How filler metal in welding wire affect welding quality?
For welding wire contains Si, Mn, S, P, Cr, AI, Ti, Mo, V and other alloying elements. How these alloying elements affect the welding performance is explained separately below.
Silicon is commonly used in welding wire deoxidation element, it can prevent iron and oxidation combined, and can be reduced in the melt pool FeO. but alone with silicon deoxidation, the generated SiO2 high melting point (about 1710 ℃), and the particles of the product is small, it is difficult to float from the melt pool, easy to cause the weld metal slag.
The role of manganese is similar to that of silicon, but the deoxidation ability is slightly worse than silicon. Manganese deoxidation alone, the generated MnO density is larger (15.11g/cm3), but also not easy to float from the dissolution pool. Manganese in the welding wire, in addition to deoxidation, but also and sulfide combined with the generation of manganese sulfide (MnS), and was removed (desulfurization), so it can reduce the tendency of thermal cracking caused by sulfur. Because of the silicon and manganese deoxygenation alone, it is difficult to remove the generation of deoxygenation. MnO-SiO2 has a low melting point (about 1270℃) and small density (about 3.6g / cm3), which can coalesce into a large slag in the melt pool and float out to achieve a good deoxidation effect. Manganese is also an important alloying element in steel, is also an important hardening element, it has a great impact on the toughness of the weld metal. When the Mn content < 0.05% weld metal toughness is very high; when mn content > 3% and very brittle; when Mn content = 0.6 ~ 1.8%, the weld metal has high strength and toughness.
Sulfur is often present in steel in the form of iron sulfide and is distributed in a network at the grain boundaries, thus significantly reducing the toughness of steel. Iron plus iron sulfide eutectic temperature is low (985 ℃), therefore, in the hot processing, due to the processing temperature is generally 1150 ~ 1200 ℃, and iron and iron sulfide eutectic has melted, resulting in processing cracking, this phenomenon is the so-called “sulfur thermal embrittlement”. This nature of sulfur makes steel in the welding of thermal cracking. Therefore, the content of sulfur in steel is generally strictly controlled. Ordinary carbon steel, high quality carbon steel and high quality steel is the main difference is the amount of sulfur, phosphorus content. As mentioned earlier, manganese has the role of desulfurization, because manganese can form a high melting point with sulfur (1600 ℃) of manganese sulfide (MnS), which is distributed in the grains of grain. In the hot processing, manganese sulfide has enough plasticity, thus eliminating the harmful effects of sulfur. Therefore, it is beneficial to maintain a certain amount of manganese in steel.
Phosphorus in steel can be completely dissolved in the ferrite. It is second only to carbon on the strengthening of steel, so that the steel strength and hardness increase, phosphorus can improve the corrosion resistance of steel, while plasticity and toughness is significantly reduced. Especially at low temperatures when 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 and plasticity and toughness is not reduced. Chromium has a strong corrosion and acid resistance, so austenitic stainless steel generally contains more chromium (13% or more). Chromium also has a very strong resistance to oxidation and heat resistance. Therefore, chromium is also widely used in heat-resistant steels, such as 12CrMo, 15CrMo 5CrMo, etc. steels all contain some amount of chromium . Chromium is an important constituent element of austenitic steel and ferritization, and it improves oxidation resistance and mechanical properties at high temperatures in alloy steels. In austenitic stainless steels, when the total amount of chromium and nickel is 40% and Cr/Ni = 1, there is a tendency to thermal cracking; when Cr/Ni = 2.7, there is no tendency to thermal cracking. So generally 18-8 steel Cr/Ni = 2.2 to 2.3 or so, chromium in the alloy steel is easy to produce carbide, so that the alloy steel thermal conductivity becomes poor, easy to produce chromium oxide, so that welding caused by difficulties.
Aluminum is one of the strong deoxidizing elements, so use aluminum as a deoxidizer, not only can produce less FeO, and easy to make FeO reduction, effectively inhibit the chemical reaction of CO gas generated in the melt pool, improve the ability to resist CO porosity. In addition, aluminum can also be combined with nitrogen and the role of nitrogen fixation, so it can also reduce nitrogen porosity. But with aluminum deoxidation, the generated AI2O3 melting point is very high (about 2050 ℃), to the solid state in the molten pool, easy to cause weld slag. At the same time, the aluminum wire easily caused by spatter, aluminum content is too high will also reduce the weld metal resistance to thermal cracking, so the amount of aluminum in the wire must be strictly controlled, should not be too much. If the amount of aluminum in the wire is properly controlled, the hardness, yield point and tensile strength of the weld metal are slightly increased.
Titanium is also a strong deoxidizing element, and can also be synthesized with nitrogen TiN and play a role in nitrogen fixation, improve the weld metal resistance to nitrogen porosity. If the Ti and B (boron) in the weld tissue content is appropriate, the weld tissue can be refined.
Molybdenum in alloy steel can improve the strength and hardness of steel, refine the grain, prevent tempering brittleness and superheating tendency, improve high temperature strength, creep strength and lasting strength, containing less than 0.6% molybdenum, can improve plasticity, reduce the tendency to produce cracks, improve impact toughness. Molybdenum has the tendency to promote graphitization. Therefore, the general molybdenum-containing heat-resistant steel such as 16Mo, 12CrMo, 15CrMo, etc. contains about 0.5% of molybdenum. Molybdenum in the alloy steel content in 0.6 ~ 1.0%, molybdenum will make the alloy steel plasticity and toughness decline, increasing the alloy steel quenching tendency.
Vanadium improves the strength of steel, refines the grain, reduces the tendency of grain growth and improves hardenability. Vanadium is a strong carbide forming element and the carbides formed are stable below 650°C. It has an age-hardening effect. Vanadium carbides are high temperature stable and therefore increase the high temperature hardness of the steel. Vanadium can change the distribution of carbides in the steel, but vanadium tends to generate refractory oxides, increasing the difficulties of gas welding and gas cutting. The vanadium content in the general weld is about 0.11%, which can play a nitrogen fixation effect, turning unfavorable to beneficial.