Hydrogen Embrittlement Characteristics Of Two Tempered Martensitic Steel Alloys For High-Strength Bolting

Abstract Hydrogen embrittlement threshold curves were derived for two quenched and tempered steel grades, AISI 4135 and AISI 4340, at varying hardness ranging from 33 to 54 HRC. For each material, hydrogen was introduced (i) by zinc electroplating as a worst case condition for internal hydrogen embrittlement and (ii) by imposing cathodic potential of 1.2 V as a worst case condition for environmental hydrogen embrittlement. Overall, AISI 4135 exhibited lower thresholds than AISI 4340, making it the more susceptible of the two alloys. The findings demonstrate although hardness and/or strength have a first-order effect on hydrogen embrittlement susceptibility, difference in chemistry leading to differences microstructural characteristics must also be considered. Below hardness of 39 HRC, both alloys were not susceptible to internal hydrogen embrittlement, a finding that is consistent with common industry practice and fastener electroplating standards that do not mandate baking of electroplated fasteners with specified hardness below 39 HRC. 
 
Introduction High strength and ultrahigh strength threaded mechanical fasteners are broadly characterised by tensile strengths ranging from 1000 to 2000MPa. They are often used in critical components, typically comprising load bearing bolted joints in aircraft structures, ships, ground vehicles, bridges, tunnels and buildings. High strength threaded steel fasteners are also susceptible to hydrogen embrittlement (HE). Important considerations in the prevention of HE are the selection and heat treatment of materials used to manufacture threaded fasteners. Typically, fasteners specifications only place broad restrictions on material selection but do not mandate the selection of a particular grade of steel.
 
This paper is part of a larger body of work aimed at better defining the relationship between chemical composition, microstructural characteristics and HE susceptibility of high-strength steel used for manufacturing mechanical fasteners. It is generally understood that strength and/or hardness have a first-order effect on HE susceptibility in high-strength steel. In this study, observations made on two alloys illustrate differences in relative susceptibility that are dependent on chemistry and heat treatment, and ultimately on microstructure. The principal investigative technique used is incremental step loading (ISL) based on test method ASTM F1624.1 In brief, this approach consists of performing a mechanical test on a notched square bar specimen that has either been pre-charged with hydrogen or is cathodically charged during the test under potentiostatic control. The test methodology is designed to measure the HE cracking threshold of the material. Under standardised test conditions, the measured cracking threshold of the specimen is a function of the metallurgical and mechanical properties of the material. 

Date : 24.01.2018