Effect of Active Screen Plasma Nitriding on Mechanical Properties of Spheroidal Graphite Cast Iron

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. fatigue (Shimadzu, Japan, H-6) test.

Screen Furnace wall (Cathode) (Anode)

Radiation thermometer Sample

Insulator

Stage

FigureFigure 1.1.Schematic Schematic ofof devicedevice usedused forfor activeactive screenscreen plasmaplasmanitriding nitriding (ASPN) (ASPN) treatment. treatment.

In the Suga-type abrasive wear test, the abrasion wheel with SiC emery paper #600 rotated against the reciprocating sample surface under a load of 6.4 N. When the sample

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Metals 2021, 11, 412 3 of 9 In the Suga-type abrasive wear test, the abrasion wheel with SiC emery paper #600 rotated against the reciprocating sample surface under a load of 6.4 N. When the sample was reciprocated for a cycle, the abrasion wheel was rotated 0.9°, and the emery paper was reciprocated for a cycle, the abrasion wheel was rotated 0.9◦, and the emery paper was changed after one revolution of the wheel (400 cycles of reciprocating motion of the was changed after one revolution of the wheel (400 cycles of reciprocating motion of the sample) to ensure the sample surface always contacted with fresh abrasive. The amount sample) to ensure the sample surface always contacted with fresh abrasive. The amount of of wear of sample was measured by electronic scale every 200 cycles of reciprocating mo- wear of sample was measured by electronic scale every 200 cycles of reciprocating motion tion of sample. Each sample was tested for 2000 cycles to determine the amount of wear. of sample. Each sample was tested for 2000 cycles to determine the amount of wear. The The Ono-type rotary bending fatigue test was performed under a rotating speed of 3400 Ono-type rotary bending fatigue test was performed under a rotating speed of 3400 rpm. rpm. Stresses of different magnitudes were applied to the round bar sample, the number Stresses of different magnitudes were applied to the round bar sample, the number of of repetitions until the sample broke was measured, and the stress that did not break even repetitions until the sample broke was measured, and the stress that did not break even 7 whenwhen the the sample sample was was repeatedly repeatedly stressed stressed 10 107 timestimes was was defined deﬁned as as the the fatigue fatigue strength. strength. ToTo evaluate evaluate the the corrosion corrosion resistance resistance of of the the nitride nitride layer layer obtained obtained by by ASPN ASPN treatment, treatment, a a corrosioncorrosion test test was was performed performed on on the the plate plate sample sample having having the the thickest thickest nitride nitride layer. layer. The The corrosioncorrosion test test was was performed performed by by immersing immersing the the ASPN-treated ASPN-treated sample sample in in 1 1 N N hydrochloric hydrochloric acid,acid, sulfuric sulfuric acid, acid, and and nitric nitric acid acid for for 5 5 h. h.

3.3. Results Results and and Discussion Discussion FigureFigure 2 shows the X-rayX-ray diffractiondiffraction patternspatterns ofof thethe non-treatednon-treated andand ASPN-treatedASPN-treated plateplate samples. samples. Diffraction Diffraction peaks peaks of of Fe 2-32–3NN and and Fe Fe4N4N were were detected detected in in the the ASPN-treated samples.samples. Furthermore, Furthermore, with with an an increase increase in inthe the treatment treatment temperature, temperature, the theintensity intensity of the of diffractionthe diffraction peaks peaks corresponding corresponding to α to-Feα decreased-Fe decreased and and that that of Fe of2-3 FeN2–3 comparativelyN comparatively increased.increased. It is It considered is considered that thatthis thisis because is because the nitrided the nitrided layer layerbecame became thicker thicker as the astreat- the menttreatment temperature temperature increased. increased.

α-Fe Fe2-3N Fe4N graphite

843K

813K

Intensity / a.u. / Intensity 783K

non-treated

20 30 40 50 60 70 80 2θ / degree

FigureFigure 2. 2. X-rayX-ray diffraction diffraction patterns patterns of of ASPN-treated ASPN-treated plate plate samples. samples.

