Cold cracking of slag pot

The cold crack is caused by the casting stress exceeding the strength of the alloy when the slag pot is in the elastic state. Cold cracking often occurs in the tensile part of the slag pot, especially where there is stress concentration. Therefore, the tendency of cold cracking in the slag pot is closely related to the formation stress of the slag pot. The factors affecting the cold cracking are basically the same as those affecting the stress of the slag pot.

The characteristics of cold cracking are different from hot cracking, the shape is continuous straight line or smooth curve, and often breaks through the grain rather than along the grain boundary. The cold crack is clean and has a metallic luster or a slight oxidation color. This shows that the cold crack is formed at a lower temperature.

The main results are as follows: 

(1) the large slag pot with complex shape is easy to form cold crack. Some cold cracks are often found after cleaning, and some are listed because of the large residual stress in the slag pot, which is vibrated during cleaning and handling.

(2) the composition and melting quality of the alloy have great influence on the cold cracking. For example, the carbon and chromium elements in steel can improve the strength of steel, but reduce the thermal conductivity of steel, so when the content of these elements is relatively high, it can increase the cold cracking tendency of steel. Phosphorus can increase the cold brittleness of steel. when the amount of phosphorus in steel is more than 0.1%, its impact toughness decreases sharply and its cold cracking tendency increases obviously. Similarly, when the phosphorus content in the ash tank is more than 0.5%, there are often a large number of reticulated phosphorus eutectic, and the cold cracking tendency increases obviously. When the deoxidization of the steel is poor, the oxidized inclusions gather on the grain boundary, which reduces the impact toughness and strength of the steel and promotes the formation of cold crack. A similar situation occurs when other non-metallic inclusions in steel increase.

(3) the microstructure and plasticity of steel have great influence on the formation of cold crack. For example, low-carbon austenitic steel (Ni-Cr acid-resistant steel), which can withstand high temperature (1050-1150 ℃), immersed in cold water quenching. Although this steel has low thermal conductivity and can produce large thermal stress (no phase transformation stress), cold cracks are rarely formed. This is because the mechanical properties of low carbon austenitic low body steel are characterized by elasticity and high plasticity. The larger thermal stress formed during quenching will soon exceed the elasticity, resulting in plastic deformation of the slag pot, so cold cracking will not be formed.

For the same low-body high manganese steel, the carbon content is on the high side, and when the slag pot is cooled, more carbon is precipitated on the Austrian-low body crystal boundary, which increases the brittleness of the steel. Therefore, this kind of slag pot is easy to form cold crack not only when it is cooled sharply in water, but also when it is put in the air in advance.