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Matters needing attention during heat treatment

  When manufacturing mold parts, heat treatment processes are usually used 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, failures are often caused by small details that are not worth mentioning rather than technical key problems, nor are they caused by errors in the specific application of the typical theories mentioned in the books. Lessons should be learned and cautioned. Today I have sorted out some minefields in the heat treatment process for you, as follows:

  Hardened parts that require higher hardness and larger dimensions cannot be made of carbon steel

  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 constant, 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, due to its poor hardenability, its quenching hardness and size effect are more obvious. When the designed part cross-section size is larger than the critical quenched 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, when using these data, attention must be paid to the influence of the size effect on the mechanical properties.

  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 as the section size increases (this phenomenon is called the size effect), especially for steel with low hardenability, the size effect is particularly obvious.

  Hardened parts with complex shapes cannot be selected from steel with large deformation

  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 workpiece, which will cause the workpiece to deform or even crack and be scrapped.

  To eliminate the side effects produced during quenching, we must try to reduce the quenching cooling rate. To be able to harden at a lower cooling rate, steel grades with good hardenability and small deformation must be selected.

Matters needing attention during heat treatment (2)

  In the quenching oil tank, water should be strictly prevented from entering

  Oil is a commonly used quenching agent for some 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 to 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 double medium quenching, which should be managed in place and separated regularly.

  The design and manufacture of quenching fixtures cannot be manufactured 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 carried out 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 prevents the free extension of the workpiece;

  •  The size and weight of the fixture are too large or too heavy to be used;
  • Fixtures that affect the cooling of the workpiece in structure should not be used;
  • 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 breaks due to repeated hardening during repeated flashing, and has a short service life.

  The surface medium-frequency and high-frequency induction hardened workpieces must undergo preliminary heat treatment

  The workpiece is quenched by medium-frequency induction heating equipment and high-frequency induction heating equipment and has a higher surface hardness, higher strength, and higher fatigue strength than ordinary quenched ones. These superior performances are mainly due to the fact that high and intermediate 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 in the hardened layer after quenching The martensite needles are extremely small, and the carbides have a high degree of dispersion.

  These superior organizations 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, reducing the performance of the hardened layer, or appear soft spots after quenching. Therefore, the high and medium frequency quenched parts should be normalized or quenched and tempered before quenching to obtain a fine and uniform structure.

Matters needing attention during heat treatment (1)

  The distance between gas carburizing workpieces should not be too small

  Gas carburizing uses 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 cementite, not only the workpieces cannot be in contact with each other when the furnace is installed, but also the spacing cannot be made too small, otherwise it will make the furnace atmosphere 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 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 needs 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 is to complete the transformation of retained austenite to martensite during tempering and cooling so that the retained austenite The transformed martensite is then transformed into tempered martensite.

  It is difficult to achieve the above-mentioned structural transformation if long-term tempering is used. Insufficient tempering will result in insignificant secondary hardening, the poor dimensional stability of the workpiece, greater brittleness, and low service life.

  High carbon steel with network carbides is not suitable for spheroidizing annealing

  In order to reduce hardness and obtain better processing performance, high carbon steel is not prone to overheat, 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 the network carbides exist, it will prevent the spheroidization from proceeding.

  For high-carbon steel with a severe network carbide structure, normalizing treatment must be used before spheroidizing annealing to eliminate network carbides and then spheroidizing annealing.

  END

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