Application of a Prototype Thermoplastic Treatment Line in Order to Design a Thermal Treatment Process of Forgings with the Use of the Heat from the Forging Process

materials

Article Application of a Prototype Thermoplastic Treatment Line in Order to Design a Thermal Treatment Process of Forgings with the Use of the Heat from the Forging Process

Marek Hawryluk 1,* , Zbigniew Gronostajski 1, Maciej Zwierzchowski 1, Paweł Jabło ´nski 1 , Artur Barełkowski 1, Jakub Krawczyk 1, Karol Ja´skiewicz 1 and Marcin Rychlik 2

1 Department of Metal Forming, Welding and Metrology, Wroclaw University of Science and Technology, Lukasiewicz Street 5, 50-370 Wroclaw, Poland; [email protected] (Z.G.); [email protected] (M.Z.); [email protected] (P.J.); [email protected] (A.B.); [email protected] (J.K.); [email protected] (K.J.) 2 Ku´zniaJawor S.A., Kuziennicza 4, 50-370 Jawor, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-71-320-21-64

Received: 29 April 2020; Accepted: 23 May 2020; Published: 27 May 2020

Abstract: The global production of die forgings is an important branch of the motor industry for obvious reasons, resulting from the very good mechanical properties of the forged products. The expectations of the recipients, beside the implementation of the forging process, include also a range of supplementary procedures, such as ﬁnishing treatment including shot blasting, thermal treatment, and machining, in order to ensure the proper quality of the provided semi-product or the ready detail for the assembly line. Especially important in the aspect of the operational properties of the products is the thermal treatment of the forgings, which can be implemented in many variants, depending on the expected results. Unfortunately, a treatment of this type, realized separately after the forging process, is very time and energy-consuming; additionally, it signiﬁcantly raises the production costs due to the increased energy consumption resulting from the necessity of repeated heating of the forgings for such thermal treatment. The article reviews the most frequently applied (separately, after the forging process) thermal treatments for die forgings together with the devices/lines assigned for them, as well as presents an alternative (thermoplastic) method of forging production with the use of the forging heat. The paper also presents a prototype semi-industrial controlled cooling line developed by the authors, which allows the development of the assumed heat treatment of forgings directly after forging with the use of forging heat, together with sample results of conducted tests.

Keywords: thermoplastic treatment technology; controlled cooling line; heat forging; thermal treatment design

1. Introduction Hot die forging, despite being known for a long time now, is one of the most difficult production processes [1]. Currently, in addition to obtaining the assumed geometry of forged products, matrix forgings should have specific mechanical and plastic properties, which are obtained through heat treatment processes [2,3]. Since the mechanical properties are a reflection/dependent on the microstructure, therefore, it is necessary to recognize and analyze these relationships between the forging process and microstructure. The study [4] investigates the effect of various thermal treatment conditions on the microstructural evolution and correlates it with the mechanical properties obtained in the tensile strength test. The thermal treatment of die forgings is realized after the process of forging

Materials 2020, 13, 2441; doi:10.3390/ma13112441 www.mdpi.com/journal/materials Materials 2020, 13, 2441 2 of 19 and trimming in order to unify the microstructure and properties of the forgings [5,6]. Depending on the requirements for forgings, such as the chemical composition, after the forging process, annealing, hardening, and tempering processes as well as oversaturation and aging are usually carried out. In fact, since the 1960s, attempts have been made to combine the plastic deformation process with heat treatment, because in many cases, this provides a synergistic combination of the effects of dynamic processes associated with hot deformation and transformations through diffusion. A suitable combination of both treatments (metal forming and thermal) allows for the mechanical properties of forgings, which are often greater than those that can be obtained by them separately. In traditional die forging technologies, the high operational properties of the forgings are obtained through the selection of the steel with a proper content of carbon and alloy elements (unless the recipient demands a specific material grade) as well as the application of the toughening procedure. A typical cycle of the conventional thermal treatment of die forgings includes: heating of the charge to the initial forging temperature, i.e., about 1100–1300 ◦C, multi-step forging on a forging aggregate (press, hammer, etc.); the forgings after the forging process have the temperature of about 1100 ◦C and, depending on the technology, they can be hot trimmed; next, they are cooled in a box or sometimes on a controlled cooling line B-Y to ambient temperature; next, they are re-heated for normalization and cooled in air or heated to the austenitization temperature and cooled in oil, water, or another medium. The last procedures include tempering in the case of hardening, that is, re-heating the forgings to 550–650 ◦C and cooling them to the ambient temperature, straightening (calibrating) the forgings after hardening, and finishing relief annealing [7]. As it can be inferred, obtaining forgings with high strength properties as well as ductility as a result of separate forging process and next the conventional thermal treatment, requires additional operations related to heating within the given thermal treatment. This is connected with a prolonged production cycle and the accompanying additional costs of labor, device exploitation, cooling agents, and above all, the energy necessary for the repeated heating. In economically developed countries, the economic factors have enforced a reduction of the production time as well as energy consumption through the elimination of some of the mentioned thermal treatment operations. That is why one of the alternatives has become thermoplastic treatment, which takes advantage of the heat generated by the forging process, which in consequence eliminates the necessity of cooling the forgings after the forging process to the ambient temperature and their re-heating [8]. Typically, thermomechanical treatment is to change the mechanical properties of metals in the solid state (and as a result cause microstructural changes in them), which are the result of the combined action of temperature, time, and plastic deformation (the combined processes of heat treatment and plastic deformation). The application of thermoplastic processing technology needs to develop know-how joining knowledge in the field of kinetics of phase transformations during and after deformation with the technical possibilities of its application in industrial conditions. That is why the investigations performed in this field are fully justified, as they will provide many advantages, both in the scientific aspect and the financial benefits for obvious reasons.

2. Materials and Methods The main aim of work is an overview of the currently used heat treatment of die forgings and the possibility of the application of heat directly from the forging process to design more eﬃciency in thermoplastic technology. Based on it, as a novelty, it presented its own research related to the application of a prototype-controlled cooling line in order to design the three diﬀerent thermal treatment process of forgings (often carried out separately in industry) with the use of the heat from the forging process. The research was divided into two stages:

I. Carrying out various heat treatment variants in industrial conditions for a forked type forgings made of C45 steel (used in the steering gears of passenger cars). The tests were carried out to analyze such possibilities and to determine the structures and hardness obtained and to show the beneﬁts of using forging heat for direct heat treatment. Materials 2020, 13, 2441 3 of 19

II. The concept of the prototype line for the design of diﬀerent variants of forgings heat treatment with the possibility of the application of the forging heat was presented. Then, tests were carried out, in particular on the developed controlled cooling line for samples of two materials (C45 and 16MnCr5) used for the forging of yoke type, in order to show the line’s possibilities and veriﬁcation as well as its use in industrial conditions.

