First, the purpose of the experiment
1. Understand the basic heat treatment process of carbon steel
2. Study the relationship between cooling conditions and steel properties
3. Analysis of the effect of quenching and tempering temperature on the properties of steel
Second, experimental equipment and test pieces
1. Experimental equipment: SX-10M-2.5 box test resistance furnace
2. Specimen: 45 pieces of steel, 30 steel and T8 steel
3. Three pieces of specimens after 45 steel quenching
Third, the experimental principle
Heat treatment is an important metal processing method with the purpose of improving the properties of the steel (useability and process performance). The heat treatment process of steel is characterized in that the steel is heated to a certain temperature, held for a certain period of time, and then cooled at a certain cooling rate. This process changes the properties of the steel.
Fourth, the experiment content and steps
(I) Quenching heat treatment of steel
Quenching heat treatment is to heat carbon steel to AC3 or above AC1 30-50 °C, after insulation, put into a different cooling medium for rapid cooling (cooling speed is greater than the critical cooling rate) to get the martensite structure (M). The quenched structure is martensite and retained austenite.
1. Determination of quenching temperature
According to different materials, in Table 1 its critical temperature AC3 or AC1, then add 40 °C, you can get its heating temperature.
Subeutectoid steel (45 steel, 30 steel):
Heating temperature = AC3 + 40°C
Hypereutectoid steel (T10 steel):
Heating temperature = AC1 + 40°C
So the final 30 steel heating temperature = °C + 40 °C =
45 steel heating temperature = °C + 40 °C =
45 steel heating temperature = °C + 40 °C =
2. Determination of holding time
After the part is heated with the furnace to reach the required heating temperature, it must be kept warm for a period of time to ensure that the entire part reaches the required temperature evenly and sufficiently. Obviously, the holding time is related to the size and shape of the workpiece.
By measuring the size of the part and looking up table 2, calculate the holding time of the test piece.
The dimensions of the parts are cylindrical parts with a diameter of 20 mm, so the holding times of 30 steel, 45 steel, and T10 steel are:
3. The choice of cooling medium
Cooling is the key process for quenching. It directly affects the properties of the quenched steel. The quenched cooling rate is greater than the critical cooling rate to obtain supercooled martensite structure. At the same time, during the cooling process, the internal stress in the crystallization process must be controlled to prevent deformation and cracking.
In order to ensure the quenching effect, a suitable cooling medium and cooling method should be selected. In this experiment we chose room temperature water as the cooling medium.
4. Place the workpiece in the furnace, set the heating temperature of the furnace temperature controller, and start heating.
5. After the furnace has reached the set temperature, it starts timing the insulation.
6. The workpiece is released and quickly placed in water for cooling.
(b) Tempering heat treatment of steel
The martensite structure obtained after quenching the steel is hard and brittle, and there is a large internal stress inside the workpiece. The purpose of tempering is to eliminate internal stress, reduce hardness, and improve processing performance. According to different process requirements, tempering is divided into high temperature tempering, medium temperature tempering and low temperature tempering three kinds of process methods, its temperature selection and organizational performance changes are shown in Table 3.
The tempering cooling method is air cooling, that is, the workpiece is slowly cooled at room temperature after it is released.
1. Put the workpiece into the furnace, set the temperature control of the electric furnace temperature controller and start heating;
2. After the electric furnace reaches the set temperature, the time for starting insulation is started, and the holding time is 30 minutes;
3. The workpiece is baked and slowly cooled at room temperature.
