Research on Heat Treatment Process of ERW Pipe

With the development of national economy, the use of erw pipe is more and more widely. Compared with the seamless pipe, welded pipe production has the following advantages: equipment, light weight, less investment in construction, low cost; and the production of a high degree of mechanization and automation, continuous production, high-frequency welded pipe in the steel industry holds a significant proportion.

The test material is ST14 cold-rolled strip produced by Baosteel, and the strip of 0.7 mm thick is made of 8 mm steel tube by high-frequency welding. The first batch of heat treatment experiments in the production of continuous annealing furnace. Continuous annealing furnace motor speed 800r / min; adjust the voltage parameters so that the experimental temperature in the desired range, the temperature measured by the infrared thermometer. The second batch of heat treatment experiments in the experimental gas protection furnace, simulated production using continuous annealing. Test sample take the entire length of 300mm steel pipe, the sample after treatment in 50kN hydraulic universal testing machine pull test to measure the mechanical properties. At the same time, the samples were observed under light microscope.

The experiment was carried out in a continuous annealing furnace. It can be seen that 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 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 plasticity begins to decline when the annealing temperature exceeds 800  From the iron carbon phase diagram we know that in this temperature range, the material is in the ferrite and austenite two-phase region, the original part of the transformation into austenite. However, some ferrites did not change. 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 when heated to a high temperature forms fine ferrite grains after cooling, which in turn results in nonuniform grain size (as shown in FIG. 2), resulting in a decrease in 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.