Comparison of hardening hardness by material. List of steels and design considerations

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I have had the experience of scouring various websites for hardness data by material, asking, "Which material should I choose to get the hardness I need for this part?" and have scoured various websites in search of hardness data by material.　　However, most websites had information on typical steels such as S45C, but I could not find an exhaustive list that included tool steel and stainless steel, and in the end I had to piece together fragmentary information.

This article was created for designers who have had the same problems I have had in the past.　　It provides an exhaustive list of hardnesses obtained by hardening major steels by material, from carbon steel to alloy steels, tool steels, and stainless steels, which is often lacking on other sites.

Furthermore, it is not just data, but is carefully explained first from the basic principles that determine the highest hardness of steel materials, and finally, it is structured to systematically learn knowledge that is truly useful in design practice, including how to refer to reliable JIS standards and manufacturer's information.　　After reading this article, you should be able to eliminate any hesitation in material selection and give heat treatment instructions with confidence.

Comparison of hardening hardness at a glance by material

Here is a list of major steels frequently used in machine design that provides a comprehensive comparison of the hardness ranges achievable by hardening.　　Please use this information to understand the characteristics of each steel material and as a first step in material selection.

Annotation:
¹ Typical hardness as hardened (HRC): This value is the approximate hardness that would be expected from the carbon content if an almost 100% martensitic structure were obtained on the surface by ideal rapid cooling. This condition is very brittle,This hardness itself is rarely specified in the drawings.

² Practical Quenched and Tempered Hardness Range (HRC): This value is the hardness range targeted for the actual product after it has been "tempered" after quenching to balance hardness and toughness.The designer should indicate this hardness range on the drawings.

³ Precipitation hardening stainless steel: SUS630 is not hardened by quenching, but by aging treatment (precipitation hardening) after solution treatment.　Hardness is adjusted by treatment temperature (e.g., H900 is approximately 480°C).

Role of carbon content in determining quench hardness

In conclusion, what determines the highest hardness obtained by hardening steel?The upper limit is determined almost entirely by the amount of carbon contained in the steel.

This is because quenching, a heat treatment, is a process in which steel is rapidly cooled from a high-temperature state to form a very hard internal structure called martensite.The hardness of this martensitic microstructure is determined by how many carbon atoms are forcibly trapped inside itTherefore, the higher the carbon content of the original steel, the higher the highest hardness that can be achieved.

For example, the theoretical maximum hardness for general-purpose S45C (carbon content of about 0.45%) is about 63HRC, and for S55C (carbon content of about 0.55%) with higher carbon content, about 65HRC.

However,Important notes that designers should be aware ofThere is a　The fact is that once the carbon content exceeds about 0.6%, the maximum hardness hardly increases even if the carbon content is increased further.　　Selecting an expensive high-carbon steel simply for its hardness will not only not be worth the cost, but may also have the disadvantage of reduced toughness (tenacity) and increased brittleness.

Thus, the first step in proper material selection is to understand that carbon content is the most fundamental factor in determining the upper limit of its potential in terms of the hardness desired for a part.

Hardness and characteristics of carbon steel (e.g. S45C)

Carbon steel, also commonly referred to as "S-C material," is the most widely used steel material in mechanical design, as exemplified by S45C.

The greatest advantage of carbon steel is its high cost performance.　　Since it does not contain expensive alloying elements such as chromium and molybdenum, material costs can be kept low.　　In the case of S45C, surface hardness of about HRC55-60 can be obtained by appropriate heat treatment, ensuring the wear resistance required for many machine parts.

On the other hand, there are some disadvantages that designers should be aware of.　　That is,Low hardenabilityThis is the point that As mentioned above, carbon steels that do not contain alloying elements have an inferior ability to harden deep into the interior of a part. Therefore, even if one tries to harden a thick-walled part such as a shaft with a large diameter, the surface tends to be hardened but the center remains soft.

For this reason,Carbon steel is considered more suitable for applications where only the surface is selectively hardened, such as induction hardening, rather than "zub quenching" where the entire part is hardened.　The low cost is attractive, but consider the size of the components and the required strength,Careful consideration should be given to ensure that quenching properties are not insufficient.

Hardness and characteristics of alloy steels (SCM,SNCM)

Alloy steels with added alloying elements such as chromium (Cr), molybdenum (Mo), and nickel (Ni) were developed to compensate for the low hardenability of carbon steel.　Typical examples are SCM material (chrome molybdenum steel) and SNCM material (nickel chromium molybdenum steel).

The greatest advantage of alloy steels is their excellent hardenability.　　The added alloying elements work to slow down changes in the internal structure of the steel as it cools.　　This makes it possible to harden the core of the part firmly with gentle cooling, such as slow cooling with oil, even in situations where rapid cooling is required for carbon steel.

For example, SCM440 has about the same carbon content as S45C, but its hardenability is much better, so it is widely used for parts with larger cross sections, high-strength bolts and axle shafts that require high reliability.　Another major advantage of SCM440 is its ability to cool slowly, reducing heat treatment risks such as quench cracking and deformation.

