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|
|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