T. BALA[KO, J. MEDVED: INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION ... 325–332 INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION TEMPERATURES OF TWO CHROMIUM HOT-WORK TOOL STEELS VPLIV TOPLOTNEGA STANJA JEKLA NA PREMENSKE TEMPERATURE DVEH KROMOVIH ORODNIH JEKEL ZA DELO V VRO^EM Tilen Bala{ko * , Jo`ef Medved Faculty of natural sciences and engineering, University of Ljubljana, A{ker~eva cesta 12, 1000 Ljubljana, Slovenia Prejem rokopisa – received: 2023-03-07; sprejem za objavo – accepted for publication: 2023-05-18 doi:10.17222/mit.2023.822 The influence of the thermal condition of the steel on the transformation temperatures of two chromium hot-work tool steels was investigated. The steels studied were in two different thermal states: the soft -annealed state and the hardened-and-tempered state. The soft-annealed condition, i.e., the fully annealed condition, is a thermal state of steels in which the matrix is ferritic, and the carbon is chemically bonded in spherical carbides. The hardened-and-tempered condition, on the other hand, means a fully hardened-and-tempered martensitic matrix with uniformly distributed (primary and secondary) carbides. The samples were analysed in a simultaneous thermal analyser (STA) using the differential scanning calorimetry (DSC) method to determine the transformation temperatures. We also performed calculations based on the CALPHAD method to obtain the equilibrium temper- atures of the transformations. The aim of the study was to determine the influence of different thermal conditions of chromium hot-work tool steels on the transformation temperatures such as solidus/liquidus temperatures, eutectoid transformation temper- atures (A 1 and A 3), austenite solidification temperature and martensite transformation start temperatures. Since DSC analysis also measures thermal influence, we were able to determine the energies absorbed during eutectoid transformation and melting (endothermic processes) and the energies released during the solidification of -ferrite and austenite (exothermic processes), as well as the energies released during martensite transformation. It was found that hardening and tempering reduce both eutectoid transformation temperatures and that the solidification intervals are closer to those calculated. From an energetic point of view, hardening and tempering reduce the energies absorbed during melting. Keywords: thermal analysis, chromium hot-work tool steels, differential scanning calorimetry, heat treatment Avtorji so raziskovali vpliv toplotne obdelave (toplotnega stanja) jekla na premenske temperature dveh kromovih orodnih jekel za delo v vro~em. Preiskovani jekli sta bili v dveh razli~nih toplotnih stanjih: mehko `arjenem in pobolj{anem (kaljenje in popu{~anje). Mehko `arjeno stanje, tj. popolnoma `arjeno stanje, je toplotno stanje jekel, kjer imamo feritno matrico, ogljik pa je kemi~no vezan v karbide krogli~ne oblike. Po drugi strani pa kaljeno in popu{~eno stanje pomeni popolnoma utrjeno in popu{~eno martenzitno matrico z enakomerno porazdeljenimi (primarni in sekundarni) karbidi. Vzorce so avtorji preu~evali v napravi za simultano termi~no analizo (STA) z metodo diferen~ne vrsti~ne kalorimetrije (DSC) za dolo~itev premenskih temperatur. Naredili so tudi termodinami~ne izra~une na podlagi metode CALPHAD za dolo~anje ravnote`nih premenskih temperatur. Namen raziskave je bil ugotoviti vpliv razli~nih toplotnih stanj kromovih orodnih jekel za delo v vro~em na premenske temperature, kot so solidus/likvidus, temperature evtektoidne transformacije (A 1 in A 3), za~etne temperature strjevanja avstenita in temperature za~etka martenzitne transformacije. Ker analiza DSC meri tudi toplotne u~inke, so avtorji dolo~ili tudi energije, porabljene med evtektoidno transformacijo in taljenjem (endotermni procesi) in spro{~ene energije (eksotermni procesi) med strjevanjem -ferita, austenita in energije, spro{~ene med martenzitno transformacijo. Ugotovljeno je bilo, da pobolj{ano stanje zni`a obe temperaturi evtektoidne transformacije in da so intervali strjevanja bli`je tistim, ki so bili ravnote`no izra~unani. Pobolj{ano stanje prav tako zmanj{a energijo, absorbirano med taljenjem. Klju~ne besede: termi~na analiza, kromova orodna jekla za delo v vro~em, diferen~na vrsti~na kalorimetrija, toplotna obdelava 1 INTRODUCTION Tool steels have high hardness, good wear resistance, resistance to deformation and breakdown, and increased durability at elevated temperatures. 1–7 According to the AISI classification, they can be divided into nine groups, with each group having a different designation. 