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 Efficiency Heating
Coolingby 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 a