FigureFigure 3 shows the cross-sectionalcross-sectional SE imagesimages ofof thethe non-treatednon-treated andand ASPN-treatedASPN-treated plate samples, and Figure4 indicates the nitrogen mapping images obtained by EPMA plate samples, and Figure 4 indicates the nitrogen mapping images obtained by EPMA for for the ASPN-treated plate samples. In the ASPN-treated sample, the presence of a layer the ASPN-treated plate samples. In the ASPN-treated sample, the presence of a layer dif- different from the base material was conﬁrmed on the surface, in which nitrogen was ferent from the base material was confirmed on the surface, in which nitrogen was con- concentrated. The thickness of this layer was 1.9, 2.8, and 4.6 µm at 783, 813, and 843 K, centrated. The thickness of this layer was 1.9, 2.8, and 4.6 μm at 783, 813, and 843 K, re- respectively, indicating that the thickness increased as the treatment temperature increased. spectively, indicating that the thickness increased as the treatment temperature increased. From the X-ray diffraction pattern shown in Figure2, this layer is hypothesized to be From the X-ray diffraction pattern shown in Figure 2, this layer is hypothesized to be a a nitride layer composed of Fe2–3N and Fe4N. The observation that the intensity of the nitride layer composed of Fe2-3N and Fe4N. The observation that the intensity of the dif- diffraction peak corresponding to α-Fe decreased as the treatment temperature increased fraction peak corresponding to α-Fe decreased as the treatment temperature increased and that of Fe2–3N comparatively increased, as shown in Figure2, is believed to be due and that of Fe2-3N comparatively increased, as shown in Figure 2, is believed to be due to to the formation of a thicker nitride layer as the treatment temperature increased. The plausible reason behind the formation of a thicker nitride layer with the increase in treatment temperature is that the increase in treatment temperature increases the amount of

Metals 20212021,, 1111,, 412412 44 ofof 99

Metals 2021, 11, 412 4 of 9 thethe formationformation ofof aa thickerthicker nitridenitride layerlayer asas ththee treatmenttreatment temperaturetemperature increased.increased. TheThe plau-plau- siblesible reasonreason behindbehind thethe formationformation ofof aa thickerthicker nitride layer with the increase in treatment temperaturetemperature isis thatthat thethe increaseincrease inin treatmentreatmentt temperaturetemperature increasesincreases thethe amountamount ofof sput-sput- sputtering on the screen surface and thereby increases the deposits, which are the source of teringtering onon thethe screenscreen surfacesurface andand therebythereby incrincreaseseases thethe deposits,deposits, whichwhich areare thethe sourcesource ofof nitrogen, as well as the diffusion rate of nitrogen. Further inside the nitride layer, nitrogen nitrogen, as well as the diffusion rate of nitrogen. Further inside the nitride layer, nitrogen was concentrated along the grain boundaries and the presence of a diffusion layer, in which was concentrated along the grain boundaries and the presence of a diffusion layer, in nitride was formed along the grain boundaries, was conﬁrmed. The nitride layer of the which nitride was formed along the grain boundaries, was confirmed. The nitride layer ferritic spheroidal graphite FCD400 radical nitrided at 803 K for 10 h in a mixed gas of H of the ferritic spheroidal graphite FCD400 radical nitrided at 803 K for 10 h in a mixed gas2 µ andof H NH2 and3 is NH 2 to3 is 3 2 mto 3 [21 μm]. The[21]. treatment The treatment time time in this in studythis study was was 4 h, but4 h, thebut thicknessthe thickness of of H2 and NH3 is 2 to 3 μm [21]. The treatment time in this study was 4 h, but the thickness the nitrided layer of the samples treated at 783 and 813 K was similar to the thickness of of the nitrided layer of the samples treated at 783 and 813 K was similar to the thickness the nitrided layer of the radical nitrided sample. of the nitrided layer of the radical nitrided sample.

FigureFigure 3. 3.Cross-sectional Cross-sectional SE SE images images of of ASPN-treated ASPN-treated plate plate samples. samples.

