Can You Distinguish The Quenching Tempering Normalizing And Annealing

What is Quenching?

Quenching of steel is a heat treatment process in which the steel is heated to the critical temperature Ac3(hypo-eutectoid steel) or Ac1(hypereutectoid steel), holding it for a period of time to make it austenitized in whole or part, and then rapidly cooling to below MS( or isothermal near MS) at a cooling rate greater than the critical cooling rate for martensite (or bainite) transformation.

In general, the solution treatment of aluminum alloy, cooper alloy, titanium alloy, tempering glass and other materials or the heat treatment process with rapid cooling process is called quenching.

What is the purpose of quenching?

1) To improve the mechanical behavior of metal products or parts, for example: improve the hardness and wear resistance of tools and bearings, improve the elastic limit of mechanical spring, and improve the comprehensive mechanical properties of shaft parts, etc;

2) To improve the material properties or chemical characteristics of some special steels such as improving the corrosion resistance of stainless steel, increasing the permanent magnetism of magnetic steel and so on;

When the quenching cooling, it is not only need to choose the reasonable quenching medium, but also has the correct method of quenching. The common methods of quenching include single-liquid quenching, double-liquid quenching, interrupted quenching, austempering and partial quenching, etc.

What are characteristics of steel after quenching?

1) Unbalanced structures such as the martensite, bainite, and retained austenite are obtained;

2) Exists the large internal stress;

3)The mechanical properties can not meet the requirements, so the steel workpieces need to temper after quenching.

What is Tempering?

Tempering is a heat treatment process in which the quenched metal material or parts are heated to a certain temperature, after keeping warm for a period time, and then cooled in a certain method. It is an operation immediately after quenching, which is usually the last working procedure for heat treatment of work-pieces. Therefore, the combined process of quenching and tempering is called final treatment.

What is the purpose of quenching and tempering?

1) To reduce the internal stress and brittleness. Because the quenched parts have great stress and brittleness, if it is not tempered in time, the work-pieces will deform or even crack;

2) Adjust the mechanical properties of the work-pieces. After quenching, the hardness of work-pieces will be higher, the brittleness will be greater, in order to meet the different performance requirements of various work-pieces. The hardness, strength, plasticity and toughness can be adjusted by tempering;

3) Stable the work-pieces size. The micro structure can be stabilized by tempering to ensure that no deformation in the later use;

4) Improve the cutting performance of some alloy steel.

What is the function of tempering?

1) In order to improve the stabilize of the structure, the structure of the work-pieces will not change in the process of use, the geometric dimensions and performance of the work-pieces can remain stable.

2) Eliminate the internal stress, in order to improve the performance or stabilize the geometric dimensions of the work-pieces.

3) Adjust the mechanical properties of steel to meet the operation requirements.

Why does tempering have these functions?

1) When the temperature rises, the atomic activity ability is enhanced, and the atoms of iron, carbon, and other alloying elements in the steel can diffuse rapidly to realize the rearrangement and combination of atoms, which makes the unstable and unbalanced structures gradually transformed into stable and balanced structures. The elimination of internal stress is also related to the decrease in metal strength when the temperature rises.

2) In general, when the steel tempering, the hardness and strength will decrease and the plasticity will increases. The tempering temperature higher, the change of these mechanical properties is greater. Some alloy steels with higher content of alloying elements will separate out some fine metal compounds when tempered in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening.

What are the tempering requirements?

Work-pieces with different functions should be tempered at different temperatures to meet the requirement in use.

1) Tools, bearing, carburized and quenched parts, surface quenched parts are usually tempered at low temperature below 250℃. The parts have no much change after low temperature tempering, the internal stress is reduced, and the toughness is slightly improved;

2) The spring is tempered at a medium temperature at 350~500℃ to obtain higher elasticity and necessary toughness;

3) Parts made of medium carbon structural steel are often tempered at high temperature at 500~600℃ to obtain a good match of suitable strength and toughness.

When the steel is tempered at about 300℃, the brittleness will be increased, we will called it the first type of temper brittleness. Generally, it should not be tempered at this temperature range. Another, some medium-carbon alloy structural steels are also prone to brittle if they are slowly cooled to room temperature after high-temperature tempering. So this phenomenon is called the second type of temper brittleness.

The second type of temper brittleness can be prevented by adding molybdenum to steel, or cooling in the oil or water during tempering. Reheating the steel with the second temper brittleness to the original tempering temperature, it can eliminate the brittleness.

In production, the tempering is divided into low temperature tempering, medium temperature tempering, and high temperature tempering according to the requirements of work-pieces performance and the different heating temperature. The heat treatment process that combines quenching and subsequent high temperature is called quenching and tempering, which means that it has high strength and good plasticity and toughness.

Low temperature tempering: 150-250℃, M(martensite)-tempering, reduce internal stress and brittleness, improve the plasticity, the work-pieces have higher hardness and wear resistance. Also be used to make measuring tools, cutting tools and rolling bearings, etc.

Medium temperature tempering: 350-500℃, T(troostite)-tempering, with high elasticity, the work-pieces have plasticity and hardness. Workers often use these work-pieces to make springs, forging dies etc.

High Temperature tempering: 500-650℃, S(sorbite)-tempering, with good comprehensive mechanical properties. They are often used to make gears, crankshafts, etc.

What is Normalizing?

Normalizing is a heat treatment that improves the toughness of steel. After the steel members are heated to 30~50℃ above the Ac3 temperature, they will keep a warm for a period of time and then cooled out of the furnace.

