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Abstract
Fatigue crack growth (FCG) tests were conducted on a medium-Mn steel annealed at two intercritical annealing temperatures, resulting in different austenite (γ) to ferrite (α) phase fractions and different γ (meta-)stabilities. Novel in-situ hydrogen plasma charging was combined with in-situ cyclic loading in an environmental scanning electron microscope (ESEM). The in-situ hydrogen plasma charging increased the fatigue crack growth rate (FCGR) by up to two times in comparison with the reference tests in vacuum. Fractographic investigations showed a brittle-like crack growth or boundary cracking manner in the hydrogen environment while a ductile transgranular manner in vacuum. For both materials, the plastic deformation zone showed a reduced size along the hydrogen-influenced fracture path in comparison with that in vacuum. The difference in the hydrogen-assisted FCG of the medium-Mn steel with different microstructures was explained in terms of phase fraction, phase stability, yielding strength and hydrogen distribution. This refined study can help to understand the FCG mechanism without or with hydrogen under in-situ hydrogen charging conditions and can provide some insights from the applications point of view.
Highlights
As a strong candidate for the third-generation advanced high-strength steels (AHSS), medium-Mn steels have attracted significant attention from both industry and academia due to their excellent combination of production cost and mechanical properties [1,2,3]
When hydrogen was in-situ charged, both the IA555 and the IA700 specimens showed an acceleration in the fatigue crack growth rate (FCGR), but the degree of such acceleration was much higher for the IA700 sample
A refined study focusing on the Fatigue crack growth (FCG) behavior has been conducted on two SENT specimens from a medium-Mn steel with different microstructures, respectively, in alternative vacuum hydrogen plasma environments
Summary
As a strong candidate for the third-generation advanced high-strength steels (AHSS), medium-Mn steels have attracted significant attention from both industry and academia due to their excellent combination of production cost and mechanical properties [1,2,3]. Medium-Mn steels consist of an ultrafine-grained (UFG) microstructure with face-centered cubic (FCC) austenite (␥), body-centered cubic (BCC) ferrite (␣) and sometimes body-centered tetragonal (BCT) martensite (␣’) phases in different proportions. The characteristics of such microstructure can be tailored by intercritical annealing (IA) at different temperatures, which has a substantial influence on the material’s mechanical performance [2,4,5]. Until now most of the studies on medium-Mn steels have been focusing on the microstructure - tensile property relationship at the ambient environment. A critical problem limiting the use of such steels is the hydrogen embrittlement, given that hydrogen is ubiquitous and quite easy to be absorbed in the materials during processing and service
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