Figure 4. BSE images and nitrogen mapping images obtained by EPMA for ASPN-treated plate FigureFigure 4. 4.BSE BSE images images and and nitrogen nitrogen mapping mapping images images obtained obtained by EPMA by EPMA for ASPN-treated for ASPN-treated plate samples.plate samples.samples. Figure5 shows the results of GDS analysis of the ASPN-treated plate samples. Concen- tration of nitrogen on the surface of the ASPN-treated sample was conﬁrmed, and nitrogen was observed to diffuse further inside as the treatment temperature increased. In all

Metals 2021, 11, 412 5 of 9

Figure 5 shows the results of GDS analysis of the ASPN-treated plate samples. Con- Metals 2021, 11, 412 5 of 9 centrationFigure of 5 nitrogenshows the on results the surface of GDS of thanalysise ASPN-treated of the ASPN-treated sample was plate confirmed, samples. and Con- ni- trogencentration was of observed nitrogen to on diffuse the surface further of insidethe ASPN-treated as the treatment sample temperature was confirmed, increased. and ni-In alltrogen ASPN-treated was observed samples, to diffuse the nitrogen further insideconcentration as the treatment dropped temperaturesharply within increased. a few μ mIn fromallASPN-treated ASPN-treated the surface. samples, Insamples, each the sample, the nitrogen nitrogen the concentration positi concentrationon where dropped the dropped nitrogen sharply sharply concentration within within a few a substan- µfewm from μm tiallyfromthe surface. thedecreased surface. In eachcoincided In each sample, sample, with the the positionthe thickness positi whereon of where the the nitride nitrogenthe nitrogen layer concentration confirmed concentration in substantially Figure substan- 3. tiallydecreased decreased coincided coincided with thewith thickness the thickness of the of nitride the nitride layer layer conﬁrmed confirmed in Figure in Figure3. 3. 7 7 6 6 5 5 4 4 3 3 2 843K N contentN (mass%) 2 813K843K N contentN (mass%) 1 783K813K 1 0 783K 0 0 10203040 0 10203040Distance from surface / μm Distance from surface / μm Figure 5. GDS analysis of ASPN-treated plate samples. FigureFigure 5. 5. GDSGDS analysis analysis of of ASPN-treated ASPN-treated plate plate samples. samples. AccordingAccording to to the the results results of of GDS GDS analysis analysis of of the the ASPN-treated ASPN-treated samples samples for for regions regions According to the results of GDS analysis of the ASPN-treated samples for regions furtherfurther inside inside the the nitride nitride layer, layer, nitrogen nitrogen diff diffusedused up up to to depths depths of of approximately approximately 180, 180, 250, 250, further inside the nitride layer, nitrogen diffused up to depths of approximately 180, 250, andand 300 300 μµmm at treatment temperaturestemperatures ofof 783,783, 813, 813, and and 843 843 K, K, respectively. respectively. From From this this result, re- and 300 μm at treatment temperatures of 783, 813, and 843 K, respectively. From this result,it was it was considered considered that that the totalthe total thickness thicknes of thes of nitridethe nitride layer layer (thickness (thickness of the of combinedthe com- binedsult,layer it layer ofwas the ofconsidered nitride the nitride and that theand the diffusionthe total diffusion thicknes layers) layers)s obtainedof theobtained nitride in thisin layer this study study(thickness was was a maximum aof maximum the com- of ofbinedapproximately approximately layer of the 300 300nitrideµ m.μm. and In In addition, addition,the diffusion itit hashas layers) beenbeen reportedobtainedreported thatin this the study total was thickness a maximum of of the the nitrideofnitride approximately layer layer was was 200–300 300 200–300 μm. μ µInm,m, addition, as as obtained obtained it has by by beenplasma plasma reported nitriding-based nitriding-based that the total treatment treatment thickness of of ferrite- ferrite-of the basednitridebased spheroidal spheroidallayer was 200–300graphite graphite μ castm, cast as iron iron obtained at at 843 843 byK K forplasma for 4 4 h h [22,23]. [nitriding-based22,23]. Thus, Thus, it it wastreatment was confirmed conﬁrmed of ferrite- that that similarbasedsimilar spheroidal thickness thickness ofgraphite of the the total total cast nitride nitride iron layerat layer 843 wa was Ks forattained attained 4 h [22,23]. by by ASPN ASPN Thus, treatment. treatment. it was confirmed The The diffusion diffusion that layersimilarlayer of of thickness the the ASPN-treated ASPN-treated of the total sample samplenitride was layer was thicker thicker was attained than than the the by diffusion diffusionASPN treatment. layer layer (150 (150 The μµm) diffusionm) of of the the radicallayerradical of nitrided nitridedthe ASPN-treated sample sample [21]. [21 ].sample was thicker than the diffusion layer (150 μm) of the radicalFigureFigure nitrided 6 shows sample the [21]. surface hardness of thethe ASPN-treated plate samples. The surface hardnesshardnessFigure increased increased 6 shows by the by ASPN surface ASPN treatment. treatment.hardness This of This thcane ASPN-treatedbe can attributed be attributed to plate the toformationsamples. the formation The of a surfacenitride of a layerhardnessnitride (a combined layer increased (a combined layer by ASPN consisting layer treatment. consisting of nitride This of andcan nitride bediffusion attributed and diffusionlayers) to the on layers)formation the surface. on theof Further-a surface.nitride more,layerFurthermore, (a the combined surface the hardness surfacelayer consisting hardness increased of increased nitrideas the treatment and as the diffusion treatment temperature layers) temperature on increased. the surface. increased. This Further- is con- This sideredmore,is considered the to surfacebe due to to behardness the due formation to increased the formation of aas thicke the oftreatmentr anitride thicker layer temperature nitride with layer increase increased. with in increase the This treatment is in con- the temperature.sideredtreatment to be temperature. due to the formation of a thicker nitride layer with increase in the treatment temperature. 800 800 ） 700 ） 700 0.98N

