When the casting is complex in shape and uneven in thickness, due to the different cooling rates of different parts in the cooling process after pouring, great stress often occurs in the casting. It not only weakens the strength of the casting, but also leads to deformation and even cracking due to the redistribution of stress after subsequent cutting. Therefore, large and complex high-precision castings (such as machine tool beds, racks, etc.) should be annealed before cutting and sometimes even after rough machining to eliminate internal stress. ?

Annealing to eliminate internal stress is usually to slowly heat the casting to 500-560℃, keep the temperature for a period of time (keep the temperature for one hour every 10 mm section), and then cool it to 150-200℃ with the furnace at a very slow speed and take it out. At this time, the internal stress of the casting is basically eliminated. ?

It should be pointed out that annealing temperature over 560℃ or holding time is too long, which will cause graphitization and reduce the strength and hardness of castings, which is not suitable. ?

2. Softening and annealing to eliminate some white spots;

When the casting solidifies, due to the rapid cooling rate, white structure is easy to appear on the surface or some thin-walled parts, which increases the hardness and brittleness of the casting, and it is not easy to cut and peel off when it is used. At this time, the casting must be heated above the precipitation temperature, softened and annealed to eliminate the white mouth. ?

Generally, the softening annealing to eliminate the blank is to heat the casting to 850-950℃ and keep the temperature for 65,438 0-3 hours, so as to decompose the * * * crystal cementite, that is, the first stage graphitization, and then decompose the second stage cementite and the cementite precipitated by * * * in the process of slow cooling with the furnace, that is, the second stage graphitization, and wait for slow cooling with the furnace to 5000. If the casting is not graphitized at a faster cooling rate in the second stage, gray cast iron with pearlite matrix will eventually be obtained, thus improving the strength and wear resistance of the casting. ?

Thirdly, surface quenching:

The purpose of surface quenching is to improve the surface hardness and wear resistance of gray cast iron. The methods of surface quenching include high frequency induction heating surface quenching, flame heating surface quenching and contact electric heating surface quenching.

Gray iron is cast iron obtained by complete graphitization in the first stage. All or most of the carbon exists in the form of flake graphite, and the fracture is gray, hence the name. Include general gray cast iron, nodular cast iron, vermicular cast iron, inoculation cast iron and rare earth gray cast iron.

Gray cast iron is called gray cast iron because its fracture is light gray. This product is cheap and widely used. Gray cast iron accounts for more than 80% of the total output of cast iron.