Heating method of stainless steel seamless pipe

Due to the inherent physical properties of stainless steel seamless pipe, such as low thermal conductivity, linear expansion coefficient, and strength under different temperature differences, etc., and the heat treatment of stainless steel welded pipes requires high temperature, rapid heating and rapid cooling, these factors lead to large-diameter thin-walled stainless steel welded pipes in the heat treatment process It is very easy to deform, if the heat treatment method is not appropriate, not only will the performance of the product not be improved, but will make its performance worse, which will greatly affect the on-site construction and the service life of the pipeline. The heating methods of post-weld heat treatment of stainless steel welded pipe mainly include gas heating, resistance heating and induction heating.

 

1 Physical properties of stainless steel

 

The main parameters for evaluating the cold and hot processing characteristics of stainless steel are linear expansion coefficient, thermal conductivity and strength. The linear expansion coefficient of austenitic stainless steel is about 50% larger than that of carbon steel, but the thermal conductivity is only about 1/2 of that of carbon steel. Strength at room temperature is very different from strength at elevated temperatures.

 

2 Solution heat treatment of stainless steel

 

According to different standards, grades and uses, the heat treatment process of stainless steel is different. Austenitic stainless steels are usually set by heat treatment. Solution treatment, stabilization treatment, and stress relief treatment, but stainless steel is mainly subjected to solution treatment after welding.

 

2.1 Solution treatment

 

Solution treatment of industrial austenitic stainless steel seamless pipe is a process of heating the welded pipe to the solution temperature, holding it for a period of time, and then cooling it rapidly. The main purpose of solution treatment is to re-dissolve the harmful precipitates in the weld and heat-affected zone into the matrix structure, fully eliminate the influence of stress and cold work hardening, so as to ensure the mechanical properties and corrosion resistance. The products meet the relevant standards and technical requirements of the project. The solution treatment temperature of stainless steel seamless pipe is high. When the large-diameter stainless steel welded pipe is heated and cooled, the temperature of different parts is different. Pipes are easily deformed, especially thin-walled pipes.

 

2.2 Process requirements

 

In order to prevent harmful precipitates during heating and cooling, stainless steel welded pipes require rapid heating and cooling, especially for super austenitic stainless steel and duplex stainless steel materials. After the furnace temperature reaches the process temperature and holding time, it is directly put into the pipe for charging and heating. , to ensure that the tube has a high heating rate. When the tube body reaches the process temperature and is kept warm for a certain period of time, the temperature of the tube body cannot drop too much before entering the water, so it is necessary to strictly control the distance and time from the furnace to the water cooling.

 

3 heat treatment heating method

 

Usually, the diameter and wall thickness of industrial stainless steel pipes, most of the wall diameter ratio S/D≤3%, and the single length is generally 6m or 12m. With the improvement of manufacturing capacity of enterprises, in order to reduce the amount of construction site, currently 12m/piece mainly for delivery. When different heat treatment heating methods are used, the heating time of stainless steel welded pipes of the same specification and brand varies greatly. The longer the heating time, the greater the temperature difference between different parts during the heating and cooling process, and the greater the deformation.

 

3.1 Gas heating

 

The box furnace is usually heated by gas, and the gas enters and burns from different positions in the furnace to realize the heating of the entire furnace and welded pipes in the furnace. When the process temperature is reached, the welded pipes in the furnace are insulated. When the gas heating method is used, the tube is placed horizontally in the hearth of the box furnace. The large-diameter thin-walled stainless steel welded pipe has large deformation, and it is difficult to form after heat treatment, and the stress will increase during forming. In addition, it must be preheated in advance before starting the furnace, the preheating time is long, and the production cost of small batches is high. Therefore, the gas heating method is not suitable for solution treatment of large-diameter thin-walled stainless steel welded pipes, but is more suitable for stabilization heat treatment or stress relief treatment.

