New approach of H-embrittlement - Degassing and hydrogenation

The phenomenon of hydrogen embrittlement associated with steels hardened beyond 30 HRC and coated leads to delayed fracture once the part is placed under stress, resulting in the total loss of integrity of metal structures. This phenomenon affects many structural components in industry, and in particular fasteners in the automotive and aerospace industries, where tempered steels are used. To improve the corrosion resistance or tribological behavior of a system, electrolytic or chemical surface treatment operations are generally carried out using various processes. These operations lead to the introduction of hydrogen, which can lead to embrittlement of the metal part. Furthermore, with the aim of lightening structures and increasing their lifespan, the mechanical properties such as fracture resistance of metal components are revised upwards, with the consequence of increasing the sensitivity of these products to hydrogen-assisted damage. This sensitivity can occur at relatively low hydrogen concentrations (well below the solubility limit). In view of this problem, it is necessary to carry out a so-called "degassing" treatment for tempered, electrolytically or chemically coated parts that are mechanically stressed during use. Numerous standards (ISO 9588, ASTM B850, DIN 50979) set the minimum degassing time at between 190°C and 220°C, depending on the product's breaking strength. These standards also introduce a maximum inter-operation time (3 to 4 h) between the 1 ^re "hydrogen-generating" operation in the coating range (pickling or electrolytic or chemical surface treatment) and the start of "degassing".

It is important to remember that the "outgassing" time is given as an indication and is based solely on the breaking strength of the coated part. Other important parameters are not taken into account, such as the shape and volume of the part, the conditions of use of the product (e.g. tightening torque), the type and thickness of the coating, the chemical composition of the steel, inclusion cleanliness and the state of precipitation, all of which can influence the degree of sensitivity of the product to hydrogen embrittlement. However, while the "degassing" process under the conditions described above has enabled products to be desensitized to hydrogen-induced delayed fracture, from an industrial point of view, the mechanisms of desensitization to hydrogen embrittlement are still open to discussion. This debate is the subject of a great deal of applied research, both in the laboratory and in industry, with the launch of field research programs that will be described in the rest of this document.

In view of this, we felt it necessary to shed some light on the phenomena involved in "outgassing" treatment, based on real...