Bainitic microstructures formed by split isothermal transformation in an Fe-C-Si-Mn-Mo steel
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I.
SPLIT isothermal transformation (SIT) of bainitic microstructures is an important phenomenon from an industrial point of view both for wrought steels and for weld deposits. Many steel assemblies, for example, in power generation plants, operate at 565 ~ and these service conditions cause isothermal transformation of the initial microstructure, which usually consists of a mixture of allotriomorphic ferrite and bainite.tq In multilayered weld deposits, SITs are inevitable because the deposition of every new layer reheats those deposited previously. Weld microstructures often contain substantial quantities of intragranularly nucleated bainite, the so-called acicular ferrite. Therefore, the SIT is equally important to weld deposits. All these examples emphasize the importance of the SIT process. Although a large amount of data is available on the tempering of quenched steels,[2-6j relatively little attention has been given to the SIT of bainite. The present study deals with a detailed microstructural analysis of SIT of bainite in a high strength steel containing a strong carbide forming element, Mo. Particular attention is also given to the effect of SIT on bainite morphology with reference to the transformation mechanism. II.
Table I.
INTRODUCTION
Chemical Composition of the Alloy Used
C 0.22
A1
Chemical Composition (Wt Pct) Si Mn 2.03 3.0
Mo 0.7
Austenitization
Tempering
E ite formation
Time
Fig. l--Schematic representation of the heat-treatment cycles used for the SIT of bainite.
EXPERIMENTAL PROCEDURES
The steel used in the present study was prepared as a 20kg vacuum induction melt, forged and hot-rolled to 10-ramdiameter bars. The final chemical composition of the steel is given in Table I. The alloy was initially hot swagged down to 6-mm-diameter rods. The swagging involves a successive reduction in the diameter in approximately l-ram steps. After removing 2 mm from the surface by machining, these were further swagged down to 3-mm rods at ambient temperature in order to avoid oxidation. All specimens were homogenized at 1250 ~ for 3 days, while sealed in quartz tubes containing a partial pressure of high purity argon. To avoid surface degradation during the subsequent heat treatments, all specimens were plated with nickel. The nickel plating process consisted of two stages: (a) nickel striking and (b) nickel plating. Striking was carried out in a solution made up of 250-g nickel sulfate and 27-mL con-
ASHRAF ALl is Senior Engineer with the Metallurgy Division, Dr. A.Q. Khan Research Laboratories, Kahuta, Rawalpindi 46000, Pakistan. Manuscript submitted June 22, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
centrated sulfuric acid and distilled water, amounting to 1 L in all, at 50 ~ with a current density of 7.75 mA/mm2 for 3 minutes. The plating solution consisted of 140-g nickel sulfate, 140-g anhydrous sodium sulfate, 15-g ammonium chloride, and 20-g boric acid, which made up to 1 L with distilled water. The plating was carried out at 50 ~ with a current density of 0.4 mA/mm2 for 15 minut
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