First．Types of annealing
1.Full annealing and isothermal annealing
Complete annealing is also called recrystallization annealing, generally referred to as annealing. This annealing is mainly used for castings, forgings and hot-rolled sections of various carbon steels and alloy steels with hypoeutectoid compositions, and sometimes also used for welded structures. Generally used as the final heat treatment of some non-heavy workpieces, or as the pre-heat treatment of some workpieces.
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.
Second.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.
Thrid. The purpose of steel tempering
1. Reduce brittleness, eliminate or reduce internal stress. Steel parts have a large internal stress and brittleness after quenching. If they are not tempered in time, they 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 aggregate 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, and there are many types, it requires versatility in the process,
For versatility, box furnace 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 batches of carburizing parts, pit gas carburizing furnaces can be used.
4. For the production of large quantities of automobile, tractor gears and other parts, a continuous carburizing production line or a box-type multi-purpose furnace can be selected.
5. Rolling furnace and roller-hearth furnace are the best choice for heating and mass production of sheet blanks for stamping parts.
6. For batches of shaped parts, push rod type or conveyor belt type 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. Steel balls and rollers can be heat treated with an internal spiral rotary tube furnace.
9. Non-ferrous metal ingots can be produced in mass production using pusher furnaces, and small non-ferrous metal parts and materials can be air-circulated heating furnaces.
Heating defects and control
First,Overheating
We know that overheating during the heat treatment process is most likely to cause the austenite grains to be coarse, which reduces the mechanical properties of the parts.
1. General overheating: The heating temperature is too high or the holding time at high temperature is too long, which causes the austenite grains to coarsen and is called overheating. Coarse austenite grains will reduce the strength and toughness of the steel, increase the brittle transition temperature, and increase the tendency of deformation and cracking during quenching. The cause of overheating is that the furnace temperature meter is out of control or mixing (usually caused by ignorance of the process). The superheated structure can be annealed, normalized or tempered at high temperature for many times, and then re-austenitized under normal conditions to refine the grains.
2. Fracture heredity: After reheating and quenching the steel with overheated structure, although the austenite grains can be refined, sometimes there are still coarse granular fractures. There are many theoretical controversies about fracture heredity. It is generally believed that impurities such as MnS have been dissolved into austenite and concentrated in the crystal interface due to excessive heating temperature, and these inclusions will precipitate along the crystal interface during cooling. It is easy to fracture along the coarse austenite grain boundary when subjected to impact.
3. Inheritance of coarse structure: When steel parts with coarse martensite, bainite, and Widmanite structure are re-austenized, they are heated to the conventional quenching temperature at a slow speed, or even lower, the austenite crystal The grains are still coarse, and this phenomenon is called tissue heredity. To eliminate the heritability of coarse tissues, intermediate annealing or multiple high temperature tempering treatments can be used.
Second, Overburn
Excessive heating temperature will not only cause coarse austenite grains, but also local oxidation or melting of the grain boundaries, which leads to weakening of the grain boundaries, which is called overburning. The performance of steel deteriorates severely after overburning, and cracks are formed during quenching. The burned tissue cannot be recovered and can only be scrapped. Therefore, avoid over-burning during work.
Third,Decarburization 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 surface carbon concentration, which is called decarburization. The surface hardness, fatigue strength and resistance of decarburized steel after quenching The abrasiveness is reduced, and the residual tensile stress on the surface is easy to form surface mesh cracks.
During heating, the iron and alloys and elements on the surface of the steel react with oxygen, carbon dioxide, and water vapor in the medium (or atmosphere) to form an oxide film. The phenomenon is called oxidation. The dimensional accuracy and surface brightness of the workpiece deteriorated after oxidation at high temperature (generally above 570 degrees), and the steel parts with poor hardenability of oxide film are prone to quenching soft spots.
In order to prevent oxidation and reduce decarburization, the measures include: coating the surface of the workpiece, sealing and heating with stainless steel foil packaging, heating with a salt bath furnace, heating with a protective atmosphere (such as purified inert gas, controlling the carbon potential in the furnace), flame burning furnace (Make the furnace gas reductive)
Fourth,Hydrogen embrittlement
When high-strength steel is heated in a hydrogen-rich atmosphere, the plasticity and toughness decrease is called hydrogen embrittlement. Hydrogen embrittlement can also be eliminated for workpieces with hydrogen embrittlement (such as tempering, aging, etc.). The use of vacuum, low hydrogen atmosphere or inert atmosphere heating can avoid hydrogen embrittlement.
