What is quenching? Types and Drawing Instructions for Designers to Know

Here, the heat treatment used for machine partsTypes and Drawing Instructions for Quenching that Designers Need to Know."　I am making a note about the

Many people are aware that "quenching" is simply a process to harden steel.　I had a bitter experience in the past when I caused quenching cracks in an important part due to inadequate instructions, which made me fundamentally change my way of thinking about it.Jig design　and ... andMachine Composition Materials　The characteristics of the system can be utilized for adjusting the characteristics of the

Many websites explain the different types of hardening and individual techniques.　However, there is not much information that provides a consistent learning flow from the basic metallurgical principles of why steel hardens to how this is incorporated into the design and ultimately into the drawing instructions that are communicated to the manufacturing floor without misunderstanding.

This article aims to fill that gap. First, the scientific principles of how hardening hardens steel and the importance of tempering, which is essential to adjusting performance, will be revealed.　Then, I will go through the comparison of various treatment methods and design tips to prevent defects, and finally, I will explain how to write specific drawing instructions that accurately convey the designer's intentions in a comprehensive and optimal learning flow.

Basic principle of quenching to harden steel

Principle of steel hardened by quenching

When the designer directs "hardening," the essence of the process is to redesign the steel's atomic-level structure and bring out new properties　The first two are located at　　Quenching is the process of heating steel to a specific high temperature followed by rapid cooling,Heat treatment technology that dramatically improves hardness, strength and wear resistanceIt is.

To understand this process, the designer must first understand the heating process.　　When steel is heated to a high temperature range of approximately 800°C to 900°C, its internal microstructure changes to a special state called "austenite," which allows for high carbon solid solution.This preparation stage determines what characteristics will be produced in the subsequent cooling process　Therefore, it is the starting point for all quenching processes.

Austenite to martensite

As mentioned earlier, steel heated to high temperatures is in an austenitic state.Rapid cooling from this state using a cooling medium such as water or oil dramatically changes the internal structure of the steel to a very hard state called "martensiteI will do so.

If cooled slowly, the atoms will align over time, forming a relatively soft and stable tissue.　　However,The purpose of quenching is to not allow this stabilization and to not allow time for the atoms to migrate by rapid cooling.　　As a result, the atoms are locked in an irregular, distorted state, and this internal "strain" is the source of martensite's high hardness.　　This transformation to martensite is the core phenomenon of steel hardening.

Improved wear resistance and fatigue strength

The martensitic microstructure obtained by quenching provides an extremely high resistance to deformation from external forces due to the large strain in the atomic structure.　　This property provides the excellent hardness and wear resistance required for machine parts.

In particular, when a surface hardening treatment such as induction hardening is applied, a compressive force (compressive residual stress) acts on the hardened surface layer.　　This force is,Effect of inhibiting the initiation and propagation of micro cracks that are the starting point of fatigue fractureThe fatigue strength of the product is　　Therefore, the life, or fatigue strength, of components subjected to repeated loading can be significantly increased.

Toughness lost in exchange for hardness

The high hardness obtained by quenching comes at a price that must be understood by the designer　The "brittleness" of the material is the "brittleness" of the material.　　It is the "brittleness" of the material. While martensitic structures are very hard, their impact tenacity, or toughness, is significantly reduced.

Steel in its hardened state is as hard as glass, but brittle.It is.　　Therefore, when used as a machine part, the risk of chipping or fracture at the slightest impact is very high. Since the part cannot be used as a practical component in this condition, the following process is absolutely essential to properly adjust the hardness and restore the toughness.

Tempering essential for hardened parts

Removal of residual stresses to prevent placement cracks

Tempering is a heat treatment in which steel is heated again to an appropriate temperature after quenching to restore toughness.　One of the most important purposes of tempering from the designer's point of view is to "remove residual stress" to ensure component reliability.

Due to rapid cooling and microstructural changes during quenching, an extremely large amount of invisible force is accumulated inside the part.　　If this residual stress is left unchecked, the part will suddenly crack after a period of time.The fatal flaw of "placement cracking".and cause dimensional deviations after machining.　　Tempering greatly reduces these dangerous internal stresses, ensuring part reliability and dimensional stability.To avoid this risk, heat treatment manufacturers, in principle, do not leave time after quenching.

