Physical principles of simulation and optimization of laser-induced surface hardening of steels (original) (raw)
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Phase transformations during the decomposition of austenite below Ms in a low-carbon steel
Materials Characterization, 2014
Tel. +32 (0) 934 51365), (Fax. +32 (0) 934 51204) Nacionality: Chinese 2 ArcelorMittal Global R&D Gent. Pres. J.F. Kennedylaan 3, 9060 Zelzate -Belgium c cecilia.fojer@arcelormittal.com (Tel. +32 (0) 934 51202), (Fax. +32 (0) 934 51204) . Technologiepark 903, 9052 Zwijnaarde -Belgium. e J.Sietsma@tudelft.nl, (Tel. +31 (0) 15 278 2284), (Fax. +32 (0) 15 278 6730) Nacionality: Dutch 1 Department of Materials Science and Engineering, Ghent University. Technologiepark 903, 9052 Zwijnaarde -Belgium. f roumen.petrov@ugent.be(Tel. +32 (0) 933 10457), (Fax. +32 (0) 926 45833) Nacionality: Bulgarian Abstract: The purpose of this study is to investigate and understand the phase transformations during the decomposition of austenite, which occurs during isothermal treatments below the martensite start temperature (M s ) in a low-carbon steel. Isothermal holding treatments after rapid cooling to various temperatures (forming a controlled volume fraction of initial martensite) were carried out in a dilatometer. Results obtained by dilatometry, microstructural characterization and hardness were analyzed. This combination of results shows that the microstructures formed below the M s temperature are mainly bainitic, mixed with tempered martensite. The kinetics of isothermal bainite formation was described by a nucleation-based transformation model. The complex competition and interactions between their transformation mechanisms during the isothermal holding at different temperature regimes are discussed.
Austenite phase formation in rapidly solidified Fe–Cr–Mn–C steels
Acta Materialia, 1999
ÐSteels having compositions (wt%) 0.05±0.5C, 12.5±20Cr, 8±25Mn and 0±0.51N have been chillblock melt-spun to ribbons in order to investigate systematically, by X-ray diractometry and electron microscopy, the eects of rapid solidi®cation and of solute concentrations on the formation of the austenite phase. The austenite is most easily formed at (wt%) 16Cr±8Mn for 0.3C ribbons while a'-martensite or emartensite was observed at lower concentrations of Cr or Mn and a-ferrite appeared for Cr >18 wt%. The volume fraction of austenite in the steel ribbons studied was found, by multiple regression analysis, to obey the equation 7 94 26X8g wn  8X4 À 0X08wn À 0X44gr À gr À 17X7 2. Thus, the eect of Mn on g formation followed a non-linear function, containing an interaction term including the Cr and Mn contents, and ®rst-and second-order terms involving the Mn concentration. This indicates the ranges over which Mn is a g-former or an a-former. Iso-austenitic lines, constructed on the basis of this new equation, are nearly orthogonal to those in the Schaeer diagram for Cr±Mn steels so that use of the latter for prediction of the austenite content in the present case would be inappropriate.
Equilibrium Phases and Constituents in the Fe-C System The Phases of Iron and Its Alloys
Hubertus Colpaert, updated and translated by André Luiz V. da Costa e Silva Ó ASM International 2017 This chapter introduces the basic concepts of the phases and constituents present in the simpler steels in conditions near equilibrium. The discussion focuses on the iron-carbon (Fe-C) system. Understanding these concepts is fundamental for a proper understanding of the transformations that occur during steel solidification, heat treatments, and thermomechanical processing. Discussion of the Fe-C system is of paramount importance to understanding many phenomena that also occur in complex alloy steels. The combination of chemical composition and structure defines the properties and hence the performance of steels. Chemical composition is controlled essentially during the steelmaking processes, although the composition close to the part surface can be affected by thermochemical treatments in the solid state. Structure, on the other hand, is altered by the combination of deformation and temperature change, normally grouped under the name of thermome-chanical treatments. Producing phase transformations between the two main crystal structures of iron, body-centered cubic (BCC) and face-centered cubic (FCC) and forming structures other than equilibrium structures with interesting properties in terms of application and with reasonable stability are possibly the two main reasons that explain the wide use of steels as industrial materials. A main objective of this book is to present the structures that result from applying different thermomechanical treatments of steels with various chemical compositions, in particular the size scale evaluated through metallography. Underlying the effect of these treatments on steels with different chemical compositions is the concept of phase transformation. The way steel undergoes phase transformations in the different stages of its processing is a critical factor in defining the features and properties of the structures formed. While the object of this book is not the in-depth study of phase transformations, the reader should be familiar with the concepts that control these transformations. Here, only the fundamental aspects essential to understanding the features presented are discussed; excellent literature is available on the subject at different levels of complexity in [1-4]. At atmospheric pressure, iron can have two solid crystal structures (BCC or FCC), depending on temperature, and is able to transform into the liquid state (the conditions under which iron is present as a vapor are less interesting to metallurgy). Alloying elements added to iron may help make one or the other structures more stable. They may also form new, important phases in steel. For this reason, the first important information related to the possible structures of an iron-base alloy is to understand the equilibrium state of the alloy at different temperatures. In metallurgy this information is classically presented in the equilibrium phase diagrams. These diagrams are constructed directly from the collection and consolidation of the results of experiments.
