Comparative Study of the Tempering Behavior of Different Martensitic

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Article Comparative Study of the Tempering Behavior of Diﬀerent Martensitic Steels by Means of In-Situ Diﬀractometry and Dilatometry

Martin Hunkel 1,*, Juan Dong 1 , Jeremy Epp 1,2 , Daniel Kaiser 3, Stefan Dietrich 3, Volker Schulze 3 , Ali Rajaei 4, Bengt Hallstedt 4 and Christoph Broeckmann 4

1 Leibniz Institut für Werkstofforientierte Technologien–IWT, Badgasteiner Straße 3, 28359 Bremen, Germany; [email protected] (J.D.); [email protected] (J.E.) 2 MAPEX Center for Materials and Processes, University of Bremen, Bibliothekstraße 1, 28359 Bremen, Germany 3 Institut für angewandte Materialien-Werkstoffkunde, Karlsruher Institut für Technologie, Engelbert-Arnold-Str. 4, 76131 Karlsruhe, Germany; [email protected] (D.K.); [email protected] (S.D.); [email protected] (V.S.) 4 Institut für Werkstoffanwendungen im Maschinenbau, RWTH Aachen University, Augustinerbach 4, 52062 Aachen, Germany; [email protected] (A.R.); [email protected] (B.H.); [email protected] (C.B.) * Correspondence: [email protected]

Received: 24 September 2020; Accepted: 6 November 2020; Published: 10 November 2020

Abstract: Martensitic steels are tempered to increase the toughness of the metastable martensite, which is brittle in the as-quenched state, and to achieve a more stable microstructure. During the tempering of steels, several particular overlapping effects can arise. Classical dilatometric investigations can only detect effects by monitoring the integral length change of the sample. Additional in-situ diffractometry allowed a differentiation of the individual effects such as transformation of retained austenite and formation of cementite during tempering. Additionally, the lattice parameters of martensite and therefrom the tetragonality was analyzed. Two low-alloy steels with carbon contents of 0.4 and 1.0 wt.% and a high-alloy 5Cr-1Mo-steel with 0.4 wt.% carbon were investigated by dilatometry and in-situ diffractometry. In this paper, microstructural effects during tempering of the investigated steels are discussed by a comparative study of dilatometric and diffractometric experiments. The influence of the chemical composition on the tempering behavior is illustrated by comparing the determined effects of the three steels. The kinetics of tempering is similar for the low-alloy steels and shifted to much higher temperatures for the high-alloy steel. During tempering, the tetragonality of martensite in the steel with 1.0 wt% carbon shifts towards a low carbon behavior, as in the steels with 0.4 wt.% carbon.

Keywords: steel; tempering; dilatometry; in-situ diﬀractometry

1. Introduction Martensite formation within steels is a widely researched phenomenon due to its industrial relevance and scientiﬁc interest. Depending mainly on the chemical composition, the properties of martensite vary in a large range. The most important alloying element controlling the mechanical properties of a steel is carbon. The hardness increases from 35 HRC for ultra-low carbon steels up to 66 HRC for Fe-0.8%C after quenching [1]. Applying a tempering after quenching, the properties of the as-quenched martensite can be balanced according to the application’s requirements. During tempering,

Materials 2020, 13, 5058; doi:10.3390/ma13225058 www.mdpi.com/journal/materials Materials 2020, 13, 5058 2 of 15 several transformation and recovery eﬀects occur. These tempering eﬀects are described by the tempering stages [2,3]:

(0) Redistribution of carbon atoms (<100 ◦C) (1) Formation of transition carbides (100–200 ◦C) (2) Loss of tetragonality (100–200 ◦C) (3) Decomposition of retained austenite (170–300 ◦C) (4) Formation of cementite (250–350 ◦C) (5) Recovery (6) Formation of secondary precipitates if possible.

Studies have shown that the formation of transition carbides in stage (1) triggers the loss of tetragonality in stage (2) [4,5]. Stages (3) and (4) have a continuous overlap over a wide range [6–8]. It should be noted that the temperature range of the tempering effects depends strongly on the heating rate and on the chemical composition. Therefore, the given temperature ranges are considered only as a rough estimation. Alloying elements have a strong impact on the stability of the as-quenched microstructure and the tempering behavior of the steel. Therefore, the effects during tempering vary with the chemical composition. Quenched and tempered steels like SAE 4140 have a mostly martensitic microstructure and small amounts of retained austenite. The most important effect during tempering is the precipitation of carbides [9]. In bearing steels like SAE 52100, this martensitic transformation is not complete at room temperature, resulting in a martensitic microstructure including considerable amounts of retained austenite. The retained austenite transforms during tempering accompanied by the precipitation of carbides and the loss of martensite tetragonality [10,11], and results in a different tempering behavior compared to SAE 4140. In high-alloyed hot working steels like SAE H13, the alloying elements provide enhanced wear and corrosion resistance. The microstructure of this steel group shows a secondary hardness maximum by increasing the tempering temperature, where secondary precipitates are formed [12,13]. The steels cited are all Cr- or Cr-Mo-alloyed. Depending on the Cr-Mo- and C-content, the tempering behavior differs significantly. Microstructure evolution in steels is widely studied by dilatometry [14] or differential scanning calorimetry [2]. The advantages are a straightforward procedure and a high resolution. The main disadvantage is the integral measurement of length or heat change. Due to the integral measurement, superposed effects of carbide precipitation and loss of tetragonality cannot be separated clearly. A measurement technique to observe individual mechanisms is the time-resolved investigation of microstructure evolutions by the X-ray diffraction method [15]. Most in-situ X-ray measurements were done by using synchrotron radiation due to the much higher intensity than by conventional X-ray diffraction measurements. By in-situ synchrotron X-ray measurements, the transformation of retained austenite during tempering under stress was researched for a steel SAE 52100 [16]. It was shown that the applied stress accelerates the transformation of retained austenite. Also, by in-situ synchrotron X-ray measurements, the decrease of carbon content in martensite during tempering was determined [16,17]. The evolution of the tetragonality and the precipitates were determined for a high-alloyed martensitic steel [18]. Combined experiments using dilatometry and in-situ X-ray diffraction experiments clearly show the effects leading to a length change. In this work, dilatometry and X-ray diffractometry were applied to investigate the tempering effects in three steel grades for a characteristic representation of different steel groups. The SAE 4140, SAE 52100, and SAE H13 steels were chosen, having the main alloying elements Cr and Mo, besides C. The tool steel SAE H13 is additionally alloyed with Si and V. These elements have a significant influence on the tempering behavior by retarding the cementite formation (Si) and vanadium carbide (VC) formation. The variation in the chemical composition leads to a significantly different tempering behavior, which is the main issue to investigate in this work. In the following, first material Materials 2020, 13, 5058 3 of 15 and methods and then the experimental results are described. Finally, the results are discussed to enlighten the similarities and differences in the tempering behavior due to the chemical composition.

2. Materials and Methods

2.1. Materials Three diﬀerent steels were investigated in this study: the low-alloyed medium-carbon steel SAE 4140 (EN 42CrMo4), the low-alloyed high-carbon steel SAE 52100 (EN 100Cr6), and the high-alloyed medium-carbon steel SAE H13 (EN X40CrMoV5-1). This choice allows the investigation of the eﬀect of carbon as well as substitutional alloying elements on the tempering behavior. All applied steels were conventionally continuous cast and bar rolled. The respective compositions determined by emission spectral analysis are given in Table1.

Table 1. Mean chemical composition determined for the three applied steels (in wt.%).

