Views: 0 Author: Site Editor Publish Time: 2026-02-17 Origin: Site
Producing high-performance Hexagonal Steel Tubing involves more than just cold drawing a round mother tube through a shaped die. While the cold-drawing process imparts the necessary geometric profile, it creates a significant metallurgical challenge: uneven work hardening. The severe deformation required to form the six sharp corners results in a dense dislocation structure, making the corners significantly harder and more brittle than the flat faces. If left untreated, this internal stress creates a high risk of corner cracking during subsequent machining, bending, or dynamic service loading.
Engineers and procurement officers face a constant trade-off. You need the tight dimensional tolerances of a cold-finished product, but you also require specific mechanical properties—whether that is ductility for flaring or high tensile strength for torque transfer. Heat treatment is the bridge between these conflicting requirements. This guide serves as a decision framework, helping you select the precise thermal process—from Stress Relieving to Solution Annealing—required to optimize your tubing for its specific end application.
Geometric Stress: Cold-drawn hexagonal tubes carry uneven residual stress distribution, with peaks at the corners; heat treatment is often mandatory to prevent corner cracking during machining or service.
Material Dictates Process: Low-carbon variants benefit from Normalizing (+N), while stainless hexagonal steel tubes (like 304) require Solution Annealing (+AT) to restore corrosion resistance.
Precision Trade-offs: High-temperature treatments can induce warping; "Bright Annealing" or vacuum furnaces are critical for precision hexagonal steel tubes where surface finish cannot be compromised.
Specification Matters: Ordering to correct delivery codes (e.g., EN 10305 +SR vs. +C) reduces TCO by eliminating the need for third-party post-processing.
The geometric complexity of hexagonal tubing introduces metallurgical variables that do not exist in standard round tubes. Understanding these variables is the first step in preventing catastrophic component failure.
When a round tube is drawn through a hexagonal die, the material at the corners undergoes more severe plastic deformation than the material at the flats. In metallurgical terms, this causes a massive pile-up of dislocations—imperfections in the crystal lattice—specifically at the six vertices. This phenomenon is known as the "Corner Effect."
Without thermal intervention, these corners act as stress concentrators. If you use an untreated shaped hexagonal steel tube in a high-torque application, such as an impact wrench extension or a hydraulic coupling, the stored energy at the corners can initiate brittle fractures. Even simple machining operations, like cutting the tube to length or turning the OD, can release these residual stresses unevenly, causing the part to snap or warp unexpectedly.
The condition of the tube upon delivery defines its usability. We generally categorize the status into "As-Drawn" or various heat-treated states:
As-Drawn (+C): The tube is delivered directly after the final cold draw. It possesses high tensile strength and a hard surface, which is excellent for wear resistance. However, elongation is very low. If you attempt to bend or flare a +C tube, it will likely crack.
Heat Treated: By applying heat, we restore the grain structure. This allows for secondary operations like welding, mandrel bending, or threading. The goal is to reset the material's ductility without sacrificing too much of the strength gained during cold working.
The manufacturing origin of the tube heavily influences the heat treatment strategy. For a seamless hexagonal steel tube, the primary goal is usually homogenization—ensuring the grain structure is uniform from the ID to the OD.
Conversely, a welded hexagonal steel tube introduces the Heat Affected Zone (HAZ) along the longitudinal seam. The HAZ often contains brittle martensitic structures or enlarged grains due to the rapid heating and cooling during welding. Heat treatment becomes vital here not just for the hex shape, but to normalize the weld seam so its mechanical properties match the base metal. Without this, the weld line becomes a predictable point of failure under pressure.
Selecting the right heat treatment is not about guessing; it is about matching the process to the intended mechanical load. Below is a framework comparing the core processes used for carbon and alloy steel hex tubes.
| Process | Delivery Code (EN 10305) | Temperature Range | Primary Benefit | Ideal Application |
|---|---|---|---|---|
| Stress Relieving | +SR | 500°C – 600°C | Reduces internal tension; maintains hardness. | Precision shafts, machining stock. |
| Normalizing | +N | 870°C – 920°C | Uniform grain size; removes directionality. | Structural columns, load-bearing parts. |
| Soft Annealing | +A | 650°C – 750°C | Maximum ductility; lowest hardness. | Tubes requiring bending or flaring. |
| Quench & Temper | +QT | 840°C Quench / 550°C Temper | High strength combined with toughness. | Impact tools, high-pressure automotive parts. |
This is the preferred method for precision hexagonal steel tubes where dimensional stability is paramount. The process involves heating the steel below its lower critical temperature. At this range, the microstructure does not change (no phase transformation occurs), but the internal elastic stresses locked in by cold drawing are relaxed.
