There are three classes of tubing:
- Seamless (4130 etc) tubing is produced by using a extrusion or rotary piercing process.
- Drawn-over-mandrel (DOM) tubing is made from cold drawn ERW tube that is drawn through a die and over a mandrel to create such characteristics as dependable weld integrity, dimensional accuracy, and an excellent surface finish.
The difference between pipe and tubing
Pipe and tube are different, tubing has tighter engineering requirements and in some cases made with a higher quality alloy than a pipe.
The actual dimensions of pipe are usually not the nominal dimensions: A 2” pipe will not actually measure 2 inch in either outside or inside diameter.
Tubing is specified by:
- Inside diameter (ID)
- Outside diameter (OD)
- Wall thickness.
4130 is a Seamless tubing, and is produced by using a extrusion or rotary piercing process. Chromoly is specified by the “SAE” Society of Automotive Engineers. the Alloy elements of 4130 are chromium and molybdenum,
this is why its often called chromoly or cromo. Chromoly has an excellent strength to weight ratio, is easily welded and are considerably stronger and harder than standard 1020 mild seamed and DOM tube steels.
The numbers 4130 defines the chemical composition of the steel.
- 41 means a low alloy steel containing 1 % chromium and 0.2 % molybdenum.
- 30 means a carbon content of 0.30%.
- In addition there will be around 0.2-0.5 % silicon, 0.5-1.0 % manganese, and under 0.1 % of each of a dozen or so other elements whose presence is unavoidable.
- The remaining 97-98 % is iron.
4130 chromoly tubing began its first use in the aircraft industry. Race chassis builders began using the tubing for its strength to weight ratio.
4130 tubing is used for anything from bicycle frames, AK-47 receivers, tubular chasis, suspension parts, aircraft parts and air frames etc… Ive used cromo for suspension components, cages / tube chassis and machined parts.
I couldn’t find a video for seamless tube manufacturing process on youtube, but i did find a link to TIMKENs (baring company) website that has a good video with the seamless tubing process. Check it out: How steel is made
DOM tubing, or “drawn over mandrel” and is made from cold drawn ERW (electric resistant welded) tube that is drawn through a die and over a mandrel to create a dependable weld integrity, accurate dimensions and an excellent surface finish. DOM is also commonly known as seamless tube. The machinability is good to excellent, the weldability is excellent. DOM is typically grade 1020, but may also be 520 or 1026. The 1026 grade is normally for OD’s larger than 2″ and wall thicknesses heavier than .156″.
This tubing is also can be used in tube chassis and cages. many people prefer DOM over 4130 due to the fact that it more malleable and tends to bend further before sheering or braking.
To get a similar strength as chromoly, you would have to increase the wall thickness substantially.
Here is a good video on how mild seamed tubing is made or the first steps in the DOM manufacturing process:
Mild Steel (ERW)
Mild steel or ERW tube is manufactured by the electric-resistance welding process, are often referred to as ERW or EW, among other names.
These tubes have a welded seam on the inside and are tend to be more affordable than seamless DOM and 4130.
Mild erw may occasionally be referred to as Grade 1008-1010. Excellent weldability and good machinability. Mild tubing is extremely soft and malleable compared to 4130. Its not as used in motor-sports as DOM & 4130 because of the potential bending or splitting at the seam. Its safer to use DOM or 4130 for structural cages or components that are under high stress.
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