Steel C Cr Mo Mn Si S V Ni Cu N SAE 4140 0.45 0.99 0.16 0.71 0.21 0.02 - - - <0.005 SAE 52100 0.97 1.50 0.10 0.43 0.21 0.004 - 0.24 0.1 <0.005 SAE H13 0.40 5.37 1.34 0.30 0.97 - 1.22 - - <0.005

2.2. Heat Treatment Typical austenitizing and quenching conditions depend on the steel composition. Consequently, the usual heat treatment conditions were applied for each of the three steels. The initial state of the SAE 4140 steel was quenched and tempered. The SAE 4140 dilatometer samples were austenitized at 920 ◦C for 60 s in order to comply with induction heating. Afterwards, the samples were quenched with maximum quenching rate within approximately 7 s to 30 ◦C. The X-ray diffraction (XRD) samples were austenitized using the same conditions in a vacuum furnace with subsequent quenching into oil. The hardness of the specimens after quenching was 730 HV0.1. The dilatometer samples were tempered 1 1 with a heating rate of 5 K min− (0.083 K s− ) up to 600 ◦C. The bearing steel SAE 52100 was delivered in a spheroidized annealed condition. This steel is typically austenitized in the two-phase field austenite-cementite. The austenitizing temperature applied was 850 ◦C. The austenitizing duration was 45 min to ensure a homogeneous distribution of carbon 1 within austenite. The dilatometer specimens were quenched with 20 K s− to ambient temperature. 1 The specimens were immediately heated up to 600 ◦C with a heating rate of 2 and 5 K min− 1 (0.083 K s− ). The XRD samples were austenitized under the same conditions in a vacuum furnace 1 1 with quenching into oil and a heating rate of 2 K min− (0.033 K s− ) for the subsequent tempering. It is known that the tempering behavior of SAE 52100 depends strongly on the quenching condition [10]. Therefore, the quenching was controlled as closely as possible. The hot work tool steel SAE H13 contains the highest amounts of alloying elements among the three investigated steels. Chromium, molybdenum, and vanadium are elements with a high affinity towards carbon and carbide formation. The initial state of the SAE H13 steel was the soft annealed condition, including V- and Cr-rich carbides, which were characterized by means of energy dispersive X-ray fluorescence (EDX) analysis. Due to the high stability of the V-rich carbides, the SAE H13 is typically austenitized at higher temperatures in order to dissolve carbides in austenite. Dilatometer samples were austenitized at 1050 ◦C for 15 min and subsequently quenched to ambient temperature within 1 100 s, with a cooling rate of 6 K s− above the martensite start temperature, Ms 400 ◦C. A martensitic microstructure with roughly 5% retained austenite and a hardness of 650 HV5 was formed. Samples 1 1 were then heated up with constant rates of 5 K min− (0.083 K s− ) and tempered at 650 ◦C for 1 h. Metallographic examinations of an as-quenched and of a quenched and tempered state are found in the Supplementary Material (Figures S3–S5). Materials 2020, 13, x FOR PEER REVIEW 4 of 15

2.3. Experimental Methods Materials 2020, 13, 5058 4 of 15 2.3.1. Dilatometry

2.3. ExperimentalThe induction Methods dilatometer TA Instruments DIL805 (New Castle, DE, USA) was used for the dilatometric measurements. The samples were cylinders of 10 mm in length and 4 mm in diameter. To2.3.1. ensure Dilatometry through-hardening, hollow samples with 3 mm inner diameter were chosen for SAE 4140. For furtherThe induction analyzing, dilatometer the measured TA Instruments length change DIL805 ∆𝑙 was (New normalized Castle, DE, with USA) respect was of used the forinitial the lengthdilatometric 𝑙 of measurements.the specimen to The get samples the relative were cylinders length change, of 10 mm dilatation in length 𝜀=∆𝑙/𝑙 and 4 mm . inQuenching diameter. dilatometersTo ensure through-hardening, are inferior in precision hollow with samples respect with to 3high-precision mm inner diameter dilatometers were chosen due to forunavoidable SAE 4140. temperatureFor further analyzing, gradients thewithin measured the device. length The change transfor∆l wasmation normalized kinetics were with respectanalyzed of theby the initial curves length of the dilatation and the dilatation rate with respect to the temperature 𝑑𝜀⁄ 𝑑𝑇 during the heat l0 of the specimen to get the relative length change, dilatation ε = ∆l/l0. Quenching dilatometers treatment.are inferior The in precisionlatter one with shows respect the effects to high-precision more pronounced dilatometers [14]. In duethe curves to unavoidable of the dilatation temperature rate, transformationsgradients within are the observed device. The as positive transformation or negati kineticsve peaks, were where analyzed the thermal by the curves expansion of the coefficient dilatation definesand the the dilatation baseline. rate with respect to the temperature dε/dT during the heat treatment. The latter one shows the effects more pronounced [14]. In the curves of the dilatation rate, transformations are 2.3.2.observed In-Situ as positiveX-Ray Diffraction or negative peaks, where the thermal expansion coefficient defines the baseline. For the in-situ X-ray measurements, a diffractometer (Bruker-AXS, Karlsruhe, Germany) with a 2.3.2. In-Situ X-Ray Diffraction built-in heating unit was used [19]. The diffractometer is equipped with a θ/2θ goniometer, a rotating anode,For and the a in-situ very X-rayfast position-sensitive measurements, adetector diffractometer (PSD). (Bruker-AXS,The anode material Karlsruhe, is Cobalt, Germany) and with the radiationa built-in line heating used unitis Iron-filtered was used K [19α1/2]. with The λ di =ff ractometer1.788893/1.792801 is equipped Å. The withbeam a withθ/2θ agoniometer, size of Ø 3 mma rotating is supplied anode, by and a power a very of fast 34 kV position-sensitive and 200 mA. Measurements detector (PSD). were The performed anode material in the is2θ Cobalt,-range fromand the 43° radiation to 103° with line an used increment is Iron-filtered of 0.1° K andα1/ 2a withscan λspeed= 1.788893 of 0.2/ 1.792801s. Å. The beam with a size of Ø 3 mmThe istempering supplied experiments by a power of are 34 enabled kV and 200in a mA.self-bu Measurementsilt miniaturized were furnace, performed see Figure in the 21,θ which-range hasfrom a 43heating◦ to 103 wire,◦ with a sample an increment holder, of and 0.1 ◦aand window a scan made speed by of Kapton 0.2 s. in order to let the X-ray beam pass Thethrough. tempering The samples experiments were are heated enabled by in heat a self-built conduction miniaturized using resistance furnace, see heating Figure 1of, whichthe wire. has Samplea heating sizes wire, are asmall sample discs holder, with thickness and a window of 2 mm made and bydiameter Kapton of in22 ordermm (SAE to let 4140, the X-raySAE 52100) beam orpass diameter through. of 15 The mm samples (SAE H13). were The heated temperature by heat conductionis measured usingby a K-thermocouple resistance heating inserted of the in wire. the sampleSample and sizes controlled are small discsby a withpower thickness control ofsystem. 2 mm andIn order diameter to protect of 22 mm the(SAE sample 4140, surface SAE52100) from oxidation,or diameter the of furnace 15 mm (SAEwas evacuated H13). The and temperature purged by is nitrogen measured gas by 3 a times; K-thermocouple thereafter, the inserted tempering in the experimentssample and controlled were started. by a The power recorded control XRD system. diffrac In ordertograms to protect were evaluated the sample by surface the Rietveld from oxidation, method [20].the furnace The lattice was evacuatedparameters and of purged martensite by nitrogen and th gase phase 3 times; contents thereafter, of martensite, the tempering austenite, experiments and cementitewere started. were The evaluated. recorded XRDThe esti diffmatedractograms statistic were error evaluated of the bydetermination the Rietveld methodof the phase [20]. Thecontent lattice is aboutparameters 2–3 ma%. of martensite Unfortunately, and the phasethe contents contents of of tr martensite,ansition and austenite, alloyed and carbides cementite were were too evaluated. small to detectThe estimated during fast statistic in-situ error measurements. of the determination of the phase content is about 2–3 ma%. Unfortunately, the contents of transition and alloyed carbides were too small to detect during fast in-situ measurements.

Figure 1. Diffractometer used for in-situ X-ray measurements with furnace (enlarged with glowing sample).

