Changes in internal elements during precision casting of 304 stainless steel:

Changes in internal elements during precision casting of 304 stainless steel:

The metallographic structure of 304 stainless steel containing 18% chromium and 8% nickel after cooling should be F+A+carbides, but its eutectoid transformation did not occur in time under casting cooling conditions, resulting in A+carbides. When chromium carbide appears in steel, its corrosion resistance is greatly reduced, causing a chromium poor layer and damaging the passivation film of chromium, resulting in a significant increase in intergranular corrosion. To eliminate carbides in cast steel, on the one hand, quenching treatment can be carried out; On the other hand, adding an appropriate amount of titanium and niobium to the steel can form titanium carbide or niobium carbide, thereby avoiding the formation of chromium carbide and poor chromium layer. In order to generate stable titanium carbide or niobium carbide, the amount of titanium added should generally be greater than 5 times the carbon content, while the amount of niobium added should generally be more than 8 times the carbon content. In addition, alkaline furnace lining is generally used for stainless steel melting, as acidic furnace lining can mix small SiO2 inclusions in the steel, greatly reducing its plasticity and toughness.

For the smelting of 304 stainless steel precision casting steel materials, it is required to remove impurities such as oil and dirt. The returning material must be sprayed off with sand and oxide skin, and can only be put into the furnace for smelting after being clean. After the steel melt pool is completely melted, low-carbon manganese iron and low-carbon silicon iron are added as required for alloying and pre deoxidation.

The order, time, and method of adding various alloying elements have a significant impact on the recovery rate. The order and time of adding alloy elements are determined based on the following requirements: the added alloy elements should be able to melt as soon as possible to make the composition uniform; The recovery rate of alloy elements should be high, which reduces the burning loss of alloy elements and does not increase impurities in the steel; The impurities and gases brought in by alloy elements must be able to be removed.

Precision machining