Article Effect of Carbon Content on the Properties of -Based Powder Metallurgical Parts Produced by the Surface Rolling Process

Yan Zhao 1,2, Di Chen 3,4, Dekai Li 3,4, Jingguang Peng 1,2,3,4,* and Biao Yan 1,2,* 1 School of and Engineering, Tongji University, Shanghai 201804, ; [email protected] 2 Shanghai Key Laboratory for R&D and Application of Metallic Functional Materials, Shanghai 201804, China 3 Shanghai Automotive Co. Ltd., Shanghai 201908, China; [email protected] (D.C.); [email protected] (D.L.) 4 Shanghai Powder Metallurgy Automobile Material Engineering Research Center, Shanghai 201908, China * Correspondence: [email protected] (J.P.); [email protected] (B.Y.); Tel.: +86-159-2124-5327 (J.P.); +86-21-6958-2007 (B.Y.)

Received: 8 November 2017; Accepted: 24 January 2018; Published: 26 January 2018

Abstract: In recent years, the rolling densification process has become increasingly widely used to strengthen powder metallurgy parts. The original composition of the rolled powder metallurgy blank has a significant effect on the rolling densification technology. The present work investigated the effects of different carbon contents (0 wt. %, 0.2 wt. %, 0.45 wt. %, and 0.8 wt. %) on the rolling densification. The selection of the raw materials in the surface rolling densification process was analyzed based on the pore condition, structure, , and friction performance of the materials. The results show that the 0.8 wt. % carbon content of the surface rolling material can effectively improve the properties of iron-based powder metallurgy parts. The samples with 0.8 wt. % carbon have the highest surface hardness (340 HV0.1) and the lowest surface friction coefficient (0.35). Even if the dense layer depth is 1.13 mm, which is thinner than other samples with low carbon content, it also meets the requirements for powder metallurgy parts such as gears used in the auto industry.

Keywords: powder metallurgy; rolling; surface densification; carbon content

1. Introduction Powder metallurgy (PM) parts have become increasingly widely used in the auto industry [1,2]. Compared with forging, the advantages of the PM process are that it is suitable for complexly shaped parts, the raw material utilization rate is high, and the production cost is reduced [3,4]. However, the inherent in traditional single pressed and sintered products is one of the major factors causing reduction in the mechanical properties of PM parts [5,6]. Although new advances in powder metallurgy such as hot isostatic pressing (HIP), selective laser melting (SLM), and high-velocity compaction technology are used to manufacture high-performance components with homogeneous high , the facilities required for these processes are more specialized and the production costs are higher compared with those of the surface rolling process [6–8]. Therefore, in order to expand the application field of PM parts, surface densification is a cost-effective and highly precise method enabling the surface of parts to be densified, while still keeping the internal porous state. Surface densification can greatly enhance the mechanical properties and surface properties of PM parts while retaining the advantages of powder metallurgy; specifically, maintaining the light weight and damping capacity of PM parts [6–10]. Rolling densification belongs to the secondary processing process in the PM industry [11]. In the rolling process, the dense layer is obtained by deformation on the surface of the sintered

Metals 2018, 8, 91; doi:10.3390/met8020091 www.mdpi.com/journal/metals Metals 2018, 8, 91 2 of 11 blank [12]. This surface densification process takes only 8–15 s for each blank; therefore, the efficiency is significantly higher than other densification methods [13]. The original powder chemical composition affects the structure and mechanical properties of the sintered blanks and then has an impact on the effect of surface rolling. Because the original powder composition is complex, the roles of different elements will be mutually influenced. Consequently, this work studied the effect of single element carbon on surface rolling densification. Carbon exists in mainly in the form of and . The of decreases rapidly with an increasing proportion of cementite and graphite. It is noteworthy that the orientation of lamellae to the anisotropic mechanical properties in the materials, and the rolling process will result in a highly controllable change in the direction of the carbide lamellae [12,14]. Meanwhile, the finer carbide lamellae can be beneficial for wear resistance. Consequently, the effects of carbon content are a priority research area for surface rolling in this work. The carbon content added to the sintered steel was generally between 0.2 wt. % and 1.0 wt. % in the previous published literature, and the form was graphite. However, the previous literature mostly focused on comparing the properties of rolling parts with those of wrought parts as well as researching rolling process parameters such as rolling speed and rolling pressure. For the material used in the surface rolling process, the previous literature indicated that the rolling process requires a soft material with sufficient core hardness in order to allow the densification of the surface region while macro deformation is prevented [15]. However, there is no specific experiment or research to support this assumption and define an appropriate carbon content in surface rolling sintered materials. Therefore, this paper investigates the influence of the gradient of carbon content on the densification of surface rolling in order to provide evidence for the selection of surface rolling PM materials. In PM sintered steel, there is a threshold value of 1.0 wt. % for the carbon content. When the carbon content is higher than 1.0 wt. % the tensile strength of the parts will decrease, and deformation is more likely to occur in the material. In addition, the alloying elements will also be segregated, leading to a reduction in the cutting performance and plastic properties of the material. Therefore, in this paper, a range of 0~0.8 wt. % carbon was chosen to investigate the effect of carbon content on the performance of surface densification. Results obtained from this work will enrich the basic knowledge of PM surface rolling technology and provide a reference for the selection of surface densification materials.

