Formation of cementite in tempered Fe–Co–C alloys
Introduction
In Fe–Co–C alloys, undesirable grain coarsening results from the specific austenite orientation variants that form after the γ→α→γ transformations. Typically, throughout a given martensite crystal, the austenite crystals that nucleate possess the same crystallographic orientation as the original γ phase. The cause of this structural heredity is unclear. Models proposed to explain this phenomenon consider the epitaxial nucleation of austenite and the restriction of austenite orientation variants by it. These restrictive structural factors may include retained austenite (Kunz, 1965), martensite crystal boundaries (Schastlivtsev and Koptseva, 1976, Sorokin and Sagaradze, 1978), second phase precipitates (Sagaradze and Vaseva, 1976) and dislocation structure (Vovk et al., 1987).
Tempering of martensite before reheating prevents austenite returning to its original orientation and also limits grain coarsening (Televich et al., 1994, Televich and Prihod’ko, 1994). Some differences between cementite formed in tempered and rapidly heated alloys may cause the variation in the final austenitic structure. However, crystallographic characterisation of cementite formed after short-term tempering in Fe-based alloys has received little attention (Televich et al., 1994b).
Three crystallographic orientation relationships (OR) between cementite and matrix ferrite (martensite) in tempered steels have been observed. They are:
The Bagaryatskii OR (Bagaryatskii, 1950):
the Isaichev OR (Isaichev, 1947):
and the Pitsch–Petch (P-P) OR (Petch, 1953):
In all cases cited above, ORs were determined using selected area electron diffraction (SAED). Recently, Zhang and Kelly (1998) have reviewed the OR between cementite and tempered martensite in plain carbon eutectoid steel. Rather than use SAED, they used Kikuchi line patterns. Using this more precise technique, they reported no Bagaryatskii OR between cementite and the recrystallised matrix in tempered martensite. Instead, in samples tempered at 650°C, where recrystallisation had already taken place, they observed the OR: (103)c within 4° of with no unique parallelism of close packed directions.
The present investigation outlines observed differences in cementite crystallography between rapidly heated and long-term tempered Fe–Co–C alloys.
Section snippets
Materials and methods
Two experimental Fe–Co–C alloys with different Co contents, which are the basis for many steel grades, were chosen for this investigation. Their compositions are given in Table 1. Both ingots were cast in a vacuum furnace and then hot rolled into 20×20×100 mm bars. Then they were homogenised for 12 h at 1050°C in argon, terminated by a water quench. Samples of 3 mm thickness were cut and heat treated in the drop-hearth furnace in argon for 1 h at 1050°C, followed by water quench to produce a
Results
After austenising and quenching, all samples contain highly dislocated lath martensite. After both long-term and short-term tempering, Fe3C particles, with lattice parameters: a=0.452 nm, b=0.508 nm and c=0.673 nm precipitated within martensite crystals and at the crystal boundaries.
After rapid tempering, in both alloys cementite particles were elongated (∽60–190 nm long), plate-like and were oriented in the same direction in each α-crystal. The crystallographic orientations between the cementite
Discussion
Similarly to earlier results (Zhang and Kelly, 1998, Pereloma et al., 1999), the present study shows, within the accuracy permitted with the MBED technique, that the Isaichev OR was present in tempered samples after both short-term and long-term tempering treatments. Neither the duration of tempering, nor the Co content affected the type of OR. These data agree with the results from the edge-to-edge matching model proposed by Kelly and Zhang (1999). This model offers an explanation of the
Conclusions
The orientation relationships between cementite and martensite in two tempered Fe–Co–C alloys have been studied. In both alloys after short-term and long-term tempering the orientation relationships were shown to obey the Isaichev OR. However, after rapid tempering, only one carbide variant was found in each α-crystal, while after long-term tempering, up to three variants were present. This might account for the observed crystallographic reversibility in rapidly heated alloys, contrary to the
Acknowledgements
The authors would like to thank Dr R.V. Televich from the IMP, Ukraine, for providing material for this research and carrying out the rapid tempering. This project was supported by the Faculty of Engineering New Staff Member Research Grant, Monash University.
References (18)
- et al.
Crystallography of spheroidite and tempered martensite
Acta Mater.
(1998) - et al.
Interpretation of Electron Diffraction Patterns
(1967) Possible mechanism of martensite decomposition
Dokl. Acad. Nauk SSSR
(1950)- et al.
Crystallography of cementite formation during decomposition of carbon martensite
Izv. Akad. Nauk SSSR, Ser. Fiz.
(1974) Orientation of cementite in tempered carbon steel
Zh. Tekhn. Fiz.
(1947)- Kelly, P.M., Zhang, M.X., 1999. Edge-to-edge matching—a new approach to the morphology and crystallography of...
Die Austenitisierung von martensitisch-austenitishen Gefungen
Neue Hutte
(1965)- et al.
Tempering of high carbon martensite
Trans. ISIJ
(1972) - Pereloma, E.V., Timokhina, I.B., Swenser, S.P., 1999. Crystallography of cementite in tempered Fe–Ni–C alloys, in:...