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Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Overview of Vacuum Hardening Furnace

1.1 Overall design of vacuum hardening furnace

Vacuum hardening furnace is mainly composed of furnace body (including quenching oil tank), evacuation system, heating power supply, control cabinet and so on. The furnace body is a horizontal structure, and the material is loaded and unloaded in the horizontal direction. The front end of the non-through batch vacuum hardening furnace is provided with a furnace door for loading and unloading. The furnace body has at least two chambers, one is the heating chamber and the other is the cooling chamber, the cooling chamber is in the front and the heating chamber is in the rear. All furnace chambers are enclosed in a vacuum furnace shell. There are cooling area, quenching oil tank, charging lifting and conveying mechanism in the cooling chamber. Two adjacent chambers should be separated by a vacuum-tight valve. The design of the furnace body should take into account that the charge can be transferred according to the predetermined heating, cooling or quenching cycle, and finally the charge is placed next to the furnace door to be discharged. The rear end of the heating chamber shall be provided with a sealed furnace door for maintenance. There is a charging furnace door at the front end of the through-batch vacuum hardening furnace, and a discharging furnace door at the rear end. The furnace body has at least two furnace chambers, one heating chamber and one cooling chamber. The heating chamber is in the front and the cooling chamber is in the rear. All furnace chambers are in a vacuum furnace shell. Other requirements are the same as non-through batch vacuum hardening furnace.

The furnace chamber of the continuous vacuum hardening furnace is composed of three furnace chambers enclosed in a vacuum furnace shell, one end is an air-cooled chamber, the middle is a heating chamber, and the other end is an oil quenching chamber. Each room can work independently. The chambers should be separated by vacuum-tight valves. The charge can be charged from the front or side of the end chamber. The moving direction of charge can be changed according to needs, that is, from air cooling room, heating room to oil quenching room, or from oil quenching room, heating room to air cooling room. Consideration should be given to the number of baskets in the heating chamber at any one time. The furnace body should be water-cooled. Before the vacuum hardening furnace is powered on, the exhaust system should be able to pump the furnace chamber to a predetermined vacuum. During the heating phase, the input heating power can be adjusted. In the quenching stage, the charge should be quenched under different vacuum degrees and in neutral gas (including inert gas, the same below). The control of the vacuum hardening furnace should meet the requirements corresponding to its technical level.

1.2 Materials

All materials in the heating chamber should be adapted to the atmosphere, vacuum and temperature specified in the design; various materials should not react with each other at the working temperature.

For Class B and Class C furnaces, chromium-containing materials must not be used in areas where the temperature in the vacuum state exceeds 1000 °C, and should also be avoided for Class A furnaces.

1.3 Furnace shell

The furnace shell can be an integral structure or a combination of multiple sections. The cylinder, head and furnace door of the furnace shell should be water-cooled, and their design and manufacture should refer to the relevant national standards for pressure vessels. The inner surface of the furnace shell should be clean and smooth. The inner wall can be made of anti-oxidation material or ordinary carbon steel. In the latter case, the inner walls of Class C furnaces shall be galvanized or coated with other materials to prevent oxidation. The surface of the electroplating or coating layer should be smooth and clean, there should be no obvious inhalation and degassing during work, and there should be no peeling and chemical reaction.

1.4 Furnace door

Unless otherwise specified, furnace doors may be of the side-hinged, top-hinged, vertically lifting, or sliding type suspended from rails. There should be a vacuum valve inside the furnace body to separate the heating chamber from other furnace chambers. There should be an observation window with a sealed structure on the outer furnace door.

