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Casting Internal Stress Heat Treatment
Oct 16, 2017

Casting internal stress heat treatment
I. Heat treatment of HT:
Can not change the shape of graphite and eliminate the harmful effects of flake graphite, only to eliminate Casting Casting stress, stable size. Eliminate the white tissue to reduce the hardness to improve its processing performance, increase the surface hardness and wear resistance.
1. Aging treatment: the shape of complex Castings due to the uniformity of the wall thickness of the foundry in the Casting process to produce internal stress Casting deformation and cracking, aging treatment is to eliminate this stress. Aging treatment of natural aging and artificial aging. Natural aging is the Castings open for several months or more months or even longer, so that the natural slow deformation of the Casting to eliminate stress, this method has a long production cycle, eliminating the stress is not complete, has been less used. (60 ~ 100 ℃ / h) heated to 520-550 ℃, after a period of time with the furnace with a slow speed (20 ~ 30 ℃ / h) cooling to 150 (2 ~ 30 ℃ / h) -200 ℃, baked air cooling, when the Casting stress is basically eliminated, if the heating is too high (more than 560 ℃) or holding time is too long, but the pearlite decomposition resulting in Casting strength and hardness reduction.
2. Graphite Annealing: Casting cooling solidification in the surface or some of the thin section, due to the rapid cooling of the white hair tissue, so that the hardness and brittleness of the Casting increased, not easy cutting, the processing technology: the Casting Heated to 900 ~ 960 ℃ insulation 1-4h, and then with the furnace cooling. Elimination of white mouth tissue is mainly solved by Casting process.
Second, QT heat treatment: through heat treatment can greatly adjust and improve the performance of QT to meet the different requirements. Commonly used heat treatment processes are: annealing, normalizing and isothermal quenching.
1. Annealing: divided into the elimination of Casting stress annealing, temperature annealing and high temperature annealing.
a. Elimination of stress Annealing: QT stress is 1-2 times larger than HT, for other heat treatment is no longer the iron nodules are often required to eliminate stress annealing
b. low temperature annealing: the purpose is to make the Casting of pearlite in the FeC graphitization to obtain ferrite sphere, improve the plasticity and toughness. The process is to heat the Casting to 720-760 ° C. Insulation for some time after the furnace cooled to 600 ℃ baked air cooled.
c. high temperature annealing: due to the large white ball tendons, and thus in the Casting organization is often free cementite in order to make free cementite decomposition (to eliminate white) for high temperature annealing.
Normalizing
a. Austenitic normalizing purpose is to obtain pearlitic ductile iron, such as QT700-2, QT600-3 as-cast structure without cementite as the process: Casting 870 ~ 940 ℃ 1-3h, and then baked air-cooled. As the ductile iron has a greater internal stress, some factories are also used after the high temperature tempering tempering. Aspergillus organization cementite volume fraction ≥ 3% when the process for the Casting 950 ~ 980 ℃ 2-3h860 ~ 880 ℃ 1-2h baked air cooled.
b. Partial austenitizing normalizing: the use of lower heating temperature of the internal organization of the reverse part of the austenite liquefaction. After the normalization of the organization still retain part of the ferrite, thereby enhancing the plasticity and toughness, but the strength is slightly lower than the high temperature normalizing. No cementite process: Casting 880 ~ 900 ℃ 1-4h, baked air-cooled. Cementite volume fraction ≥ 3%, Casting 920 ~ 980 ℃ 2-3h820 ~ 880 ℃ 1-2h baked air-cooled.
3. Isothermal Quenching: After quenching is made of bainite and austenite, this organization has a high overall mechanical properties (higher strength and toughness) and thus has a good wear resistance, the process is: Casting 860 ~ 920 ℃ for a period of time to be fully austenitized immediately into the temperature of 250 ~ 350 ℃ salt bath furnace isothermal 0.5-1.5h. Then remove the air cooling.
Heat treatment of Castings: Castings are generally heat treatment, the purpose is to refine the grain, to eliminate the Weishi organization and Casting stress, carbon steel heat treatment methods are fully annealed, normalizing and tempering. Alloy elements have improved the role of hardenability, so low-alloy Castings are mainly quenching plus tempering or normalizing and tempering.
1. Complete annealing: the Casting heated to the austenitic temperature (above the critical temperature above 30 ~ 50 ℃) and insulation for some time (according to the Casting material and wall thickness to determine), with the furnace cooled to 200 ~ 300 ℃ baked air, 600 ℃ 2h830 ~ 850 ℃ 3-4h200 ~ 300 ℃, baked air-cooled. (Pay attention to start heating speed is slow, the speed of 100 ℃ / h heating to 600 ~ 650 ℃ and then a little warmer temperature)
2. Normalizing: Casting 830 ~ 900 ℃ 2-3h, baked air cooled
3. Normalizing + tempering: Casting 600 ℃ 2h830 ~ 850 ℃ 3-4h400 ~ 450 ℃ 500 ~ 600 ℃ 200 ~ 300 ℃, baked air cooled.