3. Subject Matter The thermal treatment technologies can be implemented by means of furnaces of different constructions, sizes, and power supply types, working in various cycles, atmospheres, and at different temperatures. In view of such diversity, it seems important to select the key parameters ensuring an efficient thermal treatment, in which the die forgings can obtain the proper structure and properties. This is especially important in the case of elements with complex geometry, which are problematic in their treatment, and their thermal treatment process is hard to control [9–11]. The most frequently expected structure is the temper sorbite obtained in steel forgings as a result of toughening through hardening and tempering. For the thermal treatment of forgings, chamber, pusher, and belt furnaces powered by gas burners or electric heaters are used. Much better effects are obtained on a technological line applying belt furnaces, on which the thermal treatment runs in a nitrogen shield. Slightly worse effects in respect of the forging quality (due to decarburization) but with similar efficiency were obtained in chamber furnaces. The selection of the thermal treatment technology should be determined mainly by the quality of the obtained microstructure and properties of the forgings. In the second place, one should take into the account the process efficiency, the possibility of its automatization, and thus the efficiency of the production process itself. The availability as well as the possibility of implementing thermal treatment procedures have been included in Figure1. At present, among those mentioned in Figure1, the most commonly used types of thermal treatment implemented at a large scale in the production of steel forgings are normalization, toughening, and isothermal annealing, which are usually performed separately from the forging process [12–15]. For stainless and austenitic steels, oversaturation is applied. For aluminum forgings, oversaturation and aging are usually performed. In the case of copper alloys, the most frequently applied procedures are oversaturation and aging [16]. In turn, for mono-phase titanium alpha alloys, partial annealing at lower temperatures is implemented as well as full annealing at higher temperatures. For pseudo-alpha alloys, normalization is applied. Diphase titanium alloys can be subjected to normalization as well as aging and tempering. Titanium beta alloys undergo toughening [17]. Interesting research using thermomechanical treatment for TiAl titanium alloy. The canned-forging and subsequent heat treatments of Ti-43Al-4Nb-1.5Mo alloy have been conducted, during which the hot deformation behavior, flow softening mechanism, microstructure evolution, and mechanical properties were investigated [18]. In turn, for magnesium alloy forgings, oversaturation as well as artificial and natural aging are applied. In addition, less often, recrystallization and relief annealing are implemented [19–23]. In the case of nickel alloys, heat treatment is primarily used to shape the properties of carbides prior to putting these alloys into service, ensuring that their desired structures and morphology are obtained. Depending on the chemical composition, requirements for operational properties and anticipated operating conditions, nickel alloys can be subjected to six basic types of heat treatment: normalization, homogenization, stress relief annealing, stress equalization, oversaturation, and precipitation hardening [24]. Materials 2020, 13, 2441 4 of 19 Materials 2020, 13, x FOR PEER REVIEW 4 of 19

Figure 1. Division of the thermal treatmenttreatment procedures of die forgings.

InFor the stainless implementation and austenitic of a thermal steels, treatment oversatu process,ration theis applied. key device For is thealuminum furnace inforgings, which theoversaturation process takes and place aging [25 are–27 usually], and as performed. well as conditions In the case of heatof copper exchange alloys, [28 the–33 ].most In thisfrequently aspect, weapplied distinguish procedures between are oversaturation chamber, tunnel, and pusher,aging [16]. and In belt turn, furnaces, for mono-phase as well as titanium others, alpha with specialalloys, constructionspartial annealing and applicationsat lower temperatures [34–37]. Beside is theimplemented furnace, the as technological well as full line annealing includes additionalat higher instrumentation,temperatures. For such pseudo-alpha as cooling lines alloys, [38 ],normalization quenching tanks, is applied. washers, Diphase shot blasting titanium machines, alloys feeders,can be conveyors,subjected to trolleys,normalization and platforms as well as [ 39aging,40]. and The tempering. whole stand Titanium or line beta for alloys thermal undergo treatment toughening can be organized[17]. Interesting in different research ways, using depending thermomechanical on the needs and treatment possibilities for TiAl of the titanium forging producer. alloy. The A canned- popular solutionforging and is the subsequent application heat of a chambertreatments furnace, of Ti-43A or al-4Nb-1.5Mo few furnaces alloy placed have next been to each conducted, other in one during row togetherwhich the with hot the deformation quenching tanksbehavior, and washers.flow soften Theing whole mechanism, is operated microstr by anucture industrial evolution, car, which and makesmechanical it possible properties to transport were investigated the charge [18]. between In turn, the particularfor magnesium devices. alloy A forgings, characteristic oversaturation feature of suchas well stands as artificial is the operation and natural and aging treatment are applied. of the whole In addition, charge, whichless often, is placed recrystallization on one pallet and and relief then heated,annealing cooled, are implemented and washed [19–23]. as a whole. In the This case causes of nickel certain alloys, diffi heatculties treatment connected is primarily with the thermalused to inertiashape the of the properties system, whichof carbides is quite prior hard to toputting heat and thes evene alloys harder into to service, cool at ensuring a sufficiently that hightheir rate. desired An examplestructures of and such morphology a stand has beenare obtained. shown in Depending Figure2a. Another on the chemical solution iscomposition, a technological requirements line in which for theoperational product properties is transported and inanticipated a continuous operating manner condit lengthwiseions, nickel on the alloys platform can be or subjected the belt. to Then, six basic the particulartypes of heat elements treatment: of the normalization, stand (furnaces, homogenizat tanks, washers)ion, stress are placedrelief anneal in oneing, line stress and connectedequalization, to conveyorsoversaturation, and feeders. and precip In solutionsitation hardening of this kind, [24]. each detail (in belt furnaces) or a small portion of the productIn the (in implementation pusher furnaces) of undergoes a thermal thetreatment treatment proc iness, an the individual key device manner, is the whichfurnace eliminates in which the eprocessffect of collectivitytakes place and[25–27], the problems and as well connected as conditio withns transporting of heat exchange a large charge. [28–33]. An In example this aspect, of such we adistinguish solution has between been shown chamber, in Figure tunnel,2b. pusher, and belt furnaces, as well as others, with special constructions and applications [34–37]. Beside the furnace, the technological line includes additional instrumentation, such as cooling lines [38], quenching tanks, washers, shot blasting machines, feeders, conveyors, trolleys, and platforms [39,40]. The whole stand or line for thermal treatment can be organized in different ways, depending on the needs and possibilities of the forging producer. A popular solution is the application of a chamber furnace, or a few furnaces placed next to each other in one row together with the quenching tanks and washers. The whole is operated by an industrial

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car, which makes it possible to transport the charge between the particular devices. A characteristic feature of such stands is the operation and treatment of the whole charge, which is placed on one pallet and then heated, cooled, and washed as a whole. This causes certain difficulties connected with the thermal inertia of the system, which is quite hard to heat and even harder to cool at a sufficiently high rate. An example of such a stand has been shown in Figure 2a. Another solution is a technological line in which the product is transported in a continuous manner lengthwise on the platform or the belt. Then, the particular elements of the stand (furnaces, tanks, washers) are placed in one line and connected to conveyors and feeders. In solutions of this kind, each detail (in belt furnaces) or a small portion of the product (in pusher furnaces) undergoes the treatment in an Materialsindividual2020 ,manner,13, 2441 which eliminates the effect of collectivity and the problems connected 5with of 19 transporting a large charge. An example of such a solution has been shown in Figure 2b.

(a) (b)

Figure 2. ExemplaryExemplary images of ( a) the the stand stand for for the the thermal thermal treatment treatment of forgings in a chamber furnace and (b) the standstand forfor thethe thermalthermal treatmenttreatment basedbased onon aa beltbelt furnace.furnace.

