Carbon tool steel, according to its different impurity content, can be divided into high quality carbon tool steel (T7 ~ T13) and high quality carbon tool steel (T7A ~ T13A).
With the increase of carbon content, the hardness and wear resistance of carbon tool steel are gradually increased, but the toughness is gradually decreased.
Eutectoid carbon tool steel (T8), easy to overheat when heated, easy to deform and crack when quenched.
Carbon tool steel is widely used because of its wide source, low cost and good processing performance. However, the thermal hardness (red hardness) of this type of steel is low, and when the operating temperature exceeds 250 ° C, the hardness and wear resistance of the steel are sharply reduced. And its hardenability is poor, when the diameter or thickness of the tool is greater than 15mm, even in water can not be quenched. Therefore, it is mainly used for the manufacture of hand tools, as well as low-speed and small cutting amount of machine tools.
The heat treatment of carbon tool steel includes nodularization annealing, quenching and low temperature tempering.
(1) Annealing
Carbon tool steel is cooled by air after forging. Because of high hardness, it is not easy to cut; And the organization is flake pearlite, easy to local overheating during quenching heating, easy deformation and cracking during cooling process, does not meet the requirements of quenching, so it should be carried out spheroidizing annealing to improve the organization, reduce hardness, improve cutting performance, eliminate forging stress, and prepare the organization for quenching. The microstructure after annealing is granular pearlite.
(2) Quenching
Quenching and tempering are the final heat treatment of the tool, and the quality of heat treatment directly affects the life of the tool.
① quenching heating temperature and heating time. Quenching is the most critical process to determine the quality of the tool. The quenching of the tool not only guarantees high hardness, but also high strength, toughness and minimal deformation. Therefore, the quenching heating temperature should be determined according to the critical point of the steel, and should consider the performance requirements, workpiece shape, size and heating, cooling media and other influencing factors.
For hypoeutectoid steel (or eutectoid steel), complete quenching should be used to make the ferrite in the original structure completely dissolve into austenite to prevent soft points after quenching; For hypereutectoid steel, incomplete quenching should be used, that is, the heating temperature is 30 to 50℃ above Ac1. In order to ensure the uniform distribution of small particles of excess carbide on the martensite matrix after quenching.
When determining the quenching heating temperature of various carbon tool steels, the complex shape, small deformation and small size should be considered, and the lower limit of heating temperature should be taken.
And large size, simple shape, deformation requirements are not high, the heating temperature to take the upper limit.
The quenching heating time of carbon tool steel can be calculated according to the heating time coefficient, and the salt furnace is 25 ~ 30s/mm; Electric furnace 80 ~ 120s/mm.
Quenching heating temperature of carbon tool steel
(3) tempering
The tool should be tempered immediately after quenching to prevent cracking caused by excessive quenching stress.
In order to maintain high hardness and improve the plasticity and toughness of hardened steel, carbon tool steel is generally tempered at low temperatures (150 ~ 180 ° C). The choice of tempering temperature should be determined according to the hardness requirements of the tool.
① When high hardness is required (> 62HRC) should be tempered at 150 ~ 160℃;
In order to improve mechanical properties (plasticity and toughness), it can be tempered at 180 ° C.
② The hardness is slightly lower (60 ~ 61HRC), and the thread tool (such as die) can also be tempered at 180 ~ 210℃.
The tempering stability of carbon tool steel is low, and too high tempering temperature (above 250℃) will cause a significant decrease in hardness, so the tempering temperature should be strictly controlled in production.
Due to the low tempering temperature, the tempering time must be sufficient, generally 2 ~ 4h. If the tempering is insufficient, the brittleness of the tool is large, and the blade is easy to break in use, or it is easy to crack during subsequent grinding.