（ 600 0.98N

（ 600 500 500 400 400 300 300 200

Vickers microhardnessVickers / HV 200 100 Vickers microhardnessVickers / HV 100 non-treated 783K 813K 843K non-treated 783K 813K 843K FigureFigure 6. 6. SurfaceSurface hardness hardness of of ASPN-treated ASPN-treated plate plate samples. samples. Figure 6. Surface hardness of ASPN-treated plate samples. On the nitride layer of the ASPN-treated sample in Figure4, a nitrogen-enriched layer with a thickness of 1.0 to 1.5 µm can be conﬁrmed, and the nitrogen-enriched layer is a

deposition layer consisting of nitride sputtered from the active screen. It is considered Metals 2021, 11, 412 6 of 9 Metals 2021, 11, 412 6 of 9

On the nitride layer of the ASPN-treated sample in Figure 4, a nitrogen-enriched Metals 2021, 11, 412 layerOn with the a nitridethickness layer of 1.0of tothe 1.5 ASPN-treated μm can be confirmed, sample in and Figure the nitrogen-enriched4, a nitrogen-enriched layer6 of 9 layeris a deposition with a thickness layer consisting of 1.0 to 1.5of nitrideμm can sputtered be confirmed, from theand active the nitrogen-enriched screen. It is considered layer isthat a deposition by applying layer the consisting ASPN treatment, of nitride a sputtered nitride layer from was the activeformed screen. on the It surfaceis considered of the that by applying the ASPN treatment, a nitride layer was formed on the surface of the spheroidalthat by applying graphite the cast ASPN iron by treatment, nitrogen a diffusion nitride layer from wasthe deposition formed on layer the surface as in the of case the spheroidal graphite cast iron by nitrogen diffusion from the deposition layer as in the case ofspheroidal other materials graphite [5]. cast iron by nitrogen diffusion from the deposition layer as in the case of other materials [5]. of otherFigure materials 7 shows [5 ].the cross-sectional hardness of the ASPN-treated plate samples. The Figure 7 shows the cross-sectional hardness of the ASPN-treated plate samples. The hardnessFigure of 7all shows ASPN-treated the cross-sectional samples decrease hardnessd offrom the the ASPN-treated surface toward plate the samples. inside. TheThe hardness of all ASPN-treated samples decreased from the surface toward the inside. The possiblehardness reason of all ASPN-treatedis that a nitrogen-rich samples diffusio decreasedn layer from was the formed surface closer toward to the the surface inside. and The possible reason is that a nitrogen-rich diffusion layer was formed closer to the surface and thepossible nitrogen reason concentration is that a nitrogen-rich decreased toward diffusion the layer inside, was as formed indicated closer in Figure to the surface 5. Further, and theinthe the nitrogen nitrogen region concentration concentrationnear the surface, decreased decreased the hardness toward toward improved the the inside, inside, toward as as indicated indicated the inside in in Figure Figure with 5.increase5. Further, Further, in inthein the thetreatment region region near neartemperature. the the surface, surface, It is the theconsidered hardness hardness thimproved improvedat the higher toward toward the treatmentthe the inside inside temperature,with with increase increase the in in thehigherthe treatment treatment is the nitrogentemperature. temperature. concentration It It is is considered considered close thto thatat the the the surface, higher higher theresulting the treatment treatment in the temperature, temperature, formation ofthe the a higherharderhigher isdiffusion is the the nitrogen nitrogen layer. concentration concentrationThe hardness closeof close the to toASPN-treated the the surface, surface, resulting resultingsample was in in the theabout formation formation the same of of asa a harderthatharder of diffusionthe diffusion radical layer.layer. nitrided The The sampleha hardnessrdness (740 of of HV)the the ASPN-treated ASPN-treated[21]. sample sample was was about about the the same same as as thatthat of of the the radical radical nitrided nitrided sample sample (740 (740 HV) HV) [21]. [21]. 900 900 ） 800 ） 800

