Carbon Steel Pipe Surface Roughness

What is carbon steel pipe surface roughness?

The surface roughness of carbon steel pipe refers to the small spacing and the unevenness of small peaks and valleys on the machined surface. The distance (wave pitch) between the two wave crests or two wave troughs is very small (below 1mm), which belongs to the micro-geometric error. The smaller the surface roughness of the carbon steel pipe, the smoother the surface.

The surface roughness of carbon steel pipe is generally formed by the processing method and other factors, such as the friction between the tool and the surface of the part during processing, the plastic deformation of the surface metal when the chips are separated, and the high-frequency vibration in the process system, etc. Due to different processing methods and workpiece materials, the depth, density, shape and texture of marks left on the machined surface are different.

Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products. Ra is generally used for marking.

Common range of surface roughness (Ra) of carbon steel pipe:

Hot-rolled seamless pipe:
Surface roughness: 6.3~12.5 μm
Surface features: obvious oxide scale, need pickling or sandblasting

Cold-rolled/cold-drawn pipe:
Surface roughness: 0.8~3.2 μm
Surface features: relatively smooth, may have slight rolling lines

Precision cold-drawn pipe:
Surface roughness: 0.4~1.6 μm
Surface features: close to mirror surface, suitable for high-precision hydraulic system

Welded steel pipe:
Surface roughness: 3.2~6.3 μm
Surface features: slightly rough in the weld area, need grinding

Key factors affecting roughness:

Production process:
Hot-rolled pipe has high roughness due to high-temperature oxide scale, and cold-rolled/cold-drawn pipe obtains a smoother surface through die extrusion.

Die/roller status:
Worn rolls or drawing dies will increase surface ripples (such as "bamboo knots" on cold-drawn pipes).

Subsequent treatment:
Sandblasting/polishing: Ra can be reduced to below 1.6 μm (such as decorative stainless steel composite tubes).
Pickling/phosphating: remove oxide scale and form uniform roughness (Ra 2.5~5 μm).

Measures to improve roughness:

Machining:
Use turning and grinding to control Ra at 0.4~3.2 μm (such as hydraulic cylinder barrel).

Chemical treatment:
Passivation after pickling to form a uniform surface (Ra 2~5 μm).

Coating technology:
Before spraying epoxy resin, sandblasting is required to enhance bonding strength.

The influence of surface roughness on carbon steel parts is mainly manifested in the following aspects:

1. Affect wear resistance. The rougher the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, the greater the frictional resistance, and the faster the wear.
2. It affects the stability of cooperation. For clearance fit, the rougher the surface is, the easier it is to wear, so that the clearance gradually increases during the working process; for interference fit, due to the flattening of microscopic peaks during assembly, the actual effective interference is reduced. connection strength.
3. Affect fatigue strength. There are large wave valleys on the surface of rough parts, which are sensitive to stress concentration like sharp corner notches and cracks, thus affecting the fatigue strength of the parts.
4. Affect corrosion resistance. The rough surface of the parts easily allows corrosive gases or liquids to penetrate into the inner metal layer through the microscopic valleys on the surface, causing surface corrosion.
5. Affect the sealing. Rough surfaces do not fit tightly, and gas or liquid leaks through the gaps between the contact surfaces.
6. Affect the contact stiffness. Contact stiffness is the ability of the joint surface of a part to resist contact deformation under the action of external force. The stiffness of the machine is largely determined by the contact stiffness between the various parts.

7. Affect the measurement accuracy. The surface roughness of the measured surface of the part and the measuring surface of the measuring tool will directly affect the accuracy of the measurement, especially in the precise measurement.

In addition, surface roughness will have varying degrees of influence on the coating, thermal conductivity and contact resistance of parts, reflectivity and radiation properties, resistance to liquid and gas flow, and current flow on the conductor surface.

Selection suggestions:
High precision requirements (such as medical equipment): select cold-drawn tubes (Ra≤1.6 μm) + electrolytic polishing.
Anti-corrosion coating: select hot-rolled tubes after sandblasting (Ra 4~6 μm).
Economical structural parts: directly use hot-rolled tubes (Ra 6.3~12.5 μm).

Conclusion:
The surface roughness of carbon steel pipe is an important parameter for engineering selection, and it needs to be selected comprehensively based on functional requirements (seal/fluid/coating) and cost. Generally, cold-rolled pipes are better than hot-rolled pipes, but special scenarios (such as coating adhesion) may require a certain degree of roughness.