Delayed Cracking of Advanced High Strength Steel Solutions

Mentor: Dean Kanelos - - Nucor Steel
Co- - Principal Investigator: Eric Batt - ArcelorMittal
Co- - Principal Investigator: Curt Horvath – General Motors Company

Project Manager: Jonathan Smith, Auto/Steel Partnership

W. Wu – AK Steel

• To develop a test method for ranking the relative susceptibility of zinc coated advanced high strength steels (AHSS) and ultra-high strength steels (UHSS) to hydrogen assisted cracking.
• To avoid the most common concerns with existing tests:
− Artificial “charging” with hydrogen concentrations far above what
would be expected in automotive environment
− Development of a test that is not relevant to thin sheet steels
− Development of a test that is complicated and/or requires R&D type
of equipment

• To expand on the previous work done by the A/SP STHT which resulted in a draft procedure for determining the relative susceptibility of bare AHSS/UHSS
• To develop a representative test for zinc coated steels

• A test that is relatively easy to run and adapted for new grade validation
• A test that can be modified for specific OEM pass/fail criteria.
• A test that is applicable to sheet steel with or without additional manufacturing (coating, welding, etc.) or in-service corrosion inputs

Note: : t It s is commonly thought that the risk for hydrogen assisted cracking cannot be completely eliminated (in all potential processes/environments) unless tensile strengths are restricted below 800 MPa

• A bend test consisting of samples pre-strained to >/=70% of their yield strength shows promise for evaluating hydrogen susceptibility
• Some materials/microstructure combinations with high tensile strengths have been shown as susceptible to hydrogen assisted cracking when exposed to test conditions
• Hydrogen related cracks occur very early in the test.

• Multiple test labs have seen similar results when susceptible materials are evaluated to the new test method.
• The A/SP Sheet Steel Harmonization Task Force has developed a draft test method for testing uncoated steels.
• When testing zinc coated steels with 0.1N HCl, the generation of hydrogen during dissolution can lead to premature fracture.

• Samples are strain-gauged and bent to the desired stress levels
• Fixtured samples are immersed in 0.1 N hydrochloric acid

Observation:
Susceptible materials at high
strains, immersed in 0.1N HCl,
exhibit large cracks after relatively
short exposures to the acid solution

• Goal: Work with AET Integration, Inc. to determine adaptations needed to current draft procedure to allow testing of grades with zinc coatings
• Comparison of different concentrations of hydrogen source (NH 4 SCN) to previously studied HCl.
• Testing of dual phase, fully martensitic and press hardened grades.

• Use of NH 4 SCN resulted in a significantly lower attack on the coatings than the previously used HCl
• The lower rate of attack on the zinc coatings in the NH 4 SCN solutions (ie low mass loss), likely resulted in low/little hydrogen evolution and lower amounts of hydrogen absorption

• The use of a NH 4 SCN solution appeared to greatly reduce or eliminate hydrogen evolution of zinc coated AHSS’s

• The use of NH 4 SCN does show promise as an alternative test solution to .1N HCL, however, lower concentrations than those tested need to be evaluated for use on zinc coated substrate

• Investigate lower concentrations of NH 4 SCN
• Investigate techniques for coating removal
• Follow with acid immersion bend test
• Test coated substrate to SEP1970 – Tensile Specimen with Punched Hole
• Others?