Steel Heat Treating - Types and Applications - Materials Today

Steel Heat Treating – Types and Applications

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Steel Heat treating is a process which involves cooling and heating of a metal substance at usually high temperature and conditions. It is useful for softening, hardening, and changing physical properties. Moreover, you can manufacture various metal structures like glass by passing it through different thermal techniques.

This steel heat treating is performed under certain conditions and passes through seven main processes before changing metals’ mechanical attributes. 

Annealing heat treatment

            The first process through which metal passes through is termed as Annealing. The procedure involves both cooling and heating processes. As a result, the microstructure increases, thus ensuring a change in electrical and mechanical properties.

This steel heat-treating process heats the metal at a critical temperature. After this, the metal can cool down slowly at room temperature and pressure.

To study Annealing in detail, Follow the microstructure development in Annealing and, also, types of annealing.

Most importantly, this process is carried for electrical and dimensional stability. The benefits result in the softening of metal for natural fabrication processes. The hardness is reduced, and the ductility property of the solid mineral is enhanced.

Steel annealing types all
Steel annealing types all

Moreover, the Annealing boosts up the electrical conductivity with changes in mechanical state. Sometimes, the Annealing metal is also allowed for the thermal process in hot furnaces. This process must be performed at low cooling temperature and pressure. This whole process of heating, cooling, and then repeating is continuously performed for 4 to 8 hours.  

Normalizing – Steel Heat treating

After done with Annealing, the metal is passed through the process of Normalizing. The Normalizing process is part of the steel heat treating method.

In this method, the solid metal is heated above 40* degree Celsius. As a result, the Normalizing process forms pure grain homogeneous structures that have more unique properties. Most importantly, the process is only applicable to ferrous metals, and it is different from Annealing.

Hypo eutectoid steel annealed microstructure
Normalized Structure – Steel heat treating

The process for scaling is mentioned in article, “Scaling of Microstructure“.

The main goal or stress Normalizing is to reduce stress from the heat-treating prestressed machining, and casting induced stress is also removed by this method. The temperature goes up, then the critical temperature and then allowed to cool down in the air.

This technique should be performed with all safety precautions and in perfect desired conditions for accurate results. Besides, the Normalizing method is most common in plate mills for the mass production of forging material. This process should only be done on ferrous metals or alloys like steels. It is the second part of the heat treating of steel.

Quenching heat treatment

The third process of heat treatment of steel is named as Quenching. This process is also called hardening due to the characteristics of metals. In this technique, the solid metal is first heated above the conditions and then quickly allowed to cool down.

This process is termed as rapid cooling for maintaining the mechanical or physical properties. This ensures that the structure of homogeneous metal remains constant.  Quenching is recommended for ferrous alloys, which harden the metal and decrease the ductility. After heating above the critical temperature, the metal can cool down by passing it through the nitrogen air, water, or any polymer.

Microstructure of martensite and martensitic transformation
Microstructure of Martensite after Quenching – Steel heat treating

This process is used for hardening the metal as in the previous method of Normalizing, and the steel was softened. The Quenching process has one drawback as it can make the metal brittle. However, this issue can be resolved by heating it again.

Hardness is simple approach to measure properties of quenched microstructure. Vicker hardness is non-destrutive hardness method while Rockwell is commonly used industrial hardness method. In case of non-destructive hardness test, Brinell hardness test is used.

Case Hardening – Steel Heat treating

The next process is called Case hardening. This is used to solidify alloying metal solid, thus ensuring greater efficiency in wear resistance of machine parts. Moreover, this process is a thermochemical diffusion that allows the dissemination of nitrogen or carbon on the upper surface of alloying metals.

The Case hardening is the fourth step in Steel heat treatment and is used to harden alloying metal substances through diffusion. The diffusion of carbon forms a thin layer of a harder alloy on the surface of metals.

This Case hardening technique is completed in 20 to 30 minutes of the constant procedure. This formed metal is more complicated, with strong forces as compared to the process of Carburizing.

The benefits of Case hardening are producing more durable, more stringent, and potent substances. This increases the life span of solid metals together with the quality of being easily weld due to more flexibility attributes. The alloying metals are more malleable with hardness higher than produced in Annealing or normalizing.

Carburizing – Steel Heat treating

After Case hardening, the fifth process of steel heat treatment is called as Carburizing. Before diving deeper, let us know the basics of Carburizing.

