1 Materials Research, Vol. 8, No. 4, 417-423, 2005 2005. The quenching and partitioning process : background and Recent Progress John G. Speera, Fernando C. Rizzo Assun ob*, David K. Matlocka, David V. Edmondsc Advanced Steel Processing and Products Research center , a Colorado School of Mines, Golden, CO 80401, USA. b Department of Materials Science and Metallurgy, Pontif cia Universidad Cat lica, 22453-900 Rio de Janeiro - RJ, Brazil c School of process , Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom Received: July 19, 2004; Revised: April 8, 2005. A new process concept, quenching and partitioning (Q&P) has been proposed recently for creating steel microstructures with retained austenite. The process involves quenching austenite below the martensite- start temperature, followed by a partitioning treatment to enrich the remaining austenite with carbon, thereby stabilizing it to room temperature.
2 The process concept is reviewed here, along with the thermodynamic basis for the partitioning treatment, and a model for designing some of the relevant processing temperatures. These concepts are applied to silicon-containing steels that are currently being examined for low-carbon TRIP sheet steel applications, and medium-carbon bar steel applications, along with a silicon-containing ductile cast iron. Highlights of recent experimental studies on these materials are also presented, that indicate unique and attractive microstructure/property combinations may be obtained via Q&P. This work is being carried out through a collaborative arrangement sponsored by the NSF in the USA, CNPq in Brazil, and the EPSRC in the United Kingdom. Keywords: carbon partitioning , retained austenite, martensite 1. Introduction 2. background and Q&P Fundamentals High strength ferrous alloys containing significant fractions of Carbon partitioning concept retained austenite have been developed in recent years, and have important commercial applications.
3 In sheet steels, for example, Carbon partitioning between martensite and retained austenite carbon-enriched metastable retained austenite is considered beneficial is usually ignored in quenched steels, because the temperature is because the TRIP phenomenon during deformation can contribute normally too low for substantial amounts of carbon diffusion to occur to formability and energy absorption. In gear and bearing surfaces, after quenching , and because carbon supersaturation in martensite is austenite is considered to provide damage tolerance in rolling/sliding ordinarily eliminated by a different mechanism, viz. carbide precipita- contact fatigue applications. In thicker section structural applica- tion during tempering. Consequently, while carbon-enriched retained tions, retained austenite may provide enhanced resistance to fracture. austenite has been identified in martensitic steels for some time2, Similarly, austempered ductile cast iron materials develop favorable the thermodynamics of carbon partitioning between martensite and property combinations through a microstructure of fine ferrite plates retained austenite has been scarcely considered.
4 Recently, a model in combination with carbon-rich retained austenite. has been developed to address carbon partitioning from as-quenched Steels with substantial amounts of carbon-enriched retained martensite into austenite, under conditions where competing reac- austenite are typically produced by transforming at low tempera- tions such as bainite, cementite or transition carbide precipitation are tures, leading to a microstructure containing carbide-free bainite suppressed1. The model predicts the endpoint of partitioning , when that consists of bainitic ferrite laths with interlath retained austenite. martensite ( ferrite) is in metastable equilibrium with austenite. Alloying additions such as Si or Al are made to suppress cementite Metastable equilibrium between austenite and ferrite is not a new precipitation that usually accompanies bainite formation. Recently, concept3, and equilibrium ( orthoequilibrium) and paraequilibrium an alternative processing concept, quenching and partitioning (or concepts are well understood at sub-critical temperatures for condi- Q&P), has been developed for the production of austenite-contain- tions where partitioning of slow-moving substitutional elements is ei- ing steels, based on a new understanding of carbon partitioning ther complete or absent, respectively.
5 It must be recognized, however, hypothesized between martensite and retained austenite1. This paper that transformations occurring under equilibrium or paraequilibrium reviews the fundamental elements of the process concept, and recent necessarily involve interface migration and thus require short range experimental investigations to examine the Q&P processing response movements of iron and substitutional atoms, even when long-range of two commercial Si-containing steels and a commercial Si-contain- substitutional diffusion is precluded as in the paraequilibrium case. ing ductile cast iron. When the position of the martensite/austenite interface is effectively *e-mail: 418 Speer et al. Materials Research a constrained, as we consider to apply for carbon partitioning between f CPE xCaCPE + f CPE. g xCgCPE = xCalloy (3). martensite and retained austenite at relatively low temperatures, then and the relationship between the phase fractions of and is sim- even short-range diffusional movements of iron and substitutionals ply: are precluded, and it is not possible for a ferrite/austenite mixture to reach equilibrium in the Fe-C system (or paraequilibrium in a f CPE g + f CPE = 1 (4).)
