What is "Normalizing"? A Thorough Explanation of Purpose, Effects, and Drawing Instructions

Here.Heat treatmentwhich is one of the"Normalization."　I am making a note about the

Normalizing" is a headache in heat treatment drawing instructions.　If the heat treatment is chosen incorrectly, not only will the performance of the part not be realized, but it will also cause fatal problems such as distortion and machining defects in subsequent processes.　　　Many information sites explain the basic definition of tempering, but why is it effective in certain situations?Essential difference from annealing　is and how critical it is to the post-process, from the designer's point of view, is not enough.

In this article, we will first clarify the clear difference between annealing and tempering from a metallurgical viewpoint, and then explain the specific effects of annealing, as well as the correct drawing instruction method and design considerations to maximize the effects of annealing.

By systematically learning this information, which is often discussed in bits and pieces on other sites, the goal is to be able to confidently direct heat treatment.

What is Annealing? Purpose and Difference from Annealing

Tempering process and important temperature control

Normalizing is a heat treatment to return the microstructure of steel to a homogeneous and fine state, or "standard (normal) state.　　The process consists of three major steps: "heating," "holding," and "cooling," and proper temperature control in particular is key to the success or failure of the process. Here is the key point, though,Annealingindicates word or phrase being definedThe objectives are different.

What is the basic purpose of annealing?
Annealing, or annealing, is a heat treatment in which a metal material is heated to a specific temperature and then slowly cooled to soften it and make it easier to process.

Heating (austenitization)

First, the steel is heated to a temperature at which its microstructure changes completely to a state called "austenite" (gamma iron).　　This temperature depends on the amount of carbon contained in the steel and is determined based on an iron-carbon equilibrium state diagram.

• Sub-eutectoid steel (carbon content < 0.77%): temperature about 50°C above the A3 transformation point.
• Hypercovalent steel (carbon content > 0.77%): temperature about 50°C above the Acm transformation point.

Retention (Equal heat)

Next, the material is held at that temperature for a certain period of time.　　This is the time required to austenitize the material uniformly, not only on the surface but also in the center, and is generally 1 to 2 hours per 25 mm thickness of the material.This holding process resets the non-uniform microstructure created by forging, rolling, etc.

cooling

And the final cooling process is the most important point that characterizes the tempering process.The steel is removed from the heating furnace and cooled in the atmosphere (air cooling).　　This "air cooling" is,It is an intermediate cooling rate, faster than "furnace cooling," which is a slow cooling process in a furnace, and slower than quenching, which is a rapid cooling process in water or oil.This exquisite cooling rate results in a finer microstructure, as described below, and improves the mechanical properties of the steel.

Applied Technology for Normalization

In addition to standard air cooling, special tempering techniques exist for different purposes.

What are the effects of tissue miniaturization?

The fundamental reason why tempering improves the properties of steel is that it refines the "grain" of the metallurgical structure.　　The interior of steel that has undergone processing such as forging and casting is in a non-uniform state, with coarsened crystal grains and stretching in one direction.

Annealing is a process that resets this non-uniform microstructure and reconstructs it into a microstructure consisting of uniform, fine crystal grains. In researching the principle in depth for this article, I came across a very interesting law called the "Hall-Petch relationship," which states that the strength of a polycrystalline metal is the inverse of the square root of the average size of its grains.　　This is a rule of thumb that states that the strength of a polycrystalline metal is inversely proportional to the square root of the average size of its grains.

In other words, the smaller the crystal grains, the stronger the material.　　This is because the movement of "dislocations," which cause metals to deform, is impeded by the boundaries between crystals (grain boundaries).　　When grain size is reduced by tempering, the total area of these grain boundaries increases.　　As a result, the barriers to dislocation movement increase, and more force is required to deform the material, thus increasing its strength and hardness.　　It can be understood that tempering is not just a heat treatment, but a scientific approach to unlocking the potential of a material at the micro level.

Changes in mechanical properties due to tempering

Result of tissue refinement and homogenization,Normalized steel has significantly improved mechanical properties compared to the as-rolled or annealed condition.Not only does it become harder, but it is also transformed into a highly reliable material with a good balance of strength and toughness (tenacity).

In particular, the designer will benefit from improvements in tensile strength, yield point (the force at which the material begins to deform), and impact value, which is the resistance to impact.

Using S45C, a typical carbon steel for machine structural use, as an example, the table below shows its effects.

Table: Comparison of mechanical properties of S45C (rolled vs. tempered)

Note: Data for raw rolled material is limited because it is usually subjected to heat treatment.

As can be seen from the table, there is no significant change in hardness itself, but tensile strength and the upper limit of yield point have improved, as has elongation, which indicates the tenacity of the material.　　This indicates that tempering is not just a process to harden the material, but also a process to bring out tougher and more stable performance by preparing the structure.This characteristic of "moderately hard and moderately sticky" is the true value of yaki-narashi.