Low temperature tempering with emphasis on hardness

Low-temperature tempering is tempering performed in the relatively low temperature range of approximately 150°C to 250°C.　　Designers choose this method when they wish to eliminate the aforementioned detrimental residual stresses while minimizing the loss of the high hardness achieved by quenching.

This treatment is applied to parts where wear resistance is a top priority, such as cutting tools, precision measuring gauges, bearings, etc.The heat treatment process is a process of "tempering" the martensite.　　From the heat treatment engineer's point of view, heating in this temperature range transforms hard, brittle "hardened martensite" into "tempered martensite" with a slight recovery of tenacity.

High-temperature tempering with emphasis on toughness

High-temperature tempering is performed in the high temperature range of approximately 400°C to 650°C and is also specifically referred to as "tempering.　　The designer's objective in choosing this treatment is to dramatically increase toughness at the expense of sacrificing some hardness,Obtain a tough material with an excellent balance of strength and tenacity　This is to

It is widely used for shafts, gears, high-tensile bolts, and other structural components that need to withstand high impact loads and repetitive loads.　　The key to performance design is for the designer to select (or ask the processing manufacturer to suggest) an appropriate tempering temperature for this tempering temperature according to the performance required of the part.　　Let us take SCM435, a typical structural alloy steel, as an example to see the relationship between tempering temperature and mechanical properties.

Example of tempering characteristics of SCM435 (AISI 4140)

*Note: The temperature range of 250-400°C may fall into the low-tempering brittle zone and should be considered carefully in design. Data are representative values; actual values may vary depending on steel composition and conditions.

Tempering brittleness temperature range to avoid

Tempering temperatures are not freely selectable, and there is a "dangerous temperature range" that must be avoided by the designer.　　Treatment in a specific temperature range will not improve toughness, but on the contrary, will make the material brittleTempering embrittlement occurs.The reason for this is to

This critical temperature range varies somewhat with the steel grade, but two general ranges are known.

• Low temperature tempering brittleness: approx. 250°C to 400°C
• High temperature tempering brittleness: approx. 450°C to 550°C

Avoiding these temperature ranges when tempering is indicated on the drawings is critical to ensuring component reliability.

What is secondary hardening of tool steel?

For ordinary steel materials, hardness decreases as tempering temperature is increased.　However, for tool steels that contain many special alloying elements such as tungsten and molybdenum, tempering at high temperatures such as 500°C to 600°C will conversely reduce the hardness.A special phenomenon called "secondary hardening" that increases hardnesscan be seen.

From the heat treatment engineer's point of view, this phenomenon is caused by the formation of new very hard special carbides in the microstructure due to heating.　　This property is actively used to enhance the performance of high-performance molds and cutting tools that need to maintain hardness at high temperatures.

The difference between tempering and annealing/normalizing.

The term "tempering" is sometimes confused with "annealing" or "normalizing" because they sound similar.　　However, these are different heat treatments with completely different purposes and processes.　　It is important for the designer to clearly understand these differences in order to give the correct instructions.

Briefly, the objectives of each are as follows

• Tempering: Tempering is used to give "tenacity (toughness)" to steel that has been hardened by quenching.Adjustment processIt is.
• AnnealingSteel: To make the steel as "soft" as possible to facilitate machining.Preparation ProcessIt is.
• NormalizingTo improve the metallurgical structure of steel to be "uniform and fine" and to improve its properties.Preparation ProcessIt is.

Relationship between each heat treatment and its position in the process

Most importantly,Quenching and tempering are a series of processes that are always performed as a setThis means that the　　On the other hand, annealing and tempering arePretreatment."It is usually done after quenching and tempering, because the hardness and toughness gained would be lost. This is because the hardness and toughness that have been obtained will be lost.

• Annealing as a pretreatment: When a material is too hard to be machined, annealing is first used to soften the material and make it easier to cut.　After the shape is completed, the final strength is obtained by quenching and tempering.
• Normalizing as a pre-treatment: The metal structure that has become non-uniform due to forging, etc., is prepared to a uniform state once before quenching.　This ensures more homogeneous and reliable quenching results.

Comparison of each heat treatment

Thus, although each heat treatment is an independent process, each plays an important role at the appropriate stage in the larger flow to complete a single part.　　As a designer, you must understand these differences and look at the entire manufacturing process of a part.

Type of quenching selected by application

Many types of quenching　exist.　　The following are notes on excerpts from five of the most typical types of quenching.

