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I.Types of Annealing

1. Complete annealing and isothermal annealing

Complete annealing, also known as weight crystallization annealing, is commonly referred to as annealing. This type of annealing is mainly used for casting, forging, and hot-rolled profiles of various carbon and alloy steels with hypoeutectoid compositions, and sometimes also for welding structures. It is usually used as the final heat treatment for some non heavy parts, or as a pre heat treatment for certain workpieces.

2. Spheroidization annealing

Spheroidal annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel grades used in the manufacture of cutting tools, measuring tools, and molds). Its main purpose is to reduce hardness, improve machinability, and prepare for future quenching.

3. Stress relief annealing

Stress relief annealing, also known as low-temperature annealing (or high-temperature tempering), is mainly used to eliminate residual stresses in castings, forgings, welded parts, hot-rolled parts, cold drawn parts, etc. If these stresses are not eliminated, it will cause deformation or cracks in the steel parts after a certain period of time or during subsequent cutting processes.

II.The commonly used cooling media during quenching are saltwater, water, and oil. Workpieces quenched with salt water are easy to obtain high hardness and smooth surfaces, and are not prone to soft spots that are not hardened during quenching. However, they are prone to severe deformation and even cracking of the workpiece. However, using oil as a quenching medium is only suitable for quenching some alloy steels or small-sized carbon steel workpieces with high stability of undercooled austenite.

III.The purpose of steel tempering

1. Reduce brittleness, eliminate or reduce internal stress. Steel parts have significant internal stress and brittleness after quenching, and if not tempered in time, they often deform or even crack.

2. Obtain the required mechanical properties of the workpiece. After quenching, the workpiece has high hardness and brittleness. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted through appropriate tempering to reduce brittleness and obtain the required toughness and plasticity.

3. Stable workpiece size

4. For certain alloy steels that are difficult to soften after annealing, high-temperature tempering is often used after quenching (or normalizing) to allow appropriate aggregation of carbides in the steel, reduce hardness, and facilitate cutting processing.
Selection of furnace type

The furnace type should be determined based on different process requirements and the type of workpiece
For products that cannot be mass-produced, have unequal sizes of workpieces, and have a variety of types, it is required that the process has universality
Multi functional, box type furnace can be selected.
When heating long shafts, long screws, pipes, and other workpieces, a deep well electric furnace can be used.

3. For small batches of carburized parts, a well type gas carburizing furnace can be selected.
For the production of large quantities of automotive, tractor gears and other parts, continuous carburizing production lines or box type multi-purpose furnaces can be selected.

When heating and mass producing stamping sheet metal blanks, rolling furnaces and roller bottom furnaces are selected.

6. For batches of standardized parts, push rod or conveyor belt resistance furnaces (push rod furnaces or casting belt furnaces) can be used in production

7. Small mechanical parts such as screws, nuts, etc. can be equipped with vibrating bottom furnaces or mesh belt furnaces.

8. Steel balls and rollers can be heat treated using an internal spiral rotary tube furnace.

9. For large-scale production of non-ferrous metal ingots, a push rod furnace can be used, while for small non-ferrous metal parts and materials, an air circulation heating furnace can be used.

Heating defects and control

I.Overheating phenomenon

We know that overheating during the heat treatment process can lead to the coarsening of austenite grains, resulting in a decrease in the mechanical properties of the parts.

1. General overheating: Heating temperature is too high or holding time at high temperature is too long, causing coarsening of austenite grains, which is called overheating. Coarse austenite grains can lead to a decrease in the strength and toughness of steel, an increase in the brittle transition temperature, and an increased tendency for deformation and cracking during quenching. The cause of overheating is the loss of control of the furnace temperature instrument or mixing of materials (often due to lack of understanding of the process). Overheated tissue can be annealed, normalized, or subjected to multiple high-temperature tempering processes, and then re austenitized under normal conditions to refine the grain size.

2. Fracture inheritance: Steel with overheated structure, after reheating and quenching, can refine austenite grains, but sometimes coarse granular fractures still appear. There is much controversy over the theory of fracture inheritance. It is generally believed that impurities such as MnS were dissolved into austenite and enriched at the grain interface due to excessive heating temperature. However, these inclusions will precipitate along the grain interface during cooling, and are prone to fracture along coarse austenite grain boundaries when impacted.

3. Inheritance of coarse structure: When steel parts with coarse martensite, bainite, and bainite structures are re austenitized, the austenite grains are still coarse when heated slowly to the conventional quenching temperature, or even lower. This phenomenon is called tissue inheritance. To eliminate the heritability of coarse tissue, intermediate annealing or multiple high-temperature tempering treatments can be used.

II.Overburning phenomenon

Excessive heating temperature not only causes coarse austenite grains, but also leads to local oxidation or melting of grain boundaries, resulting in weakened grain boundaries, which is called overburning. The performance of steel deteriorates severely after overburning, and cracks form during quenching. The burnt tissue cannot be restored and can only be scrapped. Therefore, excessive burning should be avoided in work.

III.Decarbonization and oxidation

When steel is heated, the surface carbon reacts with oxygen, hydrogen, carbon dioxide, and water vapor in the medium (or atmosphere) to reduce the concentration of surface carbon, which is called decarburization. After quenching, the surface hardness, fatigue strength, and wear resistance of decarburized steel decrease, and residual tensile stress is formed on the surface, which can easily form surface network cracks.

The phenomenon in which iron and alloys on the surface of steel react with elements such as oxygen, carbon dioxide, and water vapor in the medium (or atmosphere) to form an oxide film during heating is called oxidation. High temperature (generally above 570 degrees) workpieces deteriorate in dimensional accuracy and surface brightness after oxidation, and steel parts with poor hardenability due to oxide films are prone to quenching soft spots.

Measures to prevent oxidation and reduce decarburization include coating the surface of the workpiece, sealing and heating it with stainless steel foil, heating it with a salt bath furnace, heating it with a protective atmosphere (such as purified inert gas, controlling the carbon potential inside the furnace), and using a flame combustion furnace (making the furnace gas reducing)

IV.Hydrogen embrittlement phenomenon

The phenomenon of reduced plasticity and toughness of high-strength steel when heated in a hydrogen rich atmosphere is called hydrogen embrittlement. Workpieces with hydrogen embrittlement can also be eliminated through dehydrogenation treatment (such as tempering, aging, etc.), and heating with vacuum, low hydrogen atmosphere, or inert atmosphere can avoid hydrogen embrittlement.