The Effect of Chip Binding on the Parameters of the Case-Hardened Layer of Tooth Surfaces for AMS 6308 Steel Gears Processed by Thermochemical Treatment

materials

Article The Effect of Chip Binding on the Parameters of the Case-Hardened Layer of Tooth Surfaces for AMS 6308 Steel Gears Processed by Thermochemical Treatment

Robert Fularski 1,* and Ryszard Filip 2,*

1 Department of Gears, Pratt & Whitney Rzeszów S.A., Hetmańska120, 35-078 Rzeszów, Poland 2 Department of Materials Science, Faculty of Mechanical Engineering and Aeronautics, Ignacy Łukasiewicz Rzeszów University of Technology, Powstańców Warszawy 12, 35-959 Rzeszów, Poland * Correspondence: [email protected] (R.F.); ryfi[email protected] (R.F.)

Abstract: The following article describes influence of pressure welded or bound chips to the gear tooth flank and/or the tooth root on a carburized case and surface layer hardness of Pyrowear 53 steel gears, machined by Power Skiving method. This paper is focused only on one factor, the chips generated while forming gear teeth by power skiving, which could result in local changes in the carburized case parameters as a negatively affecting point of mechanical performance of the carburized case. The chips, due to the specifics of the power skiving process and the kinematics of tooth forming, could be subject to the phenomena of pressure welding or binding of chips to the tooth. During the carburizing stage of the downstream manufacturing processes, the chips form a diffusion barrier, which ultimately could result in localized changes in the carburized case. This work was an attempt to answer the question of how and to what extent the chips affect the case hardening. Performed simulations of chips by a generating cupper “spots”, mentioned in the study, Citation: Fularski, R.; Filip, R. The represent a new approach in connection with minimization of errors, which could appear during Effect of Chip Binding on the carbon case depth and case hardness analysis for typical chips, generated during the machining Parameters of the Case-Hardened Layer of Tooth Surfaces for AMS 6308 process—assurance that a complete chip was bound to the surface. Hardness correlation for zones, Steel Gears Processed by where the chip appears with areas free of chips, gives simple techniques for assessment. Performed Thermochemical Treatment. Materials tests increased the knowledge about the critical size of the chip—1.5 mm, which could affect the case 2021, 14, 1155. https://doi.org hardening. Obtained experimental test results showed that the appearance of chip phenomena on /10.3390/ma14051155 the gear tooth might have a negative impact on a carburized case depth and hardened layer.

Academic Editor: Marcin Nabiałek Keywords: skiving; cutting; circumferential peeling; chip; carburized case; case hardness; gears; surface layer; welding; binding Received: 28 January 2021 Accepted: 24 February 2021 Published: 1 March 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Despite intense technical progress, gears continue to be available in a broad range of published maps and institutional afﬁl- different applications. This includes common household products, automotive products, iations. aircraft gearboxes, and other applications in which the transmission of high power is important. Given their area of application, the gears used in aircraft gearboxes should feature a high quality of manufacturing, according to the stringent requirements related to gear tooth geometry and retention of the surface layer parameters, including carburizing case. Materials used for their production must have resistance for: abrasion, contact stress Copyright: © 2021 by the authors. on the surface layer, bending strength at the tooth base, and also require properties to Licensee MDPI, Basel, Switzerland. withstand cyclic variable loads in the core. Heavy duty gears used in aviation gearboxes are This article is an open access article distributed under the terms and usually made from high-strength alloy steels like AMS 6308 [1] (the chemical composition ® conditions of the Creative Commons of Pyrowear 53 is presented in Table2). The AMS 6308 is a relatively new steel originally Attribution (CC BY) license (https:// prepared for military products, but for about a dozen years, has been increasingly used in creativecommons.org/licenses/by/ civil aviation. Due to the growing of industry interest, it has become the subject of scientiﬁc 4.0/). considerations, too [1–4].

Materials 2021, 14, 1155. https://doi.org/10.3390/ma14051155 https://www.mdpi.com/journal/materials Materials 2021, 14, 1155 2 of 16

The properties of carburizing case of this steel may change not only by modification of the thermochemical processing parameters, but also as a result of machining, which includes both abrasive and cutting processes. It is also known that usage of quite new techniques, e.g., electro-pulsing, ultrasonic surface layer treatment presented by authors of publications [5–8], could improve the surface structure, mechanical properties, and microstructure, in some cases. However these technics cannot be directly applied to gear teeth surfaces improvement due to the specific geometric shape of gear teeth. While the phenomena which occur during machine cutting or abrasive machining process and their effect on the surface layer are widely discussed in the available references [9–18] and the industry norms applied in the assessment of microstructure, the question of machining prior to case carburizing of the surfaces to be exposed to considerable and dynamically variable loads is less extensively detailed in research work. However, it is known, that certain external factors related to machining could result in local changes in the carburized case parameters to a point capable of negatively affecting the mechanical performance of the carburized case. This work is focused on one of the factors, the chips generated while forming gear teeth by power skiving [19,20], an increasingly popular tooth forming technique in the industry, capable of significantly improving machine cutting efficiency in comparison to the Fellows process, for example. Each sector of industry strives to optimize the manufacturing processes and reduce the production time at each step of the manufacturing cycle. In recent years, thanks to advances in technology, engineering and the drive for machines have created the opportunity for implementing a new method of gear tooth cutting in serial production, known as power skiving, or simply skiving. Power skiving was developed and patented by Julius Wilhelm von Pittler in 1910 [21,22], but due to its high processing and engineering requirements, which include high rigidity of the machine tool and the cutting tool holder, the high ratio of workpiece-to-tool spindle synchronization, and the required cutting tool durability, it was many years before power skiving became achievable for machine builders or the machine engineering industry. Advancements in various scientific disciplines have resulted in the development of new grades of steel, cutting tool coatings, and rigid machine tools combined with precision drive units and, in relation to the potential offered by the technology, rekindled interest in power skiving as a process capable of significantly improving machine cutting efficiency—for the forming of gear teeth. Skiving has certain unique requirements, which include the inclination angle of the tool spindle centerline to the workpiece centerline. In skiving, it is the angle that generates the cutting speed, the vector of which depends on the vectors of tangential velocity of the workpiece and the tangential velocity of the cutting tool. The dependencies of skiving kinematics, the specifics of skiving, and the requirements of skiving are discussed further by the authors of [19–25], for example. A significant factor which must always be considered in any approach to skiving is the inclination angle Σ (Figure1), the optimum value of which is approximately 20 ◦, for effective machining efficiency. Note, however, that this inclination angle cannot always be achieved due to the expected resultant geometry of the gear being processed. Given the specific nature and requirements of skiving, it is necessary at the tool design stage, especially concerning the cutting edges, to plan for all the expected phenomena related to chip formation and their removal from the cutting zone. As power skiving involves high rotational speeds, which translates into reduced gear manufacturing duration compared to the Fellows tooth forming process, the chips generated by the cutting feed cannot always be removed from the cutting zone due to the geometry of the gear being processed. A typical example of this involves internal tooth gears (Figure1—Internal gear), where after chip formation during the cutting process, the chips are subjected to a centrifugal force which obstructs clearing of the cutting zone. The difficult removal of the chips from the cutting zone can also be observed in the processing of external tooth wheels, where the feed moves vertically upward and gravity causes the generated chips to move across the cutting zone. Figure2 shows an example of the double helical gear cutting process. The process is split in two Materials 2021, 14, x 3 of 17