Effects of tempering and precautions designers should know

In what situations can quenching be an effective tool?　Here, we explain practical knowledge ranging from specific effects such as improved machinability and suppression of quenching distortion, to advantages and disadvantages for use in design, and the correct way to indicate them on drawings.

Improvement of machinability and its effect on cutting

One of the primary objectives of applying tempering is to improve machinability, or ease of cutting.　　What was particularly impressive was the effect on low-carbon steels.　　I used to have a vague understanding that annealing simply "makes machining easier," but it was not until I researched the reasons for this that I learned that, on the contrary, if the material becomes too soft through annealing, chips can be connected for a long time and become difficult to handle, or the surface finish of the workpiece can deteriorate.

When low-carbon steel is annealed, it tends to become excessively soft, causing the blade to bite into the material during cutting.　In contrast, annealing provides moderate hardness by refining the microstructure. This hardness makes it easier to break up chips during cutting and improves free-cutting performance.

Also, in medium carbon steels, the annealing process makes the microstructure more uniform, which stabilizes resistance during cutting.　This process is expected to have the advantage of reducing wear of tool cutting edges, extending tool life, and improving machining accuracy.　　Thus, tempering is positioned as an important pre-processing step that greatly affects the efficiency and quality of the subsequent machining process.

Effect of suppressing distortion during quenching

One of the most valuable applications of tempering is its role as a pretreatment for quenching.　Especially for parts that require high dimensional accuracy, such as gears, quenching is an indispensable process to control deformation (distortion) that occurs during quenching.

When steel is rapidly cooled from high temperature for quenching, its internal structure changes to martensite, a hard structure.　　This process is accompanied by a large volume change, which generates stress inside the part, causing distortion and, in the worst case, quench cracking.

If the material microstructure is non-uniform before quenching, this transformation to martensite will occur unevenly from place to place, and internal stresses will be generated unevenly, leading to large distortions.

Therefore, quenching is performed in advance to prepare a homogeneous and fine microstructure. This ensures that the microstructural changes during quenching proceed uniformly throughout the part and equalize the generation of internal stresses. As a result, dimensional changes after quenching become more predictable and deformation and distortion can be dramatically reduced.

Application of tempering according to steel grade and purpose

The purpose of tempering varies depending on the steel grade and the subsequent process.

Case studies in design and on site

Let's look at the advantages and disadvantages of tempering through specific case studies that can occur in design and manufacturing.

Cases where advantages were utilized

• Case 1: Stabilization of quality (large forged crankshaft)
  Crankshafts for large engines are parts that are prone to variations in mechanical properties due to partial coarsening of the crystalline structure during the forging process.　　If machining and surface hardening are performed without modification, the risk of partial strength deficiency and fatigue fracture increases.　　Therefore, by applying tempering before machining, the entire shaft is reset to a homogeneous and fine microstructure.　This results in stable machinability no matter which part is machined, and greatly improves the reliability and durability of the final product.

• Case 2: Improvement of machinability (mass production of low carbon steel collars)
  Cutting is the main process for collars (tubular parts) made of low-carbon steel (S25C, etc.), which are mass-produced by automatic machines.　　If the material is processed in its rolled state, it is so sticky that chips are connected for a long time, tangling the machine and causing it to stop frequently.　　By adding tempering as a pre-processing step, the material is given the proper hardness and chips are broken into smaller pieces. As a result, machining speed can be increased, tool life is prolonged, and productivity is dramatically improved.

• Case 3: Post-process deformation control (carburizing and quenching of precision gears)
  Gears made of hardened surface hardening steel (SCM420, etc.), for which high precision is required, are carburized and quenched to harden the surface.　　However, if carburizing and quenching is performed with a non-uniform structure of the material, the heat treatment causes significant deformation, which results in a loss of tooth profile accuracy and many rework man-hours and defective products.　　Therefore, we provide quenching and tempering before gear cutting. This homogenizes the internal structure of the material so that deformation during carburizing and quenching is kept to a minimum and within a predictable range. As a result, the amount of grinding in the subsequent process can be reduced, which directly leads to cost reduction and quality stabilization.

Cases of overlooked disadvantages and precautions

• Case 1: Restriction by material (mold parts made of high-alloy tool steel)
  A designer instructed "HNR" to a mold part to be made of a high-alloy tool steel (e.g. SKD11) in the same sense as carbon steel.　　SKD11 is a steel material with high alloy content and "air hardenability," which means that it can be hardened even by air cooling.　　As a result, after the tempering process, the part became so hardened that it could not be machined at all.　　The parts were scrapped, resulting not only in wasted material and heat treatment costs, but also in delivery delays.

