T. BALA[KO et al.: EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES ... 617–626 EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES OF TWO HOT-WORK TOOL STEELS WITH INCREASED THERMAL CONDUCTIVITY VPLIV TOPLOTNEGA STANJA JEKLA NA PREMENSKE TEMPERATURE DVEH ORODNIH JEKEL ZA DELO V VRO^EM S POVI[ANO TOPLOTNO PREVODNOSTJO Tilen Bala{ko 1* , Jaka Burja 1,2 , Jo`ef Medved 1 1 Faculty of Natural Sciences and Engineering, University of Ljubljana, A{ker~eva cesta 12, 1000 Ljubljana, Slovenia 2 Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia Prejem rokopisa – received: 2023-06-05; sprejem za objavo – accepted for publication: 2023-10-04 doi:10.17222/mit.2023.902 The aim of our study was to investigate how different thermal conditions affect the transformation temperatures of two hot-work steels with high thermal conductivity. We focused on two conditions: soft annealing, and quenching and tempering. Soft anneal- ing results in a ferritic steel matrix with spherical carbides, while quenching and tempering result in a fully hardened and tem- pered martensitic matrix with secondary and tempering carbides. We analysed samples using a simultaneous thermal analysis (STA) and differential scanning calorimetry (DSC) to determine the transformation temperatures. Controlled heating and cool- ing allowed us to observe the energy changes associated with the phase transformations. The equilibrium temperatures were cal- culated using the CALPHAD method. Our study investigated the influence of thermal conditions on different transformation temperatures, including solidus/liquidus temperatures, austenite solid transformation temperatures (A1 and A3), austenite solidi- fication temperatures and bainite and/or martensite transformation temperatures. A DSC analysis was used to quantitatively measure the transformation temperatures and energy absorption during the endothermic processes (austenite solid transforma- tion and melting) and to study the energy release during the exothermic processes (solidification and transformation). The re- sults showed that soft annealing reduced the solidification interval and the solidus temperature, while energy absorption in- creased during melting. Conversely, quenching and tempering reduced the austenite solid transformation temperatures and energy release during solidification, including -ferrite solidification. Keywords: thermal analysis, hot-work tool steels, differential scanning calorimetry, heat treatment Namen na{e raziskave je bil raziskati, kako razli~ne toplotne obdelave vplivajo na transformacijske temperature dveh jekel za vro~e delo s povi{ano toplotno prevodnostjo. Osredoto~ili smo se na dve toplotni obdelavi: mehko `arjene ter kaljene in popu{~anje (pobolj{anje). Posledica mehkega `arjenja je feritna jeklena matrica s sferi~nimi karbidi, medtem ko kaljenje in popu{~anje povzro~i popolnoma utrjeno in popu{~eno martenzitno matrico z enakomerno porazdeljenimi sekundarnimi in popu{~nimi karbidi. Vzorce smo analizirali s simultano termi~no analizo (STA) in diferen~no vrsti~no kalorimetrijo (DSC), da smo dolo~ili transformacijske temperature. Nadzorovano segrevanje in ohlajanje nam je omogo~ilo opazovanje energijskih sprememb, povezanih s faznimi transformacijami. Ravnote`ne temperature smo izra~unali z metodo CALPHAD. Na{a {tudija je preu~evala vpliv toplotnih obdelav na razli~ne temperature transformacije, vklju~no s temperaturami solidus/likvidus, temperaturami transformacije avstenita (A1 in A3), za~etnimi temperaturami strjevanja avstenita in temperaturami transformacije bainita in/ali martenzita. Analiza DSC je bila uporabljena za merjenje transformacijskih temperatur in absorpcije energije med endotermnimi procesi (transformacija avstenita v trdnem in taljenje) ter za {tudij spro{~anja energije med eksotermnimi procesi (strjevanje in transformacija). Rezultati so pokazali, da mehko `arjeno stanje zmanj{a interval strjevanja in solidus temperaturo, medtem ko se je med taljenjem pove~ala absorpcija energije. Nasprotno pa je pobolj{ano stanje zni`alo temperature transformacije avstenita v trdnem in spro{~anje energije med strjevanjem, vklju~no s strjevanjem -ferita. Klju~ne besede: termi~na analiza, orodna jekla za delo v vro~em, diferen~na vrsti~na kalorimetrija, toplotna obdelava 1 INTRODUCTION Tool steels are known for their hardness, wear resis- tance, resistance to deformation and fracture, and endur- ance at high temperatures. 