A perspective on the morphology of bainite
- PDF / 3,865,854 Bytes
- 13 Pages / 594 x 774 pts Page_size
- 63 Downloads / 231 Views
I.
INTRODUCTION
IN today's
marketplace, hundreds of thousands of tons of bainitic steels are manufactured each year to meet various customer requirements. Applications include plate and bar for high-strength steel structures and heavy forgings for nuclear reactors, pressure vessels, and steam turbines. These steels are produced through thermomechanical treatment followed by various continuous cooling processes; to our knowledge, no commercial bainitic steels are produced by isothermal transformation. Bainitic steels are selected for their favorable balance of strength, toughness, and economy. Bainitic steels exhibit a high degree of property uniformity in thick sections and are often chosen for high-temperature applications due to their favorable creep properties compared to other microstructures. These steels include Ni-Cr-Mo (e.g., HY-80) and Cr-Mo steel plate and heavy forgings for nuclear reactor and pressure vessel components, lowcarbon Cu-Ni-Cr-Mo (e.g., ASTM grade A710) steel plate for ship construction, the so-called ultralow-carbon bainitic (Mo-B) steels for gas transmission lines, and NiCr-Mo-V, Ni-Mo-V, and Cr-Mo-V heavy forgings for steam turbine rotors. Many of the widely used alloy steels, such as AISI 4340 and 8620, are also produced with a bainitic microstructure in many applications. In addition, bainitic microstructures are found in many steel weldments, forming in both the weld metal and heat-affected zone (e.g., Reference 1). The morphological features of ferrous martensites have been rather well characterized over the past 40 years, and international conferences are held regularly which
B.L. BRAMFITT, Research Fellow, and J.G. SPEER, Supervisor, are with Homer Research Laboratories, Bethlehem Steel Corporation, Bethlehem, PA 18016. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
are devoted exclusively to the martensitic transformation (e.g., Reference 2). In comparison, the characterization of bainitic microstructures and properties is much less complete. I31 Bainite has received relatively little attention, and a great deal of effort will be required to understand the bainitic transformation more fully, particularly of bainite which forms during continuous cooling. In this review, the morphology of continuously cooled bainites will be examined and compared with that of isothermally transformed bainites. The bainitic morphologies and the definition of bainite will be reviewed in historical context, from a somewhat "industrial" perspective, hopefully providing a focal point for achieving an international consensus on the interpretation of bainitic microstructures. While a current overview cannot ignore distinctive features of the bainitic transformation mechanism, we hope to sidestep some of the controver
Data Loading...
