vacuum furnace mould heat treatment

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It used to be the main means of heat treatment of mould in that year to use medium temperature and high temperature salt bath furnace for heating, low temperature alkali bath and nitrate bath for staged isothermal cooling and tempering.

However, there are many disadvantages in the heating salt bath furnace: the barium chloride raw materials used are toxic, the waste gas and waste residue produced pollute the environment, when quenching and deoxidizing the slag, the employees are exposed to high temperature radiation in front of the furnace, the working conditions are hard, when the quenching oil is cooled, the black smoke rises straight to the roof, and it is difficult to collect and dispose the waste residue of alkali bath and nitrate bath, which are dirty and disorderly in the heat treatment workshop One of the reasons for the poor. Therefore, it is no longer possible to sacrifice the environment and the health of employees for immediate benefits. Relevant government departments have strictly phased out and banned the construction of new bath equipment.

Many enterprises have been advancing with the times, using vacuum furnace for vacuum heat treatment
The vacuum furnace has no waste gas, smoke, waste water, waste residue and environmental pollution, which improves the working conditions. It is called bright heat treatment, clean heat treatment and green heat treatment. No oxidation, no decarburization, no alloy element dilution, degreasing and degassing, vacuum purification, avoid hydrogen embrittlement, realize graded isothermal quenching, control heating and cooling, and lay a foundation for automatic and flexible production.

Vacuum Heat Treatment Furnace

Compared with the salt bath furnace, the vacuum furnace also has disadvantages in fast and local heating, because in some specific situations, this operation is also required.

All alloy tool steels listed in GB / T 1299-2000, as well as high-speed tool steels, stainless steels and bearing steels, are suitable for vacuum high-pressure air quenching and oil quenching. The commonly used steel grades are Cr12MoV, D2, 1.2379, K110, dc11, SLD, qc11, xw42, K340, DC53, 4Cr5MoSiV1, H13, 1.23 4, W302, s k D61, dha1, DAC, 8402, qd61, 8407, dh31, dac55, QDN, w6mo5cr4v2co5 M2, 1.3343, skd51, W6Mo5Cr4V2, CO5, M35, 1.3243, mh55, 3cr2nimo, P20, 1.2738, px5, 718, 4Cr13, 9Cr18, 420, 440C, 17-4PH, 17-7hp, GCr15, 65Nb, LD, Gd, Hm1, HM3, Y4, Y10, GR steel, etc. The die parts of punching, drawing, extruding, bending, rolling, hot forging, warm forging, die casting and plastic mould made of these steel grades have obtained the stable and reliable structure and performance of both rigidity and toughness through vacuum heat treatment, so as to ensure the improvement of the service life of the mould.
Horizontal double chamber high-pressure air quenching and oil quenching vacuum furnaces are suitable for die quenching. Parts with similar volume or wall thickness should be processed in the same furnace to set proper and unified heat preservation time, including heat treatment process parameters such as heating temperature, heating speed, heat preservation time and cooling speed, which are crucial to the performance of parts.

The temperature difference between the surface and the center of the parts and the uneven heating and cooling are the main causes of the quenching deformation and cracking. Quenching and subsequent heat preservation are for the full dissolution of alloy elements and homogenization of austenite.

The quenching and heating temperature shall be set according to the data given in the relevant technical data, and the upper limit shall be taken for the occasions requiring high secondary hardening and tempering stability, and the lower limit shall be taken for the occasions preventing deformation and cracking. The holding time refers to the residence time after the center of the thickest part of the whole furnace reaches the quenching temperature, which is related to factors such as the power of the electric furnace, the charging quantity, the charging method and the size of the parts.

vertical gas quenching furnace

It is feasible to estimate the holding time h with H = 30 + thickness / 50 × 10min. When the quenching temperature is over 1000 ℃, it is generally preheated twice: 580 ℃ for the first time and 850 ℃ for the second time. The first preheating and holding time is twice as long as the quenching and holding time h, and the second preheating and holding time is about H / 3.
When heated, too high vacuum will cause the alloy elements in the material to volatilize. The physical quantities PA (Pascal), B a R (bar 1bar = 105Pa) and Torr (Torr, 1torr = 133.322pa) are commonly used to mark the vacuum degree, and there is a certain conversion relationship between them. Low vacuum degree is adopted in preheating stage, and high vacuum degree is adopted in heating and heat preservation stage, generally between 10-1bar, 10-2bar and 10-3bar. The cooling speed of high pressure gas quenching is related to the cooling water flow of heat exchanger, gas type, temperature and flow rate, gas pressure, air flow form and distribution, workpiece size and charging capacity. Some studies have shown that the cooling rate of 10 bar low temperature nitrogen is 30% – 40% higher than that of 6 bar, and that of 20 bar is more than 80% higher than that of 6 bar.

During oil quenching, the vacuum quenching oil with good quality should be selected, and the carburization of workpiece during cooling should be prevented by technological improvement.

The purpose of tempering is to eliminate stress and obtain required structure and properties. Die tempering is usually carried out in a well furnace with nitrogen protection after vacuum pumping.