This is connected with a prolonged production cycl cyclee as well as the the accompanying accompanying costs costs of of labor, labor, operation and exploitation exploitation of of devices, devices, cooling cooling agents agents,, and and most most of of all all energy energy consumption, consumption, which which is isneeded needed for for the the heating heating procedures, procedures, and and also also with with environment environment pollution. pollution. Currently, Currently, in in order to eliminate the the costly costly and and time-consuming time-consuming toughening toughening process, process, we we can can observe observe a development a development in the in thefield field of designing of designing chemical chemical compositions compositions and technologies and technologies of die of forging die forging production, production, which which takes takesplace placein four in grade four gradegroups: groups: pearlitic–ferritic pearlitic–ferritic steels, bainitic steels, bainiticsteels, multi-phase steels, multi-phase steels with steels a ferritic– with a ferritic–bainitic–pearliticbainitic–pearlitic structure structure and steels and steels with with a disl a dislocationocation martensite martensite structure. structure. Another Another observed direction of die forging development is the groupgroup of steels whose chemical composition and manner of cooling after after hot hot forging forging make make it itpossible possible to toobta obtainin a product a product with with a bainitic a bainitic structure: structure: lower, lower, fine- fine-grained,grained, and non-carbide. and non-carbide. A condition A condition for the for obtainin the obtainingg of a structure of a structure of this of type this typeis ensuring is ensuring a fine- a fine-grainedgrained austenite austenite structure structure after after the theend end of ofdeform deformationation and and a avery very strict strict control control of of the cooling process after the deformation deformation [6]. [6]. So, So, an an alternative alternative method method of of producing producing die die forgings forgings with with the the use use of oftoughening toughening makes makes it possible it possible to limit to limit the production the production costs costsand reduce and reduce the energy the energy consumption, consumption, which whichmaintains maintains the required the required properties properties of the of theforgings; forgings; this this method method is isa acombination combination of of regulated accelerated cooling cooling directly directly after after the the forging forging process. process. This This means means the theuse useof the of so-called the so-called forging forging heat heatand no and necessity no necessity of cooling of cooling the forgings the forgings down down to the to ambient the ambient temperature temperature and then and re-heating then re-heating them themto the to required the required temperature temperature (high-temperature (high-temperature thermoplastic thermoplastic treatment). treatment). Research Research on the on theuse use of ofthermoplastic thermoplastic technology technology is isbeing being carried carried out out more more and and more more often, often, which which aims aims to to compare the obtained mechanicalmechanical properties properties of of forged forged products products and and the economicsthe economics of such oftechnology such technology with current with methodscurrent methods (separate (separate forging and forging subsequent and subsequent heat treatment heatof treatment forgings). of The forgings). study [41 The] compares study [41] the mechanicalcompares the properties mechanical of metalproperties sheets of withmetal a sheets ferritic–martensitic with a ferritic–martensitic structure made structure of steel made C-Mn of withsteel micro-additionsC-Mn with micro-additions Nb and Ti produced Nb and by Ti means produced of an energy by means saving of thermoplastic an energy saving treatment thermoplastic technology, withtreatment those technology, obtained after with the those conventional obtained hardening after the conventional from a temperature hardening above from AC1 a. Suchtemperature studies wereabove also AC1 verified. Such studies in [42], whichwere also deals verified with the in impact [42], which of the usedeals of ThermoPlasticwith the impact Treatment of the use (TPT) of throughThermoPlastic forging Treatment on the structure (TPT) through and mechanical forging propertieson the structure of micro-alloy and mechanical steel. Forgings properties produced of micro- by thisalloy method steel. Forgings were subjected produced to temperingby this method in the were temperature subjected range to tempering from 550 toin 650the temperature◦C, which resulted range infrom obtaining 550 to 650 the properties:°C, which resulted Rp0.2, in in the obtaining range of the 925–993 properties: MPa; Rm, Rp0.2, in thein the range range of 978–1061 of 925–993 MPa. MPa; In turn,Rm, in the the article range [43 of] examined978–1061 MPa. the impact In turn, of selectedthe article heat [43] treatment examined parameters the impact on of the selected mechanical heat propertiestreatment ofparameters a steam turbine on the blade mechanical operating proper at a temperatureties of a steam of up toturbine 700 ◦C. blade operating at a temperatureIn addition, of up nowadays, to 700 °C. together with the development of new steel grades, thermo plastic treatment is also increasingly used, due to the fact that such steels contain alloying elements that predispose this type of treatment. One of its applications is the use of such a solution for Fe-C-Al-based alloys that would make it possible to design high-strength construction steel not containing elements regulated as critical by the UE (European Union), such as Nb, V, and Mo. The work [44] showed that by using TPT (thermo plastic treatment), it is possible to obtain properties of alloys that do not contain critical elements that are comparable with the mechanical properties of traditional alloys. In addition, TPT is also used for other Ti [45] and Al [46]-based alloys. The mentioned studies also present a research-based confirmation of the economical and mechanical advantages of the method. In the available literature on the subject, you can find many other interesting studies on TPT, in which the authors used numerical modeling [47] regarding microstructural changes after different TPT Materials 2020, 13, x FOR PEER REVIEW 6 of 19

In addition, nowadays, together with the development of new steel grades, thermo plastic treatment is also increasingly used, due to the fact that such steels contain alloying elements that predispose this type of treatment. One of its applications is the use of such a solution for Fe-C-Al- based alloys that would make it possible to design high-strength construction steel not containing elements regulated as critical by the UE (European Union), such as Nb, V, and Mo. The work [44] showed that by using TPT (thermo plastic treatment), it is possible to obtain properties of alloys that do not contain critical elements that are comparable with the mechanical properties of traditional alloys. In addition, TPT is also used for other Ti [45] and Al [46]-based alloys. The mentioned studies also present a research-based confirmation of the economical and mechanical advantages of the

Materialsmethod.2020 In, the13, 2441available literature on the subject, you can find many other interesting studies on6 TPT, of 19 in which the authors used numerical modeling [47] regarding microstructural changes after different TPT variants [48], as well as the impact of changes in heating and cooling parameters immediately variantsafter forging [48], ason well selected as the mechanical impact of changesproperties in heatingof forgings, and coolingincluding parameters in [49–51] immediately for the aviation after forgingindustry. on selected mechanical properties of forgings, including in [49–51] for the aviation industry.

4.4. The Die Forging Process with SelectedSelected HeatHeat TreatmentTreatment VariantsVariants FigureFigure3 3 shows shows an an example example of of a a typical typical die-forging die-forging process process of of forked forked forgings, forgings, followed followed by by a a separateseparate heatheat treatmenttreatment ofof forgings.forgings. In the case of such a traditional forging processprocess (Figure(Figure3 3a),a), after after forgingforging isis followedfollowed controlledcontrolled coolingcooling ofof thethe forgingsforgings onon thethe coolingcooling lineline (free(free cooling)cooling) wherewhere atat thethe endend ofof thethe procedure,procedure, thethe forgingsforgings fallfall intointo thethe container;container; theirtheir temperaturetemperature isis thenthen aroundaround 610610 ◦°CC (Figure(Figure3 3b).b). It It is is close close to tothe thetransformation transformation temperature temperature for for this this material: material:C45 C45 steelsteel (1.0503(1.0503 according according toto DINDIN standard).standard).

(a) (b)

FigureFigure 3.3. The views of (a) typicaltypical die-forging process an exampleexample with a separate heat treatment, ((bb)) photosphotos ofof forkedforked forgingsforgings duringduring andand afterafter aa controlledcontrolled coolingcooling process.process.