1,7 Hot-work tool steels have the designation H and are nor- mally used at higher temperatures and are divided into three groups: chromium, tungsten and molybdenum steels. 1,7 They are resistant to softening at elevated tem- peratures, even when exposed to them for prolonged pe- riods of time and/or cyclic temperature loads. 1–5 Group-H steels are mainly used for the production of tools for the die-casting of light metals, the extrusion of polymers, forging, etc. 1–5,8 It is well known that heat-treatment processes are un- avoidable in hot-work tool steels. Normally, the heat treatment of hot-work tool steels is divided into two parts: (1) the heat treatment during the manufacturing process and (2) the final heat treatment that is usually carried out after machining. 3 From the end-user’s point of view, the (2) final heat treatment is the most critical, since proper control of the final heat-treatment process Materiali in tehnologije / Materials and technology 57 (2023) 4, 325–332 325 UDK 536.6:621.9.025.6:669.26 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(4)325(2023) *Corresponding author's e-mail: tilen.balasko@ntf.uni-lj.si ensures the achievement of the intended final micro- structural constituents reflected in the desired mechani- cal properties. There are three important steps in the final heat-treatment process: austenitising, quenching (harden- ing) and tempering. Temperatures, soaking/tempering times and cooling rates are the main parameters we need to consider to achieve the desired mechanical proper - ties. 1,3,6,7,9–11 This, together with the chemical composi- tion, is the main reason why hot-work tool steels are re- sistant to softening at elevated temperatures. 1,3,4,6,9 There are several studies on the influence of elevated tempera- tures on the properties of hot-work tool steels 12–18 and studies on the influence of heat treatment on micro- structure and mechanical properties. 19–25 On the other hand, to the best of our knowledge, there are no studies on the influence of heat treatment on transformation tem- peratures such as A 1 ,A 3 ,M S ,B S ,T L ,T S , the solidifica- tion onset temperature of austenite, the solidification in- terval and the energies absorbed/released during the heating and cooling of hot-work tool steel. These tem- peratures, especially the eutectoid reaction temperatures, are important for the use of hot-work tool steels. This is because the closer the operating temperature is to the temperature of the start of the eutectoid reaction (A 1 ), the faster the steel will soften when used at elevated temper- atures. Normally, these temperatures are determined by hardening the steel at austenitisation temperatures where the microstructure consists of austenite. So, our question was, how do these temperatures change for steel in the hardened-and-tempered condition compared to those in the soft-annealed condition? Be- cause normally steels are used in the hardened-and-tem- pered condition. And how does this affect the energies absorbed and released during heating or cooling? The topic is also interesting from the point of view of the 3D printing or additive manufacturing of tool steels, both in terms of energies and transformation temperatures. Dif- ferential scanning calorimetry (DSC) was performed to determine the effects of heat treatment on the transfor- mation temperatures and the energies absorbed or re- leased during heating and cooling. Two different chro- mium hot-work tool steels were investigated in two thermal states. The first was the soft-annealed condition, which we used as a reference since tool steels are usually supplied in the "as-delivered" condition, i.e., in the fully annealed condition in which the matrix is ferritic and the carbon is chemically bonded in spherical carbides. 7 The second thermal condition was the hardened and tem- pered, i.e. fully hardened-and-tempered martensitic ma- trix with uniformly distributed (primary and secondary) carbides. 