What is White Cast Iron | Heat Treatment of White Cast Iron

White cast iron is a great material to make the blade of shot blasting machines and parts of mud pumps. What are the features of white cast iron and what heat treatments are commonly applied to it?

What is White Cast Iron?

The section of white cast iron is bright white and exists in iron in the form of cementite. White cast iron is hard and brittle, not easy to cut, and rarely used directly for casting parts. Generally, the carbon content of white cast iron is about 2.5%, of which the silicon content is below 1%. No graphite precipitation, except a small amount of carbon dissolved in ferrite, most of it exists in cast iron in the form of cementite.

Heat Treatment of White Cast Iron

The hardness of white cast iron is high, it is very brittle, so it is rarely used to make machine parts directly. Generally speaking, it is very important to select a reasonable heat treatment process according to the chemical composition and working conditions of cast iron. The commonly used heat treatment processes are stress relief annealing, softening annealing or normalizing, quenching, and tempering. In addition, most white cast iron parts are malleable iron billets, which need to be graphitized to decompose most or all of the cementite to form flocculent or spherical graphite, converting white cast iron into malleable cast iron.

1) Stress relief annealing

It is mainly used for high silicon and high chromium alloy white cast iron. Because of the large internal stress in casting, parts will crack during vibration or environmental change if the residual internal stress is not removed in time. Generally, the annealing heating of iron castings is relatively low, while for high alloy white iron castings, the structure of white iron is relatively stable, so the annealing temperature is relatively high, and the process specification is 800~900 ℃ for 1 to 4 hours, after the heat preservation is completed, the furnace shall be cooled to 100~150 ℃ and discharged for air cooling.

2) Soft annealing

For some alloy white iron castings with higher hardenability, there is a large amount of martensite in the cast iron, but there is less cementite, and the hardness is very high. Parts made of such materials need to be machined, so the softening annealing process must be used to transform the martensite into pearlite so that the hardness of the cast iron can be reduced to the extent that it can be machined.

3) Quenching and tempering

It is mainly used for low silicon, low phosphorus, and sulfur wear-resistant white iron castings with low carbon content. Because of its high alloy element content, carbides are difficult to dissolve in austenite. Quenching treatment (heating temperature of 950-1000 ℃) can dissolve more carbides in austenite, and obtain martensite structure with higher hardness after cooling, thus ensuring good wear resistance of iron castings. In order to reduce internal stress and avoid cracking, it can also be quenched in stages with nitrate, and finally tempered at 180-200 ℃ for 1-3h.

4) Isothermal quenching

The purpose of quenching is to obtain a certain amount of bainite to ensure good comprehensive mechanical properties. The carbon content of white cast iron is generally 2.2%~2.5%, and the sulfur and phosphorus content should be less than 0.1%. After the heat preservation is completed at 890~910 ℃ austenite temperature, the required mechanical properties can be obtained by holding the temperature for 90 min in 280~300 ℃ nitrate solution and air cooling out of the furnace.