 

3.2 Resistance heating

 

In order to reduce the deformation of the pipe and avoid the influence of gas heating on the material properties, a well-type resistance furnace can be used for heating. In the well-type resistance furnace, the resistance wire is arranged on the inner wall of the furnace, and the furnace and pipeline are heated by resistance heat. When the well-type resistance furnace is used for heating, since the welded pipe enters the furnace vertically, the heat treatment deformation of the welded pipe is small. The heated tube is first lifted from the furnace, then moved to the upper cooling pool, and then slowly enters the cooling pool to cool. The whole process takes a long time, especially for a single A6m tube. The top tube comes out of the oven first, but It is the last water inlet cooling, and the temperature of the upper part of the water inlet pipe is lowered more.

 

Resistance heating heat treatment must be preheated before it is released from the furnace, and the preheating and heating time is long; during the heat treatment heating process, the temperature rise rate of different parts of the welded pipe is different, and it is not suitable for solid solution of stainless steel welded pipes with a delivery length of A6m/piece deal with.

 

3.3 Induction heating

 

The basic principle of induction heat treatment is to put the workpiece into the induction coil, the induction coil through a certain frequency of alternating current, alternating magnetic field is generated around. , and the electric energy of the current is transformed into heat energy, so as to realize welding pipe heating. The structure of induction heating furnace is shown in Figure 4. According to Joule's law, the power consumed by the resistance of the current through the loop for a period of time is converted into heat Q=I2Rt. Where, I is the induced current, R is the loop resistance, and T is the heating time. The resistance of austenitic stainless steel is about 5 times that of carbon steel, so the induction heating speed is fast and easy to control, which can shorten the heating time.

 

The welding pipe adopts the rotary forward way through the induction coil, the temperature of the whole section is uniform, which can well control the deformation, the pipe reaches the process temperature and hold the heat for a certain time immediately into the water cooling. cooling device adopts the annular spray ring, and the sprinkler holes are tilted at a certain Angle to ensure that the cooling water will not flow into the induction coil and affect the temperature measurement of the infrared thermometer. The annular spray ring can be designed in the form of single ring and multi-hole and multi-angle or multi-ring and multi-hole and multi-angle, and the water will finally spray to the specified area. Because the water spray to the welded pipe has a certain Angle and distance , in order to make the welded pipe leave the induction coil to be cooled as quickly as possible, the spray ring and furnace outlet distance can be adjusted according to the actual forward speed of the pipe , to ensure that the pipe starts spraying cooling within 1min after the pipe is out of the oven. Spray cooling cools the whole section of the welded pipe at the same time, and the temperature is basically the same, which further controls the deformation of the welded pipe. Compared with the previous two heating and cooling methods, induction heating and spray cooling form can make the welded pipe in the sensitization temperature interval for the shortest time and the fastest cooling rate.

 

3.4 Comparison of heating methods

 

The choice of heating method must give priority to product performance, followed by economic benefits. The above three kinds of heating methods have their advantages and disadvantages, including induction heating due to the fast heating, heating and cooling process on the properties of small steel negative impact, especially large diameter thin-walled tube deformation is small, and is suitable for steel strip continuous production of welded pipe on-line heat treatment, so than the other two ways are more suitable for large diameter thin-walled stainless steel tube welding solid solution treatment. Solid solution treatment is a special process in the production process of stainless steel welded pipe. The selection of heating mode needs to consider the comprehensive influencing factors such as shaping, surface quality, production efficiency and batching manufacture.

 

4 conclusions

 

The solution treatment of large-diameter thin-wall stainless steel welded pipe should adopt induction heating, which can well control the deformation caused by heat treatment; The induction heating method has the shortest heating time and the fastest cooling speed, which can the negative effect of heat treatment on the metallographic structure and prevent the secondary precipitation of harmful phases. To solve the problem of large temperature difference between inside and outside of stainless steel welded pipe with wall thickness ≥ 20mm during induction heating, that it is time. suggested is suggested that measures to reduce skin effect and end effect of induction heating should be studied in the future, so as to expand the application range of induction heating and improve the heat treatment process.