Bake Hardenability Index: A New Laboratory Test ProcedureSeptember 21, 2023
Most steels display hardening behavior when they undergo a pre-straining and bake treatment. The objective of A/SP Steel Testing and Harmonization Team‘s Project #06 – STRAIN & BAKE BEHAVIOR OF AHSS reevaluated the current ASTM strain and bake process for AHSS (including DP590 and 3rd Gen 1180 steels) at a pre-strain of 2%, 5%, 8% and 10%, and then considered the development of a new test procedure. All initial testing work was performed by POSCO Laboratories, South Korea, and then repeated and replicated by General Motors.
ASTM Standard (ASTM A653/A653M) based bake hardenable tests were performed at a pre-strain of 2%, 5% and 10% using DP590 and 3rd Gen 1180 steels by both POSCO and General Motors. The test results were compared to ensure testing consistency. The team determined that it is not possible to evaluate the Bake Hardenability Index (BHI) in high tensile and low elongation materials using the current ASTM standard, especially at the higher pre-strain rates.
To resolve this issue, as shown in the figure below, a new procedure was proposed based on the pre-straining of a large specimen (e.g. KS-1B) and then fabricating the final specimen (e.g. ASTM E8) from the gauge section.
- As-Is: pre-strain → BH treatment → re-tension
- To-Be: pre-strain large specimen (e.g. KS-1B) → fabricate final specimen (e.g. ASTM E8) from the gauge section → bake hardening heat treatment → re-tension
- Uniform pre-strain is distributed in the final specimen → abnormal failure due to the non-uniform strain distribution can be avoided
The new procedure was then used in a new round of tests using the same two materials and two laboratories. The results from the new test were then compared to the present testing methodology. The new procedure validated the belief that more accurate BHI values can be obtained in large pre-strain conditions when compared to the present test standard.
You can download this new test procedure here. We encourage you to use it and provide feedback!