Acta Materialia, 1997
The dissolution of globular Mz3C6carbide in FeW-C and Fe-Me-C steels during austenitization has been studied by means of transmission electron microscopy(TEM). It was observed that the M23C6 carbide formed at 700"Cdecomposesby two differenteutectoid reactions. In the early stage of dissolution, before the ct+y transformation k completedat @JSkXIitiZatiOn temperaturethe M23C6 carbide transforms into M6C+ ctwith the formation of spheroidal M6Cprecipitates. This first reaction takes place whilethe carbide is still surroundedby ferrite. After thecr-+y transformation, theM23C6 carbide decomposesinto M6C+ Yin F+W-c steels and into FezMoC+ y or M6C+ Yin Fe-M~C steels. In the latter cases a rod-like or lamellar structure was observed. It was established that phase boundary migration during ki23c6 carbide dissolution initiates the precipitation reaction at the interface. The precipitates were characterised by X-ray diffraction and by TEM-EDS. The orientation relationship betweenM6Cand austenite and M6Cand martensite after M23C6 decompositionhas been determined.The results obtained indicate that in both F*Mo-C and FeW -C steels, h'f13c6 carbide is not stable in the temperature range of 8OO-1OOO"C. The observed results will be discussedin terms of local equilibrium at the phase interfaces during the reactions. 0 1997Acta Metallurgic Inc. R&urn&La dissolution du carbure globulaire M23C6 lors d'une austhitization a ete &tudi&par microscopic dectronique i transmission (TEM). Le carbure forrn~a 700"C se dissout suivant deux r&actionseutectoides diff&entes. La premi~re r~action, ki23c6+kf6c + u avec formation de particles sph~roidalesM6C,a~t~observ~tant que la matrice environnanteu n'a pas encoretransform+en aust~nite y. Apres Cetk transformation a+y la d~compositiondu carbure M23C6 COIIthIUe Selon la F%CtiOII M,,C,+M,C + y clans les alliages F&W-C, et selon M23C6+M6C + y ou M23C6-+Fe2MoC + y clansles alliages F+M&C. Ces reactions produisent une microstructure eutectoide lamellaire ou en b~tonnets. Ellessent initi~esi l'interfacemouvante.Les&14ments de la microstructureont &t& charact&isespar rayons X et par TEM-EDS. Le rapport d'orientation du carbure M6C avec l'aust~nite ou martensite a && determine. Les r&dtats montrent que le carbure M23C6 n'estpas stable clansl'intervallede temprhatures de 80&1000°C.Les rtactions observies sent discuttes en supposant un Aquilibrelocal aux interfaces. Zusammenfassung-Mit Hilfe der Transmissionselektronenmikroskopie(TEM) wurde die Auflosung globularer h'f23c6 Karbide bei der Austenitisierungvon F*W-C und F*M*C Stahlen analysiert. Die zuvor bei 700"CgebildetenKarbide losten sich nach zweiverschiedeneneutektoiden Reaktionen auf. Die erste Reaktion, MZ3C6+M6C + % mit Bildung von kugelforruigenM6C Teilchen wurde beobachtet, solangedas M23C6 Karbid noch von Ferrit (IX) umgebenwar.NachderFerrit-Austenitumwandlung u-w der umgebendenMatrix erfolgte die weitereAuflosung in denF+W'-CStahlengema~M23C6aM6C + y, in den F&M&C Stahlenentwedernach M23C6+M6C + y oder nach MZ3C6+FejMoC + y. Dabei bildete sichein lamellaresoder stabchenforrnigeseutektoidesGefiige.Es wurde festgestellt,da13diese Reaktionen an der bewegtenPhasengrenzeinitiiert werden.Die Gefugebestandteilewurdenmit Rontgenbeugungund rnit TEM-EDS charakterisiert und der Orientierungszusammenhangzwischen M6Cund Austenit und Martensit bestimmt.Die Beobachtungenhaben gezeigt, da13 dasM23C6 Karbid im Berekh m 80~loo130c nicht stabil ist. Die beobachteten Reaktionen werden unter der Annahme eines lokalen Gleichgewichts an den Phasengrenzendiskutiert. 'f' Dmitry V. Shtansky, Max-Planck-SocietyPost-Doctoral dissolution m;ght be ;~plained by the presence of Fellow, on leave from I.P. Bardin Central Research alloying elements. The dissolution of cementite in Institute for Iron and Steel Industry, 2nd Baumanskaya chromium-containing alloys was examined by Molin-St.,
Journal of Nuclear Materials, 2014
Using the methods of dilatometry and differential scanning calorimetry, critical points of phase transformations in the low-activation ferritic-martensitic steel EK-181 (RUSFER-EK-181) are identified. The characteristic temperature intervals of precipitation of carbide phases are revealed. It is shown that particles of the metastable carbide M 3 C are formed within the temperature range (500-600)°C. Formation of the stable phases M 23 C 6 and V(CN) begins at the temperatures higher than T = 650°C. An important feature of microstructure after tempering at T = 720°C is high density of nanoparticles (610 nm) of vanadium carbonitride V(CN).
