Martensite in Steel

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  • Materials Science and Engineering A273275 (1999) 4057

    Martensite in steel: strength and structure

    George Krauss *Colorado School of Mines, Golden, CO 80401, USA

    Abstract

    This paper reviews the strengthening mechanisms associated with the various components of martensitic microstructures insteels and other ferrous alloys. The first section examines the experiments and strengthening theories associated with FeNi andFeNiC alloys, in which the martensite, because of subzero Ms temperatures, can be evaluated with carbon atoms trapped inoctahedral interstitial sites. The evaluation of strengthening in these alloys has been limited to interpreting yield strength ofunaged, untempered martensite in terms of interstitial solid solution strengthening. The second section reviews strengthening ofmartensitic FeC alloys and low-alloy carbon steels with above-room-temperature Ms temperatures. In these alloys, it isimpossible to prevent C diffusion during quenching, and strengthening of martensite becomes dependent on static and dynamicstrain aging due to carbon atom interaction with dislocation substructure. In all alloys the dominant strengthening component ofmartensitic microstructures is the matrix of martensitic crystals, either in lath or plate morphology, but secondary effects due toother microstructural components such as carbides and retained austenite are also discussed. 1999 Elsevier Science S.A. Allrights reserved.

    Keywords: Martensite; Steels; Strengthening mechanisms

    www.elsevier.com:locate:msea

    1. Introduction

    Martensite in steels over the millennia has been usedto do work, to do battle, and to support mechanicalloads. Applications range from ancient elegantlycrafted hand tools and swords [1,2] to current high-strength, high-fatigue resistant, high-wear resistantparts for machines, tools and dies, power transmission,gears and shafts, and demanding load-bearing struc-tures such as aircraft landing gear. Hardened mi-crostructures in steels require the generation of theparent phase austenite, the formation of martensitecrystals by diffusionless, shear-type martensitic trans-formation, and adjustment of final strength and tough-ness by tempering. The essential atomic configurationsdo not change with time, but the combinations ofphases, crystal morphologies, and crystal substructuresin hardened steels are endless and the processing tech-niques to produce optimized microstructures continu-ously evolve, with surface hardening by induction,plasmas and lasers being the most recent innovations.

    The purpose of this paper is to review the structuralreasons for the high strength and hardness of marten-site in ferrous alloys. Excellent state-of-the-art reviewsregarding the origins of the strength of unaged oruntempered martensite have been written by Cohen[35] and Owen [6], but because of the high mobility ofcarbon, the explanations developed have been based onexperiments in ironnickelcarbon alloys where carbondiffusion can be suppressed because of subzero Mstemperatures. Christian [7] has also reviewed thestrength of martensite and effectively related it to thestructural changes produced by the lattice and lattice-invariant deformations characterized by the crystallo-graphic theory of martensitic transformation. Inlow-alloy steels and ironcarbon alloys, however, car-bon diffusion cannot be suppressed, and to generateuseable high-strength microstructures, is even promotedby low-temperature tempering. Thus the present reviewwill take a broader view of the strength of martensiticmicrostructures, incorporating the many effects of car-bon and as well as other phases and structures inhardened steels. Hardened microstructures in plain car-bon and low alloy carbon steels are widely used, andscientific insights combined with experience gained inpractical applications should help in defining the futureperformance limits of martensitic microstructures.

    * Tel.: 1-303-6740670; fax: 1-303-6700797.E-mail address: gkrauss@mines.edu (G. Krauss)

    0921-5093:99:$ - see front matter 1999 Elsevier Science S.A. All rights reserved.

    PII: S0921 -5093 (99 )00288 -9

  • G. Krauss : Materials Science and Engineering A273275 (1999) 4057 41

    This review will first describe the hardness and gen-eral carbon-dependent features of hardened microstruc-tural systems in steels. Then the results of studies onFeNi and FeNiC alloys will be reviewed. Finally,the properties and dynamics of deformation in hard-ened carbon steels will be discussed.