TheThe forgings,forgings, after after reaching reaching the the ambient ambient temperature temperatur ine ain box, a box, are are transported transported to shot to shot blasting blasting and afterand after it go it to go oil to hardening, oil hardening, cooling, cooling, and tempering.and tempering. The durationThe duration of the of entire the entire production production cycle cycle and thatand forgingsthat forgings are heated are heated twice twice (during (during quenching quenching and tempering) and tempering) should should be highlighted, be highlighted, which resultswhich inresults higher in energyhigher andenergy refrigerant and refrigerant consumption consumption (oil or polymer).(oil or polymer). On this On basis, this asbasis, part as of part the study,of the astudy, decision a decision was made was tomade try toto usetry heatto use from heat the from forging the forging process, process, and then and carry then outcarry research out research work forwork three for dithreeﬀerent different heat treatment heat treatment variants variants immediately immediately after the after forging the process.forging Figureprocess.4 presentsFigure 4 thepresents paths the of thepaths performed of the performed investigations investigations for the following: for the following: oil hardening oil hardening directly fromdirectly the from forging the (markedforging (marked in blue), in oil blue), hardening oil hardening after the after forging the fo withrging special with special annealing annealing at 870 ◦ atC 870 (marked °C (marked in gray), in coolinggray), cooling down from down the from forging the temperature forging temperature 1200 to 600 ◦1200C (temperature to 600 °C below(temperature the transformation below the oftransformation perlitic), re-heating of perlitic), and annealingre-heatingat and 870 annealing◦C, and the at next870 °C, oil and hardening the next (orange oil hardening course). (orange For all thecourse). three For chosen all the variants, three chosen after variants, oil hardening, after oi thel hardening, tempering the treatment tempering was treatment performed was (accordingperformed for(according this forging). for this The forging). assumed The levelsassumed of temperatures levels of temperatures were determined were determined based oﬀ basedof the off analysis of the foranalysis this material, for this material, i.e., steel i.e., C45, steel as well C45, as as the well carried as the out carried investigations. out investigations. For C45 For steel, C45 the steel, perlitic the transformationperlitic transformation with free with cooling free takes cooling place takes at about place 660 at◦ Cabout (therefore, 660 °C the (therefore, forgings werethe forgings cooled down were tocooled 600 ◦ C),down while to 600 the °C), temperature while the of temperature austenitization of austenitization is in level of 860–880 is in level◦C. of 860–880 °C.

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Figure 4. TheThe paths paths scheme scheme of of the the proposed proposed thermal treatmen treatmenttt variants variants directly directly after after the the forging forging process.

Then, forfor all all implemented implemented variants variants (one (one forging forg wasinging randomly waswas randomlyrandomly selected selectedselected from each), fromfrom SEM each),each), (scanning SEMSEM electron(scanning microscope) electron microscope) microstructure microstructure analysis and analysis microhardness and microhardness measurements measurements were performed. were Theperformed. samples The for structuralsamples for tests structural were etched tests with were 3.5% etched Nital with reagent 3.5% (Figure Nital reagent5). Hardness (Figure measurements 5). Hardness weremeasurements carried out were using carried the Vickers out using method the Vickers at 10 N loadmethod (Figure at 106). N Figure load (Figure5a shows 6). a Figure typical 5a structure shows a for tempered martensite with numerous fine precipitation of carbides (Fe C) in a spherical form.3 Of typical structure for tempered martensite with numerous fine precipitation3 of carbides (Fe3C) in a course,spherical there form. are Of also course, elongated there are forms also ofelongated these precipitates. forms of these When precipitates. comparing When the microstructurecomparing the formicrostructure forgings made for forgings in accordance made in with accordance Option 2, with it di OptionOptionffers significantly 2,2, itit differsdiffers significantlysignificantly from Option fromfrom 1 (Figure OptionOption5b). 11 One(Figure can 5b). notice One the can precipitation notice the ofprecipitation ferrite at the of boundaries ferriteite atat thethe of ancientboundariesboundaries austenite ofof ancientancient grains, austeniteaustenite which indicates grains,grains, awhich typical indicates sorbit structurea typical sorbit after quenching.structure after In quenching. SEM images, In SEM it is visible images, as it darker is visible areas as darker that do areas not containthat do not precipitates. contain precipitates.

(a)

Figure 5. Cont.

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(b)

(c)

(d)

FigureFigure 5.5. SEM scanning microscopic analysis after threethree proposed treatment variants (hardening) directlydirectly afterafter thethe forgingforging process:process: (a) technology I, (b) technologytechnology II, ((c)) technologytechnology III,III, andand ((d)) thethe placesplaces ofof thethe metallographicmetallographic specimensspecimens waswas sampled.sampled.

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FigureFigure 6. Microstructural 6. Microstructural test results test results for the threefor the selected three thermalselected treatmentthermal treatment variants:thermal variants: improvement. thermal improvement. Figure5c shows the results of microstructural tests for forgings made in accordance with the path forFigure variant 5c shows 3. In this the case, results you of can microstructural also observe ferrite tests for precipitation forgings made at the in former accordance austenite with grain the boundariespath for variant (dimensions 3. In this 30–50case, youµm), can as also well observe as fine ferrite inclusions precipitation of Fe3C at carbides the former in a austenite spherical grain and elongated form. The results presented above in the hardening process of forgings made of C45 steel boundaries (dimensions 30–50 µm), as well as fine inclusions of Fe3C carbides in a spherical and (microstructure,elongated form. hardness)The results with presented the time–temperature above in the hardening parameters process presented of forgings confirm made that byof correctlyC45 steel selecting(microstructure, them, the hardness) properties with required the bytime–tempe the final recipientrature canparameters be obtained. presented What’s more,confirm depending that by oncorrectly the expectations selecting them, of the the recipient properties by changing required these by the parameters, final recipient you cancanadjust be obtained. the properties What’s ofmore, the finisheddepending product. on the A expectations comprehensive of the heat recipient treatment by waschanging carried these out forparamete each variant.rs, you Thecan treatmentadjust the afterproperties the hardening of the finished process product. enters the A processcomprehensiv of tempering.e heat treatment Therefore, was there carried are very out smallfor each diff variant.erences inThe the treatment structure after of the the material. hardening process enters the process of tempering. Therefore, there are very smallYou differences can see from in the the structure measurement of the material. results (Figure 5), basically regardless of the heat treatment variantYou used can (fromsee from the the proposed measurement ones), theresults hardness (Figure values 5), basically are in theregardless range ofof 235–255the heat HV.treatment Such valuesvariant are used also (from within the theproposed range accordingones), the hardne to thess recipient’s values are requirements. in the range of Therefore, 235–255 HV. it can Such be concludedvalues are thatalso the within proposed the range heat treatment according variants to the carriedrecipient’s out directlyrequirements. from the Therefore, forging process it can dobe notconcluded have a negativethat the proposed impact on heat both treatment the microstructure variants carried and the out hardness directly of from the forgings the forging obtained process after do separatenot have/independent a negative impact heat treatment. on both the It microstructu seems that hardeningre and the directly hardness from of th thee forgings forging temperature obtained after is possible,separate/independent but from a technological heat treatment. point It of seems view, that it is hardening unjustified. directly The high from hardening the forging stress temperature generated canis possible, cause product but from deformation a technological and/or crackingpoint of during view, theit is cooling unjustified. process. The high hardening stress generatedIn addition, can cause carrying product out the deformation proposed heat and/ treatmentor cracking variants during directly the cooling from the process. forging temperature is of courseIn addition, economically carrying justified, out the bothproposed in terms heat of tr processeatment effi variantsciency anddirectly production from the costs. forging It is obviouslytemperature important is of course to precisely economically define justified, the cooling both times in terms and rates of process to ensure efficiency adequate and temperature production controlcosts. It over is obviously a specific important range of times to precisely and temperatures define the cooling times and rates to ensure adequate temperature control over a specific range of times and temperatures 5. The Concept of the Prototype Line for the Design of Different Variants of Forgings Heat Treatment with Possibility the Application of the Forging Heat 5. The Concept of the Prototype Line for the Design of Different Variants of Forgings Heat TreatmentTo examine with andPossibility analyze the impactApplication of selected of the time–temperature Forging Heat parameters on the heat treatment of forgings, the authors developed and built a prototype experimental line to test different heat To examine and analyze the impact of selected time–temperature parameters on the heat treatment variants. The line is based on the so-called high temperature thermoplastic treatment treatment of forgings, the authors developed and built a prototype experimental line to test different enabling the simulation and optimization of the thermal treatment of processes carried out in industrial heat treatment variants. The line is based on the so-called high temperature thermoplastic treatment conditions. Based on the research and analysis of the results obtained on the experimental line, an enabling the simulation and optimization of the thermal treatment of processes carried out in industrial line will then be designed and built. Ultimately, such an industrial line will enable the design industrial conditions. Based on the research and analysis of the results obtained on the experimental and conduct of specific heat treatment in industrial conditions in die forges. line, an industrial line will then be designed and built. Ultimately, such an industrial line will enable The concept of the prototype line is based on any (depending on the requirements of the developed the design and conduct of specific heat treatment in industrial conditions in die forges. thermal processing technology) arrangement of four key devices: mechanical press for hot and hot The concept of the prototype line is based on any (depending on the requirements of the forging (P), controlled cooling line (L), chamber furnace for heating the charge/forgings (F), and developed thermal processing technology) arrangement of four key devices: mechanical press for hot and hot forging (P), controlled cooling line (L), chamber furnace for heating the charge/forgings (F),

