Cold Drawn Seamless (CDS) is a mechanical steel tube produced from a solid billet and cold-worked to final dimensions, primarily governed by ASTM A519. It is the standard for high-pressure hydraulic cylinders and telescoping booms requiring tight concentricity for Skiving and Roller Burnishing (SRB). It fails when welded without stress relief, causing bore distortion, or when substituted with Hot Finished Seamless (HFS) in precision applications due to spiral eccentricity.
COMMON FIELD QUESTIONS ABOUT COLD DRAWN TUBE
Why does my boring bar chatter on HFS tubing but not CDS?
This is caused by spiral eccentricity inherent to the Hot Finished Seamless (HFS) rotary piercing process. Unlike CDS, HFS wall thickness varies helically along the tube length. This forces the boring tool into an interrupted cut scenario—alternating between heavy chip loads and light rubbing—which induces harmonic vibration and chatter.
Can I use Skiving and Roller Burnishing (SRB) tools on HFS to save money?
Generally, no. SRB tools require a starting wall consistency within 5-10% to float effectively. HFS eccentricity often exceeds this limit, causing the skiving knife to "dig" deeply on the thick wall side and "skip" on the thin side, potentially shattering carbide cutters and ruining surface finish.
Why do piston seals fail prematurely in ERW/DOM tubing?
In high-pressure applications (>2500 PSI), Electric Resistance Welded (ERW) tubing can experience non-uniform expansion or "ballooning" at the weld seam due to metallurgical discontinuity. This gap allows the seal to extrude under pressure, leading to "nibbling" failures where the seal is sheared off during the retraction stroke.
1. The Machining Allowance Trap: HFS vs. CDS Economics
Procurement departments often mandate the switch from Cold Drawn Seamless (CDS) to Hot Finished Seamless (HFS) (ASTM A519 Hot Finished) citing a 20-30% reduction in raw material tonnage costs. For the manufacturing engineer, this represents a false economy. The savings in steel are systematically erased by the operational penalties required to correct the geometry of HFS.
The "Spiral Eccentricity" Factor
HFS is pierced hot. If the mandrel deviates even 1 degree, the tube develops a spiral wall variance. To ensure the bore "cleans up" to the final dimension, engineers must specify significantly heavier wall thicknesses for HFS compared to CDS. You are forced to buy more steel solely to turn it into scrap chips. Furthermore, the varying cut depth requires reducing spindle speeds (RPM) to manage tool deflection, increasing cycle times by 40-60%.
Does HFS ever clean up at standard machining allowances?
Rarely. Standard allowances of 0.030"–0.060" are insufficient for HFS due to the spiral run-out. You typically need 0.125" or more per side to guarantee a clean bore, effectively doubling the material waste compared to CDS.
2. Hydraulic Failure Modes: ERW and Weld Seam Fatigue
While Drawn Over Mandrel (DOM) tubing (ASTM A513 Type 5) offers better concentricity than HFS, it introduces a weld seam risk in high-cycle hydraulic applications. The "Zipper Failure" is a catastrophic failure mode specific to ERW/DOM tubing in cold environments or high-shock applications (e.g., excavator breakers).
Microscopic oxides, known as "hook cracks," can remain trapped in the bond line during the upset welding process. Under high hoop stress, these act as stress risers. Unlike seamless CDS which fails via ductile bulging, a compromised ERW seam can unzip longitudinally—instantaneously dropping load holding capability.
Is NDT sufficient to catch ERW hook cracks?
Standard ultrasonic testing (UT) can miss fine hook cracks at the bond line. For cylinders operating above 2500 PSI, the only engineering-safe mitigation is to specify seamless CDS to eliminate the anisotropic weld zone entirely.
3. Summary Data: Operational Comparison
Feature
Cold Drawn Seamless (CDS)
Hot Finished Seamless (HFS)
DOM (Welded & Drawn)
ASTM Standard
A519 Cold Drawn
A519 Hot Finished
A513 Type 5
Concentricity
High (Suitable for SRB)
Poor (Spiral Eccentricity)
High (Uniform Wall)
Machining Mode
High Speed / Lights Out
Low RPM / Babysitting
High Speed / Hard Spots
Pressure Limit
Extreme (>5000 PSI)
Medium (Wall limited)
Medium (<3000 PSI rec.)
Engineering Takeaway: Do not specify HFS if the surface finish requirement is stricter than 16 Ra or if using SRB tooling; the tooling breakage risk outweighs raw material savings.
When cold drawn tube Is the Wrong Choice
Direct Welding Without SRA: CDS contains high residual stress from cold working. Welding trunnions or ports directly to the tube without a subsequent Stress Relief Anneal (SRA) will cause the bore to distort/shrink, seizing the piston.
High-Temperature Service: The mechanical properties of cold drawn steel degrade rapidly above 500°F (260°C) as the material stress relieves itself; utilize hot finished or alloy grades instead.
Cost-Sensitive Structural Legs: For static structural applications where bore tolerance is irrelevant (e.g., agricultural equipment legs), CDS is overkill. Use HFS.
4. Specialized Technical FAQ
Which tubing is required for Skiving and Roller Burnishing (SRB)?
CDS is the mandatory substrate for SRB. The skiving process relies on a floating head that follows the existing bore center. HFS has too much wall variation, which causes the tooling to crash. While DOM can be skived, the weld seam hard spots often leave lines in the burnished finish, leading to seal bypass.
How does the "Machining Allowance" dictate the buy-vs-make decision?
If you use CDS, you can purchase "Near Net Shape" sizes (e.g., 0.030" cut depth). If you use HFS, you must factor in the "Eccentricity Penalty," often purchasing a tube with 0.150"+ excess wall. When calculating ROI, the cost of the extra steel weight plus the 3x longer machining cycle usually makes the "cheaper" HFS more expensive per finished part.
Why did the cylinder bore go oval after machining CDS?
This is likely due to chucking stress release. CDS has high residual hoop stress. If clamped too tightly in a 3-jaw chuck during machining, the tube deforms. When released, it springs back. To fix this, use full-wrap pie jaws or specify Stress Relieved (SR) condition CDS to lower the residual energy before machining.
Is DOM safe for 5000 PSI hydraulic applications?
Generally, no. At 5000 PSI, the hoop stress on the cylinder wall approaches the yield limits where the weld seam integrity becomes the critical weak point. Even with 100% NDT, the risk of fatigue failure at the seam (zipper split) is statistically significant. Seamless CDS is the required standard for high-pressure safety factors.