1–9 As per the AISI classifica- tion system, these steels can be categorized into nine distinct groups, each bearing its own unique designa- tion. 2,7,9 Hot-work tool steels, which bear designation H, find their use in the area of elevated temperatures and are strategically divided into three different groups: chro- mium, tungsten and molybdenum steels. 1,2,6,7,9 These steels exhibit remarkable resistance to thermal softening at high temperatures and retain their structural integrity during prolonged exposure and/or cyclic temperature changes. 1–9 Group H steels are mainly used for making tools that are used in various processes such as die-cast- ing of light metals, polymer extrusion, forging operations and more. 1–5,9,10 In addition, the new generation of hot-work tool steels exhibits even higher thermal con- ductivity. Studies have shown that steels with increased thermal conductivity have better mechanical properties at higher temperatures, a longer tool life, better casting quality, higher resistance to thermal fatigue and better cooling performance compared to conventional tool Materiali in tehnologije / Materials and technology 57 (2023) 6, 617–626 617 UDK 669.01:62-971 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(6)617(2023) *Corresponding author's e-mail: tilen.balasko@ntf.uni-lj.si (Tilen Bala{ko) steels. 11–14 Extensive research has highlighted the pro- nounced influence of Mn, Ni, Mo and Cr on the thermal conductivity of steels. 15,16 For this reason, the steels ex- amined have an increased Mo content. It is generally accepted that heat treatment processes are essential in the field of hot-work tool steels. As a rule, heat treatment of hot-work tool steels involves two distinct stages: (1) heat treatment during the manufactur- ing process and (2) final heat treatment, which is usually carried out after machining. 5 From the perspective of end-users, the final heat treatment (2) is of utmost impor- tance, as careful control of this phase ensures the achievement of the desired microstructural components and thus the desired mechanical properties. There are three crucial steps in the final heat treatment: austenitising, quenching and tempering. To achieve the desired mechanical properties, the temperatures, duration of immersion/tempering and cooling rates must be care- fully considered. They are the most important parameters to consider. 2,4,5,7,17–19 The fundamental property of steel, together with the complicated interplay of chemical composition, forms the main basis for the remarkable re- sistance of hot-work tool steels to thermal softening at elevated temperatures. 2,4–6,17 High thermal conductivity hot-work tool steels require a three-step tempering pro- cess. A number of studies were carried out to investigate the effects of elevated temperatures on the properties of hot-work tool steels 20–26 and to explore the influence of heat treatment on the microstructure and mechanical properties. 27–33 However, to the best of our knowledge, there is a notable absence of studies exploring the effects of heat treatment on transformation temperatures, includ- ing A 1 ,A 3 ,M S ,B S ,T L ,T S , as well as on the austenite so- lidification temperature, solidification interval and ener- gies absorbed or released during a heating and cooling process of hot-work tool steels. These temperatures are of immense importance, especially in the field of hot-work tool steel use. This is because the proximity of the operating temperature to the start of the transforma- tion to austenite (A 1 ) has a direct influence on the soften- ing rate when the steel is exposed to elevated tempera- tures. Typically, these temperatures are determined by quenching the steel at austenitisation temperatures when the microstructure is predominantly austenitic. Our primary investigation therefore relates to the changes that can be observed at these temperatures when we compare steel in the quenched and tempered condi- tion with steel in the soft annealed condition. In theory these changes should influence the heating/melting stage but not the cooling/solidification stage. This is of partic- ular importance since steels are usually used in the quenched and tempered condition. We are also interested in how these changes affect the energies absorbed and re- leased during a heating and cooling process. Further- more, this topic is of great importance in the