1.5 Heating chamber

The heating area of the vacuum hardening furnace should be surrounded by a heat shield. The heat shield can be a metal radiation screen, carbon felt or graphite felt, or a single-layer metal radiation screen lined with refractory fiber products, etc. All materials used should be heat resistant and not sag, crack and peel during work. The design should consider the deformation caused by thermal expansion and contraction, as well as the heat loss through the heat shield to a minimum. When it is required that only metal radiation screens can be used in the heating chamber and other heat insulating materials cannot be used, the heating element materials can be selected from graphite fiber braids, graphite rods or tubes, molybdenum sheets or molybdenum rods, etc. The hearth should be made of materials whose properties are compatible with the working conditions of the vacuum hardening furnace. During the operation of the furnace, it can be used normally within the specified period, and the hearth should be able to withstand the maximum loading without damage or obvious deformation. Under normal service conditions, the service life of heat shield, heating element and hearth shall not be less than 3000 h, 6000 h and 8000 h for Class A, Class B and Class C furnaces respectively. The service life of the heating element is limited to the fact that its input power under the highest working voltage is less than 15% of the rated power. The shell of the heating chamber should be equipped with a thermocouple lead-out device for measuring the uniformity of the furnace temperature in the working area.

1.6 Vacuum oil quenching system

The volume of the quenching oil tank shall be sufficient to accommodate the charge equal to the maximum load, plus an oil equivalent to not less than 8.5 L of oil per kilogram of steel charge. The quenching oil tank should be equipped with a cooling system. When the steel material whose weight is equal to the maximum load is vacuum quenched at 1100 ℃, the temperature rise of the quenching oil should not exceed 25 ℃. The cooling system should be able to reduce the temperature of the quenching oil by at least 17 °C within 1 h. There should be an oil circulation device in the oil quenching system to circulate the quenching oil through the quenching zone. The amount of oil passing through the quenching zone per minute should not be less than 4 times the oil capacity of the quenching zone. The oil quenching system shall have the following indication, control, interlocking, alarm and other necessary facilities:

a) In the range of 35 ℃ ~ 200 ℃, the quenching oil temperature over-temperature audio alarm device can be set arbitrarily and the interlocking device to prevent quenching when the quenching oil is over-temperature;

b) Quenching oil level indicator, low oil level audio alarm device and interlocking device that prevents the quenching mechanism from starting and cuts off the heating power supply when the oil level is low;

c) Automatic temperature adjustment devices in pressure-type water-cooled or oil-cooled systems;

d) A limit interlocking device that prevents the charge from being transferred from the heating chamber to the rack when the lifting frame is in an inappropriate position.

When otherwise stipulated or required, there shall be an electric heater in the quenching oil tank, so that the oil temperature can rise from 20°C to 80°C within 6 hours. The oil temperature should be arbitrarily set within the range of 25°C to 85°C, and can be controlled within ±15°C of the set value.

1.7 Air extraction system

Each furnace chamber of the vacuum hardening furnace can be equipped with an evacuation system respectively. It is also possible to use several chambers to share a system, controlled by vacuum valves. The evacuation system consists of necessary vacuum pumps, pipelines, valves, cold traps, control systems, vacuum gauges, etc. An automatic valve should be installed in the system so that it can be automatically closed in the event of a power failure to prevent air and vacuum pump oil from entering the furnace.

For Class C furnaces, the pumping process of each chamber should be programmable. For Class B furnaces, the vacuum degree in the heating chamber should be automatically controlled.

2 Performance requirements of vacuum hardening furnace

2.1 Maximum loading capacity of vacuum hardening furnace

Unless otherwise required, when the vacuum hardening furnace works or tests at or below 1100°C, its maximum loading capacity should not be less than (370×S) kg; when it is above 1100°C, it should not be less than (300×S) kg, Among them, S is the bottom area of the working area of the heating chamber, and the unit is m2.

2.2 Heating capacity and furnace evacuation capacity

Under the condition that the furnace temperature and the charge are at the ambient temperature, load the charge whose weight is equal to the maximum load into the furnace. Start the extraction system. Heating starts when the furnace reaches a predetermined vacuum. For a vacuum hardening furnace with a working area volume not greater than 0.6 m³, the furnace temperature should be able to rise to 1100°C within 1 h after the heating starts; for a vacuum hardening furnace with a working area volume greater than 0.6 m³ but not exceeding 1.5 m³, it should be within 1.5 h Reach 1100°C; for class C furnace, the vacuum degree of the furnace chamber should be able to reach the specified working vacuum degree at the same time, for class B and class A furnaces, the time to reach the working vacuum degree can be extended by 25% and 50% respectively. For the vacuum hardening furnace with a working area larger than 1.5 m³, its heating capacity and evacuation capacity shall be agreed upon by both parties.

