Tempering Behavior of a Si-Rich Low-Alloy Medium-Carbon Steel

Abstract

Owing to the addition of Si, 0.33C-1.8Si-1.44Mn-0.58Cr steel exhibits a unique tempering behavior. The tempering takes place in two distinct sequential stages that are significantly different from those in steels containing 0.2–0.5 wt.% of Si. Stage I is associated with the precipitation of transition carbides in a paraequilibrium manner, can take place in temperatures ranging from ~200 to ~474 °C, and concurrently increases strength, ductility, and toughness. Stage II is associated with the decomposition of retained austenite to bainitic ferrite and transition carbides. As a result, no significant effect of overlapping of Stage I with Stage II takes place. Stage III does not occur at temperatures below ~474 °C, since the precipitation of cementite in a orthoequilibrium manner is suppressed by the addition of 1.8 wt.% of Si. It was shown that a major portion of carbon atoms redistributes to Cottrell atmospheres under quenching. During low-temperature tempering at 200–400 °C, the precipitation of transition carbides consumes a large portion of carbon atoms, thereby increasing the number of ductile fractures and improving the impact toughness without strength degradation. The formation of chains of cementite particles on boundaries takes place in Stage IV at a tempering temperature of 500 °C. This process results in the full depletion of excess carbon from a ferritic matrix that provides increased ductility and toughness but decreased strength.

Relationships between Strength, Ductility and Fracture Toughness in a 0.33C Steel after Quenching and Partitioning (Q&P) Treatment

“…Two accompanying processes usually occur during Q&P processing. The first process is the precipitation of the η(Fe 2 C)-or ε(Fe 2.4 C)-transition carbides in a martensitic matrix that leads to additional carbon depletion from supersaturated martensite [2][3][4][5]. This lowtemperature tempered martensite is often termed primary martensite [2].…”

“…The second process is the decomposition of untransformed austenite to bainitic ferrite. Q&P steels are usually alloyed with ≥1.5 wt.% Si, which suppresses the formation of cementite during Q&P processing if the T p and T q temperatures are below 400 • C [3,4,6,7]. The partitioning step is followed by air cooling or water quenching, which may result in the formation of secondary untempered martensite with a higher carbon content compared to the primary martensite [2,4,5,8,9].…”

“…Q&P steels are usually alloyed with ≥1.5 wt.% Si, which suppresses the formation of cementite during Q&P processing if the T p and T q temperatures are below 400 • C [3,4,6,7]. The partitioning step is followed by air cooling or water quenching, which may result in the formation of secondary untempered martensite with a higher carbon content compared to the primary martensite [2,4,5,8,9]. Thus, the Q&P process produces a multiphase structure consisting of carbon-depleted martensite with transition carbides, carbon-enriched martensite, carbondepleted bainitic ferrite and carbon-enriched RA [1][2][3][4][5][8][9][10].…”

“…The partitioning step is followed by air cooling or water quenching, which may result in the formation of secondary untempered martensite with a higher carbon content compared to the primary martensite [2,4,5,8,9]. Thus, the Q&P process produces a multiphase structure consisting of carbon-depleted martensite with transition carbides, carbon-enriched martensite, carbondepleted bainitic ferrite and carbon-enriched RA [1][2][3][4][5][8][9][10]. For low-and medium-carbon steels, both the primary and the secondary martensite usually hold the Kurdjumov-Sachs (K-S) orientation relationship with the austenite matrix, which can be determined by electron backscatter diffraction (EBSD) analysis [5,[11][12][13].…”

“…Two types of RA distinguished by morphology and carbon content may evolve in Q&P steels [2,3,5,8,11,14]. Blocky-type RA is usually located in the triple junctions of prior austenite grains (PAGs) and martensite packets [4,11,15]. Q&P steels with blocky-type RA have a carbon content of above 0.3 wt.% and exhibit elongation-to-failure, δ, ≥15% with a product of strength and elongation (PSE), UTS × δ (MPa × %), ranging from 2 × 10 4 to 4 × 10 4 MPa•% due to transformation of blocky-type RA to strain-induced martensite under tension [1,8,11,[16][17][18].…”