Understanding Heat Treatment: Quenching, Tempering, Normalizing, Annealing

To make metal parts perform well, a heat treatment process is often essential. Heat treatment typically includes three phases: heating, soaking, and cooling. Depending on how fast you cool, when you hold temperature, and how you reheat, you’ll get quenching, tempering, normalizing, or annealing. It’s important to understand what each process does and when to apply them.

In many manufacturing setups, including precision CNC machining, the right heat treatment ensures that parts meet hardness, toughness, and dimensional stability standards. Richconn, for example, uses deep knowledge of these processes to deliver machinable, high-performance parts.

Quenching: What It Means & Why It’s Used

Quenching is when metal—especially steel—is heated above a critical temperature (Ac3 for hypo-eutectoid steels or Ac1 for hyper-eutectoid steels), held until it becomes austenitic, and then cooled rapidly past the critical cooling rate down below Ms (martensite start). This produces a martensitic or bainitic structure. In non-ferrous metals (aluminum, titanium, copper), rapidly cooling from a solutionizing temperature is also called quenching.

The purpose of quenching:

(1) to improve the mechanical properties of the metal into material or parts. For example: to improve the hardness and wear resistance of tools, bearings, etc., to improve the elastic limit of the spring, to improve the overall mechanical properties of shaft parts.

(2) improve the material properties or chemical properties of some special steel. Such as improving the corrosion resistance of stainless steel, increase the permanent magnetism of magnetic steel.

Quenching and cooling, in addition to the need for a reasonable choice of quenching medium, but also the correct quenching method, commonly used quenching methods, mainly single-liquid quenching, double-liquid quenching, graded quenching, isothermal quenching, localized quenching and so on.

Steel workpiece after quenching has the following characteristics:

① obtained martensite, bainite, residual austenite and other unbalanced (i.e., unstable) organization.

② the existence of large internal stress.

③ mechanical properties can not meet the requirements. Therefore, steel workpiece quenching generally have to go through the tempering

Tempering: Taming Hardness & Stress

Tempering means reheating a previously quenched metal to a lower temperature (below the critical temperature) and holding it there before cooling. This step adjusts properties and relieves stresses.

The main purposes of quenching and tempering are:

(1) reduce internal stress and reduce brittleness, quenched parts exist a lot of stress and brittleness, such as no timely tempering often produce deformation or even cracking.

(2) adjust the mechanical properties of the workpiece, workpiece quenching, high hardness, brittleness, in order to meet the different performance requirements of a variety of workpieces can be adjusted by tempering, hardness, strength, plasticity and toughness.

(3) Stabilize the size of the workpiece. By tempering can make the metallurgical organization tends to stabilize, in order to ensure that no further deformation in the future use of the process.

(4) Improve the cutting performance of certain alloy steels.

The role of tempering is to:

(1) improve the stability of the organization, so that the workpiece in the use of the process no longer occurs in the organization of the transformation, so that the geometry and performance of the workpiece to maintain stability.

② eliminate internal stresses in order to improve the performance of the workpiece and stabilize the workpiece geometry.

③ adjust the mechanical properties of steel to meet the requirements of use.

In production, often according to the requirements of the workpiece properties. According to the different heating temperatures, the tempering is divided into low-temperature tempering, medium-temperature tempering, and high-temperature tempering. Quenching and subsequent high-temperature tempering combined heat treatment process known as tempering, that is, in a high degree of strength at the same time, but also good plastic toughness.

(1) low-temperature tempering: 150-250 ℃ , M back, reduce internal stress and brittleness, improve the plastic toughness, high hardness and wear resistance. Used in the production of gauges, cutting tools and rolling bearings.

(2) medium temperature tempering: 350-500 ℃, T back, with high elasticity, a certain degree of plasticity and hardness. Used for making springs, forging molds, etc.

(3) high temperature tempering: 500-650 ℃, S back, with good overall mechanical properties. Used for making gears, crankshafts, etc.

Normalizing: A Middle Ground Treatment

Normalizing sits between quenching and annealing. The steel is heated above its critical temperature (Ac3 + 30–50 °C), held, then air-cooled. This cooling rate is faster than annealing but slower than quenching.

Normalizing has the following purposes and uses:

① on the sub-eutectic steel, normalizing to eliminate casting, forging, welding parts of the overheating coarse crystalline organization and Wei’s organization, rolled material in the banded organization; refinement of grain; and can be used as a quenching before the pre-heat treatment.

