Annealing and positive heat treatment
(1) annealing and normalizing of steel
Annealing and normalizing are the preparation heat treatment processes widely used in production. For some machine parts with little stress and low performance requirements, they can also be used as final heat treatment.
Isothermal annealing will cool the austenitized steel to the pearlite to form temperature isothermal heat preservation, so that the super-cooled austenite will be transformed into the pearlite and air cooled to room temperature.
Spheroidizing annealing above the eutectoid carbon steel is heated to Ac1 20 ~ 30 ℃, insulation 2 ~ 4 h, the lamellar cementite does not completely dissolved broken into tiny chains or dotted, scattered on the austenitic matrix, in subsequent slow cooling process, or at the original tiny carburizing constitution as the core, or in the austenite zhongfu carbon produce new core area, form a uniform granular cementite.
Homogenizing annealing (diffusion annealing) the workpiece heated to a temperature of about 1100 ℃, insulation 10 ~ 15 h, along with the slow cooling furnace to 350 ℃, then empty out cold. After the workpiece is homogenized and annealed, the austenite grain is very thick, which must be completely annealed or normalized to refine the grain and eliminate the overheating defect.
The stress relieving the workpiece with slow furnace heating to 500 ~ 650 ℃, the heat preservation, along with the slow cooling furnace to 200 ℃ from air cooling. Mainly used to eliminate machining stress.
Recrystallization annealing heat the material above recrystallization temperature.
Complete annealing is used for casting and forging of carbon steel and alloy steel. Isothermal annealing is used in austenite stable alloy steels. Spheroidizing annealing is used for eutectoid steel, hypereutectoid steel and alloy tool steel. Uniform annealing is used for casting ingots of high quality high alloy steels with high quality requirements and for alloy steels with severe segregation of components; De-stressed annealing is used for casting, forging, welding, cold stamping and machining parts. Recrystallization annealing is mainly used to remove process hardening.
The eutectoid carbon steel is heated to above Ac3 30 ~ 50 ℃, eutectoid carbon steel is heated to Accm above 30 ~ 50 ℃, thermal insulation, air cooling method called normalizing. It is suitable for carbon steel and medium and low alloy steel, because the austenite of high alloy steel is very stable, and martensite structure can be obtained even when it is cooled in air. For low carbon steel, low carbon and low alloy steel, grain refinement, hardness improvement (140 ~ 190HBS) and machining performance improvement. For hypereutectoid steel, the elimination of secondary mesh carburizing is beneficial for spheroidizing annealing.
(2) residual stress annealing
Always will inevitably have a general machinery products in machining surface residual stress exists, if the products without the proper stress annealing treatment, the improper exposure to heat source (such as sunlight, heat engine >, will produce the phenomenon of deformation, the other by the residual stress in the common sense highly concentrated in a particular local area, such as surface, welding area and so on, so will reduce local products of mechanical strength. To avoid these problems, we must use residual stress annealing treatment.
This processing is to put the products slowly and evenly heated to a temperature below the change point of, then as the temperature for a period of time, in a slow and uniform gradually cooled down, is the most important thing in the process must keep the products in the same area of cooling speed, otherwise after cooling, due to differences in district cooling rate, can cause the emergence of the residual stress again. This is especially true for products with complex shapes.
Because a stress annealing takes advantage of the slight creep of atoms at high temperatures to rearrange the atoms' positions to eliminate the stress. Therefore, the annealing temperature of the tensile stress varies with the creep ability of the material. Common materials for resistance to creep. Such as the low alloy steel usually used the annealing temperature is 595 ~ 675 ℃, but high chromium alloy steel in 900 ~ 1065 ℃. We can use the lower temperature and longer time to achieve the same effect as the shorter time and the higher temperature to eliminate the branch stress.
Compared with annealing, normalizing treatment should be preferred for its high mechanical properties, simple price increase, short production cycle and low energy consumption. The current application is as follows:
1. Final heat treatment as a common structural part
2. Improve the machinability of low carbon steel and low carbon alloy steel
3. Preheat treatment of the more important parts of medium carbon structural steel.
4. Eliminate the stroke-shaped secondary carburized steel, and prepare tissue for spheroidization annealing
5. For some large or complex shaped parts, quenching may have the risk of cracking, and normalizing is often used instead of quenching and tempering as the final heat treatment of such parts. At this time it can not be called normal fire, but is called empty quenching related. In order to increase the hardness of mild steel, normalizing temperature can be appropriately raised.