Materials 2020, 13, 2441 10 of 19 Materials 2020,, 13,, xx FORFOR PEERPEER REVIEWREVIEW 10 of 19 oiland/polymer oil/polymer hardening hardening baths baths (Q). (Q). Thanks Thanks to this,to this, through through any any conﬁguration configuration of of devices, devices, itit willwill bebe possiblepossible toto performperform anyany thermoplasticthermoplastic treatment,treatment, including,including, e.g.,e.g., bothboth normalizationnormalization andand quenchingquenching (Figure(Figure7 7).). WithWith thethe rightright selectionselection ofof thesethese keykey devices,devices, itit isis possiblepossible totosimulate simulate almostalmost anyany heatheat treatmenttreatment process,process, bothboth asas aa separateseparate oneone andand immediatelyimmediately afterafter thethe forgingforging processprocess (Figure(Figure8 ).8).

Figure 7. The flowchart ﬂowchart of a test station for the processing of thermoplastic.

Figure 8. Exemplary arrangement of the test stand for thermoplastic treatment.

New in thethe proposedproposed solution,solution, i.e., the entire experimentalexperimental line, is the prototype line for thethe controlled coolingcooling of forgings.forgings. It is built of a chamber with a length of 3–4 mm (with(with thethe possibilitypossibility ofof attaching oror removingremoving shielding shielding panels panels or or adding adding one one segment) segment) and and a width a width of 500 of mm,500 mm, together together with awith conveyor a conveyor belt for belt transporting for transporting forgings forgings/charge/charge material. material. Directly aboveabove thethe chamber there areare 4 mechanical fans, each with a power of over 600 W, which allow youyou toto pumppump airair into into the the chamber chamber and and its its circulation circulation to to cool cool the the analyzed analyzed elements. elements. The The speed speed of theof the ventilators ventilators and and the the tape tape feed feed is regulatedis regulated by by means means of frequencyof frequency converters. converters. Owing Owing to theto the use use of aof high a high velocity velocity ratio, ratio, it isit is possible possible to to fully fully load load the the line line with with the the weight weight of of up up to to 150 150 kgkg/m./m. TheThe sideside wallswalls ofof thethe chamber chamber were were designed designed in in such such a waya way so thatso that the the biggest biggest possible possible stream stream of air of would air would ﬂow aroundflow around the forgings the forgings moving moving on the on belt. the An belt. important An important aspect aspect was such was a such construction a construction design design which wouldwhich makewould it make possible it possible for the forging for the to forging be cooled to be uniformly cooled onuniformly each side, on including each side, the including bottom. Forthe thisbottom. reason, For athis conveyor reason, belt a conveyor made of belt a heat-resisting made of a heat-resisting net with big net meshes with wasbig meshes used, which was used, minimized which theminimized air resistance the air and resistance reduced and the reduced ground etheﬀect. ground effect.

5.1. Exemplary Tests underunder Semi-IndustrialSemi-Industrial ConditionsConditions InIn orderorder to to verify verify the the developed developed line, line, preliminary preliminary tests tests were were carried carried out on out a semi-industrialon a semi-industrial stand usingstand feedstock,using feedstock, i.e., two i.e., steels: two st 16MnCr5eels: 16MnCr5 (1.7131 (1.7131 according according to DIN) to DIN) and C45. and Then,C45. Then, test samples test samples with dimensions: 35 20 mm and 35 40 mm were prepared to further analyze the eﬀect of volume with dimensions:∅ ×⌀35 × 20 mm and∅ ⌀×35 × 40 mm were prepared to further analyze the effect of volume changes. Twelve identical samples were made for each of both materials; for each geometry, i.e.,i.e., 2424 samples in total for each steel (12 with dimensions 35 20 mm and 12 with dimensions 35 40 mm). samples in total for each steel (12 with dimensions∅ ⌀×35 × 20 mm and 12 with dimensions∅ × ⌀35 × 40 mm).Separate heating was used for each sample because of the possibility of scale formation in the case ofSeparate repeated heating heating was of used the same for each element. sample Each because test consisted of the possibility of heating of thescale sample formation material in the in ancase oven of repeated to a temperature heating of just the above same element. 1100 ◦C andEach then test subjectingconsisted of it toheating a controlled the sample cooling material process in an to oven to a temperature just above 1100 °C and then subjecting it to a controlled cooling process to about 400 °C on a controlled cooling line. In addition, to analyze the impact of fan speeds, a different

Materials 2020, 13, 2441 11 of 19

about 400 ◦C on a controlled cooling line. In addition, to analyze the impact of fan speeds, a different Materials 2020, 13, x FOR PEER REVIEW 11 of 19 fan speedMaterials was 2020 used, 13, x FOR for PEER each REVIEW sample in a given series (dimensions and material), which was11 changedof 19 by 5 Hz using a frequency converter in the range from 0 Hz (which was 0 rpm) to 55 Hz (which was fan speed was used for each sample in a given series (dimensions and material), which was changed fan speed was used for each sample in a given series (dimensions and material), which was changed respectivelyby 5 Hz 1650using rpm)a frequency for maximum converter fan in the speed. range The from belt 0 Hz speed (which for was all measurements0 rpm) to 55 Hz was(which constant was at by 5 Hz using a frequency converter in the range from 0 Hz (which was 0 rpm) to 55 Hz (which was aroundrespectively 2 m/min. 1650 Preliminary rpm) for testsmaximum showed fan thatspeed. the The change belt speed in belt for speed all measurements had no significant was constant effect at on the respectively 1650 rpm) for maximum fan speed. The belt speed for all measurements was constant at samplearound cooling 2 m/min. rate. ThePreliminary value of tests the showed belt speed that wasthe ch adoptedange in belt in suchspeed a had way no that significant for the effect switched on off around 2 m/min. Preliminary tests showed that the change in belt speed had no significant effect on fans,the a single sample sample cooling was rate. cooled The value from of the the forgingbelt speed temperature was adopted from in such 400 a wayC. During that for eachthe switched process, the the sample cooling rate. The value of the belt speed was adopted in such a ◦way that for the switched off fans, a single sample was cooled from the forging temperature from 400 °C. During each process, temperatureoff fans, ofa single the middle sample ofwas the cooled sample from was the recordedforging temperature using a K-type from 400 thermocouple °C. During each with process, a diameter the temperature of the middle of the sample was recorded using a K-type thermocouple with a of 1.2the mm. temperature During each of the test, middle the cooling of the timesample was was measured recordedas using the dia ffK-typeerence thermocouple between the with time a when diameter of 1.2 mm. During each test, the cooling time was measured as the difference between the the samplediameter with of 1.2 the mm. forging During temperature each test, the of coolin 1100g◦ Ctime reached was measured 400 ◦C. Inas the Figures difference9 and between 10, the coolingthe time when the sample with the forging temperature of 1100 °C reached 400 °C. In Figures 9 and 10, processestime when for both the materialssample with (with the forging dimensions temperatur 35 40e of mm) 1100 were°C reached discussed. 400 °C. In Figures 9 and 10, the cooling processes for both materials (with dimensions× 35 × 40 mm) were discussed. the cooling processes for both materials (with dimensions 35 × 40 mm) were discussed.