7 2 EXPERIMENTAL PART Two chromium hot-work tool steels with the chemi- cal composition given in Table 1, measured by wet chemical analysis and infrared absorption after combus- tion with ELTRA CS-800, were investigated. At the beginning, the investigated steels were heat treated. The heat-treatment processes used are listed in Table 2 and the soaking time was 30 minutes in all cases. Hardening was carried out in oil, followed by tem- pering in the Bosio EUP-K 6/1200 chamber furnace. The tempering time was 2 hours for all individual tempering stages and steels examined. As an atmosphere of air was used, 2 mm of the steel surface was milled off due to de- carburisation and oxidation during heat treatment. Table 2: Heat treatment processes for steels Sample Hardness (HRC) Austenitiza- tion temper- ature (°C) First temper- ing (°C) Second tem- pering (°C) Dievar 42–44 1025 550 630 H11 42–44 1020 550 620 To ensure that the heat treatment was successful, we measured the Vickers hardness with an Instron Tukon 2100B. The average values of the measurements can be found in Table 3. We do not include these values in the results because we only wanted to check that the heat treatment was successful and to ensure that the steels studied were in two different thermal conditions. Since the steels studied are in two different states, i.e., soft-an- nealed and hardened-and-tempered, the first ones were named Dievar and H11 (soft annealed) and the second ones DievarHT and H11HT (hardened and tempered). Table 3: Measured hardness of the samples Sample Hardness/HV 10 Dievar 198 DievarHT 440 H11 180 H11HT 478 Since the microstructures of all the steels examined have already been analysed in both thermal states, we have not carried out any further metallographic analysis. It is known that the microstructure of hot-work tool steels in the soft-annealed condition consists of a ferritic matrix and carbon chemically bonded in spherical car- bides. 7 The hardened-and-tempered microstructure, on the other hand, consists of a martensitic matrix with uni- formly distributed (primary and secondary) carbides. 7 In T. BALA[KO, J. MEDVED: INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION ... 326 Materiali in tehnologije / Materials and technology 57 (2023) 4, 325–332 Table 1: Chemical composition of the hot-work tool steels examined, given in percent by weight Sample C Si Mn P S Cr Ni Mo V W Fe Dievar 0.34 0.17 0.44 0.008 0.001 5.05 0.19 2.37 0.54 / Bal. H11 0.36 0.97 0.54 0.015 0.002 5.05 0.09 1.22 0.38 / Bal. Dievar steels, the hardened-and-tempered microstructure consists of a martensitic matrix, M 23 C 6 ((Cr,Mo,Fe) 23 C 6 ), M 6 C ((Mo,Fe,V) 6 C) and M 2 C ((Mo,V ,Cr) 2 C) plus small amounts of VC and Ti(CN). 26,27 The microstructure of H11 steel consists of a martensitic matrix, M 23 C 6 ((Cr,Mo,Fe) 23 C 6 ), small amounts of VC and Ti(CN). 27,28 After heat treatment, the samples for DSC analysis with dimensions h =4m ma n d = 4 mm were pre- pared. The DSC analysis was carried out with a NETZSCH STA (Simultaneous Thermal Analyzer) Jupi- ter 449C. First, CALPHAD (CALculation of PHAse Diagrams) simulations were performed with Thermo-Calc 2023a software using the TCFE10 (TCS Steel and Fe-alloys Database) thermodynamic database. The calculations re- sulted in equilibrium transformation temperatures for all steels investigated. DSC analysis was performed in the NETZSCH STA Jupiter 449C instrument, using a protective Ar5.0 atmo- sphere with a flow rate of 30 mL·min –1 throughout the experiment. The temperature programme was the same for all steels studied, heating and cooling rates were 10 °C min –1 , samples were heated from room tempera- ture to 1550 °C and then cooled to room temperature. Empty Al 2 O 3 crucibles were used as a reference and the masses of the samples varied between 390 mg and 410 mg. The DSC heating and cooling curves were used to determine the experimental transformation tempera- tures of the steels studied. The analysis is well known and is often used to determine the transformation tem- peratures of metal alloys. 29–33 3 RESULTS AND DISCUSSION Since the steels studied are in two different states, i.e., soft annealed and hardened and tempered, the first were named Dievar and H11 (soft annealed) and the sec- ond DievarHT and H11HT (hardened and tempered). To avoid confusion and errors, we refer to the soft-annealed thermal state as this is the thermal state in which hot-work tool steels are normally supplied by the manu- facturer. Heat treatment refers to the hardened-and-tem- pered condition, as hardening and tempering of the steel in the soft-annealed condition is usually carried out be- fore hot work steels are used, to obtain the specified me- chanical and other properties. 