Causes of warpage on the inner surface of seamless steel pipes

Seamless steel pipe is machined from a billet made. Small-scale steel plant, due to lack of processing capacity, not the purchase is processed into steel billets themselves but purchase thick-walled steel, after cold drawn back to the factory several times, get a smaller size seamless annealed, then to the fine steel processing plant for further processing. Part of the inner pipe surface roughness manufacturers has sent a higher inner surface of the steel pipe boring roller, to obtain a bright inner surface. But in the process of rolling boring is sometimes found inside the steel pipe with tiny warping. Alice skin may be produced during the boring roll BUE, it may be of questionable quality steel itself.

BUE refers to a boring role, appearing in the tip of the small pieces of metal adhesion and high hardness thereof. When the surface of the steel pipe roll boring since the workpiece material is pushed crack, the chip on the front of the tool generates a lot of pressure, and friction generates a lot of cutting heat. In such a high temperature and pressure, that part of the chip in contact with the tool in front due to friction, the flow rate is relatively slowed down, forming a “stay level.” When the friction material, once greater than the binding force between the internal lattice, the “retention layer” in some of the material will be attached to the front near the tip of the tool to form a BUE. BUE if left on the machined surface, the formation of burrs, increases surface roughness. The seamless inner surface of the warp and the substrate is connected to the base from the warped out, no foreign matter adheres to the substrate after extrusion. The BUE should be adhered to in the seamless surface shall not connect with the inner surface of the steel pipe from the matrix microstructure of view. Therefore, the inner surface of the seamless warp should be created not by boring BUE roll forming, but not foreign matter formation. The material passes through the two cold-drawn, cold-drawn both after each recrystallization annealing, after annealing hopes to obtain uniform fine ferrite and pearlite equiaxed grains. But a microscopic sample of the material obviously coarse grains and the distribution is very uneven and find locally Wilcoxon, it indicates the material during heat treatment significantly overheating. Coarse grains produced after overheating, destroyed the original dense grain structure, after cold drawing pearlite and ferrite became banded pearlite and ferrite easier stratification. After boring in steel rolling, the tool to the workpiece with great pressing force, then pipe it is prone to warping. Obviously overheating in the heat treatment, is the main reason for warping generation.

How to prevent the inner surface of seamless steel pipe from warping?

Since the main reason for warping is produced during the heat treatment of steel overheating, it is necessary to control the annealing temperature by controlling the annealing temperature after cold-drawn steel to make to obtain uniform fine equiaxed ferrite and pearlite. Uniform fine grains combine to make the grain more sophisticated so that the strength and toughness of steel are good.

Manufacturers of cold-drawn steel pipe in order to save costs, often reduce the number of drawings, and each time the minimum diameter as the pipe is pulled, so it may cause excessive pulling over steel pipe can withstand the maximum, and increased internal stresses pipe, steel pipe under the external influence by extrusion defects such as cracks easily. Therefore, controlled heat treatment temperature while controlling each drawing cold-drawn steel margin is very necessary.