Effect of thermal cycling on the mechanical properties of 350-grade maraging steel
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I.
INTRODUCTION
AUSTENITE can be introduced in the microstructure through two different methods of heat treatment. When the alloy is isothermally heated at the temperature at which the dissolution of the intermetallic precipitates occurs, austenite starts nucleating at regions where the nickel content is high.t~] Prolonged aging at this temperature leads to the formation of a substantial amount of austenite, and the austenite so formed is known as "reverted austenite." In the second method, the steel is subjected to thermal cycling between room temperature (RT) and the temperature (Ay) at which the austenitic transformation is completed. When rapidly cooled from this temperature, without allowing the austenite to attain equilibrium composition, the solute-rich austenite remains untransformed and the solute-depleted austenite transforms to martensite, t2] The austenite formed by this treatment is known as "retained austenite." The effect of austenite on the mechanical properties has received much attention in the past, t3,4] and it has recently been demonstrated that the effect depends upon the morphology of the austenite.[5] In this investigation, austenite was retained in a 350-grade commercial maraging steel by thermal cycling, and its effects on RT mechanical properties were evaluated by carrying out tension and Charpy impact tests on samples with different fractions of austenite. The influence of the stability of retained austenite on the deformation behavior was also examined at subambient temperatures. II.
EXPERIMENTAL PROCEDURE
The material used in this investigation was a 350-grade commercial quality maraging steel. The chemical composition of the alloy is given in Table I. The material was
U.K. VISWANATHAN and R. K/SHORE, Radiometallurgy and Metallurgy Divisions, respectively, are with the Bhabha Atomic Research Centre, Bombay 400 085, India. M.K. ASUNDI, Metallurgical Consultant, is with Vibha, Bandra (East), Bombay 400 051, India. Manuscript submitted January 24, 1994. METALLURGICALAND MATERIALS TRANSACTIONS A
received in the form of a forged bar 70 mm in diameter in a double solution-annealed condition. The solution annealing consisted of a first anneal at 950 ~ for 2 hours followed by air cooling and a second anneal at 820 ~ for 3.5 hours followed by air cooling. Tension specimens of 4-mm diameter and 20-mm-gage length and standard Charpy Vnotch specimens of 10 X 10 • 55 mm were fabricated from the forged bar in such a way that the major axes of the specimens were parallel to the length of the bar. The investigation was conducted on the samples given one of two specific heat treatments: (1) direct aging of the solution-annealed material at 510 ~ for 3 hours (hereafter called "conventionally aged") or (2) thermal cycling of the solution-annealed material, followed by aging at 510 ~ for 3 hours. The thermal cycling treatment consisted of heating the material at a specific heating rate between RT and the predetermined Ai temperature of 750 ~ After thermal cycling, the samples were quenched in water
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