the chips from the cutting zone can also be observed in the processing of external tooth wheels, where the feed moves vertically upward and gravity causes the generated chips to move across the cutting zone. Figure 2 shows an example of the double helical gear cutting process. The process is split in two phases where cutting tool feed fa (Figure 1, External gear) for each rim has an opposite direction. When a chip moves between the cutting tool blade and the tooth being formed, unfavorable effects may result, such as fusion or binding of the chips to the tooth surfaces (Figures 3 and 4). Given the potential for the effect to occur, as described in [19,20], for example, research was undertaken to determine the effect of binding a chip, generated during machining of power skiving method to tooth surfaces, seen as a specific foreign object, on the properties characteristic of the case hardening of gears made of Pyrowear® 53 steel. The described case can be dangerous, especially for gears utilized in aircraft gearboxes, which must meet stringent reliability demands that translate directly into flight safety. The gears in aircraft gearboxes usually transfer high power, and the teeth are cyclically exposed to variable loads, which contribute to variable states of stress on the tooth flanks and in the root areas. The variance of the properties of the surface layer, consisting of reduced carbon concentrations in certain areas of the carburized case on the tooth surface, for example, due to the presence of a barrier formed by chips bonded to the same surfaces during carburizing, and which may affect more than just the hardness. It can lead to much more dangerous effects related to extreme pressure, resulting in strain or surface wearing, and hence to failure of the gear or the whole gearbox. The research described Materials 2021, 14, 1155 3 of 16 further in this paper focuses on presenting the effect of chips which, in the case of a poorly performed tooth forming manufacturing process upstream of the thermochemical processing or limited understanding of the issue, may cause localized reduction in hardness phasesin the areas where of cutting interest tool on the feed tooth fa (Figure surface,1, Externalfollowing gear) the processes for each rim intended has an to opposite improve direction.the mechanical performance of a surface layer exposed to high loads.

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FigureFigure 1.1.Basic Basic kinematicskinematicsof ofpower powerskiving skivingfor forinternal internaland and external external gears. gears.

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FigureFigure 2.2. DoubleDouble helicalhelical geargear afterafter powerpower skivingskiving machining.machining.

When a chip moves between the cutting tool blade and the tooth being formed, unfavorable effects may result, such as fusion or binding of the chips to the tooth surfaces (FiguresFigure 2.3 Double and4). helical gear after power skiving machining.

Figure 3. Bound chip on the tooth involute surface, after power skiving machining.

FigureFigure 3.3. BoundBound chipchip onon thethe toothtooth involuteinvolute surface,surface, afterafter powerpower skivingskivingmachining. machining.

Figure 4. Bound chip on the tooth root, after power skiving machining.

2. Test Material and Methodology The material chosen for testing was a double helical gear based on Pyrowear® 53 alloy steelFigure (Figure 4. Bound 2) chipwith on the the concentrations tooth root, after of power chemical skiving composition machining. as shown in Table 1 and steel properties shown in Table 2. 2. Test Material and Methodology TableThe 1. Chemical material composition chosen for testingof the PYROWEAR was a double® 53 helical (AMS 6308)gear basedsteel [26 on]. Pyrowear® 53 alloy steelCr (FigureNi 2) withC the concentrationsMo Cuof chemicalV compositionMn Sias shown in TableFe 1 and steel properties shown in Table 2. 1.0 2.0 0.11 3.25 2.0 0.1 0.4 0.9 the rest Table 1. Chemical composition of the PYROWEAR® 53 (AMS 6308) steel [26].

Cr Ni C Mo Cu V Mn Si Fe 1.0 2.0 0.11 3.25 2.0 0.1 0.4 0.9 the rest

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Figure 2. Double helical gear after power skiving machining.

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Figure 3. Bound chip on the tooth involute surface, after power skiving machining.

FigureFigure 4.4. BoundBound chipchip onon thethe toothtooth root,root, afterafter powerpower skivingskiving machining.machining.

2. TestGiven Material the potential and Methodology for the effect to occur, as described in [19,20], for example, research was undertakenThe material to chosen determine for testing the effect was of a double binding helical a chip, gear generated based on during Pyrowear machining® 53 alloy of powersteel (Figure skiving 2) method with the to concentrations tooth surfaces, of seen chemical as a specific composition foreign as object, shown on thein Table properties 1 and ® characteristicsteel properties of shown the case in hardening Table 2. of gears made of Pyrowear 53 steel. The described case can be dangerous, especially for gears utilized in aircraft gearboxes, which must meet stringentTable 1. Chemical reliability composition demands of that thetranslate PYROWEAR directly® 53 (AMS into flight6308) steel safety. [26]. The gears in aircraft gearboxes usually transfer high power, and the teeth are cyclically exposedCr toNi variable loads,C whichMocontribute Cu to variableV statesMn of stressSi on the toothFe flanks and1.0 in the2 root.0 areas.0.11 The variance3.25 of2 the.0 properties0.1 of0. the4 surface0.9 layer, consistingthe rest of reduced carbon concentrations in certain areas of the carburized case on the tooth surface, for example, due to the presence of a barrier formed by chips bonded to the same surfaces during carburizing, and which may affect more than just the hardness. It can lead to much more dangerous effects related to extreme pressure, resulting in strain or surface wearing, and hence to failure of the gear or the whole gearbox. The research described further in this paper focuses on presenting the effect of chips which, in the case of a poorly performed tooth forming manufacturing process upstream of the thermochemical processing or limited understanding of the issue, may cause localized reduction in hardness in the areas of interest on the tooth surface, following the processes intended to improve the mechanical performance of a surface layer exposed to high loads.

