Constant-Amplitude Fatigue Behavior of Five Carbon or Low-Alloy Cast Steels at Room Temperature and −45°C

Abstract

Five common carbon or low-alloy cast steels—SAE 0030, SAE 0050A, C-Mn, Mn-Mo and AISI 8630—were subjected to constant-amplitude fatigue tests at room temperature and at the common low climatic temperature of −45°C (−50°F). Tests included smooth specimen axial low and high cycle fatigue and compact type (CT) specimen crack growth behavior from 10−5 n/cycle (4 × 10−4 in./cycle) to threshold values at 10−10 m/cycle (4 × 10−9 in./cycle). Three of the five steels had nil ductility transition temperatures above the low test temperature. Despite this, all five cast steels showed equivalent or better fatigue resistance at the low temperature, except for some very short life low-cycle fatigue tests and for some very high fatigue crack growth rates where fracture was imminent. Scanning electron fractographic analysis indicated that ductile type fatigue crack growth mechanisms with or without striations occurred for all steels at both test temperatures except for a few interdispersed cleavage facets in 0050A steel at very high crack growth rates. At the low temperature, monotonic and cyclic stress-strain properties Su, Sy, and Sy' increased by an average of about 10%, fatigue limits increased from 2 to 25%, and ΔKth increased from 0 to 90%. No consistent correlations existed between fatigue and monotonic properties at either temperature. Mean stress effects at near ΔKth levels appeared to be influenced by crack closure.