Figure 9. The cooling curves of a samples made of 16MnCr5 ⌀35 × 40 mm from forging temperature FigureFigure 9. The 9. The cooling cooling curves curves of of a samplesa samples made made ofof 16MnCr5 ⌀3535 × 4040 mm mm from from forging forging temperature temperature 1100 °C to 400 °C. ∅ × 1100 ◦1100C to °C 400 to ◦400C. °C.

Figure 10. The cooling curves of a samples made of C45 35 × 40 mm from forging temperature 1100 Figure 10. The cooling curves of a samples made of C45 35 × 40 mm from forging temperature 1100 Figure°C 10. to 400The °C. cooling curves of a samples made of C45 35 40 mm from forging temperature 1100 ◦C °C to 400 °C. × to 400 ◦C.

Materials 2020, 13, 2441 12 of 19 Materials 2020, 13, x FOR PEER REVIEW 12 of 19

The cooling temperature temperature of of the the onset onset of of the the phase phase transformation transformation of ofaustenite austenite into into perlite perlite can can be beobserved observed as asthe the cooling cooling rate rate increases. increases. Of Of course, course, it itoccurs occurs much much faster faster at at higher higher cooling cooling speeds. Comparing the results obtained for 16MnCr5 steel (F (Figureigure 99)) withwith respectrespect toto C45C45 steelsteel (Figure(Figure 1010),), phase transformation isis less visible due to the higher carbon content and the the lack lack of of alloying alloying elements. elements. Figure 11 presents the distribution for hardnesshardness measurements (as the average of three measurement points)points) for for both both materials: materials: 16MnCr5 16MnCr5 and and C45 C45 as aas function a function of fan of speedfan speed for samples for samples with dimensionswith dimensions 35 2035 mm,× 20 mm, as mechanical as mechanical properties. properties. ×

Figure 11.11. The distribution for hardness measurements for both materialsmaterials (16MnCr5 and C45) as a function of fanfan speedspeed forfor samplessamples withwith dimensionsdimensions 3535 × 40 mm. × For thethe graph graph presented presented in Figure in Figure 11, di ff11,erences differen in hardnessces in resulthardness from result the material from characteristicsthe material (highercharacteristics carbon (higher content: carbon a higher content: amount a ofhigher perlite amount in the material of perlite structure). in the material The presence structure). of perlite The determinespresence of mostperlite of determines the hardness, most as of is the casehardness, with C45as is steel. the case The with differences C45 steel. in hardnessThe differences between in thehardness two materials between arethe aroundtwo materials 30–40 HB.are Asaround the fan30–40 speed HB. increases, As the fan the speed hardness increases, increases the hardness for both materials.increases for For both customers materials. who For do customers not require who the structuredo not require of martensite the structure tempered of martensite in the forging, tempered there isin thethe possibilityforging, there of using is the controlled possibility cooling of using from controlled the forging cooling temperature. from the forging For yoke-type temperature. forgings For madeyoke-type of both forgings materials, made the of hardnessboth materials, should the be hardness in the range should suitable be in forthe C45range (175–260 suitable HB) for C45 steel, (175– and for260 16MnCr5HB) steel, (150–240 and for HB). 16MnCr5 Hence, (150–240 on this basis, HB). itHence, can be concludedon this basis, that it by can choosing be concluded the appropriate that by fanchoosing speed the (controlled appropriate cooling) fan speed on the (controlled developed cooling) line, it ison possible the developed to achieve line, the it is assumed possible hardness. to achieve the assumedOn the other hardness. hand, Figure 12 presents sets of cooling courses for two different materials and two cooling rates for the same, but only slightly smaller sample sizes (35 20 mm), because there were no On the other hand, Figure 12 presents sets of cooling courses for× two different materials and two significant differences in the waveforms for larger (35 40 mm) and smaller (35 20 mm) sample sizes cooling rates for the same, but only slightly smaller sample× sizes (35 × 20 mm), ×because there were no ofsignificant the same differences material. in the waveforms for larger (35 × 40 mm) and smaller (35 × 20 mm) sample sizesAfter of the compiling same material. any two cooling courses for samples with the same dimensions and cooling parameters, but for different materials, we can also notice that despite the more energetic transformations occurring in the material at C45, the cooling time is ultimately the same. With an increase of the cooling rate, the temperature of the transformation lowers. This is clearly visible for the material C45. It is an important parameter that should be taken into consideration while designing thermal treatment processes.

Materials 2020, 13, x FOR PEER REVIEW 13 of 19

Materials 2020, 13, 2441 13 of 19 Materials 2020, 13, x FOR PEER REVIEW 13 of 19

Figure 12. The cooling curves of 16MnCr5 and C45 samples with the dimensions ⌀35 × 20 mm with the ventilator speed of 900 rpm and 1650 rpm from forging temperature 1100 °C to 400 °C.

After compiling any two cooling courses for samples with the same dimensions and cooling

parameters, but for different materials, we can also notice that despite the more energetic FigureFigure 12. 12.The The cooling cooling curves curves of of 16MnCr5 16MnCr5 and and C45 C45 samples samples with with the dimensionsthe dimensions35 ⌀3520 × mm 20 withmm with the transformations occurring in the material at C45, the cooling time is ultimately∅ × the same. With an increaseventilatorthe ventilatorof the speed cooling speed of 900 ofrate, rpm 900 the andrpm temperature1650 and rpm1650 fromrpm of fr forging omthe forgingtransformation temperature temperature 1100 lowers. ◦1100C to °C 400This to◦ C. 400is clearly °C. visible for the material C45. It is an important parameter that should be taken into consideration while designing ToAfter be able compiling to optimize any thetwo times cooling of designed courses for thermal samples treatments, with the which same are dimensions important and in terms cooling of thermal treatment processes. theparameters, production but time, for based different on the collectedmaterials, data we and can measured also notice cooling that times despite as well the as themore dependence energetic To be able to optimize the times of designed thermal treatments, which are important in terms of the oftransformations cooling times on occurring temperature, in the a material fan speed at diagram C45, thewas cooling developed time is (Figureultimately 13). the For same. example, With the an production time, based on the collected data and measured cooling times as well as the dependence of cooling characteristicsincrease of the for cooling the sample rate, the with temperature dimensions of 35the transformation40 mm made of lowers. C45 (Figure This is13 clearly–light cyanvisible line) for times on temperature, a fan speed diagram was develope∅ × d (Figure 13). For example, the characteristics for werethe material determined C45. based It is an on important the times parameter measured forthat a should series of be 12 taken such into bars, consideration with diﬀerent while speeds designing of fans the sample with dimensions ⌀35 × 40 mm made of C45 (Figure 13–light cyan line) were determined (Figurethermal 10 treatment). processes. based on the times measured for a series of 12 such bars, with different speeds of fans (Figure 10). To be able to optimize the times of designed thermal treatments, which are important in terms of the production time, based on the collected data and measured cooling times as well as the dependence of cooling times on temperature, a fan speed diagram was developed (Figure 13). For example, the characteristics for the sample with dimensions ⌀35 × 40 mm made of C45 (Figure 13–light cyan line) were determined based on the times measured for a series of 12 such bars, with different speeds of fans (Figure 10).

