How to solve the problem of gray iron castings that are not hard

The hardness of gray iron castings is directly related to the grade, such as the user unit requires the material to be HT200 material gray iron castings, in the casting according to the material grade accurate batching, the casting hardness should meet the HT200 material casting hardness requirements 163-255HBS, the test results are qualified within the standard floating range, if the casting hardness meets the material requirements, but can not meet the use requirements can be used to the surface of the casting to carry out nitriding treatment or quenching treatment and other processes to solve the problem of insufficient hardness.

The common solutions for gray iron castings are not hard enough:

1. Flame quenching (suitable for single pieces or large workpieces that need to increase local hardness)

Use a baking gun (with oxygen plus acetylene) to heat the surface temperature of the gray iron casting workpiece to 900-1000°C, then quickly cool it down with cold water or put the workpiece into the quenching tank to cool. The quenching depth is about 2-6 mm, and the hardness is about 40-48HRC. This method will cause local deformation of large workpieces, and after quenching, the grinding machine can be used to grind to restore the original machining accuracy due to the increase in hardness. The disadvantage of this method is that the temperature is difficult to control and requires experienced personnel to operate.

2. Intermediate frequency induction quenching: The common frequency of intermediate frequency quenching of gray iron castings is 2500-8000Hz, and the surface of the workpiece is quickly heated to the quenching temperature and then sprayed with water or put into the quenching tank for cooling, the thickness of the quenching layer is about 3-5 mm, and the hardness is > 50HRC. This quenching process is suitable for large, medium and small parts, by adjusting the anode current, frequency and heating time, the quenching temperature and depth can be controlled, the advantage is that the quenching quality is stable, the workpiece deformation is small.

3. High-frequency induction quenching: This quenching process is suitable for small workpieces such as wrenches, sockets, gears and machine tool guide rails to improve the hardness. The frequency of use during operation is 200-300KHz, the heating speed is 200-1000°C per hour, the penetration layer is relatively shallow, the quenching layer is about 1-2 mm, the higher the quenching temperature, the higher the hardness. The advantages are stable quenching quality, less oxidation and decarburization, and small deformation of the workpiece.

4. Electrical contact self-cooling quenching, the principle is that the electrode is in contact with the surface of the workpiece, the contact is heated to the austenitic temperature with low voltage and high current, and then the electrode is moved to the entire quenching area in order, and the area cools naturally after heating. The quenching depth is about 0.2-0.3 mm, and the hardness is more than 50HRC.

The user unit or gray iron casting production unit can choose the above quenching methods to improve the hardness of gray iron castings according to the actual size of the casting and the actual situation of the need to improve the hardness.