2. Test Material and Methodology The material chosen for testing was a double helical gear based on Pyrowear® 53 alloy steel (Figure2) with the concentrations of chemical composition as shown in Table2 and steel properties shown in Table1.

Table 1. PYROWEAR® 53 (AMS 6308) steel properties (typical) [27].

Ultimate Tensile Core Achievable Tempering Alloy Yield Strength %EI %RA K Toughness Strength Hardness IC Surface Hardness Temperature √ Pyrowear®Alloy 965 MPa (140 354–434 HV 126 MPa × m 674-722 HV (59–63 1172 MPa (170 ksi) 16 67 √ 204 ◦C (400 ◦F) 53 ksi) (36–44 HRC) (115 ksi × in) HRC) Hardness conversations from HRC to HV per ASTM E140

Table 2. Chemical composition of the PYROWEAR® 53 (AMS 6308) steel [26].

Cr Ni C Mo Cu V Mn Si Fe 1.0 2.0 0.11 3.25 2.0 0.1 0.4 0.9 the rest

Prior to tooth cutting by power skiving, the gear was plated with copper, using a galvanic process, to mask the surfaces not to be case-hardened. This operation was intended to accurately represent a typical manufacturing process involving the case-hardening of gears. Materials 2021, 14, x 5 of 17 Materials 2021, 14, x 5 of 17

Table 2. PYROWEAR® 53 (AMS 6308) steel properties (typical) [27]. Table 2. PYROWEAR® 53 (AMS 6308) steel properties (typical) [27]. Ultimate Achievable Yield Ultimate Core KIC Achievable Tempering Alloy Yield Tensile Core %EI %RA KIC Surface Tempering Alloy Strength Tensile Hardness %EI %RA Toughness Surface Temperature Strength Strength Hardness Toughness Hardness Temperature Strength Hardness Pyrowear® 965 MPa 1172 MPa 354–434 HV 126 MPa × √m 674-722 HV 204 °C Pyrowear® 965 MPa 1172 MPa 354–434 HV 16 67 126 MPa × √m 674-722 HV 204 °C Alloy 53 (140 ksi) (170 ksi) (36–44 HRC) 16 67 (115 ksi × √in) (59–63 HRC) (400 °F) Alloy 53 (140 ksi) (170 ksi) (36–44 HRC) (115 ksi × √in) (59–63 HRC) (400 °F) Hardness conversations from HRC to HV per ASTM E140 Hardness conversations from HRC to HV per ASTM E140 Materials 2021, 14, 1155 5 of 16 Prior to tooth cutting by power skiving, the gear was plated with copper, using a Prior to tooth cutting by power skiving, the gear was plated with copper, using a galvanic process, to mask the surfaces not to be case-hardened. This operation was in- galvanic process, to mask the surfaces not to be case-hardened. This operation was intended to accurately represent a typical manufacturing process involving the case-hard- tended to accurately represent a typical manufacturing process involving the case-hard- Afterening power of gears. skiving After of power the gear skiving with aof Gleason the gear 600 with PS a (Gleason-Pfauter Gleason 600 PS Maschinenfabrik(Gleason-Pfauter ening of gears. After power skiving of the gear with a Gleason 600 PS (Gleason-Pfauter GmbH,Maschinenfabrik Ludwigsburg, GmbH, Germany), Ludwigsburg it was,visually Germany inspected), it was forvisually the presence inspected of chipsfor the on pres- the Maschinenfabrik GmbH, Ludwigsburg, Germany), it was visually inspected for the pres- toothence of flanks chips and on roots. the tooth A × 10flanks magnifying and roots. glass A was×10 usedmagnifying as a visual glass aid. was Having used confirmedas a visual ence of chips on the tooth flanks and roots. A ×10 magnifying glass was used as a visual thataid. theHaving gear confirmed wheel featured that the no gear foreign wheel objects featured in the no form foreign of chips, objects a number in the form of processes of chips, aid. Having confirmed that the gear wheel featured no foreign objects in the form of chips, werea number performed of processes to deposit were the performed copper plating to deposit on certain the copper areas toplating simulate on thecertain presence areas ofto a number of processes were performed to deposit the copper plating on certain areas to chipssimulate on thethe toothpresence surfaces, of chips which on wasthe thentooth subject surfaces, to downstreamwhich was then carburizing subject to and down- heat simulate the presence of chips on the tooth surfaces, which was then subject to down- treatment.stream carburizing For this purpose,and heat thetreatment. designated For this areas purpose, of the gear the designated (selected teeth areas flanks) of the were gear stream carburizing and heat treatment. For this purpose, the designated areas of the gear protected(selected teeth by paraffin, flanks) followedwere protected by the selectiveby paraffin, removal followed of the byparaffin the selective coat toremoval create smallof the (selected teeth flanks) were protected by paraffin, followed by the selective removal of the holesparaffin in specificcoat to create areas tosmall enable holes their in galvanicspecific areas copper to plating.enable their The galvanic described copper methodology plating. paraffin coat to create small holes in specific areas to enable their galvanic copper plating. wasThe useddescribed to generate methodology copper was spots, used which to generate simulated copper the spots real chips, which appearing simulated on the teeth real The described methodology was used to generate copper spots, which simulated the real surfaces.chips appearing The described on teeth technique surfaces. reliablyThe described bound technique that whole reliabl surfacey bound of simulated that whole chips sur- to chips appearing on teeth surfaces. The described technique reliably bound that whole sur- theface gear of simulated teeth, which chips would to the not gear be possibleteeth, which in the would case of not using be possible real chips in for the verification case of using of face of simulated chips to the gear teeth, which would not be possible in the case of using theirreal chips influences for verification on the hardened of their layer.influences Furthermore, on the hardened that gave layer. additional Furthermore assurance, that about gave real chips for verification of their influences on the hardened layer. Furthermore, that gave theadditional sizes of assurance the simulated about chips. the sizes The of galvanic the simulated copper chips. plating The of thegalvanic test specimen copper plating (gear additional assurance about the sizes of the simulated chips. The galvanic copper plating wheel)of the test created specimen selected (gear areas wheel) featuring created copper selected “spots” areas varying featuring in size,copper representing “spots” varyin chipsg of the test specimen (gear wheel) created selected areas featuring copper “spots” varying inin differentsize, representing locations chips on the in gear different teeth (Figureslocations5 andon the6). gear teeth (Figures 5 and 6). in size, representing chips in different locations on the gear teeth (Figures 5 and 6).

