Seamless and welded are mainly two types stainless steel tube & stainless steel pipe, there is a debate whether seamless is better than welded for a long time.
Although with improved metallurgy and welding processes in welded tubes, the arguments typically focus on structural integrity and corrosion resistance in weld area.
Obviously, seamless and welded tubes & pipes difference is manufacturing process.
Welded tube manufacturing process.
Depending on the outer dimension, wall thickness and final application, there are different ways of manufacturing welded tubes and pipes, and different manufacturing method.
Strip welded tubes & pipes.
Welded tube start from stainless steel strip and coil that is passed through grooved rollers till the shape is formed and the free edges are properly shaped for welding, cold forming is performed step by step from flat strip into a round profile, the edges are welded together as they approach the welding rolls.
Typical welding method for strip welded tubes is traditionally autogenous tungsten inert gas (TIG), TIG welding method have some advantages that can effectively protect weld seam.
After welding outside and inside grinding of the weld seam, solution annealing or stress relieving may be necessary by application requirements.
- Coil
- Inspection and analysis
- Slitting
- Auto-welding
- Seam-grinding
- Solution treatment
- Cut to length
- Straightening
- Picking
- End-facing
- Eddy current test
- Hydrostatic test
- Final inspection
- Marking
- Packing
- Shipping
Today, most of the modern welding lines are equipped with inline induction annealing, then straitening and calibration to control dimension of tube & pipe, cut to standard or special lengths, de-burred, NPD & DT testing and inspection should be carried out as standard or requirement.
Heavy wall tubes & pipes
When large outside diameter or heavy wall or both welded tubes & pipes required, there is a different welding method and process, for large diameter and heavy wall pipes, EFW, ERW and other welding methods can be used.
Starting from stainless steel plate or sheet, the forming is done in a roller bending machine or in a hydraulic press, then some steps are same with strip welded tubes.
- Plate
- Inspection
- Plasma-cutting
- Edging
- Bending
- Seam cleaning
- Forming
- Auto- welding
- Degreasing
- Anneaspanng
- Roundness caspanbrating
- End-facing
- Eddy current test
- Hydrostatic test
- Final inspection
- Marking
- Packing
- Shipping
For testing and inspection, large diameter sizes, radiography test (RT) and hydrostatic test (HT) are often required.
For large diameter welded pipes, double welding method can be accepted.
Seamless Tube Manufacturing Process
Seamless tubes are generally made in complex steps starting with the drilled hollows from billets, by cold drawing and cold rolling manufacturing process with cold drawing & cold rolling machines.
To control outside diameter and wall thickness, seamless type dimension is difficult to control compared to welded tube, cold work improves the mechanical properties and tolerances.
Cold pilgering is the preferred production process since this technique provides a high forming rate, narrow tolerances and good productivity yields, but cold drawing gives narrower tolerance ranges and better surface quality.
- Round bar
- Inspection and analysis
- Cut to length
- Centering
- Peel
- Heating
- Perforation
- Inspection of size
- Pickspanng
- Inspection of surface
- Lubricating
- Cold drawing / cold rolspanng
- Degreasing
- Anneaspanng
- Straightening
- Cut
- Picking / passivating
- Final inspection
- Marking
- Packing
- Shipping
Solution annealing should be made through offline annealing furnace, then straightening and pickling, these are important steps in manufacturing seamless stainless steel pipe and tubes.
Advantages of welded tubes & pipes
- Lower cost compared to the equivalent seamless alternative.
- Short lead time.
- Better surface finishing of outside and inside, the internal material surface for welded tubes can be checked prior to manufacture.
- Thinner wall thickness, and more uniform.
- Tighter tolerance.
Advantages of seamless tubes & pipes
- Heavy and thick wall thickness.
- No weld seam. generally be considered as better properties and corrosion resistance
- Seamless pipes have better ovality, or roundness
How to choose tubes & pipes, welded or seamless?
Despite many advantages with welded tubes, seamless tube is still treated better than welded type, especially used in rough environment, considered as higher strength, higher pressure, and better corrosion resistance.
In reality, seamless & welded manufacturing process both can provide tubes & pipes with quality, reliability and excellent corrosion resistance, generally complement each other.
Depending on specific application and cost aspect, decide which type is better choice to use.
Tubes and pipes can be produced in both seamless and welded types depending on application requirement, with Austenitic stainless steels, duplex and special grades.