In order for metal workpiece to have required working properties, a heat treatment process is often necessary. Heat treatment process generally includes three processes of heating, heat preservation and cooling. It is divided into quenching, tempering, normalizing, annealing, etc. depending on process. Can you distinguish it?

1. Quenching, tempering, normalizing, annealing, do you know
clearly?
In order for metal workpiece to have required working properties, a heat treatment process is
often necessary. Heat treatment process generally includes three processes of heating, heat
preservation and cooling. It is divided into quenching, tempering, normalizing, annealing, etc.
depending on process. Can you distinguish it?
 What is quenching?
Quenching of steel is to heat steel to a temperature above critical temperature Ac3
(hypegmatized steel) or Ac1 (hyper-eutectoid steel), hold it for a period of time, make it all or
part austenitized, and then rapidly cooling to a temperature below Ms (or isothermal near Ms) at
a cooling rate greater than critical cooling rate. Solution treatment of materials such as aluminum
alloys, copper alloys, titanium alloys, tempered glass, or heat treatment process with a rapid
cooling process is also referred to as quenching.
 Purpose of quenching:
1) Improve mechanical properties of metal materials or parts. For example, improve hardness
and wear resistance of tools, bearings, etc., improve elastic limit of spring, and improve
comprehensive mechanical properties of shaft parts.
2) Improve material properties or chemical properties of certain special steels. Such as improving
corrosion resistance of stainless steel, increasing permanent magnetity of magnetic steel.
When quenching and cooling, in addition to reasonable selection of quenching medium, there
must be a correct quenching method. Commonly used quenching methods mainly include
single-liquid quenching, two-liquid quenching, fractional quenching, austempering, and local
quenching.
 Steel workpieces have following characteristics after quenching:
1 Unbalanced(unstable) structures such as martensite, bainite, and retained austenite are
obtained.

2. 2 There is a large internal stress.
3 Mechanical properties can not meet requirements. Therefore, steel workpieces are generally
tempered after quenching
 What is tempering?
Tempering is a heat treatment process in which a quenched metal material or part is heated to a
certain temperature and then cooled in a certain manner after being kept for a certain period of
time. Tempering is an operation performed immediately after quenching, and is usually the last
heat treatment of workpiece. Therefore, combined process of quenching and tempering is called
final treatment.
 Main purposes of quenching and tempering are:
1) Reduce internal stress and reduce brittleness. Quenching parts have great stress and
brittleness. If they are not tempered in time, they will often deform or even crack.
2) Adjust mechanical properties of workpiece. After workpiece is quenched, hardness is high and
brittleness is large. In order to meet different performance requirements of various workpieces,
hardness, strength, ductility and toughness can be adjusted by tempering.
3) Stabilize workpiece size. By tempering, metallographic structure can be stabilized to ensure
that deformation does not occur during subsequent use.
4) Improve cutting performance of certain alloy steels.
 Role of tempering is:
1 Improve stability of structure, so that workpiece no longer undergoes tissue transformation
during use, workpiece geometry and performance are stable.
2 Eliminate internal stress to improve performance of workpiece and stabilize geometry of
workpiece.
3 Adjust mechanical properties of steel to meet requirements of use.
Reason why tempering has these effects is because atomic activity is enhanced when
temperature is raised, atoms of iron, carbon and other alloying elements in steel can be diffused
faster, realizing rearrangement and combination of atoms, thereby unstable unbalanced tissue is
gradually transformed into a stable equilibrium organization. Elimination of internal stress is also
related to decrease in metal strength at elevated temperatures. In general, when steel is

3. tempered, hardness and strength are lowered, and plasticity is improved. The higher tempering
temperature, the greater change in these mechanical properties. Some alloy steels with a high
content of alloying elements will precipitate some fine metal compounds when tempered in a
certain temperature range, which will increase strength and hardness. This phenomenon is called
secondary hardening.
 Tempering requirements:
Workpieces with different uses should be tempered at different temperatures to meet
requirements of use.
1 Tools, bearings, carburized and quenched parts, and surface hardened parts are usually
tempered at a low temperature below 250℃. After low temperature tempering, hardness does
not change much, internal stress decreases, and toughness is slightly improved.
2 Spring is tempered at 350-500 ℃ at medium temperature to obtain high elasticity and
necessary toughness.
3 Parts made of medium carbon structural steel are usually tempered at a high temperature of
500 to 600℃ to obtain a good fit of strength and toughness.
When steel is tempered at around 300℃, its brittleness is often increased. This phenomenon is
called the first type of temper brittleness. Generally should not temper in this temperature range.
Some medium carbon alloy structural steels tend to become brittle if they are slowly cooled to
room temperature after tempering at high temperatures. This phenomenon is called second type
of temper brittleness. Addition of molybdenum to steel, cooling in oil or water during tempering
prevents second type of temper brittleness. This brittleness can be eliminated by reheating
second type of tempered brittle steel to original tempering temperature.
In production, tempering is often divided into low temperature tempering, medium temperature
tempering, and high temperature tempering according to requirements of performance of
workpiece and heating temperature. Heat treatment process combined with quenching and
subsequent high temperature tempering is called quenching and tempering, that is, it has high
strength and good plastic toughness.
1) Low temperature tempering: 150-250℃, M tempering, reduce internal stress and brittleness,
improve plastic toughness, have high hardness and wear resistance. Used in production of
measuring tools, tools and rolling bearings.
2) Medium temperature tempering: 350-500 ℃ , T tempering, with high elasticity, a certain
plasticity and hardness. Used to make springs, forging dies, etc.
3) High temperature tempering: 500-650℃, S tempering, with good comprehensive mechanical
properties. Used to make gears, crankshafts, etc.