FigureFigure 5.5. GearGear samplesamplewith with copper copper “spots” “spots” varying varying in in size, size, representing representing chips chips locations locations close close to pitchto pitchFigure diameter 5. Gear sampleof the gear with teeth. copper “spots” varying in size, representing chips locations close to diameterpitch diameter of the of gear the teeth. gear teeth.

Figure 6. Gear sample with copper “spots” varying in size, representing chips in different loca- Figure 6. Gear sample with copper “spots” varying in size, representing chips in different loca- Figuretions on 6. theGear gear sample teeth. with copper “spots” varying in size, representing chips in different locations ontions the on gear the teeth. gear teeth. The gear wheel was then thermochemically treated based on the parameters speci- TheThe geargear wheelwheel was was then then thermochemically thermochemically treated treated based based on on the the parameters parameters speciﬁed speci- fied for Pyrowear® 53 (AMS 6308). The properties of the teeth surface layer were achieved forfied Pyrowear for Pyrowear® 53® (AMS 53 (AMS 6308). 6308). The The properties properties of of the the teeth teeth surface surface layer layer were wereachieved achieved by thermochemical treatment processes, which consisted of the following steps and temperature parameters recommended by the producer: Gas carburizing (870–930 ◦C), oil

quenching/hardening (905–920 ◦C), subzero treatment (below −75 ◦C), and tempering (150–290 ◦C) [2]. The preparation of the test specimens then began. For this purpose, samples were cut from the gear and metallographic specimens were prepared for selected sections, for which the cutting plane was perpendicular to the surface of the areas reﬂecting the simulated presence of chips. The specimens were etched in Nital 5%, a specialist product for microsections. Hardness testing was done with a FM-700 micro-hardness testing machine (Future- Tech Corp., Kanagawa, Japan (Figure7). The hardness test method was HV 0.5, ref. ASTM Materials 2021, 14, x 6 of 17

by thermochemical treatment processes, which consisted of the following steps and temperature parameters recommended by the producer: Gas carburizing (870–930 °C), oil quenching/hardening (905–920 °C), subzero treatment (below −75 °C), and tempering (150–290 °C) [2]. The preparation of the test specimens then began. For this purpose, samples were cut from the gear and metallographic specimens were prepared for selected sections, for which the cutting plane was perpendicular to the surface of the areas reflect- Materials 2021, 14, 1155 ing the simulated presence of chips. The specimens were etched in Nital 5%, a specialist6 of 16 product for microsections. Hardness testing was done with a FM-700 micro-hardness testing machine (Future- Tech Corp., Kanagawa, Japan (Figure 7). The hardness test method was HV0.5, ref. ASTM E384-17.E384-17. The uncertainty uncertainty for for the the range range 240 240–600–600 HV HV0.50.5 waswas ±9± HV9 HV0.5, and0.5, andfor the for range the range >600 >600HV0.5 HV was0.5 ±was17 HV±170.5. HV0.5.

FigureFigure 7. HardnessHardness research research station station equipped equipped with with microhardness microhardness tester, tester, Future Future-Tech-Tech Corp., Corp., Type: Type:FM-700. FM-700.

3.3. TestTest ResultsResults andand AnalysisAnalysis TheThe teststestsprovided provided data data on on the the case case hardness hardness values, values, which which enabled enabled an analysisan analysis of the of effectthe effect of chips of chips on the onparameter. the parameter. The testsThe tests also allowedalso allowe visualizationd visualization of the of changes the changes in case in hardeningcase hardening depth depth under under large large chips chips over theover diffusion the diffusion case hardened case hardened layer. layer. The tests The were tests performedwere performed on two on characteristic two characteristic areas areas of the of gear, the thegear, pitch the diameterpitch diameter and the and tooth the tooth root. Theroot. locations The locations where where the chips the chips were were simulated simulated by copper by copper ‘spots’ ‘spots’ were were selected selected due todue the to stressesthe stresses present present during during typical typical engagement engagement with with the tooth the tooth on the on mating the mating gear wheel.gear wheel. The simulationThe simulation was repeated was repeated for the for cases theof cases a large of a chip large near chip the near pitch the circle, pitch as circle, represented as repre- by thesented analytical by the resultsanalytical for zonesresults 3A for and zones 4A 3A Dp and and 4A for Dp the and cases for of the a small cases chip of a withinsmall chip the pitchwithin circle the pitch (zone circle 3C) and (zone the 3C) tooth and flank the tooth (4A Ds). flank The (4A assignment Ds). The assignment of the chip sizeof the to chip the areassize to of the interest areas was of interest undertaken was withundertaken reference with to thereference presence to ofthe chips presence observed of chips during ob- powerservedskiving during testspower and skiving optimization tests and of optimi powerz skiving.ation of power skiving. TheThe analysisanalysis ofof hardnesshardness in chipchip zone 3A (Figure(Figure5 5)) inin cross-sectioncross-section FF-F-F (Figure(Figure8 8)) demonstrateddemonstrated a significant significant hardness hardness reduction reduction in in3A 3A (Figure (Figure 9) 9in) comparison in comparison to the to adja- the adjacentcent areas. areas. The Thelocali localizedzed increase increase in hardness in hardness on both on bothsides sides(Figure (Figure 10) were 10) werea result a result of the of the hardness tests in the zone identification location. Having observed this hardness hardness tests in the zone identification location. Having observed this hardness reduc- reduction, tests were performed to image the carburized case depth within 3A. The test tion, tests were performed to image the carburized case depth within 3A. The test showed Materials 2021, 14, x showed reduction of carburized case depth in the area of the simulated chip appearing7 of 17 reduction of carburized case depth in the area of the simulated chip appearing (Figure 11). (Figure 11).

