Cold Drawn Seamless (CDS) is a precision steel tube produced by drawing hot-finished stock through a die (and usually over a mandrel) to increase yield strength and surface finish. Governed primarily by ASTM A519, it is the standard for hydraulic cylinders and linear shafting. It fails via catastrophic warping ("banana effect") during asymmetric machining or longitudinal splitting due to unmitigated residual hoop stress.
COMMON FIELD QUESTIONS ABOUT COLD DRAWN TUBE
Why does the tube bow immediately after cutting a keyway?
This is the release of asymmetric residual stress. Cold drawing creates a high-tension outer skin and a compressive core. Cutting a keyway breaks the tensile containment on one side, allowing the compressive core to push the tube into a bow (concave toward the cut).
Why did the bore diameter shrink after turning the OD?
Turning down the OD removes the tensile "hoop" layer that was compressing the core. Once this outer constraint is removed, the compressive internal stresses relax outward, causing the part to lengthen and the bore to constrict slightly.
What is the loud "pop" heard when parting off sink-drawn tubing?
That sound is the audible release of extreme tensile hoop stress. Unlike mandrel-drawn tubing, sink-drawn (air-drawn) tubing has high tension on the OD. Deep grooving weakens the structure until the remaining material can no longer contain the stress, causing it to snap or split longitudinally.
The Hidden Killer: Hoop Stress & Longitudinal Cracking
Cold drawn seamless (CDS) tubing offers superior surface finish and dimensional tolerances compared to hot finished counterparts, but it carries a "hidden battery" of stored energy: residual hoop stress. Unlike hot rolled steel, which is relatively stress-neutral, cold drawn tubing is essentially a pre-loaded spring. The direction and intensity of this stress depend entirely on the drawing method.
The "Skin Effect" (Mandrel Drawn)
Most precision CDS is drawn over a mandrel. This creates a stress profile where the OD surface is under high tensile stress (+) and the ID/Core is under high compressive stress (-). When you turn down the OD ("skinning the onion"), you remove the tensile containment layer. The compressive core, now unbalanced, expands longitudinally.
The "Time Bomb" (Sink Drawn)
Tubes drawn without a mandrel (Sinking) have a dangerous profile: extreme compressive hoop stress on the ID and high tensile hoop stress on the OD. The ID is constantly trying to expand, held back only by the tensile strength of the OD. In environments with ammonia or chlorides, this stored energy accelerates stress corrosion cracking (SCC), often splitting the tube while it sits on the shelf (Season Cracking).
Is it safe to galvanize cold drawn tube without heat treatment?
No. The thermal shock of the zinc bath (approx. 830°F) combined with the existing high residual stress can trigger liquid metal embrittlement or instantaneous stress cracking in the bath.
Machining Distortion Profiles
Understanding the specific stress state allows you to predict distortion direction. The following table outlines how standard operations trigger specific geometric failures.
Operation
Stock Removal Layer
Stress Mechanism
Resulting Distortion
OD Turning
Outer Skin (Tensile)
Core compression releases
Length Growth & Bore Collapse
ID Boring
Inner Skin (Compressive)
Outer tension releases
OD Expansion & Length Shrinkage
Keyway/Slot
Asymmetric OD
Unbalanced release
"Banana" Bow (Concave toward cut)
Parting Off
Radial Cut
Axial Stress release
Face Dishing (Concave/Convex ends)
Engineering Takeaway: If the tube ID is less than 50% of the OD, residual stresses are exponentially higher; machining these heavy-wall tubes without prior Stress Relief (SR) is almost guaranteed to result in movement.
How do I machine heavy wall CDS without warping?
Use the "Rough and Rest" protocol. Rough machine the tube to within 0.040", unchuck it, and let it sit (or perform a low-temp Stress Relief) to allow the primary stress movement to occur before the final finish cut.
The ASTM A519 "Straightness Trap"
Engineers often specify ASTM A519 believing it guarantees a straight tube. It does not. The standard straightness tolerances are generous (typically 1/8" in 5 feet), and for many sizes or "special" wall thicknesses, the specification states straightness is "mutually agreed upon between purchaser and producer."
If you do not explicitly define a straightness tolerance (e.g., "0.010 inch per foot max TIR") on the purchase order, mills can ship tubes that are visibly bowed, and they will be technically compliant with the standard.
Does ASTM A519 guarantee concentricity?
No. While CDS has better concentricity than hot finish, ASTM A519 specifies tolerances for OD and Wall Thickness or OD and ID. Concentricity is a result of these variables, not an independent guarantee, unless specifically negotiated.
Stress Relief (SR) vs. Annealing: The Trade-off
To prevent distortion, the material must be thermally treated. However, "Annealing" is a broad term that can ruin the mechanical properties you paid for.
1. Stress Relief (SR) - The Sweet Spot
Process: Heat to ~950°F - 1100°F, hold, and air cool. This removes ~85-90% of residual stress while retaining most of the yield strength and hardness gained from cold drawing. This is ideal for hydraulic cylinders and linear shafting.
2. Soft Anneal / Dead Soft - The Danger Zone
Process: Heat above the upper critical temperature (~1600°F) and furnace cool. This removes 100% of residual stress but wipes out the cold work hardening. A 1018 CDS tube with 70 ksi yield strength will drop to ~40 ksi (Hot Rolled properties).
Can I restore yield strength after a full anneal?
Not via heat treatment alone. Once cold drawn steel is fully annealed, the work hardening is lost. Strength can only be restored by cold working the material again or engaging in expensive quench and temper processes (if the alloy allows).
When cold drawn tube Is the Wrong Choice
Heavy Asymmetric Machining: If milling >30% of the cross-section on one side (e.g., long keyways), the part will warp. Use Hot Finished Seamless (HFS) or Stress Relieve before cutting.
High-Heat Service: If the component operates above 500°F, the tube may stress relieve itself in service, leading to unpredictable dimensional changes.
Caustic Environments: Without stress relief, the high residual hoop stress makes CDS highly susceptible to Season Cracking in the presence of chlorides or ammonia.
Troubleshooting FAQ: Cold Drawn Tubing
How do I prevent "cigar" distortion when turning long shafts?
This shape (fat in the middle) is caused by tailstock pressure combining with residual stress release. To fix this, reduce tailstock pressure to the absolute minimum required to hold the part, or switch to a "Rough and Rest" machining strategy to allow the stress to equalize before the final pass.
Why does "Season Cracking" occur in inventory without load?
Season cracking is a form of stress corrosion cracking (SCC) driven by the internal residual hoop stress from the cold drawing process. If the storage environment contains trace ammonia or chlorides, the chemical attack targets the grain boundaries under tensile stress, causing the tube to split spontaneously.
Comparison: When is Hot Finished Seamless (HFS) safer than CDS?
HFS is the safer choice when significant material must be removed asymmetrically (like milling deep pockets) or when the part requires no stored internal energy. While HFS has looser dimensional tolerances, it is stress-neutral and will not bow or warp during heavy machining.
How does Stress Relief (SR) affect ASTM A519 yield strength compliance?
A proper Stress Relief (950°F-1100°F) typically lowers yield strength by 10-15%, but usually keeps the material within the minimum requirements of ASTM A519 for cold worked tubing. Full annealing, however, will drop the strength below the standard's "as-drawn" minimums.
Does specifying "ASTM A519" automatically prevent bowed tubing?
No. The default straightness tolerance in ASTM A519 is very loose (often 1/8 inch in 5 feet) and frequently defaults to "mutually agreed upon." You must explicitly state a straightness requirement (e.g., 0.010"/ft) on your purchase order to ensure machine-ready stock.