Characteristic of normalizing

1) The cooling speed is faster than annealing and lower than quenching.

2) When normalizing, the crystal grains of the steel can be refined in a slightly faster cooling, it is not only can get the satisfied strength, but also the toughness (AKV value) will have a significantly improved, and reduce the cracking tendency of the steel members.

3) After some low-alloy hot-rolled steel plates, low-alloy steel forgings and castings are normalized, the comprehensive mechanical properties of the materials can be greatly improved, and the cutting performance is also improved.

What is the purpose and application of normalizing?

1) Hypo-eutectoid steel: the normalizing is used to eliminate the overheated coarse-grain structure and widmanstatten structure of the cast, forging, and weldment, the banded structure in the rolled material, refined grain, and can be used as pre-heated treatment before quenching.

2) Hypereutectoid steel: the normalizing can eliminate the network secondary cementite and refine the pearlite. It is not only can improve the mechanical properties, but also facilitates the subsequent spheroidizing annealing.

3) Low-carbon deep drawing sheet steel: the normalizing can eliminate the proeutectoid cementite in the grain boundary to improve the deep-drawing performance.

4) Low-carbon steel and low-carbon low alloy steel: adopting the normalizing will obtain more flake pearlite structure, and increase the hardness to HB140-190. Another, it can avoid the phenomenon of “sticking knife” during cutting, and improve the machinability. It is more economical and convenient to use normalizing for medium carbon steel when both normalizing and annealing are available.

5) Ordinary medium carbon structural steels: normalizing can be used instead of quenching and high temperature tempering when the requirements of mechanical properties are not high, which is not only easy to operate, but also make the structure and size of steels more stable.

6) High temperature normalizing(150-200℃ above Ac3): It can reduce the component segregation of castings and forgings due to the high diffusion rate at high temperature. The coarse grains after high temperature normalizing can be refined by the second lower temperature normalizing.

7) Some low and medium carbon alloy steels used in steam turbines and boilers: they are often use the normalizing to obtain bainite structure, and then through the high temperature tempering, these low and medium carbon alloy steels will have a good creep resistance when used at400-500℃.

8) In addition to steel parts and steels, normalizing is also widely used for heat treatment for ductile iron to obtain a pearlite matrix and improve the strength of ductile iron.

Because of the characteristic of normalizing is air cooling, the ambient temperature, stacking method, airflow and workpiece size will affect the structure and performance after normalizing.

The normalizing structure can also be used as a classification method for alloy steel. In general, alloy steels are divided into pearlite steel, bainite steel, martensitic steel and austenitic steel based on the structure obtained by heating a sample with a diameter of 25 mm to 900 ℃ and air cooling.

What is annealing?

Annealing is a metal heat treatment process, in which the metal is slowly heated to a certain temperature, kept for enough time, and then cooled at a suitable speed. It is divided into full annealing, partial annealing and stress relieving. The mechanical properties of annealed materials can be tested by tensile test pr hardness test. Many steels are supplied in annealed heat treatment state; the steel hardness is tested by Rockwell hardness tester to test HRB hardness. For the sheet steel, steel strips and thin-walled steel pipes, the surface Rockwell hardness tester can be used to test HRT hardness.

What is the purpose of annealing

1) To improve or eliminate various structural defects and residual stresses in the process of casting, forging, rolling and welding, and prevent the deformation and cracking of the workpice;

2) Soften the work pieces for cutting;

3) Refine the grains and improve the structure to increase the mechanical properties of the workpiece;

4) Prepare for the final heat treatment (quenching and tempering).

Commonly used annealing process

1) Full annealing: It is used to refine the coarse overheated structure of poor mechanical properties after casting, forging and welding of medium and low carbon steel. Heating the workpiece to 30-50℃ above the temperature at which all ferrite is transformed into austenite, keep it for a period of time, and then slowly cool down with the furnace. During the cooling process, the austenite transforms again to make the steel structure finer.

2) Spheroidizing annealing: It is used to reduce the high hardness of tool steel and bearing steel after forging. When the workpiece is heated to 20-40 ℃ above the temperature at which austenite begins to form, and cool slowly after holding, the lamellar cementite in pearlite becomes spherical during cooling process, and then reducing the hardness.

3) Isothermal annealing: It mains used to reduce the high hardness of some alloy structural steels with high nickel and chromium contents for cutting. Generally, it is cooled to the most unstable temperature of austenite at a faster rate, and after holding for a proper time, the austenite is transformed into troostite or sorbite, and the hardness can be reduced.

4) Recrystallization annealing: this is a way to eliminate the hardening phenomenon (increase the hardness and decrease the plasticity) of the metal wire and sheet during cold drawing and cold rolling. The heating temperature is generally 50-150 ℃ below the temperature at which austenite begins to form. Only in this way can the work-hardening effect be eliminated and the metal can be softened.

5) Graphitizing annealing: making the cast iron (containing a lot of cementite) into malleable cast iron with good plasticity. The process operation is to heat the casting to about 950 ℃ and cool it properly after holding for a certain time, so that the cementite decomposes and forms flocculent graphite.

6) Diffusion annealing: Homogenize the chemical composition of alloy casting and improve their performance. On the premise of no melting, the casting is heated to the highest temperature as possible and kept warm for a long time. After the diffusion of various elements in the alloy tends to be evenly distributed, and then cooled slowly.

7) Stress relieving: Eliminate the internal stress of steel castings and weldments. After heating the steel product, the temperature at which austenite begins to form is below 100-200℃, the internal stress can be eliminated by keeping a warm and then cooled.