General Knowledge of Alloy Steel Pipes

General knowledge of alloy steel pipes: the brief introduction, chemical compositions, heat treatment perfomance and smelting equipment.

1. The brief introduction of alloy steel pipes
Alloy steel pipes have hollow sections. A large number of alloy steel pipes are used for conveying fluid pipelines, such as the transportation pipelines of oil, natural gas, gas, water and some solid materials. Compared with solid steel, such as round steel, the alloy steel pipe has the same bending and torsional strength and is lighter in weight.

Alloy steel pipes have good mechanical properties. It is mainly used in power plants, nuclear power, high pressure boilers, high temperature superheaters and reheaters of high-temperature high-pressure pipelines and equipment. It is made of high quality carbon steel, alloy structural steel and heat resistant stainless steel materials, and then it is made by hot rolling (crowded, expanding) or cold rolling (drawing). The greatest advantage is that it can be recycled by 100%. What’s more, it is in line with the national strategy of environmental protection, energy saving and resource saving. The national policy encourages the expansion of the application field of high-pressure alloy pipes. At present, the proportion of China’s alloy pipes consumption in the total steel is only half of that of the developed countries. The expansion of the application of alloy pipes can provide a broader space for the development of the industry.

According to the research of the expert group of alloy pipes branch of China Association of Special Steel, the demand for the length of high-pressure alloy pipes in China will increase by 10-12% annually in the future. The alloy pipe is that the steel pipe can be defined in accordance with the production of material, as the name implies that it is the alloy of the pipe. The seamless pipe is that the steel pipe is defined in accordance with the production process (seamed or seamless).

2. Chemical compositions
C: 0.08 ~ 0.15
Si: 0.17 ~ 0.37
Mn: 0.40 ~ 0.70
Cr: 0.90 ~ 1.20
Mo: 0.25 ~ 0.35
V: 0.15 ~ 0.30

3. The influence of alloying elements
Carbon (C): Carbon is the main element in steel. With the increase of carbon content in steel, the strength and hardness of steel at normal temperature increase. However, the plasticity, toughness and welding energy decrease. Therefore, the carbon content of steel for boiler pressure components is generally 0.1%~0.25%.
Mn: Mn can improve the strength, hardness and wear resistance of steel at normal temperature. When the content is high, the welding stress will increase. Mn can increase the short-time strength of steel at high temperature, but has no obvious effect on the tensile strength and creep limit.

Molybdenum (Mo) and Cr (Cr): Both Mo and Cr can improve the strength of steel. Chromium has an obvious effect on improving the microstructure stability of steel at high temperature, such as resisting nodulizing, graphitization and resisting high temperature oxidation. It can improve the corrosion resistance. However, the steel with high chromium content has strong welding crack sensitivity and high temperature difference stress. Molybdenum has an obvious effect on increasing the tensile strength height of steel. Molybdenum has a tendency to graduate and chromium can be added to prevent diseases. The coexistence of the two elements can improve the comprehensive properties of steel.

Vanadium (V): V in steel can improve the microstructure stability at high temperature, and can offset the negative effect of chromium on welding property.
Titanium (Ti): Ti can improve the tensile strength of steel. It can also improve the weld ability of steel in the case of anti-alloy steel.
Tungsten (W): W can improve the tensile strength and high temperature hardness of steel.
Silicon (Si): Silicon can improve the strength, wear resistance and oxidation resistance of steel. Coexistence with chromium can improve the high temperature oxidation resistance, but also improve the corrosion resistance in flue gas.
Niobium (Nb): Nb has the same effect as titanium and can improve the thermal strength of steel.
Boron (B): Boron has a prominent role in improving the hardenability of steel. The thermal strength and durable ductility of the steel can improve in the heat resistant steel.

4. The heat treatment performance
The heat treatment process of alloy steel can be divided into low-carbon alloy steel, medium-carbon alloy steel and high-carbon alloy steel. The low-carbon alloy steel generally needs to be carburized, quenched and tempered. The medium-carbon alloy steel generally needs to be quenched and tempered. Some of them also need to be quenched on the surface. The high-carbon alloy steel generally needs quenching and tempering.

For example:
Low-carbon alloy steel 18CrMnTi: carburizied at 920~950℃, oil quenched at 850~870℃, tempered at 180~200℃, surface hardness HRC58~67, the core HRC30~45
Medium-carbon alloy steel 40CrMnMo: oil quenched at 840~850℃, water or oil at 630~650℃, hardness HB 302~341
High-carbon alloy steel Cr12MoV: oil quenching at 950~1000℃, tempering at 150~180℃, HRC60~64

5. The smelting process
Smelting equipment: crucible resistance furnaces and gas continuous melting furnaces
Process specifications: the material used in smelting should be placed in a dry and pollution-free place. It should be treated accordingly before use. The surface contaminants of melting furnaces should be removed and dried before use.
Refining treatment: removal of gases, nonmetallic inclusions and other harmful elements from the alloy.