We shall take the following two factors into consideration when selecting steel, the one is the machinability of the steel, the other is the vervice character during the processing, we hope the steel has a low strength and high elongation, which make ti easy to cut, stamp or form. But in the service of steel, we hope it has high strength, good impact performance to suffer extreme service condition. For these two reasons, we should select suitable steel form its mechanical properties.  Main Mechanical Properties Include Yield Strength  The yield strength or yield point of a material is defined in engineering and materias science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its orignal shape whem the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permanent  and non-reversible.  Tensile Strength Tensile strength is indicated by the maximum stress before the break of specimen. In general, it indicates when necking will occur.  Elongation  Elongation , or percent elongation at break, is defined as  the change in gauge length after break per unit of the original gauge length. A high enlongation means the material can stand great permanet deformation before break, or high deformability.   The parameter yield strength, tensile strength, enlongation are measured by tensile test.  Impact Energy Impact energy, or toughness, is determined by the energy absorbed by the specimen during fracture in the impact test. It is measured in units of joules. Impact energy indicates material's resistance to impact load. It is tested by charpy V-notch test.  If welding is required during the process, we should consider the welding performance of the steel.  Welding  For the steel, welding is a fabrication to combine different pieces of steel together. In the welding, normally the binding sites melt together and cool to form a strong joint, such as electric arc welding, gas welding and electric resistance welding.  Weldability Weldability, also known as joinablility, of a material refiers to its alibityy to be welded. Most steels can be welded,but some are easier to weld than others. It greatly influences weld quality and is an important factor in choosing which welding process to use.