• Case 2: Effect of dimensions (wall thickness) (cast steel bracket with large wall thickness difference)
  Normalizing was indicated on a cast steel bracket with a complex shape consisting of thick ribs and thin plate sections.　　After treatment, the thin section cooled relatively quickly by air cooling, resulting in a fine, hard microstructure, while the center of the thick ribs cooled slowly, resulting in a coarse, soft microstructure.　　This variation in hardness caused instability in the behavior of the drill as it moved between the hard and soft areas during subsequent drilling, resulting in inaccurate machining.

• Case 3: Oxidized scale generation (precision mating shaft)
  A shaft part requiring H7 tolerance in the final finish was instructed to be tempered. However, the designer determined the material dimensions without considering the thickness of the oxide scale (black skin) that would be formed on the surface by tempering and the "grinding allowance" required to remove the scale. After heat treatment, the surface was ground to remove the scale, and eventually the shaft diameter fell below the lower tolerance limit, rendering the part unusable.

Advantages and disadvantages of utilizing it in design

As mentioned earlier, while tempering has many advantages, it is not a universal process and several disadvantages and caveats exist. The designer must comprehensively understand these and determine their application.

Advantages

• Stabilization of quality: homogenization and refinement of the microstructure and reduction of mechanical property variations.
• Improved mechanical properties: Increases strength, especially toughness (tenacity).
• Improved machinability: Facilitates cutting, especially of low-carbon steels.
• Reduced deformation in post-processing: Increased dimensional stability during quenching.
• Residual Stress Removal: Remove internal stresses caused by forging, rolling, etc.

Disadvantages and Cautions

• Cost and time: Heat treatment is an additional process, which increases cost and time.
• Limitations by material: High alloy steels such as tool steel and stainless steel are not suitable for tempering because they are prone to "air quenching," in which a hard martensitic structure is formed simply by air cooling.
• Size (wall thickness) effects: For parts with large wall thickness, differences in cooling rates between the surface and center of the part can cause variations in microstructure and hardness.
• Generation of oxide scale: Heating and cooling in air causes the formation of an oxide film called "black scale" on the surface. Since this needs to be removed in the subsequent process, the material dimensions must be designed to allow for the "grinding allowance" for this.

It is important to understand these characteristics and determine whether or not tempering is necessary, taking into consideration the performance required of the part and the overall manufacturing process.

Drawing instruction method to communicate correctly

Once the designer has decided to apply tempering, the drawings must contain the correct instructions in order to accurately communicate the intent to the manufacturing floor.　　Heat treatment instructions are extremely important technical information to assure component performance.

The process symbol based on the Japanese Industrial Standards (JIS) for normalizing is "HNR".　　This is an abbreviation for "Heat treatment - Normalizing.　　In older drawings and literature, the term "burn standard" is sometimes used, but in the current JIS standard, HNR is the official symbol.

The description on the drawing is usually clearly stated near the title column of the drawing or in the notes section, such as "Heat treatment: HNR".

According to JIS, it is permissible to omit the leading "H" and use "NR" as long as it is not confusing with other processing symbols.　　However,To prevent misunderstandings and oversights in the field, it is most reliable and strongly recommended to use the full "HNR" without abbreviation.

The designation "HNR" on a drawing is more than a mere note. It is a definitive technical instruction by the designer that defines the reference metallographic condition that the part should have and specifies the prerequisites for subsequent processing and final product performance.

Summary for selecting the best firing method

In this article, we have discussed all aspects of "tempering" that machine designers should know. Finally, we will summarize the key points of this article to help you choose the best heat treatment.

• Normalizing is a heat treatment that returns the steel structure to the "standard state" of homogeneity and fineness.
• The purpose is to homogenize the microstructure, improve mechanical properties, machinability, etc.
• The main purpose of annealing is "softening", while the main purpose of tempering is "toughening".
• The difference is in the cooling method: air cooling for tempering and furnace cooling for annealing.
• The process consists of three steps: "heating," "holding," and "cooling.
• Heating temperature is set about 50°C higher than the A3 or Acm point depending on the carbon content of the steel
• The cooling rate of air cooling is the key to achieving grain refinement.
• Grain refinement improves strength and toughness
• Tensile strength, yield point and elongation are improved in carbon steels such as S45C
• Moderate hardness is obtained in low carbon steels and machinability is improved.
• Distortion and deformation can be greatly reduced when performed as a pretreatment for quenching
• Advantages include stable quality, improved performance, improved workability, and reduced deformation.
• Disadvantages are increased cost, material limitations, dimensional effects, and oxidation scale generation.
• High-alloy steel is not suitable for tempering because it is easily hardened by air cooling
• Drawing instructions should clearly indicate the JIS symbol "HNR".

That's it.