6 Multicomponent alloy systems). The metastable / equilibrium in Example CPE calculations have been reported previously , where 1. the case of an immobile or constrained interface, is therefore termed it was shown that most of the carbon in the steel is expected to parti- constrained paraequilibrium or CPE. Paraequilibrium and CPE. tion to the austenite, and quite high levels of carbon enrichment are derive fundamentally from the immobility of iron and substitutionals possible. The dependence of the metastable CPE condition on alloy in comparison to carbon and other interstitials. Consequently, these carbon content, temperature, and the as-quenched austenite and two conditions are considered by the authors to be closely related, martensite phase fractions was also illustrated. While the detailed although this view is not held universally4 and remains the subject calculations are not difficult, it was found that the austenite composi- of discussion5.
7 Tion at constrained paraequilibrium can be closely approximated by Constrained paraequilibrium is essentially defined by one thermo- assuming that virtually all of the carbon in the martensite partitions to dynamic requirement, and one key matter balance constraint. First, the austenite, and then applying the appropriate carbon matter balance carbon diffusion is completed under constrained paraequilibrium based on the amount of retained austenite present after quenching9. conditions when the chemical potential of carbon is equal in the fer- The results of the constrained paraequilibrium model suggested a rite and austenite. Ignoring effects of alloying on carbon activity, this new process , whereby austenite is formed at high temperature (either requirement may be represented using results of Lobo and Geiger6,7. by full austenitization or intercritical heat treatment), followed by for the Fe-C binary system as follows: cooling to a temperature carefully selected (between Ms and Mf) to 76, 789 - - (169, 105 - ) xCc (1) control the fractions of martensite and retained austenite, and finally xCc = xCa $ e RT by a thermal treatment that accomplishes the desired carbon partition- a g where x and x represent the mole fractions of carbon in ferrite and C C ing to enrich the austenite with carbon and stabilize some (or all) of austenite.
8 The relevant thermodynamics are embedded in Equation 1. it to room temperature. This process sequence and the corresponding This thermodynamic condition may be understood by comparing microstructural changes are illustrated schematically in Figure 2 10. the schematic Gibbs molar free energy vs. composition diagram in The process assumes that carbon supersaturation is relieved by dif- Figure 1a representing metastable equilibrium in the Fe-C system, fusion into retained austenite, and is referred to as quenching and with constrained paraequilibrium in Figure 1b. In (ortho) equilibrium, or paraequilibrium in higher order alloys, a g there are unique ferrite and austenite compositions (xEQ and xEQ ). satisfying the common tangent construction whereby the chemical potentials of both carbon and iron are equal in both phases (mCa = mCg and mFE. a = mFE. g ). (In paraequilibrium, the same construction would apply G. if the vertical axis at the composition of pure iron were replaced by the appropriate composition in multicomponent space representing ' A MA# MG#.)
9 The relative fractions of iron and substitutional elements in the alloy). In constrained paraequilibrium, the thermodynamic condition that the chemical potential of carbon is equal in both phases requires MA&E MG&E. only that the tangents to the ferrite and austenite free energy curves must intersect the carbon axis at a single point. This condition can be satisfied by an infinite set of phase compositions8, and examples XA%1 XG%1. of two such conditions are given in Figure 1b, one which is associ- &E #. ated with phase compositions (xCaP-EII and xCgP-EII) having a higher carbon (a). concentration than the equilibrium phase compositions, and one as- sociated with phase compostions (xCaP-EI and xCgP-EI ) having lower carbon levels than equilibrium. The actual CPE phase compositions must also satisfy the unique matter balance constraint associated with the stationary / interface. This second constraint requires that the G. number of iron (and substitutional) atoms is conserved in each phase M#A )) M#G )).
10 During carbon partitioning . Mathematically, this matter balance for '. iron may be represented by: A. g f CPE (1 xCgCPE ) = f ig (1 xCalloy) (2). where x is the overall carbon content of the steel (in atom fraction, alloy C. recognizing also that in Fe-C binary alloys, 1 xC = xFE ), f ig is the M#A ) M#G ). g mole fraction of retained austenite before partitioning begins, and f CPE. and xCgCPE represent the austenite amount and carbon concentration, X#A ) A )). 0% X#0% X#G )0% X#G )). 0%. &E #. respectively, at constrained paraequilibrium when carbon partition- ing is complete. (A small change in austenite fraction is consistent (b). with transfer of carbon atoms across the interface). Constrained Figure 1. Schematic molar Gibbs free energy vs. composition diagrams il- paraequilibrium is achieved when Equations 1-2 above, and Equa- lustrating metastable equilibrium at a particular temperature between ferrite tions 3-4 below are satisfied, where the mass balance for carbon is and austenite in the Fe-C binary system.