FigureExemplarily, 1. Diffractometer the diﬀraction used for patterns in-situ forX-ray 20, measur 300, andements 500 ◦withC are furnace shown (enlarged for SAE with 52100 glowing in Figure 2. At 20sample).◦C, 20 ma.% austenite, 3 ma.% cementite, and 87 ma.% martensite could be detected. The austenite peak vanished at 300 ◦C. At 500 ◦C, the cementite increased to an amount of 10 ma.%. The tetragonality Materials 2020, 13, x FOR PEER REVIEW 5 of 15 Materials 2020, 13, x FOR PEER REVIEW 5 of 15 Exemplarily, the diffraction patterns for 20, 300, and 500 °C are shown for SAE 52100 in Figure 2. AtExemplarily, 20 °C, 20 ma.% the austenite,diffraction 3 patterns ma.% cementite, for 20, 300, and and 87 500 ma.% °C aremartensite shown for could SAE be 52100 detected. in Figure The 2.austenite At 20 °C, peak 20 vanishedma.% austenite, at 300 °C. 3 ma.% At 500 cementite, °C, the cementite and 87 ma.% increased martensite to an amount could be of detected.10 ma.%. The Materialsaustenitetetragonality2020 peak, 13 resulted, 5058vanished in aat broad, 300 °C. asymmetric At 500 °C, peakthe cementite of martensite increased at 20 °C to duean amount to peak ofsplitting. 10 ma.%. At5 of The300 15 tetragonality°C, the peak of resulted martensite in a broad,was more asymmetric symmetric peak due of to martensite the loss of at tetragonalit 20 °C due toy. Thepeak martensite splitting. At peaks 300 °C,were the shifted peak of to martensite smaller angles was morebecause symmetric of thermal due expansion. to the loss Theof tetragonalit diffractiony. patterns The martensite for SAE peaks 4140 resulted in a broad, asymmetric peak of martensite at 20 ◦C due to peak splitting. At 300 ◦C, the peak wereand SAE shifted H13 to determined smaller angles at 20, because 300, and of 500 thermal °C are expansion.found in the The Supplementary diffraction patterns Material for (Figures SAE 4140 S1 of martensite was more symmetric due to the loss of tetragonality. The martensite peaks were shifted and SAES2). H13 determined at 20, 300, and 500 °C are found in the Supplementary Material (Figures S1 to smaller angles because of thermal expansion. The diﬀraction patterns for SAE 4140 and SAE H13 and S2). determined at 20, 300, and 500 ◦C are found in the Supplementary Material (Figures S1 and S2).

Figure 2. Diffraction patterns at 20, 300, and 500 °C from in-situ X-ray diffraction (XRD) for SAE 52100.

Figure 2. DiffractionDiffraction patterns at 20, 300, and 500 °C◦C fr fromom in-situ in-situ X-ray diffraction diffraction (XRD) for SAE 52100. 3. Results and Interpretation 3. Results and Interpretation 3.1. Comparison of Dilatometer Results 3.1. Comparison of Dilatometer Results 3.1. Comparison of Dilatometer Results The tempering behavior of the three steels was compared using the same heating rate of 5 K The tempering behavior of the three steels was compared using the same heating rate of 5 K min–1. min–1The. In Figuretempering 3, the behavior measured of dilatationthe three steelsis plotted, was andcompared the resulting using dilatationthe same heatingrate is presented rate of 5 inK In Figure3, the measured dilatation is plotted, and the resulting dilatation rate is presented in Figure4. minFigure–1. In 4. Figure The thermal 3, the measured expansion dilatation coefficient is plotted,is about and 14 the× 10 resulting−6 K−1 in dilatationthe first stage rate isof presented tempering. in The thermal expansion coefficient is about 14 10 6 K 1 in the first stage of tempering. Tempering FigureTempering 4. The effects thermal can beexpansion detected coefficientas a peak-like is× about de−viation 14− × from 10−6 theK−1 thermalin the first expansion stage of coefficient tempering. in effects can be detected as a peak-like deviation from the thermal expansion coefficient in Figure4. TemperingFigure 4. The effects differences can be detected in the tempering as a peak-like behavior deviation for thefrom three the thermalsteels can expansion be clearly coefficient seen. The in The differences in the tempering behavior for the three steels can be clearly seen. The thermal expansion Figurethermal 4. expansion The differences coefficient in theof th temperinge final martensite behavior is 14.5for the× 10 −three6 K−1 forsteels SAE can 4140 be and clearly SAE 52100seen. andThe coefficient of the final martensite is 14.5 10 6 K 1 for SAE 4140 and SAE 52100 and 16.5 10 6 K 1 thermal16.5 × 10 expansion−6 K−1 for SAE coefficient H13. of the final× martensite− − is 14.5 × 10−6 K−1 for SAE 4140 and SAE× 52100− and− for SAE H13. 16.5 × 10−6 K−1 for SAE H13.

1 FigureFigure 3.3. MeasuredMeasured dilatationdilatation forfor thethe threethree usedused steelssteels temperedtempered withwith aa heatingheatingrate rateof of 5 5K Kmin min−−1. Figure 3. Measured dilatation for the three used steels tempered with a heating rate of 5 K min−1.

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−1 Figure 4. Dilatation rate for the three used steels tempered with a heating rate of 5 K min . 1 Figure 4. Dilatation rate for the three used steels tempered with a heating rate of 5 K min− . 3.2. Dilatometry and Phase Content of SAE 4140 Figure 4. Dilatation rate for the three used steels tempered with a heating rate of 5 K min−1. 3.2. Dilatometry and Phase Content of SAE 4140 The resulting dilatometric curve with the derivative with respect to temperature is shown in Figure3.2. DilatometryThe 5. resultingBetween and approximately dilatometric Phase Content curve 80–200of SAE with 4140°C, the a dip derivative in the dilatation with respect derivative to temperature can be found. is shown Usually, in FiguretemperingThe5. Betweenresulting stage 1, approximately dilatometricthe precipitation curve 80–200 ofwith ◦𝜀C,-carbides, the a dip derivative in theis found dilatation with to respect occur derivative into thistemperature can temperature be found. is shown Usually,regime in tempering[2,3,21],Figure 5. whereas Between stage 1, a theapproximately recent precipitation paper [22] 80–200 of εclaims-carbides, °C, athat dip is this in found the dip dilatation to is occur due into derivative thisa carbon temperature canredistribution be found. regime Usually, [during2,3,21], whereastempering.tempering arecent Fromstage paper2401, the to [400precipitation22] claims°C, a second that of this 𝜀dip-carbides, dip is visible, is due is towhichfound a carbon isto much occur redistribution more in this pronounced temperature during tempering. compared regime Fromto[2,3,21], the 240 first whereas to one. 400 ◦InC, a this arecent second regime, paper dip the is [22] visible, decomposition claims which that is this muchof retained dip more is due pronouncedaustenite to a carbon as comparedwell redistribution as the to formation the ﬁrst during one. of Incementitetempering. this regime, is Fromknown the 240 decomposition to tooccur 400 [2,8].°C, aof Thesecond retained plateau dip austenite isin visible, the dip as which wellthat can as is the muchbe formationobserved more pronouncedat of approximately cementite compared is known 260– to300to occurthe °C firstmight [2,8 one.]. indicate The In plateau this the regime, decomposition in the the dip decomposition that canof retained be observed of austenite, retained at approximately whichaustenite results as 260–300 well in a asdilatation◦ Cthe might formation increase, indicate of thecomparedcementite decomposition isto known the carbide of to retained occur precipitation, [2,8]. austenite, The plateauwhich results resultin thes dip inin aa that dilatationdecrease. can be increase, Afterobserved reaching compared at approximately 400 to°C, the solely carbide 260– an precipitation,increase300 °C might in the indicate which thermal results the expansion decomposition in a decrease. coefficient of After retained is found, reaching austenite, indicating 400 ◦whichC, no solely further results an changes increasein a dilatation with in the increasing increase, thermal expansiontemperature.compared coeto theﬃcient carbide is found, precipitation, indicating which no further result changess in a decrease. with increasing After reaching temperature. 400 °C, solely an increase in the thermal expansion coefficient is found, indicating no further changes with increasing temperature.

