Three phenomena of multi-purpose furnace heat treatment heating temperature
1 General overheating: The heat treatment heating temperature is too high or the holding time is too long at high temperature, which causes the austenite grain coarsening to be called overheating. Coarse austenite grains will reduce the strength and toughness of the steel, increase the brittle transition temperature, and increase the deformation and cracking tendency during quenching. The cause of overheating is out of control or mixing of the furnace temperature instrument (often caused by the process). The superheated structure can be re-austenitized under normal circumstances to refine the grain after annealing, normalizing or multiple high-temperature tempering.
2 Fracture inheritance: After heat-treating steel with overheated structure, after reheating and quenching, although austenite grains can be refined, sometimes coarse grain fractures still appear. The theory of fracture genetics is controversial. It is generally believed that due to excessive heating temperature, impurities such as MnS have dissolved into austenite and enriched at the crystal interface, and these inclusions will precipitate along the crystal interface when cooling. When impacted, it is easy to fracture along coarse austenite grain boundaries.
3 Genetics of coarse structure: When austenitizing steel parts with coarse martensite, bainite, and weinite structure are re-austenitized, they are slowly heated to the conventional quenching temperature, and even lower, the austenite grains Still coarse, this phenomenon is called tissue heredity. To eliminate the heritability of coarse tissue, intermediate annealing or multiple high-temperature tempering can be used.
The furnace type should be determined according to different process requirements and the type of workpiece
1. For those that cannot be produced in batches, the sizes of the workpieces are not equal, and there are many types that require versatility and versatility in the process, and a box furnace can be selected.
2. When heating long shafts, long screw rods, pipes and other workpieces, a deep well electric furnace can be selected.
3. For small quantities of carburizing parts, a well gas carburizing furnace can be selected.
4. For the production of large quantities of automobile, tractor gear and other parts, a continuous carburizing production line or a box-type multi-purpose furnace can be selected.
5. For mass production of sheet metal blanks for stamping parts, rolling furnaces and roller hearth furnaces are preferred.
6. For batches of shaped parts, a pusher type or conveyor resistance furnace (pusher furnace or cast furnace) can be selected for production.
7, small mechanical parts such as: screws, nuts, etc. can choose to use the bottom furnace or mesh belt furnace.
8. Steel ball and roller heat treatment furnaces can use the inner spiral rotary tube furnace.
9, non-ferrous metal ingots can be used in large-scale production of pusher furnace, and non-ferrous metal parts and materials can be used air heating furnace.