6061-O temper extruded rod may be used as feed stock in forming processes for automotive pressure vessel applications. Key parameters for forming are the strength and hardness of the material. The purpose of this paper was to reduce variation in hardness to achieve a process capability index of 1.33 or greater. Among the process steps affecting hardness, annealing is the most critical. Initially, the process showed unacceptable hardness variation.
Initial anneal recipes called for a 4-hour soak at 775°F (413°C). Initial process capability for hardness was a Cpk of 1.12, with tensile strength readings very close to the upper specification limit. Initial temperature uniformity surveys of the anneal oven showed a large variation in temperature distribution, with some areas of the oven staying below 650°F (343°C). Initial improvement efforts focused on soak time. While this did greatly improve the material, the improvement did not have enough of an effect on hardness to achieve a good capability. In fact, Cpk was lower than the initial data.
Review of TUS data drove the conclusion that a higher soak temp, with an intermediate soak time was the only reasonable input left to adjust. After making adjustments to allow the system to run at a higher set point, a new recipe was run at 825°F (441°C) for 6 hours, with a slow ramp down. The strength remained well below the USL. Final Cpk for hardness is 1.97. Additionally, the mean, median, and mode are within 0.17% of each other.