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Blade is an essential equipment in machining, its cutting performance directly affects the product processing quality and productivity. The cutting performance of the blade is closely related to the organisational structure of its material. Heat treatment technology is an important method to improve the hardness, wear resistance, toughness and other properties of the blade by changing the organisational structure of the material. The heat treatment process of the blade is to heat it to a certain temperature, hold it for a certain period of time, and then cool it at an appropriate rate, so that its organisational structure produces corresponding changes, thus improving the performance of the insert.
Process flow of heat treatment
Heating
Heating temperature: Depending on the type of material of the blade (e.g., stainless steel, high-speed steel, carbide, etc.) and the desired properties, the appropriate heating temperature is determined. Generally speaking, the heating temperature should be controlled above the phase change point of the material to ensure that the internal organisation of the blade is transformed.
Heating method: Electric furnace, gas furnace and other heating equipment can be used to ensure uniform heating and avoid local overheating or overcooling.
Heating time: The length of the heating time depends on the size of the blade, the material and the required performance, generally need to be maintained for a period of time in order to make the temperature inside and outside the blade consistent.
Heat preservation
The purpose of heat preservation: To make the internal tissue of the blade fully transformed under the heating temperature to achieve the expected organisational structure.
Holding time: The length of the holding time depends on the thickness of the blade, the heating temperature and the material properties. Generally, the holding time should be long enough to ensure adequate tissue transformation, but too long a holding time will increase costs and man-hours.
Cooling
Cooling method: The choice of cooling method has an important effect on the performance of the blade. Common cooling methods include water quenching, oil quenching and air cooling. Water quenching cooling speed, applicable to the need for high hardness occasions; oil quenching cooling speed moderate, applicable to most industrial blades; air cooling is slower cooling speed, applicable to some special materials or the need to avoid quenching and cracking occasions.
Cooling speed: The cooling speed determines the hardness and toughness of the blade. Rapid cooling can get high hardness, but may also increase the brittleness; slow cooling is conducive to reduce internal stress, improve toughness.
Tempering
Tempering Purpose: Tempering is used to remove internal stresses and brittleness produced during the hardening process, while maintaining a certain level of hardness and toughness.
Tempering temperature: The choice of tempering temperature depends on the material, hardness and desired properties of the blade. In general, the tempering temperature should be lower than the quenching temperature to avoid re-austenitising the tissue.
Tempering Time: The length of the tempering time depends on the tempering temperature and the blade material. In general, the tempering time should be long enough to ensure that internal stresses are adequately removed, but too long a tempering time can result in a loss of hardness.
Heat treatment methods and characteristics of blades
Homogenisation
Homogenisation, also known as ‘diffusion annealing’, is the process of heating at high temperatures for a long period of time to fully diffuse the internal chemical composition. The heating temperature varies depending on the type of steel. Large steel ingots are usually homogenised between 1200°C and 1300°C, high carbon steels between 1100°C and 1200°C, and generally forged or rolled steel is heat treated between 1000°C and 1200°C.
Complete annealing treatment
Complete annealing treatment is to heat the sub-eutectic steel to 30~50℃ above the Ac3 temperature, and the over-eutectic steel to 50℃ above the Ac1 temperature, and keep it at this temperature for a sufficient time to make it become a single-phase organisation of Wolstedtian body (sub-eutectic steel) or a mixed organisation of Wolstedtian body and Xieming carbon body, and then soften the steel by furnace cooling in order to get the best ductility and fine grain organisation of the steel.
Spheroidal Annealing
The main purpose of spheroidal annealing is to improve the cutting performance and machining plasticity of steel by making the internal layer or network carbide coalesce into a sphere through heat treatment, especially the high carbon tool steel is the need for such annealing treatment.
Common spheroidal annealing treatment includes: (1) in the steel A1 temperature above, below the repeated heating, cooling several times, so that A1 perversion of the precipitation of the snow Ming carbon and iron, continue to adhere to the growth in the above spheroidal carbide. (2) heated to steel A3 or above the temperature of the Acm, the beginning of the carbide completely solidified in the Wolters Kluwer Tian body after cooling, and then in accordance with the above method of spheroidal treatment. Make carbide spheroidisation, but also to increase the toughness of steel after quenching, to prevent quenching and cracking, but also to improve the mechanical properties of steel after quenching and tempering, to improve the service life of steel.
