MECHANISMS OF SLOW CRACK GROWTH IN HIGH-STRENGTH STEELS.
Technical rept. Jul 65-Dec 66,
AEROJET-GENERAL CORP SACRAMENTO CA
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The metallurgical nature of slow crack growth in 18 nickel maraging steel and D6AC low-alloy steel was investigated using the stress-wave-analysis technique SWAT as a new tool for monitoring incremental crack growth. The two heats of maraging steel investigated were found to be insensitive to hydrogen and aqueous environment for the sustained-load periods of time investigated. D6AC, on the other hand, heat treated to produce a variety of microstructure and strength levels was susceptible to slow crack growth under all conditions investigated involving elevated temperature, hydrogenation and aqueous environment. An inverse relationship was found between secondary-incubation time and crack-growth rate. At elevated temperature the secondary-incubation time was found to increase at first and then reach a steady-state condition. For those conditions were there was a relatively small amount of carbon in interstitial solid solution, the steady-state secondary-incubation time was relatively long and independent of microstructure. For that condition where a relatively larger amount of carbon was in solid solution, the secondary-incubation time was relatively short and spontaneous strain-aging embrittlement was most apparent. In the latter case, the kinetics of both secondary-incubation time and crack-growth rate suggested the embrittling mechanism to be carbide precipitation. D6AC subjected to prior hydrogenation and water environment produced a large amount of stress-wave emission. Based upon experimental secondary-incubation times, a theoretical interpretation of hydrogen embrittlement indicated that gross diffusion of hydrogen from one crack site to the next was the controlling mechanism.
- Properties of Metals and Alloys