Carbon Segregation in CA6NM Castings: Impact on Hardness, Weldability, and Acceptance

The white paper explores the impact of carbon segregation on the properties of CA6NM alloy castings. The study aims to identify the inconsistency of carbon content in different locations of a casting and its impact on hardness, weldability, and acceptance of the casting. The findings highlight the need to maintain low carbon levels throughout the casting and apply a proper heat treatment cycle to achieve the required performance standards.

CA6NM alloy castings are commonly used in oil and gas applications exposed to corrosive environments. To maintain their superior performance and limit sensitivity to sulphide stress corrosion cracking, the castings need to be relatively soft. NACE standards limit the maximum hardness of CA6NM alloy to 23 HRc.

Achieving this performance requires a low carbon level throughout the casting and proper heat treatment. However, the occurrence of local micro and macro segregation of carbon can significantly affect the final hardness, making it difficult and sometimes impossible to soften the casting.

The white paper concludes that maintaining a low carbon level throughout the casting and applying the correct heat treatment cycle is essential to achieve the required performance standards for CA6NM alloy castings. Weld repairs should also meet NACE requirements and undergo double tempering. The study highlights the need to consider other residual elements responsible for alloy hardening, such as vanadium and nitrogen, when creating a melt recipe.