Adding calcium to steel improves its corrosion resistance, fatigue strength, impact toughness and plasticity. It also increases its machinability, casting performance and high-temperature performance. It can also help to change the composition, quantity and form of non-metallic inclusions.
Rutile-potassium electrodes produce slag with a low oxygen level and can be used with either direct or alternating current. They provide a quiet, smooth arc.
Adding calcium to steel is a process used since a long time in casting irons but it only started being applied for alloying light metals like aluminum a few years ago. It can be added in a wide range of conditions and it has a high influence on the final quality of the alloys.
The most common way of adding calcium is through cored wires. In this way, the steelmaker can easily adapt the chemical composition of the molten metal according to its needs. For instance, CaFe or CaSi cored wires are preferred for low Silicon content steel grades.
The best way to control the calcium treatment is by using protective slag blankets or inert gas to prevent reoxidation. It is also important to use refractory shrouded nozzles to protect the liquid metal from splashing and splattering. By this, the steelmaker can guarantee a constant and safe level of addition. By adjusting the concentration and introducing different levels of Ca into the slag, the metallurgical performance can be improved significantly.
Adding calcium is a powerful desulphurizer and deoxidizer. It also improves the mechanical properties of steel by changing the shape of inclusions. For example, it transforms inter dendritic Al2O3 galaxies into fine Type III inclusions that will not clog continuous caster nozzles. It can also modify the shape of sulphide inclusions by forming a CaS shell around them.
In a 60t Consteel furnace, solid calcium cored wires and pyrite are added after vacuum degassing (VD) refining to modify the inclusions in 95CrMo steel. The results show that the addition of calcium greatly improved the casting performance, thermal cracking resistance, cold heading strength and welding properties of the steel.
The modification of magnesium aluminate spinel (MAS) inclusions in the steel is analyzed and found that the calcium treatment has a good effect on them, while the modification of oxide-sulfide duplex inclusions depends on their size, morphology and S content. Therefore, it is suggested that combining magnesium and calcium treatments is a good method for balance the castability of molten steel and controlling inclusion size.
Welding materials containing Ca improve the fluidity and cleanliness of cast irons and steels. The violent agitation that accompanies the addition of Ca to liquid metal also helps to reduce gas content.
Calcium treatment alters the composition, size and shape of oxides, sulphides and silicate inclusions in liquid steel to give better control over their morphology. This gives a more uniform weld bead and reduced directional anisotropy.
Adding Ca to basic fluxes increases the oxygen level of the slag and decreases hydrogen diffusion levels in out-of-position welding. It also makes the slag more soluble in water and less toxic.
Calcium can be added to thoriated or pure tungsten electrodes to increase current-carrying capacity and reduce arc sensitivity. This reduces electrode wear and extends electrode life. It also improves arc stability and provides a more stable welding zone. This improves production and quality for critical applications. It also makes a more efficient electrode for low-temperature, low-slag welding.
During electrode welding, metal stick electrodes are connected to an electric current from a welding power supply. An arc is struck between the electrode and the work piece, melting the electrode end and a portion of the base metal to form a weld pool. The slag and flux coating on the electrode protect the weld pool from atmospheric contamination.
Rutile-potassium (EXX13): This type of electrode has a high potassium content that helps provide arc ionization and works well with alternating current. It can also be used with direct current of either polarity. This type of electrode produces a quiet, smooth-running arc.
Low hydrogen-sodium (EXXX5): This type of electrode has a lower sodium content than other rutile-based types. It has a higher weld speed and provides medium penetration. It can be used in flat position and in making horizontal fillet welds.
Calcium in the flux reduces the number of inclusions and modifies their shape. This prevents the inclusions from clogging continuous caster nozzles.
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