3.1 CALPHAD calculations Calculations were carried out to determine the equi- librium transformation temperatures for the steels stud- ied. The focus was on the eutectoid transformation tem- peratures, the liquidus, solidus and austenite solidification onset temperatures. The two calculated equilibrium diagrams are shown in the following figures. These are so-called "property diagrams", which show the amount of thermodynami- cally stable equilibrium phases as a function of tempera- ture. The diagrams shown (Figure 1) were calculated T. BALA[KO, J. MEDVED: INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION ... Materiali in tehnologije / Materials and technology 57 (2023) 4, 325–332 327 Figure 1: Amount of all thermodynamically stable equilibrium phases as a function of temperature: a) and b) for Dievar steel and c) and d) for H11 steel based on the chemical composition of the steels Dievar and H11. From the data calculated for the above dia- grams, we have determined the transformation tempera- tures for the steels studied and compiled them in a table (Tables 4 and 5). If we first look at the solidification interval (Table 4), there are no major differences. The closest solidification interval is that of H11 steel, followed by Dievar. The dif- ferences between these two are minimal, which was to be expected as the chemical composition is very similar. Table 4: Transformation temperatures of the investigated steels, calcu- lated with the CALPHAD method Sample Liquidus (°C) Austenite (°C) Solidus (°C) Solidifica- tion interval (°C) Dievar 1491 1443 1418 73 H11 1484 1444 1414 70 The eutectoid transformation temperatures (Table 5) are also very similar, which consequently also applies to the two-phase field intervals. Table 5: Equilibrium temperatures of the eutectoid transformation of the steels examined, calculated with the CALPHAD method Sample Ae 1 (°C) Ae 3 (°C) Two-phase field interval (°C) Dievar 808 832 24 H11 814 843 29 3.2 DSC analysis In the following we have summarised the results of the DSC analysis in tables and the diagrams of the heat- ing and cooling curves of all the samples examined are also shown. First, the DSC heating curves are shown (Figure 2), from which we can determine the solidus temperature, the eutectoid transformation temperatures (Ac 1 and Ac 1 ), the Curie point and the energies absorbed during eutectoid transformation and melting (endother- mic processes). On the other hand, from the DSC curves during cooling (Figure 3) we can determine the liquidus temperature, the starting temperature of austenite solidi- fication and the starting temperatures of martensite trans- formation. Under the aspect of energies, we can deter- mine the released energies (exothermic processes) during the solidification of -ferrite, austenite and the released energies during the martensite transformation. The results are discussed selectively by steel type and at the end a comparison of the results is made and dis- cussed. At this point we must add that we were also able to determine the Curie temperature from the DSC heat- ing curves (Figure 2). There were slight changes in Cu- rie temperatures between the samples in the hard- ened-and-tempered condition and in the soft-annealed condition. For Dievar steel it was 766.4 °C and 771.4 °C for the hardened-and-tempered and soft-annealed sam- ples, respectively. For H11 steel the trend was almost the same: the temperature was 758.4 °C and 759.8 °C for the hardened-and-tempered and soft-annealed samples, re- spectively. First, we will discuss the influence of heat treatment on the DSC heating curves (Figure 2), from which we can determine the solidus temperature, the eutectoid transformation temperatures (Ac 1 and Ac 1 ), the Curie point and the energies absorbed during eutectoid trans- formation and melting (endothermic processes). The re- sults of the DSC heating curves are summarised in Ta- ble 6 and Table 7. The next results (Table 6) refer to the eutectoid trans- formation temperatures and are discussed selectively by steel grade, starting with Dievar steel, where both tem- peratures (Ac 1 and Ac 3 ) are lower in the case of the hard- ened-and-tempered sample. Consequently, the interval of the two-phase field is 4.8 °C smaller for the hard- ened-and-tempered sample. The same trend continues for H11 steel, where both temperatures (Ac 1 and Ac 3 ) are lower in the case of the hardened-and-tempered sample, but the interval of the two-phase field remains almost the same, the difference being only 0.3 °C. T. BALA[KO, J. MEDVED: INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION ... 