Ten lessons that must be paid attention to in the process of mold heat treatment
When manufacturing mold parts, a heat treatment process is usually adopted in order to achieve the required hardness and strength. The metal heat treatment process is to change the surface or internal structure of the material and obtain the required performance by means of heating, heat preservation and cooling in the solid state of the metal material. However, in actual operation, it is often caused by failures rather than technical key problems caused by some small details that are not worth mentioning, and it is not a mistake in the specific application of the typical theories mentioned in the book. Lessons should be learned and a warning. Today, the editor has sorted out some minefields in the heat treatment process for you, as follows.
Carbon steel cannot be used for quenched parts that require higher hardness and larger dimensions
The achievable hardness of the surface of the part after quenching depends on the hardenability of the steel, the section size and the quenching agent. When other conditions are fixed, as the size of the part increases, its surface hardness decreases after quenching. Therefore, the effect of quenching hardness and size must be considered when designing and selecting the material of quenched parts. For carbon steel, because of its poor hardenability, its quenching hardness and size effect are more obvious. When the designed part’s cross-sectional size is larger than the critical quenching diameter of the selected steel, the predetermined hardness requirement cannot be reached. Therefore, alloy steel with better hardenability should be used for this kind of workpiece.
The mechanical property data of the materials listed in the manual cannot be simply applied in the mechanical design
The number of mechanical properties listed in various manuals is generally based on data obtained by testing small-sized specimens that can be hardened. Therefore, we must pay attention to the influence of size effect on mechanical properties when using these data. When the diameter (thickness) of the part is similar to the critical hardening diameter of the material, the data in the manual can be used as the basis for design and material selection. When the size of the part is larger than the critical diameter of the material, the mechanical properties of the steel will decrease with the increase of the section size (this phenomenon is called the size effect), especially for the steel with low hardenability, the size effect is particularly obvious.
Steel with large deformation cannot be used for hardened parts with complex shapes
For workpieces with complex shapes, due to the effect of thermal stress and structural stress during quenching, large internal stresses will be generated inside the workpieces, resulting in deformation or even cracking of the workpieces and scrapped. To eliminate the side effects produced during quenching, we must try to reduce the quenching cooling rate, and to be able to harden at a lower cooling rate, steel grades with better hardenability and small deformation must be selected.
Strictly prevent water from entering in the quenching oil tank
Oil is a commonly used quenching agent for certain small-section alloy steels. However, if water is unintentionally brought into ordinary quenching oil, and the oil is not water-soluble, the oil will emulsify with water to form an emulsion. The cooling capacity of this medium is comparable. Poor oil. If the oil is a non-emulsified liquid, water and oil layered exist, and the water is located at the bottom of the oil tank, which may cause quenching deformation and cracking of the workpiece during quenching. If the water layer is thick, the rapidly vaporized water during quenching may cause an explosion.
Sometimes it is unavoidable to use water and oil dual medium quenching, which should be managed in place and separated regularly.
The design and manufacture of quenching fixtures cannot be manufactured at will without principle
In order to ensure that the quenched workpiece can be reasonably heated and immersed in the quenching agent in the correct way to improve production efficiency, it is often necessary to design and manufacture some fixtures in production. The quality of the quenching fixture design has a great relationship with the quality of the product, so the quality of the quenching fixture Design and manufacturing cannot be done at will, and the following requirements must be met:
- (1) Fixtures and hangers that cannot withstand the load given by the workpiece during red heat, and the deformation of the fixture during heating and cooling prevent the free extension of the workpiece.
- (2) The size and weight of the fixture are too large or too heavy to be used.
- (3) Fixtures that affect the cooling of the workpiece in structure should not be used.
- (4) High-carbon steel should not be used as the material of the fixture, and low-carbon steel is best, because high-carbon steel is difficult to weld and easy to break from the fracture, which affects quenching. High-carbon steel is easy to oxidize and decarburize, and it breaks due to repeated hardening during repeated flashing, and has a short service life.
High, medium frequency and surface hardened parts must undergo preliminary heat treatment
Compared with ordinary quenched workpieces, high and intermediate frequency surface quenching has higher surface hardness, higher strength and higher fatigue strength. These superior properties are mainly due to the high and medium frequency heating is a kind of rapid heating without heat preservation. This heating condition causes uneven austenite composition, refinement of austenite grains and substructures, and the hardened layer after quenching The martensite needles are extremely small, and the carbides have a high degree of dispersion.
These superior organization and excellent performance can only be obtained under the small original organization. If there are large pieces of free ferrite in the original structure, the thickness of the hardened layer will be uneven after quenching, which will affect the uniformity of the hardness of the hardened layer, reduce the performance of the hardened layer, or appear soft spots after quenching. Therefore, the high and medium frequency quenched parts must be normalized or quenched and tempered before quenching to obtain a fine and uniform structure.
The distance between gas carburizing workpieces should not be too small
Gas carburizing relies on a fan to make the atmosphere intensively circulate in the furnace to achieve a uniform atmosphere in the furnace. In order to achieve the purpose of good circulation of furnace gas in the carburizing tank, the distance between the workpieces should not be too small. Especially for some small cementites, not only the workpieces cannot be in contact with each other when the furnace is installed, but the spacing cannot be too small, otherwise it will make the atmosphere in the furnace difficult to circulate. The atmosphere in the furnace is uneven, and even causes a dead angle in the furnace part, resulting in poor carburization. Under normal circumstances, the gap between the workpieces should be 5-10mm.
The quenched repair parts of high carbon and high alloy steel should not be quenched directly
High-carbon high-alloy steel has a low Ms point and a large quenching specific volume. Therefore, the quenched part has a large internal stress. If it is directly re-quenched, it is easy to deform and crack. Therefore, an annealing treatment must be performed before re-quenching to eliminate its internal stress.
High-alloy molds with high temperature quenching cannot be used for a long time tempering instead of multiple tempering
High-alloy molds that are quenched at high temperatures need to be tempered multiple times, such as hot forging dies made of 3Cr2W8 steel that need to be tempered more than twice. This is because these high-temperature quenched high-alloy workpieces have more retained austenite in the structure after quenching. The purpose of multiple tempering makes the retained austenite complete the transformation to martensite during tempering and cooling, so that the retained austenite The transformed martensite is then transformed into tempered martensite. If a long-term tempering is used, it is difficult to achieve the above-mentioned structural transformation. Insufficient tempering will lead to insignificant secondary hardening, poor dimensional stability of the workpiece, greater brittleness, and low life.
High carbon steel with network carbides is not suitable for spheroidizing annealing
In order to reduce the hardness and obtain better processing performance, high carbon steel is not prone to overheating, deformation and cracking during quenching. Generally, spheroidizing annealing is adopted. But before the spheroidizing annealing, there should be no serious network carbides in the steel. If there are network carbides, the spheroidization will be prevented. For high carbon steel with serious network carbide structure, normalizing treatment must be used before spheroidizing annealing to eliminate network carbides, and then spheroidizing annealing.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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