EN 10325:2006

Overview

EN 10325:2006 defines a standardized method to determine the increase in yield strength of steel caused by heat treatment used in industrial paint processes - the Bake-Hardening-Index (BH2). The procedure quantifies how a 2 % plastic pre-strain followed by a controlled oven cycle (simulating automotive paint baking) raises yield or proof strength. This test is particularly relevant for steels that exhibit bake-hardening behavior and are commonly used in the automotive industry.

Key topics and technical requirements

• Scope: Measurement of yield strength increase (BH2) for steels showing bake-hardening mechanisms.
• Reference standard: Tensile testing and test-piece geometry in accordance with EN 10002-1.
• Pre-strain: A plastic pre-strain of (2 ± 0.05) % is applied under a constant testing rate.
• Heat treatment (simulation of paint-bake):
  • Test-piece temperature must reach 170 °C within 7 minutes.
  • Holding time is (20 ± 0.5) minutes starting when the test-piece reaches 168 °C.
  • Oven control: test-piece temperature maintained at 170 °C ± 2 °C.
  • Temperature measurement device: resolution ≤ 1 °C and measuring uncertainty ± 2 °C.
  • After hold, test piece is air-cooled to room temperature (23 ± 5 °C).
• Measurement:
  • BH2 is defined as the difference between the yield/proof strength before and after the specified heat treatment.
  • Relevant symbols: Rp2,r (stress at 2 % pre-strain), ReL,t (lower yield after heat treatment), Rp0,2,t (0.2 % proof after heat treatment).
  • Elastic modulus assumption: 200 000 MPa (for calculation context given in the standard).
• Testing conditions: Tensile testing follows EN 10002-1 procedures; the initial testing rate is controlled until ReL or Rp0.2 is determined; all tensile tests for a given steel must use the same actual testing rate.
• Test report: When BH2 testing is combined with conventional tensile tests, include BH2 in the report. If performed separately, report must reference EN 10325 and provide test-piece identification, sampling location/direction and the BH2 result.

Applications and users

• Primary users: Metallurgical laboratories, materials testing laboratories, automotive OEMs, tier suppliers, quality/verification engineers, and R&D teams working on steels with bake-hardening characteristics.
• Practical applications:
  • Evaluating formability and post-painting strength of automotive body steels.
  • Material selection and specification for panels that undergo paint-bake cycles.
  • Quality control and supplier acceptance testing for bake-hardenable steels.
  • Research into alloy and processing effects on bake-hardening response.

Related standards

• EN 10002-1:2001 - Metallic materials - Tensile testing - Method of test at ambient temperature (normative reference used for tensile methods and test-piece geometry).

Keywords: EN 10325:2006, Bake-Hardening-Index, BH2, steel, heat treatment, tensile testing, EN 10002-1, yield strength, bake hardening, automotive industry.