Hot Rolled Tubing vs Cold Rolled Tubing

In the as-welded tubing industry, the design engineer has several options for size, shape, wall thickness, steel grade, and tubing strength. Often overlooked, however, is the material type. Hot Rolled Black (HRBL), Hot Rolled Pickled and Oiled (HRPO), and Cold Rolled (CR) are the most common material types offered.

Each material type has benefits and potential drawbacks. It really is specific to the needs of the application, fabrication, and customer expectations. Strength, formability, surface quality, paintability, plating, dimensional consistency, steel availability, and steel price are important factors for the design engineer to consider.

High-Frequency Electric Resistance Welded (HF ERW) tubing utilizes the three material types, again, depending upon what is required of the final tubing part, and whether the application is exposed or unexposed in its end use.

HRBL Tubing is produced using Hot Rolled steel with a tight mill scale left intact on the top and bottom surfaces which equate to the tube OD and ID surfaces. Applications utilizing HRBL tubing are commonly for structural and mechanical applications with minimal surface requirement concerns. HRBL tubing can produce a dirtier surface due to the mill scale which can flake off during fabrication such as bending, flattening, expansion, and swaging operations. Components that need subsequent painting or E-coating have the scale mechanically removed from the tube OD prior to painting (e.g. shot blasting, tumbling).

HRPO Tubing is produced using Hot Rolled steel having been acid pickled cleaned and oiled in sheet form to remove all mill scale from both top and bottom surfaces, which equate to OD and ID surfaces. HRPO tubing applications range from structural to mechanical. These tubes are ready for cleaning and painting systems. Where cleaner fabrication environments are warranted, HRPO tubing offers a cleaner option. Some fabrications are better suited for HRPO tubing. Mandrel bending, swaging, expansion, roll grooving, significant flattening are some fabrications that would benefit from HRPO. Most paint or coating systems employ a cleaning stage that removes remnant oils and grease prior to coating. Where HRBL would need acid cleaned, HRPO avoids those harsh chemicals in the coating systems.

CR Tubing is produced using Cold Rolled steel. CR steel takes HRPO coil and cold reduces the thickness to the desired final CR thickness. This amount of cold reduction can exceed 70%, so CR Full Hard steel must be annealed back to regain its formability. A temper pass will ensure uniform elongation to avoid stretcher-strains, luder- lines, and cross-breaks. CR tubing is commonly used for surface critical applications, tighter dimensional control, and/or lighter wall thicknesses outside of available Hot Rolled capabilities.

Comparisons for material types are shown below*:

Characteristic	CR Tubing	HRPO Tubing	HRBL Tubing
Surface Critical: Free of Scale. Free of pitting and minor mechanical flaws	Best – guarantees available. Free of pitting and flaws	Good – limited guarantees available. Some minor pitting and flaws permissible	N/A – no guarantees
Wall Thickness Dimensional Control	Best – half standard vs HR	Good – Industry Standard without further processing	Good – Industry Standard
Formability	Good – slight loss of ductility	Best	Fair – lubrication limitations
Cleanliness: Equipment, Handling, Tooling	Best	Good	Poor
Bendability: Mandrel Bending	Best	Good	N/A-Limited lubrication on mandrel
Ductility: Ability to deform uniformly	Best	Good	Fair
Price	Fair	Good	Best
Thin Materials	Best	Limited below .060”	Limited below .060”
Large Tube Sizes	Good – limited in strength and wall thickness to avoid roll marks	Fair – roll mark prone at higher strengths and thicker walls	Best
Availability for Advanced High Strength Steels	Best – All Grades	Fair – Limited grade availability	N/A
Plain Carbon Steels	All Grades	All Grades	All Grades
High Strength Low Alloy Steels	All Grades	All Grades	All Grades
NiCr Surface Plating	Best – Ra surface requirements available	Poor – minor pitting and grainy surfaces renders HRPO a poor plating choice	N/A
Surface Texture Finishes for Exposed Applications	Best – Temper passed Matte surface finishes can be tailored for specific requirements for exposed applications	Fair – limited matte surfaces. Limited exposed applications	N/A

*Strength levels (tension, compression, shear, and torsional) are subject to steel grade, size, shape, and wall thickness (tube configuration). Minor differences for the same configuration can be seen between CR and HR versions for strength, and CR typically has slightly less % elongation than HR.

By: David Haghiri

VP of Technical Service / Quality Assurance at Lock Joint Tube

Hot Rolled Tubing vs Cold Rolled Tubing

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