When choosing the optimal material for a threaded fastener, it is important to know both the demands on the bolts and the mechanical properties of the clamped parts.
High working temperature
The most common property classes for threaded fasteners are 8.8 and 10.9. However, these are rarely suitable for service temperatures exceeding 150 °C as their creep characteristics will result in significant stress relaxation. When the temperature is high in the bolted joint, (e.g., if the bolts are connecting to the exhaust manifold to the engine) it is recommended to use a material that does not lose its mechanical properties at elevated temperatures.
Bulten's solution to solve this is a B14 or variants of stainless steel. The best option depends on the working temperature and the load on the bolted joint.
Knowledge of the strength of a fastener, and the forces it can transfer, is decisive when selecting the right product. In the past, each country had its unique regulations as regards the choice of material, strength requirements, markings, and testing methods. However, it soon became evident that it would be more practical to agree on common mechanical properties, classification, and identification of fasteners. Standardized fasteners would be easier to trade from one country to another. The material (carbon steel or alloyed) and tempering temperatures used for producing bolts, screws and studs are specified in ISO 898-1.
When choosing a bolt material for elevated temperatures, thermal expansion characteristics should be considered for both the fastener and the clamped part. If a “stiffer” fastener material is used, there is a risk for settlement during thermal expansion – resulting in a permanent plastic deformation which will increase the risk of fatigue breakdown.
Bulten's solution to handle the thermal elongation is to produce bolts in aluminum, optimized for aluminum applications or bolts made of stainless steel if the need for high clamping force is essential.
There is a risk of galvanic corrosion: If two metals are electrically connected by direct contact or by an electrolyte, a sufficient potential difference between the two metals might produce a flow of electron from less noble metal (anode) to the noble metal (cathode), giving a higher corrosion rate on the less noble metal.
Austenitic stainless steel has very good impact strength at low temperatures. Many austenitic grades has excellent fatigue strength, some even better than special carbon steel. Particular good are duplex grades and some precipitation hardenable grades.
In some applications, it is important to have stainless steel material with very low relative magnetic permeability that do not get magnetised when exposed to magnetic fields.