Selection of furnace type
1. What determines the furnace type?
1. For workpieces that cannot be produced in batches, have unequal sizes, and have many types, and require universality and versatility in technology, a box furnace can be selected.
2. When heating long shafts, long screws, pipes and other workpieces, a deep well electric furnace can be selected.
3. For small batches of carburized parts, a well gas carburizing furnace can be selected.
4. For large-scale production of automobile and tractor gear parts, a continuous carburizing production line or a box multi-purpose furnace can be selected.
5. For mass production of stamping parts, it is best to use a rolling furnace or a roller hearth furnace.
6. For batches of fixed parts, a push rod or conveyor belt resistance furnace (push rod furnace or casting belt furnace) can be selected in production.
7. Small mechanical parts such as screws, nuts, etc. can be selected with a vibrating bottom furnace or a mesh belt furnace.
8. The heat treatment of steel balls and rollers can be carried out in a rotary tube furnace with an internal spiral.
9. Pusher furnaces can be used for large-scale production of non-ferrous metal ingots, while air circulation heating furnaces can be used for small non-ferrous metal parts and materials.
Heating defects and control
2. What is overheating?
We know that overheating during heat treatment is most likely to cause the coarsening of austenite grains, which will reduce the mechanical properties of parts.
1. General overheating: Overheating is caused by excessively high heating temperature or too long holding time at high temperature, which causes the coarsening of austenite grains. Coarse austenite grains will reduce the strength and toughness of steel, increase the brittle transition temperature, and increase the tendency of deformation and cracking during quenching. The cause of overheating is the loss of control of the furnace temperature instrument or mixing of materials (often caused by ignorance of the process). Overheated tissue can be re-austenitized under normal circumstances to refine the grains after annealing, normalizing or multiple high-temperature tempering.
2. Fracture inheritance: For steel with overheated tissue, although the austenite grains can be refined after reheating and quenching, coarse granular fractures sometimes still appear. There are many theoretical controversies about the generation of fracture inheritance. It is generally believed that the impurities such as MnS were dissolved into austenite and enriched at the grain interface due to the excessive heating temperature. When cooling, these inclusions will precipitate along the grain interface and easily break along the coarse austenite grain boundary when impacted.
3. Inheritance of coarse structure: When steel parts with coarse martensite, bainite, and widmanstattenite structures are re-austenitized, they are slowly heated to the conventional quenching temperature, or even lower, and their austenite grains are still coarse. This phenomenon is called tissue inheritance. To eliminate the inheritance of coarse structure, intermediate annealing or multiple high-temperature tempering treatments can be used.
3. What is overburning?
Excessive heating temperature not only causes coarse austenite grains, but also local oxidation or melting of grain boundaries, resulting in weakening of grain boundaries, which is called overburning. After overburning, the performance of steel deteriorates seriously, and cracks are formed during quenching. Overburned structures cannot be restored and can only be scrapped. Therefore, overburning should be avoided during work.
4. What are decarburization and oxidation?
When steel is heated, the carbon on the surface reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere), reducing the surface carbon concentration, which is called decarburization. After quenching, the surface hardness, fatigue strength and wear resistance of decarburized steel are reduced, and residual tensile stress is formed on the surface, which is easy to form surface network cracks.
When heated, the iron and alloy elements on the surface of the steel react with oxygen, carbon dioxide, water vapor in the medium (or atmosphere) to form an oxide film, which is called oxidation. After high-temperature (generally above 570 degrees) workpieces are oxidized, the dimensional accuracy and surface brightness deteriorate, and steel parts with poor hardenability of oxide films are prone to quenching soft spots.
Measures to prevent oxidation and reduce decarburization include: coating the surface of the workpiece, heating with stainless steel foil packaging, heating in a salt bath furnace, heating in a protective atmosphere (such as purified inert gas, controlling the carbon potential in the furnace), and flame combustion furnace (making the furnace gas reducing)
5. What is hydrogen embrittlement?
The phenomenon of reduced plasticity and toughness of high-strength steel when heated in a hydrogen-rich atmosphere is called hydrogen embrittlement. Hydrogen embrittlement can also be eliminated by dehydrogenation treatment (such as tempering, aging, etc.) of workpieces with hydrogen embrittlement. Hydrogen embrittlement can be avoided by heating in a vacuum, low-hydrogen atmosphere or inert atmosphere.
