the source of nitrogen hole in the casting
The nitrogen absorbed by liquid metal comes from a variety of sources, and there are two main categories:
One is the nitrogen contained in the molten metal itself before pouring.
For the gray cast iron smelted in the cupola, the scrap steel in the charge is an important source of nitrogen. The alkaline electric arc furnace scrap has a nitrogen content of 60ppm to 140ppm, and the scrap steel is more than 35%. It is possible to produce nitrogen holes. The method to prevent the nitrogen hole caused by the high nitrogen content of the molten iron is to add ferrotitanium to the molten iron to reduce the tendency of cast iron to white mouth and to select the recarburizer correctly.
The second is nitrogen contained in resin sand.
The nitrogen contained in resin sand comes from resin and curing agents, nitrogen accumulated in reclaimed sand, nitrogen-containing additives in molding sand, and nitrogen in paint. The production of iron castings with resin sand is more likely to produce nitrogen holes. This is because when the molten iron is poured into the mold, the N-containing resin is heated to decompose NH3, and the NH3 dissociates on the surface of the molten metal. A considerable part of [N] atoms enter the surface of the molten metal at the metal interface of the mold, and diffuse from the surface to the inside, causing the surface metal liquid to be saturated with nitrogen. During the solidification of the metal, the solubility of nitrogen drops sharply, and nitrogen will be precipitated to form nitrogen pores.When the nitrogen content in the molten iron is greater than 100ppm, nitrogen holes are often generated. The total nitrogen content of the molten metal before solidification = the total nitrogen content of the molten metal itself before pouring + the amount of nitrogen that the resin sand decomposes and invades the molten metal.The nitrogen content in the molten metal is mainly controlled by the raw material scrap steel. The sand-to-iron ratio is too high and the molding sand cannot be burned through. This is an important factor leading to the high residual N content of the reclaimed sand. The detection of the burning loss is to monitor the residue in the resin sand. The main measure of N quantity.The method to prevent nitrogen holes caused by the high nitrogen content of resin sand is to select low nitrogen resin.
Anyang Huatuo Metallurgy Co., Ltd has in the raw materials for casting for more than 20 years, always provide qualified carbon, can supply the various grade of carbon include the type with low content of nitrogen, if any interests welcome to consult.