2.3 Evacuation time of empty furnace

For the vacuum hardening furnace with a working area volume not greater than 0.6 m³, which has been dried and degassed and not loaded with charge, in the case of a cold furnace, each furnace chamber should be able to pump air to the required working vacuum degree within 20 minutes; the working area volume vacuum hardening furnaces larger than 0.6 m³ but not larger than 1.5 m³ should be able to evacuate to the required working vacuum within 30 minutes. For the vacuum hardening furnace with a working area larger than 1.5 m³, the evacuation time shall be agreed upon by both parties.

2.4 Pressure rise rate

For the pressure rise rate of the vacuum hardening furnace, when carbon felt or carbon felt plus fiber felt insulation screen is installed in the heating furnace, the influence of the fiber felt’s easy moisture absorption should be considered. The pressure rise rate can be measured before the fiber felt is installed and should meet the requirements. The following provisions:

Class A furnace: <2.00 Pa/h;

Class B furnace: <1.30 Pa/h;

Class C furnace: <0.65 Pa/h

2.5 Furnace temperature uniformity

The furnace temperature uniformity of the vacuum hardening furnace should not exceed the following regulations: Class A furnace: ±10 °C; Class B furnace: ±7.5 °C;

Class C furnace: ±5.0 ℃.

2.6 Furnace temperature stability

The furnace temperature stability of the vacuum hardening furnace should not exceed the following regulations: Class A furnace: ±3.0 ℃;

Class B furnace: ±2.0 ℃;

Class C furnace: ±1.0°C.

3 test method

3.1 Test conditions

When carrying out the empty furnace (without charge) test to determine the empty furnace evacuation time, ultimate vacuum degree, pressure rise rate, rated temperature, rated power, empty furnace loss and surface temperature rise, the quenching oil tank may not contain oil. The test shall start when the furnace temperature is equal to the ambient temperature, and the furnace temperature shall be set at the rated temperature.

3.2 Measurement of pressure rise rate

According to regulations, close the vacuum valve when the ultimate vacuum is reached. The first reading starts approximately 15 min after closing the vacuum valve. The test shall be carried out under the following two conditions respectively: a) when the vacuum valve between the chambers is closed and the adjacent chamber is at atmospheric pressure, test the pressure rise rate of the heating chamber alone; b) the valve between the chambers is open state, test the pressure rise rate of the whole furnace; c) For the furnace equipped with carbon felt, in order to prevent the influence of moisture absorption of carbon felt, it can be carried out without installing carbon felt before the test or after the carbon felt is heated and baked to remove moisture. The test results should meet the requirements.

3.3 Measurement of Furnace Temperature Uniformity

Follow the rules. The test temperature is divided into two levels, namely 550 °C and 1100 °C. The test shall be carried out in the hot state under the two test temperatures respectively. During the test, three sets of data were measured under different pressures between 130 Pa and the ultimate vacuum, and then the arithmetic mean of the three was calculated. The furnace temperature uniformity under the two test temperatures should meet the requirements.

3.4 Heating test

The purpose of this experiment is to measure the heating capacity and air extraction capacity of the vacuum hardening furnace, and to check the thermal operation of the vacuum hardening furnace. Unless otherwise required, the heating test of the vacuum hardening furnace is carried out according to the following method: under the condition that the furnace chamber and the charge are at ambient temperature, load the charge equal to the specified maximum load into the heating chamber. Furnace charges can be steel with a cross-sectional thickness not exceeding 25mm, or it can be agreed between the supplier and the purchaser. Charges should be clean and dry and should be arranged as evenly as possible in the work area.