FigureFigure 8.8. LocalizationLocalization ofof cross-sectioncross-sectionF-F, F-F, zone: zone:3A. 3A.

Figure 9. Pattern of hardness verification. Paths depth (distance from tooth surface): 0.1 mm and 0.3 mm. Cross-section F-F, zone: 3A.

800 0.1 mm

780 0.3 mm

760

740

0.5 720

700

680 Hardness Hardness HV

660

640 3A

620

600 1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 Impression (dent) no.

Figure 10. Microhardness measurement results. Paths depth (distance from tooth surface): 0.1 mm and 0.3 mm. Cross-section F-F, zone: 3A.

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Figure 8. Localization of cross-section F-F, zone: 3A.

FigureFigure 9. 9. PatternPattern of ofhardness hardness verification. veriﬁcation. PathsPaths depth depth(distance (distance from tooth from surface) tooth: 0 surface):.1 mm and 0.1 mm and 0.3 mm. Cross-section F-F, zone: 3A. 0.3Figure mm. 9. Pattern Cross-section of hardness F-F, verification. zone: 3A. Paths depth (distance from tooth surface): 0.1 mm and 0.3 mm. Cross-section F-F, zone: 3A. 800 0.1 mm 800 0.1 mm 780 0.3 mm 780 0.3 mm 760 760

740740 0.5 0.5 720720

700700

680

Hardness Hardness HV 680 Hardness Hardness HV 660 660 640 3A 640 3A 620 620 600 600 1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 1 5 9 13 17Impression21 25 (dent)29 no.33 37 41 45 49 53 57 Impression (dent) no. Materials 2021, 14, x FigureFigure 10 10.. MicrohardnessMicrohardness measurement measurement results. Paths results. depth Paths (distance depth from (distancetooth surface) from: 0.1 mm tooth surface): 0.18 mmof 17 and 0.3 mm. Cross-section F-F, zone: 3A. andFigure 0.3 10 mm.. Microhardness Cross-section measurement F-F, zone: results. 3A. Paths depth (distance from tooth surface): 0.1 mm and 0.3 mm. Cross-section F-F, zone: 3A.

FigureFigure 11.11. Waviness of carburized case depth. Magnification: Magniﬁcation: 12 12.5.5×.× Cross. Cross-section-section F-F, F-F, zone: zone: 3A. 3A. EtchedEtched sample—Nitalsample—Nital 5%.5%.

Another analysis of hardness in chip zone 3C (Figure 5) in cross-section G-G (Figure 12) did not reveal a significant hardness reduction in 3C (Figure 13) in comparison to the adjacent areas (Figure 14). Given this, no further tests were done on the specimen to image the carburized case depth in zone 3C.

Figure 12. Localization of cross-section G-G, zone: 3C.

Figure 13. Pattern of hardness verification. Paths depth (distance from tooth surface): 0.1 mm and 0.3 mm. Cross-section G-G, zone: 3C.

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Materials 2021, 14, 1155 8 of 16 Figure 11. Waviness of carburized case depth. Magnification: 12.5×. Cross-section F-F, zone: 3A. Etched sample—Nital 5%.

Another analysis of hardness in chip zone 3C (Figure5) in cross-section G-G (Figure 12) Another analysis of hardness in chip zone 3C (Figure 5) in cross-section G-G (Figure did not reveal a signiﬁcant hardness reduction in 3C (Figure 13) in comparison to the adja- 12)Figure did 11. notWaviness reveal of carburized a significant case depth. hardness Magnification: reduction 12.5×. Cross in -3Csection (Figure F-F, zone: 13) 3A. in comparison to the centadjacentEtched areas sample areas (Figure—Nital (Figure 5%. 14 ). Given14). Given this, this, no further no further tests tests were were done done on the on specimen the specimen to image to image the carburizedthe carburized case case depth depth in zone in zone 3C. 3C. Another analysis of hardness in chip zone 3C (Figure 5) in cross-section G-G (Figure 12) did not reveal a significant hardness reduction in 3C (Figure 13) in comparison to the adjacent areas (Figure 14). Given this, no further tests were done on the specimen to image the carburized case depth in zone 3C.

FigureFigure 12.12. LocalizationLocalization ofof cross-sectioncross-section G-G,G-G, zone:zone: 3C.3C. Figure 12. Localization of cross-section G-G, zone: 3C.

Figure 13. Pattern of hardness verification. Paths depth (distance from tooth surface): 0.1 mm and Figure 13. Pattern of hardness veriﬁcation. Paths depth (distance from tooth surface): 0.1 mm and Materials 2021, 14,0 x.3 mm. Cross-section G-G, zone: 3C. 9 of 17

0.3 mm. Cross-section G-G, zone: 3C. Figure 13. Pattern of hardness verification. Paths depth (distance from tooth surface): 0.1 mm and 0.3 mm800. Cross-section G-G, zone: 3C. 0.1 mm 780 0.3 mm

760

740

0.5 720

700

680 Hardness Hardness HV 3C 660

640

620

600 1 5 9 13 17 21 25 29 33 37 41 45 49 53 Impression (dent) no.