Figure 5. Dilatation and dilatation rate as a function of temperature for SAE 4140 during tempering Figure 5. Dilatation and dilatation1 rate as a function of temperature for SAE 4140 during tempering with a heating rate of 5 K min− . with a heating rate of 5 K min−1. Figure6 shows the evolution of the phase contents of martensite, retained austenite, and cementite Figure 5. Dilatation and dilatation rate as a function of temperature for SAE 4140 during tempering duringFigure tempering, 6 shows as the determined evolution byof the in-situ phase X-ray cont analysis.ents of martensite, A retained retained austenite austenite, content and of with a heating rate of 5 K min−1. approximatelycementite during 5 ma.%tempering, was foundas determined at the beginning, by in-situ X-ray which analysis. decomposes A retained between austenite 270 and content 300 ◦ ofC. At the same time, the content of cementite starts to increase. This substantiates the assumption that the approximatelyFigure 6 shows5 ma.% the was evolution found at theof thebeginning, phase cont whichents decomposes of martensite, between retained 270 andaustenite, 300 °C. and At plateau observed in the derivative of the tempering dilatation between 270 and 300 C is caused by the thecementite same time, during the tempering, content of ascementite determined starts by to in-s increase.itu X-ray This analysis. substantiates A retained the assumption austenite◦ content that the of decomposition of retained austenite into ferrite and cementite. approximately 5 ma.% was found at the beginning, which decomposes between 270 and 300 °C. At the same time, the content of cementite starts to increase. This substantiates the assumption that the

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Materialsplateau 2020 observed, 13, x FOR in PEERthe derivative REVIEW of the tempering dilatation between 270 and 300 °C is caused7 of by 15 the decomposition of retained austenite into ferrite and cementite. plateau observed in the derivative of the tempering dilatation between 270 and 300 °C is caused by Materials 2020, 13, 5058 7 of 15 the decomposition of retained austenite into ferrite and cementite.

Figure 6. Phase content evolution during tempering of martensitic SAE 4140 determined by in-situ X- rayFigure measurement. 6. Phase content evolution during tempering of martensitic SAE 4140 determined by in-situ Figure 6. Phase content evolution during tempering of martensitic SAE 4140 determined by in-situ X- X-ray measurement. rayWith measurement. further increasing tempering temperature, a continuous increase in the cementite content With further increasing tempering temperature, a continuous increase in the cementite content can be observed. This is contradictory to the dilatometry and literature results [2,8], which indicate can be observed. This is contradictory to the dilatometry and literature results [2,8], which indicate the the cementiteWith further precipitation increasing to tempering be finished temperature, by reaching a continuousabout 400 °C. increase However, in the one cementite should contentkeep in cementite precipitation to be ﬁnished by reaching about 400 C. However, one should keep in mind canmind be that observed. the amount This isof contradict cementiteory is atto thethe dedilatometrytection threshold and◦ literature of the X-rayresults measurements, [2,8], which indicate which that the amount of cementite is at the detection threshold of the X-ray measurements, which introduces theintroduces cementite large precipitation scatter in the to quantitativebe finished byvalues. reaching about 400 °C. However, one should keep in mindlarge scatterthat the in amount the quantitative of cementite values. is at the detection threshold of the X-ray measurements, which introduces large scatter in the quantitative values. 3.3. Dilatometry and Phase Content of SAE 52100 3.3. DilatometryFigure 77 shows shows and Phase thethe dilatation dilatationContent of and andSAE dilatation dilatation52100 raterate during during tempering. tempering. TheThe baselinebaseline ofof dilatationdilatation rate is the thermal expansion coefficient of about 14⋅10−66 K−11. There is the evidence of 4 events given rate is the thermal expansion coeﬃcient of about 14 10− K− . There is the evidence of 4 events given by theFigure deviation 7 shows from the this dilatation baseline. and The dilatation negative peakrate· duringat 125 °Ctempering. and the positive The baseline peak atof 260dilatation °C are by the deviation from this baseline. The negative peak⋅ − at6 125−1 ◦C and the positive peak at 260 ◦C are rateclearly is the visible. thermal There expansion is a broad coefficient negative ofpeak about around 14 10 300 K °C.. There Additi isonally, the evidence there is of a small4 events negative given clearly visible. There is a broad negative peak around 300 ◦C. Additionally, there is a small negative bypeak the at deviation about 360 from °C. this baseline. The negative peak at 125 °C and the positive peak at 260 °C are clearlypeak at visible. about 360 There◦C. is a broad negative peak around 300 °C. Additionally, there is a small negative peak at about 360 °C.

1 Figure 7. DilatationDilatation and and dilatation dilatation rate rate of of SAE SAE 52100 52100 during during tempering tempering with with a heating a heating rate rate of of2 K 2 Kmin min−1. − . Figure8 shows the phase contents of retained austenite, cementite, and martensite. The retained Figure 7. 8 Dilatation shows the and phase dilatation contents rate of of SAE retained 52100 during austenite, tempering cementite, with a heatingand martensite. rate of 2 K Themin− 1retained. austenite transforms in the temperature range between 200 and 300 ◦C corresponding to the positive peakaustenite at 260 transformsC in Figure in the7 temperature. The initial range cementite between content 200 and of 300 about °C 4corresponding ma.% corresponds to the positive to the Figure 8 ◦shows the phase contents of retained austenite, cementite, and martensite. The retained amount of undissolved carbides after hardening, and is consistent to previous investigations [19]. austenite transforms in the temperature range between 200 and 300 °C corresponding to the positive The cementite formation during tempering starts at the same temperature, which the retained austenite

Materials 2020, 13, x FOR PEER REVIEW 8 of 15 peakMaterials at 2602020 ,°C 13 , inx FOR Figure PEER 7. REVIEW The initial cementite content of about 4 ma.% corresponds to the amount8 of 15 of undissolved carbides after hardening, and is consistent to previous investigations [19]. The peak at 260 °C in Figure 7. The initial cementite content of about 4 ma.% corresponds to the amount cementite formation during tempering starts at the same temperature, which the retained austenite of undissolved carbides after hardening, and is consistent to previous investigations [19]. The startsMaterials to2020 transform., 13, 5058 The formation of cementite is much slower than the transformation of the retained8 of 15 cementite formation during tempering starts at the same temperature, which the retained austenite austenite, leading to a continuous increase of cementite over the entire temperature range. starts to transform. The formation of cementite is much slower than the transformation of the retained Corresponding to the decrease of austenite and the increase of cementite, the martensite/ferrite startsaustenite, to transform. leading to The a continuous formation ofincrease cementite of iscementite much slower over thanthe entire the transformation temperature ofrange. the content changes. retainedCorresponding austenite, to leadingthe decrease to a continuous of austenite increase and th ofe cementiteincrease of over cementite, the entire the temperature martensite/ferrite range. Correspondingcontent changes. to the decrease of austenite and the increase of cementite, the martensite/ferrite content changes.

Figure 8. Phase contents of SAE 52100 during tempering determined by in-situ X-ray measurement. Figure 8. Phase contents of SAE 52100 during tempering determined by in-situ X-ray measurement. 3.4. DilatometryFigure 8. Phase and Phase contents Content of SAE of 52100 SAE H13during tempering determined by in-situ X-ray measurement. 3.4. Dilatometry and Phase Content of SAE H13 3.4. DilatometryFigure 9 shows and Phase the dilatationContent of andSAE theH13 dilatation rate of the SAE H13 sample, measured by Figure9 shows the dilatation and the dilatation rate of the SAE H13 sample, measured by dilatometry. dilatometry. Similar to the other two steel grades, two stages of slight volume change can be observed SimilarFigure to the 9 other shows two the steel dilatation grades, two and stages the ofdilatation slight volume rate changeof the canSAE be H13 observed sample, that measured are attributed by that are attributed to the ε-carbide precipitation (100–200 °C) and cementite formation (380–500 °C). todilatometry. the ε-carbide Similar precipitation to the other (100–200 two ◦steelC) and grades, cementite two stages formation of slight (380–500 volume◦C). change can be observed that are attributed to the ε-carbide precipitation (100–200 °C) and cementite formation (380–500 °C).