field of 3D printing or additive manufacturing of tool steels as it in- cludes both the energies involved and the transformation temperatures. To investigate these aspects, we performed differential scanning calorimetry (DSC) experiments to explore the influence of heat treatment on the transfor- mation temperatures and energies absorbed or released during heating and cooling cycles. We studied two differ- ent hot-work tool steels in two different thermal states. The first condition corresponded to the soft annealed condition, which served as our reference point since tool steels are usually supplied in the "as-delivered" condi- tion, in which the matrix has a ferritic structure and spherical carbides. 7 The second thermal state corre- sponded to the quenched and tempered (QT) condition, which was characterised by a completely hardened and tempered martensitic matrix, in which secondary and tempering carbides were evenly distributed. 7 2 EXPERIMENTAL PART Two high thermal conductivity hot-work tool steels, namely HTCS-130 and W600, were investigated and their chemical compositions are given in Table 1. The compositions were determined with a wet chemical anal- ysis and infrared absorption after combustion with ELTRA CS-800. The heat treatment of the steels studied began with the implementation of the prescribed heat treatment pro- cedures listed in Table 2. In all cases, a uniform soaking time of 30 min was used. After quenching in oil, temper- ing was performed using a Bosio EUP-K 6/1200 cham- ber furnace. The duration of each tempering stage, for both steels investigated, was 2 h. Due to the air atmo- T. BALA[KO et al.: EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES ... 618 Materiali in tehnologije / Materials and technology 57 (2023) 6, 617–626 Table 1: Chemical compositions of the investigated hot-work tool steels in weight percent Sample C Si Mn P S Cr Ni Mo V W Fe W600 0.32 0.12 0.23 0.005 0.001 0.11 2.10 3.20 / 1.90 Bal. HTCS-13 0 0.31 0.07 0.08 0.007 0.001 0.10 0.04 3.20 0.01 1.90 Bal. Table 2: Heat treatment processes employed for the steels investigated Sample Hardness (HRC) Austenitization tem- perature (°C) First tempering (°C) Second tempering (°C) Third tempering (°C) W600 42–44 1090 540 590 640 HTCS-130 42–44 1080 540 590 640 sphere during heat treatment, the steel surface was milled 2 mm deep to mitigate the effects of decarburisation and oxidation. To verify the effectiveness of the heat treatment pro- cess, Vickers hardness measurements were carried out using an Instron Tukon 2100B instrument. The average values obtained with these measurements are shown in Table 3. It is important to note that these values were not included in the analysis of the results as they were only used to confirm the successful completion of the heat treatment and determine different thermal conditions of the steels tested. Table 3: Measured hardness of the samples Sample Hardness (HV 10) W600 197 W600QT 422 HTCS-130 179 HTCS-130QT 438 The microstructural properties of hot-work tool steels in different thermal states have been well documented. In the soft annealed condition, these steels exhibit a microstructure consisting of a ferritic matrix with carbon chemically bonded in globular carbides. 7 In contrast, the quenched and tempered microstructure exhibits a martensitic matrix with evenly dispersed primary and secondary carbides. 7 In particular, the W600 steel exhib- its a microstructure comprising a martensitic matrix to- gether with M 6 C ((Mo,Fe,V) 6 C) and MC ((W,Mo)C) car- bides. In addition, traces of (Nb,Cr)CN, (V,Nb)N and TiN (less than 0.1 mass percent) are present in this steel. 34,35 On the other hand, the HTCS-130 steel has a microstructure characterised by a martensitic matrix with small amounts of bainite, accompanied by M 6 C ((Mo,Fe,V) 6 C) and MC ((W,Mo)C) carbides. 