How to design an intermediate frequency induction heat treatment production line?

Intermediate frequency induction heat treatment line equipment introduction

The secondary coil of an intermediate frequency quenching transformer is an AC induction coil. The workpiece is placed in an ac induction coil and heated by an alternating magnetic field. Intermediate frequency induction steel pipe quenching and tempering, the heat treatment production line has many characteristics, such as less investment, quick effect, low production cost, less equipment maintenance cost, and less maintenance. Moreover, the traditional heat treatment method is broken, which greatly improves production efficiency and reduces production costs.

Intermediate frequency induction forging furnace

1.Yield, specification, and variety

1.1 Output and maximum specification of pipe expansion

The annual output is expected to be more than 30,000 t. The maximum size of tube expansion reaches 700mm.

1.2 Types and steel

It can deal with low carbon steel, medium carbon steel, and alloy steel whose alloy content is less than 5%. The varieties correspond to petroleum casing series, hydraulic prop series, boiler series, pipeline series, and seamless pipe for fluid transportation.

2.Production technology of each operation area

The process work is divided into a raw material area, heat treatment area, straightening area, flaw detection area, hydraulic operation area, and chamfering operation area. The heat treatment operation is divided into 3 quenching lines and 2 tempering lines.

The technological process is as follows:

Seamless pipe → grinding → induction heating → water quenching → heating in doubt → tempering → sizing → flaw detection → straightening → water pressure → chamfering → check for storage.

2.1 Raw material operation area

Unloading, storage, marking, grinding, and cleaning of steel pipes are carried out in the raw material operation area. After marking, the steel pipes are entered into the heat treatment operation area through the rotating roller table. The main purpose of the rotary roll table is to rotate the steel pipe forward and prevent the pipe from bending after quenching.

Requirements for raw materials: the maximum length is 1400mm; Outside diameter ∅219 ~ ∅470mm; Wall thickness 6 ~ 40 mm; Straightness requirements: the full-length straightness shall not exceed 0.2%; The maximum deviation within 1.5mm from the pipe end shall not be greater than 3.18mm.

2.2 Heat treatment zone

Heat treatment production line can realize quenching + tempering, normalizing + tempering, normalizing, tempering, and other processes, which can meet the heat treatment needs of different kinds of steel.

2.2.1 quenching area

An intermediate frequency induction coil is used for heating, and the maximum temperature is designed to be 1040℃. Processing speed is 5mm per second; The steel is fed by the rotating roller, and the quenching is treated by internal spray and external spray. The internal spray is supplied by the pressure of the limiting pipe, and the external spray is quenched by water tank spray. Intermediate frequency induction furnace parameters: power 400kW, current 550A.

Quenching process is a heat treatment process in which steel is heated to a certain temperature above AC3 or AC1 point, kept for a certain time, and then cooled at an appropriate rate to obtain martensite and/or Bainite structure.

After quenching, hardness and strength can be improved, but it must be tempered to optimize the performance of strength and toughness.

The quenching temperature is determined according to the composition, microstructure, and different performance requirements of the steel. When selecting the quenching heating temperature of the workpiece in production practice, it is necessary to take into account the influence of many factors such as chemical composition, technical requirements, size, shape, original structure, intermediate frequency heating equipment, and cooling medium, and adjust the heating temperature appropriately. The maximum intermediate frequency heating temperature of 1040℃ can basically meet the quenching requirements of all kinds of steel.

In order to complete the microstructure transformation, carbide dissolution, and austenite homogenization of all parts inside and outside the workpiece, a certain time must be held at the quenching heating temperature, namely the holding time [2]. Under the condition of medium frequency induction heating, the required quenching temperature can be reached between the steel tube and the middle of the induction coil. Under the premise of appropriately increasing the quenching temperature, the remaining stroke can basically make the structure uniform.

2.2.2 back to the fire area

An intermediate frequency induction coil is also used for tempering, and the maximum temperature is designed to be 800℃. Adopt a rotating roller way to enter steel; Medium frequency induction furnace parameters: power 350kW, current 550A.

Because the maximum temperature is designed at 800℃, high temperature and low temperature tempering process requirements can be guaranteed.

