Mechanical Behavior of Fresh and Tempered Martensite in a CrMoV-Alloyed Steel Explained by Microstructural Evolution and
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INTRODUCTION
WEAR-RESISTANT steels with excellent combination of strength/hardness and toughness have wide applications, for example tipper bodies, which usually suffer frequent loading and unloading of heavy and sharp rocks, steel scrap, concrete with rebars, etc. This harsh application environment puts significant requirements on the performance of steels used for the tipper bodies. The steels need to have a high resistance towards dent formation and abrasion to increase the lifetime of the product. These property requirements are generally met by combining a martensitic microstructure and precipitation-hardening, which generates a combination of ultra-high strength/hardness and good toughness.[1,2] The diffusionless nature of the martensitic transformation leads to a martensitic microstructure that is defect-rich, with high supersaturation of alloying elements and a hierarchic microstructure consisting of packets, blocks, sub-blocks, and laths.[3–11] The TAO ZHOU and PETER HEDSTRO¨M are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, 100 44 Stockholm, Sweden. Contact e-mail: [email protected] JUN LU is with the School of Materials Science and Engineering, University of Science and Technology Beijing, Beijng 100083, China. Manuscript submitted March 4, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
martensitic microstructure is not so ductile and thus a tempering treatment is required subsequently to the quench-hardening. During tempering, the initial martensitic microstructure will evolve towards the equilibrium state of ferrite and carbides. The microstructural evolution can be divided into the following concurrent processes: (i) recovery with dislocation annihilation and residual stress relief[12,13]; (ii) precipitation of various carbides heterogeneously at defects like boundaries and dislocations[14–16]; (iii) grain growth.[9,10] The clarification of the respective contribution of each phenomenon on mechanical properties is vital for the optimization of wear-resistant steels to extend their service life. In CrMoV-alloyed martensitic wear-resistant steel, the additions of Cr, Mo, and V can increase hardenability during cooling after austenitization and strength/ hardness by precipitation-hardening during tempering. Six possible carbide phases may precipitate in this alloy system depending on alloying content and heat treatment conditions.[17–19] Precipitation of these multiple carbides together with the evolution of the martensitic microstructure during tempering determines the mechanical properties of the alloy. The present work aims at exploring the microstructure–property relationship of wear-resistant CrMoV steels. The mechanical behavior measured by tensile testing of a low-alloy CrMoV martensitic steel in quenched and tempered conditions is correlated with the quantitative
microstructural evolution. Modeling of carbide precipitation supports the experimental characterization to understand the microstructural evolution. Finally, yield strength modeling attempts
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