Refinement of Cementite in High Strength Steel Plates by Rapid Heating and Tempering

Abstract

The precipitation behavior of cementite in low carbon steels at various heating rates from 0.3 to 100 K/s has been studied using a high-frequency induction heating apparatus. The materials used in this study were steel platesfor welded structures: 610 and 780 MPa class steel plates with a mixed microstructure of bainite and martensite.Cementite was observed using a carbon extraction replica method and the hardness and toughness were also examined. When heated at the conventional slow rate of 0.3 K/s, relatively large cementite particles with an average diameter of 72 nm precipitated at the lath boundaries, whereas when heated at a rapid rate over 3.0 K/s, cementite precipitated both within the laths and at the lath boundaries, and the cementite was refined down to an average diameter of 54 nm. With such refinement of the cementite, the toughness was improved. On the other hand, the hardness was irrespective of the heating rate and was dependent on the tempering parameter. TEM observations of the cementite precipitation behavior during the rapid heating process revealed that cementite begins to precipitate at the lath boundaries at about 773 K and within the laths at about 873 K. It is concluded that rapid heating especially from 773 to 873 K contributes to the cementite refinement and consequently the improvement in toughness. The effect of alloying elements such as chromium, molybdenum or silicon on the cementite growth during the rapid heating and tempering treatment is also discussed.

Tempering of Martensite in Dual-Phase Steels and Its Effects on Softening Behavior

“…[24,25] Corroborating the present work, Furuhara et al also reported difference in locations of cementite precipitation in nonisothermal and isothermal tempering at 923 K (650°C). [24] In nonisothermal tempering, finer cementite precipitated within laths and interlath regions compared to that at prior-c and block boundaries, whereas isothermal holding (3600 seconds) resulted in growth of the cementite at all locations, but the trend in cementite size at location remained similar.…”

“…This may be because DP M steel has lean chemistry, whereas the steel of Furuhara et al [24] has high carbon and Cr content, which restricts growth of the cementite in tempering. [25] From the aforementioned locations and size of cementite in both the tempered structures of DP M steel, it was concluded that precipitation of cementite is controlled by diffusion of carbon atoms to lattice defects in the following order: prior-c grain boundaries and block boundaries (larger cementite), lath boundaries (fine interlath cementite), and dislocations (finer intralath cementite). This finding is very consistent with earlier reports on isothermal tempering of low-carbon martensite describing cementite precipitation in the early stages of tempering and its consequent coarsening at the same locations by increasing temperature.…”

“…Another interesting aspect was that various tempering reactions, which take place during the conventional high-temperature (third stage) tempering of martensite, were easily delayed in nonisothermal tempering, i.e., coarsening and spheroidization of the precipitated carbides, and complete recovery of lath substructure, which is consistent with earlier reports on low-carbon martensitic steel. [24][25][26] 3. Coarsening kinetics of cementite The coarsening of cementite particles occurs by the diffusion of solute atoms down the concentration gradients in the matrix and can be predicted by a power-law relationship: [41] …”

“…[7] According to work on induction heating rates, which may be the only open literature on nonisothermal tempering of fully martensitic steel, [24] increasing the heating rate would result in refinement of cementite particles. [25] This was ascribed to the increased temperature required for cementite precipitation and higher nucleation rate of cementite during rapid heating. [24] Furthermore, delay in commencement of cementite spheroidization to 873 K (600°C) was also observed during rapid heating, which compares to the effects at lower temperatures, e.g., 773 K (500°C) in slow heating.…”

Effect of Uniform Distribution of Fine Cementite on Hydrogen Embrittlement of Low Carbon Martensitic Steel Plates

“…Nagao et al 21) reported that the main nucleation site of cementite shifts from lath boundary to within lath during the heating process of tempering. The dislocation density is greatly increased after quenching, especially in the steels subjected to ausforming.…”