Microstructural effects on the stress corrosion cracking behavior of medium and high strength steels
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I.
INTRODUCTION
for each of these steels is given in Table II. All austenitizing, intercritical annealing, and high temperature tempering treatments were conducted in a large volume lead bath, and the low temperature tempering and isothermal transformations were conducted in a Wood's metal bath. Intercritical anneals were carried out between the AI andA3 temperatures and all tempers were subcritical, below the A~ temperature. The requisite approximate A~ and A3 temperatures 7 for 300M and HY-130 are approximately 735 ~ and 850 ~ and 650 ~ and 810 ~ respectively. They were not determined for the H13, as no intercritical treatments were carried out on that steel. The stress corrosion cracking (SCC) response as a function of heat treatment was determined on 11.9 mm thick, bolt-loaded double cantilever beam (DCB) type specimen immersed in 3.5 wt pct NaC1 aqueous solution at ambient temperature. The HY-130 DCB's were machined so that crack propagation occurred parallel to the rolling direction, while the 300M DCB's were machined so that crack propagation occurred normal to the rolling direction. Each DCB was machined from a heat treated blank, fatigue precracked on a closed loop, servo hydraulic testing system, and then bolt loaded to a constant displacement, monitored by a clip extensometer mounted across the starter notch plus fatigue crack. All HY-130 specimens were coupled to zinc to increase the input hydrogen fugacity. Specimens were immersed in the test solution and the crack length was monitored optically to the nearest 0.1 mm on either submerged or wet specimens, as the crack grew within 15 pct deep machined side grooves, whose function was to encourage crack propagation on a single plane normal to the loading
THEuse of high-strength steels in engineering applications is often limited by their susceptibility to stress corrosion cracking (SCC). It has been asserted and demonstrated in a number of reviews 1-5 that microstructure is one of the metallurgical variables which affects the SCC behavior of steels. However, while it is recognized that it is very important to optimize material design for engineering applications under SCC conditions, the understanding of how to achieve this is fairly limited. The major barrier is an incomplete appreciation of how specific microstructural features alone, or in concert, contribute to the environmental response of the material over a range of strength levels. This paper describes in some detail the resistance to SCC of three steels, of moderate to high strength levels, for a variety of microstructural changes obtainable by relatively conventional heat treatment. The contributions of strength level and microstructure has been, to the extent possible, independently examined and evaluated. The detailed results of one of these important variables, retained austenite, are presented and modeled in a companion paper. 6
II.
EXPERIMENTAL PROCEDURE
The three steels studied were a high strength tool steel, HI3 (designated below as "H"), and the low-alloy steels, 300M, essentially
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