36 After the heat treatment, samples with dimensions of h=4mmand = 4 mm were prepared for the subse- quent DSC analysis. The DSC analysis was conducted using a NETZSCH STA (simultaneous thermal analyser) Jupiter 449C instrument. To further improve our understanding, CALPHAD (CALculation of PHAse Diagrams) simulations were carried out with the Thermo-Calc 2023a software, using the TCFE10 (TCS steel and Fe-alloys database) thermo- dynamic database. The aim of these simulations was to calculate the equilibrium transformation temperatures for the steels studied, thus gaining valuable insights into their thermal behaviour. The DSC analysis was carried out with the NETZSCH STA Jupiter 449C device under a protective Ar5.0 atmosphere with a constant flow rate of 30 mL·min –1 . A standardised temperature programme was used for the steels investigated, with heating and cooling rates set at 10 °C min –1 . The samples were heated from room temperature to 1550 °C and then cooled back to room temperature. To ensure accurate measurements, empty Al 2 O 3 crucibles were used as the reference material. The masses of the samples were be- tween 390 and 410 mg. By analysing the DSC heating and cooling curves, the experimental transformation tem- peratures of the steels studied were determined. This method of analysis is widely accepted and is often used to determine the transformation temperatures of various metal alloys. 37–41 Following the DSC analysis, the samples were pre- pared for a microstructural analysis by grinding, polish- ing and etching with Nital. This preparation technique allowed better visibility of the microstructure under a light microscope. The microstructural analysis was per- formed using a Nikon Microphot FXA microscope equipped with a 3CCD video camera, model Hitachi HV-C20A. Light microscopy served as the primary method for evaluating and examining the resulting microstructure after the DSC analysis. 3 RESULTS AND DISCUSSION In order to clearly distinguish between the two ther- mal states of the steels examined, specific names were assigned to them. The steels in the soft annealed condi- tion were named W600 and HTCS-130 (soft annealed), while the steels in the quenched and tempered condition were named W600QT and HTCS-130QT (quenched and tempered). This naming was chosen to avoid confusion or possible errors in the analysis and interpretation of the results. It is important to note that the soft annealed con- dition reflects the typical condition in which hot-work tool steels are supplied by manufacturers. On the other hand, the term "heat treatment" refers specifically to the quenched and tempered condition, as the soft annealed steels are usually quenched and tempered prior to use to achieve the desired mechanical and other properties. 3.1 CALPHAD calculations In order to determine the equilibrium transformation temperatures for the steels studied, thorough calculations were carried out. The main focus was on determining the austenite solid transformation temperatures as well as the liquidus, solidus and austenite solidification tempera- tures. Two main equilibrium diagrams, known as "property diagrams", were constructed to show the thermodynami- cally stable equilibrium phases as a function of tempera- ture. These diagrams, shown in Figure 1, were calcu- lated based on the specific chemical compositions of the W600 and HTCS-130 steels. Using the data derived from these diagrams, the transformation temperatures were determined for the steels studied. These temperature val- ues were summarised and organised in tables (Tables 4 and 5) for an easy reference and analysis. Upon examining the solidification interval presented in Table 4, it can be observed that there are no signifi- T. BALA[KO et al.: EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES ... Materiali in tehnologije / Materials and technology 57 (2023) 6, 617–626 619 cant differences between the steels studied. This finding is attributed to the fact that the main difference in their chemical composition lies mainly in the Ni content and slightly in the Mn content. Table 4: Transformation temperatures of the investigated steels, calcu- lated using the CALPHAD method Sample Liquidus (°C) Austenite (°C) Solidus (°C) Solidification interval (°C) W600 1491 1460 1408 83 HTCS-130 1502 1451 1423 79 When examining the austenite solid transformation temperatures, shown in Table 5, a remarkable difference is noted. This difference can be attributed to the influ- ence of the chemical composition, in particular the in- creased Ni content and the slightly increased Mn content in the W600 steel. It appears that the addition of Ni and Mn causes the Ae 1 and Ae 3 temperatures to be lowered while, at the same time, the interval of the two-phase field to be lengthened. The reason for this is that both Ni and Mn are austenite stabilizing elements. Table 5: Equilibrium temperatures of the austenite solid transforma- tion of the steels examined, calculated using the CALPHAD