328 Materiali in tehnologije / Materials and technology 57 (2023) 4, 325–332 Figure 2: Heating DSC curves of the investigated samples: a) Dievar and b) H11 In summary, hardening and tempering lowers both eutectoid transformation temperatures (Ac 1 and Ac 3 ), but the interval of the two-phase field remains almost the same as for the soft-annealed samples. Table 6: Eutectoid transformation temperatures of the investigated samples, with corresponding two-phase field interval Sample Ac 1 /°C Ac 3 /°C Two-phase field inter- val/°C Dievar 848.8 888.8 40.0 DievarHT 845.2 880.4 35.2 H11 855.3 901.5 46.2 H11HT 850.3 896.2 45.9 The following table (Table 7) compiles the energies absorbed during eutectoid transformation and melting. It is obvious that in the case of Dievar steel, the sample in the hardened-and-tempered state absorbed less energy during the eutectoid transformation and melting. The same trend continues for H11 steel, where the absorbed energy during melting is lower for the sample in the hardened-and-tempered state. However, there is a differ- ence in H11 steel: the energy absorbed during the eutectoid transformation is higher for the sample in the hardened-and-tempered state. The main reason for this is the microstructure, i.e., the final hardness after temper- ing, which was higher (478 HV10) than for Dievar steel (440 HV10). This means that even more alloying ele- ments were dissolved in the martensitic matrix, which was also not sufficiently tempered and consequently more energy was needed for the eutectoid transformation than for the Dievar steel in the hardened-and-tempered state with lower hardness after tempering. From the results obtained, it appears that hard- ened-and-tempered samples absorb less energy during melting than samples in the soft-annealed condition. This is somehow to be expected as the martensitic matrix is metastable from a thermodynamic point of view com- pared to the ferritic matrix, more alloying elements are dissolved in the matrix and the carbides are small and homogeneously distributed in the sample. 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. There were also no major differences in the thermal state of the sam- ples with regard to energy absorption during the eutectoid transformation. Table 7: Absorbed energies during eutectoid transformation and melt- ing (endothermic processes) of the investigated samples Sample Energies/J·g –1 Eutectoid transfor- mation Melting Dievar –11.410 –194.0 DievarHT –9.388 –147.2 H11 –11.110 –184.6 H11HT –13.640 –165.2 Next, we discuss the influence of heat treatment on solidification, i.e., the cooling DSC curves (Figure 3), from which we can determine the liquidus temperature, the starting temperature of austenite solidification and the starting temperatures of martensite transformation. From the aspect of energies we can determine the re- leased energies (exothermic processes) during the solidi- fication of -ferrite, austenite and the released energies during the martensite transformation. The results of the DSC curves during cooling are summarised in Ta- bles 8–10. In the case of the Dievar steel the samples in the hardened-and-tempered condition have higher liquidus, solidus and austenite solidification temperatures (Ta- ble 8). Consequently, the solidification interval is also extended, but only by 5 °C. For H11 steel the tendency is different, there are smaller differences in the liquidus and austenite solidification temperatures than for Dievar steel. On the other hand, the differences in solidus tem- peratures are almost the same compared to Dievar steel. In this case the solidification interval for steels in both thermal states remains almost the same: the difference is only 1.9 °C. The reason for this is probably the hardness of the hardened-and-tempered sample, which is higher than for the Dievar steel. T. BALA[KO, J. MEDVED: INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION ... Materiali in tehnologije / Materials and technology 57 (2023) 4, 325–332 329 Figure 3: Cooling DSC curves of the investigated samples: a) Dievar and b) H11 The results show that the liquidus, solidus and aus- tenite solidification temperatures hardly differ, which means that the solidification interval also remains almost the same. We continue with the results concerning the released energies (exothermic processes) obtained when cooling the samples (Table 9). The samples of Dievar steel in the hardened-and-tempered condition released less energy during solidification than the soft-annealed samples. Only the energy released during the martensite transfor- mation was higher. Interestingly, the opposite is true for the H11 steel: the hardened-and-tempered samples re- leased more energy during solidification than the soft-an- nealed samples and less energy during martensite trans- formation. Overall, the