Heat treatment of steel: Refers to the process of heating, heat preservation and cooling of solid steel appropriately to obtain the required structure and properties.
Heat treatment can be used not only to strengthen steel and improve the service performance of mechanical parts, but also to improve the technological performance of steel.
The common point is that only the internal organizational structure is changed, without changing the shape and size of the surface.
The heat treatment process can significantly improve the mechanical properties of steel, increase the strength, toughness and service life of parts, and improve hardness and wear resistance.
Therefore, important machine parts and tools must be heat treated.
Heat treatment can also improve the processing performance of the part, thereby improving processing productivity and quality.
Therefore, heat treatment plays a very important role in the machinery manufacturing industry.
Let's take #45 steel and 40Cr steel as examples.
High-temperature tempering after quenching is called “quenching and tempering” in production. The parts after quenching and tempering have good comprehensive mechanical properties and are widely used in various important structural parts, especially connecting rods, screws, gears and shafts that work under alternating load.
However, the surface hardness is low and is not wear-resistant.
The surface hardness of parts can be improved by quenching and tempering + surface quenching.
1. Steel #45 – high quality medium carbon structural steel
#45 steel is named in GB, named S45C in JIS, 1045080M46 in ASTM, and C45 in DIN;
#45 steel is a high-quality carbon structural steel, with chemical composition: carbon (C) content of 0.42 ~ 0.50%, Si content of 0.17 ~ 0.37%, Mn content from 0.50 ~ 0.80%, Cr content <= 0.25%.
The cold and hot processing performance is good, the mechanical performance is good, the price is low, and the supply is wide, so it is widely used.
Its biggest weakness is that parts with low hardenability, large section dimensions and high requirements should not be used.
The recommended temperature for heat treatment of #45 steel: normalizing at 850°C, quenching at 840°C, tempering at 600°C.
① #45 steel is qualified if its hardness is greater than HRC55 (up to HRC62) after quenching and before tempering.
The highest hardness in practical application is HRC55 (high frequency quenching HRC58).
② The heat treatment process of carburizing and quenching is not adopted for #45 steel.
Quenching and tempering of #45 steel: the quenching temperature of #45 steel is A3+(30~50) ℃. In practical operation, it is generally considered the upper limit.
Higher quenching temperature can accelerate the heating speed of the workpiece, reduce surface oxidation and improve work efficiency.
To homogenize the austenite of the part, a sufficient retention time is required.
If the actual amount of cargo is large, it is necessary to extend the retention time appropriately.
Otherwise, insufficient hardness may occur due to uneven heating.
However, if the waiting time is too long, coarse grain defects and severe oxidation decarburization will also occur, affecting the quenching quality.
We believe that the heating and holding time should be extended by 1/5 if the amount of charge is greater than that specified in the process document.
As the hardenability of #45 steel is low, 10% saline solution with a high cooling rate must be used.
After the workpiece is cooled in water, it must be quenched but not completely cooled.
If the workpiece is cooled in salt water, it may crack.
This is because when the part is cooled to about 180℃, austenite quickly transforms into martensite, resulting in excessive structural stress.
Therefore, when the quenched part is quickly cooled to this temperature range, the slow cooling method should be adopted.
Because the outlet water temperature is difficult to control, it must be operated with experience. When the workpiece in the water stops shaking, the outlet water can be cooled by air (oil cooling is better if possible).
Furthermore, the workpiece must move rather than remain static when entering the water. It should move regularly according to the geometric shape of the workpiece.
The static cooling medium plus the static workpiece will lead to uneven hardness and stress, resulting in large deformation and even cracking of the workpiece.
The hardness of quenched and tempered #45 steel parts should reach HRC56~59, and the possibility of large section is less, but it cannot be lower than HRC48.
Otherwise, it means that the part has not been fully quenched, and there may be sorbite or even ferrite structure in the structure, which is still retained in the matrix after tempering, and the purpose of quenching and tempering cannot be achieved.