method Sample Ae 1 (°C) Ae3 (°C) Two-phase field interval (°C) W600 708 785 77 HTCS-130 804 861 57 3.2 DSC analysis We summarised the results of the differential scan- ning calorimetry (DSC) analysis in tables and produced diagrams illustrating the heating and cooling curves of all the samples studied. Figure 2 shows the DSC heating curves, which allow us to determine important tempera- tures such as the solidus temperature, the austenite solid transformation temperatures (Ac 1 and Ac 3 ) and the ener- gies absorbed during austenite solid transformation and melting (endothermic processes). Similarly, Figure 3 shows the DSC curves during cooling, which provide in- formation on the liquidus temperature, the initial temper- ature of austenite solidification and the initial tempera- ture of bainite transformation. As part of the energy analysis, we can also determine the energies released (exothermic processes) during the solidification of -fer- rite, austenite and the subsequent bainite transformation. Let us first examine the effects of heat treatment on the DSC heating curves shown in Figure 2. These curves provide valuable information on the solidus temperature, the austenite solid transformation temperatures (Ac 1 and Ac 1 ) and the energy absorbed during austenite solid transformation and melting, which are representative of endothermic processes. The results obtained from the DSC heating curves are summarised and clearly pre- sented in Tables 6 and 7. These tables summarise the measured temperatures and the corresponding energy T. BALA[KO et al.: EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES ... 620 Materiali in tehnologije / Materials and technology 57 (2023) 6, 617–626 Figure 1: Amounts of all thermodynamically stable equilibrium phases as a function of temperature: a) and b) for W600 steel; c) and d) for HTCS-130 steel values associated with the observed thermal transforma- tions. The results compiled in Table 6 refer to the austenite solid transformation temperatures although in the cases of the W600 and HTCS-130 steels, both temperatures (Ac 1 and Ac 3 ) are lower for the quenched and tempered sample. The interval of the two-phase field remains al- most the same, in the case of the W600 steel, the differ- ence is only 1.9 °C. In the case of HTCS-130, however, the interval of the two-phase field is 3.8 °C smaller for the quenched and tempered sample. The main reason for the differences between the Ac 1 and Ac 3 temperatures of the W600 and HTCS-130 steels is the Ni content, as it stabilizes the -region in the phase diagram. The differ- ence in the Mn content also contributes a little, as it is also an austenite stabilizing element. Table 6: Austenite solid transformation temperatures of the investi- gated samples, with the corresponding two-phase field interval Sample Ac 1 (°C) Ac 3 (°C) Two-phase field in- terval (°C) W600 717.8 777.0 59.2 W600QT 714.5 775.6 61.1 HTCS-130 755.9 802.0 46.1 HTCS-130QT 741.5 783.8 42.3 The following table (Table 7) compiles the energies absorbed during austenite solid transformation and melt- ing. For both steels studied, the energy absorbed during austenite solid transformation is lower for the quenched and tempered samples. However, the difference is the largest for the W600 steel, being –10.216 J·g –1 , while the difference for HTCS-130 is minimal. On the other hand, the energies absorbed during melting are also lower for the quenched and tempered samples of both steels. How- ever, the largest difference is found for HTCS-130 where the quenched and tempered sample absorbs –75.53 J·g –1 less energy to melt compared to the soft annealed sam- ple. From the results obtained, it appears that the quenched and tempered samples absorb less energy to melt than the samples in the soft annealed condition. This is to be expected as, from a thermodynamic point of view, the martensitic matrix is less stable during heating than the ferritic matrix; more alloying elements are dis- solved in the matrix and the carbides are small and ho- mogeneously distributed in the samples. In contrast, the samples in the soft annealed condition have a thermody- namically stable ferritic matrix with large spherical car- bides that are inhomogeneously distributed. It seems that especially an increased Ni content and a slightly in- creased Mn content influence the austenite solid trans- formation energies because for the W600 steels the