Heating element in the heating chamber of a vacuum furnace
Are working in the use of vacuum furnace, the main is to heat, need to require rapid heating rate in the process of heating, heating material must be selected to ensure that good thermal conductivity, and does not produce deformation at high temperature and large amounts of heat loss, to ensure that in use process over a period of time performance is stable, in selecting heating element according to their chemical properties required for processing products, in order to avoid the material of heating elements affect the quality of the workpiece. Therefore, the heating element should be selected reasonably.
The commonly used heating elements can be divided into two categories: metal type and non-metal type.
1. The non-metal electrothermal materials mainly include graphite (cloth, pipe and bar), which is characterized by low price, easy processing, large radiation area and good thermal impact resistance. Mainly rely on radiation heat transfer, good working conditions, but volatile at high temperature, prevent vacuum discharge (less than 100V). The characteristics of graphite heating elements: graphite with high temperature resistance, small thermal expansion, strong thermal shock resistance characteristics, its mechanical strength under 2500 ℃ temperature rise and improve, best around 1700 ℃, more than all of the oxide and metal. The graphite material has high melting point, low vapor pressure and low carbon content in the atmosphere of the vacuum furnace. It will react with oxygen and water vapor in the residual gas to produce the purification effect, greatly simplifying the vacuum system and reducing the cost.
2. Metal type can be divided into pure metal and alloy type. Pure metal electrothermal materials mainly include: molybdenum, tungsten, niobium, etc. used in high temperature vacuum furnaces. Electrothermal alloy material mainly nickel chrome, iron chromium aluminum two pa, can only use 1000 ℃ for 10-1 is used for low temperature and low in vacuum furnace. Nickel-chromium alloy and ferrochromium aluminum alloy can only be used in the range of low temperature and moderate vacuum. Molybdenum, tantalum and tungsten are volatile when heated in vacuum. Characteristics of stainless steel heating elements: small size, high power, fast thermal response, high temperature control accuracy, high comprehensive thermal efficiency, wide application range, strong adaptability, high heating temperature, long life and high reliability. High degree of mechanization, can be bent into various shapes according to the requirements of customers, lightweight, easy to disassemble. Simple structure, less material, low cost, long service life, high thermal conversion, and energy saving.
Selection of vacuum
The medium vacuum (general limit 4x10-1p a) can be selected for quenching heat treatment. The two-stage pumps (front-stage pumps and roots pumps), such as bearing steel, die steel and structural steel, can be used for vacuum heat treatment.
For some products requiring high vacuum, such as (high temperature alloy, titanium alloy, magnetic material, etc.) vacuum heat treatment or high temperature brazing. Three stage pump can be selected. In general, the vacuum limit is required to be 10-4pa.
Nitrogen should not be used as a cooling gas when treating titanium alloy in vacuum furnace, because titanium and nitrogen react at high temperature to form golden titanium nitride.
The movable connection part of the vacuum furnace is all sealed with o-type rubber ring.
When the workpiece is hardened in a vacuum, a vacuum quenching oil should be used. This oil has a low saturation vapor pressure.
The maintenance of vacuum furnace should be in the state of vacuum or pure nitrogen, to avoid the usual time when not inhaled, moisture absorption.
During the heating process, the workpiece and the materials in the furnace will be deflated, so that the vacuum will drop.
The heating temperature of vacuum tempering, vacuum annealing, vacuum solid solution treatment and vacuum aging treatment is generally the same as that of conventional treatment.
The vacuum return furnace shall have a rapid cooling device. The cooling water pressure should be greater than 0.2mpa and the flow rate should be adjustable.