energy released during solidification is lower for the Dievar steel in the hardened-and-tempered condition than for the samples in the soft-annealed con- dition. The deviation concerns steel H11, where more energy was released during solidification of the hard- ened-and-tempered sample. This is related to the solidifi- cation interval, which remains almost unchanged in the case of steel H11 (Table 8). At the end of the cooling another peak could be ana- lysed on the DSC curves, which belonged to the martensite transformation (Figure 3 and Table 10). For Dievar steel, the starting temperature of martensite trans- formation was higher for the sample in the hard- ened-and-tempered condition than for the sample in the soft-annealed condition, the difference being 46.5 °C. However, this was reversed for H11 steel, i.e., the martensite transformation temperature was lower for the sample in the hardened-and-tempered condition than for the sample in the soft-annealed condition, but the differ- ence was only 8 °C. The reasons for this are the same as before (the higher hardness of the hardened-and-tem- pered samples resulting in a less-tempered martensitic matrix, more alloying elements dissolved in the matrix, etc.). The other reason that also needs to be considered is the interval of the two-phase field (between A 1 and A 3 ), and the results show that the H11 steel has almost the same interval in both cases (Table 10). Table 10: Starting temperatures of the martensite transformation of the investigated samples Sample M S/°C Dievar 395.4 DievarHT 441.9 H11 375.9 H11HT 367.9 Overall, we were able to determine the Curie temper- atures from DSC heating curves for both steels studied. The eutectoid transformation temperatures (Ac 1 and Ac 3 ) are the next temperatures to be considered. Overall, it can be seen that hardening and tempering lower both eutectoid transformation temperatures. However, the in- terval of the two-phase field remains almost the same as for the samples that are in the soft-annealed state. If we consider the absorbed energies during eutectoid transfor- mation and melting (endothermic processes). It seems that hardening and tempering reduce the energies ab- sorbed during the eutectoid transformation. The results of the H11 steel stand out, the reasons being the same as before. The alternative results of energies absorbed dur- ing melting were consistent for all the samples studied. Hardening and tempering lowers the energy absorbed during melting in all cases. This is to be expected to some extent as the martensitic matrix of the hard- ened-and-tempered samples is metastable, has more al- loying elements dissolved and there are mainly fine and small carbides homogeneously distributed in the sample. In the soft-annealed samples, on the other hand, there is a stable ferritic matrix with large spherical carbides that are not homogeneously distributed, and in general the microstructure is closer to equilibrium than in the hard- ened-and-tempered samples. Based on the solidification (DSC cooling curves) we can assume that there are minor differences between the T. BALA[KO, J. MEDVED: INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION ... 330 Materiali in tehnologije / Materials and technology 57 (2023) 4, 325–332 Table 8: Liquidus, solidus and austenite solidification temperatures, with corresponding solidification interval of the investigated samples Sample Liquidus/°C Austenite/°C Solidus/°C Solidification interval/°C Dievar 1487.8 1374.3 1426.8 61.0 DievarHT 1496.9 1399.3 1430.9 66.0 H11 1486.1 1360.5 1414.5 71.6 H11HT 1480.6 1354.1 1410.9 69.7 Table 9: Released energies during solidification and martensite transformation (exothermic processes) of the investigated samples Sample Energies/J·g –1 -ferrite solidification Austenite solidification Entire solidification ( -ferrite and austenite) Martensite transformation Dievar 119.60 17.47 137.07 45.84 DievarHT 109.40 10.60 120.00 55.25 H11 110.40 16.45 126.85 30.18 H11HT 124.30 18.38 142.68 23.89 liquidus and solidus temperatures. The solidification in- terval also remains almost the same for both thermal states, which means that there are no major influences of the heat treatment on the solidification of the steels stud- ied. As far as the energies released during solidification are concerned, the energies in the Dievar steels are lower in the hardened-and-tempered samples than in the soft-annealed samples. However, this is reversed for the H11 steel, as the hardness of the hardened-and-tempered samples was higher than for the