−1 1 Figure 9. 9. DilatationDilatation and and dilatation dilatation rate rate of of SAE SAE H13 H13 during during tempering tempering with with a heating a heating rate rate of 5 of K 5min K min. − .

−1 TheFigure evolution 9. Dilatation of and the dilatation content rate of retainedof SAE H13 austeniteaust duringenite tempering and cementite with a heating during rate heating of 5 K min and . 1 h of tempering at at 650 650 °C◦C is is shown shown in in Figure Figure 10. 10 The. The reta retainedined austenite austenite stays stays stable stable up to up 650 to °C 650 and◦C then and The evolution of the content of retained austenite and cementite during heating and 1 h of transformsthen transforms over overthe holding the holding time. time. Cementite Cementite cont contentent shows shows only only a asmall small positive positive trend trend during tempering at 650 °C is shown in Figure 10. The retained austenite stays stable up to 650 °C and then heating up, which correspondscorresponds qualitativelyqualitatively to to the the second second peak peak of of the the dilatation dilatation rate rate shown shown in in Figure Figure9. transforms over the holding time. Cementite content shows only a small positive trend during 9.However, However, the the content content of cementiteof cementite is apparently is apparently less thanless than the scattering the scattering of the of measured the measured data, sodata, that so a heating up, which corresponds qualitatively to the second peak of the dilatation rate shown in Figure quantitativethat a quantitative conclusion conclusion cannot cannot be drawn. be drawn. During Du thering tempering the tempering at 650 ◦ C,at 650 more °C, cementite more cementite is formed, is 9. However, the content of cementite is apparently less than the scattering of the measured data, so formed,and a clear and rise a clear of the rise phase of the content phase cancontent be observed. can be observed. The precipitation The precipitation of fineV-, of fine Mo-, V-, and Mo-, Cr-rich and that a quantitative conclusion cannot be drawn. During the tempering at 650 °C, more cementite is Cr-richcarbides carbides during the during tempering the tempering of the high-alloy of the high steel-alloy SAE H13steel is SAE another H13 major is another difference, major compared difference, to formed, and a clear rise of the phase content can be observed. The precipitation of fine V-, Mo-, and comparedthe tempering to the behavior tempering of SAE behavior 4140 and of SAE SAE 52100 4140 [12 and]. The SAE investigation 52100 [12]. ofThe these investigation nano-sized carbidesof these requiresCr-rich carbides a highly resolvedduring the measurement tempering of technique the high-alloy such as steel transmission SAE H13 is electron another microscopy major difference, (TEM) andcompared cannot to be the observed tempering by means behavior of dilatometry of SAE 4140 and and X-ray SAE di ff52100raction. [12]. Thus, The theinvestigation formation ofof thesethese carbides is not discussed in this work. Materials 2020, 13, x FOR PEER REVIEW 9 of 15

Materials 2020, 13, x FOR PEER REVIEW 9 of 15 nano-sized carbides requires a highly resolved measurement technique such as transmission electron nano-sizedmicroscopy carbides (TEM) andrequires cannot a highly be observed resolved by me meansasurement of dilatometry technique and such X-ray as transmission diffraction. Thus,electron the microscopyformation of (TEM) these and carbides cannot is benot observed discussed by in means this work. of dilatometry and X-ray diffraction. Thus, the Materials 2020, 13, 5058 9 of 15 formation of these carbides is not discussed in this work.

Figure 10. Measured microstructure evolution during tempering of SAE H13 during heating to 650 Figure 10. Measured−1 microstructure evolution during tempering of SAE H13 during heating to 650 C Figure°C with 10. 5 MeasuredK min (left microstructure) and holding evolutionat 650 °C (duringright), determinedtempering of by SAE in-situ H13 X-ray during measurement. heating to 650 ◦ with 5 K min 1 (left) and holding at 650 C(right), determined by in-situ X-ray measurement. °C with 5 K min− −1 (left) and holding at 650◦ °C (right), determined by in-situ X-ray measurement. 3.5. Lattice Parameters and Tetragonality of Martensite 3.5. Lattice Parameters and Tetragonality of Martensite 3.5. LatticeThe lattice Parameters parameters and Tetragonality aM and cM offor Martensite martensite are plotted in Figure 11 for the three steels. The The lattice parameters a and c for martensite are plotted in Figure 11 for the three steels. lattice parameters for SAE 4140M and MSAE H13 increase linearly with temperature. Between 100 and The latticeThe lattice parameters parameters for SAE aM and 4140 cM and for SAEmartensite H13 increase are plotted linearly in Figure with temperature.11 for the three Between steels. The 100 200 °C, for SAE 52100, the lattice parameter cM drops significantly and the lattice parameter aM latticeand 200 parametersC, for SAE for 52100, SAE 4140 the lattice and SAE parameter H13 increa c dropsse linearly signiﬁcantly with temperature. and the lattice Between parameter 100 and a increases◦ nonlinearly. M M 200increases °C, for nonlinearly. SAE 52100, the lattice parameter cM drops significantly and the lattice parameter aM increases nonlinearly.

Figure 11. Evolution of martensite lattice parameters during tempering determined by in-situ X-rayFigure measurement. 11. Evolution of martensite lattice parameters during tempering determined by in-situ X-ray Figuremeasurement. 11. Evolution of martensite lattice parameters during tempering determined by in-situ X-ray The ratio between the lattice parameters aM and cM of martensite, which describes the tetragonality, measurement. is givenThe in Figureratio between 12. For SAE the 52100,lattice the parameters tetragonality aM drops and sharplycM of martensite, between 100 which and 200 describes◦C. For both the SAEtetragonality, 4140 and SAEis given H13, inthe Figure tetragonality 12. For SAE decreases 52100, the linearly tetragonality with the drops temperature. sharply between The decrease 100 and is The ratio between the lattice parameters aM and cM of martensite, which describes the larger for SAE 4140. Despite the signiﬁcantly larger carbon content of SAE 52100, the tetragonality is tetragonality,200 °C. For both is given SAE in4140 Figure and 12.SAE For H13, SAE the 52100, tetragon the tetragonalityality decreases drops linearly sharply with between the temperature. 100 and nearly the same in the temperature region of 200–400 C as for SAE 4140 and SAE H13. 200The °C. decrease For both is largerSAE 4140 for SAE and 4140.SAE H13,Despite the the tetragon significantly◦ality decreases larger carbon linearly content with theof SAE temperature. 52100, the The decrease is larger for SAE 4140. Despite the significantly larger carbon content of SAE 52100, the

Materials 2020, 13, x FOR PEER REVIEW 10 of 15 tetragonality is nearly the same in the temperature region of 200–400 °C as for SAE 4140 and SAE H13.Materials 2020, 13, 5058 10 of 15

