The Susceptibility to Hydrogen Induced Stress Cracking for Alloy 718 and Alloy 725
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The Hydrogen Induced Stress Cracking (HISC) susceptibility of Alloy 718 and Alloy 725 where examined and compared. Pre-charged samples of each alloy where stepwise loaded during polarization in Cortest Proof rings. A safe load was found from this stepwise loading. To confirm that these load levels were safe, pre-charged samples were loaded to this load for 30 days during cathodic polarization. After fracture the fracture surfaces were examined in scanning electron microscope and the hydrogen concentration were measured by hydrogen melt extraction. It was found that both alloy are severely embrittled by hydrogen. Both the ductility and stress at fracture were reduced by the same amount in both alloys. The constant load tests revealed a safe stress at 123,8 % of YS for Alloy 725 and 120,4 % of YS for Alloy 718. Based on this it was concluded that Alloy 725 is somewhat more resistant to HISC than Alloy 718.