Effects of Carbon Content and Tempering Treatment on the Mechanical Properties and Sulfide Stress-Corrosion Cracking Resistance of AOD-Refined CA6NM

Cast Alloy CA15 is presently being used in sour gas environments in accordance with National Association of Corrosion Engineers (NACE) Standard MR-01-75, which requires that the maximum hardness not exceed Rockwell C22. There has been effort to replace CA15 with CA6NM, which has improved toughness properties. However, the 22HRC maximum has been difficult to reproduce in CA6NM. The argon oxygen decarburization (AOD) method of refining stainless steel produces a material with excellent impact properties and allows lower carbon levels to be consistently achieved that were previously difficult to attain. Charpy impact values and tensile properties are described, comparing AOD versus conventional methods of melting. An experimental hardness correlation has been suggested for the alloy. A new version of CA6NM with 0.03 percent maximum carbon, which may appropriately be called CA3NM, shows excellent sulfide stress-cracking (SSC) resistance using the standard NACE test. A threshold value in excess of 90 percent of material yield strength was achieved. The AOD method of refining improves mechanical properties, including toughness, and allows low carbon contents to be achieved with ease. Resultant lower hardness values can then be attained which satisfy NACE requirements for SSC applications.