In this article, we will learn what is normalizing, definition, process, advantages, applications.
What is Normalizing? Definition
Steel and other alloys have a wide range of applications in engineering practice, each needing a unique set of qualities. They may be subjected to bending in one location while twisting in another. They may be required to bear various sorts of pressures and to have hardness, particularly red hardness, along with toughness and an anon-brittle cutting edge as tool materials.
They may be needed to withstand static or dynamic loads, to rotate at extremely high speeds, to function in highly corrosive fluids, to have an exceptionally hard skin with a robust core, to be subjected to fatigue and creep, and so on. It is critical that the material utilized for any project has the appropriate mechanical properties for the application.
Heat treatment procedures are frequently employed to change the mechanical characteristics of a metal, with normalizing being one of the most prevalent. Normalization, like annealing, is a form of heat treatment. Annealing, normalizing, and other heat treatments are useful techniques for increasing the strength and resilience of metallic materials. In this article, we shall get to know more about Normalizing, process, limitations, and many more, so without any further ado let’s begin!
What is Normalizing?
- Heat treatment describes a range of industrial, thermal, and metallurgical techniques used to change the physical and, in certain cases, chemical qualities of a material.
- Heat treatment is the process of heating or cooling a material to severe temperatures in order to obtain the desired result such as hardening or softening.
- Case hardening,
- Precipitation strengthening,
- Normalizing, and
- Quenching are all heat treatment processes.
- Although the phrase “heat treatment” refers to techniques in which heating and cooling are done specifically for the aim of modifying characteristics, heating and cooling frequently occur accidentally during various industrial operations, for example, hot shaping or welding.
- Normalization is a sort of annealing technique used to relieve stress and improve ductility and toughness qualities in hardenable steels following cold work.
- Normalizing is a heat treatment method used to control internal material stress. It decreases the rate of corrosion on a metallic surface while also increasing the steel’s strength and hardness.
- It is the process of heating a material to a high temperature and then cooling it to room temperature by subjecting it to room temperature air after it has been heated. This heating and slow cooling change the microstructure of the metal, lowering its hardness and increasing its ductility.
- During this process, the material is heated to temperatures ranging from 750 to 980 degrees Celsius (1320 to 1796 degrees Fahrenheit). The exact heat used for treatment varies and is dependent on the amount of carbon in the metal.
- Normalizing may be used in place of traditional hardening when the size or shape of the item is such that liquid quenching may result in cracking, distortion, or excessive dimensional changes, depending on the mechanical qualities required.
- Thus, items with complex shapes or sharp changes in a section can be normalized and tempered as long as the qualities acquired are satisfactory.
Why Does Normalizing Require?
- Normalizing is frequently undertaken because another operation has lowered ductility and increased hardness, either intentionally or unintentionally.
- It is utilized because it causes microstructures to restructure and become more ductile. This is crucial since it enables the metal extra formable and machinable while also reducing residual stresses in the material, which could contribute to unexpected failure.
- Normalizing iron and steel components increases their hardness and strength. Furthermore, normalizing aids in the reduction of internal stresses caused by operations like forging, casting, machining, forming, or welding.
- The steel is heated just above its upper critical temperature and kept there for a long enough period of time to allow new, fine grains to form and high-energy grain forms to coalesce, a process known as grain refinement.
- Normalizing also increases microstructural homogeneity and heat treatment responsiveness (e.g., annealing or hardening), as well as stability by imparting “thermal memory” for subsequent lower-temperature treatments. Normalization is frequently used for parts that require maximal toughness and are subjected to impact.
- When big cross-sections are normalized, they are also tempered to reduce stress and control mechanical qualities more precisely.
- Many gear blanks are normalized prior to machining so that dimensional changes like as growth, shrinkage, or warpage can be better controlled during later hardening or case hardening.
- Soak times for normalizing are normally one hour per inch of cross-sectional area, with a maximum of two hours at temperature.
- It is vital to remember that the part’s mass or workload can have a substantial impact on the cooling rate and consequently the final microstructure.
- Thinner pieces cooled faster and become harder after normalization than thicker ones. In an annealing process, the hardness of the thin and thicker parts is roughly the same after furnace cooling.
Normalizing is usually done in order to:
- Increase machinability
- Enhance dimensional stability
- Make changes to and/or refine the grain structure.
- Make a microstructure that is homogeneous.
- Banding is reduced.
- Ductility is improved
- When hardening or case hardening it provides a more consistent response.
- Normalizing is not usually required for low-carbon steels. However, when these steels are normalized, no negative effects occur.
- Annealed castings with reasonably homogeneous wall thickness and section sizes are more commonly used than normalized castings.
- Other castings benefit from normalizing, particularly those with a complex shape or interconnected thick and thin portions that are prone to high levels of residual stresses. The content of the castings (which impacts hardenability), as well as the cooling time, determine the normalized microstructure.
