Basic knowledge of metal heat treatment (4)
1. Types of annealing
1. Complete annealing and isothermal annealing
Complete annealing is also called recrystallization annealing, generally referred to as annealing. This annealing is mainly used for all kinds of carbon steel and gold steel with hypoeutectoid composition. “target=_blank> alloy steel castings, forgings and hot rolled profiles, sometimes also used For welding structure, it is usually used as the final heat treatment of some non-heavy parts, or as the pre-heat treatment of some parts.
2. Spheroidizing annealing
Spheroidizing annealing is mainly used for hypereutectoid carbon steels and alloy tool steels (such as steel grades used in manufacturing cutting tools, measuring tools, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for subsequent quenching.
3. Stress relief annealing
Stress relief annealing is also called low temperature annealing (or high temperature tempering). This annealing is mainly used to eliminate residual stress in castings, forgings, welded parts, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, it will cause the steel parts to deform or crack after a certain period of time or in the subsequent cutting process.
2. When quenching, the most commonly used cooling media are brine, water and oil. Salt water quenched workpieces are easy to obtain high hardness and smooth surface, and it is not easy to produce soft spots that are not hardened, but it is easy to cause serious deformation of the workpiece and even cracks. The use of oil as the quenching medium is only suitable for the quenching of some alloy steels or small-sized carbon steel workpieces with relatively large stability of undercooled austenite.
3. The purpose of steel tempering
1. Reduce brittleness and eliminate or reduce internal stress. There is a large internal stress and brittleness of steel parts after quenching. If not tempered in time, the steel parts will often deform or even crack.
2. Obtain the required mechanical properties of the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted through appropriate tempering to reduce the brittleness and obtain the required Toughness, plasticity.
3. Stable workpiece size
4. For some alloy steels that are difficult to soften by annealing, high-temperature tempering is often used after quenching (or normalizing) to properly accumulate carbides in the steel and reduce the hardness to facilitate cutting.
Choice of furnace type
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, there are many types, and the process requires versatility and versatility, box furnaces can be used.
2. When heating long shafts, long screw rods, pipes and other workpieces, deep-well electric furnaces can be used.
3. For small batch of carburizing parts, pit gas carburizing furnace can be used.
4. For the production of large quantities of automobile, tractor gears and other parts, continuous carburizing production lines or box-type multi-purpose furnaces can be selected.
5. When heating the sheet blanks of stamping parts for mass production, it is best to use rolling furnaces and roller hearth furnaces.
6. For batches of shaped parts, push rod or conveyor belt resistance furnaces (push rod furnaces or cast belt furnaces) can be selected for production.
7. Small mechanical parts such as screws, nuts, etc. can be used in vibrating hearth furnace or mesh belt furnace.
8. The internal spiral rotary tube furnace can be used for the heat treatment of steel balls and rollers.
9. The push rod furnace can be used for mass production of non-ferrous metal ingots, and air circulation heating furnaces can be used for small non-ferrous metal parts and materials.