Investigation of stress corrosion cracking of the cast and forged steel in water
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I.
INTRODUCTION
P R E V I O U S work I showed that the fracture toughness of the ZG-18 ultra-high strength cast steel was higher than that of forged steel with the same compositions, but the ductility and impact toughness of the former were much lower than that of the latter. One of the purposes of the present work is to determine whether or not the stress corrosion cracking (SCC) susceptibility of the cast steel differs from that of forged steel and to examine the cause of the difference, if it exists. The nucleation and propagation of SCC have been previously observed in ultrahigh strength steels in water and interpreted in terms of a hydrogen induced delayed cracking process. 2,3,4 In addition, the electrochemical method of measuring hydrogen permeation has been shown to be a useful tool for investigating the mechanism of SCC in water. 5 Therefore, the effect of the cathodic and anodic polarization on both the da/dt of SCC and the hydrogen permeation flux has also been investigated. The propagation rate of SCC of ultrahigh strength steel in water may be controlled by the rate of transport of hydrogen ion, the rate of the discharge of hydrogen ion on the crack surface, or the rate of diffusion of hydrogen atom to cracking site. The activation energy of crack growth must be equal to the apparent diffusion activation energy of hydrogen in the same material if the propagation rate of SCC, i.e., the da/dt, is controlled by the rate of diffusion of hydrogen atom. For this reason, the activation energy of these two processes was measured.
II.
EXPERIMENTAL PROCEDURES
The chemical compositions of the steel are as follows: C = 0.23 pet, Si = 0.63 pct, Mn = 0.85 pct, Cr = WU-YANG CHU, Associate Professor, and CHI-MEI HSIAO, Professor, are at the Departmentof Metal Physics, Beijing Universityof Iron and Steel Technology, China. WEN-XUE LI and YAN-GUO ZHANG, formerly Students at Beijing University of Iron and Steel Technology, are now with the Chang-Zhi Steel Company and Central Iron and Steel Research Institute, respectively. Manuscript submittedOctober 11, 1983. METALLURGICALTRANSACTIONSA
1.30 pct, Mo = 0.53 pct, P = 0.015 pct, S = 0.005 pct. One-half of a heat-treated steel was directly cast in the modified WOL constantly deflected specimens with thickness of 20 mm; the other half was forged into the same WOL specimens. The code and heat treatment of the specimens are listed in Table I. The stress intensity factor Kj of the crack tip of the constant deflection specimen is 2
K, = VEF (a Iw)/W"2C (a/w)
[1]
where W is the width of the specimen, E is Young's modulus, V is the initial displacement, F is a function of a / w (a is crack length):
F(a/w) = 30.96(a/w) '/2- 195.8(a/w) 3/2 + 730.6(a/w) 5/2 - l186(a/w) 7/z + 254.6(a/w) 9/2
[2]
and C(a/w) is the compliance. We calibrated C(a/w) by putting the knife edge at the surface of the specimen. 6 During crack propagation, the length of the crack on the surfaces of the specimen was measured by means of a tool microscope. The average length on the two surfaces a and ti
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