One disadvantage is that the material cost is still higher than that of carbon steel.　　However, there are many situations where the use of alloy steel is essential to ensure reliability and performance after heat treatment in modern design, where larger parts and higher strength are required.

Hardness and characteristics of tool steel (SK material)

Tool steel, as the name implies, is a steel material used in molds and cutting tools for processing metals, and is characterized by extremely high hardness and excellent wear resistance.

Tool steel has a very high carbon content and also contains large amounts of alloying elements such as chromium (Cr), tungsten (W), and molybdenum (Mo).This results in very high hardness exceeding HRC60 after quenching.

A typical steel grade is SKD11 for cold dies and molds, which is used as a standard material for precision dies and molds due to its excellent characteristics of very little dimensional change after quenching. However, while it is extremely hard, it also has low toughness and is brittle, making it unsuitable for applications subject to strong impacts.

There are also tool steels, such as SKD61 for hot molds, whose hardness does not decrease easily even in high-temperature environments.

Tool steels are expensive materials used for special applications, but their high hardness and wear resistance provide superior performance that cannot be replaced by other steels.　　It is important to understand the characteristics of these materials when involved in the design of molds and jigs.

Hardness and characteristics of stainless steel

Stainless steel is known for its excellent corrosion resistance, but not all stainless steels are hardened by quenching.　Hardness can be increased by quenching mainly in a type of stainless steel called "martensitic" stainless steel.

Martensitic stainless steels, unlike common austenitic stainless steels (SUS304, etc.), contain carbon as a component.　　Therefore, heat treatment can cause martensitic transformation and increase hardness.

Typical steel grades include SUS420J2 and SUS440C.SUS420J2 is hardened to a hardness exceeding HRC50 by quenching, and is used for cutlery, valve parts, shafts, etc., where corrosion and wear resistance are required.　　On the other hand,SUS440C is a material that can achieve the highest level of hardness among stainless steels, reaching HRC58 or higher.　Because of its excellent wear resistance, it is used in precision parts such as bearing balls, inner and outer rings, and nozzles.

However, it should be noted that while martensitic stainless steels can increase hardness, they are inferior to austenitic stainless steels in terms of corrosion resistance.　　Material selection should be made in consideration of the environment in which the product will be used, and whether rust resistance or hardness should be prioritized.

Design considerations to take advantage of hardening hardness

Material selection considering hardenability and mass effect

When selecting materials for parts, it is dangerous to look only at the target hardness to determine the steel grade.Particular attention should be paid to the "mass effect" and the related "hardenability".

What is the mass effect?

The mass effect refers to the phenomenon in which the hardness after quenching differs depending on the size of the part (cross-sectional area), even if it is the same steel material.　　Specifically, the thicker and thicker the part, the significantly slower the cooling rate in the center.　　Due to this difference in cooling rate, even if the surface is fully hardened, the interior is not fully hardened."Raw" conditionIt is a good idea.

Hardenability and its evaluation method

A major factor in this mass effect is "hardenability," a property inherent to the material.　Hardenability is the ability to "how deeply hardenable" a product is by quenching.The following table shows the hardenability of the product.　　Several indices exist to objectively evaluate this hardenability.

Jominy test and hardenability curve

The most typical evaluation method is the "Jominy test (one end quenching method)" specified in JIS G 0561It is.　　In this test, a specimen of round bar machined to the specified dimensions is heated and then cooled by spraying water on only one end face (one end).

The "hardenability curve (Jominy curve)" plots the relationship between the distance of the specimen from the water-cooled edge and its hardness.　　The hardness is highest at the water-cooled end because of the most rapid cooling, and decreases as the cooling rate slows away from the water-cooled end.The slower this hardness decline curves, the better the material is rated as having "good hardenability.

For example, the hardness of S45C, a carbon steel, drops rapidly just a short distance from the water-cooled edge, while SCM440, an alloy steel, maintains a high hardness and a gentle curve for a relatively long distance.　　This shows that the hardenability of SCM440 is superior to that of S45C.

Indicator called critical diameter

Another practical indicator is the "critical diameter.　　This is defined as "the largest diameter that will yield a martensitic structure of 50% or greater at the center of the part.　　ThisThe larger the critical diameter of the material, the more firmly it can be hardened to the core, even in thicker parts.

Hardenability is improved by adding alloying elements such as chromium (Cr), molybdenum (Mo), and manganese (Mn).　　Therefore,It is extremely important for the designer to choose a material that is hardenable enough for the thickest part of the part (dominant wall thickness) and a material with a large critical diameter to prevent internal strength deficiencies due to mass effectswill be.

Balance of hardness and toughness by tempering

Steel that has attained its highest hardness through quenching is actually very brittle, like glass.　　In this "left-unquenched" state, it cannot be used for most mechanical parts because it can be easily cracked by even the slightest impact.

Therefore, a process called "tempering" is always required after quenching.　　Tempering is a heat treatment in which quenched steel is reheated to an appropriate temperature to slightly reduce hardness, but at the same time to remove internal strains that cause brittleness and to restore tenacity, or "toughness.

