1、 General overheating
The austenitic grain coarsening caused by too high heating temperature or long holding time at high temperature is called overheating. The coarse austenite grain can reduce the strength and toughness of the steel, increase the brittle transition temperature and increase the deformation cracking tendency during quenching. The reason of overheating is that the furnace temperature instrument is out of control or mixed materials (the overheated structure often occurs when the process is not understood. After annealing, normalizing or repeated high-temperature tempering, the grains can be refined by re austenitizing under normal conditions.
2、 Fracture heredity
After reheating and quenching, austenite grains can be refined, but sometimes coarse granular fracture appears. It is generally believed that the impurities such as MNS dissolved into austenite and enriched in the crystal interface due to the high heating temperature. However, these inclusions will precipitate along the grain boundary during cooling, which is easy to fracture along the coarse austenite grain boundary when impacted.
3、 Inheritance of coarse tissue
When steel parts with coarse martensite, bainite and widmanstatten structure are re austenitized, the austenite grains are still coarse when they are slowly heated to the conventional quenching temperature or even lower. This phenomenon is called structural heredity. In order to eliminate the heredity of coarse structure, intermediate annealing or multiple high temperature tempering can be used.
How to control the temperature of high temperature heat treatment of mesh belt furnace:
For rolled or extruded products, metal stress can be relieved by stretching after quenching; for forgings, metal stress can be relieved by stretching or extrusion after quenching.
When the difference between the temperature ranges exceeds 10 ℃, any 10 ℃ temperature range (15 ℃ for 6061) can be used in the whole temperature range, as long as there are no exceptions or restrictions specified in the table or in the applicable data specification.
Cold working after solution heat treatment and before aging.
The low temperature of 482 ℃ can be used, as long as each heat treatment batch can meet the requirements of the applicable data specification after testing, and the test data analysis proves that the data meet the specified scope of the specification and have repeatability.
The temperature of 6061 aluminum clad plate should not exceed 538 ℃.
Other temperature ranges may be used for specific sections, conditions and requirements.
It is necessary to realize that under some conditions, the 7075 alloy will melt when heated above 482 ℃, and measures should be taken to prevent such problems. In order to reduce the diffusion between aluminum clad layer and matrix, 7075 with thickness less than or equal to 0.5 mm should be solution heat treated in the range of 454 ℃ ~ 499 ℃.
On 2018-08-08 11:45:46