Soft annealing treatment
Soft annealing heat treatment procedure is to heat the workpiece to 600 ° C to 650 ° C range (A1 temperature below), maintained for a period of time after the air-cooled, the main purpose is to make the workpiece to work hardening soften again, back to the original toughness, in order to be able to further processing. This heat treatment method is often implemented repeatedly in the cold working process, so it is also called process annealing. Most of the metal in the cold working, material strength, hardness will increase with the amount of processing and become larger, but also lead to the material ductility reduced, the material becomes brittle, if you need to further processing, it must be softened by the first annealing heat treatment to continue processing.
Relaxation Annealing
The main purpose of relaxation annealing heat treatment is to remove the residual stress caused by forging, casting, machining or welding, this residual stress often leads to the reduction of strength of the workpiece, long-lasting deformation, and the toughness of the material, ductility has a negative impact, so the relaxation annealing heat treatment for the size of the workpiece of the longitude of the requirements of the strict, there are safety concerns about the mechanical components are very important. Relaxation annealing heat treatment procedure is to heat the workpiece to the appropriate temperature below the A1 point, and keep it for a period of time (not as long as the softening annealing heat treatment), and then slowly cooled to room temperature. Particular attention should be paid to the slow heating rate, especially for large objects or workpieces with complex shapes, otherwise the effectiveness of relaxation annealing will be greatly reduced.
Normalisation
Normalisation heat treatment has two important functions, one is to make the workpiece crystalline grain refinement and improve the mechanical properties of the material; the other purpose is to regulate the size or distribution of carbides in the rolling or casting organisation, so as to facilitate the subsequent heat treatment of carbides easily solidified in the material, in order to enhance the material cutting properties, and homogenisation of the material. Normalised heat treatment heat treatment procedures, the workpiece is heated to A3 (sub-eutectic steel) or Acm (over-eutectic steel) point temperature above the high temperature of 30 ℃ to 60 ℃ (this is the normalisation temperature) to maintain a period of time, the material to become homogeneous Worcesterian body, and then placed in the air to allow it to cool. Estimation of normalisation time, you can hold the temperature for 30 minutes per 25mm thickness to estimate the time required to hold the temperature. Normalisation heat treatment can be divided into two-stage normalisation, constant temperature normalisation and secondary normalisation and other improved normalisation heat treatment.
Hardening
The main purpose of quenching treatment is to cool the steel rapidly in order to obtain a very hard asahida bulk organisation. There are three essentials, one without the other, for the quenching treatment of steel, which are:
Heating for a period of time in the Voss field body region (i.e., Voss field bodying).
Cooling in such a way as to avoid Ar’ (boreal body) metamorphism.
Hardening of the steel by the production of asafoam or toughened bodies.
Hardening can be carried out in two procedures, one is heating and the other is cooling. The usual heating temperature, also known as the quenching temperature or Wolstianising temperature, varies according to the heat-treated steel. The quenching temperature for sub-eutectic steels is in the range of 30°C to 60°C above the Ac3 temperature, while the quenching temperature for eutectic and hyper-eutectic steels is in the range of 30°C to 60°C above the Ac1 temperature. Cooling should be divided into two stages to cool, steel from the heating furnace to take out the steel pieces, has been cooled to Ar ” perversion before the critical area, to try to cool quickly; in Ar ” below the temperature region need to adopt a slow cooling method, otherwise prone to cause steel Quenching crack or quenching deformation, this temperature region is also known as the danger zone.
Tempering
General tempering treatment is often implemented after quenching, in order to eliminate the adverse effects of quenching treatment and retain and play the efficacy of quenching, its main purpose is to make the quenching of the organisation generated by the metamorphosis or precipitation more stable (so that the formation of the tempering of the Maten dispersion), to reduce the residual stress and to improve the relevant mechanical properties (to enhance the ductility of the material). Tempering temperature is different, will produce a different combination of mechanical strength and ductility, the general tempering temperature is mostly below 600 ℃, because of higher tempering temperature, any steel will show a rapid softening trend, this time, the gradual cohesion of carbide and spheroidal, fat particles will be re-crystallised and grow into a continuous base, is the main reason for the softening.
