Speaker
Description
Grain boundary weakening caused by grain boundary wetting is a potential precursor for liquid metal embrittlement in a Zn coated press hardening 20MnB8 steel. Galvanized press hardened steel samples, deformed by bending, and not deformed (flat) steel samples were analyzed by means of electron backscatter diffraction (EBSD), Auger electron spectroscopy (AES), energy dispersive X-ray analysis (EDX) and transmission electron microscopy (TEM) on the nanometer scale. Sample cross sections were prepared by Ar ion milling and subsequently analyzed via EBSD. Measurement of all possible phases such as bcc iron, different Zn/Fe phases (Zn-ferrite, gamma, delta) and ZnO was successful. We showed micro cracks which were formed between prior austenite grains and identified structures developed after micro crack formation. Zn as well as oxygen was detected by Auger electron spectroscopy on top of the micro crack surface. Zn/Fe phases were present at the wedge shaped crack tips, smaller than 100 nm in size. Zn distribution indicated that Zn penetrated from the crack tip further into the martensite bulk. For a complete picture, including the material state before micro cracking, we used electrolytic galvanized resp. Zn coated and not deformed samples. The thermal press hardening treatment was equally to hot dip galvanized 20MnB8. Cross sections were prepared by breaking the sample in a fracture stage and characterizing the interface coating steel by Auger spectroscopy. A Zn signal could be detected up to a depth of 25 µm near to the interface steel coating by those Auger measurements. By TEM EDX measurements, Zn could be found at prior austenite grain boundaries near to the interface coating steel. The quantity was one atomic layer or even less. The effect of Zn at prior austenitic grain boundaries on micro crack formation, due to grain boundary weakening, can not be ruled out from a physical characterization point of view.
Keywords
PHS
LME
TEM
AES
EBSD
EDX