Performance Effect of ERW Pipe Heat Treatment Process

1, The annealing temperature on the performance
When the annealing temperature is lower, the strength of the sample is higher, but the plasticity is poor. With the increase of annealing temperature, the tensile strength gradually decreases and the elongation increases continuously, which is mainly the result of the gradual elimination of stress and hardening in the pipe during the annealing process. However, when the annealing temperature exceeds 800 °C, not only the strength continues to decrease, but also the elongation begins to decrease.

We know that erw pipe welding leads to work hardening and welding stresses during forming and welding. If the annealing temperature is low, the stress and hardening can not be fully eliminated, so the strength of the annealed pipe is higher but the plasticity is poor. With the annealing temperature increases, the stress and hardening gradually eliminated, so that the strength of the pipe to reduce the plasticity.

But why the ductility begins to decline when the annealing temperature exceeds 800 . The reason is that in this temperature range the material is in the two-phase zone of ferrite and austenite and the original tissue part is transformed into austenite but there is also a part ferrite has not changed. It can be known by calculation that during the forming of the pipe, about 10% of the cold deformation occurs in the material; due to the small degree of cold deformation, the material rarely recrystallizes during annealing. These unaltered ferrites grow during annealing and the coarser ferrite grains remain after annealing at higher temperatures. On the other hand, the austenite formed by heating to a high temperature forms fine ferrite grains after cooling, which in turn leads to non-uniform grain size, thereby decreasing the strength and ductility.

When the annealing temperature is 920 , the pipe also has better strength and plasticity. As the welding process, not only in the weld formation of a small amount of martensite and other non-equilibrium weld microstructure, but also heat-affected zone coarse grains, which are adversely affected the performance. Only heated to Ac3 above the temperature, the organization of all austenitic, in order to eliminate these effects, so that the weld and the parent material to the same organization, that is, to get the fine structure, thereby improving the mechanical properties of the pipe.

2, The cooling rate of performance impact
In order to simulate the case of continuous annealing, the sample was heated at 920  for 2 minutes and then cooled at different speeds. As mentioned earlier, when heated at 920 , the base metal and weld metal in the pipe are all austenitized and the austenite is transformed into a new structure upon cooling. Cooling rate is different, the formation of the organization will be different performance is not the same.

In the case of cooling in the furnace, the cooling rate is very slow, forming a large amount of ferrite and a small amount of pearlite. With the cooling rate increases, the intensity has greatly improved. 8 #, 9 #, and 11 # samples were air-cooled, air-cooled and spray-cooled, respectively. The cooling rates increased in turn and their intensities increased correspondingly, but their elongation decreased in turn. This is mainly due to the formation of a small amount of bainite or martensite during rapid cooling, as well as thermal stress; the faster the cooling rate, the greater the amount of bainite or martensite, and the thermal stress Also larger, resulting in increased strength, decreased plasticity.
 
The 10 # sample in the high temperature stage of fast cooling (air-cooled), 650  after cooling in the protective atmosphere, so that the formation of fine ferrite and pearlite, so that the welded pipe to obtain higher strength and better ductility.

3, The production should pay attention to the problem
From the above research results, if the user requirements of the pipe has good ductility, but not excessive strength requirements, we can use the continuous annealing temperature between 700 ~ 800 , in this case, the temperature range Wide, easy to control in the production process.

If the user requires the pipe at the same time has a high strength and good ductility, then annealing at 700 ~ 800  is not up to the requirements. Because of the combination of skin effect, proximity effect and heat conduction during high-frequency welding, the gradient of the peak temperature of welding thermal cycle at the edge of the tube blank is caused, and the melting zone, partial melting zone and overheated tissue zone appear feature area. As a result, the unbalanced structure and the coarse texture around the weld adversely affect the performance of the welded pipe; to eliminate these effects, the heat treatment temperature must be raised above Ac3. But the temperature should not be too high, otherwise it will deteriorate the performance. This requires strict control in the actual production process annealing temperature, the annealing temperature is guaranteed at about 920 .