When manufacturing mold parts, usually in order to achieve required hardness and strength, a heat treatment process is adopted. Metal heat treatment process refers to metal material in solid state through heating, heat preservation and cooling means to change surface or internal structure of material and obtain desired performance. However, in actual operation, it is often because of some small details that are not worth mentioning cause failure rather than technical key, or a mistake in specific application of typical theories mentioned in books, and we should learn from them and use them as commandments. Today, we compiled some notes about heat treatment process for you, as follows.

 

Hardness that can be achieved on the surface after quenching depends on hardenability, cross-sectional size and quenching agent of steel. When other conditions are constant, as size of part increases, its surface hardness decreases after quenching. Therefore, when designing and selecting materials for quenched parts, effects of quenching hardness and size must be considered. For carbon steel, due to its poor hardenability, its quenching hardness and size effect are more obvious. When designed cross-sectional size of part is larger than critical hardening diameter of selected steel type, predetermined hardness requirement cannot be reached. Therefore, alloy steel with better hardenability should be selected for this kind of workpiece.

Number of mechanical properties listed in various manuals is generally based on data obtained by testing small-size specimens that can be hardened. Therefore, when using these data, we must pay attention to effect of size effect on mechanical properties. When diameter (thickness) of part is close to critical hardening diameter of material, data in manual can be used as basis for design and material selection. When part size is greater than critical diameter of material, as cross-sectional size increases, mechanical properties of steel will decrease (this phenomenon is called size effect), especially for low hardenability steel, size effect is particularly obvious.

Due to effects of thermal stress and tissue stress during quenching, a large amount of internal stress will be generated inside workpiece, which will cause workpiece to deform or even crack and be scrapped. To eliminate side effects produced during quenching, we must try to reduce quenching cooling rate, to be able to harden at a lower cooling rate, it is necessary to use steel with better hardenability and small deformation.

It is a common quenching agent for some small-section alloy steels. However, if water is accidentally brought into ordinary quenching oil, and oil is not water-soluble, oil will emulsify with water to form an emulsion, Cooling capacity of this medium is worse than that of oil. If oil is a non-emulsion, water and oil exist in layers. Water is located at the bottom of oil tank, which may cause workpiece to deform and crack during quenching. If water layer is thick, rapidly vaporized water may cause an explosion during quenching. Sometimes it is inevitable to use water-oil dual-medium quenching, management should be in place and separated regularly on request.

In order to ensure that quenched workpiece can be heated reasonably and immersed in quenching agent in correct way to improve production efficiency, it is often necessary to design and manufacture some fixtures in production. Design of quenching fixture has a great relationship with quality of product. Therefore, design and manufacture of quenching fixture cannot be carried out at will. It must meet following requirements:
(1) Load cannot withstand workpiece during red heat, and fixture is deformed during heating and cooling, fixture that hinder free elongation of workpiece should not be used. (2) Volume and weight of fixture are too large and too heavy to be used. (3) Fixtures that affect cooling of work piece should not be used. (4) High-carbon steel should not be used as material for fixture. It is better to use low-carbon steel, because high-carbon steel is difficult to weld and easy to break from fracture, affecting quenching. High carbon steel is easy to be oxidized and decarburized, and it is broken due to repeated hardening during repeated flashing, and service life is short.

Compared with ordinary hardened workpieces, high and medium frequency surface quenching has higher surface hardness, higher strength and higher fatigue strength. These superior properties are mainly due to high and medium frequency heating is a kind of rapid heating without heat preservation. This heating condition results in uneven austenite composition, refinement of austenite grains and substructures, martensite needles in hardened layer after quenching are extremely fine, an dispersion of carbides is extremely high. These superior structures and excellent performance are only obtained under small original structure. If there is a large free ferrite in original structure, thickness of hardened layer will be uneven after quenching, which will affect uniformity of hardness of hardened layer, reduce performance of hardened layer, or appear soft spots after quenching. Therefore, high- and medium-frequency quenched parts should be normalized or quenched and tempered before quenching to obtain a fine and uniform structure.

Gas carburizing is to rely on fan to make atmosphere circulate strongly in furnace to achieve a uniform atmosphere in furnace. In order to achieve purpose of good furnace gas circulation in carburizing tank, distance between workpieces should not be too small. Especially for some small cementite, when furnace is installed, not only can not contact each other between workpieces, but also can not make distance too small, otherwise it will make furnace atmosphere difficult to circulate. Atmosphere in furnace is uneven, even causing dead spots in furnace and causing poor carburization. Under normal circumstances, gap between workpieces should maintain a gap of 5 ~ 10mm.

High-carbon high-alloy steel has a lower Ms point and a larger specific volume for quenching. Therefore, there is a greater internal stress in quenched parts. 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-temperature quenched high-alloy molds must be tempered multiple times. For example, hot forging dies made of 3Cr2W8 steel should be tempered more than twice. This is because these high-temperature quenched high-alloy workpieces have more residual austenite in structure after quenching. Purpose of multiple tempering is to complete transformation of retained austenite to martensite during tempering and cooling, then transform retained austenite transformed martensite into tempered martensite. If a long-term tempering is adopted, it is difficult to achieve above-mentioned structural transformation, which is insufficient tempering, lead to insignificant secondary hardening, poor dimensional stability of workpiece, large brittleness, and low life.

In order to reduce hardness and obtain better processing performance, high carbon steel is not prone to overheating, deformation and cracking during quenching, and spheroidizing annealing is generally used. But before spheroidizing annealing, there should be no serious network carbides in steel. If there are network carbides, it will prevent spheroidization from proceeding. For high-carbon steels with severe network carbide structure, normalizing treatment must be used before spheroidizing annealing to eliminate network carbide, and then spheroidizing annealing.