Nanoscale Investigation of Microcracks and Grain Boundary Wetting in Press Hardened Galvanized 20MnB8 Steel

TL;DR

This study investigates the nanoscale mechanisms of microcrack formation and grain boundary wetting in zinc-coated 20MnB8 steel, revealing how zinc-induced phases weaken grain boundaries and contribute to embrittlement during heat treatment.

Contribution

It provides detailed nanostructural insights into zinc-induced grain boundary weakening and microcrack development in press hardened steel, using advanced microscopy techniques.

Findings

Zn/Fe intermetallic phases are present at crack surfaces and tips.

Zinc wetting occurs along grain boundaries before cracking.

Grain boundary weakening by zinc wetting is necessary for microcrack formation.

Abstract

Grain boundary wetting as a preliminary stage for zinc induced grain boundary weakening and embrittlement in a Zn coated press hardened 20MnB8 steel was analyzed by means of electron backscatter diffraction, Auger electron spectroscopy, energy dispersive X-ray analysis, atom probe tomography and transmission electron microscopy on the nanometer scale. Microcracks at prior austenite grain boundaries were observed and structures that developed after microcrack formation were identified. Zn/Fe intermetallic phases with grain sizes smaller than 100 nm in diameter are present at the crack surfaces and at the wedge-shaped crack tips. In order to get a complete picture, including the microstructure before cracking, an undeformed, electrolytically coated reference sample which underwent the same heat treatment as the press hardened material was investigated. Here, Zn, in the order of one atomic layer or less, could be found along prior austenite grain boundaries several micrometers away from the actual Zn/Fe phases in the coating. Such a grain boundary weakening by Zn wetting of prior austenitic grain boundaries during austenitization and/or hot forming is a necessary condition for microcrack formation.