VDA 235-205 (11/2024)

In addition to fasteners of property classes 8.8 to 12.9 in accordance with DIN EN ISO 898-1 and recommendation VDA 235-204 with martensitic microstructure, in this recommendation requirements for ultra-high strength fasteners with bainitic microstructure of property classes 12.8U to 16.8U are specified.

The development of such fasteners is intended to take into account the increased requirements for the weight reduction of parts and components in the vehicle and thus the reduction of the overall vehicle weight and, in particular, the increase in the efficiency of engines with comparable or improved functional properties compared to high-strength fasteners in accordance to DIN EN ISO 898-1.

The main characteristic of ultra-high strength fasteners in accordance with this recommendation is the lower bainitic instead of the martensitic, heat treatment to achieve ultra-high tensile strength values in combination with excellent ductility and fatigue properties.

Compared to martensitically quenched and tempered screws made of conventional materials (e.g. in accordance with DIN EN ISO 898-1), high-strength and ultra-high-strength screws with a lower bainitic microstructure show a lower sensitivity to hydrogen-induced brittle fracture with the same strength.

Production-related hydrogen embrittlement (internal hydrogen embrittlement, IHE) can be reliably avoided by appropriate process control (see 4.4 and 5), so that the assessment of the sensitivity to hydrogen-induced brittle fracture resulting from operational load (environmental hydrogen embrittlement, EHE) is of crucial importance for the user.

As with conventional high-strength materials, the sensitivity to EHE increases with increasing strength for bainitic ultra-high-strength materials. For the selection of applications, the desired property class shall therefore be evaluated in conjunction with the environmental potential hydrogen loading, e.g. due to corrosion processes