a glow discharge is generated between the furnace distributed under the terms and wall and the screen, rather than on the surface of the material to be treated, the constituent conditions of the Creative Commons Attribution (CC BY) license (https:// atoms of the screen and its nitrides in addition to nitrogen molecules, atoms, ions, and creativecommons.org/licenses/by/ electrons exist in the plasma formed on the screen surface. The nitrides formed on the 4.0/). screen flow through the gas stream in the furnace and reach the surface of the material to Metals 2021, 11, 412. https://doi.org/10.3390/met11030412 https://www.mdpi.com/journal/metals Metals 2021, 11, 412 2 of 9 Metals 2021, 11, 412 treated, the constituent atoms of the screen and its nitrides in addition to nitrogen mole-2 of 9 cules, atoms, ions, and electrons exist in the plasma formed on the screen surface. The nitrides formed on the screen flow through the gas stream in the furnace and reach the surface of the material to be treated, causing diffusion of nitrogen atoms into the material be treated, causing diffusion of nitrogen atoms into the material to be treated. Therefore, to be treated. Therefore, no edge effect, arcing, and hollow cathode discharge occur. no edge effect, arcing, and hollow cathode discharge occur. Attempts have been made to evaluate the hardness and wear resistance of ASPN- Attempts have been made to evaluate the hardness and wear resistance of ASPN- treated austenitic stainless steel [6,9,10]. From the analysis, it was found that the formation treated austenitic stainless steel [6,9,10]. From the analysis, it was found that the forma- of a nitride layer improves the fatigue strength of ASPN-treated austenitic stainless steel tion of a nitride layer improves the fatigue strength of ASPN-treated austenitic stainless [19,20]. Since ASPN treatment can be processed at a relatively low temperature, there is steel [19,20]. Since ASPN treatment can be processed at a relatively low temperature, there little structural change or deformation due to thermal effects, and a dense hardened layer is little structural change or deformation due to thermal effects, and a dense hardened can be obtained in a short time. In addition, ASPN treatment can form a uniform nitride layer can be obtained in a short time. In addition, ASPN treatment can form a uniform layer on the surface of the material. Spheroidal graphite cast iron is a material with a wide nitride layer on the surface of the material. Spheroidal graphite cast iron is a material range of uses, such as in automobile parts. By applying ASPN treatment, the use of sphe- with a wide range of uses, such as in automobile parts. By applying ASPN treatment, the useroidal of spheroidalgraphite cast graphite iron is castexpanded, iron is expanded,and it can be and expected it can be to expectedbe used under to be usedspecial under load specialconditions. load However, conditions. to However,date, there to have date, been there no havereports been on nothe reports application on the of applicationASPN treat- ofment ASPN in treatmentcast iron. inTherefore, cast iron. in Therefore, this study, in thiswe subjected study, we subjectedspheroidal spheroidal graphite cast graphite iron, castequivalent iron, equivalent to FCD400, to FCD400,to ASPN totreatment ASPN treatment and evaluated and evaluated the resulting the resulting structure structure and me- andchanical mechanical properties properties to ascertai ton ascertain the effect the of ASPN effect oftreatment ASPN treatment on the mechanical on the mechanical properties propertiesof cast iron. of cast iron. 2.2. MaterialsMaterials andand MethodsMethods FerriticFerritic spheroidal spheroidal graphite graphite cast iron,cast equivalentiron, equivalent to FCD400 to (Fe-3.5mass%C-3.0mass FCD400 (Fe-3.5mass%C-%Si- 0.02mass%P-0.01mass%S-0.1mass%Ti-0.1mass%Cr-0.6mass%Mn),3.0mass%Si-0.02mass%P-0.01mass%S-0.1mass%Ti-0.1mass%Cr-0.6mass%Mn), was used, andwas threeused, typesand three of plate types samples of plate (10 mmsamples× 20 (10 mm mm× 5 mm× 20 formm structure × 5 mm observation, for structure20 observation, mm × 40 mm 20 ×mm5 mm × 40 for mm wear × 5 test, mm and for 15wear mm test,× 20 and mm 15× mm5 mm × 20 for mm corrosion × 5 mm test) for and corrosionφ15 mm test) round and barϕ15 samples mm round (parallel bar samples portion (parallel length of portion 25 mm, length parallel of 25 portion mm, parallel diameter portion of 8 mm) diameter were prepared.of 8 mm) Figurewere prepared.1 shows a Figure schematic 1 shows of the a deviceschematic used of for the ASPN device treatment. used for ASPN The ASPN treat- treatmentment. The was ASPN performed treatment using was aperformed direct current using plasma a direct nitriding current device.plasma Thenitriding sample device. was placedThe sample in an insulatedwas placed state, in an and insulated a screen state, (diameter and a of screen 170 mm (diameter and height of 170 of 220mm mm) and height made ofof mesh-like220 mm) SUS304made of was mesh-lik placede aroundSUS304 it.was The placed ASPN around treatment it. wasThe performedASPN treatment under was the followingperformed conditions: under the treatmentfollowing timeconditions: of 4 h, treatment treatment temperatures time of 4 h, treatment of 783–843 temperatures K, pressure ofof 100783–843 Pa, and K, atpressure a ratio of of N 1002/H Pa,2 = and 1:1. Theat a ASPN-treatedratio of N2/H2 samples= 1:1. The were ASPN-treated evaluated by samples X-ray diffractionwere evaluated (RIGAKU, by X-ray Japan, diffraction RINT-2550V) (RIGAKU, test using Japan, CuK RINT-2550V)α radiation, test cross-sectional using CuKαSEM radi- observationation, cross-sectional (JEOL, Japan, SEM JSM-6060LV), observation electron(JEOL, Japan, probe JSM-6060LV), microanalysis electron (EPMA; probe JEOL, Japan,micro- JXA-8800),analysis (EPMA; glow discharge JEOL, Japan, spectrometry JXA-8800), (GDS; glow HORIBA,discharge Japan,spectrom GD-Profileretry (GDS; 2) HORIBA, analysis, VickersJapan, GD-Profiler microhardness 2) analysis, (Matsuzawa, Vickers Japan, microhardness MXT50) test, (Matsuzawa, Suga-typeabrasive Japan, MXT50) wear (Suga test, TestSuga-type Instruments, abrasive Japan, wear NUS-ISO-3) (Suga Test test, Instruments, and Ono-type Japan, rotary NUS-ISO-3) bending fatiguetest, and (Shimadzu, Ono-type Japan,rotary H-6)bending test.