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Management of sodium nitrate as heat treatment salt bath

VIEWS: times Release Date:2020-07-03

The nitrate bath quenching agent is prepared from sodium nitrate, sodium nitrite and potassium nitrate, and is widely used for the grading and isothermal quenching of various steel parts. The melting point, fluidity and water content of the salt bath all have an impact on the cooling performance of the salt bath itself, which also affects the hardness value and uniformity, hardenability of the heat-treated parts, and even causes quenching cracks and deformation. A good nitrate bath is very important.

Management of sodium nitrate as heat treatment salt bath

   1. Look at the melting point


   Various formulations of nitrate have a certain melting point range. Put the newly prepared nitrate into the furnace and heat it to melt it, remove the surface floats and sediments, obtain a clean melt, re-melt after the furnace is condensed, note the melting point, and then record the corresponding melting point for each molten salt in order to Strengthen management. If it is found that the melting point rises to indicate aging, the higher the melting point, the lower the cooling capacity of the salt bath. In order to maintain a more stable melting point, the slag removal operation should be carried out before each furnace shutdown, and then new salt is added to the original liquid level.


   2. Look at the color


   Normal nitrate bath color is light yellow, high transparency, "clear as a mirror", good fluidity, the workpiece surface after quenching is light blue. If the salt bath becomes thicker and the color becomes darker, indicating aging, it should be adjusted.


   3. Look at moisture


  Moisture has the greatest influence on the cooling capacity of the nitrate bath. Because the nitrate deliquesces and contains crystal water, the nitrate bath generally has a water content of about 0.5-1.0%. After quenching into the workpiece, the salt bath boils and has a slight feel. If the workpiece is quenched, "water powder" splash is found (visible to the eyes), and the hand feels strong, indicating that the water content is too high, and the workpiece after quenching must be high in hardness, and even the phenomenon of quenching cracking or excessive deformation occurs. At this time, the temperature of the nitrate can be increased to reduce its cooling capacity accordingly, or the nitrate bath can be heated for a period of time to allow a part of the water to evaporate.


  If you find that the hardness of the quenched parts is low, you can slowly add a little water. According to the information, adding 0.12% water, the cooling capacity can be doubled compared to those without, therefore, the control of moisture is very necessary for the quality of quenching. The water content in nitrate can be determined by physical methods.


   4. Look at the salt residue


The quenching of the workpiece into the nitrate bath will bring some neutral salt, which has a great influence on the aging of the nitrate bath, that is, the decrease in cooling capacity. The neutral salt will increase the melting point of the nitrate bath, thicken, and flow The properties become worse and the infusibles increase. Therefore, it is possible to reduce the introduction of neutral salts as much as possible. If the salt residue fracture is black, it indicates that the neutral salt bath deoxidation fishing residue is not clean. If the salt residue fracture shows coarse crystals, it indicates that a large amount of chloride salt is mixed in the nitrate salt bath.


   5. Look at the slag


  Like neutral salt management, slag removal is a necessary operation, and slag removal is to remove insoluble materials. There are many nitrate aging residues, and fishing residues is an effective means to maintain the cooling capacity of the nitrate bath. The method is generally "first high and then low", that is, the temperature of the nitrate bath is raised to 30--50 ℃ higher than the normal operating temperature, and then the temperature naturally drops, and is slightly lower than the normal operating temperature. . If there are many slags and large blocks, it indicates that the aging is serious, depending on the situation to decide whether to update. In production, slag fishing is performed before quenching into the workpiece, which plays a certain role in ensuring the cooling capacity of the nitrate bath.

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