Designers need to understand this trade-off relationship between hardness and toughness and find the optimal balance point depending on the performance required of the part.

For example, for tools for which wear resistance is the highest priority, "low-temperature tempering" is used, which causes little reduction in hardness.　　On the other hand, "high temperature tempering (tempering)" is applied to parts that are subject to impact, such as shafts and gears, to significantly improve toughness at the expense of hardness to some extent.

It should be absolutely noted here that there is a dangerous temperature range, called "temper brittleness," where toughness is significantly reduced.　　Tempering in the temperature range of approximately 250-350°C and 450-550°C must be avoided unless there is a special reason. The designer must achieve the optimum balance of hardness and toughness in a safe temperature range.

How to refer to drawing instructions and JIS standards

Drawings are the final deliverables that accurately convey the designer's intentions to the manufacturing site and produce parts with the desired quality. If instructions regarding heat treatment are unclear, it may lead directly to defective products.

Basics of Drawing Instructions

There are several manners in which to indicate hardness in the drawings.

First,Hardness should always be specified in a range, such as "HRC 48-52" rather than a single value such as "HRC 50".This range is set in the list shown at the beginning of this article.Practical quenched and tempered hardness range."is very helpful.

The ranges shown in that table are for each material.Generally targeted hardness rangeIt is.　　Depending on the application of the part and the required performance (e.g., whether wear resistance or toughness is important), specific target values are set within that range.Generally, a range of 4 to 5 points in HRC allows for slight variations that cannot be avoided in the heat treatment process and enables realistic quality control.will be.

Next,Clearly describe the treatment. Be sure to clearly state "quenching and tempering" instead of just "hardening.By doing so, we tell them that the tempering process is mandatory to ensure toughness. (In actual one-off or prototype drawings, even if "tempering" is not mentioned, they will perform quenching and tempering as a set.)

especiallySurface hardening like induction hardening　In the case of the "surface hardness" and "effective hardened layer depth", it is important to clearly indicate the extent to be hardened in the figure, as well as to indicate the "surface hardness" and the "effective hardened layer depth".It is.　　The "effective hardened layer depth" is the depth from the surface at which the specified hardness is maintained, and is a very reliable and professional method of indication in assuring component performance.

How to Use Reliable Sources of Information

If you need materials not listed in this article or require more detailed data, the following sources can be useful.

How to utilize JIS standards

The basis for these instructions is JIS (Japanese Industrial Standards). For example, carbon steel for machine structural use is specified in "JIS G 4051.　　JIS standards can be viewed by anyone with free user registration on the JISC (Japan Industrial Standards Committee) website. It is important to make it a habit to always consult primary information as the basis for design.

• JISC (Japan Industrial Standards Committee) Web site: JISC (Japan Industrial Standards Committee) https://www.jisc.go.jp/

Refer to the steel manufacturer's technical documents.

While JIS standards establish general requirements, the technical documents issued by individual steelmakers provide more detailed data on their products (e.g., detailed tempering curves, CCT diagrams, fatigue strength data, etc.).　　In particular,For manufacturer's own modified steel grades (e.g., Daido Steel's DC53), manufacturer's documentation is the only source of informationThis will be the case.　　When designing critical components, be sure to obtain and review the official technical data from the material manufacturer.

• Daido Steel Co:https://www.daido.co.jp/
• Pro Materials Corporation (formerly Hitachi Metals, Ltd.):https://www.proterial.com/
• JFE Steel Corporation:https://www.jfe-steel.co.jp/
• Kobe Steel, Ltd:https://www.kobelco.co.jp/
• Sanyo Special Steel Co:https://www.sanyo-steel.co.jp/

Understanding hardening hardness for optimal design

In this article, we have discussed the hardening hardness of steel, from its basic principles to specific data for each material, as well as design considerations. Finally, we summarize some important points that machine designers should keep in mind.

• Maximum hardness of steel is determined primarily by carbon content
• Maximum hardness reaches a plateau when carbon content exceeds about 0.61 TP3T
• The ability to put hardness into the core of the part (hardenability) is improved by alloying elements
• Carbon steel (S45C, etc.) is cost-effective but has low hardenability
• Alloy steels (such as SCM440) have excellent hardenability and are suitable for large and critical parts
• Tool steel (e.g. SKD11) has extremely high hardness and wear resistance
• In stainless steels, martensitic steels (SUS420J2, SUS440C, etc.) are hardened by quenching.
• Large component size makes it difficult to harden to the core (mass effect)
• Select a hardenable material that is appropriate for the maximum wall thickness of the part, taking into account mass effects
• Since parts are brittle after quenching, they must be tempered to give them toughness.
• There is a trade-off between hardness and toughness.
• Avoid processing at temperatures in the dangerous range of tempering brittleness
• Drawings should clearly indicate heat treatment method and "range" of hardness
• Specifying the "effective hardening layer depth" is the key to guaranteeing performance in surface hardening
• JIS standards are a reliable source of information on which to base designs

That's it.