Temper embrittlement
Tempering treatment to avoid several temperatures will produce tempering embrittlement range, these embrittlement temperature range depending on the type of steel, including: (1) 270 ° C to 350 ° C embrittlement (also known as low-temperature tempering embrittlement or A embrittlement), the majority of carbon steel and low alloy steel, are in this temperature range of the occurrence of embrittlement phenomenon; (2) 400 ° C to 550 ° C embrittlement, the usual structural use of alloy steel in this temperature range will produce embrittlement phenomenon (3) 475 ℃ embrittlement (especially Cr content of more than 13% of the fat grain system of stainless steel); (4) 500 ℃ to 570 ℃ embrittlement, for tool steel or high-speed steel heated in this temperature range, the precipitation of a uniform distribution of carbides, resulting in a secondary hardening effect, but also prone to cause embrittlement.
Hemp quenching treatment
The main purpose of hemp quenching treatment is to reduce the huge difference in temperature between inside and outside of the workpiece during quenching, and to make the inside and outside of the workpiece together at lower temperatures to produce a metamorphosis of the hemp field, which can avoid quenching rupture and minimise the quenching deformation without loss of hardness of any quenching. The main operation procedure is to quench the steel into the temperature at the Ms point slightly on the hot bath, briefly hold the temperature so that the temperature inside and outside the workpiece is the same, and then put forward the air-cooled, so that the workpiece to form the asada body metamorphosis of the heat treatment method.
Asahi Tempering
Hemp tempering treatment is the steel quenching into the Ms and Mf temperature range between the hot bath, after a long time to hold the temperature, so that part of the supercooled alloy Wolstedtian body metamorphosis into Hemp field bulk, part of the metamorphosis into the lower variable tough body. After this heat treatment, it is not necessary to carry out tempering treatment, and can reduce the sharp degree of general quenching and tempering; its final organisation is a mixture of tempered asada and toughened body, so it has a combination of high hardness and high toughness. The main disadvantage is the need to maintain a constant temperature for a long time, less economical in industrial applications.
Voss Tempering
Worth tempering treatment is a more special heat treatment method, the main procedure is to quench the steel into the temperature between the nose of the S curve and Ar” (Ms point) temperature between the hot bath, until the supercooled Worth field body completely metamorphosed into a toughened body before removing the air-cooled heat treatment method, also known as toughened quenching, which does not need to retempering treatment. Worth tempering is the most important feature is that you can get high hardness, high toughness of both materials, in general, the higher the metamorphosis temperature, the lower the hardness, but can enhance the low-temperature toughness; metamorphosis of the temperature closer to the Ms temperature, the resulting strength, hardness are greatly increased, and the elongation and sectional shrinkage also increased, quite suitable for small-scale production of large quantities of workpieces.
Key parameters in heat treatment
Heating temperature
Heating temperature is an important factor affecting the organisation and properties of the material, too high a heating temperature will lead to grain growth and coarsening; too low a temperature will cause insufficient grain recrystallisation.
Holding time
The length of holding time also has an effect on the organisation and properties of the material. In general, the holding time should be long enough to ensure adequate grain growth, but too long can lead to increased costs and man-hours.
Cooling rate
Cooling rate is an important factor in the hardness of a material. Rapid cooling results in higher hardness, but can also result in increased material brittleness.
Cooling medium
Cooling medium can be water, oil, brine, etc. The cooling effect of different mediums is also different, and needs to be selected according to the specific situation.
Caveat
The surface of the blade should be checked for cracks, inclusions and other defects before heat treatment to avoid fracture during heat treatment.
The heating process should strictly control the heating temperature and heating time to avoid local overheating or overcooking.
The cooling process should pay attention to the cooling speed and the choice of cooling medium to avoid excessive internal stress and quenching cracks.
The tempering process should strictly control the tempering temperature and tempering time to obtain the desired combination of hardness and toughness.
The blade should be inspected after heat treatment, including hardness test, metallographic analysis, etc., to ensure that its performance meets the requirements.
Heat treatment of industrial blades is a complex and critical process that requires strict control of the parameters and conditions of each step. Through a reasonable heat treatment process, the performance and service life of blades can be significantly improved to meet the needs of various industrial applications.