Dievar steel. This means that the samples were not sufficiently tempered com- pared to the others and even more alloying elements were dissolved in the martensitic matrix. This results in more energy being released during the solidification of a hardened-and-tempered sample than a soft-annealed sample. There is no trend in terms of energies released during martensite transformation and no general obser- vations can be made. Only one other thing can be ex- plained from the DSC cooling curves, and that is the starting temperatures of martensite transformation. The martensite transformation temperature is higher for Dievar steel in the hardened-and-tempered condition. However, the results for H11 steel differ from this, as ex- plained earlier. A final point to analyse is the comparison of the cal- culated equilibrium transformation temperatures with the experimental temperatures obtained by DSC analysis. As far as the austenite solidification temperatures are con- cerned, the calculated temperatures were closer to those of the soft-annealed samples in the case of the Dievar steel and closer to those of the hardened-and-tempered samples in the case of the H11 steel. However, if we con- sider the solidification interval, there is a tendency for the results of the hardened-and-tempered samples to be closer to the calculated values for all the steels studied. 4 CONCLUSIONS In summary, then, we can write the following: • Chromium hot-work tool steels hardly differ in terms of liquidus and solidus temperatures. The solidifica- tion interval remains also almost the same for both thermal states, which means that there are no major influences of heat treatment on the solidification of chromium hot-work tool steels; • we were able to determine the Curie temperatures, which were lower for hardened-and-tempered sam- ples; • hardening and tempering reduce both the energy ab- sorbed in the eutectoid transformation and in the melting; • we could determine the starting temperature of the martensitic transformation, hardening and tempering reduce both eutectoid transformation temperatures. However, the interval of the two-phase field remains almost the same as for the samples in the soft-an- nealed state; • the calculated liquidus and solidus temperatures are closer to the temperatures determined for the soft-an- nealed samples; • the solidification intervals of the hardened-and-tem- pered samples are closer to the calculated values. 5 REFERENCES 1 G. Roberts, G. Krauss, R. Kennedy, Tool Steels: 5 th ed., ASM Inter- national, Materials Park 1998, 364 2 C. R. Sohar, Lifetime controlling defects in tool steels, Springer, Berlin 2011, 224 3 R. A. Mesquita, Tool steels: properties and performance, CRC Press, Boca Raton 2016, 245 4 ASM handbook, volume 1: Properties and selection: irons, steels, and high-performance alloys, ASM International, Materials Park 1990, 1063 5 W. F. Hosford, Iron and Steel, Cambridge University Press, Cam- bridge 2012, 310 6 Heat Treater’s Guide: Practices and Procedures for Irons and Steels, ASM International, Metals Park 1995, 904 7 C. Højerslev, Tool steels, Risø National Laboratory, Roskilde 2001, 25, http://orbit.dtu.dk/files/7728903/ris_r_1244.pdf, January 2021 8 J. Sjöström, Chromium martensitic hot-work tool steels - damage, performance and microstructure, Doctoral thesis, Karlstad Univer- sity, 2004, 53, http://www.diva-portal.org/smash/get/diva2:24899/ FULLTEXT01.pdf, 21.01.2013 9 G. Krauss, Steels: Processing, Structure, and Performance, ASM In- ternational, Materials Park 2015, 682 10 G. E. Totten, Steel Heat Treatment: Metallurgy and Technologies, CRC Press, Boca Raton 2006, 848 11 G. N. Haidemenopoulos, Physical Metallurgy: Principles and De- sign, CRC Press, Boca Raton 2018, 476 12 Q. Zhou, X. Wu, N. Shi, J. Li, N. Min, Microstructure evolution and kinetic analysis of DM hot-work die steels during tempering, Mater. Sci. Eng. A, 528 (2011) 18, 5696–5700, doi:10.1016/j.msea. 2011.04.024 13 A. Medvedeva, J. Bergström, S. Gunnarsson, J. Andersson, High- temperature properties and microstructural stability of hot-work tool steels, Mater. Sci. Eng. A, 523 (2009) 1–2, 39–46, doi:10.1016/ j.msea.2009.06.010 14 Z. Zhang, D. Delagnes, G. Bernhart, Microstructure evolution of hot-work tool steels during tempering and definition of a kinetic law based on hardness measurements, Mater. Sci. Eng. A, 380 (2004)1 , 222–230, doi:10.1016/j.msea.2004.03.067 15 N. Mebarki, D. Delagnes, P. Lamesle, F. Delmas, C. Levaillant, Rela- tionship between microstructure and mechanical properties of a 5% Cr