dif- ference between the soft annealed and the quenched and tempered sample was larger compared to HTCS-130. On the other hand, these two elements also influence the melting energy, as the distance between the soft annealed and the quenched and tempered sample was smaller for the W600 steel than for HTCS-130. Table 7: Absorbed energies during austenite solid transformation and melting (endothermic processes) of the investigated samples Sample Energies (J·g –1 ) Austenite solid transformation Melting W600 –18.350 –170.4 W600QT –8.134 –152.3 HTCS-130 –10.210 –158.7 HTCS-130QT –9.991 –83.17 With the following analysis, we examine the effects of heat treatment on the DSC curves associated with so- lidification (Figure 3). These curves provide a valuable insight into the liquidus temperature, the initial tempera- ture of austenite solidification and the initial temperature at which bainite transformation occurs. By examining the energetic aspects, we can determine the energies re- leased (exothermic processes) during the solidification phases of -ferrite and austenite, as well as the energies released during bainite transformation. Tables 8 to 10 provide a comprehensive summary of the DSC curves obtained during the cooling process. T. BALA[KO et al.: EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES ... Materiali in tehnologije / Materials and technology 57 (2023) 6, 617–626 621 Figure 2: Heating DSC curves of the investigated samples: a) W600 and b) HTCS-130 steels First, we investigate the influence of heat treatment on solidification (Table 8). The results for the W600 steel show that the quenched and tempered samples have higher liquidus and austenite solidification temperatures compared to the other condition. Conversely, the solidus temperature is slightly lower for the quenched and tem- pered samples compared to the soft annealed samples. In addition, the solidification interval is larger for the quenched and tempered samples, with a difference of 11.8 °C. Similar trends can be observed for the HTCS-130 steel where the solidification interval is even larger than for the W600 steel, 31.3 °C. This also shows that the quenched and tempered samples have higher austenite solidification temperatures but lower solidus temperatures. In both cases, however, the solidification interval is wider in the samples subjected to quenching and temper- ing. The main difference lies in the austenite solidifica- tion temperature, which is particularly pronounced in the HTCS-130 steel. From the results, it appears that both al- loying elements (Ni and Mn) influence the solidification interval, which is narrower for W600 than for HTCS-130 in both thermal conditions. The difference becomes even more evident when comparing the solidification intervals of both steels under both thermal conditions, where W600 has an interval of 11.8 °C and HTCS-130 has an interval of 31.3 °C. Table 8: Liquidus, solidus and austenite solidification temperatures, with the corresponding solidification interval of the investigated sam- ples Sample Liquidus (°C) Austenite (°C) Solidus (°C) Solidification interval (°C) W600 1493.5 1392.8 1447.3 46.2 W600QT 1500.6 1400.9 1442.6 58.0 HTCS-130 1507.1 1376.2 1447.5 59.6 HTCS-130QT 1500.6 1432.0 1409.7 90.9 We continue with the results concerning the energies released (exothermic processes) obtained when cooling the samples (Table 9). For both steels studied, less en- ergy is released during -ferrite formation and complete solidification for the samples in the quenched and tem- pered condition. Furthermore, the energy released during bainite transformation is almost the same for the W600 steel in both thermal states. However, for the HTCS-130 steel, ferrite precipitation occurs before bainite transfor- mation. The energy released during ferrite and bainite transformation is lower for the quenched and tempered sample. There is no ferrite precipitation in W600, which is due to the high Ni content. The energy during austen- ite solidification is almost the same in both thermal states of the W600 steel. Minor differences occur in the HTCS-130 steel where more energy is released for the quenched and tempered sample (4.84 J·g –1 ). If we look only at the total solidification and compare the results for both steels, it is obvious that in the case of the W600 steel there is little change between the two thermal states compared to the results for HTCS-130. Table 