The following metals and alloys can be normalized:
- Alloys based on Nico, such as Nilo 6 * and Pernifer 6 *.
- Tool steel, carbon steel, stainless steel, and cast iron are all iron-based alloys.
Not all metals necessitate the thermal normalizing process. Normalization is uncommon in low-carbon steels, for example. However, if such steels are normalized, no harm will be done to the material. Additionally, when iron castings have a uniform thickness and similar section sizes, they are typically annealed rather than normalized.
Normalizing Heat Treatment
- Normalizing heat treatment is a technique used on ferrous materials. The goal of the normalizing heat treatment is to improve the material’s mechanical characteristics by refining the microstructure.
- The ferrous metal is heated above the transformation range to the austenite phase and then cooled in still air at room temperature. Normalizing heat treatment balances structural imperfections and softens the material for subsequent processing.
- Cold working techniques such as forging, bending, and hammering harden and make materials less ductile. The same is true for the heat-affected area near the welded part.
- This material’s ductility and softness can be restored with normalizing heat treatment. This treatment is also utilized before any subsequent surface hardening in order to improve reaction to the desired hardness.
Heat Treatment Process
- The first and most critical point to make is that steel is a one-of-a-kind metal. Other metals, such as copper, bronze, and iron, can be heat treated to soften or return them to their normal state. Steel, on the other hand, is unique in that it can undergo structural changes during heat treatment due to the iron-carbon alloy.
- Steel’s susceptibility to heat treatment is due to the alloying of the two elements. This is one of the reasons why steel has been the most important industrial metal since its discovery. Steel is the only metal that can be toughened and tempered.
- The three stages of heat treatment are heating, soaking, and cooling. The various processes are Normalizing, Quenching, Tempering, Hardening, Annealing. In order to normalize the heat treatment process, the metal is heated in a furnace. The furnace is kept at a temperature of 750-980 °C (1320-1796 °F), depending on the carbon content of the material.
- For 1-2 hours, the material is maintained at a temperature higher than the austenite temperature. If the vacuum furnace is run at less than 1 bar pressure, the ferrites are cooled to room temperature in still air or Nitrogen until all of the ferrites transform to austenite, and then cooled to room temperature in still air or Nitrogen.
- Carbon is more soluble in iron in the austenite phase. Normalizing heat treatment results in a more uniform carbide size, which facilitates subsequent heat treatment procedures and yields a more consistent end product.
- The following describes the normalization procedure. The metal is heated from “a” to “b” and held at that temperature for a period of time. It is then cooled in still air to the ambient temperature “d.”
Carbon Steel Normalizing
- Carbon content in carbon steel ranges from 0.12 to 2%. Steel becomes harder, tougher, and less ductile as the percentage of carbon content increases.
- Normalizing is not frequently required for low-carbon steels. They can, however, be normalized based on the requirement.
- Normalizing heat treatment of carbon steel involves heating it to a temperature 55 °C (131 °F) above the austenitic temperature, Ac3, (which lies between 750-980 °C / 1320-1796 °F), as shown in the picture below, this is also called as “holding temperature.”
- The temperature is held for one hour every 25 mm (0.984 in) thickness. The technique ensures that 100% of the steel is converted to austenite. The steel is then cooled to room temperature in still air.
- This procedure results in a finer, more consistent pearlite structure. Pearlite is a two-phase layered structure composed of cementite (iron carbide) and α-ferrite.
- This method differs from annealing in that the heated metal is cooled slowly and at a set rate inside the furnace during annealing. Normalized steel has more strength and hardness than annealed steel, and the method is less expensive because it is cooled directly with air.
- Carbon steel thickness can have a substantial impact on the cooling rate and consequently the final microstructure. After normalizing, bigger components cool down slower and become more ductile than thinner pieces.
- After normalization, the areas of steel with 0.80 percent carbon are pearlite, whereas the areas with low carbon are ferrites. The process of atomic diffusion redistributes carbon atoms between ferrite (0.022 percent by weight) and cementite (6.7 percent by weight).
- The amount of pearlite in annealed steel with the same carbon content is greater. This is due to the eutectoid composition moving to a lower value and the development of cementite.
- Fine-grained pearlite microstructures are harder than coarse-grained pearlite microstructures. Normalizing decreases the carbon steel’s inherent tensions. It also improves microstructural homogeneity, thermal stability, and heat treatment responsiveness.
Examples of Normalizing in Commercial Practice
The following are some common examples of normalizing in commercial practice:
- Prior to machining, gear blanks are normalized so that dimensional variations such as growth, shrinkage, or warpage can be better controlled during subsequent hardening or case hardening.
- Cast and wrought structure homogenization and Improvements in machinability and grain size refinement of casting cast structures.