Figure 13. The cooling curves of 16MnCr5 and C45 samples with the dimensions 35 20 mm, 35 ∅ × ∅ × 40 mm and 35 60 mm, from forging temperature 1100 ◦C to 400 ◦C, in the function of the speed ∅ × of ventilator.

Materials 2020, 13, x FOR PEER REVIEW 14 of 19

Materials 2020, 13, x FOR PEER REVIEW 14 of 19 Figure 13. The cooling curves of 16MnCr5 and C45 samples with the dimensions ⌀35 × 20 mm, ⌀35 × 40Figure mm and 13. ⌀ The35 × cooling 60 mm, curves from forgingof 16MnCr5 temperature and C45 samples1100 °C withto 400 the °C, dimensions in the function ⌀35 × of20 the mm, speed ⌀35 × of Materials 2020, 13, 2441 14 of 19 ventilator.40 mm and ⌀35 × 60 mm, from forging temperature 1100 °C to 400 °C, in the function of the speed of ventilator. Analyzing thethe obtainedobtained cooling cooling curve curve runs, runs, it canit can be seenbe seen that that for thefor twothe ditwofferent different materials materials used, whichused, which areAnalyzing forged are forged inthe the obtained industrialin the industrial cooling process, curve process, the runs, cooling th ite cancooling times be seen for times the that for same for the samplethe same two sample geometrydifferent geometry materials are similar are (Figuresimilarused, 13which(Figure). Some are 13). forged diff erencesSome in thedifferences may industrial be caused, mayprocess, e.g.,be thcaus bye slightlycoolinged, e.g., times di ffbyerent forslightly the emissivities same different sample of materials, emissivitiesgeometry which are of causesmaterials,similar di ff (Figureerentwhich intensities causes13). Some different of differences scale intensities formation may of onbe scal samples.cause formationed, e.g., It can by alsoon slightly samples. be seen di thatfferentIt can individual alsoemissivities be seen runs thatof are materials, which causes different intensities of scale formation on samples. It can also be seen that parallel,individual regardless runs are ofparallel the geometry, regardless/volume of the of geometry/volume the samples. of the samples. individual runs are parallel, regardless of the geometry/volume of the samples. 5.2. The Initial Testing of the Yoke ForgingForging UsedUsed inin CarCar SteeringSteering GearsGears 5.2. The Initial Testing of the Yoke Forging Used in Car Steering Gears First, cooling curves were determined during an in industrialdustrial process on an industrial cooling cooling line, line, First, cooling curves were determined during an industrial process on an industrial cooling line, referred toto asas B-YB-Y (marked(marked as as dashed dashed lines). lines). On On the the other other hand, hand, cooling cooling curves curves for for the the same same forgings forgings as referred to as B-Y (marked as dashed lines). On the other hand, cooling curves for the same forgings inas thein the forgings forgings industry, industry, heated heated in the in furnacethe furnace to 1100 to ◦1100C and °C cooled and cooled to 400 to◦C 400 were °Cdetermined were determined on the as in the forgings industry, heated in the furnace to 1100 °C and cooled to 400 °C were determined constructedon the constructed cooling cooling line. In addition,line. In addition, two variants two werevariants adopted were onadopted the prototype on the prototype line for controlled line for on the constructed cooling line. In addition, two variants were adopted on the prototype line for cooling:controlled the cooling: maximum the maximum fan speed fan was speed used was for coolingused for a cooling single forginga single andforging forging and coolingforging cooling (with a controlled cooling: the maximum fan speed was used for cooling a single forging and forging cooling thermocouple)(with a thermocouple) cooled on cooled the line on andthe line placed and between placed between other heated other forgings, heated forgings, as is the caseas is inthe industry case in (with a thermocouple) cooled on the line and placed between other heated forgings, as is the case in (Figureindustry 14 (Figure). 14). industry (Figure 14).

Figure 14. Cooling course of a yoke forging within the scope of 1100–400 °C in the function of time Figure 14. Cooling coursecourse ofof aa yokeyoke forging forging within within the the scope scope of of 1100–400 1100–400◦C °C in thein the function function of timeof time for diforﬀfor erentdifferent different ventilator ventilator ventilator speeds sp speedseeds (pictures (pictures (pictures from from from industrial industrialindustrial cooling cooling process). process).process).

ThisThis was was to to determine determine the the eeffect effectffect ofof coolingcooling forfor aa singlesingle forging forging and and placed placed with with other other forgings forgings (additional(additional heat heat source), source), because because all all previous previous teststests on on the line line werewere were carried carried carried out out out on on on individual individual individual forgings. forgings. forgings. During each test, the temperature was recorded in the back of the forging (body) and in one of its During each test, the temperature was recorded in the back of the forging (body) and in one of its arms (fork) using shielded K-type thermocouples (Figure 15) due to forging dimensional differences arms (fork) using shielded K-type thermocouples (Figure 15)15) due to forging dimensional differences differences (volume difference between the fork and body). (volume didifferencefference betweenbetween thethe forkfork andand body).body).

(a) (b) (a) (b)

Figure 15. The forgings with the place of the thermocouples assembly marked on the prototype controlled cooling line: (a) in the fork and (b) in the body of forging. Materials 2020, 13, x FOR PEER REVIEW 15 of 19

Figure 15. The forgings with the place of the thermocouples assembly marked on the prototype controlled cooling line: (a) in the fork and (b) in the body of forging. Materials 2020, 13, 2441 15 of 19

As can be seen in Figure 14, for cooling forgings on an industrial BY line from forging temperatureAs can be 1100 seen °C in to Figure 400 °C, 14 the, for cooling cooling time forgings is about on an600 industrial s. While for BY a line prototype from forging line for temperature controlled 1100cooling,◦C towith 400 maximum◦C, the cooling fan efficiency time is about (1650 600 rpm), s.While the cooling for a prototype time was linereduced for controlled by almost cooling, half to withabout maximum 360 s. On the fan other efficiency hand, (1650 the difference rpm), the coolingbetween time the wasmeasuring reduced points by almost (fork and half body) to about is about 360 s. On50 °C. the In other addition, hand, the the cooling difference time between from forging the measuring temperature points to (fork400 °C and for body) a single is aboutforging 50 placed◦C. In addition,between the the remaining cooling time forgings from forging increased temperature by about to90 400s. ◦C for a single forging placed between the remainingThe test forgings results increased presented by above about on 90 the s. developed controlled cooling line are preliminary but promisingThe test results. results Therefore, presented conducting above on our the research, developed as controlledwell as developing cooling linesuch are devices preliminary is justified, but promisingespecially results.in view Therefore, of the indisputable conducting use our research,of thermoplastic as well as treatment. developing As such demonstrated devices is justified, by the especiallyselection inof viewappropriate of the indisputable time–temperature use of thermoplastic parameters, treatment. it is possible As demonstrated to simulate by thealmost selection any ofconditions appropriate that time–temperature occur in industrial parameters, conditions, it incl is possibleuding die to forges, simulate where almost it should any conditions bring measurable that occur infinancial industrial benefits. conditions, including die forges, where it should bring measurable financial benefits.