0.25N 700 non-treated （

0.25N 700 non-treated （ 783K 600 783K 600 813K 500 813K 843K 500 400 843K 400 300 300 200

Vickers microhardnessVickers / HV 200

Vickers microhardnessVickers / HV 100 100 0 50 100 150 200 250 300 350 0 50 100Distance 150 from surface 200 / μ 250m 300 350 Distance from surface / μm

FigureFigure 7. 7. Cross-sectionalCross-sectional hardness hardness of of ASPN-treated ASPN-treated plate plate samples. Figure 7. Cross-sectional hardness of ASPN-treated plate samples. FigureFigure 88 showsshows thethe Suga-typeSuga-type abrasiveabrasive wearwear testtest resultsresults ofof thethe ASPN-treatedASPN-treated plateplate samples.samples.Figure With With 8 shows the the ASPN ASPN the Suga-typetreatment, treatment, abrasivethe the amount amount wear of of wear weartest results decreased decreased of the and and ASPN-treated the the wear wear resistance resistance plate samples.increasedincreased With as as the thethe treatment treatmentASPN treatment, temperature temperature the amount increased. increased. of wear The The decreased possible possible andreason reason the iswear is that that resistance the the hard hard increasednitridenitride layer layer as thebecame became treatment thicker thicker temperature as as the the treatment treatment increased. temperature temperature The possible increased increased reason and and isthe the that depth depth the of ofhard the nitridehardenedhardened layer layer layer became increased, increased, thicker as as as shown shown the treatment in in Figure temperature 77.. increased and the depth of the hardened layer increased, as shown in Figure 7. 16 16

14 14

12 12

10 10 Wear mass loss / mg

Wear mass loss / mg 8 8

6 6 non-treated 783K 813K 843K non-treated 783K 813K 843K

FigureFigure 8. 8. Suga-typeSuga-type abrasive abrasive wear wear test test results results of of ASPN-treated ASPN-treated plate plate samples. samples. Figure 8. Suga-type abrasive wear test results of ASPN-treated plate samples. Figure9 shows the cross-sectional SE images of the ASPN-treated samples after immersion in acid. Severe corrosion and unevenness was observed on the surface of the non-treated sample, whereas the ASPN-treated sample, though slightly corroded, did not

exhibit unevenness found in the non-treated sample. It is considered that the formation of Metals 2021, 11, 412 7 of 9

