What Is Cold Drawing?

Cold drawing is a kind of processing technology of materials. For metal materials, cold drawing refers to drawing in order to achieve a certain shape and certain mechanical properties, while the material is at normal temperature. Compared with hot forming, cold drawn products have the advantages of high dimensional accuracy and good surface finish.

Tensile test

Cold drawing is to make the HPB300 grade steel bar with a diameter of 6 ~ 8 mm forcefully pass through a special tungsten alloy wire drawing die hole with a decreasing diameter at room temperature to plastically deform the bar to change its physical and mechanical properties.

Before cold drawing, the raw materials shall be inspected as necessary. For steels whose steel number is unknown or without a factory certificate, samples shall be taken for inspection. In the case of irregular, round, barbed, stiff, damp steel bars with irregular cross-sections, they should not be used for drawing, so as not to damage the wire drawing die and affect quality. The steel bar must be rolled and derusted before cold drawing. The rust removal device can use a wire drawing machine reel and a wire rod turret. Among them, there are 3 to 6 slotted peeling wheels that are staggered or staggered up and down. It is also possible to mechanically remove rust using a waste wire drawing die with a diameter substantially the same as that of the reinforcing bar.

In order to facilitate the rebar to pass through the wire drawing die, the head of the steel bar needs to be thinned (length 150 ~ 200 mm) and rolled to a diameter smaller than 0.5 ~ 0.8 mm than the wire drawing hole so as to pass through the hole smoothly. In order to reduce the number of rolling heads, butt welding can be used to connect the reinforcing bars, but the convex seam at the weld should be smoothed with a wheel file to protect the equipment and the drawing die.

Before operation, the equipment should be inspected and run once at no load as usual. When installing the wire drawing die, distinguish the front and back sides. Tighten the fixing bolts after installation.

In order to reduce wire drawing force and wire hole loss, lubricant should be applied during drawing. Generally, a lubricating box is installed before the drawing die, so that the viscous lubricant of the reinforcing steel enters the drawing die.

The wire drawing speed should be controlled at 50 ~ 70 m / min. The continuous reinforcement of steel bars should not be more than 3 times. If it is necessary to pull again, the internal stress of the steel bar should be eliminated, and the low temperature (600 ~ 800 ) annealing treatment should be used to soften the steel bar. After heating, take it out and bury it in the sand to cool it slowly. The cooling rate should be controlled within 150 / h.

The finished product of wire drawing should be inspected for sand holes, groove marks, and skin defects at any time. In order to change the wire drawing die or adjust the speed at any time. ^[2]

1) The situation of cold tensile stress control:

For steel bars of grades II, III, IV and No. 5 steel, which are used as prestressed steel bars after cold drawing, cold drawing stress control shall be used. However, after the steel bar was cold drawn, it was checked that the maximum cold drawing rate exceeded the value specified in the specification, and mechanical energy experiments were also performed.

2) Control requirements for cold drawing rate measurement:

When the cold drawing rate is controlled by the cold drawing rate, the control value should be determined by experiments. Requirement during the test: There must be no less than 4 test specimens in the same batch in the same furnace. Each sample must be measured for the corresponding cold-drawing rate according to the cold-drawing stress specified in the specification, and the average value of the test pieces is taken as the furnace The actual cold drawing rate of the batch of steel bars. If the strength of the steel bar is high, when the average cold drawing rate is lower than 1%, the cold drawing rate of the steel bar should still be controlled according to the 1% cold drawing rate.

3) Cold drawing control of different furnace batches:

For mixed and indistinguishable rebars, the cold drawing rate cannot be used to control the cold drawing, and multiple connected steel bars must be cold drawn, and the cold drawing rate and control stress of each must meet the specifications.

4) Cold drawing speed control:

In order to fully deform the steel bar, it is necessary to properly control the cold drawing speed, generally 0.5-1.0 is appropriate. At the same time, it is required that after the cold drawing reaches the specified stress and cold drawing rate, stop pulling immediately after 2-3, and then relax the steel bar and end the cold drawing to give the steel bar sufficient time to deform.

5) Cold drawing control method:

In cold drawing, only the cold drawing rate or cold drawing stress control is called single control. The cold drawing rate and cold drawing stress are applied at the same time in cold drawing, which is called double control. Using single control, the construction is simple and convenient. However, for steel bars with uneven materials, it is impossible to test one by one (testing one by one, labor and materials, it is impossible to do so, and the cold drawing rate of some of the same steel bars is not the same). The cold drawing quality cannot be guaranteed. The dual control method can avoid the above problems. In cold drawing, if the control stress has been reached and the cold drawing rate does not exceed the allowable value, it can be considered as qualified. However, if the cold-drawing rate has been reached and the cold-drawing stress has not reached the control stress, this kind of steel bar should be used with reduced strength. For prestressed reinforcement, a dual control method must be used.