More tensile strength the 'harder' the material.....when something is 'hard' it is brittle....
Temperature cycling will harden a material further.....
The relationship between stress and strain is known as the Youngs Modulus, this ratio is also known as the Elastic Modulus....how resistant a material is to 'Plastic Deformation' i.e. how resistant it is to be permanently deformed....gone beyond its tensile strength and has transitioned the Yeild point of a material.
Hardness is a materials resistance deformation to an applied compressive force (scratch, indentation and rebound).
These two properties to together can determine the materials Toughness....the amount of energy the material can absorb before fracture.....
The higher the tensile strength of a material, the more brittle it becomes(harder)....the material is then normalised or annealed to reduce this brittle nature (hardness) and makes it tougher.....but a small amount of Tensile Strength is lost in this de-hardening process.
Normalising is done by heating a material to a certain temperature and held there for a period of time determined by material thickness (the A2 point seems to ring a bell) and then let to cool down at a normal (say room) temperature...this de-stresses the material by allowing the atoms to settle down......
A constant heating and cooling can lead to Case Hardening...this were the outer 'skin' of the material gets harder, but the core is still 'soft' The work piece will have some ductility due to its flexible core, but the outer 'case' is more brittle.
Now the boring bit is over.......back to your question....
In structural dynamics, the grade of bolt is critical for a given application, as a bolts tensile strength is used during pre-loading calculations to ensure the optimum mechanical fixing to prevent bolt creep, bolt elongation and failure.
In the context you guys are using the bolts...i.e. to hold the exhaust manifold on, you won't be tightening the bolt to a point it will fail due to the heat expansion effect, as there will be more than enough tensile strength left in the bolt to take up this additional amount of tensile stress.
As a for instance an 8.8 M10 bolt (3/*" is 9.5mm) is recommended to torque to 56.3Nm.......the RAVE torque value for manifold bolts is 55Nm....so an 8.8 is a perfect fitment.
Yours are grade 12.9 and this is good for 92Nm......and if you tighten to 55Nm...there is plenty of elongation/expansion/tensile strength available for any heating or hardness effects.
Bored yet?????