Whole part quenching to increase the strength of the entire part

Whole quenching, also called "zubu quenching," is a method in which the entire part is heated uniformly and then cooled entirely by immersing it in oil or water.　　This theoretically hardens the entire part, from the surface to the center.

Designer's Perspective

The best choice when uniform strength and toughness are required throughout the entire part.　It is suitable for applications such as tools, molds, and bolts, where the entire part must support the load. However, since the entire part is heated and cooled, thermal deformation (distortion) tends to be greater than with other methods, which must be considered in design.

Engineer's Perspective

The treatment temperature is set at approximately 800-900°C, followed by rapid cooling with oil or water. Care must be taken in selecting a cooling method for parts with large wall thicknesses, since the cooling rate in the center is slower and the "mass effect," a phenomenon in which hardness is reduced compared to the surface, is likely to occur.

High-frequency quenching to harden only the surface

high-frequency quenching　is a surface hardening method that uses the principle of electromagnetic induction to selectively and rapidly heat and cool only the surface layer of a component.　　Because of the "skin effect," in which high-frequency currents are concentrated on the surface of a conductor, the interior is not heated and only the surface can be hardened while retaining its original toughness.

Designer's Perspective

It is best suited when both the surface, which requires wear resistance, and the interior (core), which requires toughness to withstand impacts, are required.　Typical applications include gear tooth surfaces and shaft bearings. The greatest advantage is that only the necessary portion can be hardened, and since the entire surface is not heated, thermal distortion is very small.

Engineer's Perspective

The area and depth of curing are controlled by precisely controlling the shape, frequency, output, and heating time of the heating coil. The heating cycle is extremely short, only a few seconds, and can be incorporated into automated lines for mass production.

Flame hardening suitable for large parts

Flame hardening is a method in which a high-temperature flame using oxyacetylene gas or other gases is blown directly from a burner to heat the surface and then cool it.

Designer's Perspective

Because it does not require a dedicated coil like high-frequency quenching, it is suitable for very large parts, parts with complex shapes, or small-lot production where the cost of producing a dedicated coil is not worth it. The main applications include sliding surfaces of machine tools and large gears.

Engineer's Perspective

Uniformity of heating temperature and depth of the hardened layer is highly dependent on the operator's skill in adjusting the flame, moving speed, and controlling distance. Since the accuracy of temperature control is inferior to that of induction hardening, skilled techniques are required to ensure consistent quality.

Carburizing and quenching to modify the surface

Carburizing and quenching is not just a heat treatment, but a "thermochemical treatment" that changes the chemical composition of the steel surface.　Steel materials that are low in carbon content and do not harden when quenched as they are, are heated to high temperatures in an atmosphere rich in carbon. This allows carbon to permeate the surface layer, resulting in a high-carbon steel, and only the surface is hardened by subsequent quenching.

Designer's Perspective

This method is ideal when you want to combine conflicting characteristics in a single part: extremely high hardness and wear resistance on the surface and high toughness to absorb impact in the core. This method is used for parts used under severe conditions, such as high-performance gears for automobiles.

Engineer's Perspective

Because treatment is performed at a high temperature of approximately 900-950°C for several hours or more, thermal distortion tends to be larger than in other surface hardening methods. Therefore, dimensional design based on the assumption of post-treatment distortion removal and finishing is important.

Nitriding treatment with advantage of low distortion

Nitriding is another type of thermochemical treatment　But at a lower temperature than carburizing (about 500°C to 550°C), nitrogen atoms penetrate the steel surface.　This nitrogen reacts with alloying elements in the steel to form extremely hard nitrides, which harden the surface. Since martensitic transformation is not used, quenching (rapid cooling) process is not required.

Designer's Perspective

It is a more viable option than any other hardening treatment when dimensional accuracy is a top priority. Due to the low treatment temperature, there is little dimensional change or distortion as is associated with quenching. Therefore, it can be applied to precision molds and precision machine parts after final finishing.

Engineer's Perspective

It is characterized by a very shallow hardened layer and a long processing time.　　Although the hardness is very high, it may not be suitable for applications where large impact loads are applied, so the environment in which the part will be used must be carefully considered.

Design and materials to prevent quenching defects

Shape design to prevent baking cracks and distortion

Defects such as "quench cracking" and "distortion" in quenching are mainly caused by the non-uniform temperature and microstructural changes that occur inside the part during rapid cooling.　The risk of these defects can be significantly reduced not only by devising manufacturing processes, but also by consideration at the design stage.

