Alloy steel (part 1)

Introduction

Alloy steel is a kind of steel with other alloying elements added in addition to carbon and steel. It’s an iron-carbon alloy formed by adding an appropriate amount of one or more alloying elements on the basis of ordinary carbon steel. By adding different elements and adopting appropriate processing technology, special properties such as high strength, high toughness, wear resistance, corrosion resistance, low and high temperature resistance, as well as non-magnetic properties can be obtained.       

Function

Carbon (C)

As the carbon content in steel increases, the yield point and tensile strength increase, but the plasticity and impact properties decrease. In addition, carbon can increase the hot shortness and aging sensitively of steel. When the carbon content exceeds 0.23%, the welding performance of the steel will deteriorate. Therefore, the carbon content of the low alloy structural steel used for welding generally does not exceed 0.20%. The high carbon content will also reduce the atmospheric corrosion resistance of the steel. The high carbon steel in the open stock yard is prone to corrosion.   

Silicon (Si)

Silicon is added as a reducing agent and deoxidizer in the steel making process, so the killed steel contains 0.15-0.30% silicon. If the silicon content in steel exceeds 0.50-0.60%, silicon is regarded as an alloying element. Silicon can significantly improve the elastic limit, yield point and tensile strength, so it is widely used as spring steel. By adding 1.0-1.2% to quenched and tempered structural steel, the strength can be increased by 15-20%. The combination of silicon and molybdenum, tungsten, chromium, etc., has the effect of improving corrosion and oxidation resistance, as well as can produce heat-resistant steel. Low-carbon steel with silicon content of 1-4% has extremely high magnetic permeability and is used as silicon steel in the electrical industry. An increase in the amount of silicon will reduce the welding performance of steel.

Manganese (Mn)

In the steel making process, manganese is a good deoxidizer and desulfurizer. Generally, the manganese content in steel is 0.30-0.50%. When adding more than 0.70% to carbon steel, it is considered “manganese steel”. Compared with ordinary steel, it not only has sufficient toughness, but also has higher hardness and strength， improves the hardenability and the hot workability of steel. For example, the yield point of 16Mn steel is 40% higher than that of A3. Steel containing 11-14% manganese has extremely high wear resistance and is used in excavator buckets, ball mill linings, etc. The increase of manganese content weakens the corrosion resistance of steel and reduces the welding performance.

Phosphorus (P)

Under normal circumstances, phosphorus is a harmful element in steel. It increases the cold brittleness of steel, is bad welding performance, reduce plasticity, so that the cold bending performance is bad. Therefore, phosphorus content in steel is usually less than 0.045%, and high quality steel is required to be lower.

Sulfur (S)

Sulfur is also a harmful element under normal circumstances, causing steel to produce hot brittleness, reducing the ductility and toughness of steel, and causing cracks during forging and rolling. Sulfur is also detrimental to welding performance, reducing corrosion resistance. Therefore, the sulfur content is generally required to be less than 0.055%, and the high quality steel is required to be less than 0.040%. Adding 0.08-0.20% sulfur to steel can improve machinability, usually called free machining steel.             

Note: 

1. The prefix "G" means carbon steel and alloy steel, and "H" means steel with hardenability requirements.

2. a≤0.10%, b＝0.10%～0.20%, c＝0.15%～0.30%, d＝0.20%～0.40%, e＝0.30%～0.60%

3. AISI grades with the prefix E are electric furnace steels.

4. The chemical composition of H steel with hardenability requirements is basically taken from ASTM A304, and the chemical composition of carbon steel and alloy steel with prefix G is basically taken from ASTM A29. The chemical composition specified in A29. The chemical composition specified in A29 is used for the following technical conditions: 

Hot rolled alloy steel: ASTM A322-2007, A434-2006, A739-2006

Cold finished alloy steel: A331-2000, A434-2006, A696-2006

5. The chemical composition is only applicable to structural profiles, plates, strips and welded pipes.

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