Polishing method of stainless steel seamless pipe

Polishing method of stainless steel seamless pipe

Unless the finished stainless steel seamless pipe is used deep underground, pipes in normal use need to be polished to ensure a bright and beautiful surface. Currently commonly used polishing methods mainly include mechanical polishing, chemical polishing, electrochemical polishing, etc.

1. Mechanical polishing
The mechanical polishing method uses a polishing wheel or polishing belt for processing. The polishing belt uses the abrasives in the polishing agent to batch polish the surface of the stainless steel seamless pipe, so that the surface of the stainless steel seamless pipe can achieve a smooth polishing effect.

Mechanical polishing can obtain a mirror-bright surface less than 0.4μm. Parts with simple shapes can be polished with hard polishing wheels or polishing tapes, and parts with complex shapes can be polished with soft polishing wheels. Batch finishing is used for large quantities of small parts. Batch polishing can be subdivided into drum polishing, vibration machine vibration polishing, centrifuge centrifugal polishing and rotation polishing methods.

The amount of grinding on the surface of stainless steel seamless pipes by mechanical polishing is very low, so it is difficult to polish rough surfaces. At this time, the polishing process needs to be done in advance, and the polishing wheel and polishing belt are dipped in polishing paste for grinding, which is divided into rough grinding, medium grinding and fine grinding. The surface roughness of the finely ground stainless steel pipe can reach 0.4μm. In order to meet other personalized requirements, such as scale removal, deburring, welding slag or pipe surface matting, etc. Surface treatments such as sandblasting, shot peening, and wire wheel brushing can also be used. Stainless steel wire wheel polishing can effectively reduce iron contamination.

2. Chemical polishing
Chemical polishing involves immersing parts such as stainless steel seamless pipes in a suitable solution. Since the solution will dissolve the protruding parts on the surface of the stainless steel pipe faster, the surface of the stainless steel pipe can be flattened to achieve the purpose of polishing. Usually chemical polishing has a weak polishing ability and only slightly improves brightness. However, it saves labor and time compared to mechanical polishing, and it can also process the inner surface of small parts.

There are several aspects that need to be paid attention to during the chemical polishing process. Since the polishing speed is fast but the finish is poor, it is more suitable for pre-polishing. Chemical polishing will increase surface activity, and the work surface must be passivated to ensure corrosion resistance.

3. Electrochemical polishing
Electrochemical polishing can increase the reflective properties of the surface of stainless steel seamless pipes, improve corrosion resistance, reduce the surface hardness of the workpiece, and reduce the friction coefficient due to the reduction of surface roughness. Electrochemical polishing can also be used to remove burrs on parts.

Compared with mechanical polishing, the characteristics of electrochemical polishing are as follows
1) Electrochemical polishing will form a passivated surface on stainless steel seamless pipes and improve the corrosion resistance of the material, which cannot be achieved by mechanical processing.
2) Electrochemical polishing has certain requirements for the substrate. For example, when the metallographic structure is uneven, an uneven polished surface will be formed, and deep scratches cannot be polished flat. Mechanical polishing, on the other hand, has lower requirements on the substrate.
3) The production efficiency of electrochemical polishing is also higher than that of mechanical polishing, but be careful not to enlarge large workpieces.

The surface current density of electrochemically polished steel pipes must remain uniform so that the polished surface brightness will be uniform. Due to the large current during electrochemical polishing, the contact area between the fixture and the stainless steel seamless pipe must be large enough to form good contact. Otherwise, it may easily cause local overheating and burn out. A final note is that some processes used to polish austenitic stainless steels cannot be used to polish martensitic stainless steels because they are prone to corrosion.

What is austenitic stainless steel pipe?

Austenitic stainless steel pipe refers to stainless steel pipe with austenitic structure at room temperature. When the steel contains about 18% Cr, about 8% to 10% Ni, and about 0.1% C, it has a stable austenite structure. Austenitic chromium-nickel stainless steel includes the famous 18Cr-8Ni steel and the high Cr-Ni series steel developed by increasing the Cr and Ni content and adding elements such as Mo, Cu, Si, Nb, Ti, etc. Austenitic stainless steel is non-magnetic, has high toughness and plasticity, but low strength and cannot be strengthened through phase transformation. It can only be strengthened by cold working. If elements such as S, Ca, Se, and Te are added, it will have good machinability.

Austenitic stainless steel pipe has good weldability, low temperature toughness and non-magnetic properties. Its characteristics are that the carbon content is less than 0.1%, Cr and Ni are combined to form a single-phase austenite structure, and it has good cold working properties. It has strong deformation ability, high corrosion resistance and plasticity. It can be cold-drawn into extremely fine steel wires, and cold-drawn into extremely thin steel strips or steel pipes. At the same time, after a large amount of deformation, the strength of the steel is greatly increased because in addition to the cold work hardening effect, the deformation-induced martensitic transformation is also superimposed. Austenitic stainless steel pipes have good resistance to uniform corrosion, but there are still some problems in resistance to local corrosion. The main problems existing in the welding of austenitic stainless steel pipes are: intergranular corrosion of welded joints, stress corrosion cracking of welded joints, hot cracks of welded joints, etc.

Welding method of austenitic stainless steel pipe

There are many welding methods for austenitic stainless steel pipes, such as manual welding, gas shielded welding, submerged arc welding, plasma welding, etc. The most commonly used welding method is manual welding (MMA), followed by gas metal arc welding (MIG/MAG) and joining inert gas welding (TIG).

1. Manual arc welding. The heat of electrode arc welding is relatively concentrated, the heat affected zone is small, and the welding deformation is small; it can adapt to the process requirements of various welding positions and different plate thicknesses, and the equipment used is simple. During the construction process, in order to control the weld energy and prevent intergranular corrosion, the welding speed must be controlled, and the welding can be performed quickly while ensuring penetration. At the same time, in order to reduce the heat of the welding pool and improve the corrosion resistance of the weld metal, the electrode is not allowed to swing laterally during welding, and narrow weld bead technology is used to speed up the cooling rate. The width of the weld is generally not greater than 2 times the diameter of the electrode. The thickness of each layer of multi-layer welding is not greater than 3mm to reduce heat input and facilitate gas escape. The interlayer temperature is not higher than 150°C.

2. Junji ammonia shielded welding is an ideal welding method for welding austenitic stainless steel pipes. Due to the good protective effect of ammonia and the high excess coefficient of alloy elements, the composition of the weld is easy to control; due to the concentrated heat source, the linear energy during welding is very small, the cooling effect of ammonia, the welding heat affected zone is narrow, the weld strength and plastic toughness are excellent, and after welding No slag cleaning is required, and all-position welding and mechanized welding can be performed.

In the actual welding process, the arc length should be shortened as much as possible without short-circuiting the arc, and the arc voltage is generally controlled within the range of 9~20V. In order to prevent the air flow from destroying the protection of the molten pool, the welding speed should not be too fast. At the same time, in order to improve production efficiency, the residence time of welded joints at dangerous temperatures of 450~850°C should be minimized, which will help improve the corrosion resistance of stainless steel welded joints.