After tempering, the process requirements of toughness and plasticity can be obtained by reducing brittleness, eliminating, or reducing internal stress.

2.2.3 Water quenching method

Internal spray and external spray are used to quench, the internal spray is used to spray and internal spray is used to quench, and external spray is used to spray high pressure pipe ring in box tank, and internal spray and external spray can ensure the hardenability of thick wall steel tube. Spray water temperature should be controlled below 35℃, water pressure is 3MPa, water flow is 5L/s.

The quenching medium chooses water with a wide source, low price, and stable composition. By controlling the temperature of the water, the martensite can be quenched without causing too much quenching stress.

Considering the hardenability of the pipe wall is too thick, you can add an appropriate amount of salt and alkali in the water, so that the high-temperature workpiece immersed in the cooling medium, in the steam film stage precipitated salt and alkali crystal and immediately burst, the steam film will be destroyed, the surface of the workpiece oxide skin is also blasted, so as to improve the cooling ability of the medium in the high temperature region.

2.2.4 Circulating water cooling

A cooling water tower is used for circulating water cooling to ensure the water temperature is less than 35℃.

2.2.5 Heating defects and control

First, attention should be paid to overheating and overburning to avoid excessive heating temperature, which will lead to large austenite grains and local oxidation or melting of grain boundaries, resulting in grain boundary weakening. In severe cases, the performance deteriorates seriously after overfiring, and cracks are formed during quenching. The overburned tissue cannot be recovered and can only be scrapped. So you really need to control the heating temperature.

In addition, attention should be paid to avoid decarburization and oxidation to ensure the surface hardness, fatigue strength, and wear resistance of the steel after quenching.

2.3 straightening zone

(1) After tempering, the hydraulic quick-open six-roller straightener can be used for straightening.

(2) Straightening process parameters. The straightening temperature is normal temperature ~650℃. Bending after straightening: tube body less than 1.5/1000 mm; Pipe end less than 1/1000mm; Ellipticity meets different delivery requirements; No indentation shall be allowed on the surface of steel pipe after straightening.

2.4 inspection area

(1) Adopt the ultrasonic flaw detection machine and magnetic leakage flaw detection unit.

(2) Check the types of defects and measurement items. Types of defects to be checked: longitudinal, transverse, and stratified defects on internal and external surfaces; Measurement items: outside diameter, wall thickness, ovality. The geometrical dimensions were measured by the ultrasonic method.

The detection accuracy is divided into wall thickness measurement accuracy and outside diameter measurement accuracy. The measurement accuracy of wall thickness requires ±30 m. Accuracy of outside diameter measurement requires ±50 m.

2.5 Hydraulic operation area

Inspection seamless steel tube strength and holes and other internal defects.

The hydrostatic test was conducted according to API Spec 5CT and API Spec 5L, and the test pressure was 35~100MPa.Test medium: emulsion; Pressure holding time: 5~300s; Pressurization mode: high pressure pump plus booster cylinder; Pressure retaining form: breakwater retaining pressure or pressurized cylinder retaining pressure.

2.6 Finishing operation area

Check and store the finished products, chamfer the steel pipe.

2.7 Other Equipment

Lifting transport equipment 3 sets, specifications for the main hook 20/5T crane. Mainly used for equipment maintenance, raw material lifting, tool replacement, accident treatment, intermediate pipe material lifting, etc.

Cold bed and steel transfer bench 5. The distribution places are raw materials and quenching front stand, quenching and tempering front stand, tempering back stand, flaw detection and water pressure front stand, water pressure back stand.

3. Auxiliary parts and workshop labor staffing

Electrical control parts, water supply and drainage facilities (including clean water system, new water supply system for production, domestic fire water system, safe water supply system, etc.), heating, air conditioning, ventilation, etc., shall be designed according to relevant national standards and regulations; The completion shall meet the requirements of environmental protection; Occupational safety and health meet the requirements of relevant national laws and regulations.

4 shifts and 3 reverse operations are adopted, including production line, auxiliary equipment operation, and management personnel, etc., the whole workshop needs a total of 60 people.


Adopting medium frequency induction heating to treat seamless steel tube is feasible, it has the characteristics of small batch and many specifications, and has the value of popularization and application.

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