Figure 14. MicrohardnessFigure 14. Microhardness measurement measurement results. results. Paths Paths depth depth (distance (distance from tooth tooth surface) surface):: 0.1 mm 0.1 mm and 0.3 mm. Cross-section G-G, zone: 3C. and 0.3 mm. Cross-section G-G, zone: 3C. Another microsection of zone 3C was made as cross-section G1-G1 (Figure 15) and had a hardness distribution analysis was performed perpendicular to the outer surface of the surface layers, as shown by the measurement results in Figure 16. The hardness distribution shown for pattern (path) 4 (Figure 17) did not deviate from other hardness distribution patterns in the area of interest and which indicates a negligible or lack of influence of small chips on the effect of carbon diffusion or hardness after thermochemical treatment adjacent to the pitch circle.

Figure 15. Localization of cross-section G1-G1, zone: 3C.

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800 0.1 mm

780 0.3 mm

760

740

0.5 720

700

680 Hardness Hardness HV 3C 660

640

620

600 1 5 9 13 17 21 25 29 33 37 41 45 49 53 Impression (dent) no.

Materials 2021, 14, 1155 9 of 16 Figure 14. Microhardness measurement results. Paths depth (distance from tooth surface): 0.1 mm and 0.3 mm. Cross-section G-G, zone: 3C.

AnotherAnother microsectionmicrosection ofof zonezone 3C3C waswas mademade asas cross-sectioncross-section G1-G1G1-G1 (Figure(Figure 15 15))and and hadhad aa hardnesshardness distribution analysis was was performed performed perpendicular perpendicular to to the the outer outer surface surface of ofthe the surface surface layers, layers, as asshown shown by bythe the measurement measurement results results in Figure in Figure 16. 16The. The hardness hardness dis- distributiontribution shown shown forfor pattern pattern (path) (path) 4 (Figure 4 (Figure 17) 17did) did notnot deviate deviate from from other other hardness hardness dis- distributiontribution patterns patterns in the in thearea area of interest of interest and andwhich which indicates indicates a negligible a negligible or lack or of lack influ- of inﬂuenceence of small of small chips chips on onthe the effect effect of of carbon carbon diffusion diffusion or or hardness hardness after thermochemicalthermochemical treatmenttreatment adjacentadjacent totothe thepitch pitch circle. circle.

Materials 2021, 14, x 10 of 17

FigureFigure15. 15.Localization Localization ofof cross-sectioncross-sectionG1-G1, G1-G1,zone: zone: 3C.3C.

FigureFigure16. 16.Pattern Pattern ofof hardnesshardnessverification. verification. Cross-sectionCross-sectionG1-G1, G1-G1,zone: zone: 3C.3C. Further, the data for the chip presence in zone 4A Dp (Figure6) for cross-section 800 H - H (Figure 18) was analyzed and demonstrated a significant hardness reduction in 1 4A Dp (Figure 19) in comparison to the adjacent areas, as illustrated by the hardness distribution pattern (Figure 20). For 4A Dp, a hardness distribution analysis was performed2 perpendicular to the outer surface of the carburizing layer, as shown by the measurement 700 3 pattern (path) in Figure 21. The hardness distribution pattern for the Dp1 set (Figure 22) markedly deviated from the hardness distribution pattern of the Dp2 pattern (path),4 the spread of which was typical for a normal distribution hardness, and formed5 a control 600 (crosscheck test) for this test. The analysis of the pattern (path) revealed the significant6 effect of chips on carbon diffusion and ultimately the case hardness after thermochemical 7

0.5 treatment. To better illustrate the effect of the chip on the carburized case, additional tests were performed to image the carburized case depth in zone 4A Dp, the results of which are 500 shown in Figure 23 as a distinct reduction in carburized case depth within the area of the chip, as shown by the Dp1 pattern (path). The hardness distribution chart (Figure 20) shows

Hardness Hardness HV another sector with an observable case hardness reduction, including two measurement points within the tooth root area. Its interpretation is shown further later. 400

300

200 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Impression (dent) no.

Figure 17. Microhardness measurement results. Cross-section G1-G1, zone: 3C.

Further, the data for the chip presence in zone 4A Dp (Figure 6) for cross-section H - H (Figure 18) was analyzed and demonstrated a significant hardness reduction in 4A Dp (Figure 19) in comparison to the adjacent areas, as illustrated by the hardness distribution pattern (Figure 20). For 4A Dp, a hardness distribution analysis was performed perpendicular to the outer surface of the carburizing layer, as shown by the measurement pattern (path) in Figure 21. The hardness distribution pattern for the Dp1 set (Figure 22) markedly deviated from the hardness distribution pattern of the Dp2 pattern (path), the spread of

Materials 2021, 14, x 10 of 17

Materials 2021, 14, 1155 10 of 16

Figure 16. Pattern of hardness verification. Cross-section G1-G1, zone: 3C.

800 1 2

700 3 4 5 600 6

7 0.5

500

Materials 2021, 14, x Hardness HV 11 of 17 Materials 2021, 14, x 11 of 17 400

which was typical for a normal distribution hardness, and formed a control (crosscheck which300 was typical for a normal distribution hardness, and formed a control (crosscheck test) for this test. The analysis of the pattern (path) revealed the significant effect of chips on carbon diffusion and ultimately the case hardness after thermochemical treatment. To better illustrate the effect of the chip on the carburized case, additional tests were per- better200 illustrate the effect of the chip on the carburized case, additional tests were performed to image1 2 the carburized3 4 5case depth6 7 in zone8 4A9 Dp,10 the 11results12 of which13 14 are shown in Figure 23 as a distinct reduction in Impressioncarburized (dent) case no. depth within the area of the chip, as shown by the Dp1 pattern (path). The hardness distribution chart (Figure 20) shows another sector with an observable case hardness reduction, including two measurement Figureother 17.sectorMicrohardness withFigure an observable 17. measurement Microhardness case results. measurement hardness Cross-section results. reduction, Cross G1-G1,-section including zone:G1-G1,3C. zone: two 3C. measurement points within the tooth root area. Its interpretation is shown further later. points within the toothFurther, root area.the data Its for interpretation the chip presence is in shown zone 4A further Dp (Figure later. 6) for cross-section H - H (Figure 18) was analyzed and demonstrated a significant hardness reduction in 4A Dp (Figure 19) in comparison to the adjacent areas, as illustrated by the hardness distribution pattern (Figure 20). For 4A Dp, a hardness distribution analysis was performed perpendicular to the outer surface of the carburizing layer, as shown by the measurement pattern (path) in Figure 21. The hardness distribution pattern for the Dp1 set (Figure 22) markedly deviated from the hardness distribution pattern of the Dp2 pattern (path), the spread of

FigureFigure18. 18.Localization Localizationof ofcross-section cross-sectionH-H, H-H,zone: zone:4A 4ADp. Dp.

