It is expected that the spring has the ability to change its shape in the elastic field under any load and to return to its original state with the removal of the load. At this point, the springiness of the material used in the construction of the bow gains importance. Spring steel is the steel material used to make various springs.  The springing ability of steel material depends on its ability to store energy during elastic deformation. This amount of energy is called the spring limit of the material and the areas under the tensile curve within the elastic limits are measured. Since the elastic modulus at high operating temperatures and in highly alloyed steels varies depending on temperature and chemical composition, this property should be taken into account in the calculations.  The yield limit is the point at which there is a certain increase in elongation without an increase in stress, and the yield limit of a steel material can be greatly modified by its chemical composition, heat treatment and cold forming. It is expected that spring steels should have a high spring and yield limit, but a plastic deformation capability that is plastic enough to be formed.  In steel materials, the phenomenon of crack formation caused by load changes is called fatigue and the maximum stress in loading is less than the maximum tensile strength of the material. This is 90% of the reason why spring elements do not fulfill their duties, i.e. why they break. For this reason, the fatigue time of the steel material used in spring making is as important as its springing ability. This time is determined by fatigue testing, but the exact determination of the actual working stresses of fatigue tests is very difficult in practice.  The fatigue limit is the minimum stress at which the material fatigues and ruptures under cyclic loading, and the material works safely at loads below this stress (Figure 2). Fatigue cracking is usually caused by stress raisers within the material, such as a slag inclusion (residue) or an excessively sharp molding radius.  Initially the crack progresses slowly, but the rate of progress gradually increases. In many cases of disuse, the crack surface shows signs like growth rings on a tree trunk. These marks are called “clam shells” or “stop lines” in the literature. The growth of the crack and the cessation of crack expansion as a result of changes in the loading of the part can be observed with these lines.  In conditions where carbon steel cannot be used, alloying elements are added to the spring steel during the melting process and these elements increase the tensile strength, hardness and toughness of the steel.  Effects of Chemical Composition on the Properties of Spring Steels:  Silicium: It increases the tensile strength of steel without reducing its ductility and toughness. This element facilitates the hot working of steel and prevents scaling, while increasing the tendency for surface decarburization during heat treatment. Heat treatment must therefore be carried out under controlled atmosphere conditions.  Manganese: The addition of manganese to spring steels facilitates ingot casting, and although the outer surface of spring steels without manganese hardens over a short distance, the center remains soft. This element facilitates the rolling, forging and drawing of steel, while at the same time accelerating the process of rapid and deep hardening. This accelerates the process of deep hardening. Quenching should be done in oil to prevent cracks caused by this deep hardening property.  Chrome: With the addition of this element, tensile strength, hardness and toughness increase, while the need for carbon is reduced and the resistance of steel to acid and alkali increases. As with manganese, chromium increases the hardening depth and raises the hardening temperature. When it comes to stainless steels, it helps prevent corrosion.  Vanadium: The addition of this element increases the tensile strength, toughness and impact resistance of steel. It also allows the grain size to be controlled at high temperatures. With these features, it is used in springs operating at high temperatures.  Molybdenum: Like chromium, it increases hardness and requires quenching. Molybdenum increases the toughness of steel and provides the ability to be processed at high temperatures.  Boron: With the addition of a very small amount of 0.0005-0.003% to the steel, its hardenability increases while its ductility does not decrease. Boron steels are used in springs operating under high stresses, Tugsten: It increases the strength of steel at high temperatures.  Nickel: Nickel lowers the hardening temperature and increases the tendency for steel to harden in oil rather than water. Although it contributes little to hardenability, it increases wear and toughness. Nickel is used in stainless spring steels together with chromium.  Phosphorus and Sulfur: Minimizing non-metal phosphorus and sulfur in spring steels improves their quality. Generally, it is desired to reduce phosphorus, which is 0.035% (max) in quality spring steels, to 0.025% or even below 0.020%, and sulfur, which is 0.040% (max), to below 0.025%, even below 0.010% in extreme applications.