2. Materials and Methods

2.1. Preparation of PM Components The raw material of the surface-densified samples was Höganäs powder. The steel powder was pre-alloyed powder with 0.85 wt. % Mo mixed with 1.40 wt. % Cu and different contents of graphite (0 C, 0.2 C, 0.45 C, and 0.8 C) in the -mix way. Table1 below shows the chemical composition of the PM materials, which have the same content of Cu and Mo and only differ in the carbon content. The element Cu is used for improving the strength of the PM parts. The element Mo is used for enhancing the of PM parts, as they undergo a high-frequency quenching process after rolling. The size of these powders is about 150 µm. According to our production experience, the parts with 1.0 wt. % carbon content are susceptible to after sintering as a great amount of cementite is generated in them. So, the values of carbon content up to 1.0 wt. % were not studied in this research.

Table 1. Chemical composition ( %) of powder metallurgy (PM) material.

Specimen Astaloy85Mo Graphite Cu 0 C - 1.4 0.2 C 0.2 1.4 Bal. 0.45 C 0.45 1.4 0.8 C 0.8 1.4 Metals 2018, 8, 91 3 of 11

TheMetals blanks 2018, 8 were, x FOR pressedPEER REVIEW on a mechanical press under a pressure of 650~700 MPa. The3 of 11 sintering process was performed via a single normal-temperature sintering (1393 K × 30 min) route in a continuousThe furnace blanks under were pressed an ammonia on a mechanical dissolving press .under a pressure Ammonia of 650~700 MPa. dissolving The sintering is used as a Metalsprocess 2018 was, 8, x FORperformed PEER REVIEW via a single normal-temperature sintering (1393 K × 30 min) route3 inof 11a protectivecontinuous atmosphere furnace to under protect an ammonia the parts dissolving from oxidation atmosphere. during Ammonia the sintering dissolving process. is used as It doesa not have anyprotective chemicalThe blanks atmosphere reaction were pressed to with protect on the athe mechanical parts. parts from The press ox carbonidation under potential aduring pressure the wasof sintering 650~700 controlled process. MPa. The byIt sinteringdoes the not protective ammoniaprocesshave dissolving any was chemical performed atmosphere reaction via witha sosingle thatthe parts.normal-temperature the carbon The carbon content potential sintering of the was blank(1393 controlled K was × 30 maintained by min) the routeprotective atin thea same level ascontinuousammonia the original dissolving furnace ones. underatmosphere All of an the ammonia part so that thedissolving carbon were contentatmosphere. well of controlled the Ammoniablank was by dissolving compacting.maintained is at used the After same as sintering,a the densitiesprotectivelevel as of the greenatmosphere original parts ones. to with All protect of each the the part of parts densities the from different wereoxidation well carbon controlledduring contents the by sintering compacting. were process. measured After It sintering, does based not on the havethe densities any chemical of green reaction parts withwith each the parts.of the Thediff erentcarbon carbon potential contents was werecontrolled measured by the based protective on the Archimedes drainage method, with the values fluctuating between 7.08 and 7.12 g/cm3. The average ammoniaArchimedes dissolving drainage atmosphere method, with so thatthe valu the carbones fluctuating content between of the blank 7.08 andwas 7.12maintained g/cm3. The at the average same 3 densityleveldensity of the as theof sintered theoriginal sintered samples ones. samples All of was the was part then then densities calculated calculated were wellat about controlled 7.10 7.10 g/cm by g/cm compacting.3. Sample. Sample photographs After photographs sintering, are are shown intheshown Figure densities in 1Figure, and of green the1, and dimensions parts the withdimensions each are of outer are the outer diff diametererent diameter carbon (OD) (OD) contents = 25= 25 mm, mm,were inner inner measured diameter based (ID) (ID)on = the 15 = 15 mm, thicknessArchimedesmm, = 8thickness mm. drainage = 8 mm. method, with the values fluctuating between 7.08 and 7.12 g/cm3. The average density of the sintered samples was then calculated at about 7.10 g/cm3. Sample photographs are shown in Figure 1, and the dimensions are outer diameter (OD) = 25 mm, inner diameter (ID) = 15 mm, thickness = 8 mm.

