Mid-range unhardened vs the structural standard. When 5.8 is enough and when to step up.
Class 5.8 bolts (proof stress 380 MPa) occupy a middle ground — stronger than 4.6/4.8 but without the heat treatment of 8.8. They're common in European general engineering where loads are moderate but class 4.6 feels insufficient.
Class 8.8 (proof stress 580 MPa) is about 53% stronger. The jump comes from quenching and tempering, which transforms the microstructure. This heat treatment also gives 8.8 bolts better fatigue resistance.
If the application sees any vibration, cyclic loading, or if the bolted joint is structural, 8.8 is the correct choice. Class 5.8 works well for static loads in non-critical applications — equipment guards, covers, non-structural brackets.