Duplex stainless steels (DSS's) are frequently used in oil and gas production and are subsequently subjected to cathodic protection. There is now growing concern about the cathodic evolution of hydrogen produced from this protection system, which may diffuse into the alloy and cause an embrittled condition. DSS's have a microstructure that is a mixture of austenite and ferrite and combines the advantages of these grades, whilst minimising their deficiences. In this research, Zeron 100 DSS was studied in six conditions to investigate the effects of hydrogen embrittlement (HE) on the various strengths and microstructures. The six conditions wer~ as follows: as-received, cold worked, age-hardened (475°C embrittlement), high temperature heat treated, rod and powder. To simulate service environments, 3.5% wt NaCI solution at ambient temperature with an applied potential of -1.1 V (SCE) was used. The effect of pre-charging for up to 550 hours at 80°C was also investigated. Test methods included slow strain rate testing (SSRT), monitoring of transient crack propagation (TCP) using circumferentially notched tensile specimens using a DC potential drop method, acoustic emission CAE) and some conventional bolt loaded fracture mechanics specimens. Test results were correlated with the varying microstructures and environmental conditions and consisted of mechanical properties, threshold crack growth including transient effects and AE data. In this work transgranular cleavage cracks were obtained in the susceptible ferrite phase as a direct result of HE; the depth of these cracks implied a high hydrogen concentration throughout the specimen. The austenite failed by ductile tearing and acted as a physical barrier to the propagation of cleavage cracks. As a result of SSR testing the best material was found to be the powder material; the fine equally dispersed austenite phase caused a lowering of the effective K value. The worst material was the high temperature heat treated type because it contained more ferrite (11:1 72%). The age-hardened material was also susceptible because of the hard and brittle ex' phase. However, regardless of the environment the UTS remained virtua]]y unchanged for each individual material, indicating that most cracking occurred in the post-UTS stage of the test. With the TCP test a lowering of the fracture load was found when an HE environment was used; daldt vs Kq curves were produced, however the DC potential drop equipment could not accurately measure crack growth because of the bridging effect of the austenite phase. The most susceptible microstructures were again the age-hardened and heat treated types. The hydrogen evolution reaction (HER) was also investigated by creating a fresh surface on the as-received DSS and studying the changes in the HER. This work showed that the effect of scratching is irreversible. Also the oxide film can not be totaHy reduced electrochemica]]y and only mechanical methods can remove the oxide films entirely. Fina]]y a means of detecting "475°C embrittlement" of DSS's was investigated using an electrochemical technique in 5M HCI. i-E curves were produced which showed the reactivation of the ferrite and austenite phases in the as-received material. By age-hardening at 475°C the two reactivation peaks merged.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:631722 |
| Date | January 1994 |
| Creators | Hutchings, D. |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.0022 seconds