15 Chapter 6: Heat Treating
OBJECTIVE
After completing this unit, you should be able to:
- Correctly harden a piece of tool steel and evaluate your work.
- Correctly temper the hardened piece of the tool steel and evaluate your work.
- Describe the proper heat treating procedures for other tool steels.
Safety
The following procedures are suggested for a safe heat treating operation.
1. Wear heat-resistant protective clothing, gloves, safety glasses, and a face shield to prevent exposure to hot oils, which can burn skin.
2. Before lighting the furnace, make sure that air switches, exhaust fans, automatic shut-off valves, and other safety precautions are in place.
3. Make sure that there is enough coolant for the job. Coolant will absorb heat given off by the metal as it is cooling, but if there is insufficient coolant, the metal will not cool at the optimal speed.
4. Make sure that there is sufficient ventilation in the quenching areas in order to maintain desired oil mist levels.
5. When lighting the furnace, obey the instructions that have been provided by the manufacturer.
6. During the process of lighting an oil or gas-fired furnace, do NOT stand directly in front of it.
7. Make sure that the quenching oil is not contaminated by water. Explosions can be results of moisture coming into contact with the quenching oil.
8. Before taking materials out of the liquid carburizing pot, make sure that the tongs are not wet and that they are the correct tongs for the job.
9. Make sure that an appropriate fungicide or bacterial inhibitor has been mixed into the quenching liquid.
10. When quench tanks are not being used, always cover them.
11. Use a nonflammable absorbent to clean leaks and oil spills. This should be done immediately.
12. If possible, keep tools, baskets, jigs, and work areas free from oil contamination.
13. Before breaks and before moving on to the next task, wash your hands thoroughly.
14. If any skin trouble is shown or suspected, report to your instructor and get medical help.
15. Fumes from the molten carburizing salt bath should not be inhaled, because carbon monoxide is a product of the carburizing process.
16. Make sure there is good ventilation in the work area.
17. Be on the lookout for contamination from pieces of carburized metal.
18. Do not take oil-soaked clothes or equipment to areas where there are food or beverages.
19. Do not take food or beverages where oils are either being used or stored.
Procedure
The first important thing to know when heat treating a steel is its hardening temperature. Many steels, especially the common tool steels, have a well established temperature range for hardening. O-1 happens to have a hardening temperature of 1450 – 1500 degrees Fahrenheit.
To begin the process:
1. Safety first. Heat treating temperatures are very hot. Dress properly for the job and keep the area around the furnace clean so that there is no risk of slipping or stumbling. Also, preheat the tongs before grasping the heated sample part.
2. Preheat the furnace to 1200 degrees Fahrenheit.
3. When the furnace has reached 1200 degrees Fahrenheit, place the sample part into the furnace. Place the sample part into the center of the oven to help ensure even heating. Close and wait.
4. Once the sample part is placed in the furnace, heat it to 1500 degrees Fahrenheit. Upon reaching this temperature, immediately begin timing the soak for 15 minutes to an hour (soak times will very depending on steel thickness).
Table 1: Approximate Soaking Time for Hardening, Annealing and Normalizing Steel
|
Thickness Of Metal (inches) |
Time of heating to required Temperature (hr) |
Soaking time (hr) |
|
up to 1/8 |
.06 to .12 |
.12 to .25 |
|
1/8 to 1/4 |
.12 to .25 |
.12 to .25 |
|
1/4 to 1/2 |
.25 to .50 |
.25 to .50 |
|
1/2 to 3/4 |
.50 to .75 |
.25 to .50 |
|
3/4 to 1 |
.75 to 1.25 |
.50 to .75 |
|
1 to 2 |
1.25 to 1.75 |
.50 to .75 |
|
2 to 3 |
1.75 to 2.25 |
.75to 1.0 |
|
3 to 4 |
2.25to 2.75 |
1 to 1.25 |
|
4 to 5 |
2.75 to 3.50 |
1 to 1.25 |
|
5 to 8 |
3.50 to 3.75 |
1 to 1.50 |
Soak time is the amount of time the steel is held at the desired temperature, which is in this case 1500 degrees Fahrenheit.
5. When the soak time is complete, very quickly but carefully take the sample out with tongs. Place the sample part into a tank of oil for quenching. Move the sample part around as much as possible while it is quenching.
6. Once the sample part has been quenched down to around 125 degrees Fahrenheit, begin the tempering process. To temper the sample part it must be placed into the furnace at 375 degrees Fahrenheit. Allow it to soak for 2 hours, then remove the sample part and allow it to cool to room temperature. The sample part should now be approximately at a hardness of 60 RC.
Austenitize and Air-Cool:
- This heat treatment is usually done by the manufacturer, which results in it being called the as-received condition
- To reach this state, a process called normalizing (also called the thermal history) is done. Normalizing 1045 steel usually consists of these steps:
- Austenitize: Place the steel in the furnace at 1562°F in the austenite range, and keep it there for an hour until the metal has reached its equilibrium temperature and corresponding solid solution structure.
- Air-cool: Take the steel out of the furnace and let it air-cool to room temperature.
Austenitize and Furnace-Cool (Annealing):
- This process is also referred to as annealing. During annealing, the steel goes through the following temperature histories:
- Austenitize: Place the steel in the furnace at 1562°F in the austenite range, and keep it there for an hour until the metal has reached its equilibrium temperature and corresponding solid solution structure.
- Furnace-Cool: cool the steel slowly in the furnace. Allow the temperature to drop from 1562°F to 1292°F over a ten hour period.
- Air-cool: Take the steel out of the furnace and let it air-cool to room temperature.
Austenitize and Quench:
- Austenitize: Place the steel in the furnace at 1562°F in the austenite range, and keep it there for an hour until the metal has reached its equilibrium temperature and corresponding solid solution structure.
- Quench: quickly remove the steel from the furnace, plunge it into a large container of water at room temperature, and stir vigorously. When using 1045 steel, room temperature water is used as the quenching medium.
Quench: Rapidly remove material from furnace, plunge it into a large reservoir of water at ambient temperature, and stir vigorously.
For 1045 steel, the quenching medium is water at room temperature (for other steels, other quenching media such as oil or brine are used).
4. Austenitize, Quench, and Temper:
- Austenitize: Place the steel in the furnace at 1562°F in the austenite range, and keep it there for an hour until the metal has reached its equilibrium temperature.
- Quench: Quickly remove the steel from the furnace, plunge it into a large container of water at room temperature, and stir vigorously.
- Temper:
- Bring the steel to the tempering temperature and hold it there for about 2 hours.
- There is a range of different tempering temperatures. For 1045 steel the range is from 392 to 932°F.
- The different temperatures lead to differences in mechanical properties.
- Lower temperatures give higher yield strength but lower toughness and ductility.
- Higher temperatures give lower strength but increase toughness and ductility.
- Air-cool: Take the steel out of the furnace and let it air-cool to room temperature.