- Segregated, cored, and dendritic features, as well as non-uniform characteristics, characterize cast metals and alloys.
- Similarly, wrought metals and alloys that have undergone mechanical processing such as forging, rolling, extrusion, and so on have a non-uniform structure and characteristics. By normalizing, these structures and attributes are made homogeneous.
- In a few circumstances, once the steel has been hot or cold wrought, a normalizing heat treatment is required to restore its original mechanical properties.
- Due to the high mechanical stresses imposed on the part and the differences in the microstructure, it is extremely rare for a forging to be used without some type of heat treatment.
- Normalizing is one of the most basic heat treatments for refining (or normalizing) the microstructure and equalizing the effects of the temperature range to which the material was subjected during the forging operations. Normalizing forgings is extremely advantageous to any following hardening procedures.
- Pearlite, which is irregularly shaped and somewhat large but varies in size, is found in steels that have experienced plastic deformation. Normalizing is a heat treatment that is applied to steel in order to refine its crystal structure and provide a more consistent and desired grain size distribution.
Difference Between Normalizing, Tempering, Annealing, & Quenching
- The procedure is distinct: The procedure of heating the work piece to the final temperature at which all free ferrite transforms to austenite during heating is known as normalizing, generally between 727 ° C and 912 ° C) or Acm (The crucial temperature line for complete austenitizing of eutectoid steel during real heating is defined as Acm.) 30 50 °C, after holding for a period of time, remove the metal that has been cooled in the air or sprayed with water, sprayed or blown from the furnace Heat treatment process. Tempering refers to the process of cooling down steel after quenching, hardening, or normalization treatment at a specific rate after being submerged for an amount of time well below the critical temperature. The steel is quenched by heating it to a temperature more than the critical cooling rate Ac3 (hypoeutectic steel) or Ac1 (hypereutectic steel), retaining it for a period of time to totally or partially austenitize it, and then cooling at a temperature greater than the critical cooling rate. For the martensite (or bainite) transition heat treatment method, rapid cooling down below Ms (or isothermal around Ms) is required.
- The material organization modifications are diverse: After normalizing, the structure of hypoeutectoid steel is ferrite + pearlite, eutectoid steel is pearlite, and hypereutectoid steel is pearlite + secondary cementite. Tempering at low temperatures yields the martensite structure; tempering at medium temperatures yields the trophite structure, and tempering at high temperatures yields the sorbite structure. Following annealing, the grains are refined, the structure is modified, and structural faults are removed. Quenching causes super cooled austenite to undergo martensite or bainite transformation, resulting in martensite or bainite structure and, eventually, an asymmetrical structure driven by martensite (sometimes obtaining bainite or maintaining Phase austenite).
- The following are the outcomes of changes in material properties: During normalizing, the steel’s crystal grains can be refined at a little faster cooling rate, resulting in not only sufficient strength but also significantly improved toughness (AKV value) and reduced breaking tendency of the component. The complete mechanical characteristics of the material can be substantially enhanced after normalizing the low-alloy hot-rolled steel plates, low-alloy steel forgings, and castings, as well as the cutting performance. Upon tempering, it will be quenched and normalized for a period of time before being immersed in a moderate temperature for a period of time, which can boost the precipitation of some of the carbides while also eliminating some of the residual stress brought about by rapid cooling, thus improving the material’s toughness and flexibility. Annealing can reduce hardness while improving machinability. Remove residual stress, stabilize the size, and limit the likelihood of deformation and cracking; refine the grain, correct the structure, and remove tissue flaws. Improve material performance, uniform material structure; and composition, or prepare tissue for future heat treatment. Quenching may substantially increase steel’s stiffness, hardness, wear resistance, fatigue strength, and toughness, allowing it to meet the various needs of various mechanical components and tools. Through quenching, it can also meet particular physical and chemical qualities of some special steels, such as ferromagnetism and corrosion resistance.
Differentiating between Normalizing & Annealing
Both annealing and normalizing have significant advantages for a variety of industrial applications. High temperatures are used in these two heat treatments to heat materials above their recrystallization point and then progressively cool them down. While several important stages differ, the fundamental purpose of both is usually to increase ductility via a microstructural change.
- While the general process of normalizing is similar to that of annealing and stress reduction, there are significant distinctions in the procedure and in the end product.
- Normalizing is distinguished from annealing in that the metal is heated to a higher temperature before being withdrawn from the furnace for air cooling rather than furnace cooling.
- Many manufacturing engineers are often perplexed about when to specify normalizing and when to require annealing. There is a logical basis for this because, in many cases, the normalizing and annealing procedures are the same.
- For example, by heating over the transformation range and cooling in air, very-low-carbon steel can be nearly totally annealed.