FigureFigure 1. Photographs 1. Photographs of of samples samples and and the the cutting cutting after after rolling. rolling. 2.2. Surface Rolling Apparatus and Methods 2.2. Surface Rolling Apparatus and Methods The rolling densificationFigure 1. Photographsprocess was of carried samples out and on the a cuttingFLEX afterM20 rolling.HCN rolling machine from TheEscofier, rolling Chalon-sur-Saône, densification process France, as was shown carried in Figu outre 2. on During a FLEX the rolling M20HCN process, rolling two roller machine dies from 2.2. Surface Rolling Apparatus and Methods Escofier,were Chalon-sur-Sa spinning in theône, same France, direction as shownand at the in sa Figureme speed.2. During Through the moving rolling both process, of the roller two dies roller dies with Thepressure rolling on densification the components, process the was surface carried area out was on densified. a FLEX M20 During HCN the rolling process, machine the rolling from were spinning in the same direction and at the same speed. Through moving both of the roller dies Escofier,speed was Chalon-sur-Saône, 30 r/min and the France,feeding asspeed shown of thein Figu rollerre dies2. During was 0.6 the mm/s. rolling After process, reaching two rollerthe preset dies with pressureweredistance, spinning on the the roller in components, the dies same stopped direction the the surface feed and atat 0.8 areathe mm sa wasme an speed.d densified. kept theThrough distance During moving for the1 s.both process,These of theparameters theroller rolling dies of speed was 30 r/minwiththe rolling pressure and process the on feeding the have components, been speed studied of the in surface rollerour previous diesarea waswas research 0.6densified. mm/s. [16], andDuring After it was the reaching found process, that the thewith preset rolling these distance, the rollerspeedparameters dies was stopped 30 we r/min can the produceand feed the at feedingPM 0.8 parts mm speed with and of a kept surfthe rollace the erdense-layer distance dies was for0.6 deeper mm/s. 1 s. Thesethan After 0.3 parametersreaching mm and the a density preset of the rolling processdistance,higher have beenthan the 7.5 studiedroller g/cm dies3. This in stopped our meets previous the the feed performance at research 0.8 mm usean [16d requirements. ],kept and the it distance was found for 1 thats. These with parameters these parameters of we canthe produce rolling PMprocess parts have with been a studied surface in dense-layer our previous research deeper [16], than and 0.3 it mm was andfound a that density with these higher than parameters we can produce PM parts with a surface dense-layer deeper than 0.3 mm and a density 7.5 g/cm3. This meets the performance use requirements. higher than 7.5 g/cm3. This meets the performance use requirements.

Figure 2. Schematic view of the experimental apparatus.

Figure 2. Schematic view of the experimental apparatus. Metals 2018, 8, 91 4 of 11 Metals 2018, 8, x FOR PEER REVIEW 4 of 11

In the presentpresent work,work, thethe porespores inin thethe materialmaterial werewere observedobserved byby aa metallographicmetallographic microscopemicroscope (Olympus, bx51, Tokyo, Japan),Japan), andand thethe depthdepth andand distributiondistribution of the dense layer were analyzed by image processingprocessing software. software. The The hardness hardness distribution distribu ontion the on cross-section the cross-section of the sample of the was sample measured was bymeasured a micro by Vickers a micro hardness Vickers testerhardness (DHV-1000 tester (DHV-1000 Shanghai, Shanghai, Shanghai, Shanghai, China) with China) a test with load a oftest 0.98 load N andof 0.98 an indentationN and an indentation of 10 s. Each of hardness 10 s. Each point hardness on the distributionpoint on the figure distribution is the average figure hardnessis the average value ofhardness five different value indentationsof five different at the indentations same distance at the from same the surface.distance The from roughness the surface. detector The roughness (Mitutoyo, SJ-410,detector Kawasaki, (Mitutoyo, Japan) SJ-410, detected Kawasa theki, roughness Japan) detected level of the the roughness sample surface level of after the rolling. sample The surface structure after androlling. morphology The structure of the and samples morphology were of examined the samp byles SEMwere (Camexamined Scan, by Apollo300, SEM (CamCambridge, Scan, Apollo300, UK). TheCambridge, deformation UK). and The frictional deformation properties and frictional of the microspheres properties of were the microspheres tested by nano-scratch were tested experiments by nano- (CSM,scratch NST, experiments Peseux, Switzerland).(CSM, NST, Peseux, Switzerland). 3. Results 3. Results 3.1. Surface-Densified Layer Properties 3.1. Surface-Densified Layer Properties The porosity distribution on the cross-section in the surface layer is shown in Figure3. It can be seenThe that porosity as the carbon distribution content on increases, the cross-section the overall in levelthe surface of porosity layer in is theshown densified in Figure layer 3. increases. It can be Itseen is obvious that as the that carbon the porosity content of increases, 0 C is the the lowest overal ofl alllevel the of specimens. porosity in The the porositydensified of layer 0 C fluctuatesincreases. lessIt is thanobvious 2% inthat a large the porosity region (0–1.2 of 0 C mm is the from lowest the surface). of all the Therefore, specimens. the The dense porosity layer depthof 0 C (defined fluctuates as porosityless than <3.84%2% in a[ 13large]) of region the 0 C(0–1.2 samples mm isfrom the thickestthe surface). in this Therefore, work. The the porosity dense layer of 0.2 depth C is obviously (defined higheras porosity overall <3.84% than [13]) that ofof 0the C in0 theC samples surface layer,is the andthickest it has in some this work. fluctuations The porosity after 0.7 of mm 0.2 withC is aobviously mostly upward higher overall trend. Thethan porosity that of 0 distributions C in the surface of 0.45 layer, C and and 0.8 it has C show some large fluctuations fluctuations after and 0.7 instabilitymm with ina themostly dense upward layer region. trend. TheThe porosity porosity declines distributions first and of then 0.45 rises C and from 0.8 the C surface show tolarge the centerfluctuations in the and 0.45 instability C and 0.8 Cin samples. the dense layer region. The porosity declines first and then rises from the surface to the center in the 0.45 C and 0.8 C samples.

