*Review* **Heuristic Design of Advanced Martensitic Steels That Are Highly Resistant to Hydrogen Embrittlement by** ε**-Carbide**

**Michio Shimotomai**

MOTP LLP, Masago 3-17-4-1303, Mihama Ward, Chiba 261-0011, Japan; shimotomai317@gmail.com

**Abstract:** Many advanced steels are based on tempered martensitic microstructures. Their mechanical strength is characterized by fine sub-grain structures with a high density of free dislocations and metallic carbides and/or nitrides. However, the strength for practical use has been limited mostly to below 1400 MPa, owing to delayed fractures that are caused by hydrogen. A literature survey suggests that ε-carbide in the tempered martensite is effective for strengthening. A preliminary experimental survey of the hydrogen absorption and hydrogen embrittlement of a tempered martensitic steel with ε-carbide precipitates suggested that the proper use of carbides in steels can promote a high resistance to hydrogen embrittlement. Based on the surveys, martensitic steels that are highly resistant to hydrogen embrittlement and that have high strength and toughness are proposed. The heuristic design of the steels includes alloying elements necessary to stabilize the ε-carbide and procedures to introduce inoculants for the controlled nucleation of ε-carbide.

**Keywords:** steel; martensitic steel; ε-carbide; tempering; hydrogen embrittlement; mechanical strength; inoculant; materials design

**Citation:** Shimotomai, M. Heuristic Design of Advanced Martensitic Steels That Are Highly Resistant to Hydrogen Embrittlement by ε-Carbide. *Metals* **2021**, *11*, 370. https://doi.org/10.3390/met11020370

Academic Editor: Andrea Di Schino

Received: 14 December 2020 Accepted: 19 February 2021 Published: 23 February 2021

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#### **1. Introduction**

High-strength low-carbon martensitic steels yield low-cost environmentally efficient materials. Wider use of these materials could improve energy savings and reduce the carbon footprint of many products. The use of such steels in martensitic condition provides a good weldability and a high strength, which makes them attractive materials for structural applications. However, an increase in steel strength enhances hydrogen embrittlement (HE). For example, the strength of steel bolts for automobiles has been limited to 1400 MPa because of HE. In hydrogen-powered vehicles, steel with a high resistance to HE is required for cost and weight reductions [1]. A promising approach to develop steels with a high strength and a low HE susceptibility may be through the use of ε-carbide precipitates in steels. In this work, a conceptual design of advanced steels with a high resistance to HE is proposed based on a literature survey and on experimental results of the influence of ε-carbide on the HE susceptibility of steel.

The paper is organized as follows. Section 2 presents a critical literature review on hydrogen absorption of ε-carbide and steel precipitation strengthening. Section 3 presents preliminary experimental results on the influence of ε-carbide in martensitic steel on the HE susceptibility. In Section 4, a steel design with a high strength that is compatible with a low HE susceptibility is proposed. The design includes alloying elements that are required to stabilize ε-carbide and inoculants to nucleate ε-carbide precipitates. The final section provides concluding remarks.
