Vacuum furnace for mould heat treatment

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In the past, the main means of mould heat treatment were heating in medium-temperature and high-temperature salt bath furnace, grading isothermal cooling and tempering in low-temperature alkali bath and nitrate bath.

However, there are many shortcomings in heating salt bath furnace: toxic barium chloride raw materials, polluting environment with waste gas and slag, high temperature radiation in front of furnace during quenching operation and deoxidizing dregs removal, hard working conditions and difficulties in collecting and disposing waste slag from alkali bath and nitrate bath, which are dirty and disorderly in heat treatment workshop. To this end, the immediate benefits can no longer be achieved at the expense of the environment and the health of employees. Relevant government departments have strictly ordered the phase-out and prohibition of new bathroom equipment.

Many enterprises have been keeping pace with the times. Vacuum furnace is used for vacuum heat treatment of dies. The figure is vacuum gas quenching furnace.

Vacuum furnace has no waste gas, smoke and dust, no waste water, waste residue, no environmental pollution, and improved working conditions. It is called bright heat treatment, clean heat treatment and green heat treatment. Vacuum furnace heating workpiece has no oxidation, decarburization, alloyless element dilution, degreasing and degassing, vacuum purification, avoiding hydrogen embrittlement, realizing graded isothermal quenching, controlling heating and cooling, laying the foundation for automation and flexible production.

Compared with the salt bath furnace heating, the vacuum furnace also has shortcomings in terms of rapid and local heating, because in some specific situations, this operation is also needed.

All alloy tool steels listed in GB/T 1299-2000, as well as high-speed tool steels, stainless steel and bearing steels, are suitable for vacuum high-pressure gas quenching and oil quenching. Commonly used steel grades are Cr12MoV, D2, 1.2379, K110, DC11, SLD, QC11, XW42, K340, DC53, 4Cr5MoSiV1, H13, 1.234, W302, S K D61, DHA1, DAC, 8402, QDH31, DAC55, Q5Mo5CoV2. M2, 1.3343, S K D 51, W6M O 5 C R 4 V 2, C o 5, M35, 1.3243, MH55, 3C r2NiMo, 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 stamping, drawing, extrusion, bending, wire rubbing, hot forging, warm forging, die casting and plastic die parts made of these steels have been treated by vacuum heat treatment to obtain both rigid and tough stable structure and properties, and to ensure that the service life of the die is prolonged.

Horizontal double chamber high pressure gas quenching and oil quenching vacuum furnace are more suitable for die quenching. Parts with similar volume or wall thickness should be processed in the same furnace to set proper and uniform heat preservation time, including heat treatment parameters such as heating temperature, heating speed, heat preservation time and cooling speed. It is very important to affect the performance of parts.

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

The quenching heating temperature should be set according to the data given by relevant technical data. The upper limit should be set in the case of high secondary hardening and tempering stability, and the lower limit should be taken in the case of preventing deformation and cracking. The holding time refers to the residence time of the thickest part of the furnace after reaching the quenching temperature. It is related to the power of the furnace, the quantity of the furnace, the way of charging and the size of the parts.
It is feasible to estimate the holding time H by H=30+thickness/50*10 min. When the quenching heating temperature exceeds 1000℃, it usually undergoes two preheating: 580℃ for the first time and 850℃ for the second time. The first preheating holding time is twice as long as quenching holding time H, and the second preheating holding time is about H/3.

When heated, excessive vacuum will cause the volatilization of alloying elements in materials. Physical quantities Pa, bar (1bar = 105Pa) and Torr (1Torr = 133.322Pa) are commonly used to label the vacuum. There is a certain conversion relationship between them. Low vacuum is used in preheating stage, and high vacuum is used in heating and holding stage, which is generally selected between 10-bar, 10-bar and 10-bar. The cooling speed of high-pressure gas quenching is related to the cooling water flow rate of heat exchanger, gas type, temperature and flow rate, gas pressure, gas flow form and distribution, workpiece size and furnace load. Some studies have shown that the cooling rate of 10 bar low temperature nitrogen gas is 30%-40% higher than that of 6 bar, and that of 20 bar is more than 80% higher than that of 6 bar.

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

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