Abstract

The resistance of steel wires to hydrogen embrittlement is very difficult to define and is therefore rarely investigated in literature. In this work a C65 steel wire was tested in hot rolled condition and after cold working. Both material states were first cathodically charged in sodium chloride (NaCl) with addition of thiourea (CH4N2S) at different current densities to determine the hydrogen content of each charging condition. In the next step mechanical properties and susceptibility to hydrogen induced stress corrosion cracking (HISCC) were determined in Slow Strain Rate Tests (SSRT). To prevent hydrogen effusion during experiments, samples were additionally further charged during the SSRT. Critical hydrogen concentrations for initiation of HISCC of both material states have been determined, whereas the critical hydrogen concentration is lower in the hot rolled state.

Introduction

The threshold hydrogen content of a material regarding hydrogen embrittlement plays an increasingly important role in corrosion research. This value indicates the hydrogen content to which the material can be used without failure. However, when determining the threshold hydrogen content, different test methods, different analysis methods and different interpretations of the results come together. This paper is intended to provide a guideline for the determination of the critical hydrogen concentration of high strength steel wire samples.

A compilation of critical hydrogen contents in the literature were summarized by Trautmann et al.1. A clear trend can be seen in the literature that the critical hydrogen content decreases with increasing strength1,2. However, the methods used to obtain the different data often differ fundamentally.

In order to set a certain hydrogen content, cathodic charging or high pressure charging is commonly used. In electrochemical hydrogen charging, hydrogen is generated cathodically on the sample surface by applying a voltage. In pressurized hydrogen charging, hydrogen molecules are separated into absorbable atoms by dissociation. The most commonly used methods of determining limit values require a determination of the mechanical properties as well as the analysis of the hydrogen content after hydrogen charging. Possible methods to determine the mechanical properties of charged materials are the use of slow strain rate tests, constant load tests, bending tests or fracture mechanics tests3-7. Possible hydrogen analysis methods are the carrier gas melt extraction, carrier gas hot extraction, thermal desorption analysis or the hydrogen collecting analysis8-10. Looking at methods that specifically produce failures, such as slow strain rate tests or crack growth measurements, there can be seen a variation of the criteria to assess a critical hydrogen content of a material. One common method is to determine the hydrogen environment embrittlement index according to ASTM-G12911:

equation (1)

Where RAe/RAc is the ratio of reduction in area between the test environment RAe and the control environment RAc.

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