General heat treatment

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General steel heat treatment includes the following techniques:

Quenching

Steel is heated to a temperature higher than its transformation point to form austenite; then the steel is quickly cooled to form martensite. This process increases the hardness and strength of the steel without changing its shape, but the steel becomes more brittle.

Tempering

Heating the steel again after quenching adds elasticity and toughness, transforming the steel to a structure with high resistance to wear. However, the hardness is slightly reduced when compared with the state after quenching.

Annealing

This process is used to improve the structure and soften the steel since semi-hard steel, hard steel and alloy steel are difficult to cut due to their hardness.

Normalizing

This process is used to make the structure finer, uniformize carbides and remove residual stresses, thus making the steel easier to cut. Since hardenability also improves, normalizing is sometimes used as a pre-process before quenching.

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

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

Surface treatment

For products such as mechanical parts, machining tools and functional parts, surface treatment may be applied to increase the hardness of only the surface layer instead of achieving uniform hardness over the entire work.

Carburizing and quenching

Nitrogen, methanol or propane gas is fed into the furnace. The gas is decomposed to cause carbon to permeate into the steel surface, creating a high-carbon steel layer only on the surface. Applying quenching and tempering to this state makes the surface harder and the deep portion moderately hard, producing a tough product.
This is a relatively deep surface hardening technique, which is widely used for automobile parts.

Nitriding

Ammonia or another gas is fed into the furnace. The gas is decomposed to cause nitrogen to permeate into the steel surface, creating a hard nitrided layer.
Nitriding methods including hard nitriding, gas soft nitriding and plasma nitriding. Nitriding is a relatively shallow surface hardening technique, which produces harder steel than that achieved by carburizing.
The process is widely used for parts such as bearings, gears and cylinder shafts.

Thermal products in this category

Typical die steels and machinery structural steels

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
[mm]
Heat treatment [℃] JIS hardness [HB] Design hardness (4 HRC Main applications
New Old
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.
  1. Manganese steel (Mangan is increased so that S38C can be oil-quenched. Example: SMn438)
  2. Boron steel (Boron is added to SCr440 to achieve the hardenability equivalent to that of SCM440. Example: SCrB440)
  3. 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 *.

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