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Why Mild Steel has two Yield Points ? 
Mild steel has two yield points because of the presence of two different crystal structures within its microstructure. These structures, known as ferrite and pearlite, have different strengths and respond differently to stress, resulting in the appearance of two yield points.
The two yield points in mild steel are the upper and lower yield points. The upper yield point occurs at a higher stress level and corresponds to the yield strength of the pearlite crystals. The lower yield point occurs at a lower stress level and corresponds to the yield strength of the softer ferrite crystals.
The two yield points in mild steel affect its mechanical properties by giving it a unique stress-strain curve. The upper yield point results in a sudden increase in strain without a significant increase in stress, while the lower yield point results in a more gradual increase in strain and stress. This makes mild steel more ductile and less prone to sudden failure compared to other types of steel.
The two yield points in mild steel disappear at higher temperatures due to the process of annealing. When heated above a certain temperature, the crystal structures of ferrite and pearlite break down, resulting in a more homogeneous microstructure. This eliminates the two yield points and results in a single yield point and a more consistent stress-strain curve.
Yes, the presence of two yield points in mild steel has practical significance in industries such as construction and manufacturing. The two yield points allow for better control over the deformation and failure of the material, making it more predictable and easier to work with. This also allows for the use of lower strength grades of steel, reducing costs and making it more accessible for various applications.