Standards Development

Scope

This proposal specifies a method for performing the secondary work embrittlement test and the determination of Secondary Work Embrittlement Temperature (SWET) of flat products (sheet and strip) made of metallic materials. The above flat products that are usually used for deep drawing or stamping into components have the thickness from 0.45 mm to 2.50 mm. The test includes the preparation of cup samples with specific geometric sizes by deep drawing process. Then, these cup samples are cooled at various low temperature and impacted by using drop-weight test equipment (with 98J capacity, initial impact energy 98J for steel materials and 49J for Al-Mg light alloys respectively). The highest temperature at which no less than 2 cups of the total 8 cups fracture in a brittle manner is determined as Secondary Work Embrittlement Temperature, SWET.

Purpose

Background

High strength interstitial-free (IF) steels sheets, new-generation of Advanced High Strength steels sheets and some light Al-Mg alloys like 5xxxx and 6xxx sheet products have the good combination of strength and ductility, at the same time, these flat products that have thin thickness are usually used for stamping into complex automotive components, meeting the requirements of lightweight passenger car body design. As well known, the deep drawing for these metallic sheets usually introduce the high level of inner stress relative to the deformation/pre-strain, causing the tendency to embrittlement at low temperature further. In the meanwhile, especially for IF steels the additive of phosphorus significantly increases the boundary strength, but also weakening the low temperature toughness of the steels sheet. The aim of the proposal is try to find the balance between the high strength/formability combination and the low embrittlement. Secondary work embrittlement (SWE) is used to describe the phenomenon that is after stamping forming (the first work) of the metallic sheet or strip, pre-strain induced internal stress appears, when affected by external loading again (especially under impact force at low temperature, the secondary work), brittle fracture is likely to occur at grain boundaries. In theoretical study, at rather low temperatures, metals with BCC structure, and some with hexagonal-close-packed lattice–HCP), the state of lowtemperature embrittlement (LTE) or cold brittleness arises. In metals with FCC and in many metals with HCP lattice, the LTE state does not occur. The physical-mechanical mechanisms for formation of the LTE state and SWE fracture of metals in different states are still insufficiently defined. So a global standardized procedure for determination of the Secondary Work Embrittlement Temperature (SWET) is strictly necessitated. To determine if a grade of metallic sheet with high ductility, especially the ultra-low carbon steels, AHSS and Al-Mg alloy sheets for deep stamping, are prone to SWE, SWET is to be determined in terms of this proposal. The SWET is defined as the highest temperature at which the fracture in a brittle manner is formed. Low SWET and SWE insusceptible are compulsorily required for the automotive components served in the cold area, relative to the automotive performance of safety guarantee. Nowadays, the SWET of flat products are evaluated by different ways, including Charpy impact on the pre-strain thin-thickness pieces, bending test and drop-weight test upon the deep-drawn cup samples. The SWET results from above individual testing methods can hardly be compared, which brings the obstacles for the quality control and the global marketing for those high-profitable metallic sheets products widely applied in the automotive industry.

Purpose

This proposal aims at the determination of SWET by means of drop-weight impact in convenience, assuring the safety of the metallic sheets after their industrial deformation process (stamping, punching…) for service. (avoid brittle cracking) SWET test has wide safety-related applications for material selection in automotive industry and for quality control and R&D requirements in metallic industry. The Drop-weight SWET test clearly reveals the influence of low temperature and deformation on the embrittlement of metallic sheets. In China/North America…, Drop-Weight SWET test has been performed long before, all of the IF, HS-IF, especially some grade of AHSS and Al-Mg alloy sheets have been tested for their SWET in metallic products and automotive companies, assuring the service safety