2018
The aim of the performed experiments was to determine the influence of deformation and of austenitization temperature on the kinetics of phase transformations during cooling of high-carbon steel (0.728 wt. % C). The CCT and DCCT diagrams for austenitization temperature 940°C and DCCT diagram for austenitization temperature 1000°C were constructed with the use of dilatometric tests. On the basis of obtained results, a featureless effect of austenitization temperature and deformation on the kinetics of phase transformations during cooling of investigated steel was observed. Critical cooling rates for the transformation of martensite in microstructure fluctuated from 5 to 7°C·s–1 (depending on the parameters of austenitization and deformation), but only at cooling rates higher than 8°C·s–1 a dominant share of martensite was observed in the investigated steel, which resulted in the significant increase of hardness.
Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe–18Ni–9Co–3.4Mo–1.2Ti) produced by SLM. We found that despite the “intrinsic heat treatment” and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (l phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations,
Phase transformations and microstructure changes in a 12%Cr-steel during tempering at 1053 K
Steel Research, 1994
The evolution of the microstructure and the phase transformations in a 12% Cr-steel during tempering at 1053 K for 30 s, 0.25, 1 and 2 h were studied. The investigations were carried out by transmission electron microscopy. For the identification of the different phases and the determination of their chemical composition electron diffraction and energy dispersive X-ray spectroscopy were used. Four different types of precipitates were found: MX, M 3 C, M 2 X and M 23 C 6. They showed the following time dependent precipitation sequence: MX-, MX + M 3 C _, MX + M 2 X + M 23 C 6-, MX + M 23 C 6. Changes in the chemical composition of the various precipitates were determined. The chemical composition of the M 23 C 6 carbide was nearly constant, whereas it varied with time for the M 2 X and MX carbonitrides. Two different types of MX particles were identified: Ti,V-rich precipitates in the quenched and short-term tempered states and V,Cr-rich particles in the longer-term tempered states. Two types of orientation relationships between the ferrrite and the M 23 C 6 were observed: (101},11(111)M 23 c 6 and (111)all(110)M 23 c 6-Phasenumwandlungen und Anderungen der Mikrostruktur in einem 12%Cr-Stahl wahrend des Anlassens bei 1053 K. Die Entwicklung der Mikrostruktur und die Phasenumwandlungen eines 12%-Cr-Stahles wurden wahrend des Anlassens bei 1053 K nach 30 s, 0.25, 1 und 2 h untersucht. Die Messungen wurden mil Durchstrahlungselektronenmikroskopie durchgefOhrt. Zur ldentifizierung der verschiedenen Phasen und zur Bestimmung ihrer chemischen Zusammensetzung wurden Elektronenbeugung und energiedispersive Rontgenspektroskopie eingesetzt. Vier verschiedene Teilchenarten wurden beobachtet: MX, M 3 C, M 2 X und M 23 C 6. Sie zeigten die folgende Ausscheidungssequenz: MX _, MX + M 3 C-, MX + M 2 X + M 23 C 6 _, MX + M 23 C 6. Veranderungen in der chemischen Zusammensetzung der verschiedenen Teilchenarten wurden beobachtet. Die chemische Zusammensetzung des M 23 C 6 war nahezu konstant, wahrend bei den Carbonitriden M 2 X und MX milder Zeit Anderungen auftraten. Zwei verschiedene Typen von MX-Teilchen wurden identifiziert: Ti,V-reiche in der abgeschreckten und den kurzzeitig angelassenen Proben und V,Cr-reiche in den langer angelassenen. Zwei Arten von Orientierungsbeziehungen M 23 C 6 /Ferrit wurden ermittelt: (101)a 11(111)M 23 c 6 und (111 la 11(11 O)M 23 c 6 .