The result is a tube that retains most of its cold-worked hardness and yield strength but will not warp when you machine a keyway or drill radial holes. It offers the best compromise between strength and stability.
Normalizing acts as a "reset button" for the steel's grain structure. By heating the tube above its upper critical temperature and allowing it to cool in still air, the elongated, stressed grains caused by the drawing process recrystallize into a uniform, equiaxed structure.
For structural applications where impact toughness is required (such as roll cages or heavy machinery frames), normalizing removes the "directionality" of the mechanical properties. A normalized tube exhibits consistent strength regardless of the direction of the load.
When the application demands extreme performance—such as in power transmission shafts or impact wrench bodies—simple cold working is insufficient. The tube must be hardened. This involves heating to the austenitizing range and rapidly cooling (quenching) to form martensite, a hard but brittle structure. This is immediately followed by tempering, which reheats the steel to a lower temperature to trade some hardness for necessary toughness.
Hex Geometry Risk: The six corners of a hexagonal tube cool much faster than the flats during quenching. If the quench media is too aggressive (like water), the thermal shock can cause "quench cracks" to initiate at the corners. For hex tubing, oil or polymer quenching is often required to moderate the cooling rate and protect the profile geometry.
If your manufacturing process involves severe deformation—such as swaging the tube ends or bending the hex profile into a complex shape—you need maximum ductility. Soft annealing creates a ferrite-spheroidized carbide structure, which is the softest state possible for steel. While this significantly lowers tensile strength, it allows the material to flow without tearing during forming operations.
Carbon steel rules do not apply to corrosion-resistant alloys. Applying a standard normalizing cycle to a stainless tube can destroy its corrosion resistance. Here is how we handle advanced materials.
For a 304 hexagonal steel tube, the standard treatment is Solution Annealing (+AT). During cold working or welding, carbon can combine with chromium to form chromium carbides at the grain boundaries. This depletes the chromium needed to protect the steel from rust.
Solution annealing involves heating the tube to around 1040°C to dissolve these carbides back into the matrix. The critical step is the cooling: it must be rapid (often water quenched or gas cooled) to freeze the structure in place. Slow cooling would allow the carbides to precipitate again, leading to sensitization and intergranular corrosion.
Duplex hexagonal steel tube contains a microstructure of roughly 50% ferrite and 50% austenite. Heat treating these alloys is a balancing act. The temperature control must be precise; deviating by just 30-50°C can upset the phase balance, reducing toughness or corrosion resistance.
The risk here is the "sigma phase." If the tube is cooled too slowly through the 600°C–900°C range, brittle intermetallic phases form, which can cause the tube to shatter like glass under impact. Cooling rates must be carefully calculated based on wall thickness.
When treating a nickel alloy hexagonal steel tube, surface quality is often the biggest challenge. Nickel alloys form stubborn oxide scales that are extremely difficult to remove via acid pickling without pitting the metal. Therefore, these alloys almost exclusively require "Bright Annealing" in a vacuum or a pure hydrogen atmosphere. This prevents oxidation from occurring in the first place, ensuring the tube emerges with a pristine, bright surface.
Heat treatment involves high temperatures that relax stresses, often resulting in movement. For precision applications, managing this distortion is as important as the metallurgical outcome.
Hexagonal tubes have a unique propensity to twist during heat treatment. Because the stress is concentrated at the corners, the release of this stress can manifest as a corkscrew-like deformation.
To mitigate this, mills often employ tension leveling or rotary straightening machines after the heat treatment process. However, mechanical straightening re-introduces a small amount of cold work (and stress). For ultra-precision applications, engineers must account for this or request "press straightening," which is slower but induces less residual stress than rotary methods.
In standard open-air furnaces, steel reacts with oxygen to form scale. For a precision hexagonal steel tube intended for hydraulic fittings or telescopic mechanisms, scale is unacceptable as it alters the dimensional tolerance and surface roughness.
