Structure and strength of lath martensite

Abstract

The discovery of a shift martensite transformation made by G. V. Kurdyumov [I] and the many-sided electron microscopic studies of martensite in various alloys [2] have made it possible to create a morphological classification of their structures. A number of systematic studies of martensite strength (including that in 29 compositions of carbon steel with equal impurity contents [3]) have given a simple dependence of the yield strength on the carbon content and on the amount of austenite. A similar dependence exists for other concentrated interstitial solutions (of oxygen and nitrogen in Nb and "Ira) [4], which means that the high martensite strength is not an anomaly. The retainment of this strength after a low-temperature tempering is not an anomaly either; other interstitial solutions of a quite high concentration behave similarly (in them the softening due to the depletion of the solution is compensated by the hardening due to the zonal stage of the decomposition). The only "anomalous" phenomenon is the high hardening of martensite in deformation (up to 4500 MPa in 10 -30% compression [5]), but this is a natural consequence of dynamic strain aging (in Fe-C and Fe -N solutions it occurs even at 20~ Therefore, the toughness of carbon martensite is low and depends on the stage of decomposition before and during the tempering and on the initial structure of the crystals.Martensite is the main structural component of high-strength steels and has a complex self-organizing structure. The lattice deformation and the accompanying accommodation strain determine the final morphology, i.e., the shape and size of the structural components, their crystallographic orientation and the packing pattern, and hence the state of the interfaces between the structural components. Analyzing the structure and mechanisms of plastic flow of martensite, we can predict many features of the behavior of this composite structure in deformation and fracture.Out of the two basic morphological types of martensite [1,2], lath martensite formed in hardening structural lowand medium-carbon steels, maraging steels, and iron or its moderately alloyed carbon-free alloys presents the greatest 140 interest for applications. Studies of random sections of this morphologically complex structure give numerous and quite contradictory data. The origin of the structure of lath martensite became clear in quenching single crystals of austenite [6][7][8][9][10]. Their initial structure does not possess interphase and intergrain boundaries, subboundaries, and dislocation substructure, and the structure of the martensite is determined only by the martensitic transformation as such. When the length of the martensite crystals attains 100 btm or more and the width of the martensite laths is several millimeters, we can study single-lath specimens in oriented sections and directly observe the relation between the crystal geometry, the morphology, the plastic defonnation, and the fracture. Such studies made it possible to determine the general rules of formation of a m...

Structure and properties of low-alloy Cr-Mo-V steel after austenitization in the intercritical temperature range

“…All types of boundaries in the mar tensite packet are relatively immobile and relatively impenetrable to slipping dislocations, according to the suggestion in [11]. In such structures, the rack size makes the greatest contribution to the strengthening; the strengthening due to the packets is 1-2 orders of magnitude smaller [12]. In the present work, we find that only one orientation of the martensite packets appears at the location of a (2-6) μm austenite grain.…”

“…When the twin disorientation Σ3 is formed, the boundary between the crystals is not aligned with the twinning plane in the bcc lattice (211) α , since it is formed by the collision of two independently growing crystals rather than by twinning [11]. Austenite decomposition leads to the formation of three pairs of mutually twinned orientations (Σ3), bound by a reflection in a single {111} plane of the γ phase that is parallel to the plane of the martensitic packet [10][11][12][13].…”

Three-dimensional model of martensitic transformations with elasto-plastic effects

“…For example, an Fe-20Ni-5Mn alloy produces individually nucleated laths [18,19] with relationships of the form (111) F k(101) B , and ½110 F jj½111 B , where thin films of retained austenite are observed. The similar relationship and lath structure is formed in Fe-(0.2-0.4)C carbon steels [20,21]. We suggest that transformation dislocations accommodate the strain and lead to an increase of the final martensite fraction.…”

Effect of austenitising temperature on microstructure and mechanical properties of 30Si2CrNi4MoNb ultrahigh strength steel

“…A similar dependence exists for other concentrated interstitial solutions as oxygen and nitrogen in Nb and Ta, the remaining of this strength after a low temperature tempering is not an anomaly, and other interstitial solutions of a quite high concentration behave similarly because the softening due to the depletion of the solution is compensated by the hardening due to the zonal stage of decomposition. 17 When the grain boundaries begin to move due to hot activation, the Nb and Mo atoms are dragged by the grain boundaries. However, the diffusion velocity of metal atoms of matrix influences the diffusion velocity of Nb and Mo atoms.…”