Tempering of 2.25 Pct Cr-1 Pct Mo Low Carbon Steels
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I.
INTRODUCTION
Table I.
L o w carbon 2.25 Cr- 1 Mo steels are used as filler materials for joining 0.5 Cr-Mo-V steam piping employed in modern power stations. ~'2 The joints are typically multipass butt welds and are laid down under conditions of severe structural restraint. The residual stresses thus developed may be of the same order of magnitude as the yield stress, and are relieved by applying a post welding heat treatment which , involves heating for several hours in the temperature range 690 to 720 ~ The carbon level of the weld is usually specified as 0.10 pct maximum, but in practice this has been found to vary between 0.02 and 0.12 pct. The present work examines the effect of a variation in the carbon level of wrought steels, made to weld metal compositions, on hardness and microstructure after tempering at 700 ~ for times up to 12 hours. In particular, the morphology, distribution, crystal structure, and composition of the carbides which are precipitated inthe initially coarse-grained bainitic/martensitic microstructure have beela investigated.
II.
EXPERIMENTAL
The steels used in the investigation were prepared by melting Japanese electrolytic iron in a vacuum induction unit, with the alloy additions being made immediately prior to vacuum casting. The ingots, each weighing 25 kg with a diameter of 85 mm, were hot worked to 14 mm square bars which, after air cooling, were machined to 12 mm diameter rods. The compositions o f the three steels examined are given in Table I. Specimens, 12 mm diameter and l0 mm long, of each steel were austenitized at 1250 ~ for 30 minutes, furnace cooled, and then re-solution treated at 1100 ~ for 3~ miIiutes before oil quenching to give a coarse-grained bainitic/martensitic structure. The specimens were then tempered for up to 12 hours at 700 ~ and water quenched. All the heat treatments were carded out under a vacuum of better than 10 -3 torr. JOHN PILLING, Graduate Student, and N. RIDLEY, Senior Lecturer, are both with Joint University of Manchester/UMIST, Department of Metallurgy, Grosvenor Street, Manchester, M1 7HS, England. Manuscript submitted March 16, 1981. METALLURGICALTRANSACTIONS A
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Steel A B C
Compositions of the Steels (Wt Pct)
C .018 .06 .09
Cr 2.34 2.25 2.30
Mo 0.99 1.00 0.99
Mn 1.02 1.00 1.00
Si 0.47 0.42 0.45
P .005 .005 .005
S .007 .007 .007
The variation in hardness of the three steels with tempering time was followed using a Vickers diamond indentation test with a load of 30 kg. The hardness values reported are the mean of five indentations per specimen. For microstructural studies, carbon extraction replicas were prepared from specimens which had been polished and lightly etched in two pet Nital. About 80 carbide particles from each sample were quantitatively analyzed using electron probe microanalysis (EPMA), and either selected area (AEI EMMA-4) or convergent beam (Philips EM400T) diffraction patterns were recorded from the precipitates. To follow qualitatively the changes in proportions of carbide types during tempering, use was ma
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