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Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

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Vacuum Heat Treating Small Parts

1.Vacuum heat treatment of small parts springs

Spring is a basic component widely used in various types of mechanical equipment, instrumentation, military products and furniture and home appliances. In some mechanical equipment and devices, spring is a key component to ensure its function and operation safety. There are many factors that affect the function and reliability of springs, including spring design, material selection, production technology, and operating conditions. Among them, the material and vacuum heat treatment have an important or even decisive impact on the performance and service life of the spring. Due to the function and usage characteristics of the spring, the spring must not only use the material used to make the door spring, but must also be strengthened through vacuum heat treatment to obtain the best mechanical properties with both high elastic limit and strength and sufficient toughness to ensure that the spring is in use. Work reliably for a long time. For springs that are required to work under alternating loads, in order to improve the fatigue performance of the spring, strengthening treatments such as shot peening or surface chemical vacuum heat treatment are usually performed. However, spring vacuum heat treatment is the most basic and important strengthening treatment method. The vast majority of springs are made of spring steel. Spring copper includes carbon spring steel, alloy spring steel and some stainless steel. Since there are many types of springs, the materials used (including material, shape and state), usage conditions and manufacturing methods vary. They are not the same, and their vacuum heat treatment methods are also different.

2.Vacuum hardening and vacuum tempering of shells

The vacuum hardening of the spring is to heat the spring to 30~50C above Ac3 (for hypoeutectoid steel) or Ac (for hypereutectoid steel), keep it warm for a certain period of time, and then rapidly cool it (cooling rate > hardening critical cooling rate Vk) to Vacuum heat treatment process to obtain martensite and/or bainite structure. After the spring is vacuum quenched, in order to eliminate stress and obtain the required structure and performance, it is heated to a certain temperature below A1, kept for a certain time, and then cooled to room temperature at an appropriate speed. This is a vacuum heat treatment process called spring vacuum. Tempering. Vacuum tempering must be carried out after Vacuum hardening of the spring. hardening and tempering are an inseparable combined vacuum heat treatment process.

2.1 Vacuum hardening of springs

Vacuum hardening of springs is to heat the workpiece to austenite and then quickly cool it to obtain a martensite structure, so that the required structure and properties of the spring can be obtained through vacuum tempering.

(1) Hardenability of steel

Harden ability is an important performance index to be considered in the composition design, material selection and vacuum heat treatment process formulation of structural steel. Hardenability is a material property characterized by the depth of the hardened layer and hardness distribution that can be obtained by Vacuum hardening of a steel sample under specified process conditions. It is an inherent attribute of the steel itself. The hardenability of steel is related to the stability of supercooled austenite, and the stability of supercooled austenite mainly depends on the chemical composition of the steel and the austenitization process. Generally speaking, the more the number and types of alloying elements in the steel and the closer the carbon content in the steel is to the eutectoid composition, the more stable the austenite will be and the better the hardenability of the steel will be.

From the austenitization process, the higher the heating temperature and the longer the holding time, the coarser the austenite grains, the more uniform the austenite composition, the more stable the austenite, and the better the hardenability of the steel. .

(2) Vacuum hardening and heating

The hardening heating process is mainly based on the heating temperature and heating and holding time determined by the heating equipment and heating method used. The purpose of heating is to make the metal structure reach a uniform austenite state, so the heating temperature is mainly determined based on the steel grade and composition. For example, carbon steel is mainly determined based on the carbon content of the steel and the Fe-Fe;C phase diagram. For hypoeutectoid steel, the hardening heating temperature is generally Ac3 +30~50C. The hardening heating temperature of eutectoid steel and hypereutectoid steel is generally Ac +30 ~50C. After Vacuum hardening, a uniform and fine martensite structure or martensite + a small amount of granular cementite (hypereutectoid steel) should be obtained. Heating temperature that is too high will cause the austenite grains to coarsen. After hardening, the martensite will become coarser and the toughness will decrease, making the properties of the steel worse. At the same time, it will increase oxidation and decarburization, increase the specific volume of martensite, and increase the hardening stress. If the heating temperature is too low, unmelted ferrite may exist, which will cause insufficient hardening hardness and worsen the performance after tempering. The hardening heating temperature of spring steel can be determined according to the critical point of phase transformation of spring steel. The transformation of the metal structure takes a certain amount of time, so even if the spring surface reaches the required heating temperature, it must be maintained at this temperature for a certain period of time to make the internal and external temperatures consistent. This should also be added to the time for the complete transformation of the microstructure and the homogenization of the tissue components.

Usually the sum of spring hardening heating and holding time is called hardening heating time. It is related to many factors such as the steel composition, original structure, part shape and size, heating furnace type, heating medium, furnace loading method, furnace temperature, etc. Therefore, it is more complicated to accurately calculate the heating time. Usually it can be initially determined based on the recommendations in the manual, and then determined through experiments based on specific conditions.

(3) Vacuum hardening and cooling

The purpose of Vacuum hardening cooling is to make the parts obtain a martensitic structure. For this reason, the hardening cooling rate must be greater than the critical cooling rate. Rapid cooling will inevitably cause large hardening stress, causing deformation and cracking of the spring. Therefore, one of the most important issues in the hardening process is to ensure that the quenched spring obtains a martensite structure, while also reducing deformation and preventing cracking of the spring. For this reason, it is very important to reasonably select the cooling medium and cooling method.