Quenching and partitioning plate steel for wear and toughness applications

Abstract

There is interest in exploring the application of quenching and partitioning (Q&P) processing to produce low-alloy steel plates with increased wear resistance and toughness for use in the mining and earth moving industries. This work investigates the application of Q&P heat treatments to 18 mm thick low-alloy steel plates to obtain microstructures containing martensite and retained austenite (RA) that are wear resistant and tough compared to more traditional as-quenched (AQ) and quench and tempered (Q&T) heat treated plates.Three experimental alloys were used in this work: Base (0.2C-1.5Mn-1.5Si-0.4Cr-0.25Mo), Base-Mo (0.2C-1.5Mn-1.5Si-0.4Cr), and Base+C (0.3C-1.5Mn-1.5Si-0.4Cr-0.25Mo). Using a thermal model and dilatometry, heat treatment paths were designed for the alloys to obtain varying microstructures with nominally similar hardness (~400 Brinell hardness for Base and Base-Mo, ~450 Brinell hardness for Base+C). Q&P heat treatments applied to "full sized" (285×180×18 mm) plates included a full austenitization step followed by an interrupted water quench, where each plate was removed from the quench at an elevated target quench temperature (TQT) and either allowed to air-cool to room temperature (1-Step Q&P) or subjected to a partitioning heat treatment step in another furnace (2-Step Q&P). AQ and Q&T heat treatments were also applied to plates of each alloy for comparison. For each alloy, 1-Step Q&P plates had the highest RA as measured by x-ray diffraction, followed by 2-Step Q&P, AQ, and Q&T plates. Microstructural characterization indicated the presence of varied amounts of tempered and/or partitioned martensite, carbide-free bainite, and untempered martensite in each plate. When results of tensile testing, Charpy V-notch (CVN) impact testing, and Bond abrasion wear testing for each heat treated plate were compared, Q&P heat treated plates performed better than Q&T plates. The higher TQT 1-Step Q&P heat treated plates in particular demonstrated the most favorable combinations of tensile properties, impact-toughness, and wear resistance, even as compared to the much harder AQ heat treated plates. The relative high quantity of RA in 1-Step Q&P plates may indicate that RA is providing a beneficial effect during mechanical deformation and may reduce microcracking during impact-abrasion wear conditions simulated by Bond abrasion testing. 1-Step Q&P processing through the methods employed in this work also represents an industrially feasible process that might be applied to obtain microstructures of martensite/bainite and RA without the use of a more complicated secondary tempering/partitioning heat treatment step. Further work would be beneficial to optimize heat treatment parameters.