Figure 3. Porosity distribution on the cross-section in the surface layer.

In this work, the variation of carbon content has little effect on the theoretical full density of the 3 PM material. Therefore, the theoreticaltheoretical full densities are aroundaround 7.87.8 g/cmg/cm 3 inin 0 0 C, C, 0.2 0.2 C, 0.45 C, and 3 0.8 C specimens. The densificationdensification effecteffect isis obtainedobtained whenwhen thethe surfacesurface densitydensity isis largerlarger thanthan7.5 7.5 g/cmg/cm3,, i.e., the porosity isis less than 3.84%. Figure 4 4 shows shows thethe depthdepth ofof thethe densifieddensified layerlayer ofof allall samples,samples, andand indicates that the densifieddensified layer depth of 0 C isis 22 mm,mm, whichwhich isis thethe thickest.thickest. As the carbon content increases, thethe depthsdepths ofof 0.20.2 C,C, 0.450.45 C,C, andand 0.80.8 CC areare 1.361.36 mm,mm, 1.321.32 mm,mm, andand 1.131.13 mm,mm, respectively.respectively. Metals 2018, 8, 91 5 of 11 Metals 2018, 8, x FOR PEER REVIEW 5 of 11 Metals 2018, 8, x FOR PEER REVIEW 5 of 11

FigureFigure 4.4. ComparisonComparison ofof depthdepth ofof thethe densifieddensified layer.layer. Figure 4. Comparison of depth of the densified layer. 3.2. Micro-Hardness Analysis 3.2. Micro-Hardness Analysis Figure 5 shows the microhardness distributions on the cross-section in the surface layer with Figure5 5 shows shows thethe microhardnessmicrohardness distributionsdistributions onon thethe cross-sectioncross-section inin thethe surfacesurface layerlayer withwith different carbon contents. The hardness values at a distance of 8 mm from the surface can represent different carboncarbon contents.contents. The The hardness hardness values values at at a distancea distance of of 8 mm8 mm from from the the surface surface can can represent represent the the hardness level of the matrix of the specimens. In Figure 5, it is shown that the hardness of the hardnessthe hardness level level of the ofmatrix the matrix of the of specimens. the specimens. In Figure In Figure5, it is 5, shown it is shown that the that hardness the hardness of the matrix of the matrix increases with the increase of the carbon content. At the same time, the hardness of the increasesmatrix increases with the with increase the increase of the carbon of the content. carbon content. At the same At the time, same the time, hardness the ofhardness the densified of the densified areas of 0 C and 0.2 C are nearly the same, indicating that the same rolling condition can areasdensified of 0 Careas and of 0.2 0 CC areand nearly 0.2 C theare same,nearly indicating the same, that indicating the same that rolling the same condition rolling can condition increase can the increase the hardness of the densified layer in 0 C and obtain the same level of densification with 0.2 hardnessincrease the of thehardness densified of the layer densified in 0 C and layer obtain in 0 C the and same obtain level the of same densification level of densification with 0.2 C. When with the0.2 C. When the carbon content of the samples increases to 0.45 wt. % and 0.8 wt. %, it shows higher carbonC. When content the carbon of the content samples of increases the samples to 0.45 increases wt. % and to 0.45 0.8 wt.wt. %,% itand shows 0.8 wt. higher %, it hardness shows higher in the hardness in the densified layer because the hardness of the matrix has reached a higher level. The densifiedhardness layerin the because densified the layer hardness beca ofuse the the matrix hardness has reachedof the matrix a higher has level. reached The highesta higher hardness level. The is 340highest HV hardnessin the densified is 340 HV layer0.1 in of the 0.8 densified C. layer of 0.8 C. highest0.1 hardness is 340 HV0.1 in the densified layer of 0.8 C.

Figure 5. Micro-hardness distributions on the cross-section in the surface layer. Figure 5. Micro-hardness distributions on the cross-section in the surface layer. 3.3. Roughness Analysis 3.3. Roughness Analysis Sven Bengtsson et al. showed that the surface roughness of surface-densified parts is better than Sven Bengtsson et al. showed that the surface roughnessroughness of surface-densifiedsurface-densified parts is better than that of shaved parts [17]. Figure 6 shows the comparison of tooth flank roughness in 0 C, 0.2 C, 0.45 thatthat ofof shavedshaved partsparts [[17].17]. FigureFigure6 6shows shows the the comparison comparison of of tooth tooth flank flank roughness roughness in 0in C, 0 0., 0.2 C, 0.45C, 0.45 C, C, and 0.8 C specimens. It can be seen that the roughness levels of 0.45 C and 0.8 C reach 0.5 μm, andC, and 0.8 C0.8 specimens. C specimens. It can It becan seen be thatseen thethat roughness the roughness levels oflevels 0.45 of C and0.45 0.8C and C reach 0.8 0.5C reachµm, which0.5 μm, is which is only half that of the 0 C and 0.2 C samples. onlywhich half is only that ofhalf the that 0 C of and the 0.2 0 C C and samples. 0.2 C samples. Metals 2018, 8, 91 6 of 11 Metals 2018, 8, x FOR PEER REVIEW 6 of 11 Metals 2018, 8, x FOR PEER REVIEW 6 of 11

