20th International Forgemasters Meeting - IFM 2017

11/4Cr- ½ Mo steels are designed for critical applications in heavy wall pressure vessels operating below 441°C. API recommended practice: API 934-C states that beyond a thickness of 100 mm, this material shows difficulty in meeting sub-zero impact toughness and percentage shear requirements at the mid-thickness, which is required by API 934 – C. API recommends the use of a higher alloyed grade, i.e. 21/4Cr-1Mo to fulfil these requirements. With increasingly challenging market demands and customer expectations, in terms of higher tempering temperatures and Post Weld Heat Treatments, it is now required to meet the above mentioned mechanical property requirements for heavy forgings thicker than the recommended 100 mm limit. To meet this demand and to understand the limitations of this material, an extensive study was carried out using the Design of Experiments (DoE) methodology to improve the Heat Treatment. The experiments were carried out on full thickness blocks (215mm) and the tensile and impact properties tested. The results were correlated to the input parameters using ANOVA. The deterioration of sub – zero impact values were also correlated with microstructural characteristics. This paper describes the DoE methodology, the experimental methods and the results of this study, illustrating the limits of this material.