Thermal analysis in a PREMOMET® steel has been performed by differential scanning calorimetry (DSC) and highresolution dilatometry. The phase transformation temperatures (Ac1, Ac3, Ms and Mf) of this steel were obtained by the two methods at different heating rates showing good agreement between both techniques. The enthalpy of α-γ transformation for this steel was measured using the thermograms acquired by DSC and microstructure was analyzed by scanning electron microscope (SEM). The results showed that this steel retained a martensitic structure for all conditions

E. Garcia-Sanchez

L. Guerra-Fuentes

L. Y. Gomez-Zamorano

M. A. L. Hernandez-Rodriguez

M. Torres-López

English

L.Y. Gomez-Zamorano

M.A.L. Hernandez-Rodriguez

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Thermal and phase transformations analysis in a PREMOMET® steel

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71METALURGIJA 56 (2017) 1-2, 71-74M. TORRES-LÓPEZ, L. GUERRA-FUENTES, L.Y. GOMEZ-ZAMORANO, M.A.L. HERNANDEZ-RODRIGUEZ, E. GARCIA-SANCHEZTHERMAL AND PHASE TRANSFORMATIONS ANALYSIS IN A PREMOMET® STEELReceived – Prispjelo: 2016-04-11Accepted – Prihvaćeno: 2016-08-20Original Scientific Paper – Izvorni znanstveni radM. Torres-Lopez, L. Guerra-Fuentes, L. Y. Gomez-Zamorano, M. A. L. Hernandez-Rodriguez, E. Garcia-Sanchez. University Autonomous of Nuevo Leon.Thermal analysis in a PREMOMET® steel has been performed by differential scanning calorimetry (DSC) and high-resolution dilatometry. The phase transformation temperatures (Ac1, Ac3, Ms and Mf) of this steel were obtained by the two methods at different heating rates showing good agreement between both techniques. The enthalpy of α-γ transformation for this steel was measured using the thermograms acquired by DSC and microstructure was ana-lyzed by scanning electron microscope (SEM). The results showed that this steel retained a martensitic structure for all conditions.Keywords: martensitic steel, phase transformation, DSC, dilatometry, microstructure.INTRODUCTIONPREMOMET® is a high strength martensitic steel, it was produced using combination of vacuum induc-tion melting (VIM) and vacuum arc remelting (VAR) and followed by casting as ingots. This steel offers an attractive combination of high strength and toughness in the quench and tempered condition [1]. A fundamen-tal investigation on the high temperature thermodynam-ic stability in martensitic steels are very important due to that the microstructural conditions at these tempera-tures should provide the required set of mechanical properties and corrosion resistance [2]. Phase transfor-mations are one of the factors that most influence in the mechanical properties of steels, especially the γ-α trans-formation [3], while the martensite formation depends on the austenite chemical composition and defects in the structure [4, 5].Different thermal analysis methods can be applied in order to analyze the phase transformation temperatures.One of them is the dilatometry, which provides in-formation on the consequences of transformations by the study of changes in the sample length, related to phase change, the lattice structure become altered and produces a change in specific volume [6, 7].DSC is a very powerful method for the non– isother-mal study of solid-state phase transformations in mate-rial science [8, 9].On the other hand phase transformation tempera-tures of PREMOMET® steel has not been investigated extensively. In this work were measured Ac1, Ac3 tem-ISSN 0543-5846METABK 56(1-2) 71-74 (2017)UDC – UDK 669.19.018:536.425:536.54.62:620.18=111peratures and the martensitic transformation using ther-mal analysis techniques like dilatometry and differen-tial scanning calorimetry (DSC). These results were correlated with microstructural analysis and microhard-ness tests.EXPERIMENTAL PROCEDURESamples of PREMOMET® steel were obtained from a billet in form of a slide of 6 inches of diameter, the material was sectioned from an ingot and was solu-bilized at 1 040 ºC, the chemical composition of this steel is shown in Table 1.For dilatometric studies a set of 4 mm diameter and 10 mm length samples were cut and machined, then were analyzed using a L78 R.I.T.A. high-resolution dilatometer. During