② eutectic steel, normalizing can eliminate the network of secondary carburization, and make the pearlite refinement, not only to improve the mechanical properties, but also conducive to the subsequent spherical annealing.

③ low carbon deep-drawn thin steel plate, normalizing can eliminate free carburization of grain boundaries to improve its deep-drawn properties.

④ low carbon steel and low carbon low alloy steel, the use of normalizing, you can get more fine flake pearlitic organization, so that the hardness increased to HB140-190, to avoid cutting the “sticky knife” phenomenon, improve the cutting processability. For medium carbon steel, in both available normalizing and annealing occasions, with normalizing is more economical and convenient.

⑤ for ordinary medium-carbon structural steel, in the mechanical properties required for the occasion is not high, can be used instead of quenching quenching and high-temperature tempering, not only easy to operate, but also to make the organization of steel and dimensional stability.

⑥ High-temperature normalizing (Ac3 above 150 ~ 200 ℃) due to the high temperature diffusion rate is higher, can reduce the composition of castings and forgings segregation. High-temperature normalizing after the coarse grains can be refined through the subsequent second lower temperature normalizing.

⑦ for some of the turbine and boiler for low and medium carbon alloy steel, often using normalizing to obtain the bainite organization, and then by high-temperature tempering, for 400 ~ 550 ℃ with good creep resistance.

⑧ In addition to steel parts and steel, normalizing is also widely used in ductile iron heat treatment, so that it obtains pearlite matrix, improve the strength of ductile iron.

Because of the characteristics of normalizing air cooling, and therefore the ambient air temperature, stacking mode, airflow and workpiece size on the organization and properties after normalizing have an impact. Normalized organization can also be used as a classification method for alloy steel. Usually according to the diameter of 25 mm specimen heated to 900 ℃, air-cooled organization, the alloy steel is divided into pearlite steel, bainitic steel, martensitic steel and austenitic steel.

Annealing: Softening, Stress Relief, and Grain Refinement

Annealing is a slower, gentler treatment: heat to a specific temperature, hold, and then cool slowly (often in furnace). This promotes atomic diffusion, recrystallization, and removal of defects.

The purpose of annealing is to:

① improve or eliminate the steel in the casting, forging, rolling and welding process caused by a variety of organizational defects, as well as residual stress, to prevent the workpiece deformation, cracking.

② soften the workpiece for cutting.

③ refine the grain, improve the organization to improve the mechanical properties of the workpiece.

④ for the final heat treatment (quenching, tempering) to prepare the organization.

Commonly used annealing process:

① complete annealing. Used to refine the medium and low carbon steel by casting, forging and welding after the mechanical properties of poor coarse superheated organization. The workpiece will be heated to ferrite all transformed into austenite temperature above 30 ~ 50 ℃, hold for a period of time, and then slowly cooled with the furnace, the austenite in the cooling process again, you can make the organization of the steel fine.

② spheroidal annealing. Used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece will be heated to the steel began to form austenite temperature above 20 ~ 40 ℃, insulation and slow cooling, in the cooling process of pearlite in the lamellar carburite into spherical, thus reducing the hardness.

③ Isothermal annealing. Used to reduce the high hardness of certain alloy structural steel with high nickel and chromium content for cutting. Generally first cooled to the austenite at a faster rate to the most unstable temperature, insulation for an appropriate period of time, the austenite transformation to tosite or sostenite, the hardness can be reduced.

④ recrystallization annealing. Used to eliminate metal wire, sheet in the cold drawing, cold rolling process of hardening phenomenon (hardness increases, plasticity decreases). The heating temperature is generally 50-150 ℃ below the temperature at which the steel starts to form austenite, and this is the only way to eliminate the work-hardening effect so that the metal can be softened.

⑤ Graphitization annealing. ⑤ Graphitization annealing. This is used to transform cast iron containing a large amount of carburization into malleable cast iron with good plasticity. Process operation is the casting is heated to about 950 ℃, insulation for a certain period of time after appropriate cooling, so that the decomposition of carburite to form a flocculent graphite.

⑥ diffusion annealing. Used to make the chemical composition of alloy castings uniform, improve its performance. Method is not melting under the premise that the casting is heated to the highest possible temperature, and a long period of heat preservation, to be a variety of elements in the alloy diffusion tends to be uniformly distributed after slow cooling.

(7) stress relief annealing. Used to eliminate the internal stress of steel castings and weldments. For iron and steel products begin to form austenite after heating the temperature below 100 to 200 ℃, insulation and cooling in the air, you can eliminate internal stress.

Comparison Table of Heat Treatment Processes