the heat treatment of high chromium cast iron

the heat treatment of high chromium cast iron
1 annealing
Because of the high hardness of the high chromium products as cast, in order to improve the machining performance of the workpiece, all the blank must be necessary to soften the annealing process.
Specific process (see Figure 1, with a wall thickness of not more than 100mm and the shape of the complex castings as an example) as follows.
The first will be the workpiece at room temperature under the heat treatment furnace, and furnace with slow heating to 400 DEG C temperature 1 ~ 2H, then the furnace temperature rise to 600 DEG C and insulation of 1 ~ 2H, to be not more than 150 DEG C / h temperature rise speed, the temperature quickly rose to 950 degrees after 2 ~ 3H insulation, then stop heating, the temperature dropped to 820 degrees Celsius natural, then can control the furnace at 10 to 15 DEG C / h temperature drop speed temperature will drop to 700 ~ 720 DEG C, and the temperature of 4 ~ 6h (the more thick soaking time should be longer) after shutdown, the workpiece depending on the circumstances with furnace cooling or baked in still air cooling to room temperature (to meet the performance requirements, pearlite, easy machining).
After the specific production, if the shape of the workpiece is simple, also can use a more rapid annealing process, namely in the temperature rise to 950 DEG C and insulation 3h after shutdown, can be around with the furnace cooling to 400 DEG C, then open the door, to cooling it to 300 DEG C, the workpiece can be released air cooling.
Annealing of the workpiece after machining, because of the high chromium white cast iron in the quenching process of size change is smaller than that of the gray cast iron and cast steel more and generally is not correct size, for according to the technological requirements of grinding of the workpiece to be leaving the amount of grinding can also be very small.
2 quenching
Thicker workpiece processing machinery will be the room temperature pack furnace, to less than 80 deg / h of temperature rising speed of the furnace temperature rise to 600 DEG C (if the work piece or shape is more complex, the temperature rose to 300 DEG C and 400 DEG C, at a temperature of 500 DEG C and 600 DEG C were given 0.5 h insulation), with no more than 150 DEG C / h of temperature rising speed of the furnace temperature rise to the quenching temperature of 950 to 980 DEG C after thermal insulation and heat preservation time is 2 ~ 4 h (depending on the thickness of workpiece of different holding time vary, the more thick heat preservation time is long), and then the workpiece is fast released empty cold, if the ambient temperature is high, the quenching should be supplemented by strong winds and water mist spray to enhanced cooling and quenching process curve as shown in Figure 2 shows.
3 temper
In order to reduce the casting residual stress and brittle, and keep the quenching to get high hardness and wear resistance, and the martensite tempering, and retained austenite decreases, deal with after quenching the workpiece to 230 to 260 DEG C tempering. Specific process: will work at room temperature download furnace, heated to 230 to 260 DEG C, insulation within 3 ~ 6 hours, then released air cooling.
Four, the performance of high chromium alloy cast iron
(1) the hardness properties in the as cast for 50 ~ 54hrc, after annealing for 38 to 42 HRC, after quenching for 60 ~ 67HRC. hardness with carbon content variation and change, low carbon good toughness and low hardness for situations in which impact load is larger; carbon and high hardness is slightly higher, is indicated for hammer load smaller occasions, showed good wear resistance.
(2) the density of high chromium white cast iron is 7.6 ~ 7 7g / cm3.
(3) the linear shrinkage rate is 1.8% ~ 2%, the volume shrinkage rate is 8% ~ 7.5%, and the fluidity is 300 ~ 500mm. at 1400.
Five, conclusion
(1) according to the performance characteristics of high chromium cast iron, the smelting process is reasonably set up to meet the requirements of the matrix organization.
(2) heat treatment is a necessary means to obtain qualified high chromium cast iron, so it is very important to develop a reasonable heat treatment process.