the tests, specimens were protected from oxidation using helium gas; the heating rates were 60 and 600 ºC/min, for this tests the samples were heat-ed up to 850 ºC for 2 min to reach austenite region in this steel, the cooling rate was 18 ºC/min for all tests.Calorimetry tests were performed in a Differential Scanning Calorimeter NETZSCH model DSC 404C Pegasus, using platinum pans for sample and reference with an argon atmosphere in order to protect the sam-ples from the oxidation. The studies were carried out using small discs, 6mm diameter and 13 - 16 mg. These samples were prepared by mechanical thinning and fi-nally electropolishing in order to eliminate the microde-formation caused by the previous step. Samples were Table 1  Chemical composition of PREMOMET® steel / %wtC Si Ni Co Mn Cr Mo V Fe0,4 1,5 3,8 0,5 0,9 1,25 0,5 0,3 Bal.72M. TORRES-LÓPEZ et al.: THERMAL AND PHASE TRANSFORMATIONS ANALYSIS IN A PREMOMET® STEEL METALURGIJA 56 (2017) 1-2, 71-74heated until 850 ºC for 2 minutes using two heating rate, 10 and 25 ºC/min and were cooled at 18 ºC/min.Samples were sectioned transversally, mechanically polished and etched with Villella´s reagent for metallo-graphic inspection after dilatometric tests, after that mi-crohardness tests were performed in each sample, using a Shimadzu microindenter with a load of 980,7 mN for 15 sec.RESULTS AND DISCUSSIONFigure 1 shows the micrograph of PREMOMET® steel as received condition, it can be observe that the material has a martensitic structure, and the hardness was measured of 640 HV, this values corresponds with a martensitic microstructure typical for this type of steels [1].curves in the region of the α-γ transformation. This is associated with a change in the mechanism of phase transformation: the formation of austenite starts by the shear mechanism and then the transformation continues as diffusive [11,12].Figure 4 and 5 show the length changes of PRE-MOMET® steel during heating and cooling rates. In the heating cycle it can be observed the α-γ transformation, which correspond to the Ac1-Ac3 temperatures, while martensitic transformation was measured during cool-ing at 18 ºC/min from the austenitic region. All transfor-mation temperatures were determined with the changes in the curve slope and calculating the first derivate func-tion.At 60 ºC/min the Ac1 temperature was measured at 698 ºC and Ac3 temperature at 757 ºC, during cooling can be observed an expansion, which represents the start and end of martensitic transformation, this were estimated at 243 ºC and finished at 172 ºC as shown in Figure 4.Figure 1 Micrograph of PREMOMET ® steel as initial condition.Figure 2 shows a DSC thermogram, the heating and cooling rates were 10 ºC/min and 18 ºC/min respective-ly, two endothermic peaks can be observed and were related to Ac1 transformation temperature, which corre-sponds to austenite temperature onset, this transforma-tion starts at 690 ºC and Ac3 temperature of 820 °C was measured. During the cooling down, it can be observed that the adiffusional formation of martensite resulted in a decomposition of austenite [10] started at 415 ºC, this value was considered as Ms temperature while Mf was measured at 364 ºC with an enthalpy formation of 170 J/g. These temperatures were estimated from the analy-sis of the energy curves using the change in the slope for each transition and complemented with the calculation of the first derivate of the curve, the same analysis was carried out on each curve at different heating rates.Figure 3 shows the thermogram with heating condi-tion of 25 ºC/min and cooling condition of 18 ºC/min, where the Ac1 temperature was measured at 692 ºC and Ac3 at 827 ºC. The martensitic transformation was ob-served in the cooling down at 407 ºC in the starts and the Mf was registered at 363 ºC with finally enthalpy formation of 180 J/g. In DSC results Ac1 and Ac3 tem-peratures were similar due to the heating rates are simi-lar, it can be observed the presence of two peaks in the Figure 3  DSC thermogram using a heating rate of 25 ºC/min showing the transformation temperatures using intersection lines method.Figure 2  DSC thermogram using a heating rate of 10 ºC/min showing the transformation