Figure 6.6. Comparison of tooth flankflank roughnessroughness inin materialsmaterials withwith differentdifferent carboncarbon content.content. Figure 6. Comparison of tooth flank roughness in materials with different carbon content. 3.4.3.4. MetallographsMetallographs andand SEMSEM AnalysisAnalysis 3.4. Metallographs and SEM Analysis FigureFigure7 7 shows shows the the metallographs metallographs of of the the densified densified layer layer of of the the specimens. specimens. Figure Figure8 8shows shows the the SEMSEMFigure imagesimages 7 ofof shows thethe densifieddensified the metallographs layerlayer ofof thethe of specimens.specimens. the densifie Fromd layer FigureFigure of 7 the7a,a, it specimens.it can can be be seen seen Figure that that the the8 shows structure structure the ofSEMof 00 CC images isis fullyfully of ferrite.ferrite. the densified TheThe blackblack layer regionregion of the isis specimens. thethe porespores leftleft From byby copperFigurecopper 7a, meltingmelting it can duringbeduring seen sintering, sintering,that the structure andand thethe reasonofreason 0 C is forfor fully thethe formationformationferrite. The ofof black darkerdarker region areasareas inisin thethe ferriteporesferrite left grainsgrains by iscopperis the melting solution during of . sintering, Comparing and the betweenreasonbetween for FigureFigure the formation7 7b–d,b–d, itit is ofis noted darkernoted that thatareas lamellar lamellar in the ferrite pearlite grains appears appears is the in solidin the the solution material material of with with copper. the the increaseComparing increase of of carbonbetweencarbon content. Figure The7b–d, The structure structure it is noted of of 0.2 that 0.2 C Candlamellar and 0.45 0.45 Cpearlite is C ferrite is ferrite appears + pearlite + pearlite in the and material and the structure the structurewith theof 0.8 increase of C 0.8 is fully C of is fullycarbonpearlite. pearlite. content. The structure of 0.2 C and 0.45 C is ferrite + pearlite and the structure of 0.8 C is fully pearlite.

Figure 7. Metallographs of the densified layer in specimens of (a) 0 C; (b) 0.2 C; (c) 0.45 C; and Figure(d) 0.8 7.C.7. MetallographsMetallographs of of the the densified densified layer layer in specimens in specimens of (a )of 0 C;(a) ( b0) 0.2C; C;(b) ( c0.2) 0.45 C; C;(c) and 0.45 (d C;) 0.8 and C. (d) 0.8 C. ThereThere isis nono carbidecarbide inin thethe 00 CC specimensspecimens inin FigureFigure8 a.8a. When When the the carbon carbon content content increases, increases, the the proportionproportionThere is ofof no pearlite carbide gradually in the 0 increases, C specimens as as shown shown in Figu in inre Figures Figure 8a. When8 8b–d.b–d. the ComparingComparing carbon content FigureFigures increases,8b–d, 8b–d, it it can canthe beproportionbe observedobserved of thatthat pearlite thethe carbidecarbide gradually layerlayer increases, isis finerfiner inasin theshownthe specimensspecimens in Figures withwith 8b–d. higherhigher Comparing carboncarbon content.content. Figures The The8b–d, carbidecarbide it can belamella observed has beenthat the formed carbide with layer a regular is finer directioin the specimensnal arrangement with higher and carbonthe interlamellar content. The spacing carbide is lamellaapproximately has been 500 formed nm on withaverage a regular in Figure directio 8d. nal arrangement and the interlamellar spacing is approximately 500 nm on average in Figure 8d. Metals 2018, 8, 91 7 of 11 lamella has been formed with a regular directional arrangement and the interlamellar spacing is approximatelyMetals 2018, 8, x FOR 500 PEER nm REVIEW on average in Figure8d. 7 of 11 As can be seen from the SEM images of these four dense layers, the pores are compressed and As can be seen from the SEM images of these four dense layers, the pores are compressed and closed under the rolling process, forming tiny elongated holes and/or triangular holes. The 0.2 C closed under the rolling process, forming tiny elongated holes and/or triangular holes. The 0.2 C sample has the most apparent boundaries between pearlite and ferrite. The microstructures of 0 C and sample has the most apparent boundaries between pearlite and ferrite. The microstructures of 0 C 0.8 C are uniform compared with those of 0.2 C and 0.45 C. and 0.8 C are uniform compared with those of 0.2 C and 0.45 C.

