Lecture 7 Carbon Steel NCKU（讲座7碳钢国立）.pdf

1 Engineering materials Lecture 7 Carbon Steel 2 Strengthening mechanisms • Crystals contain dislocations • A shear stress , applied to the slip plane of a dislocation, exerts a force b to push it forward. • When the force b exceeds R, the resistance force opposing the motion of a dislocation, crystals deform plastically, i.e. it yields. • How to increase yield strength of metals: introduce obstacles into the path of the dislocations 3 Strengthening Mechanisms • Solid solution strengthening • Precipitate strengthening • Dispersion strengthening • Strain hardening – Example: 冷軋鋼筋 4 Solid Solution Strengthening • Addition of one or more metals can increase the strength of metals. • Solute atoms, on case of substitutional solid solution, create stress fields around themselves and hinder the dislocation movement. • Distortion of lattice and clustering of like atoms also impede dislocation movement. • Example: Solid solution of 70 wt % Cu 30 wt % Zn (cartridge brass) has tensile strength of 500 MPa. Tensile strength of unalloyed copper is 330 MPa 6-42 5 Solid solution strengthening • Obstacles are formed via solid solution • Example: Brass: Zn and Cu • Example: Bronze: Sn and Cu • Example: Stainless steel: Cr, Ni and Fe 6 Precipitate strength