Figure 12. Evolution of martensite tetragonality during tempering. Figure 12. Evolution of martensite tetragonality during tempering. 4. Discussion 4. Discussion 4.1. Phase Transformations and Precipitation 4.1. PhaseFor allTransformations three steel types, and Precipitation the decomposition of retained austenite could be clearly observed. WhenFor comparing all three steel SAE types, 4140 and the SAE decomposition 52100, the in-situ of retained XRD measurements austenite could reveal be clearly that the observed. decomposition When comparingtemperature SAE range 4140 of and both SAE steels 52100, lies betweenthe in-situ approximately XRD measurements 250 and reveal 300 ◦C. that The the major decomposition difference is temperaturegiven by the range initial of content both steels of retained lies between austenite appr ofoximately 5% and 20% 250 respectively,and 300 °C. The due major to the difference significantly is givenlower by martensite the initial start content temperature of retained resulting austenite from of the 5% higher and carbon20% respectively, content in SAE due 52100. to the Insignificantly both cases , lowerthis range martensite coincides start with temperature a positive resulting peak in thefrom dilatation the higher rate. carbon This content is expected in SAE since 52100. the austenite In both cases,phase this has arange close coincides packed fcc with lattice. a positive However, peak the authorsin the dilatation of Reference rate. [23 This] stated is expected that depending since the on austenitethe carbon phase content has of a theclose retained packed austenite, fcc lattice. a contraction However, canthe occurauthors during of Reference decomposition [23] stated of retained that dependingaustenite due on tothe a lesscarbon densely content packed of fccthe latticeretained from austenite, the carbon a contentcontraction dissolved can occur in the during lattice. decompositionThis could not of be retained observed austenite for the SAEdue to 4140 a less and densely SAE 52100 packed steels fcc lattice investigated from the here, carbon which content differ dissolvedsignificantly in the in lattice. carbon This content. could not This be temperature observed for range the SAE was 4140 also and found SAE 52100 in Reference steels investigated [2,24] for a here,steel which with similar differ significantly composition in as carbon SAE 52100. content. For This the temperature medium carbon range steel was SAE also 4150 found (EN in 50CrMo4),Reference [2,24]the authors for a steel of Referencewith similar [22 composition] found a higher as SAE temperature 52100. For the range medium between carbon approximately steel SAE 4150 350 (EN and 50CrMo4),400 ◦C for the decompositionauthors of Reference of retained [22] found austenite, a higher which temperature could not be range supported between for theapproximately very similar 350SAE and 4140 400 in °C this for study. the decomposition The hot-working of retained tool steel austenite, SAE H13 which contains could a similarnot be supported carbon content for the as verythe SAEsimilar 4140, SAE which 4140 resultsin this instudy. the sameThe hot-working initial amount tool of steel retained SAE austeniteH13 contains of approximately a similar carbon 5%. contentIn contrast as tothe the SAE other 4140, two which steels, theresults decomposition in the same of retainedinitial amount austenite of onlyretained occurs austenite during theof approximatelyisothermal hold 5%. at In 650 contrast◦C. For to each the ofother the threetwo steel steels,s, the the decomposition transformation of of retained retained austenite austenite only and occursthe cementite during the formation isothermal overlap hold at as 650 reported °C. For before each of [6 the–8]. three Surprisingly, steels, the thistransformation is the also caseof retained for the austenitehigh-alloy and steel the with cementite significantly formation higher overlap transformation as reported temperature. before [6–8]. Possibly, Surprisingly, cementite this is formation the also caserelaxes for internalthe high-alloy stresses steel due with to the significantly smaller martensite higher transformation volume. Resulting temperature. the relaxing Possibly, stresses cementite triggers formationthe transformation relaxes internal of the retained stresses austenite, due to the which smaller is bainite-like. martensite The volume. transformation Resulting of the the relaxing retained stressesaustenite triggers is therefore the shifted transformation to higher temperature,of the retained as Si retards austenite, the cementite which precipitation.is bainite-like. The transformationThe cementite of the precipitation retained austenite of the is steels therefor SAEe 4140shifted and to SAEhigher 52100 temper wasature, found as fromSi retards the XRD the cementitemeasurements precipitation. starting around 250 ◦C and was not finished at the final temperature. Cementite volumeThe contentscementite observed precipitation are close of tothe the steels quantification SAE 4140 limitand SAE of the 52100 measuring was found method from applied the XRD with measurementshigh time resolution. starting Cementite around 250 precipitation °C and was is observednot finished during at the the final dilatometry temperature. measurements Cementite as volumea negative contents peak observed in the dilatation are close rate to the curve. quantifi Thiscation minimum limit doesof the not measuring have the method expected applied form ofwith the highderivative time resolution. of a sigmoid Cementite function, precipitation which is explained is observed by the during effect the of thedilatometry simultaneous measurements decomposition as a of retained austenite at the early stage of the cementite precipitation regime, as discussed above.

For the high-alloy steel SAE H13, the cementite precipitation is shifted to the holding step at 650 ◦C, Materials 2020, 13, x FOR PEER REVIEW 11 of 15

negative peak in the dilatation rate curve. This minimum does not have the expected form of the derivative of a sigmoid function, which is explained by the effect of the simultaneous decomposition of retained austenite at the early stage of the cementite precipitation regime, as discussed above. For Materials 2020, 13, 5058 11 of 15 the high-alloy steel SAE H13, the cementite precipitation is shifted to the holding step at 650 °C, similar to the transformation of the retained austenite. It is assumed that the higher Si content in the similarhigh-alloy to the steel transformation SAE H13 prevents of the retained cementite austenite. precipitation It is assumed and stabilizes that the higherthe retained Si content austenite in the high-alloy[25,26]. Independent steel SAE H13 of the prevents transformation cementite temper precipitationature, andfor all stabilizes steels, the retainedcementite austenite formation [25 ,and26]. Independentthe retained austenite of the transformation transformation temperature, overlaps, foras already all steels, reported the cementite [6–8]. formation and the retained austeniteThe transformationevolution of d𝜀/d𝑇 overlaps, of SAE as already H13 with reported increasing [6–8]. temperature shows two pronounced minima.The evolutionThe first one of d liesε/d inT ofthe SAE same H13 temperatur with increasinge range temperature as for the two shows other two steels, pronounced the magnitude minima. Theof the first minimum one lies is in comparable the same temperatureto the one of SAE range 4140 as forand the smaller two otherthan for steels, SAE the52100. magnitude This minimum of the minimumcan again be is comparableattributed to to the the precipitation one of SAE 4140 of 𝜀 and-carbides. smaller The than second for SAE minimum 52100. Thislies around minimum 470 can °C againand coincides be attributed with tothe the rise precipitation of the cementite of ε-carbides. fraction The as second measured minimum by XRD, lies although around 470 a ◦preciseC and coincidesdetermination with theof the rise beginning of the cementite of the fractioncementite as precipitation measured by XRD,is difficult although due ato precise its small determination amount. In ofcontrast the beginning to the other of steels, the cementite the minimum precipitation of d𝜀/d𝑇 is hasdiffi thecult shape due toof a its single small peak amount. as it can In be contrast expected to thefor a other single-phase steels, the transformation, minimum of sincedε/d Tthehas countera the shapecting of decomposition a single peak of as retained it can be austenite expected occurs for a single-phaseat a higher temperature. transformation, since the counteracting decomposition of retained austenite occurs at a higher temperature. 4.2. Lattice Parameter and Tetragonality 4.2. Lattice Parameter and Tetragonality Above 0.6 wt.% carbon, the lattice parameters aM and cM depend linearly on the carbon content, independentAbove 0.6 ofwt.% further carbon, alloying the latticeelements parameters [27,28]. Below aM and 0.6 cM wt.%depend carbon, linearly the lattice on the parameters carbon content, can independentdeviate from ofthe further linear alloying behavior, elements regardless [27, 28of]. the Below carbon 0.6 wt.%content carbon, and thedepends lattice on parameters the steel cancomposition deviate from[28,29]. the Figure linear 13 behavior, compares regardless the lattice of parameters the carbon with content the high and dependscarbon reference on the steel [27] compositionand a low carbon [28,29 ].reference Figure 13 [28]. compares It has theto latticebe kept parameters in mind that with only the high the carboncarbon referencein solid solution [27] and ainfluences low carbon the reference martensitic [28 ].lattice It has parameters, to be kept in while mind the that carbon only thecontent carbon bound in solid in the solution carbides influences has no theinfluence martensitic on it. lattice For SAE parameters, 4140 and while SAE the H13, carbon a ne contentgligible boundamount in of the carbides carbides is has assumed no influence after onquenching it. For SAE and 4140 the carbon and SAE content H13, a is negligible given by amountthe nominal of carbides carbon is content assumed in afterTable quenching 1. For SAE and 52100, the carbonapproximately content is4% given cementite by the was nominal detected carbon in the content as-quenched in Table1 state.. For SAEAs a 52100, result, approximately the solute carbon 4% cementitecontent in was martensite detected is in approximately the as-quenched 0.7 state. wt.% As [15]. a result, The the lattice solute constants carbon content of SAE in 52100 martensite before is approximatelytempering stage 0.7 (2), wt.% loss [of15 ].tetragonality, The lattice constants fits the high of SAE carbon 52100 reference before very tempering well. Tempering stage (2), loss stage of tetragonality,(2) was not detected fits the for high SAE carbon 4140 referenceand H13. veryAfter well. the sharp Tempering change stage in the (2) lattice was not parameters detected for SAE 414052100 and between H13. After100 and the 200 sharp °C in change Figure in 12, the the lattice determined parameters lattice for parameters SAE 52100 fit between better to 100 a low and carbon 200 ◦C inbehavior Figure 12[28]., the Both determined SAE 4140 lattice and SAE parameters H13 fit the fit better low carbon to a low reference carbon behaviorbetter. [28]. Both SAE 4140 and SAE H13 fit the low carbon reference better.

