The present study was conducted to evaluate the susceptibility of ASTM A36, A572, and A517 Grade F steels to stress-corrosion cracking (SCC) by using fatigue-pre-cracked specimens and principles of linear-elastic fracture mechanics (LEFM). Tests were conducted under total-immersion conditions in 3.5 percent sodium chloride (NaCl) solution maintained at + 22°C (+ 72°F) and pH ≅ 6.0. Single-edge-notched specimens were subjected to cantilever bending under conditions of constant dead-weight loading for test periods between 6000 and 16 000 h, or equivalently, between 8.5 months and nearly 2 years.
Results showed that these three steels exhibited a high relative fracture toughness level in air, KIx, and a high resistance to SCC as measured by the apparent level of SCC crack-growth threshold, “K*Iscc”; Each KIx and “K*Iscc” value corresponded to the application of either elastic-plastic or fully plastic stress levels that were well above the respective values of yield strength, σys.
These and earlier results, representing evaluation of eight different steels, demonstrate that structural steels in the strength range 210 to 900 MPa (30 to 130 ksi) are typically either immune or virtually immune to SCC crack growth in 3.5 percent NaCl solution. Such results were beyond the direct applicability of LEFM and were obtained despite the use of severe LEFM test conditions (sharp crack, high crack-tip stress, total immersion condition, and relatively long test durations).
Results of metallographic evaluation studies also are presented and discussed in relation to the tendency of various steels to exhibit crack extension (Δascc), crack-tip blunting, or transverse cracks or fissures or all three along the walls of the original machined notch and the original fatigue crack. Such metallographic examinations were conducted on steels from both the present and earlier studies. All such evaluations relate specifically to specimens at initial stress-intensity values, KIj, that were either at or below the respective SCC stress-intensity threshold, “K*Iscc,” for each steel.
The significance of these SCC results is discussed in relation to the generally more realistic conditions of cyclic loading (corrosion fatigue behavior) that often arc the controlling factors in the determination of useful structural life for components subjected to long-term environmental service.