4.  What is normalizing?
Normalizing is a heat treatment that improves toughness of steel. After heating steel member to
30~50℃ above Ac3 temperature, steel is cooled for a while. Main feature is that cooling rate is
faster than annealing and less than quenching. During normalizing, crystal grains of steel can be
refined in a slightly faster cooling, which not only can obtain satisfactory strength, but also can
significantly improve toughness (AKV value) and reduce tendency of member to crack. After
normalized treatment of some low-alloy hot-rolled steel sheets, low-alloy steel forgings and
castings, comprehensive mechanical properties of material can be greatly improved, and cutting
performance is also improved.
 Normalizing has following purposes and uses:
1 For sub-eutectic steel, normalizing is used to eliminate superheated coarse-grained structure
and Wei's structure of cast, forged and welded parts, banded structure in rolled material, and
grain refinement, and it can be used as a pre-heat treatment before quenching.
2 For hypereutectoid steel, normalizing can eliminate network secondary cementite and refine
pearlite, which not only improves mechanical properties, but also facilitates subsequent
spheroidizing annealing.
3 For low-carbon deep-drawn thin steel sheets, normalizing can eliminate free cementite at grain
boundary to improve its deep-drawing performance.
4 For low-carbon steel and low-carbon low-alloy steel, normalizing can obtain more fine-grained
pearlite structure, so that hardness is increased to HB140-190, avoiding "sticking knife"
phenomenon during cutting and improving machinability. For medium carbon steel, it is more
economical and convenient to use normalizing in the case where both normalizing and annealing
can be used.
5 For ordinary medium carbon structural steel, in the case where mechanical properties are not
high, normalizing can be used instead of quenching and high temperature tempering, which is
not only easy to operate, but also makes steel structure and size stable.
6 High-temperature normalizing (Ac-3 or more 150 to 200℃). Due to high diffusion rate at high
temperatures, composition segregation of castings and forgings can be reduced. Coarse grains
after high temperature normalizing can be refined by a second lower temperature normalizing.
7 For some low and medium carbon alloy steels used in steam turbines and boilers, normalizing is
often used to obtain bainite structure, then tempered at high temperature for good creep
resistance at 400-550℃.
8 In addition to steel, normalizing is also widely used in heat treatment of ductile iron to obtain a
pearlite matrix and improve strength of ductile iron.
Since normalizing is characterized by air cooling, ambient temperature, stacking mode, airflow,
and workpiece size all have an impact on organization and performance after normalizing.
Normalizing tissue can also be used as a classification method for alloy steel. Generally, according
to structure obtained by air cooling after heating a sample with 25 mm diameter to 900 ℃ ,
alloy steel is divided into pearlitic steel, bainitic steel, martensitic steel and austenitic steel.

5.  What is annealing?
Annealing is a metal heat treatment process in which metal is slowly heated to a certain
temperature for a sufficient period of time and then cooled at a suitable rate. Annealing heat
treatment is divided into complete annealing, incomplete annealing and stress relief annealing.
Mechanical properties of annealed material can be tested by tensile testing or by hardness
testing. Many steels are supplied in an annealed heat treatment state. Hardness test of steel can
be tested by Rockwell hardness tester. For thinner steel plates, steel strips and thin-walled steel
pipes, a surface Rockwell hardness tester can be used to detect HRT hardness. .
 Purpose of annealing is to:
1 Improve or eliminate various structural defects and residual stresses caused by steel during
casting, forging, rolling and welding, prevent deformation and cracking of workpieces.
2 Soften workpiece for cutting.
3 Refine grains and improve structure to improve mechanical properties of workpiece.
4 Prepare organization for final heat treatment (quenching, tempering).

6.  Common annealing processes are:
1 Fully annealing. It is used to refine coarse superheated structure of medium and low carbon
steel which has poor mechanical properties after casting, forging and welding. Workpiece is
heated above a temperature of 30 to 50℃ at which all of ferrite is transformed into austenite,
kept for a period of time, slowly cooled with furnace, and austenite is again transformed during
cooling process, so that microstructure of steel is thinned. .
2 Spheroidizing annealing. It is used to reduce high hardness of tool steel and bearing steel after
forging. Workpiece is heated above a temperature of 20 to 40℃ at which steel begins to form
austenite, and is slowly cooled after heat preservation. During cooling process, lamellar
cementite in pearlite becomes spherical, thereby lowering hardness.
3 Isothermal annealing. It is used to reduce high hardness of certain alloy structural steels with
high nickel and chromium content for cutting. Generally, it is cooled to the most unstable
temperature of austenite at a relatively rapid rate, austenite is transformed into torsite or sorbite
at a suitable temperature for a suitable period of time, and hardness can be lowered.
4 Recrystallization annealing. It is used to eliminate hardening phenomenon (hardness increase,
plasticity drop) of metal wire and sheet during cold drawing and cold rolling. Heating
temperature is generally 50 to 150℃ below temperature at which steel begins to form austenite.
Only in this way can work hardening effect be eliminated and metal softened.

7. 5 Graphitization annealing. It is used to make cast iron containing a large amount of cementite
into a malleable cast iron with good plasticity. Process operation is to heat casting to about 950℃,
and after proper cooling for a certain period of time, cementite is decomposed to form a group of
flocculent graphite.
6 Diffusion annealing. It is used to homogenize chemical composition of alloy castings and
improve their performance. Method is to heat casting to the highest possible temperature
without melting, keep for a long time, various elements in alloy tend to be uniformly distributed
and then slowly cooled.
7 Stress relief annealing. Used to eliminate internal stress of steel castings and welded parts.
After steel product is heated, austenite is formed at a temperature of 100 to 200℃ below, and
after cooling, it is cooled in the air to eliminate internal stress.