The solution is Bright Annealing. By purging the furnace with inert gases (Nitrogen or Argon) or using a vacuum, we prevent oxidation. The tube comes out with a metallic shine, eliminating the need for abrasive polishing or chemical pickling that could alter the sharp hex corners.
High-carbon steel tubes face the risk of decarburization—where carbon atoms diffuse out of the surface layer into the furnace atmosphere. A decarburized surface is soft and wears out quickly, even if the core of the tube is hard. If surface hardness is a Key Performance Indicator (KPI) for your application, you must specify "Restricted Decarburization" or ensure the heat treater uses a carbon-potential controlled atmosphere to maintain the surface carbon content.
The gap between a successful project and a failed batch often lies in the purchase order. Vague requests like "heat treated" leave too much room for interpretation. You can learn more about our quality commitment on our About Us page.
To ensure you get exactly what you need, use standard delivery condition codes, particularly from EN 10305 or equivalent ASTM standards:
+C (Hard): No thermal treatment after the final cold draw. Maximum strength, low ductility.
+LC (Soft): A slight cold pass is applied after heat treatment. This is used when you need the ductility of a treated tube but the tight tolerances of a cold-drawn size.
+SR (Stress Relieved): Ideal for parts that will undergo significant machining.
+N (Normalized): Required for parts needing uniform structural integrity in all directions.
Procurement teams should evaluate the "Make vs. Buy" decision on heat treatment. Purchasing a hex tube in the +C condition and sending it to a third-party heat treater might seem cheaper initially. However, when you factor in logistics, the risk of handling damage, and the 15-20% higher cost of small-batch processing, the Total Cost of Ownership usually favors buying mill-treated material.
Furthermore, mill-treated tubing allows the manufacturer to straighten the tubes immediately on the line, whereas third-party treaters may return warped tubes that require expensive secondary straightening.
Trust but verify. Always request Mill Test Certificates (MTC) compliant with EN 10204 3.1. Do not settle for a simple checkbox that says "Heat Treatment: Satisfactory." The MTC should list the actual soak temperature, time, and cooling medium. For critical applications in sectors like aerospace or nuclear (see our Application Areas), request a microstructure analysis report to confirm that the grains at the hex corners are fully recrystallized and free from micro-cracks.
Heat treatment for hexagonal tubing is not a generic "one-size-fits-all" process; it is a strategic tool to balance the extreme work hardening at the corners with the need for dimensional precision. Whether you are designing hydraulic components, architectural supports, or precision drive shafts, the success of your component relies on selecting the right thermal cycle.
For structural loads, prioritize Normalizing (+N) to ensure safety. For corrosive environments, demand Solution Annealing (+AT) to protect the alloy. For precision machinery, utilize Stress Relief (+SR) to maintain tight tolerances during machining. We strongly advise consulting with your metallurgical team to define the precise delivery condition code before issuing Purchase Orders. This proactive step is the most effective way to minimize machining losses and ensure long-term product reliability.
A: Yes. High-temperature treatments like Normalizing or Annealing relax internal stresses, which can cause the tube to twist or bow ("warping"). While Stress Relieving (+SR) causes minimal movement, full annealing requires the mill to perform subsequent straightening operations (like tension leveling) to restore the tube to straightness tolerances.
A: Stress Relieving (+SR) is performed at lower temperatures (approx. 500-600°C) to relax tension without changing the material's hardness or grain structure significantly. Normalizing (+N) heats the steel above its critical temperature (approx. 900°C) to fully recrystallize the grain structure, resulting in softer, tougher material with uniform properties but lower tensile strength than +SR.
A: No. Austenitic stainless steels like 304 cannot be hardened by heat treatment; heating them actually softens (anneals) the material. They can only be hardened through cold working (strain hardening). If you need a hard stainless hex tube, you must order it in the cold-drawn or "tempered" rolled condition.
A: The corners of a hexagonal tube have a higher surface-area-to-volume ratio than the flats, causing them to cool much faster during quenching. This thermal differential creates immense stress. If the material is not ductile enough or the quench media (like water) is too aggressive, brittle fractures will initiate at these high-stress corner points.
A: The best condition is usually Stress Relieved (+SR) or +LC (Cold drawn + soft). These conditions provide enough hardness to ensure the chips break cleanly (avoiding "gummy" machining) while ensuring the material is stable enough not to warp when material is removed.
