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Low alloy quenched and tempered steels used in current Naval aircraft applications, particularly the ultrahigh strength steels used in landing gear, have characteristically small critical flaw sizes and extreme susceptibility to stress corrosion cracking in a shipboard environment. Newly developed steels which develop ultrahigh strengths with secondary hardening based on precipitation of M2C carbides offer significantly larger critical flaw sizes; and while susceptible to stress corrosion cracking, their susceptibility is substantially less than that of low alloy steels. A long term test program conducted by the Naval Air Warfare Center Aircraft Division Warminster has characterized the stress corrosion cracking susceptibility of the newly developed steels. Results of the program have shown that, compared to low alloy steels, the newly developed steels show substantially reduced susceptibility to stress corrosion at short exposure times and maintain their advantage to a lesser extent at exposure times up to 1 0,000 hours. The test program has demonstrated also that 1,000 hour exposure times, characteristically used for stress corrosion tests of steels, are insufficient to establish stress corrosion thresholds (Klscc), as numerous failures were observed at exposure times between 1,000 and 1 0,000 hours. Fracture characteristics of the stress corrosion failures are shown.
The stress corrosion cracking (SCC) was investigated for AerMet 100 and 300M steels in four aqueous NaCl solutions of different concentrations (0.035-3.5%), three of which had an identical electrical conductivity (12.44 x 10(exp -4) sq m S/mol). Especially, the variation of threshold stress intensity for SCC, K(ISCC), with cathodic potential was evaluated, employing rising step load test method. The K(ISCC) increased, peaked at around the potential of -0.7 V(SCE), and then decreased with increasing potential for AerMet 100 steel. On the other hand, the K(ISCC) did not change much with potential for 300M steel. The open circuit potential and the corresponding K(ISCC) were greater for AerMet 100 steel than for 300M steel, indicating the former nobler and more SCC resistant. The SEM fractographs showed mixed cleavage and intergranular cracking, more cleavage for AerMet 100 steel but more intergranular for 300M steel, at all potentials employed.
The stress corrosion cracking of AerMet 100 and 300M steels was investigated in aqueous NaCl solutions of different concentrations (0.035-3.5%) but an identical electrical conductivity, employing rising step load test method. The threshold stress intensity for stress corrosion cracking, K(sub ISCC), increases from 15.4 MPa square root of m to 26.4 v with applied cathodic potential for AerMet 100 steel. On the other hand, K(sub ISCC) is relatively constant, 15.4-16.5 MPa square root of m, for all potentials employed, ranging from -1.2 V(sub SCE) to -0.7 V(sub SCE). The open circuit potentials and the K(ISCC) values at those potentials are greater for AerMet 100 steel than for 300M steel. These results indicate the AerMet 100 steel is nobler and more resistant to stress corrosion cracking than 300M steel. The SEM fractographs of both steels show mixed intergranular and cleavage cracking across all potential employed.
Forged parts and billets of AISI 4340, H11 tool steel, and 9% and 18% nickel alloy steels were evaluated for susceptibility to stress corrosion cracking (SCC) by an alternate immersion test. The alloys were prepared by the consumable electrode-vacuum melt practice and were heat treated as test specimens to strengths in the range from 263,000 to 290,000 psi. Polished specimens were alternately immersed in a 5% NaCl solution while sustaining a load equivalent to 75% of the ultimate strength.