Figure 8.8. SEMSEM images images of of the the densified densified layer layer in specimens in specimens of (a )of 0 C;(a) ( b0) 0.2C; ( C;b) ( c0.2) 0.45 C; C;(c) and 0.45 (d C;) 0.8 and C. (d) 0.8 C. 3.5. Friction Performance Analysis 3.5. Friction Performance Analysis In order to investigate the effect of carbon content on the wear resistance of the PM materials, we useIn order nano-scratch to investigate experiments the effect to characterizeof carbon content the densified on the wear layer resistance area of 0 of C, the 0.45 PM C, andmaterials, 0.8 C specimens.we use nano-scratch The tests experiments use constant to loading characterize mode the and densified the scratch layer load area is 10of mN.0 C, The0.45 radiusC, and of0.8 the C diamondspecimens. ball The tip tests is 2 µ usem. Theconstant scratch loading speed ismode 0.6 mm/min. and the scratch The scratch load lengthis 10 mN. is 0.5 The mm. radius The scratch of the processdiamond can ball be tip divided is 2 μ intom. The three scratch stages. speed In the is first0.6 mm/min. stage, the The sample scratch is pre-scratched length is 0.5 undermm. The a scanning scratch loadprocess of 0.5can mN. be Thedivided surface into morphology three stages. of In the the samples first stage, can be the obtained sample after is pre-scratched pre-scratching. under In the a secondscanning stage, load the of 0.5 constant mN. The scratch surface load morphology is used to scratch of the throughsamples thecan surface be obtained of the after samples pre-scratching. and obtain theIn the transverse second force–displacementstage, the constant curve.scratch According load is used to this to scratch curve, thethrough relationship the surface between of the friction samples and displacementand obtain the can transverse be calculated. force–displacement In the final stage, curv thee. instrument According uses to thethis scanning curve, the load relationship to measure thebetween change friction of residual and depthdisplacement with displacement. can be calculated. In the final stage, the instrument uses the scanningFigure load9 shows to measure the variation the change of penetrationof residual depth depth with with displacement. scratch length in different specimens. WithFigure the increase 9 shows of carbonthe variation content, of thepenetration average penetrationdepth with depthscratch in length the densified in different layer specimens. decreases, indicatingWith the increase that the of addition carbon content, of carbon the improves average thepenetration hardness depth of the in material, the densified and thelayer deformation decreases, resistanceindicatingability that the is enhanced.addition of Therefore, carbon improves the wear resistancethe hardness is improved. of the material, The penetration and the deformation depth curve inresistance 0 C is smoother ability is than enhanced. others, Therefore, and the peaks the we in thear resistance curves of 0.45is improved. C and 0.8 The C represent penetration the depth pores whichcurve in formed 0 C is on smoother the cross-section than others, of the and materials. the peaks in the curves of 0.45 C and 0.8 C represent the pores which formed on the cross-section of the materials. Metals 2018, 8, 91 8 of 11 Metals 2018, 8, x FOR PEER REVIEW 8 of 11 Metals 2018, 8, x FOR PEER REVIEW 8 of 11

Figure 9. Variation of penetration depth with scratch length. Figure 9. Variation of penetration depth with scratch length. Figure 10 shows variation of the friction coefficient with scratch length in different specimens. Figure 10 shows variation of the friction coefficient with scratch length in different specimens. The friction coefficient in 0 C shows the declining trend from the surface to the center. On the The frictionFigure coefficient10 shows variation in 0 C shows of the the friction declining coeffici trendent from with the scratch surface length to the in center. different On the specimens. contrary, contrary, the friction coefficient of 0.45 C falls first and then rises starting from 0.25 mm, while the theThe frictionfriction coefficientcoefficient of in 0.45 0 C C shows falls first the anddeclinin theng risestrend starting from the from surface 0.25 mm, to the while center. the frictionOn the friction coefficient of 0.8 C shows an entirely rising trend in the densified layer. It can be seen from coefficientcontrary, the of 0.8friction C shows coefficient an entirely of 0.45 rising C falls trend first in theanddensified then rises layer. starting It can from be seen0.25 frommm, while Figure the 10 Figure 10 that the overall level of the friction coefficient of the sample density zone decreases with thatfriction the coefficient overall level of of0.8 the C frictionshows an coefficient entirely ofrising the sampletrend in density the densified zone decreases layer. It can with be the seen increase from the increase of carbon content. The average value of the friction coefficient of 0.8 C reaches 0.4, which ofFigure carbon 10 content.that the Theoverall average level valueof the of friction the friction coefficient coefficient of the of sample 0.8 C reaches density 0.4, zone which decreases is the lowest with is the lowest of all the specimens, and that of 0.45 C and 0 C is more than 0.5 and 0.6, respectively. ofthe all increase the specimens, of carbon and content. that of The 0.45 average C and 0value C is moreof the thanfriction 0.5 coefficient and 0.6, respectively. of 0.8 C reaches 0.4, which is the lowest of all the specimens, and that of 0.45 C and 0 C is more than 0.5 and 0.6, respectively.