- The cooling rate used in normalizing is slower than that used in a quench-and-temper process but faster than that used in annealing.
- As a result of the intermediate cooling rate, the pieces will have a hardness and strength that is somewhat higher than annealed but slightly lower than quenched and tempered. Because of the slower cooling rate, normalized sections will not be as strained as-quenched sections.
- Because it does not require additional furnace time during the cool-down process, normalizing is less expensive than annealing. Thus, normalizing is a treatment that achieves a moderate improvement in strength without causing unnecessary stress.
Advantages of Normalizing Steels over Annealing
In both heat treatments, the phase shift to austenite happens during heating, and the grain size of the newly generated austenite is fine. Although some grain expansion may occur in the normalized situation, more uniform and homogenous austenite is obtained. Because of the lower temperature of transformation, faster air cooling in normalizing creates slightly different microstructures and hence characteristics than slow furnace cooling in annealing.
Normalizing is preferable over annealing for the following reasons:
- Normalizing results in improved mechanical qualities like strength and hardness (with somewhat lower ductility), i.e. if improving mechanical properties is the primary goal, then normalizing is performed. Normalized forged plain carbon shafts or even rolled stocks are not annealed.
- Mild steels have higher machinability in the normalized condition, whereas steels with 0.3 to 0.4 percent carbon have superior machinability in the annealed state.
- Annealing is recommended when there are no internal tensions in the pieces, such as in intricate shapes and crucial elements.
- From a process standpoint, normalizing has the following advantages:
- Components cool along with the furnace during annealing, whereas parts are removed from the furnace to cool in the air during normalizing. The empty hot furnace can be used to heat the next batch of components to be normalized, boosting the furnace’s productivity. The heat treatment takes less time.
- As with annealing, the furnace is cooled to low temperatures before being heated again for the next batch of parts. Here, too, fuel and heat treatment time is more important, but the consumption of fuel or power is more important.
- Normalizing and annealing have nearly identical goals, although normalizing may be favored due to improved mechanical properties and less time and expense of procedures.
- However, it cannot replace annealing for increased softness or the absence of internal tensions, especially in complex structures.
- In low carbon steels, normalizing and annealing provide nearly identical results; for example, to obtain softness, normalizing may be favored for lower cost and shorter time. However, with high carbon steels, the variation in characteristics may be significant.
A normalizing process has three major steps as follows:
- Recovery Stage- During the recovery step, a furnace or other sort of heating equipment is utilized to elevate the temperature of the material to a point where its internal tensions are released.
- Recrystallisation Stage- The material is heated above its recrystallisation temperature but below its melting temperature during the recrystallisation stage. This results in the formation of new grains with no pre-existing pressures.
- Grain Growth Stage- The new grains fully form during grain growth. This growth is slowed by exposing the material to air and letting it cool to ambient temperature. This type of cooling is known as non-equilibrium cooling.
Completing these three processes yields a material with increased ductility and decreased hardness. Following the normalizing process, additional processes that can modify mechanical properties are occasionally performed.
Equipment During Normalizing
- Normalizing equipment is available in both batch and continuous operations.
- Bell furnaces provide a cost-effective form of heat treatment as well as a variety of bell lifting systems. Continuous furnaces heat and treat metal in a continuous manner.
- The conveyor runs at a steady speed and transports the product to the proper conditions following heat treatment.
Applications of Normalizing
When compared to annealing, the inexpensive cost of the normalizing process makes it one of the most widely utilized industrial processes. As soon as the heating and holding periods are completed, the furnace is ready for the next batch.
Normalization is applied to:
- After the hardening of work that occurs throughout the manufacturing process, ferritic stainless steel stamping in the automotive industry can be normalized.
- In the nuclear sector, nickel-based alloys can be normalized following thermal microstructure changes caused by welding.
- Carbon steel can be normalized after cold rolling to prevent brittleness caused by work hardening. Restore forged or cold wrought steel’s native mechanical properties. Reduce the amount of time it takes to forge high carbon steel.
- Normalization has a wide range of practical uses in a variety of industries, including Aerospace, Agriculture, Automotive, Energy, Heavy equipment.
- In general, it is best practice to utilize normalizing when manufacturing processes are likely to put significant stress on the material or when dimensional stability is critical to the product.
Thus, these are the Normalizing ideas to keep in mind when doing a heat treatment because contemporary computer-controlled steel manufacturing techniques produce more consistent products and structures, normalizing is a less commonly used operation. Normalizing, on the other hand, is still a method of coaxing better qualities or performance out of some steels.
Normalizing is a procedure that enhances the part quality and is crucial in limiting dimensional variance during hardening and case hardening. When dimensional stability is critical or when manufacturing procedures are projected to transmit considerable quantities of stress into the material, this should be done. It aids in the prevention of several heat-treating issues.
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