Heat treatment facts

• Basics of heat treatment
• Quenching
• Tempering
• Low temperature tempering
• High temperature tempering
• Quenching hardness
• Quenching depth
• Mass effect
• Sub-zero treatment
• Dimensional instability
• Annealing
• Full annealing
• Spheroidized annealing
• Stress relief annealing
• Normalizing

Basics of heat treatment

In most cases, steel regains its inherent traits through heat treatment.
Basically the traits can be controlled by the temperature rising rate, heating temperature and holding time during heating process and cooling rate and holding time during cooling process. Also, surface treatment can change the traits further by improving toughness, wear resistance and life span.
Our main business is vacuum heat treatment, but we are also contracted to perform various surface treatments (induction hardening, carburizing, gas soft nitriding, ion nitriding, Tufftride, etc.).

Quenching

Quenching is a kind of heat treatment to harden steel.
To gain hardness, steel has to be heated to and held at the austenitizing temperature, and then cooled rapidly to transform it into martensite. This rapid cooling is termed quenching. Water, oil, air and gas are used as cooling medium.
The rules of quenching are
(1) Steel must be heated up 50°C above the A3 or A3-1 transformation point to be fully austenitized.
(2) The heated steel has to be cooled rapidly through the critical region and then slowly through the danger zone.
Regarding austenitizing temperature, the most important rule is the rapid cooling method.
Cool rapidly through the critical region and slowly through a danger zone.
To do so, we need to come up with various ideas.
Water or oil is used to cool rapidly through the critical region. But, water cools rapidly through the danger zone as well, thus causing a quench cracking and deformation, while oil has a risk of causing fire.
We can offer stable quality with the optimized heat treatment method and condition to each material and shape. This is enabled by accumulated techniques and 18 vacuum heat treatment furnaces.

Tempering

Tempering adjusts the hardness of a quenched steel to make it into a tougher steel and also removes the internal strain.
In general, the tempering temperature for structural steel to gain toughness ranges above 400°C and for steel to gain hardness around 200°C. The former is called "high temperature tempering" or "thermal refining" and the latter (below 200°C) "low temperature tempering".
The rules of tempering are
(1) Steel must be heated to below the A1 transformation point and cooled rapidly.
(2) By default, it is performed right after quenching.
That is because when left for a long time after quenching, the piece may form a season cracking. We set the standard tempering hold time at around an hour but it is considered more effective to repeat tempering 2 to 3 times for a shorter time than tempering once for a longer time.
It should be noted that depending on the tempering temperature, there is a temperature range that causes brittleness.
We can offer stable quality with the optimized heat treatment method and condition according to each material and shape. This is enabled by accumulated techniques and 18 vacuum heat treatment furnaces.

Low temperature tempering

This low temperature tempering is used for the products that require high hardness and wear resistance, like shafts and gauges.
The tempering temperature ranges around 150 to 200°C and it should be held at least for an hour.
Low temperature tempering turns the hard and brittle quenched martensite into a tough tempered martensite. It also removes stress and improves dimensional stability and wear resistance.

High temperature tempering

This high temperature tempering is used for the products that require high toughness, like shafts, gears, and tools made with high speed tool steel or hot work die steel.
To achieve high toughness, steel is heated up to around 550 to 650°C and held for at least an hour, and then cooled rapidly to prevent the formation of brittleness. As a result, troostite structure is formed at around 400°C and sorbite at around 600°C.
In either case, it is basically a mixed phase consisting of ferrite and cementite. Tempering temperature for the purpose of hardening is at around 500 to 600°C and air cooling is used.
This treatment transforms retained austenite at quenching stage into martensite. The first tempering transforms retained austenite into martensite. The second one is tempering in its true sense. For that reason, tempering has to be performed at least twice for hardening purposes.

Quenching hardness

Quenching increases hardness. In case of tool steels, elements like W (tungsten), Cr (chrome) and V (vanadium) affect the hardness. But, in case of structural steels, only C (carbon) content (%) affects hardness and other alloy elements do not.

Quenching depth

The chemical composition of the material significantly affects the depth of quenching. This trait that affects the depth of quenching is called "hardenability". "C%" affects hardenability most.
Then, B, Mn and Cr in respective order affect the depth, while Si (Silicon) and Ni (Nickel) have little influence.
The grain size at the austenitizing temperature also affects the depth. The coarser the grains are, the stronger the hardenability is, and thus steel is hardened deeper.