Metals 2021, 11, 412 7 of 9

Figure 9 shows the cross-sectional SE images of the ASPN-treated samples after immersion in acid. Severe corrosion and unevenness was observed on the surface of the non- Metals 2021, 11, 412 Figure 9 shows the cross-sectional SE images of the ASPN-treated samples after7 ofim- 9 treatedmersion sample, in acid. whereas Severe corrosion the ASPN-treated and unevenness sample, was though observed slightly on corroded,the surface did of thenot non- ex- hibittreated unevenness sample, whereas found in the the ASPN-treated non-treated sample sample,. It though is considered slightly that corroded, the formation did not ex-of thehibit nitride unevenness layer on found the surface in the improved non-treated the sample corrosion. It isresistance, considered suppressing that the formation the extent of ofthethe corrosion. nitridenitride layerlayer onon thethe surfacesurface improvedimproved the the corrosion corrosion resistance, resistance, suppressing suppressing the the extent extent ofof corrosion.corrosion.

Figure 9. Cross-sectional SE images of ASPN-treated samples after immersion in 1N HCl, H2SO4,

andFigure HNO 9.3 Cross-sectional. SE images of ASPN-treated samples after immersion in 1N HCl, H2SO4, Figure 9. Cross-sectional SE images of ASPN-treated samples after immersion in 1N HCl, H2SO4, and HNO3. and HNO3. Figure 10 shows the fatigue strength of the ASPN-treated round bar samples, and FigureFigureFigure 11 shows 1010 shows showsthe SE theimagesthe fatiguefatigue of the strengthstrength fracture ofof surface thethe ASPN-treated ASPN-treated of the non-treated roundround and barbar ASPN-treated samples, samples, andand samples.FigureFigure 11 11 It shows shows was observed thethe SESE imagesimages that ASPN ofof thethe treatmen fracturefracture surfacetsurface improved ofof thethe the non-treatednon-treated fatigue strength. andand ASPN-treatedASPN-treated The fatigue strengthsamples.samples. increased ItIt waswas observedobserved as the thattreatmentthat ASPNASPN temperat treatmenttreatmenuret improved improved increased. the the From fatigue fatigue Figure strength. strength. 11, it The The was fatigue fatigue con- firmedstrengthstrength that increased increased both the as non-treatedas the the treatment treatment and temperature the temperat ASPN-treatedure increased. increased. samples From From Figureunderwent Figure 11, it brittle was11, it confirmed wasfracture con- inthatfirmed the both rotary that the both non-treatedbending the non-treated fatigue and thetest. and ASPN-treated However, the ASPN a -treatedquasi-cleavage samples samples underwent fracture underwent brittle surface fracturebrittle was fracture in con- the firmedrotaryin the bendingnearrotary the bending fatigue surface test. fatiguein the However, non-treated test. However, a quasi-cleavage sample, a quasi-cleavage whereas fracture grain surface fracture boundary was surface confirmed fracture was nearwascon- observedthefirmed surface near in in the the the ASPN-treated surface non-treated in the sample, sample. non-treated whereas This graisample, grainn boundary boundarywhereas fracture grain fracture boundary was was confirmed observed fracture to in was theoc- curASPN-treatedobserved further in inside the sample. ASPN-treated the sample This grain as sample. the boundary treatment This fracture grai temperaturen boundary was confirmed increased.fracture to was occur Accordingly, confirmed further inside to it oc-is surmisedthecur samplefurther that asinside the the starting treatment the sample point temperature as of the the treatment crack increased. moved temperature to Accordingly, the inside increased. of it the is surmised sampleAccordingly, by that ASPN theit is treatment,startingsurmised point increasingthat of the the starting crack the fatigue moved point strength. toof thethe insidecrack The ofmoved fatigue the sample to strength the byinside ASPN of theof treatment, theASPN-treated sample increasing by ASPNsam- plethetreatment, was fatigue higher increasing strength. than that The the of fatigue the ra strength. strengthdical nitrided ofThe the fatigue sample ASPN-treated strength (approximately of sample the ASPN-treated was300 higher MPa). thanItsam- is consideredthatple ofwas the higher radicalthat this than nitrided is becausethat of sample the the ra diffusion (approximatelydical nitrided layer was sample 300 thicker MPa). (approximately in It the is consideredASPN-treated 300 thatMPa). sample this It isis [21].becauseconsidered the diffusionthat this is layer because was the thicker diffusion in the layer ASPN-treated was thicker sample in the [ASPN-treated21]. sample [21]. 400 400

350 350

300 300

250 Fatigue strength / MPa strength Fatigue 250 Fatigue strength / MPa strength Fatigue

200 200 non-treated 783K 813K 843K non-treated 783K 813K 843K FigureFigure 10. 10. FatigueFatigue strength strength of of ASPN-t ASPN-treatedreated round bar samples. Figure 10. Fatigue strength of ASPN-treated round bar samples.

