Strength–Toughness of a Low-Alloy 0.25C Steel Treated by Q&P Processing

Abstract

Quenching and partitioning (Q&P) treatments were applied to 0.25C steel to produce the microstructures that exhibit an improved balance of mechanical properties. The simultaneous bainitic transformation and carbon enrichment of retained austenite (RA) during the partitioning stage at 350 °C result in the coexistence of RA islands with irregular shapes embedded in bainitic ferrite and film-like RA in the martensitic matrix. The decomposition of coarse RA islands and the tempering of primary martensite during partitioning is accompanied by a decrease in the dislocation density and the precipitation/growth of η-carbide in the lath interiors of primary martensite. The best combinations of a yield strength above 1200 MPa and an impact toughness of about 100 J were obtained in the steel samples quenched to 210–230 °C and subjected to partitioning at 350 °C for 100–600 s. A detailed analysis of the microstructures and the mechanical properties of the steel subjected to Q&P, water quenching, and isothermal treatment revealed that the ideal strength–toughness combinations could be attributed to the mixture of the tempered lath martensite with finely dispersed and stabilized RA and the particles of η-carbide located in the lath interiors.

Quench and Tempered Embrittlement of Ultra-High-Strength Steels with Transition Carbides

“…Crack propagation in the CVN specimens was analyzed on the fractured specimens by EBSD on lateral surfaces [35]. Other details of structural and mechanical characterization were reported in previous works [16,17,20,29,35,36]. The volume fraction of RA was determined by magnetic saturation measurements using Fischer Feritscope FMP30 (Helmut Fischer Gmbh, Sindelfingen, Germany).…”

“…Sufficient ductility and fracture toughness is attained in these steels by vacuum arc remelting, the use of modified heat treatment routes, and/or alloying the steels with ≥1.5 wt.%Si [1,[7][8][9][10][11]. Additions of Si ≥ 1.5 wt.% reduce significantly the precipitation of cementite which enhances the strength and toughness of UHSS [1,4,10,[12][13][14][15][16]. The microstructure of Si-enriched UHSS consists of lath martensite with transition carbides located in the matrix and film-like retained austenite (RA) located at lath and block boundaries [1,14,17].…”

Microstructure and Mechanical Properties of Structural Steels and Alloys

“…A total of 10 papers cover a wide range of structural metals and alloys. The majority of these papers focus on the mechanisms of microstructure evolution and the mechanical properties of metallic materials subjected to various thermo-mechanical, deformation, or heat treatments [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. Readers who are interesting in structural steels may learn from comprehensive investigations of microstructure evolution and its effect on the mechanical properties of diverse steel types [ 1 , 3 , 5 , 7 , 8 ].…”

“…The majority of these papers focus on the mechanisms of microstructure evolution and the mechanical properties of metallic materials subjected to various thermo-mechanical, deformation, or heat treatments [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. Readers who are interesting in structural steels may learn from comprehensive investigations of microstructure evolution and its effect on the mechanical properties of diverse steel types [ 1 , 3 , 5 , 7 , 8 ]. Four of these papers [ 1 , 3 , 7 , 8 ] present experimental investigations of strengthening mechanisms operating in high-strength steels subjected to various promising treatments including tempforming [ 1 ] and quenching–partitioning processing [ 7 ].…”

“…Readers who are interesting in structural steels may learn from comprehensive investigations of microstructure evolution and its effect on the mechanical properties of diverse steel types [ 1 , 3 , 5 , 7 , 8 ]. Four of these papers [ 1 , 3 , 7 , 8 ] present experimental investigations of strengthening mechanisms operating in high-strength steels subjected to various promising treatments including tempforming [ 1 ] and quenching–partitioning processing [ 7 ]. Some crucial features related to structure–property relationships are detailed for high-strength rebar [ 3 ] and a high-speed axle steel [ 8 ].…”