The welding wire contains Si, Mn, S, P, Cr, AI, Ti, Mo, V and other alloy elements.
The effects of these alloy elements on welding performance are described below:
Silicon is the most commonly used deoxidizing element in welding wire. It can prevent iron from oxidizing and reduce FeO in the molten pool.
However, when deoxidized with silicon alone, the resulting SiO2 has a high melting point (about 1710 ℃), and the particles of the product are small, so it is difficult to float out of the molten pool, which is easy to cause slag inclusion in the weld metal.
The effect of manganese is similar to that of silicon, but the deoxidation ability is slightly worse than that of silicon.
The density of MnO produced by deoxidation with manganese alone is high (15.11g/cm3), and it is not easy to float out of the solution pool.
In addition to deoxidation, manganese contained in the welding wire can also combine with sulfide to form manganese sulfide (MnS) and be removed (desulfurization), so the tendency of thermal crack caused by sulfur can be reduced.
It is difficult to remove deoxidized products due to deoxidation with silicon and manganese alone. Therefore, at present, silicon manganese combined deoxidation is mostly used to compound the generated SiO2 and MnO into silicate (MnO · SiO2).
MnO · SiO2 has a low melting point (about 1270 ℃) and low density (about 3.6g/cm3).
It can agglomerate into large slag and float out in the molten pool to achieve good deoxidation effect.
Manganese is also an important alloy element and hardenability element in steel.
It has a great influence on the toughness of weld metal.
When Mn content is less than 0.05%, the toughness of weld metal is very high;
When Mn content is more than 3%, it is very brittle;
When Mn content = 0.6 ~ 1.8%, the weld metal has high strength and toughness.
Sulfur often exists in the form of iron sulfide in steel and is distributed in a network at the grain boundary, which significantly reduces the toughness of steel.
The eutectic temperature of iron plus iron sulfide is relatively low (985 ℃).
Therefore, during hot processing, the eutectic of iron and iron sulfide has melted, resulting in cracking during processing, which is the so-called “thermal brittleness of sulfur”.
This property of sulfur causes hot cracks in steel during welding.
Therefore, the content of sulfur in steel is generally strictly controlled.
The main difference between ordinary carbon steel, high-quality carbon steel and high-quality steel lies in the content of sulfur and phosphorus.
As mentioned earlier, manganese has desulfurization effect because manganese can form manganese sulfide (MnS) with a high melting point (1600 ℃) with sulfur, which is distributed in grains in granular form.
During hot working, manganese sulfide has enough plasticity, so the harmful effect of sulfur is eliminated.
Therefore, it is beneficial to maintain a certain manganese content in the steel.
Phosphorus can be completely dissolved in ferrite in steel. Its strengthening effect on steel is second only to carbon, which increases the strength and hardness of steel.
Phosphorus can improve the corrosion resistance of steel, while the plasticity and toughness are significantly reduced.
Especially at low temperature, the impact is more serious, which is called the cold kneeling tendency of phosphorus.
Therefore, it is unfavorable to welding and increases the crack sensitivity of steel.
As impurities, the content of phosphorus in steel should also be limited.
Chromium can improve the strength and hardness of steel, but the plasticity and toughness are not reduced.
Chromium has strong corrosion resistance and acid resistance, so austenitic stainless steel generally contains more chromium (more than 13%).
Chromium also has strong oxidation resistance and heat resistance.
Therefore, chromium is also widely used in heat-resistant steel, such as 12CrMo, 15CrMo, 5CrMo, etc. Steel contains a certain amount of chromium .
Chromium is an important constituent element and ferrite element of austenitic steel.
It can improve the oxidation resistance and mechanical properties at high temperature in alloy steel.
In austenitic stainless steel, when the total amount of chromium and nickel is 40% and Cr / Ni = 1, there is a tendency of hot cracking;
When Cr / Ni = 2.7, there is no hot crack tendency.
Therefore, when Cr / Ni in 18-8 section steel is about 2.2 ~ 2.3, chromium is easy to produce carbides in alloy steel, which makes the heat conduction of alloy steel worse, easy to produce chromium oxide and difficult to weld.
Aluminum is one of the strong deoxidizing elements.
Therefore, using aluminum as deoxidizer can not only produce less FeO, but also easily reduce FeO, effectively inhibit the chemical reaction of CO gas generated in the molten pool and improve the ability to resist CO pores.
In addition, aluminum can also combine with nitrogen to fix nitrogen, so it can also reduce nitrogen pores.
However, when deoxidized with aluminum, the generated AI2O3 has a high melting point (about 2050 ℃) and exists in the molten pool in solid-state, which is easy to cause slag inclusion in the weld.
At the same time, aluminum containing welding wire is easy to cause splash, and too high aluminum content will reduce the hot crack resistance of weld metal.
Therefore, the aluminum content in welding wire must be strictly controlled and should not be too much.
If the aluminum content in the welding wire is properly controlled, the hardness, yield point and tensile strength of the weld metal are slightly improved.
Titanium is also a strong deoxidizing element, and can also synthesize tin with nitriding to fix nitrogen and improve the ability of weld metal to resist nitrogen pores.
If the content of Ti and B (boron) in the weld microstructure is appropriate, the weld microstructure can be refined.
Molybdenum in alloy steel can improve the strength and hardness of steel, refine grains, prevent tempering brittleness and overheating tendency, improve high-temperature strength, creep strength and lasting strength.
When the content of molybdenum is less than 0.6%, it can improve plasticity, reduce the tendency of cracks and improve impact toughness.
Molybdenum tends to promote graphitization.
Therefore, the molybdenum content of heat-resistant steel containing molybdenum, such as 16mo, 12CrMo and 15CrMo, is about 0.5%. When the content of molybdenum in alloy steel is 0.6 ~ 1.0%, molybdenum will reduce the plasticity and toughness of alloy steel and increase the quenching tendency of alloy steel.
Vanadium can improve the strength of steel, refine grain, reduce grain growth tendency and improve hardenability.
Vanadium is a strong carbide forming element, and the carbides formed are stable below 650 ℃. It has the effect of age hardening.
Vanadium carbide has high-temperature stability, so it can improve the high-temperature hardness of steel.
Vanadium can change the distribution of carbides in steel, but vanadium is easy to form refractory oxides, which increases the difficulty of gas welding and gas cutting.
Generally, when the vanadium content in the weld is about 0.11%, it can fix nitrogen and turn disadvantage into advantage.