Like bolts from the Society of Automotive Engineers (SAE), metric bolts have specific torque values according to the grade of metal (measured by the amount of carbon in it) and diameter of the bolt. These bolts commonly come in "low grade," "8.8," "10.9", and "12.9."
Markings are usually on the bolt head itself or underneath the head on the shaft or underside of the bolt head. If no markings are present, assuming it is a low grade bolt.
We discussed metric grade and torque values in our Metric Bolt Chart, reproduced below.
Maximum Torque in ft lbs for clean, dry threads
Although this chart is not definitive and should be over-ridden by any shop manuals or application-specific information, it gives a base outline of the torque values recommended for bolts in grades and diameter.
Bolts are commonly tested in one or more ways. Tensile strength is determined by the maximum load (pulling) that the material can withstand before breaking or fracturing.
Yield Strength is the maximum load a material can stand before exhibiting a specific, permanent deformation.
Proof Load is the axial tensile load that a product must be able to withstand without evidence of any permanent set.
Most quality manufacturers test with all three measurements on random lots of their production line. Some specifications for critical designs will require a Proof Load on all bolts being used.
Torque Specs for Bolts