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Corrosion Resistance
Corrosion resistance and the fabrication of stainless steel
Induction melting is one of the efficient as well as matured methods for melting high melting point metals. Induction melting can be divided into vacuum melting and atmospheric melting. Vacuum melting can produce steel with almost no gases, but it is a expensive procedure with limited application like aerospace grade products mostly. Commercial grade products are generally smelted by atmosphere melting and uses various refining techniques for the reduction of harmful gas content in the steel as to improve the corrosion resistance and physical properties.Commercial grade products are melted in atmosphere and cannot avoid the use of recycled materials Commercial products are melted in atmosphere (not vacuum) and cannot avoid the use of recycled materials. In fact, some furnace charges consist almost entirely of remelted scraps. This is not only because of cost factors, but also because it is simply not possible nor practical tosufficiently supply refined steel, e.g. steel ingots. Repeated melting accumulates gases that are drawn into the molten metal. The gases create porosity in the casting, this may restrict their usage.
But there is solution, the most significant technical advancement impacting the steel industry was the introduction of argon-oxygen-decarburization (AOD) refining. AOD technology was invented in the first half of last century. The process allows precise gas manipulation to achieve the desired result and refines stainless steel by gradually replacing oxygen by means of blowing argon through the molten metal to eliminate impurities such as Carbon, Sulphur and Phosphorus, which not only deterioate the corrossion resistance feature but also induce higher brittleness to the steel.
Argon Oxygen Decarburization (AOD) is a refining process for ferrous metals such as stainless steel or high-grade steel that improves alloy performance through tight chemistry control and high levels of metal purity. A mixture of oxygen and argon gases is injected into the molten metal bath. The oxygen reacts with carbon in the steel, forming carbon monoxide, which is removed from the vessel. This decarburization reduces the steel’s carbon content. This results in improved properties such as:
- Weldability
- Corrosion resistance
- Tensile
- Malleability
For higher demanding applications, heat treatment and surface treatment can be performed to enhance the physical properties and corrosion resistance of the steel casting.
- +AT is the code for the heat treatment of stainless steel which has a specific term called: solution annealing. Solution annealing dissolves carbon molecules into iron, making the molecules smaller and more regular, eliminating stress and enhancing corrosion resistance. The benefits are self-evident.
- The surface treatment methods of stainless steel include pickling, passivation, electrolytic polishing and mechanical polishing. The surface quality obtained as well as the respective production cost also increases in this sequence.
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