Figure 10. Variation of friction coefficient with scratch length. Figure 10. Variation of friction coefficientcoefficient with scratch length.length. 4. Discussion 4. DiscussionPores are affected by both hydrostatic pressure and shear stress in the rolling densification process.Pores Pores are areaffected not only by compressedboth hydrostatic by hydrostatic pressure pressure,and shear but stress also ineasily the elongatedrolling densification and closed process.by shear Pores deformation. are not only The compressed maximum shearby hydrostatic stress appears pressure, on butthe alsosub-surface easily elongated rather than and onclosed the by shear deformation. The maximum shear stress appears on the sub-surface rather than on the Metals 2018, 8, 91 9 of 11

4. Discussion Pores are affected by both hydrostatic pressure and shear stress in the rolling densification process. Pores are not only compressed by hydrostatic pressure, but also easily elongated and closed by shear deformation. The maximum shear stress appears on the sub-surface rather than on the surface [9]. Therefore, the initial plastic deformation happens in the sub-surface area. Due to the overall small deformation resistance of the 0 C and 0.2 C samples, both the sub-surface and the outermost surface have severe plastic deformation during the rolling process. However, under the same rolling conditions, the deformable sub-surface in the 0.45 C and 0.8 C samples is preferentially densified, while the outermost surface does not reach the same densification as the sub-surface after rolling. This indicates that the increase of carbon content will worsen the effect of rolling compaction on density. The distribution of the density in the dense area is more unstable and non-uniform with a high carbon content. In the published work from Bengtsson, the depth of the densified layer in samples with 0.25 wt. % carbon content is 1.3 mm [15], and is at the same level as the depth in this work. When the carbon content increases in the samples, the material microstructure changes (from ferrite to pearlite), resulting in the effect of rolling treatment. Therefore, the yield strength rises and the deformation resistance ability is enhanced. In the same rolling condition, specimens with different materials will show different densification behaviors. The hardness fluctuation in the dense layer is related to the densification state of the samples. The highest hardness of 0.8 C occurred at about 0.6 mm from the surface, where the porosity is at a low state. In 0.45 C, hardness and density have the same trend of fluctuations. In 0 C and 0.2 C, hardness at a distance of 0.4–0.5 mm from the surface has a slight increase, and at the same time there is a decrease in porosity at this position. The reason for this phenomenon is that the existence of internal pores in the material leads to increasing indentation deformation and then decreasing hardness. In addition, the hardness changing rates in the densified layer decline with the increase of the carbon content. This means that the same rolling process is highly efficient in samples with low carbon content. In Bengtsson’s work, the roughness of the rolled gears with 0.3 wt. % carbon content is Ra1.13, which is lower than that of the shaved reference gear [17] but significantly higher than that of the 0.45 C and 0.8 C samples. Therefore, the rolling process provides a good surface condition for the part and can even be used as the final step in part machining. Furthermore, a high carbon content can to better surface roughness. The friction coefficient of the rolled material with a high carbon content is lower, which is mainly due to the increase of the carbide lubrication. This indicates that the addition of carbon improves the friction performance of the densified layer in PM parts and is conducive to extending the service of parts in actual working situations. The surface-densified layer, with relatively high hardness, could hinder crack initiation and propagation on and under the surface and enhance the wear properties of the PM material [6]. It has to be pointed out that the roughness should have been obtained via scratch results, but the data obtained from the scratch test in this work gives the penetration depth and transverse force but does not include the roughness. This will be improved in future research. In future research, the friction and wear behaviors of the surface-densified material need to be further studied via sliding wear tests. We can then investigate the effect of the carbon content on the wear resistance of the surface-densified PM material through wear rate, the morphologies of the worn surface, and the chemical composition of the wear debris. As a result, the wear mechanism could then be researched. Furthermore, residual stress is an important property generated by the rolling process, which controls fatigue and the corrosion performance of materials. We would like to conduct residual compressive stress measurements using an X-ray stress analyzer on the surface of rolling samples in future research. This will indicate the performance of different carbon content materials after rolling. Metals 2018, 8, 91 10 of 11

5. Conclusions The influence of carbon content on the surface rolling performance of PM materials was investigated. The results reveal that the porosity, hardness, roughness, and friction performance of surface rolled parts have different trends with different carbon contents. For the samples with 0 wt. % carbon content, an excellent and stable porosity distribution in a 2-mm-thick dense layer region was observed; however, the hardness at the surface was the lowest (due to the lower hardness of the ferrite itself, even if the sample has high densification) and the wear resistance and friction performance of 0 C performed worst in this work. As the carbon content increases, the porosity distribution has a fluctuation in the region above 0.7 mm from the surface and the dense layer depth shows a decline in the 0.2 C samples. However, the hardness value of 0.2 C is similar to that of 0 C because of the rising porosity. For the 0.45 C samples, the porosity is slightly higher than that of 0.2 C, and therefore the maximum hardness of 0.45 C—about 280 HV0.1—is about 20% higher than the maximum hardness of 0.2 C—about 230 HV0.1—in the surface region. Meanwhile, the roughness, wear resistance, and friction performance were also improved. When the carbon content is 0.8 wt. %, the hardness reaches 340 HV0.1 while the roughness is close to Ra0.5 and the friction coefficient is the lowest, about 0.35 approaching the surface. The microstructure is uniform fine pearlite lamellae in 0.8 C. However, the depth of the densified layer is 1.13 mm, which is thinner than the others, and meets the requirements for powder metallurgy parts such as gears used in the auto industry. Taking into account the application of surface-densified parts and the results obtained from this work, high carbon content provides high strength and good performance in surface rolling densification.

