손진호
2008-11-25 08:19:52
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열간 단조후 제품을 열처리(노말라이징)을 하려고 하는데 보통 단조후
상온까지 식은 제품을 재가열하여 열처리하는데 단조후 상온까지 식지
않은 제품을 열처리를 하려면 최소 몇도 정도 까지 식힌다음 해야하는지요? (0.2%C의 탄소강)
상온까지 식은 제품을 재가열하여 열처리하는데 단조후 상온까지 식지
않은 제품을 열처리를 하려면 최소 몇도 정도 까지 식힌다음 해야하는지요? (0.2%C의 탄소강)
The heat-treating process after hot forging normally depends on the required properties and metallurgy per material (e.g., strength, toughness, flow pattern, flake, mass effect, internal stress, etc.).
In order to achieve the desired mechanical properties and to relieve any internal stresses.
(1) Slow cooling in air may normalize workpieces, or followed by normalizing or normalizing-tempering, or
(2) Quenched in oil and then tempered or reheated.
Good forging practice makes it possible to control the flow pattern resulting in maximum strength of the material and the least chances of fatigue failure. These characteristics of forging, as well as fewer flaws and hidden defects, make it more desirable than some other operations (i.e. casting) for products that will undergo high stresses.
Cooling in still air or in factory tote boxes is common practice and is usually satisfactory for carbon steel or low-alloy steel forgings when cross sections are no greater than approximately 64 mm (2.5 in.). Flaking may occur on larger forgings when they are air cooled. Flakes (such as, shatter cracks or snowflakes) are short, discontinuous internal fissures attributed to stresses produced by localized transformation and decreased solubility of hydrogen during cooling. In a fractured surface, flakes appear as bright silvery areas; on an etched surface, they appear as short cracks. Flaking indicates the need for cooling to at least 175 °C (350 °F) in a furnace or cooling by burying the piece in sand or slag. An alternative method of treating large forgings made of alloy steels such as 4340 consists of cooling in air to about 540 °C (1000 °F), followed by isothermal annealing at 650 °C (1200 °F). Forgings of alloy tool steel should always be cooled slowly, as is recommended above for larger forgings of carbon and alloy steels.
For more detail, please find Metal Handbook Vol.14, ASM.
In order to achieve the desired mechanical properties and to relieve any internal stresses.
(1) Slow cooling in air may normalize workpieces, or followed by normalizing or normalizing-tempering, or
(2) Quenched in oil and then tempered or reheated.
Good forging practice makes it possible to control the flow pattern resulting in maximum strength of the material and the least chances of fatigue failure. These characteristics of forging, as well as fewer flaws and hidden defects, make it more desirable than some other operations (i.e. casting) for products that will undergo high stresses.
Cooling in still air or in factory tote boxes is common practice and is usually satisfactory for carbon steel or low-alloy steel forgings when cross sections are no greater than approximately 64 mm (2.5 in.). Flaking may occur on larger forgings when they are air cooled. Flakes (such as, shatter cracks or snowflakes) are short, discontinuous internal fissures attributed to stresses produced by localized transformation and decreased solubility of hydrogen during cooling. In a fractured surface, flakes appear as bright silvery areas; on an etched surface, they appear as short cracks. Flaking indicates the need for cooling to at least 175 °C (350 °F) in a furnace or cooling by burying the piece in sand or slag. An alternative method of treating large forgings made of alloy steels such as 4340 consists of cooling in air to about 540 °C (1000 °F), followed by isothermal annealing at 650 °C (1200 °F). Forgings of alloy tool steel should always be cooled slowly, as is recommended above for larger forgings of carbon and alloy steels.
For more detail, please find Metal Handbook Vol.14, ASM.
공지사항
Normalizing은 변태점 이상의 온도까지 올려서 강재를 Normal(표준) 상태로 돌려 놓는 것이지요
즉, 이전 과정에서 성형, 열처리, 기계가공, 용접등으로 인해 변형되고 외곡된 조직을 표준(Virgin) 상태로 돌려 놓는 것입니다.
그렇다면 그 전에 어떠한 이력을 가졌는 가는 전혀 문제가 되지 않겠지요
안정적으로 다시 Normalizing 온도까지 올려서 조직을 충분히 표준 상태로 돌려 놓고 다시 너무 빠르거나, (일부 강종에 있어서) 너무 늦지만 않도록 냉각하면 되겠습니다.
시간 절약을 위해서라면 굳이 다 식을때까지 기다리지 않아도 되겠습니다.