Is it true that the harder the material of the large bolt, the better?
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Is it true that the harder the material of the large bolt, the better?
If the hardness is too high, operating it at high temperatures for an extended period will increase this hardness and reduce its toughness. Under certain conditions, such as stress concentration, excessive initial tightening force, or the occurrence of additional bending moments during operation, brittle fracture of the bolts can occur, leading to accidents!
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No, high hardness and high tensile strength come at the cost of lower toughness.
The large bolts referred to by the poster are likely high-pressure bolts; in terms of material requirements, they need to have high tensile strength as well as good internal toughness. Therefore, quenching and tempering treatment is required to obtain the desired properties.
It’s not the case that the higher the hardness, the better; it’s better to have comprehensive mechanical properties.
Select based on pressure and temperature
The hardness of bolt material can be achieved by quenching followed by tempering at a certain temperature. The lower the tempering temperature, the higher the hardness of the bolt; higher hardness implies greater strength but reduced toughness； If the operating temperature of the bolt is higher than its tempering temperature, it is as if the bolt is tempered again during operation, and its strength will decrease. If the poster is referring to bolts with a large diameter, due to the size of the material, when tempered at the same temperature, their hardness will be lower than that of bolts with a smaller diameter (such as those under 24), meaning their strength grade is also lower. The hardness of the bolt material should be selected based on the actual operating conditions; harder is not always better.
Both hardness and tensile strength need to be taken into account in order to ensure that the bolt exhibits good toughness and tightening capacity, thereby achieving better results with less effort.