    2. Hardness and microstructure of martensitic carbonsteels

    Fig. 1 shows hardness measured as a function ofcarbon content for a variety of carbon and low alloysteels by a number of investigators [8]. The referencesfor the various investigations are given by their num-bers in [8]. For a given carbon content, there is a widerange of hardness reported, typically on the order of100 DPH units, for as-quenched steels. This scatterreflects differences in the multi-component systemswhich constitute the microstructures of hardened steels.Austenite grain size, which in turn affects the size ofmartensite crystals and the size of parallel arrays ofmartensite crystals, and thereby affects the strength of

    hardened microstructures, may vary. Varying amountsof retained austenite may also significantly affect hard-ness. The amount of retained austenite increasesmarkedly with increasing carbon content, and maydiffer from one investigation to another. In fact someinvestigators have used cooling in liquid nitrogen toreduce the amount of retained austenite for the dataplotted in Fig. 1, leading to the significant variations inhardness plotted for the high carbon steels.

    In addition to retained austenite, other phases whichmay be present in the microstructures of high-strengthhardened steels may be fine carbides produced duringquenching of low carbon steels with high Ms tempera-tures, i.e. carbides produced by autotempering, or tran-sition carbides produced by low-temperature tempering[9]. On a somewhat larger size scale, spherical carbidesundissolved during austenitizing prior to quenching,either because of insufficient time for the dissolution ofcarbides in the structures present prior to austenitizing[911], or by design in the intercritical austenitizing ofhypereuctectoid steels, may also be a significant compo-nent of hardened microstructures. For example, in52100 steel, a steel containing 1.00% C and typicallyaustenitized in the two phase austenitecementite fieldat 850C, sufficient spherical carbides are retained tolower the carbon content of the austenite to 0.55% [12].Thus, by virtue of the diffusionless martensitic transfor-mation, the carbon content of the martensite in as-quenched 52100 steel is also 0.55%. Such variations inheat treatment practice make the direct relationship ofhardness to the martensitic component of hardenedmicrostructures of carbon steels difficult to interpret.

    Coarse second-phase particles imbedded in marten-sitic matrices, either spheroidized carbides or inclusions,play a relatively small role in strengthening but play amajor role in the fracture of hardened steels [9,11]. Ifthe matrix martensite is capable of plastic flow and thesecond-phase particles are well dispersed, then the par-ticles become the sites for microvoid formation andcoalescence leading to ductile fracture. Other arrays ofsecond-phase particles, such as carbides formed onaustenite grain boundaries or between laths of marten-site, may lead to brittle fracture and various types ofembrittlement of hardened steels [10,13].

    Despite the complexity of hardened microstructures,there is no question that the deformation response ofthe martensite crystals in as-quenched steels accountsprimarily for the carbon-dependent hardness shown inFig. 1. The complex interactions between the fine struc-ture and the carbon atoms within martensite crystalsunder applied stress lead to the parabolic shape of thehardness versus carbon curve, and are the subject ofmany of the investigations described below. Based onrecent nanohardness measurements on individualmartensite crystals, the strengthening mechanismswhich operate appear to extend to higher carbon levels

    Fig. 1. Hardness of martensitic microstructures as a function of steelcarbon content [8].

  • G. Krauss : Materials Science and Engineering A273275 (1999) 405742

    Fig. 2. Nanohardness, microhardness and retained austenite as a function of carbon content in a carburized and oil quenched 4320 steel [14].

    than implied in Fig. 1 [14]. Fig. 2 compares the resultsof the nanohardness measurements to more macro-scopic hardness measurements which integrate defor-mation response of larger volumes of themicrostructure, including that of retained austenite aswell as the martensite crystals [14]. The nanohardnessmeasurements show that the hardness of individualplates of martensite attain and maintain very highvalues, close to HRC 70, at carbon contents of 0.80wt.% C and above.

    Although it is the deformation resistance of the car-bon-containing substructure of martensitic crystalswhich accounts for the high hardness and strength ofhardened steels, the shape and distribution of the crys-tals also contributes to the collective deformation be-havior of hardened microstructural systems in carbonsteels. Two major morphologies of martensitic crystalsand microstructures, now termed lath and plate, formin steels [1518]. Fig. 3 shows Ms as a function ofcarbon content and ranges of carbon content in whichthe lath and plate morphologies of martensite form[17,19,20]. Examples of light micrographs of lath andplate martensite are shown in Figs. 4 and 5 and dis-cussed below.

    Lath martensite forms in low- and medium-carbonsteels and consists of parallel arrays or stacks of board-or lath-shaped crystals. In low-carbon alloys most ofthe crystals in a parallel group have the