High temperature tempering of #45 steel after quenching, the heating temperature is generally 560 ~ 600 ℃, and the hardness requirement is HRC22 ~ 34.
Since the purpose of tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide.
However, if the drawing has hardness requirements, the tempering temperature must be adjusted according to the drawing requirements to ensure hardness.
For example, some shaft parts require high strength and high hardness;
However, for some gears and shaft parts with keyways, the hardness requirements are lower due to milling and grooving after quenching and tempering.
The tempering heat preservation time depends on the hardness requirements and the size of the workpiece.
We believe that the hardness after tempering depends on the tempering temperature and has little relationship with the tempering time, but must be penetrated back.
Generally, the tempering heat preservation time of the workpiece is more than one hour.
If #45 steel is used for carburizing, hard and brittle martensite will appear in the core after quenching and the advantages of carburizing treatment will be lost.
At present, the carbon content of carburized materials is not high, and the core strength can reach 0.30%, which is rare in the application.
0.35% have never seen examples, only introduced in textbooks.
The process of quenching and tempering + high-frequency surface quenching can be adopted, and the wear resistance is a little worse than that of carburizing.
2. 40Cr steel – structural steel alloy
40Cr belongs to GB3077 “Structural Alloy Steel”.
The carbon content of 40Cr steel is 0.37%～0.44%, which is slightly lower than that of #45 steel. The content of Si and Mn is equivalent, with 0.80%～1.10% Cr.
In the case of hot rolling supply, 1% Cr basically does not work, and the mechanical properties of the two are practically the same.
Since the price of 40Cr is about half the price of #45 steel, it is unnecessary for those who can use #45 steel for economic reasons.
Quenching and tempering treatment of 40Cr steel: The main role of Cr in heat treatment is to improve the hardenability of steel.
Due to the improvement of hardenability, the strength, hardness, impact resistance and other mechanical properties of 40Cr after quenching (or tempering) treatment are also significantly higher than those of #45 steel.
However, due to the strong hardenability, the internal stress of 40Cr during quenching is also higher than that of #45 steel.
Under the same conditions, the crack slope of 40Cr material is also greater than that of #45 steel.
Therefore, to prevent cracking of workpieces, oil with low thermal conductivity is mainly used as quenching medium during 40Cr quenching (sometimes double liquid quenching method, commonly known as water quenching and oil quenching, is also used ), while water with high thermal conductivity is used as a quenching medium for 45Cr steel.
Of course, the choice of water and oil is not absolute and is also closely related to the shape of the piece.
Water quenching can also be used for 40Cr parts with simple shapes, while oil quenching or even salt bath can be used for #45 steel parts with complex shapes.
For quenching and tempering of the 40Cr part, several parameters are specified in the process sheet.
Our actual operation experience is as follows:
(1) Oil cooling should be adopted for 40Cr parts after quenching.
40Cr steel has good hardenability, can be tempered when cooled in oil, and the tendency of parts to deform and crack is small.
However, in the case of tight oil supply, small companies can quench the uncomplicated shaped workpiece in water without cracking, but the operator must strictly control the water inlet and outlet temperature based on experience.
(2) The hardness of the 40Cr workpiece is still high after tempering, and the temperature of the second tempering will increase by 20 ~ 50 ℃, otherwise it will be difficult to reduce the hardness.
(3) After high-temperature tempering, 40Cr parts with complex shapes are quenched in oil and simply quenched in water to avoid the impact of the second type of tempering brittleness.
Parts after tempering and rapid cooling will undergo stress relief treatment, if necessary.
The maximum hardness of medium carbon steel after heat treatment is about HRC55 (HB538), σb is 600~1100MPa.
Therefore, medium carbon steel is the most used in various applications with a medium resistance level.
In addition to being used as a construction material, it is also widely used in the manufacture of various mechanical parts.
As long as medium carbon steel has sufficient temperature and retention time, it is generally possible to achieve this hardness value, and it is impossible if it does not deform.
The first is to have machining margin and then use a grinding machine for machining, and the second is surface hardening.