tempered martensitic tool steel, Mater. Sci. Eng. A, 387–389 (2004), 1-2, 171–175, doi:10.1016/j.msea.2004.02.073 16 A. Jilg, T. Seifert, Temperature dependent cyclic mechanical proper- ties of a hot work steel after time and temperature dependent soften- ing, Mater. Sci. Eng. A, 721 (2018), 96–102, doi:10.1016/j.msea. 2018.02.048 17 D. Caliskanoglu, I. Siller, R. Ebner, H. Leitner, F. Jeglitsch, W. Waldhauser, Thermal Fatigue and Softening Behavior of Hot Work Tool Steels, Proc. 6 th Int. Tool. Conf., Karlstad 2002, 707–719 18 R. Marke`i~, N. Mole, I. Nagli~, R. [turm, Time and temperature de- pendent softening of H11 hot-work tool steel and definition of an anisothermal tempering kinetic model, Mater. Today Commun., 22 (2020), 1–7, doi:10.1016/j.mtcomm.2019.100744 19 S. Kheirandish, H. Saghafian, J. Hedjazi, M. Momeni, Effect of heat treatment on microstructure of modified cast AISI D3 cold work tool steel, J. Iron Steel Res. Int., 17 (2010), 40–45, doi:10.1016/S1006- 706X(10)60140-9 20 C. J. Chen, K. Yan, L. Qin, M. Zhang, X. Wang, T. Zou, Z. Hu, Ef- fect of Heat Treatment on Microstructure and Mechanical Properties T. BALA[KO, J. MEDVED: INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION ... Materiali in tehnologije / Materials and technology 57 (2023) 4, 325–332 331 of Laser Additively Manufactured AISI H13 Tool Steel, J. Mater. Eng. Perform., 26 (2017) 11, 5577–5589, doi:10.1007/s11665- 017-2992-0 21 M. Priyadarshini, A. Behera, C. K. Biswas, D. K. Rajak, Experimen- tal Analysis and Mechanical Characterization of AISI P20 Tool Steel Through Heat-Treatment Process, J. Bio- Tribo-Corrosion, 8 (2022) 1, 1–10, doi:10.1007/s40735-021-00607-3 22 F. Huber, C. Bischof, O. Hentschel, J. Heberle, J. Zettl, K. Y. Nagulin, M. Schmidt, Laser beam melting and heat-treatment of 1.2343 (AISI H11) tool steel – microstructure and mechanical prop- erties, Mater. Sci. Eng. A, 742 (2018), 109–115, doi:10.1016/j.msea. 2018.11.001 23 I. Souki, D. Delagnes, P. Lours, Influence of heat treatment on the fracture toughness and crack propagation in 5% Cr martensitic steel, Procedia Eng., 10 (2011), 631–637, doi:10.1016/j.proeng.2011. 04.105 24 N. B. Dhokey, S. S. Maske, P. Ghosh, Effect of tempering and cryo- genic treatment on wear and mechanical properties of hot work tool steel (H13), Mater. Today Proc., 43 (2021), 3006–3013, doi:10.1016/ j.matpr.2021.01.361 25 W. R. Prudente, J. F. C. Lins, R. P. Siqueira, P. S. N. Mendes, R. E. Pereira, Microstructural evolution under tempering heat treatment in AISI H13 hot-work tool steel, Int. J. Eng. Res. Appl., 7 (2017)4 , 67–71, doi:10.9790/9622-0704046771 26 T. Bala{ko, M. V on~ina, J. Burja, J. Medved, Influence of Heat Treat- ment on the High-Temperature Oxidation Behaviour of Chro- mium-Molybdenum-Vanadium Alloyed Hot-Work Tool Steel, Mater. Tehnol., 56 (2022) 2, 233–241, doi:10.17222/mit.2022.406 27 T. Bala{ko, M. V on~ina, J. Medved, Simultaneous thermal analysis of the high-temperature oxidation behaviour of three hot-work tool steels, J. Therm. Anal. Calorim., 148 (2022), 1251–1264, doi:10.1007/s10973-022-11616-w 28 T. Bala{ko, M. V on~ina, J. Burja, B. [. Bati~, J. Medved, High-tem- perature oxidation behaviour of AISI H11 tool steel, Metals, 11 (2021) 5, doi:10.3390/met11050758 29 E. Kaschnitz, P. Hofer-Hauser, W. Funk, Electrical resistivity mea- sured by millisecond pulse-heating in comparison to thermal conduc- tivity of the hot work tool steel AISI H11 (1.2343) at elevated tem- perature, High Temp. – High Press., 49 (2020) 1–2, 75–87, doi:10.32908/hthp.v49.825 30 T. Bala{ko, J. Burja, J. Medved, Effect of Ni on solidification of du- plex low-density steels, J. Therm. Anal. Calorim., 142 (2020)5 , 1605–1611, doi:10.1007/s10973-020-10254-4 31 E. Wielgosz, T. Kargul, Differential scanning calorimetry study of peritectic steel grades, J. Therm. Anal. Calorim., 119 (2015)3 , 1547–1553, doi:10.1007/s10973-014-4302-5 32 B. Smetana, M. @aludová, S. Zlá, J. Dobrovská, M. Cagala, I. Szurman, D. Petlák, Application of high temperature DTA technique to Fe based systems, Proc. of the 19 th Int. Metallurgical and Materials Conference, Roznov pod Radhostem 2010, 357–362 33 B. Smetana, S. Zlá, J. Dobrovská, P. Kozelsky, Phase transformation temperatures of pure iron and low alloyed steels in the low tempera- ture region using DTA, Int. J. Mater. Res., 101 (2010) 3, 398–408, doi:10.3139/146.110283 T. BALA[KO, J. MEDVED: INFLUENCE OF THE THERMAL CONDITION OF STEEL ON THE TRANSFORMATION ... 332 Materiali in tehnologije / Materials and technology 57 (2023) 4, 325–332