The effect of non-metallic inclusion morphology on the hydrogen induced cracking (HIC) resistance of L80 steel

The hydrogen induced cracking (HIC) resistance of four (4) L80 casing steels, with different nominal compositions and processing conditions, was determined using the NACE TM0284 HIC test. Microstructural and inclusion characterization of each steel was undertaken using optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The morphology (equivalent diameter, aspect ratio and major length) of inclusions over identical analysis regions were measured for each steel. In total, >15,000 inclusions were characterized. Non-linear models correlating inclusion morphology with the measured values of CLR (crack length ratio) and CTR (crack thickness ratio) were developed. The models show that both CLR and CTR are complex functions of the diameter (equivalent) of oxide-based inclusions and the major length of the manganese sulfide (MnS) stringers. The models show that oxides <10 μm in diameter were found to have negligible effect on HIC resistance (CLR) while oxides >20 μm exhibited a significant and escalating negative impact on HIC resistance comparable to long (>50 μm) MnS stringers.