Figure 13. Experimental findings and literature reference [27,28] of lattice parameters aM and cM as Figure 13. Experimental findings and literature reference [27,28] of lattice parameters aM and cM as function of the carbon content at room temperature. For SAE 52100, the lattice parameters before and function of the carbon content at room temperature. For SAE 52100, the lattice parameters before and after tempering stage (2) are plotted. after tempering stage (2) are plotted. The tetragonality of the as-quenched martensite of SAE 4140 and SAE H13 is nearly equal to 1.01, while it has a significantly higher value of 1.03 in SAE 52100, due to the higher carbon content. Varying the carbon content in solution, the tetragonality of as-quenched martensite is between 1.005 and 1.015 for a carbon content below 0.5 wt.% and above 1.025 for a carbon content above 0.5 wt.% [29,30]. Materials 2020, 13, x FOR PEER REVIEW 12 of 15 Materials 2020, 13, x FOR PEER REVIEW 12 of 15 The tetragonality of the as-quenched martensite of SAE 4140 and SAE H13 is nearly equal to 1.01, Thewhile tetragonality it has a significantly of the as-quenched higher value martensite of 1.03 in ofSAE SAE 52100, 4140 due and to SAE the H13higher is nearlycarbon equalcontent. to Materials1.01,Varying while2020 the, 13it carbon ,has 5058 a significantly content in solution, higher value the tetragonality of 1.03 in SAE of as-quenched52100, due to martensite the higher is carbon between content.12 1.005 of 15 Varyingand 1.015 the for carbon a carbon content content in solution, below 0.5 the wt.% tetragonality and above of 1.025 as-quenched for a carbon martensite content is above between 0.5 1.005wt.% Whileand[29,30]. 1.015 a While rapid for a decreasea carbonrapid decrease content in tetragonality inbelow tetragonality 0.5 is wt.% found isand found during above during the 1.025 tempering the for tempering a carbon of SAE content of SAE 52100 52100above from from 0.5 1.03 wt.% 1.03 to [29,30].to approximately While a rapid 1.01 decreasebetween in100 tetragonality and 200 °C, onlyis found a small during change the temperingis found in ofthis SAE temperature 52100 from range 1.03 approximately 1.01 between 100 and 200 ◦C, only a small change is found in this temperature range for SAEtofor approximately SAE H13 H13 and and SAE SAE1.01 4140. between4140. 100 and 200 °C, only a small change is found in this temperature range for SAETheThe tetragonalityH13 tetragonality and SAE decreases 4140. decreases significantly significantly in the twoin mediumthe two carbonmedium steels carbon only above steels approximately only above approximatelyThe tetragonality 200 °C, the decreases temperature significantly range where in cementitethe two formationmedium carbonwas also steels found. only The carbonabove 200 ◦C, the temperature range where cementite formation was also found. The carbon content within approximatelycontent within 200martensite °C, the temperature𝑐 was calculated range whereby a lever cementite rule: formation was also found. The carbon martensite cM was calculated by a lever rule: content within martensite 𝑐 was calculated by a lever rule: 𝑐 = , (1) c0 cCpC cM𝑐= = − ,, (1) (1) pM using measured martensite and cementite contents 𝑝 and 𝑝, mean carbon content 𝑐, and carbon usingcontent measured of cementite martensite 𝑐 . The and tetragonality cementite contents depends 𝑝 linearly and 𝑝 on, mean the carbon contentcontent 𝑐with, and different carbon using measured martensite and cementite contents pM and pC, mean carbon content c0, and carbon contentslopes below of cementite and above 𝑐 . The0.6 wt.%tetragonality carbon [27,30,depends31]. linearly Cementite on theformation carbon leadscontent to witha decrease different of content of cementite cC. The tetragonality depends linearly on the carbon content with different slopes slopes below and above 0.6 wt.% carbon [27,30,31]. Cementite formation leads to a decrease of belowdissolved and abovecarbon. 0.6 The wt.% carbon carbon content [27,30,31 within]. Cementite marten formationsite was estimated leads to a decreasefrom the of cementite dissolved content. carbon. dissolved carbon. The carbon content within martensite was estimated from the cementite content. TheIn Figure carbon 14, content the tetragonality within martensite is plotted was against estimated the calculated from the ca cementiterbon content. content. For SAE In Figure52100, 14the, In Figure 14, the tetragonality is plotted against the calculated carbon content. For SAE 52100, the themeasurements tetragonality below is plotted 200 against°C are the not calculated plotted. carbonFor all content.three steels, For SAE a linear 52100, dependency the measurements of the measurementstetragonality on below the carbon 200 °Ccontent are notis given. plotted. The Forslope all depends three steels, on the a steel linear composition. dependency As offound the below 200 ◦C are not plotted. For all three steels, a linear dependency of the tetragonality on the carbon tetragonality on the carbon content is given. The slope depends on the steel composition. As found contentbefore, isthe given. tetragonality The slope does depends not only on thedepend steel on composition. the carbon Ascontent found [29]. before , the tetragonality does before, the tetragonality does not only depend on the carbon content [29]. not only depend on the carbon content [29].

Figure 14. Tetragonality in dependency on the calculated carbon content compared with a literature Figure 14. Tetragonality in dependency on the calculated carbon content compared with a literature Figurereference 14. [28]. Tetragonality in dependency on the calculated carbon content compared with a literature reference [28]. reference [28]. 4.3.4.3. DilatometryDilatometry 4.3. Dilatometry ComparingComparing the the dilatometer dilatometer results results with with the the in-situ in-situ X-ray X-ray measurements, measurements, the individualthe individual effects effects can becan identified beComparing identified more themore clearly. dilatometer clearly. Detected Detected results tempering temperingwith the effects in-s effects are:itu X-ray are: measurements, the individual effects can be identified more clearly. Detected tempering effects are:  A pronounced loss of tetragonality with negative peak occurs at 150 °C for SAE 52100 (tempering A pronounced loss of tetragonality with negative peak occurs at 150 ◦C for SAE 52100 (tempering  A pronounced loss of tetragonality with negative peak occurs at 150 °C for SAE 52100 (tempering stagestage (2)).(2)). TemperingTempering stagestage (2)(2) doesdoes notnot appearappear forfor SAESAE 41404140 andand SAESAE H13.H13.  stageRetained (2)). austenite Tempering transforms stage (2) withdoes anot positive appear peak for SAE at 170 4140 °C andfor SAESAE 52100H13. and 180 °C for SAE Retained austenite transforms with a positive peak at 170 ◦C for SAE 52100 and 180 ◦C for SAE  Retained austenite transforms with a positive peak at 170 °C for SAE 52100 and 180 °C for SAE 41404140 (tempering(tempering stage stage (3)). (3)). The The peak peak is more is more pronounced pronounced for SAE for 52100 SAE due52100 to thedue higher to the retained higher 4140retained (tempering austenite stage content. (3)). For The SAE peak H13, is retained more pronounced austenite transforms for SAE 52100during due holding to the at 650higher °C. austenite content. For SAE H13, retained austenite transforms during holding at 650 ◦C.  retainedFormation austenite of cementite content. and For loss SAE of tetragonalityH13, retained with austenite a broad transforms negative duringpeak occurs holding between at 650 200°C. Formation of cementite and loss of tetragonality with a broad negative peak occurs between 200  Formationand 430 °C of for cementite SAE 52100 and and loss SAE of tetragonality 4140 and between with a 490broad and negative 550 °C peakfor SAE occurs H13 between (tempering 200 and 430 ◦C for SAE 52100 and SAE 4140 and between 490 and 550 ◦C for SAE H13 (tempering andstage 430 (4)). °C for SAE 52100 and SAE 4140 and between 490 and 550 °C for SAE H13 (tempering stage (4)). stage (4)).

