Excess Solute Carbon and Tetragonality in As-Quenched Fe-1Mn-C (C:0.07 to 0.8 Mass Pct) Martensite
- PDF / 3,614,093 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 102 Downloads / 187 Views
I.
INTRODUCTION
QUENCHED and tempered medium- to high-carbon martensitic steels have been used for many years as tool and machined steels. More recently, lower-carbon martensite steels have attracted remarkable interest due to their potential for weight savings and the improved crashworthiness of structural parts in automobiles. This interest has led to a need to better understand the microstructural evolution that occurs in steels with low- and medium-carbon contents during quenching.
NAOKI MARUYAMA is with the Advanced Technology Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtsu 293-8511, Japan and also with the Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, 565-0871, Japan. Contact e-mail: [email protected] SHINICHIRO TABATA is with the Yawata Research & Development Laboratory, Nippon Steel Corporation, 1-1 Tobihata, Tobata-ku, Kitakyushu 804-8501, Japan. HIROYUKI KAWATA is with the Steel Research Laboratories, Nippon Steel Corporation, 20-1 Shintomi, Futtsu 293-8511, Japan. Manuscript submitted October 22, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Low-alloy Fe-C martensite has been extensively investigated crystallographically; for example, high-carbon martensite exceeding C:0.6 mass pct is tetragonal in structure, with its tetragonality (i.e., c/a, where a and c are lattice parameters) dependent upon the carbon contents[1]: c=a ¼ 1 þ 0:045 ½mass pct C
½1
The tetragonal structure can be understood on the basis of the Bain correspondence, where the octahedral symmetry of the interstitial carbon atoms in the parent austenite face-centered cubic (fcc) lattice is maintained in the martensite body-centered cubic lattice (bcc), leading to the increased occupancy of the interstitial z sites at octahedral positions (Oz sites) in the new lattice and the lattice expansion along the c axis.[2,3] While the initial preferred location of carbon atoms on one set of octahedral interstices is a result of the Bain mechanism, the elastic and chemical interactions between carbon and iron atoms affect the ordering behavior of the carbon atoms and the resultant tetragonality.[4–6] No consensus has been reached to explain the evolution of crystal structures in low- to medium-carbon martensites with less than C:0.6 mass pct. According to the relationship between the c/a ratio and the C or N contents
summarized by Sherby et al.,[7] an abrupt change in the crystal structure from body-centered tetragonal (bct) to bcc at a carbon level below 0.6 mass pct takes place. Hutchinson et al.[8] observed carbon segregation on lattice defects and carbide precipitation during the water-quenching of 0.1 to 0.5 mass pct C lath martensite and showed that the structure was bcc. On the other hand, tetragonality has been found in the range of 0.2 to 0.6 mass pct C for water-quenched Fe-C alloy[9] and water-quenched 4340 steels,[10] although the c/a ratio was lower than expected from Eq. [1]. Winchell and Cohen[11] found tetragonality in line with Eq. [1] in Fe-Ni-C systems with l
Data Loading...