This process is the same as Case hardening because it is also performed for making the alloying metal hard and tough. In this technique, the heat treatment involves the absorption of iron and steel. The carbon layer formed in Case hardening is absorbed with the iron alloy’s help for making the metal more durable. This process is also known as Case hardening and should be performed under the same conditions and temperatures.

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carburizing - Steel heat treating
carburizing – Steel heat treating

Likewise, case hardening, this technique also forms a more pliable and flexible alloying metal substance. Carburizing is divided into three types named as Gas carburizing, liquid carburizing, and vacuum carburizing. These three types are based upon the raw materials it uses for the absorption of carbon alloy. After this, the metal is heated with carbon monoxide and charcoal, etc.

Nitriding

The Nitriding is the sixth process of heat treatment of steels. As Carburizing uses carbon alloy to make the metal hard, this Nitriding process diffuses nitrogen gas on the surface of the solid metal substance. The nitrogen gas absorbs on the surface of the metal and makes it sturdy and more robust. This technique is recommended for low steel alloy, aluminum, and titanium.

The Nitriding process is divided into two types. Firstly, the diffusion through gaseous nitrogen gas named Ammonia and secondly with plasma ion of nitrogen. This Nitriding process is performed in 48 hours for completing one cycle. It increases wear resistance and abrasion quality of the solid metal. Most importantly, it enhances bending and contact fatigue properties with strong forces of metals. This process is carried out with strong safety precautions and under certain conditions and temperatures.  

Stress relieving

The extreme stress heat treating steel is named as stress relieving. This stress is used for removing induced stress in the alloy or metal. This stress is formed due to different processes like machining, rolling, and Annealing, etc.

The metal is firstly heated at the high temperature, and pressure is then allowed to cool down in the air. Finally, this process is essential for the proper heat treatment of steel alloy. In this procedure, the metal becomes pure free of any induced or unwanted stress on its upper layer. The last process is not much time consuming and is most accessible of all.

Conclusion

In conclusion, these were the main seven techniques of steel heat treatment. All these methods have different producers and form different solid metal type. The heat treatment process

is known for the manufacture of glass substance. Changing of physical and mechanical properties will help you to maintain the structure of the metal alloy. These seven processes should be used one by one and incorrect order.

All techniques have their own desired temperature and pressure conditions to be carried out. Start Stress annealing, it finally ends on the stress-relieving step. The metal or steel becomes tight, hard, and, most importantly, stress-free at the end of the procedure.

F.A.Q

What does heat treating do to steel?

Heat treating is applied on steel to optimize grain structure for specific properties, relieve internal hardness, creating hard case and tough core for impact applications. Depending upon cycle given during heat treatment, steel properties can be controlled.

What temperature do you heat treat steel?

Mostly, heat treatment temperature lies in region of austenite phase. With carbon percentage, appearance of austenite may very resulting in variation of heat treatment temperature. To understand importance of heat treatment temperature, follow annealing article which can explain importance austenite phase in achieving steel properties.

What are three stages of heat treatment?

What are three stages of heat treatmeIn general, three stages of heat treatment are comprised of heating, soaking and cooling of steel. During soaking, steel structure is homogenized for optimum properties throughout microstructure.

What is heat treatment process?

Heat treatment is a heating and cooling process employed on steel for achieving optimum properties and homogenized microstructure.

Does steel weaken with heat?

No, steel gets softened with heating as stresses are released and dislocations can move easily. This softening can be considered weakening but proper heat treatment can harden the structure.

How do you harden steel after heat treat?

Heat treatment process is employed for hardening steel structure. During heat treatment, after soaking steel in austenite region, it is quenched in water, brine or oil which drastically increase hardness of steel. Details can be studies in Effect of austenitizing temperature and Quenching media on hardening of steel.

What is difference between hardening and tempering?

Hardening of carbon steel improves the hardness of materials due to the formation of martensite. It is carried out by cooling the steel from austenite region in oil, water or brine solution. On the other hand, Tempering

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is employed after hardening process to induce ductility and toughness of quenched microstructure. Temperature of tempering is lower than that of hardening process.

Why is heat treatment important?

Casting of steel and primary metal working processes induce various defects like coarse microstructure, segregation of impurities, softness and stresses. All these defects produced by initial manufacturing process can be recovered or removed using heat treatment process.

What is normalizing heat treatment?

Normalizing heat treatment is employed for refining the grain structure. During this process, steel is heated in austenite region than it is cooled in air for increasing hardness.

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