Development and application of a novel tuning-fork test in studying hydrogen-induced fracture in as-quenched martensitic steels

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

TA105, Arina auditorium

Topic of the dissertation

Development and application of a novel tuning-fork test in studying hydrogen-induced fracture in as-quenched martensitic steels

Doctoral candidate

M.Sc. Renata Latypova

Faculty and unit

University of Oulu Graduate School, Faculty of Technology, Materials and Mechanical Engineering

Subject of study

Hydrogen embrittlement

Opponent

Research Professor Elina Huttunen-Saarivirta, VTT

Second opponent

Docent Rachel Pettersson, Jernkontoret

Custos

Professor Jukka Kömi, University of Oulu

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Development and application of a novel tuning-fork test in studying hydrogen-induced fracture in as-quenched martensitic steels

Hydrogen embrittlement (HE) is a well-recognized issue with ultrahigh-strength steels (UHSS) and an intensively studied subject to minimize the risk of sudden, catastrophic failures in structural applications. A novel tuning-fork test (TFT) is developed in this thesis to study HE of UHSS. The testing method utilizes constant displacement for stressing of the tuning-fork specimens by bending, combined with electrochemical hydrogen charging. With an isolated tensile stress region, crack initiation is controlled and can be monitored with different clamping arrangements. The main objectives were to create a simple and fast testing method, which allows ranking of UHSS, and to investigate the effects of prior austenite grain (PAG) structure. Traditional hydrogen permeation tests with a set-up built within this thesis complement the results.

The first part of the TFT implementation is composed of an evaluation of the HE susceptibility of 300–600 HBW martensitic steels with relative threshold stress levels. Increasing the strength level leads to lower threshold stress levels and increased susceptibility to HE, especially for steels with over 400 HBW hardness. With the addition of a loadcell to the TFT clamping system, a more precise investigation was conducted for 500 HBW steels with various PAG structures. The effect of PAG morphology (elongated/equiaxed) on HE susceptibility was studied with steels that had different alloying compositions and with steels that had different PAG morphologies but the same alloying.

An elongated PAG structure leads to enhanced resistance against HE in comparison to equiaxed PAG structures. Crack propagation rates were slower with more elongated microstructure, which is linked to transgranular quasi-cleavage crack propagation and slower hydrogen diffusion. Equiaxed microstructure exhibits partly intergranular cracking with faster crack propagation rate and hydrogen diffusion. Crack propagation was affected by the microstructure alignment of the elongated PAG structure, but the equiaxed structure did not show orientation differences. No correlation was observed between hydrogen diffusivities and different PAG boundary surface areas. However, the density of reversible hydrogen traps increased with decreasing amount of PAG boundaries, indicating that PAG boundaries may act as strong hydrogen traps in the investigated steels.
Last updated: 8.12.2022