Can the mechanical property data of materials listed in the manual be mechanically applied in the mechanical design
The number of mechanical properties listed in various manuals is generally based on the data obtained by testing small size specimens (Ø15 to 25mm) that can be hardened. Therefore, when using these data must pay attention to the effect of size effects on mechanical properties. When the part diameter (thickness) and the material critical hardening diameter is similar to the data in the manual can be used as the basis for design and material selection. When the size of the part is larger than the critical diameter of the material, with the increase in cross-sectional size, the mechanical properties of steel will be reduced (this phenomenon is known as the size effect), especially for low hardenability of steel, the size effect is particularly obvious.
For example, after tempering 45 steel, when the diameter of 100mm, its ReL or Rp0.2 (old standard σs) (yield strength) value compared to Ø30mm decreased by about 30%, toughness decreased by about 50%. Therefore, if the shape and size of the part to be kept unchanged, the material with greater hardenability should be selected instead. If not change the material, should be reduced by the mechanical properties of the corresponding increase in the size of the part cross-section, otherwise the part will not reach the actual required performance due to heat treatment and cause early failure of the parts.