Maraging steels cracking

The 18% Ni maraging steels do not display passivity and normally undergo uniform surface attack in the common environments. Of more serious consequence, however, for all high strength steels, is the degree of susceptibility to stress corrosion cracking (s.c.c.). 

The stress corrosion resistance of maraging steel has been evaluated both by the use of smooth specimens loaded to some fraction of the yield strength and taking the time to failure as an indication of resistance, and by the fracture mechanics approach which involves the use of specimens with a pre-existing crack. Using the latter approach it is possible to obtain crack propagation rates at known stress intensity factors (K) and to determine critical stress intensity factors (A iscc) below which a crack will not propagate (see Section 8.9). 

Although tests on smooth specimens indicate that cathodic protection of maraging steel is possible, tests on specimens with pre-existing cracks indicate a greater sensitivity to hydrogen embrittlement during cathodic polarisation . The use of cathodic protection on actual structures must therefore be applied with caution, and the application of less negative potentials than are indicated to be feasible in smooth specimen tests is to be recommended if it is assumed that structures contain crack-like defects. 

A further estimation of the corrosion resistance of maraging steel can be obtained from data on the rate of crack propagation. Although the rate of crack propagation has been found to be a function of stress intensity in some alloys, for many alloys and heat treatments there is a range of stress... 

It is notable that while it is possible to produce maraging steels with consistently uniform mechanical properties, the stress corrosion properties are subject to scatter, as indicated in Fig. 3.34. To a large extent this scatter is an indication of the greater sensitivity of s.c.c. resistance to metallurgical variables. Although the variation in cracking resistance is not well understood, and the reaction to certain treatments not always consistent, certain observations may be used to indicate guidelines for improved properties. 

Fig. 7.120 Corrosion-fatigue-crack-growth rate as a function of stress-in-tensity range for a maraging steel in air and 3% NaCl solution.

Figure 8.15. Effect of temperature on the stage II crack growth rate for 18Ni (250) maraging steel tested over a range of hydrogen pressures [6,7].

Gangloff, R. P., and Wei, R. P., Gaseous Hydrogen Assisted Crack Growth in 18 Nickel Maraging Steels, Scripta Metallurgica, 8 (1974), 661. 

Green, J. A. S. and Haney, E. G., Relationships Between Electrochemical Measurements and Stress Corrosion Cracking of Maraging Steel, Corrosion, January 1967, p. 5. 

Dautovich, D. P. and Floreen, S., "The Stress Corrosion and Hydrogen Embrittlement Behavior of Maraging Steels, Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, NACE-5, R. W. Staehle, et al., Eds., NACE, Houston, TX, 1971, pp. 798-815. 

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