Heating method of stainless steel welded pipe

Stainless steel welded pipe has excellent corrosion resistance and mechanical properties, widely used in different industrial projects. However, with the development of large-scale projects and equipment, the diameter of stainless steel welded pipe is increasingly large, the use of stainless steel welded pipe is mainly concerned with its corrosion resistance, usually the pressure of pipeline design is not high; Therefore, its wall diameter is relatively small. When welding stainless steel welded pipe, harmful phase and stress concentration are easily precipitated in the weld and heat affected zone, which leads to the reduction of corrosion resistance of the pipe. At the same time, residual stress and cold work hardening are easy to exist in the process of steel plate forming into steel pipe. In order to ensure the service performance of stainless steel welded pipe products, the need for heat treatment after welding, the purpose of which is to redecompose the weld and the harmful phase precipitated in the heat affected zone caused by welding, eliminate the stress and work hardening produced in the process of cold forming and welding.

Because the stainless steel inherent physical properties, such as low thermal conductivity, linear expansion coefficient and strength under different temperature differences and so on, at the same time, high temperature stainless steel welded pipe heat treatment requirements, and need to rapid heating and rapid cooling, these factors lead to large diameter thin-walled stainless steel welded pipe is very easy to deformation in the process of heat treatment, if improper heat treatment way, Not only can not improve the performance of the product, but will make its performance worse, to the field construction and pipe service life caused a great impact. The heating modes of post-welding heat treatment of stainless steel welded pipe mainly include gas heating, resistance heating and induction heating.

1 Physical characteristics of stainless steel

The main parameters for evaluating cold and hot working 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. The strength at room temperature is very different from that at high temperature.

2 Solution heat treatment of stainless steel

According to different standards, grades and purposes, stainless steel heat treatment process is different. Austenitic stainless steel is usually fixed by heat treatment. Solution treatment, stabilization treatment, stress removal treatment, but stainless steel after welding mainly for solution treatment.

2.1 Solid solution treatment

The solution treatment of industrial austenitic stainless steel welded 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 make the harmful precipitates in the weld and heat affected zone re-solution into the matrix structure, and fully eliminate the influence of stress and cold work hardening, so as to ensure that the mechanical properties and corrosion resistance of the product meet the relevant standards and project technical requirements. The solution processing temperature of stainless steel welded pipe is high. When heating and cooling, the temperature of different parts of large diameter stainless steel welded pipe is different. The pipe is easy to deformation, especially the thin-walled pipe.

2.2 Process Requirements

In order to prevent harmful precipitate phase in the process of heating and cooling, fast heating and cooling requirements of stainless steel welded pipe, especially for super austenitic stainless steel and duplex stainless steel material, furnace temperature reach the process temperature and holding time after directly put the pipe in the charging for heating, ensure pipe has high heating rate, When the pipe body reaches the process temperature and holds the heat for a certain time, the pipe body temperature can not 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 mode

Usually, industrial stainless steel welded pipe diameter, wall thickness, most of the wall diameter ratio S/D≤3%, a single length is generally 6m or 12m, with the improvement of enterprise manufacturing capacity, in order to reduce the amount of site construction, the current 12m/ branch delivery mainly. When using different heat treatment heating methods, the same specification, the same brand of stainless steel welded pipe heating time is very different, the longer the heating time, heating and cooling process of different parts of the temperature difference is greater, the greater the deformation.

3.1 Gas heating

Gas heating is usually used in box furnace, gas enters and burns from different positions in the furnace, so as to realize the heating of the whole furnace and welded pipe in the furnace. The welded pipe in the furnace is kept warm when the process temperature is reached. When the gas heating method is adopted, the pipe is horizontally placed in the chamber of the box furnace. The deformation of large-diameter thin-walled stainless steel welded pipe is large, and it is difficult to shape after heat treatment, and the stress will be increased during shaping. In addition, the furnace must be preheated in advance before opening, preheating time is long, small batch production cost is high. Therefore, gas heating method is not suitable for solution treatment of large-diameter thin-wall stainless steel welded pipe, but is more suitable for stabilization heat treatment or stress elimination treatment.

3.2 Resistance heating

In order to reduce tube deformation and avoid the influence of gas heating on material properties, well resistance furnace can be used for heating. In the well resistance furnace, the resistance wire is arranged in the inner wall of the furnace, and the resistance heat is used to heat the furnace and the pipe. When the well-type resistance furnace is used for heating, the heat treatment deformation of the welded pipe is small because the welded pipe is put into the furnace vertically. After heating the pipe from the furnace first lift, and then moved to the cooling pool above, then slowly into the pool cooling, the whole process takes a long time, especially a single length of A6m pipe, the top of the pipe first out of the oven, but it is the last into the water cooling, the upper temperature of the pipe into the water is reduced more.

Resistance heating heat treatment, before the furnace must also be preheated, preheating and heating time is long; In the process of heat treatment and heating, the heating rate of different parts of the welded pipe is not the same, which is not suitable for the solution treatment of stainless steel welded pipe with A delivery length of A6m/ branch.

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. The electromagnetic induction of alternating magnetic field produces a closed induced current in the workpiece, 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 resistivity 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. The 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 stainless steel small 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 batch manufacturing.

4 conclusion

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 reduce 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, it is suggested to extend the holding time. It 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.