Mass effect

Even when the compositions are same, thicker and larger steels are more difficult to be hardened.
So, hardness and depth change according to the mass. Thus, this effect is termed "mass effect".
When the mass effect of a piece is strong, the difference of hardness becomes greater depending on its size. Thus, the larger the piece is, the more difficult it is to be hardened.
On the contrary, when the mass effect is weak, the piece is less affected by mass effect, so even a large piece is quenched well.
In general, carbon steels have stronger mass effect, and special steels have weaker mass effect.

Sub-zero treatment

Sub-zero treatment, also called deep cold treatment, cools steels to less then 0°C.
Quenched steels contain about 10 to 30% of retained austenite.
This austenite can cause a cracking or dimensional change and lead to lowered hardness. Thus, it is necessary to form martensite artificially by cooling a piece to less then 0°C.
Sub-zero treatment is one of the methods to do so.
The cooling medium includes dry ice, carbon dioxide and liquid nitrogen.
Dry ice and alcohol (methyl, ethyl) can cool a piece down to about -80°C, carbon dioxide to about -130°C, and liquid nitrogen to about -196°C.　A sub-zero treatment down to -80°C is called normal sub-zero, while below -130°C is called super sub-zero treatment. The lower the temperature, the better the wear resistance becomes.　Treatment takes about 30 to 60 minutes after reaching that temperature. After cooling, a piece can be left in the atmosphere, but we think it's better to immerse the piece in water or hot water. After the treatment, a predetermined tempering is needed.
In our company, we perform sub-zero treatment everyday.
Our proud sub-zero equipment has a large size (effective size: H800 x W800 x D1,200) unmatched in the prefecture and has the wide range of treatment temperature (-50 to -196°C). If you have a super sub-zero or cryogenic treatment related problem, please consult us.

Dimensional instability

This phenomenon demonstrates the volume expansion caused by the transformation of retrained austenite in products like precision cold die into martensite over time.
It rarely occurs among low temperature tempered products, but is more common among high temperature tempered products because retained austenite is unstable. Sub-zero treatment or stabilization treatment is recommended as a measure.

Annealing

Annealing is a treatment, which adjusts grain size of steel and soften it.
There are various methods depending on the purpose.
(1) Let a steel heated up to 50°C above the A3 or A3-1 transformation point to be fully austenitized.
(2) At just below the Ar1 point (around 700°C), let the austenite transform into pearlite.

Full annealing

In general, annealing means full annealing, in which a steel is heated up to 50°C above the transformation point and then cooled down slowly inside a furnace.
It is not necessary to cool down slowly to room temperature, but only to the critical region. Then, the piece can be cooled rapidly. However, to minimize residual stress, it is better to cool slowly until around 400°C before choosing to cool rapidly or leave it in the ambient atmosphere.

Spheroidized annealing

This is a kind of annealing that transforms cementite or reticular cementite in pearlite into spheroidized cementite.
The typical methods include
(1) A method to heat up to just below or just above the Ac1 point.
(2) A method to heat up to just above the Ac1 point, then go back and forth between just below and just above the Ac1 point several times.
(3) To spheroidize easily or spheroidize a difficult steel like structural steel, quenching a piece then tempering it at high temperature (600 to 700°C) before annealing makes it easier.

Stress relief annealing

This is a kind of low temperature annealing that aims to remove residual stress, soften, and reduce distortion from cold forging, rolled or casting products.
The heat up temperature is above the recrystallization temperature (around 450°C) and below the A1 transformation point. It is usually around 550 to 650°C.
Slow cooling is preferable, but below 450°C, a piece can be cooled in the atmosphere. It works as a pretreatment to reduce quenching deformations.
This is also called SR treatment. With our super large vacuum heat treatment furnace (H 1,300 x W 1,300 x D 1,650), we can handle large products and mass-produced products.

Normalizing

This is a treatment to bring steel back to its standard condition (homogenized structure).
It removes the effects of previous processing, creates fine grain size and improves mechanical properties.
(1) Let steel be heated up to 50°C above the A3 transformation point to be fully austenitized.
(2) A piece can be cooled in the atmosphere or inside a furnace.