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General steel heat treatment includes the following techniques:
Poor quenching or tempering may result in problems such as insufficient hardness, cracks, distortion and deterioration of dies, tools and jigs. In processed products such as mechanical parts, machining tools and functional parts, failures such as breaking or bending may occur after the products are built into machines, which may lead to user complaints and claims for damages.
Quenching and tempering under the right heat treatment conditions for the specific type of steel is essential to ensure the quality required for the application.
Click the button on the right for information on heat treatment for typical die steels and machinery structural steels.
The general heat treatment process for steel is as follows:
Heat treatment process for steel
Since metal is heated to high temperatures in heat treatment, the metal combines with moisture and oxygen in the air to produce oxides, which is called oxidation. Removing these oxides by grinding or other methods is necessary in the post-process. There may also be decarburization, which can occur when steel is heated. The carbon in the surface layer combines with oxygen in the air, resulting in carbon reduction.
Thermal’s quenching furnaces (A series) can reduce such oxidation and decarburization by providing a nitrogen atmosphere in the heating chamber. Methanol or propane gas may be added to the atmosphere depending on the target work.
Thermal products in this category
There are various types of steel; their uses and heat treatment methods vary from one another.
The following tables 1 and 2 show the heat treatment characteristics of typical die steels and machinery structural steels specified in JIS (Japanese Industrial Standards).
Typical die steels
|Steel type||Effective thickness（mm）||Die steel properties||Main applications|
|Water cooling||Oil cooling||Air cooling||Wear resistance||Heat resistance||Toughness||Deformation by heat treatment||Machinability|
|SK3||20||10||-||2||1||2||1||9||Simple dies for small lots|
|SKS3||-||70||5||4||2 to 3||4 to 5||4 to 5||8||General dies|
|SKD11||-||510||140||7 to 8||6||3 to 4||8 to 9||3||Precision heavy-load dies|
|SKD1||-||160||20||8 to 9||5||2||7 to 8||2 to 3||Drawing dies|
|SKD61||-||360||100||3 to 4||6||9||8||7 to 8||Shrink rings, hot dies|
|SKH9||-||60||10||7||7 to 8||4 to 5||4||6||Hot forged punches|
- Note 1)
- The effective thickness is that where core hardness of HRC60 is obtained after quenching.
- Note 2)
- Die steel property rating 1 is the worst and 9 the best.
Typical machinery structural steels
|JIS symbol(3||Effective diameter(3
|Heat treatment [℃]||JIS hardness [HB]||Design hardness (4 HRC||Main applications|
|S25C||-||Normalizing||870 to 920 Air cooling||116 to 174||-||Cold forged products, light-duty carburized products|
|S35C||-||Normalizing||840 to 890 Air cooling||149 to 207||-||Products that do not require thermal refining|
|S45C||< 35||Thermal refining||830 to 880 Water cooling, 550 to 650 Quick cooling||179 to 255||14* to 26||Light-duty thermal refined products, induction hardened products|
|S55C||< 40||Thermal refining||810 to 860 Water cooling, 550 to 650 Quick cooling||212 to 277||20* to 29||Light-duty thermal refined products|
|SCr415||SCr21||< 30||Quenching||850 to 900 Oil cooling, 150 to 200 Air cooling||217 to 302||-||Small carburized products|
|SCr420||SCr22||< 35||Quenching||850 to 900 Oil cooling, 150 to 200 Air cooling||235 to 321||-||General carburized products|
|SCr440||SCr4||< 45||Thermal refining||830 to 880 Oil cooling, 520 to 620 Quick cooling||269 to 331||28 to 36||General thermal refined products|
|SCM420||SCM22||< 45||Quenching||850 to 900 Oil cooling, 150 to 200 Air cooling||262 to 352||-||Heavy-duty carburized products|
|SCM435||SCM3||< 60||Thermal refining||830 to 880 Oil cooling, 530 to 630 Quick cooling||269 to 331||28 to 36||Heavy-duty induction hardened products|
|SCM440||SCM4||< 65||Thermal refining||830 to 880 Oil cooling, 530 to 630 Quick cooling||285 to 352||30 to 38||Heavy-duty thermal refined products|
|SCM445||SCM5||< 70||Thermal refining||830 to 880 Oil cooling, 530 to 630 Quick cooling||302 to 363||32 to 38*||Heavy-duty thermal refined products|
|SNCM420||SNCM23||< 50||Quenching||Primary 850 to 900 Oil cooling, 150 to 200 Air cooling||293 to 375||-||Carburized products that require high toughness|
|Secondary 770 to 820 Oil cooling, 150 to 200 Air cooling|
- Note 1)
- In addition to the types listed above, the following structural steels may be used.
- Manganese steel (Mangan is increased so that S38C can be oil-quenched. Example: SMn438)
- Boron steel (Boron is added to SCr440 to achieve the hardenability equivalent to that of SCM440. Example: SCrB440)
- Leaded free-cutting steel (Lead is added to SCM420 to provide a free-cutting property. However, it is inappropriate for heavy-duty gears that require Hertzian stress of over 200 kgf/mm2. Example: SCM420L.)
- Note 2)
- In the case of alloy steels for automobiles, H steels with specified hardenability bands are used. Example: SCr415H)
- Note 3)
- The maximum diameter to ensure the mechanical properties specified in JIS. This limit should not be exceeded.
- Note 4)
- Hardness specified in drawings may not include the upper and/or lower limits of JIS hardness, as in the items marked with an *.