temperatures using intersection lines method.73M. TORRES-LÓPEZ et al.: THERMAL AND PHASE TRANSFORMATIONS ANALYSIS IN A PREMOMET® STEELMETALURGIJA 56 (2017) 1-2, 71-74Figure 5 shows the dilatometric curve for the steel heated at 600 ºC/min; trough this heating rate the Ac1 and Ac3 temperatures were registered at 721 ºC and 757 ºC respectively and the Ms and Mf, which corre-sponds to the martensitic transformation, were deter-mined at 241 ºC and 168 ºC.The Ac1 and Ac3 temperatures measured are similar in two techniques. Nevertheless in the martensitic trans-formation can be observed a data differences, this can be attributed to the martensitic transformation occurred at high velocities and the cooling is not controlled in the same way in both techniques employed. Additionally in both dilatometric curves, it was observed a change in the slope during the heating in the range of 450 ºC. In this region of temperatures is possible the loss of te-tragonality in the martensitic structure and the transition carbides.Figure 6 shows the microstructure after dilatometric test, this is very similar in both heating rates and it only presents a martensitic structure without evidence of pre-cipitates at least in SEM. The hardness likewise does not show a significant change, which was measure at 622 HV for a heating rate of 60 ºC/min and 632 HV for 600 ºC/min.CONCLUSIONSA comprehensive c haracterization of phase transfor-mations in a PREMOMET® steel has been performed. The Ac1 and Ac3 temperatures in the PREMOMET® steel are similar in each technique used in this investiga-tion. Ms and Mf temperatures were measured in both techniques: in DSC the martensite formation appears as an exothermic peak and in the dilatometric curves there is an evident expansion caused by the change of crystal-line structure, on other hand all the conditions analyzed in the steel shows a martensitic microstructure, this steel does not evidenced show precipitates in the micro-structure and finally the hardness was similar after dilatometric testsREFERENCES[1] T. S. Srivatsan, K.M.T.Q. M.L.S. Investigating and under-standing the cyclic fatigue fracture behavior of two high strength specialty steels. Int. Engg. Sc. & Mgmt., II, (2012), 15.[2] E. T. G. Steel Heat Treatment Metallurgy and Technolo-gies. 2nd. ed. [S.l.]: CRC Press, 205.[3] M. Gómez. Modelling of phase transformation kinetics by correction of dilatometry results for a ferritic Nb-microal-loyed steel. ISIJ International, 43, (2003) 8, 1228-1237.[4] Honeycombe, H. Badeshia. Steel Microstructure and Pro-perties. [S.l.]: Elsevier, (2006).[5] P. V. Morra. Decomposition of Iron-Based Martensite a Kinetic Analysis by Means of Differential Scanning Calo-rimetry and Dilatometry. J. Therm. Anal. Calorim. (2001) 64, 905.Figure 4  Experimental dilatometric curve of PREMOMET® steel heated at 60 ºC/min.Figure 5  Experimental dilatomeric curve of PREMOMET® steel heated at 600 ºC/min.(a)Figure 6  Microstructure after dilatometric test;...(a) 60 ºC/min, (b) 600 ºC/min.(b)74M. TORRES-LÓPEZ et al.: THERMAL AND PHASE TRANSFORMATIONS ANALYSIS IN A PREMOMET® STEEL METALURGIJA 56 (2017) 1-2, 71-74[6] M. Gojic. Thermal Analysis of Alloy Cr-Mo Steel. Journal of Thermal Analysis and Calorimetry (2004) 75, 947-956.[7] J. R. Yang. Martensitic transformations in AISI 440C stainless steel. Materials Science and Engineering A (2006), 276-280.[8] S. Primig, H. L. Transformation from continuous-to-iso-thermal aging applied on a maraging steel. Materials Science and Engineering A (2010), 526, 4399.[9] L. M. Kleiner. Phase and structural transformations in low-carbon martensitic steels. The Physics of Metals and Me-tallography (2009) 108, 2, 153-160.[10] Ferritic Steel for Fast Reactor Steam Generators. BNES, London, (1978).[11] M. L. Barnshtein. L. M. Kaputkina, S. D. Prokoshkin. Tempering of Steel. MISiS, Moscow (1997).[12] L. Guerra-Fuentes, R. Deaquino-Lara, M. A. L. Rodriguez, A. Salinas-Rodriguez, E. Garcia-Sanchez, Thermal stability and phase transformation of a FV535 steel, J Thermal Anal Calorim, Hungary, 2015.Note:  The responsible person for English language is the translatior from University Autonomous of Nuevo Leon, Mexico

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