Materials 2020, 13, x FOR PEER REVIEW 12 of 15

The tetragonality of the as-quenched martensite of SAE 4140 and SAE H13 is nearly equal to 1.01, while it has a significantly higher value of 1.03 in SAE 52100, due to the higher carbon content. Varying the carbon content in solution, the tetragonality of as-quenched martensite is between 1.005 and 1.015 for a carbon content below 0.5 wt.% and above 1.025 for a carbon content above 0.5 wt.% [29,30]. While a rapid decrease in tetragonality is found during the tempering of SAE 52100 from 1.03 to approximately 1.01 between 100 and 200 °C, only a small change is found in this temperature range for SAE H13 and SAE 4140. The tetragonality decreases significantly in the two medium carbon steels only above approximately 200 °C, the temperature range where cementite formation was also found. The carbon

content within martensite 𝑐 was calculated by a lever rule:

𝑐 = , (1)

using measured martensite and cementite contents 𝑝 and 𝑝, mean carbon content 𝑐, and carbon content of cementite 𝑐 . The tetragonality depends linearly on the carbon content with different slopes below and above 0.6 wt.% carbon [27,30,31]. Cementite formation leads to a decrease of dissolved carbon. The carbon content within martensite was estimated from the cementite content. In Figure 14, the tetragonality is plotted against the calculated carbon content. For SAE 52100, the measurements below 200 °C are not plotted. For all three steels, a linear dependency of the tetragonality on the carbon content is given. The slope depends on the steel composition. As found before, the tetragonality does not only depend on the carbon content [29].

Materials 2020, 13, 5058 13 of 15

DueFigure to the14. Tetragonality linear correlation in dependency between on cementite the calculated content carbon and content low carbon compared tetragonality, with a literature as shown reference [28]. in Figure 13, a distinction between cementite precipitation and loss of tetragonality cannot be done by dilatometry. 4.3. Dilatometry Transition carbides could not be detected by in-situ X-ray diffraction due to the fast measurements leadingComparing to low counting the dilatometer statistics. results Nonetheless, with the a furtherin-situ X-ray peak inmeasurements, the dilatometer the results individual was found, effects whichcan be could identified be interpreted more clearly. by the Detected literature tempering [5,9,10]: effects are:  A pronounced loss of tetragonality with negative peak occurs at 150 °C for SAE 52100 (tempering Formation of transition carbides leads to a negative peak between 50 and 200 ◦C for SAE 52100 andstage SAE (2)). 4140 Tempering (tempering stage stage (2) does (1)). not The appear transition for SAE carbide 4140 and peak SAE for H13. SAE H13 is between 50  Retained austenite transforms with a positive peak at 170 °C for SAE 52100 and 180 °C for SAE and 300 ◦C. 4140 (tempering stage (3)). The peak is more pronounced for SAE 52100 due to the higher Theretained thermal austenite expansion content. coe Forffi SAEcient H13, of theretained final austenite martensite transforms depends during only holding weakly at on 650 the °C. chemical Formation composition. of cementite and loss of tetragonality with a broad negative peak occurs between 200 and 430 °C for SAE 52100 and SAE 4140 and between 490 and 550 °C for SAE H13 (tempering 4.4. Influence of Chemical Composition on Tempering Behavior stage (4)). In this work, two low-alloy steels with carbon contents of 0.4 and 1.0 wt.% and a high-alloy steel with 0.4 wt.% carbon were investigated. The retained austenite content of the three steels depends mainly on the carbon content, and the retained austenite is transformed during tempering. Also, the amount of cementite depends on the carbon content. While for both low-alloy steels these transformations are in the same temperature region, they are shifted to a higher temperature range for the high-alloy steels. Significantly higher contents of Mo, Cr, or Si stabilize the martensite against cementite formation. It was already demonstrated that cementite precipitation and retained austenite transformation overlap over a wide range of temperature independently [8]. Regardless of the steel composition, this was also found in this work. The as-quenched state of the SAE 52100 has a significantly higher tetragonality than SAE 4140 and SAE H13 due to the higher carbon content. The tetragonality of SAE 52100 after quenching fits the literature reference for carbon contents higher than 0.6 wt.% [27]. After the loss of the high carbon tetragonality during tempering stage (2), SAE 52100 shows a low carbon tetragonality, as SAE 4140 and SAE H13, with a weak dependency on the dissolved carbon content in martensite. The loss of tetragonality, retained austenite content, and cementite formation results in large effects in the dilatometer curve for SAE 52100, leads to smaller effects for SAE 4140, and only to very small effects for SAE H13.

5. Conclusions Three different steels, SAE 4140, SAE 52100, and SAE H13, were investigated by dilatometry and corresponding in-situ X-ray diffraction to determine similarities and differences during tempering. The steel compositions were chosen in a way that different effects could be identified clearly. Effects analyzed were lattice parameters, martensite tetragonality, retained austenite transformation, and cementite precipitation. The main effects determined from peaks in the dilatation rate can be identified clearly by in-situ X-ray-diffraction. For the low-alloyed steels SAE 4140 and SAE 52100, these effects were found in a similar temperature range, while they were shifted to much higher temperatures for the high-alloy steel SAE H13. Nonetheless, retained austenite transformation and cementite precipitation are in the same temperature range for each steel. The lattice parameters of martensite show a typical evolution for low carbon steel (c < 0.5%). This is also valid for SAE 52100 after the loss of tetragonality during tempering. The tetragonality depends linearly on the carbon content of martensite with different slopes for each steel.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/13/22/5058/s1, Figure S1: Diffraction patterns at 20 ◦C, 300 ◦C, and 500 ◦C from in-situ X-ray diffraction for SAE 4140, Figure S2: Diffraction patterns at 20 ◦C, 300 ◦C, and 500 ◦C from in-situ X-ray diffraction for SAE H13, Figure S3: Micrographs of SAE 4140: (a) as quenched, (b) quenched and tempered up to 600 ◦C, Figure S4: Micrographs of SAE 52100: (a) as quenched, (b) quenched and tempered up to 500 ◦C, Figure S5: SEM micrographs of SAE H13: (a) as quenched, (b) quenched and tempered up to 650 ◦C. Materials 2020, 13, 5058 14 of 15

Author Contributions: Conceptualization, M.H., S.D., J.E., B.H., V.S., and C.B.; Investigation (dilatometry), M.H., D.K., and A.R.; Investigation (in-situ diffractometry), J.D. and J.E.; Writing—Original draft preparation, M.H., J.D., D.K., and A.R.; Writing—Review and editing, S.D., J.E., B.H., V.S., and C.B.; Funding acquisition, M.H., V.S., and C.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—259019485. Conflicts of Interest: The authors declare no conflict of interest.

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