Stress Corrosion Crack Growth Testing of Weld Root Defects: Test Techniques and Implications for Testing of Smaller Welding-Induced Defects

Abstract

Stress corrosion crack (SCC) initiation and growth in Alloy 82/182 weld metal has been studied extensively for bulk, smooth surface test specimens and compact tension, C(T), specimens with standard fatigue precracks, respectively. In situ monitoring is used to measure initiation times and growth rates. However, the effect of weld defects on the measured initiation times, or ability for a crack to “incubate” from a weld root defect rather than a fatigue crack has not been widely studied. Incubation has been described as the post-initiation slow crack growth regime prior to steady state crack growth. While fatigue cracks can develop during service, natural weld defects such as hot cracks, ductility dip cracks (DDC), or root defects could also exist and have the potential to grow. This test program evaluated the question: do different starting defects develop into steady-state SCC differently than a fatigue precrack? First, a methodology to quantify the development of SCC from fatigue precracks based on crack engagement evolution over time was developed. This methodology was then applied to SCC growing in C(T) specimens with the natural weld root at the notch. The nature of these root conditions varied. Tests were performed under constant load for varying times to establish the amount of intergranular SCC engagement as a function of time. Results showed that transgranular fatigue cracks tended to have longer incubation times compared to lack of fusion or weld root extensions; however, some root microstructures showed high resistance to SCC in the time tested. The implications of these results from C(T) weld root specimens to testing and quantifying SCC from smaller defects such as DDC will be discussed.