CA6-NM - Weld Procedure Qualification to NACE Requirements

CA6-NM is an alloy known for its excellent strength, ductility, hardness, and toughness. The alloy has a martensitic structure and exhibits similar properties for corrosion resistance to most environments. It is extensively used in various industries such as power generation, marine, oil extraction, and refining. CA6-NM is preferred for its machining and welding properties. This whitpaper addresses how heat treatment cycles can be employed to improve the specific parameters of the alloy.

The welding of CA6-NM is governed by NACE requirements, and the maximum hardness limit of the alloy has been set to HRC 23 to avoid susceptibility to sulphide stress corrosion cracking. The carbon content of the base metal and the filler metal should be as low as possible to maintain superior performance and limit the alloy sensitivity to sulphide stress corrosion cracking. Prior to welding, the base metal should be in a normalized and double-tempered condition, and after welding, the test coupons should receive a double-temper treatment.

CA6-NM has a medium resistance to general corrosion compared to other grades, but the low carbon/low hardness version is very resistant to sulphide stress corrosion cracking, and this is what the oil and gas industry was looking for: an alloy resistant to fluids containing various combinations of CO2 and H2S. The experiment also demonstrated that the recommendations of T.G. Gooch for a successful NACE procedure qualification were effective.

The experiment covered two welding processes: Shielded Metal Arc (SMAW) and Gas Metal Arc (GMAW) solid wire. After the final temper, all test plates were softer than 255 BHN, meeting the NACE requirements. The results obtained from this study will be useful for industries that use CA6-NM and need to comply with NACE requirements for welding procedure qualification.