Figure 19. Pattern of hardness verification. Paths depth (distance from tooth surface): 0.1 mm and FigureFigure 19. 19.Pattern Pattern ofof hardnesshardness veriﬁcation.verification. PathsPaths depthdepth (distance from tooth surface):surface): 0 0.1.1 mm and and 0.3 mm. Cross-section H-H, zone: 4A Dp. 0.30.3 mm. mm.Cross-section Cross-sectionH-H, H-H, zone: zone:4A 4ADp. Dp.

Materials 2021, 14, x 12 of 17

800 0.1 mm Materials 2021, 14, 1155 11 of 16 Materials 2021, 14, x 12 of 17 0.3 mm 780

760800 0.1 mm 0.3 mm 780 740

760

720 0.5 740 700

720 0.5

Hardness Hardness HV 680700

660 Hardness HV 680

660 640

640 620 4A Dp 620 4A Dp 600 600 1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 ImpressionImpression (dent) no. (dent) no.

Figure 20. Microhardness measurementFigureFigure 20. 20. Microhardness results.Microhardness Paths measurement depth measurement (distance results. from Paths results. tooth depth surface): Paths (distance0.1 depth mm from and(distance tooth 0.3 mm.surface) from Cross-: 0 .tooth1 mm surface): 0.1 mm section H-H, zone: 4A Dp. andand 0. 30 .mm3 mm. Cross. Cross-section-section H-H, zone: H-H, 4A zone: Dp. 4A Dp.

Figure 21. Pattern of hardness verification. Cross-section H-H.

FigureFigure 21. 21.Pattern Pattern of hardness of hardness veriﬁcation. verification. Cross-section Cross H-H.-section H-H.

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Materials 2021, 14, 1155 12 of 16 Materials 2021, 14, x 13 of 17

800 Ds Dp1 800 Ds Dp2 700 Dp1 Dp2 700

600

0.5 0.5 500

500

Hardness Hardness HV Hardness Hardness HV 400 400

300 300

200200 1 22 33 4 4 5 5 6 6 7 7 8 8 9 910 1011 1112 1312 1413 14 ImpressionImpression (dent) (dent) no. no.

FigureFigureFigure22. 22. 22.Microhardness Microhardness Microhardnessmeasurement measurement measurement results. results. results. Cross-section Cross -Crosssection-section H-H. H-H. H-H.

Figure 23. Waviness of carburized case depth. Reduction in carburized case depth within the area Dp1. Magnification: 12.5×. Cross-section H-H. Etched sample—Nital 5%. FigureFigure 23. 23.Waviness Waviness of carburizedof carburized case case depth. depth. Reduction Reduction in carburized in carburized case depth case within depth the within area the area Dp1.Dp1.The Magniﬁcation: Magnification: last of the 12.5processed 12×..5 Cross-section×. Cross areas-section of H-H.interest H Etched-H. was Etched sample—Nital 4A Ds sample (Figure— 5%.Nital 6), representing 5%. the tooth root area. The tests were performed on cross-section I-I (Figure 24). The testing revealed a hardnessTheThe lastreduction last of of the the processed in processed 4A Ds areas(Figure areas of interest25), of interestas demonstrated was 4A was Ds 4A (Figure Dsby (the6Figure), representinghardness 6), representing distribution the tooth the tooth patternrootroot area. area. (Figure The The tests 26). tests For were were zone performed performed 4A Ds, the on hardness cross-sectionon cross distribution-section I-I (Figure I-I (patternFigure 24). The showed 24). testing The a reductiontesting revealed revealed a ina hardness hardness, reduction but it must in 4Abe Dsnoted (Figure that 25here), as the demonstrated tooth root area by theis a hardnessregion where, distribution due to hardness reduction in 4A Ds (Figure 25), as demonstrated by the hardness distribution patternthe geometry (Figure of 26 the). For gear zone and 4A its Ds, adjacent the hardness teeth, the distribution kinetics of pattern the carbon showed diffusion a reduction into pattern (Figure 26). For zone 4A Ds, the hardness distribution pattern showed a reduction the surface layer is not as efficient as in the areas near the pitch circle, for example. The in hardness, but it must be noted that here the tooth root area is a region where, due to the geometry of the gear and its adjacent teeth, the kinetics of the carbon diffusion into the surface layer is not as efficient as in the areas near the pitch circle, for example. The

MaterialsMaterials 20212021,, 1414,, xx 1414 ofof 1717 Materials 2021, 14, 1155 13 of 16

ineffecteffect hardness, cancan but bebe it observedobserved must be noted inin thatFigureFigure here the20,20, tooth atat thosethose root area pointspoints is a region locatedlocated where, nextnext due tototo the zonezone Ds,Ds, wherewhere subse-subse- geometryquentquent measurementsmeasurements of the gear and its provedproved adjacent that teeth,that thethe the kineticshardnesshardness of the grewgrew carbon withwith diffusion thethe distancedistance into the fromfrom thethe toothtooth root.root. surface layer is not as efﬁcient as in the areas near the pitch circle, for example. The effect canThisThis be effect observedeffect waswas in Figure alsoalso observed observed20, at those in pointsin thethe located detaileddetailed next analysisanalysis to zone Ds, shownshown where subsequent inin FigureFigure 23,23, wherewhere thethe regionregion measurementsnearnear zonezone DsDs proved featuredfeatured that the anan hardness observableobservable grew with changechange the distance ofof thethe from carburizedcarburized the tooth root. casecase This depthdepth inin comparisoncomparison effecttoto zonezone was also Dp2,Dp2, observed whichwhich in thewaswas detailed aa controlcontrol analysis forfor shown allall teststests in Figure onon 23 thethe, where specimen.specimen. the region Note,Note, near however,however, thatthat withwith zone Ds featured an observable change of the carburized case depth in comparison to zone Dp2,aa smallsmall which chipchip was no ano control distinctdistinct for all effecteffect tests on onon the thethe specimen. casecase hardeninghardening Note, however, depthdepth that withwaswas a observed,observed, small whilewhile thethe small,small, chiplocalizedlocalized no distinct reductionreduction effect on the inin case hardnesshardness hardening mightmight depth was notnot observed, onlyonly bebe while anan the effecteffect small, ofof localized thethe chipchip,, butbut alsoalso aa lowerlower reductionconcentrationconcentration in hardness ofof might carboncarbon not onlyfromfrom be aninhibitedinhibited effect of the diffusiondiffusion chip, but also duringduring a lower carburizing,carburizing, concentration oror possiblypossibly aa slightslight of carbon from inhibited diffusion during carburizing, or possibly a slight contribution fromcontributioncontribution an error in the fromfrom measurement anan errorerror method. inin thethe measurementmeasurement method.method.

