Investment casting Heat treatment: Quenching Tempering Annealing Normalizing Different
Heat treatment is a metal thermal processing technology. In the processing, the material is heated, held, and cooled in the solid state to obtain the desired structure and properties. In the time from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment has gradually become known.We are a china investment casting foundry. Stainless steel casting, carbon steel precision casting, alloy casting investment casting is our main castings products. In our casting foundry, Quenching,Tempering,Annealing,Normalizing can be used. In this article,let us introduce the four heat treatment method and their different.
Quench hardening is a mechanical process. It make steel and cast iron alloys strengthen and harden. These metals consist of ferrous metals and alloys. This is done by heating the material to a certain temperature, dependent upon material, and then rapidly cooling the material. This produces a harder material by either surface hardening or through-hardening varying on the rate at the material is cooled. The material is often tempered to reduce the brittleness. It may increase from the quench hardening process. Items that may be quenched include gears, shafts, and wear blocks.
Tempering is a heat treatment technique applied to ferrous alloys. Such as steel or cast iron, to achieve greater toughness by decreasing the hardness of the alloy. The reduction in hardness is usually accompanied by an increase in ductility, thereby decreasing the brittleness of the metal. Tempering is usually performed after quenching. It is rapid cooling of the metal to put it in its hardest state.
Annealing , a heat treatment that alters the microstructure of a material causing changes in properties of strength, hardness, and ductility
Normalizing is an annealing process in which a metal is cooled in air after heating in order to relieve stress.
There are 3 main difference between normalizing, tempering, annealing and quenching. They are the process different, the material structure changing and the material properties changing.
1. Process different:
Normalizing is to heat the workpiece to Ac3. (Ac is the final temperature at which free ferrite is converted to austenite during heating. Generally it is between 727 °C and 912 °C) or Acm (Acm is over-eutectoid in actual heating). The critical temperature line for complete austenitization of steel) is above 30~50 °C. After holding for a period of time,a metal heat treatment process of spraying water, spraying or blowing air in the air . It can be taken out from the furnace.
The tempering treatment means that the quenched or normalized steel is cooled at a certain rate after being immersed. It is for a period of time below a critical temperature.
Annealing is a metal heat treatment process. it heats the metal to a temperature for a enough period of time . then cools it at a suitable rate (usually slow cooling, sometimes controlled cooling).
Quenching is the heating of steel to a temperature above the critical temperature of Ac3 (hypoeutectic steel) or Ac1 (hyper-eutectoid steel). Keep it for a period of time to make it all or part austenitized. Then, a heat treatment process of martensite (or bainite) transformation is performed by rapidly cooling to a temperature below the critical cooling rate to below Ms (or isothermal near Ms).
2. Material organization changes different.
the Material organization After normalizing. the sub-aluminum steel is ferrite + pearlite, the eutectoid steel is pearlite. the hypereutectoid steel is pearlite + secondary cementite. it is discontinuous.
Low temperature tempering results in martensite structure. The tissue obtained by medium temperature tempering is tempered tortoise. The high temperature tempering and quenching treatment results in a tempered sorbite structure.
After annealing, the grains are refined. the structure is adjusted, and the tissue defects are eliminated.
Quenching causes the supercooled austenite to undergo martensite or bainite transformation. A martensite or bainite structure is obtained. Finally, an unbalanced structure dominated by martensite is obtained. it is also having bainite or maintaining single-phase austenite as needed.
3. Material performance changes are different:
In the case of normalizing, the crystal grains of the steel can be refined in a slightly faster cooling. Not only does it achieve satisfactory strength, but it also significantly improves toughness (AKV value). Reduce the tendency of the member to crack. Some low-alloy hot-rolled steel sheets, low-alloy steel forgings and castings, after normalizing treatment, the comprehensive mechanical properties of the material can be greatly improved. It also improves the cutting performance.
After tempering, it will be quenched and normalized and placed in the intermediate temperature for a period of time (aging) to promote the precipitation of a part of the carbide. At the same time, it can eliminate some residual stress caused by rapid cooling. Therefore, the toughness and flexibility of the material can be improved.
Annealing can reduce the hardness. Improve machinability. Eliminate residual stresses. Stable size and reduced tendency to crack and crack. Refine grains, adjust tissue, and eliminate tissue defects. Uniform material organization and composition improve material properties or prepare tissue for future heat treatment.
Quenching can greatly improve the rigidity, hardness, wear resistance, fatigue strength and toughness of steel. Thus to meet the different use requirements of various mechanical parts and tools. Special physical and chemical properties. Such as ferromagnetism and corrosion resistance of certain special steels can also be met by quenching.
When it comes to heat treatment in the industrial field, we all know that it is annealing, normalizing, quenching, and tempering.
But For beginners, it may be very abstract. Below I will use a metaphor to explain the difference between them!
Annealing. Slowly heat the metal to a certain temperature. Keep warm for a while. Then slowly cool to room temperature. Think about it: you cooked the food but it was too hot. So you have to put it aside. Let it be cold and eat. Annealing is like this
Normalizing. It is to heat the metal to 30-50 ° C above the critical temperature. Heat treatment process in which air is cooled after a suitable period of time. It sounds a lot like annealing.
Only the normalizing cooling rate is slightly faster and the production cycle is short. Because normalizing is like cooling down.It is often faster to eat, that is, to get products faster.
So, when annealing and normalizing can also meet the performance requirements of parts. we can use normalizing as much as possible.
If annealing and normalizing are brothers, then quenching and tempering are good partners who will never leave.
Quenching. It is to heat the metal above the critical point. Now, the structure and state of the metal will change – austenitizing. We need to keep warm for a certain amount of time to let the metal make this change.
It is then cooled at a temperature greater than the critical cooling rate. To obtain a metastable martensite structure or a lower bainite structure.
This method of rapid cooling is usually like this
After quenching, a martensite structure is obtained. But the internal structure of this organizational state is extremely unbalanced.
Although the hardness is high, the plasticity and toughness are poor. It is also brittle. Therefore, the quenched metal will not be shipped as a finished product. This product that cannot be reworked is like a mobile phone screen. For example, the mobile phone screen is fragile metal. No one will want it.
Therefore, the role of tempering is reflected! After the metal is hardened, it is heated to a temperature below the critical temperature. Keep warm for a while. Allow the tissue inside the metal to be evenly distributed and then cool to room temperature. In this way, a finished product having a certain strength, hardness, and certain plasticity and toughness can be obtained.
This is the perfect example of 1+1>2! The difference between annealing, normalizing, quenching and tempering is almost the same. So “quenching” and “tempering” can make the face crisp.
Annealing, normalizing, quenching, tempering sequence
In the four processes of heat treatment, quenching and tempering are used in combination. Is the annealing and normalizing done before or after them? According to the process requirements, the overall order of the four? In addition to these four heat treatments, do you need other processing steps?
Four heat treatments, one surface treatment. That is, annealing, normalizing, quenching, tempering and surface heat treatment. There is no fixed order. The process needs to be tailored to the specific situation. For example, normalizing can be used as a residual heat treatment or as a final heat treatment.