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Influence of chemical elements on properties of steel (I)

Views:1     Author:Site Editor     Publish Time: 2019-06-03      Origin:Site

Effect of hydrogen on properties of steel

Element 1: H (hydrogen)


H is the most harmful element in general steel. Hydrogen in steel will cause hydrogen embrittlement, white spot and other defects. Hydrogen and oxygen, nitrogen, in the solid steel solubility is very small, in high temperature dissolved into steel liquid, cooling time too late to escape and accumulated in the organization of high pressure pores, steel plasticity, toughness and fatigue strength sharply reduced, serious will cause cracks, brittle fracture. "Hydrogen embrittlement" mainly occurs in martensitic steel, but is not very prominent in ferrite steel, and generally increases with hardness and carbon content.


On the other hand, H can improve the magnetic permeability of steel, but also increase coercivity and iron loss (the coercivity can increase by 0.5 ~ 2 times after adding H).


Effect of boron on properties of steel

Element 2: B (boron)


The main role of B in steel is to increase the hardenability of steel, thus saving other rare and expensive metals, such as nickel, chromium and molybdenum. For this purpose, its content is generally prescribed in the range of 0.001% to 0.005%. It can replace 1.6% nickel, 0.3% chromium or 0.2% molybdenum. It should be noted that replacing molybdenum with boron can prevent or reduce temper brittleness, while boron slightly promotes temper brittleness, so molybdenum cannot be completely replaced by boron.


Medium carbon carbon steel with boron, because of improved hardenability, can make the thickness of more than 20mm steel after the performance is greatly improved, therefore, 40B and 40MnB steel can be used to replace 40Cr, 20Mn2TiB steel can be used to replace 20CrMnTi carburizing steel. But because the role of boron with the increase of the carbon content of steel and weakened, or even disappear, in the choice of boronizing carbon steel, must consider the parts carburizing, carburizing layer of hardenability will be lower than the core of hardenability of this characteristic. Spring steel generally requires complete quenching, usually the spring area is small, use boron steel is advantageous.


Effects of carbon on steel properties

Element 3: C (carbon)


Second only to iron, C is the main element, which directly affects the strength, plasticity, toughness and welding properties of steel.


When the carbon content of steel is below 0.8%, the strength and hardness of steel increase with the increase of carbon content, while the plasticity and toughness decrease. But when the carbon content is above 1.0%, the steel strength decreases with the increase of carbon content.


With the increase of carbon content, the welding performance of steel becomes worse (for steel with carbon content greater than 0.3%, the weldability decreases significantly), cold brittleness and aging sensitivity increase, and the resistance to atmospheric corrosion decreases.


Effects of nitrogen on steel properties

Element 4: N (nitrogen)


The influence of N on the properties of steel is similar to that of carbon and phosphorus. With the increase of nitrogen content, the strength of steel increases significantly, and the plasticity, especially the toughness, decreases significantly. At the same time, increasing aging tendency, cold brittleness and hot brittleness will damage the welding property and cold bending property of steel. Therefore, nitrogen content in steel should be reduced and limited as far as possible. Generally, the nitrogen content should not be higher than 0.018%.


With the help of aluminum, niobium, vanadium and other elements, nitrogen can reduce its adverse effects and improve the properties of steel. Some brands of stainless steel, appropriately increasing the content of N, can reduce the use of Cr, can effectively reduce the cost.


Effect of oxygen on properties of steel

Element 5: O (oxygen)


O is a harmful element in steel. It is in the process of steelmaking naturally into steel, although in the end of steelmaking to add manganese, silicon, iron and aluminum to deoxidize, but it is impossible to remove all. During steel solidification, oxygen and carbon in the solution react to form carbon monoxide, which bubbles. Oxygen in steel is mainly in the form of FeO, MnO, SiO2, Al2O3 and other inclusions, which reduces the strength and plasticity of steel. Especially, it has serious influence on fatigue strength and impact toughness.


Oxygen will increase the iron loss, decrease the permeability and magnetic sensitivity, and intensify the aging effect.


Effect of magnesium on properties of steel

Element 6: Mg (magnesium)


It can reduce the quantity, size, distribution and shape of inclusions in steel. Trace magnesium can improve the size and distribution of carbide in bearing steel. When magnesium content is 0.002% ~ 0.003%, its tensile strength and yield strength increase by more than 5%, and the plasticity basically remains unchanged.


The effect of aluminum on the properties of steel

Element 7: Al (aluminum)


When aluminum is added into steel as deoxidizer or alloyed element, its deoxidation capacity is much stronger than that of silicon and manganese. The main role of aluminum in steel is to refine grain and fix nitrogen in steel, so as to significantly improve the impact toughness of steel and reduce the tendency of cold brittle and aging. For example, D grade carbon structural steel requires the acid soluble aluminum content in steel to be no less than 0.015%, and cold rolled sheet 08AL for deep stamping requires the acid soluble aluminum content in steel to be 0.015%-0.065%.


Aluminum can also improve the corrosion resistance of steel, especially when used with molybdenum, copper, silicon, chromium and other elements, the effect is better.


Al content in chromium-molybdenum steel and chromium steel can increase its wear resistance. The presence of Al in high carbon tool steels can cause quenching brittleness. The disadvantage of aluminum is that it affects the thermal machining properties, welding properties and cutting properties of steel.

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