FigureFigureFigure 24. 24.24.Localization LocaliLocalizz ofationation cross-section ofof crosscross I-I,--sectionsection zone: 4A I Ds.I--I,I, zone:zone: 4A4A Ds.Ds.

FigureFigureFigure 25. 25.25.Pattern PatternPattern of hardness ofof hardnesshardness veriﬁcation. verification.verification. Paths depth (distance PathsPaths depthdepth from tooth (distance(distance surface): fromfrom 0.1 mm toothtooth and surface)surface):: 00..11 mmmm andand 0.300..33 mm. mmmm Cross-section.. CrossCross--sectionsection I-I, zone: II--I, 4AI, zone:zone: Ds. 4A4A Ds.Ds.

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800 0.1 mm 0.3 mm 780

760

740

720 0.5

700

Hardness Hardness HV 680

660

640

620 4A Ds

600 1 3 5 7 9 11 13 15 17 19 Impression (dent) no.

Figure 26. Microhardness measurementFigure 26. Microhardness results. Paths measurement depth (distance results. from Paths tooth depth surface): (distance 0.1 mm from and tooth 0.3 surface) mm. Cross-: 0.1 mm section I-I, zone: 4A Ds. and 0.3 mm. Cross-section I-I, zone: 4A Ds.

4. Conclusions TTestsests demonstrated demonstrated that that chips chips pressure pressure welded welded or orbonded bonded to the to thesurfaces surfaces of the of gear the ® teethgear teethmade made of Pyrowear of Pyrowear 53 subject® 53 subject to downstream to downstream thermochemical thermochemical treatment treatment can nega- can tivelynegatively affect affect the process the process of carburizin of carburizing.g. Under Under certain certain circumstances, circumstances, the outcome the outcome of this of effectthis effect may may cause cause a localized a localized reduction reduction in inhardness hardness of ofthe the surface surface (carburized (carburized case) case) layer. layer. CConsequencesonsequences ofof thethe unfavorable unfavorable impact impact are are particularly particularly dangerous dangerous when when the copper the copper from fromthe coating the coating described described in the in test the methodology test methodology is attached is attached to the to tooth the tooth surface surface along along with withthe steel the chip,steel chip, which which forms forms a barrier a barrier to the to carbon the carbon diffusion diffusion process process and is and cut is together cut to- withgether the with chip the during chip during its formation. its formation. This phenomenon This phenomenon is extremely is extremely hazardous hazardous to gears to workinggears working under under high loads, high loads, where where stress concentrationsstress concentrations can lead can to lead the failure to the offailure the gear of the or gearthe entire or the gearbox. entire gearbox. TTestsests revealed that small chips (size under 1.51.5 mm) do not have a negative effect on the case hardnesshardness ininthe the area area of of the the pitch pitch circle circle but, but, given given the the geometry geometry of theof the teeth teeth and and the thedecelerated decelerated kinetics kinetics of carbon of carbon diffusion diffusion within within the tooththe tooth root, root, the chips the chips present present in this in partthis partof the of gear the couldgear could result result in small, in small, localized locali reductionszed reductions in hardness. in hardness. The larger The thelarger size the of chipsize (sizeof chip over (size 1.5 over mm) 1 present.5 mm) on present a tooth on surface a tooth during surface the during carburizing the carburizing process, the process, more it canthe moreintensify it can the intensify unfavorable the unfavorable localized reduction localized in reduction carburized in case carburized depth, reduction case depth, in carbonreduc- tionconcentration in carbonin concentration the surface layer in the obscured surface by layer the chips,obscured and localizedby the chips, reduction and locali in hardnesszed re- ductionfor those in areas hardness obscured for bythose the areas chips. obscured Given this, by it the is crucial chips. toGiven properly this, implementit is crucial and to properlyoptimize powerimplement skiving and upstream optimize of power any thermochemical skiving upstream processing, of any asthermochemical the impact of chips processing,on tooth as surfaces the impact or other of chips areas on processed tooth surfaces by thermochemical or other areas processed treatment by may thermochem- negatively ical treatment may negatively affect the performance of the components. It is prudent to

Materials 2021, 14, 1155 15 of 16

affect the performance of the components. It is prudent to improve the machining processes to a point where chip fusion or bonding does not occur. The phenomenon discussed in this work is not only related to power skiving of gear teeth. It can also be present in Fellows slotting and other machine cutting processes. It is important that effects of similar nature do not occur or are eliminated at the machining stages, especially for components which feature areas processed by carburizing. This will help to avoid problems related to localized case hardness reduction in the ﬁnished products, and operating issues with these products.

Author Contributions: Conceptualization, R.F. (Robert Fularski); methodology, R.F. (Robert Fu- larski); validation, R.F. (Ryszard Filip); formal analysis, R.F. (Robert Fularski) and R.F. (Ryszard Filip); investigation, R.F. (Robert Fularski); resources, R.F. (Robert Fularski); data curation, R.F. (Robert Fu- larski); writing of the original draft preparation, R.F. (Robert Fularski); writing of review and editing, R.F. (Ryszard Filip) and R.F. (Robert Fularski); visualization, R.F. (Robert Fularski); supervision, R.F. (Ryszard Filip). All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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