Carry out the test of the heating capacity and the extraction capacity of the hot furnace first. Set the furnace temperature at 1100 °C, start heating when the vacuum degree of the heating chamber reaches 25 Pa, and raise the furnace temperature to the set value with the rated power or according to the temperature rise program specified in the enterprise product standard, and keep the temperature for 30 min. After 1 hour or 1.5 hours after the heating starts, the furnace temperature should reach the set value, and the vacuum degree of the heating chamber should reach the specified working vacuum degree according to the time requirement. After the heating chamber has cooled down to the ambient temperature, test according to the following procedures: a) Evacuate each furnace chamber; b) Start heating when the vacuum degree of the heating chamber reaches 50 Pa, and continue to evacuate until the working vacuum degree; c) Insulate at 950 ℃ and working vacuum for 12 minutes; d) Decrease the vacuum to 25 Pa at 1000 ℃; e) Raise the temperature to 1100 ℃; f) Insulate at 1100 ℃ and 25 Pa for 30 minutes; g) Transfer the charge to the cooling chamber, and use argon or nitrogen to carry out forced circulation cooling at the maximum rate under the condition of not exceeding atmospheric pressure. The temperature of the charge should be able to drop from 1100 °C to 150 °C within 1 h. It can be measured with a surface thermometer after discharge. Class C furnaces should be able to automatically operate according to the set procedures and record complete data. For Class B and Class A furnaces, the above tests are completed with manual control and partial automatic control. At least three more heating tests with full load, furnace temperature up to 1100 ℃, and different procedures, including oil quenching tests, should be carried out. During the oil quenching test, the quenching transfer time is measured from the time when the charge is removed from the heating chamber until it is completely immersed in the oil, and a stopwatch is used for timing. When required, the vacuum hardening furnace should run for more than 24 hours accumulatively at the highest working temperature and full load. Check after the test.

3.5 Test of oil quenching system

Oil tank heating capacity test This test is only carried out when the quenching oil tank is equipped with an electric heater. When the oil tank is in a cold state and no cooling system is used for cooling, the oil tank is heated by electricity. The temperature of the quenching oil should be able to rise from 20°C to 80°C within 6 h. In the oil temperature control accuracy test, set the oil temperature at 25°C, 55°C, and 85°C. When the oil tank is heated by electricity but not cooled by the cooling system, the changes in oil temperature are recorded respectively. The change in oil temperature should be within the set value. within the ±14°C range. The test time shall not be less than 3 h. In the oil tank quenching capacity test, when the oil tank is not heated by electricity, but is cooled by the cooling system, in the later stage of the charging test heating, quench the steel material whose weight is equal to the maximum loading capacity, and record the temperature rise of the quenching oil, which should not exceed 25 ℃. Cooling capacity test of the cooling system In the above test, after the charge is quenched and discharged, the cooling system has the maximum cooling capacity, and the change of the oil temperature is recorded. The quenching oil temperature should drop by at least 17°C within 1 h.

4 Inspection rules and classification

4.1 Factory inspection items

The factory inspection items of the vacuum hardening furnace: a) General inspection; b) Safety inspection; c) Durability test of signs and writings; d) Circuit test; e) Calibration of temperature instruments; f) Inspection of water system; g) Gas circuit System inspection; h) Hydraulic system inspection (when there is such a system); i) Motion mechanism operation or action inspection; j) Interlock alarm system inspection; k) Ultimate vacuum measurement; l) Empty furnace Measurement of evacuation time; m) Measurement of pressure rise rate; n) Inspection of supporting parts, including inspection of model, specification, and factory certificate; o) Scope of supply, including inspection of completeness of factory technical documents; p) Inspection of packaging .

It should be supplemented when necessary in the enterprise product standard.

4.2 Type inspection items

Type inspection items of vacuum hardening furnace: a) All factory inspection items (under type inspection conditions); b) Measurement of heating time of empty furnace; c) Measurement of rated power; d) Measurement of rated temperature; e) Loss of empty furnace f) measurement of furnace temperature uniformity; g) measurement of furnace temperature stability; h) heating test; i) test of oil quenching system; j) measurement of water consumption; k) inspection after thermal test.

It should be supplemented when necessary in the enterprise product standard.