5.3. The Directions of Further Investigations In the nextnext stagesstages ofof research,research, itit isis assumedassumed thatthat thethe semi-industrialsemi-industrial stand for the design of thermoplastic treatment treatment will will be be expanded, expanded, including, including, among among others: others: controlled controlled cooling cooling line, line, with with air airflow flow velocity velocity measurement measurement systems. systems. Due Due to tothis this fact, fact, a ameasurement measurement in in two two axes axes was made: the longitudinal axisaxis inin respect respect of of the the feeder feeder belt belt and an thed the perpendicular perpendicular axis axis (along (along the ventilatorthe ventilator axis). axis). The measurementsThe measurements were were performed performed for the for ventilator the ventil speedator ofspeed 300 rpmof 300 to 1500 rpm rpm, to 1500 with rpm, the resolution with the everyresolution 150 rpm every on 150 the rpm whole on working the whole length working of the length conveyor. of the The conveyor. sensor was The mounted sensor was on themounted conveyor on beltthe conveyor and moved belt at and the moved speed of at 0.5 the m speed/s, which of 0.5 was m/s, similar which to was the similar earlier cases.to the Thisearlier will cases. allow This you will to determineallow you theto determine temperature the distribution temperature and distribution air flow inside and line. air Onflow the inside basis ofline. the On presented the basis diagrams of the (Figurespresented 16 diagrams and 17), we(Figures can see 16 that and the 17), speed we can along see the that belt the is muchspeed higheralong (averagethe belt is of much about higher 4 m/s) than(average in the of caseabout of 4 them/s) measurement than in the case along of the the measurement ventilator axis along (about the 2 ventilator m/s). This axis is caused(about 2 by m/s). the groundThis is caused effect, occurringby the ground as a resulteffect, ofoccurring accumulated as a result air masses, of accumulated which on air meeting masses, the which resistance on meeting of the conveyorthe resistance belt grid,of the escape conveyor through belt grid, the sides escape of thethrough device the (entry sides and of the exit). device (entry and exit).

Figure 16. Distribution of the air flow ﬂow speed in the scope of 300–1500 rpm along the ventilator axis: vertical lines (axis Y).Y).

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FigureFigure 17.17. DistributionDistribution of of the the air air ﬂow flow speed speed in thein th scopee scope of 300–1500of 300–1500 rpm rpm along along the conveyor the conveyor belt shift belt axisshift (axis axis X);(axis vertical X); vertical lines: lines: ventilators ventilators axes. axes.

AnAn increasedincreased speedspeed inin thethe casecase ofof axisaxis XX cancan bebe observedobserved inin thethe areasareas ofof thethe centralcentral ventilators,ventilators, whereaswhereas the the lowest lowest speed speed is is observed observed between between the the ventilators. ventilators. In In the the case case of of axis axis Y, Y, the the highest highest speed speed is observedis observed between between the the ventilators ventilators and and the the lowest lowest speed: speed: in in the the passages passages through through the the veryvery centercenter ofof thethe ventilators.ventilators. ItIt alsoalso schedulesschedules aa comparativecomparative analysisanalysis ofof identicalidentical forgedforged elementselements mademade ofof twotwo didifferentfferent materialsmaterials andand next next an an examination examination of the of e fftheect ofeffect the performedof the performed thermal treatmentsthermal treatments on the mechanical on the properties.mechanical The properties. future research The future will research also include will al aso cycle include of thermal a cycle treatments of thermal performed treatments on performed forgings andon forgings a comparison and a comparison of their mechanical of their mechanical properties pr withoperties the with properties the properties of forgings of forgings subjected subjected to the traditionalto the traditional thermal thermal treatment. treatment. It is also It is planned also planned to use to numerical use numerical modeling, modeling, e.g., usinge.g., using Marc Marc and Abaqusand Abaqus computing computing packages packages for so-called for so-called CFD (computationalCFD (computational fluid dynamics) fluid dynamics) analysis. analysis. Then, itThen, will beit will possible be possible to more to accurately more accurately determine determine the air flowthe air in theflow chamber, in the chamber, which will which allow will the allow cooling the chambercooling chamber to be designed to be designed in such ain way such that a way the generatedthat the generated air rotation air rotation will be used will be eff ectively.used effectively.

6.6. SummarySummary andand ConclusionsConclusions ThisThis workwork presentedpresented anan overviewoverview ofof currentlycurrently usedused heatheat treatmentstreatments withwith necessarynecessary equipmentequipment dedicateddedicated forfor thethe treatmenttreatment ofof diedie forgingsforgings investigations.investigations. OnOn thisthis basis,basis, itit hashas beenbeen demonstrateddemonstrated thatthat itit isis fullyfully justifiedjustified inin manymany casescases toto performperform thermalthermal treatmenttreatment directlydirectly usingusing thethe heatheat generatedgenerated duringduring thethe forgingforging process, process, without withou thet the need need to to cool cool to to ambient ambient temperature temperature and and reheat. reheat. As As a result a result of usingof using this this type type of treatment,of treatment, a significant a significant amount amount of energy of energy is obtained, is obtained, which which significantly significantly reduces reduces the processthe process time time and reducesand reduces production production costs. Thecosts. results The results of the research of the research show that show the properthat the selection proper ofselection time–temperature of time–temperature parameters parameters (the use of (the controlled use of coolingcontrolled lines) cooling can obtain lines) suchcan obtain mechanical such propertiesmechanical of properties forgings thatof forgings are even that better are than even assumed, better than compared assumed, to traditional compared (separate) to traditional and time-consuming(separate) and time-consuming and expensive thermal and expensive treatment th performedermal treatment without performed using heat forging.without Conductedusing heat researchforging. showedConducted that research for yoke-type showed forgings that madefor yo ofke-type both materials, forgings themade hardness of both should materials, be in thethe rangehardness suitable should for be C45 in (180–260the range HB) suitable steel for and C4 for5 (180–260 16MnCr5 HB) (150–240 steel and HB). for Based 16MnCr5 on it for(150–240 customers HB). whoBased do on not it require for customers the specific who structure do not afterrequire being the tempered specific structure in the forgings, after being there istempered the possibility in the offorgings, an application-developed there is the possibility controlled of an application-developed cooling line. Additionally, controlled through cooling the optimization line. Additionally, of the constructionthrough the optimization and the consideration of the construction of the size and and the direction consideration of the air of flowthe size in the and cooling direction chamber of the air of theflow develop in the cooling cooling chamber line, it also of the becomes develop possible cooling to line, reduce it also the becomes cost of manufacturing possible to reduce forgings the cost with of dimanufacturingfferent geometries forgings and with specific different operational geomet properties.ries and specific operational properties.

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Author Contributions: Conceptualization, M.H. and Z.G.; methodology, M.H.; validation, P.J., A.B. and J.K.; formal analysis, M.H.; investigation, A.B., K.J., M.Z. and J.K.; resources, M.R.; data curation, P.J. and M.R.; writing—original draft preparation, M.H.; writing—review and editing, M.H., P.J. and M.Z.; visualization, P.J. and K.J.; supervision, Z.G.; project administration, A.B. and M.R. All authors have read and agreed to the published version of the manuscript. Funding: This study was founded by National Centre for Research and Development, Poland (grant no. TECHMATSTRATEG1/348491/10/NCBR/2017). Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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