9: Released energies during solidification and bainite transfor- mation (exothermic processes) of the investigated samples Sample Energies (J·g –1 ) -ferrite solidifi- cation -austen- ite solid- ification Entire solidifi- cation ( -ferrite and -austenite) Bainite transfor- mation W600 141.00 13.05 154.05 65.39 W600QT 109.20 13.41 122.61 66.29 HTCS-130 135.10 11.28 146.38 78.84 HTCS-130QT 87.11 16.12 103.23 51.64 At the end of cooling, another peak can be analysed on the DSC curves of cooling, which belongs to the bainite transformation (Table 10). For the W600 steel, we are able to determine the initial bainite transforma- tion temperature, which is slightly higher for the soft an- nealed sample with onlya6° Cd i f ference. The results for the HTCS-130 steel are different as there are two peaks on the DSC cooling curve. The first belongs to the temperature of ferrite precipitation. The initial ferrite precipitation temperature is 13.5 °C higher for the soft annealed sample. The second peak, which is just below T. BALA[KO et al.: EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES ... 622 Materiali in tehnologije / Materials and technology 57 (2023) 6, 617–626 Figure 3: Cooling DSC curves of the investigated samples: a) W600 and b) HTCS-130 steels the ferrite precipitation temperature, belongs to bainite transformation. This is also confirmed by light micros- copy (Figure 5) where ferrite can be found along the grain boundaries. For the HTCS-130 steel, the ferrite precipitation tem- perature is higher for the quenched and tempered sam- ple, and the metallographic analysis also showed more ferrite than for the soft annealed sample. For the W600 steel, the initial bainite transformation temperatures are almost the same for the samples in the two thermal states and the interval for the two-phase field is also almost the same in both cases (Table 7). It seems that the addition of Ni and Mn suppresses the ferrite precipitation when we compare the results for the W600 and HTCS-130 steels. Table 10: Starting temperatures of bainite transformation of the inves- tigated samples and ferrite precipitation along the austenite grain boundaries for the HTCS-130 steel Sample Ferrite precipitation (°C) B S (°C) W600 / 522.7 W600QT / 516.7 HTCS-130 556.6 439.0 HTCS-130QT 543.1 450.9 3.3 Light microscopy Due to the peaks on the DSC cooling curve below 550 °C (Figure 3 and Table 10), we also performed light microscopy to clarify, which transformation these peaks belong to. As with the microstructures obtained with the light microscope (Figures 4 and 5), an additional expla- nation is first required to clarify why both dark and light areas are labelled as bainite. The reason is the etching ef- fect, which varies due to the orientation of bainite nee- T. BALA[KO et al.: EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES ... Materiali in tehnologije / Materials and technology 57 (2023) 6, 617–626 623 Figure 5: Microstructure of the HTCS-130 steel: a) soft annealed sample and b) quenched and tempered sample Figure 4: Microstructure of the W600 steel: a) soft annealed sample and b) quenched and tempered sample dles and makes some areas appear lighter and others darker. The following figure (Figure 4) shows the microstructure of the samples after the DSC analysis for the W600 steel in the soft annealed condition (Figure 4a) and quenched and tempered condition (Figure 4b). Based on the evaluation of the microstructure in both states, the peaks (Table 10) belong to the bainite trans- formation temperatures. The next figure (Figure 5) shows the microstructure of the HTCS-130 steel after the DSC analysis in the soft annealed condition (Figure 5a) and quenched and tem- pered condition (Figure 5b). Based on the results of the DSC analysis (Table 10), two peaks appear in this case. After the microstructural analysis, we found that the first peak belongs to the ferrite precipitation temperature along the austenite grain boundaries in both cases, as shown in Figure 5. The second peak, on the other hand, belongs to the bainite transformation temperature, as is the case with the W600 steel. 