As designers, we should be particularly conscious of shape design that avoids stress concentration.　　Sharp corners and areas where the wall thickness changes abruptly can easily become the starting point for quench cracking due to the concentration of stress during heat treatment. The basic design guideline to increase the success rate of heat treatment is to provide as large a radius of R (corner radius) as possible and to make the wall thickness as uniform as possible.

Measures against dimensional changes due to heat treatment

Dimensional changes are inevitable in principle because quenching causes volume expansion due to microstructural changes and deformation due to residual stress.　　Especially for parts that require high dimensional accuracy, it is essential to design in advance to anticipate these dimensional changes.

A specific measure that should be taken as a designer is to provide a "grinding allowance (shiro)" for finishing to the final dimensions after heat treatment.　　This allows slight distortions or dimensional changes caused by heat treatment to be absorbed in the subsequent grinding process.　　In addition, because internal stresses created by machining can contribute to distortion due to heat treatment,Heat treatment manufacturers may perform "stress relief annealing" prior to quenching if necessary.

Hardenability is the key to material selection

Hardenability is an indicator of a steel material's properties, such as "how easily and deeply it can be hardened.　　It is,Highest hardness a steel material can achieve(determined primarily by carbon content), is a very important concept.

Alloying elements such as chromium (Cr) and molybdenum (Mo) serve to improve this hardenability.　　The presence of these elements facilitates the formation of a martensitic microstructure even in the center of the part where the cooling rate is relatively slow. Therefore, the designer's selection of a material with hardenability commensurate with the size and wall thickness of the part is key to successful heat treatment.

Correct quenching instructions performed on drawings

Drawing instructions to be given by the designer

Heat treatment instructions on drawings are technical "contracts" to accurately convey the designer's intentions to the manufacturing site.　Ambiguous instructions must be clear and specific, as they can lead directly to insufficient performance and manufacturing problems.

At a minimum, the designer should include the following items on the drawings

• Treatment method: Specify which method is used, such as total quenching (tempering) or induction hardening.　　It is also common to use JIS symbols such as "HQT (hardening and tempering).
• Treatment area: For surface hardening, clearly illustrate the area to be hardened on the diagram with a thick single-dotted line or similar.
• Hardness: Specifies the required hardness.

It is an important role of the designer to ensure that this information is not over- or under-described.

Designation of HRC and effective hardened layer depth

When dictating hardness, there are several important rules that designers should follow.

HRC (Rockwell C Scale)

The hardness of steel after quenching is generally specified on a scale called the "Rockwell C scale (HRC).　　When indicating, always specify a range, such as "HRC 52-57" to allow for normal variations in manufacturing, rather than a single number such as "HRC 55".

Effective hardening layer depth

In surface hardening, such as induction hardening, the designation of "effective hardening layer depth" is extremely important along with hardness.　　This refers to the distance from the surface to the position where a specific hardness (generally specified as 550 HV in JIS) is maintained, and is a performance indicator directly related to the wear resistance and fatigue strength of the part. On the drawing, this should also be clearly indicated by a range, such as "effective hardened layer depth 0.8 to 1.2 mm".

Achieve the required performance with correct hardening

As explained in this article, proper hardening instructions are more than just specifying hardness.　　It is important to always be aware of the following points in order to give correct hardening instructions.

• Quenching is a process that hardens the steel by martensitizing the microstructure.
• After quenching, the material becomes brittle, so it is essential to restore toughness by tempering.
• Balance of hardness and toughness is adjusted by selecting the tempering temperature
• Removal of residual stress is important for preventing placement cracks and for dimensional stability
• Select the best quenching method according to the application and shape of the part
• Whole quenching is suitable for improving the strength of the entire part
• Induction hardening is effective when only the surface needs to be hardened with low distortion
• Carburizing and quenching is a process used to harden the surface of low carbon steel
• Nitriding is a surface hardening method suitable for precision parts that require minimal distortion
• Try to avoid stress concentration in the design stage.
• Avoid sharp corners and provide sufficient corner radii to prevent burning cracks.
• Select a material with hardenability appropriate for the size of the part
• Drawings clearly indicate treatment method, extent, hardness, and effective hardened layer depth
• Always specify hardness and hardened layer depth in a range.
• Integrating this knowledge will result in reliable mechanical design

That's it.