Acknowledgments: This work was funded by the “Morning Star” project supported by the Science and Technological Commission of Shanghai. Contribution and encouragement to this work from Wei Lu is greatly appreciated and acknowledged. Author Contributions: Biao Yan and Jingguang Peng conceived and designed the experiments; Yan Zhao, Di Chen, and Dekai Li performed the experiments; Yan Zhao analyzed the data; Yan Zhao and Jingguang Peng wrote the paper; Biao Yan revised the manuscript. All authors discussed and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Chen, D.; Li, D.; Peng, J.; Wang, T.; Yan, B.; Lu, W. The effect of rolling temperature on the microstructure and mechanical properties of surface-densified powder metallurgy Fe-based gears prepared by the surface rolling process. Metals 2017, 7, 420. [CrossRef] 2. Capus, J. Industry roadmap update from MPIF. Met. Powder Rep. 2012, 67, 10–11. [CrossRef] 3. Takemasu, T.; Koide, T.; Shinbutsu, T.; Sasaki, H.; Takeda, Y.; Nishida, S. Effect of surface rolling on load capacity of pre-alloyed sintered steel gears with different densities. Procedia Eng. 2014, 81, 334–339. [CrossRef] 4. Lee, J.G.; Lim, C.H.; Kim, H.S.; Hong, S.J.; Kim, M.T.; Kang, B.C.; Park, D.K.; Lee, M.K.; Rhee, C.K. Highly dense steel components prepared by magnetic pulsed compaction of iron-based powders. Powder Technol. 2012, 228, 254–257. [CrossRef] 5. Shanmugasundaram, D.; Chandramouli, R.; Kandavel, T.K. Cold and hot deformation and densification studies on sintered Fe-C-Cr-Ni low P/M . Int. J. Adv. Manuf. Technol. 2009, 41, 8–15. [CrossRef] 6. Liu, X.; Xiao, Z.Y.; Guan, H.J.; Zhang, W.; Li, F.L. Friction and wear behaviours of surface densified powder metallurgy Fe-2Cu-0.6C material. Powder Metall. 2016, 59, 329–334. [CrossRef] 7. Takemasu, T.; Koide, T.; Takeda, Y.; Kamimura, D.; Nakamoto, M. Properties of densification by surface rolling and load bearing capacity of 1.5Cr-0.2Mo high density sintered steel rollers and gears. J. Solid Mech. Mater. Eng. 2011, 5, 825–837. [CrossRef] 8. Ibrahim, A.M.; Bishop, D.P.; Kipouros, G.J. Effect of swaging and rolling post sintering treatments on the corrosion behaviour of alumix 321 PM alloy. Powder Technol. 2017, 320, 89–98. [CrossRef] Metals 2018, 8, 91 11 of 11

9. Lawcock, R. Rolling-contact fatigue of surface-densified gears. Int. J. Powder Metall. 2006, 42, 17–29. [CrossRef] 10. Forden, L.; Bengtsson, S.; Bergstrom, M. Comparison of high performance PM gears manufactured by conventional and warm compaction and surface densification. Powder Metall. 2005, 48, 10–12. [CrossRef] 11. Whittaker, D. PM structural parts move to higher density and performance. Powder Metall. 2007, 50, 99–105. [CrossRef] 12. Liu, X.; Xiao, Z.Y.; Guan, H.J.; Zhang, W. Experimental study on the surface densification of Fe-2Cu-0.6C powder metallurgy material. Mater. Manuf. Process. 2016, 31, 1621–1627. [CrossRef] 13. Yu, Y.; Linnea, F. Surface densification-an effective way to improve the performance of sintered gears. Powder Metall. Technol. 2005, 23, 62–74. [CrossRef] 14. Jones, P.; Buckley-Golder, K.; Lawcock, R.; Shivanath, R. Densification strategies for high endurance pm components. Int. J. Powder Metall. 1997, 33, 37–44. [CrossRef] 15. Bengtsson, S.; Dizdar, S.; Svensson, M. Material aspects of selectively densified transmission gears. In Proceedings of the 2000 Powder Metallurgy World Congress, Kyoto, Japan, 12–16 November 2000. 16. Peng, J.G.; Zhao, Y.; Chen, D.; Li, K.D.; Lu, W.; Yan, B.A. Effect of surface densification on the microstructure and mechanical properties of powder metallurgical gears by using a surface rolling process. Materials 2016, 9, 846. [CrossRef][PubMed] 17. Bengtsson, S.; Fordén, L.; Dizdar, S.; Johansson, P. Surface densified P/M transmission gear. In Proceedings of the 2001 International Conference on “Power Transmission Components. Advances in High Performance Powder Metallurgy Applications”, Ypsilanti, MI, USA, 16–17 October 2001.

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