3.3 Scanning electron microscopy (SEM) To confirm that ferrite is present along the austenite grain boundaries in the HTCS-130 steel, an additional SEM analysis was carried out. Using EDS (en- ergy-dispersive X-ray spectroscopy) we analysed the ar- eas along the grain boundaries and in the grains, i.e., the matrix. The values of the analysed alloying elements are given in w/%. First, we analysed the sample in the soft annealed condition (Figure 6) where Area 1 represents the analysed area along a grain boundary while Area 2 represents the area in a grain, i.e., the matrix, which is bainitic, as already confirmed by light microscopy (Fig- ure 5). Increased amounts of Mo and W are present along the grain boundary (Figure 6 – Area 1) when com- pared with those in the grain (Figure 6 – Area 2), i.e., the matrix (bainite). Based on these results, we can con- firm that ferrite is present along the austenite grain boundaries because it is a known metallurgical fact that both chemical elements (Mo and W) are alphagenic, i.e., they stabilise and support the formation of ferrite. In the next figure (Figure 7), the sample in the quenched and tempered condition is analysed. In this case, Areas 1 and 2 represent the analysed area along a grain boundary and Area 3 represents the area in a grain, i.e., the matrix, which is bainitic, as already confirmed by light microscopy (Figure 5). The results are essen- tially the same as for the sample in the soft annealed condition. There are increased amounts of Mo and W along the grain boundary (Figure 7 – Areas 1 and 2) when compared with those in the grain (Figure 7 – Area 3), i.e., the matrix (bainite). Based on these results, we can again confirm that ferrite is present along the austen- ite grain boundaries. The reasons for this have already been explained in Figure 6. 4 CONCLUSIONS In summary, we can conclude the following: • Melting is influenced by the initial thermal condition, with the samples in the soft annealed condition con- T. BALA[KO et al.: EFFECT OF STEEL’S THERMAL CONDITION ON THE TRANSFORMATION TEMPERATURES ... 624 Materiali in tehnologije / Materials and technology 57 (2023) 6, 617–626 Figure 7: EDS-analysed areas with the corresponding chemical com- positions in w/% for the HTCS-130QT steel in the quenched and tem- pered condition Figure 6: EDS-analysed areas with the corresponding chemical com- positions in w/% for the HTCS-130 steel in the soft annealed condi- tion suming more energy to melt than the samples in the quenched and tempered condition; • during melting, increased amounts of Ni and Mn (W600 steel) also increase the energy consumed dur- ing melting compared to the energy consumed during the melting of HTCS-130 steel; • quenching and tempering reduce the austenite solid transformation temperatures in both steels investi- gated. However, the interval of the two-phase field re- mains almost unchanged for both steels under both conditions analysed; • increased amounts of Ni and Mn (W600 steel) also reduce both austenite solid transformation tempera- tures (Ac 1 and Ac 3 ), compared to those determined in the case of the HTCS-130 steel; • the specimens in the soft annealed condition have a lower solidification interval and lower solidus tem- peratures. At the same time, the austenite solidifica- tion temperatures are higher for quenched and tem- pered samples in both cases. However, there are no major deviations in the liquidus temperatures; • the energies released during the solidification of the quenched and tempered samples are lower than those required for the soft annealed samples; • the same tendency is observed during both solidifica- tion and melting, i.e., higher amounts of Ni and Mn (W600 steel) also increase the energies released dur- ing solidification compared to those released during the solidification of the HTCS-130 steel; • we were able to determine the initial temperatures of the bainite transformation; • in the case of the W600 steel, the addition of Ni and Mn suppresses the ferrite precipitation that occurs in the HTCS-130 steel; • the calculated liquidus temperatures in the case of the HTCS-130 steel are closer to those determined for the quenched and tempered samples, but on the other hand, the calculated liquidus temperatures in the case of the W600 steel are closer to those determined for the soft annealed condition; • the solidus temperatures of the quenched and tem- pered samples of both steels investigated are closer to the calculated temperatures; • meanwhile, the solidification intervals of the quenched and tempered samples are closer to the cal- culated values for both steels investigated. 5 REFERENCES 1 C. 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