Cold Drawn Process

According to marketing demands, customers require us to provide seamless steel tubes with various specification, performances and precisions. We could make by different process method. Tube cold drawing is the most common process.

Tube drawing is a metalworking process to size the tube by drawing the tube through a die to required length by controlling the extensibility step by step. This process is finished at room temperature, so it is usually called cold drawing. To make continuous deformation, this process needs annealing to relieve internal stresses. Annealing also could induce ductility, soften steel tube, refine the structure by making it homogeneous and improve cold working properties.

Raw Material

Tube cold drawing begins with procuring hot rolled steel tube as the raw material. The hot rolled tube usually have a rough and scale surface for they are produce at elevated temperature. It may also exhibit variations in section and size.

Pointing

The next step is pointing. Several inches of tube’s end could be reduced in size by push pointing, rotary swaging and squeeze pointing. Thus, it could pass through the drawing die freely.

Pickling, phosphating and lubrication

Pickling helps removing abrasive scale or iron oxide on the surface of hot rolled steel tube with hydrochloric acid.

After pickling, phosphating provides surface porous coating by placing steel tube in the phosphonic acid. The porous coating on the surface not only separate the steel tube and drawing die, but also adsorb soap powder to reduce friction during cold drawing.

Lubrication is vital for this process to reduce friction and to reduce metal transfer from the tube to the die.

Drawing

The material is drawn at room temperature. It is finished on a machine called a draw bench, containing a back bench, die head and front section. Jaws on a trolley grip the tube and a hook on the back of the trolley engages a moving chain, pulling the tube through a die. Dies are most commonly sintered tungsten carbide inserts with a cobalt binder that has been shrunk-fit into a steel casing.

There are two main processes employed for the cold drawing of tube: sink or hollow drawing, and stationary plug drawing.

Sink or hollow drawing is the simplest drawing procedure. The tube is drawn with no internal support. With enough lubrication, when the tube surface goes through the outside diameter of the tube is reduce and the outside surface polished in the die, with the wall thickness undergoing increasing during drawing.

In stationary plug drawing, a slightly tapered tungsten carbide plug is threaded onto the end of a mandrel; the tube is loaded over the rod, lubricant pumped onto the OD surface and the tube is drawn. This plug makes an annular gap with the block die through which is the tube is drawn. This process reduces within close tolerance both the outside and inside diameters, and this also the wall thickness, as well as smoothing and polishing both the outside and inside surfaces. This process is slow and the area reductions are limited, but it gives the best inner surface finish of any of the processes.

Annealing

After drawing, the grain structure of steel tube is stressed and then the material becomes brittle and hard. To further process, the stress needs to be removed to make is back to normal state. Annealing is a heat treatment that alters the physical and chemical properties of the steel tube to increase the ductility. After exposed to a controlled temperature, the steel tube is softer and more workable.