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Figure 11. SE images of fracture surfacesurface of ASPN-treated samples.

4. Conclusions Conclusions InIn thisthis study,study, FCD400-equivalent FCD400-equivalent spheroidal spheroidal graphite graphite cast cast iron wasiron subjectedwas subjected to ASPN to ASPNtreatment treatment and its and microstructure its microstructure and mechanical and mechanical properties properties were evaluated: were evaluated. (1) WithWith ASPN ASPN treatment, treatment, a nitride a nitride layer layer was was formed formed on the on sample the sample surface surface and a anddiffu- a siondiffusion layer layerwas formed was formed further further inside inside the nitride the nitride layer. layer. (2) TheThe thickness thickness of of the the nitride nitride layer layer increas increaseded as as the the treatment treatment temperature temperature increased. (3) TheThe hardness hardness increased increased as as the the treatment treatment temperature temperature increased. (4) TheThe wear wear resistance resistance increased increased as as th thee treatment treatment temperature temperature increased. (5) TheThe fatigue fatigue strength strength increased increased as as th thee treatment treatment temperature temperature increased. (6) ASPNASPN treatment treatment improved improved the the corrosion corrosion resistance. resistance.

Author Contributions: Conceptualization, Y.H.; data curation, K.C.; formal analysis, K.C.; investi- Author Contributions: Conceptualization, Y.H.; data curation, K.C.; formal analysis, K.C.; investiga- gation, Y.H., K.C. and T.M.; project administration, Y.H.; supervision, Y.H.; visualization, Y.H. and tion, Y.H., K.C. and T.M.; project administration, Y.H.; supervision, Y.H.; visualization, Y.H. and K.C.; K.C.; writing—original draft, Y.H. and K.C.; writing—review & editing, Y.H. All authors have read writing—original draft, Y.H. and K.C.; writing—review & editing, Y.H. All authors have read and and agreed to the published version of the manuscript. agreed to the published version of the manuscript. Funding: This research received no external funding. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Informed Consent Statement: Not applicable. Conflicts of Interest: The authors declare no conflicts of interest. Data Availability Statement: Not applicable. References Conflicts of Interest: The authors declare no conflict of interest. 1. Tošić, M.M.; Gligorijević, R. Plasma nitriding improvements of fatigue properties of nodular cast iron crankshafts. Mater. Sci. References Eng. A 1991, 140, 469–473. 1.2. Nie,Tošić,M.M.; X.; Wang, Gligorijević,R. L.; Yao, Z.C.; PlasmaZhang, nitridingL.; Cheng, improvements F. Sliding wear of fatiguebehaviour properties of electrolytic of nodular plasma cast nitrided iron crankshafts. cast iron Mater.and steel. Sci. Surf. Eng. ACoat.1991 Technol., 140, 469–473. 2005, 200 [CrossRef, 1745–1750.] 2.3. KoneNie, X.;čná, Wang, R.; Nicoletto, L.; Yao, Z.C.;G.; Majerová, Zhang, L.; V.; Cheng, Baicchi, F. P. Sliding Influence wear of behaviour nitriding on of electrolyticthe fatigue behavior plasma nitrided and fracture cast iron micromechanisms and steel. Surf. Coat.of nodular Technol. cast2005 iron., 200 Strength, 1745–1750. Mater. [ CrossRef2008, 40, ]75–78. 4. Rolinski, E.; Konieczny, A.; Sharp, G. Nature of surface changes in stamping tools of gray and ductile cast iron during gas and plasma nitrocarburizing. J. Mater. Eng. Perform. 2009, 18, 1052–1059.

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