Cold drawn tube is seamless mechanical tubing sized by drawing through a die and over a mandrel to achieve precise tolerances and high yield strength. While governed primarily by ASTM A519, its application in sour service environments is strictly limited by NACE MR0175 / ISO 15156. It is ubiquitous in hydraulic cylinders and downhole tools but fails catastrophically via Sulfide Stress Cracking (SSC) if residual surface cold work exceeds 22 HRC.
COMMON FIELD QUESTIONS ABOUT COLD DRAWN TUBE
Why does my ASTM A519 tubing fail SSC tests despite passing mill MTRs?
Standard mill MTRs report hardness taken at the mid-wall (mid-radius) per ASTM A370. This misses the localized work-hardened "skin" (0.1–0.5mm deep) on the ID/OD caused by the drawing die. This skin often exceeds the NACE 22 HRC limit, initiating cracks even if the core is compliant.
Is "Stress Relieved" (SR) condition sufficient for NACE compliance?
Generally, no. Standard stress relief temperatures are often too low to fully recrystallize the highly strained surface grains created by cold drawing. To ensure a uniform hardness profile below 22 HRC (250 HV) throughout the wall, the tube typically requires Normalizing or Quench and Tempering (Q&T).
Can we verify surface hardness using portable Leeb or UCI testers?
No. Portable rebound (Leeb) or ultrasonic contact impedance (UCI) testers lack the depth resolution to accurately measure a thin 200-micron skin. They read the underlying bulk material. Only laboratory micro-hardness testing (Vickers/Knoop) on a polished cross-section provides valid data for surface compliance.
The Technical Disconnect: ASTM A370 vs. NACE MR0175
The primary engineering hazard with cold drawn tube in sour service is the "Compliance Gap" between manufacturing standards and application physics. ASTM A519 is a mechanical tubing specification, not a pressure vessel or line pipe standard. It does not inherently account for environmental cracking mechanisms.
The cold drawing process—pulling steel through a die to reduce OD and over a mandrel to set ID—generates extreme plastic deformation at the surface. This results in a "Skin Effect" where the outer and inner 500 microns can exhibit hardness spikes of 35–40 HRC, while the mid-wall remains a compliant 19 HRC.
Why does a thin surface layer cause total catastrophic failure?
In sour environments, H2S cracking is a surface-initiated mechanism. Once a micro-crack forms in the brittle, non-compliant skin (over 250 HV), it acts as a sharp notch. This notch raises the stress intensity factor dramatically, allowing the crack to propagate dynamically through the softer, compliant mid-wall material.
The Compliance Dispute Table
Engineers often face pushback from mills when rejecting heat lots based on surface readings. Understanding the opposing logic is critical for arbitration.
Party
Argument
Technical Reality
Mill (Supplier)
"Tested per ASTM A370 at mid-radius. Result: 19 HRC. Material is compliant."
ASTM A370 measures bulk properties. It legally satisfies the purchasing spec but ignores the physics of SSC initiation.
User (Engineer)
"Lab found 280 HV (27 HRC) on the ID surface. Reject heat."
NACE MR0175 §7.3.3 mandates the wetted surface must be <22 HRC. If the fluid touches the skin, the skin must be soft.
Engineering Takeaway: Never accept a generic "NACE Compliant" statement on an MTR for cold drawn tubing without reviewing the specific testing location protocols used by the mill.
When cold drawn tube Is the Wrong Choice
As-Drawn or Stress-Relieved in Zone 3 Sour Service: Without full heat treatment (Normalization/Q&T), the risk of surface hardness spikes is too high for critical H2S partial pressures.
Unmachined Surfaces in Fatigue Applications: The micro-defects and die marks inherent to the drawing process act as stress risers; if the tube is not honed or machined, fatigue life is compromised.
High-Pressure Hydrogen Service without Volumetric NDE: ASTM A519 does not mandate ultrasonic testing (UT) by default. Laminations or inclusions elongated by drawing can blister in high-pressure hydrogen.
Procurement Mandates: Overriding ASTM A519
To safely utilize cold drawn tube in sour environments, procurement language must explicitly close the compliance gap. Relying on standard part numbers is insufficient.
Key Ordering Specifications
Heat Treatment: Specify "Normalized" or "Quench & Tempered" rather than "Stress Relieved" (SRA). This ensures the microstructure is reset, eliminating the work-hardened skin.
Hardness Verification: Add a supplementary requirement: "Hardness testing shall include OD and ID surface measurements (HV5 or HV10) in addition to standard mid-radius tests. Max 250 HV10 on all wetted surfaces."
Surface Condition: Specify "Free of die marks, laps, and heavy oxide," as surface topology defects exacerbate hydrogen uptake.
What is the cost impact of requesting surface hardness transverses?
Adding specific micro-hardness surface transverses typically adds <5% to the testing costs but prevents 100% of "Skin Effect" failures. The cost of a single field failure outweighs the testing cost by orders of magnitude.
Specialized FAQ: Cold Drawn Tube Compliance
How do we resolve hardness disputes when field lab results contradict mill MTRs?
Disputes almost always arise from testing location. If the mill tested mid-wall (A370) and the field lab tested the surface (NACE intent), the field lab is correct regarding fitness-for-service. To resolve, request a joint Third-Party witness test focusing on a Micro-Hardness Traverse (testing every 0.1mm from surface to core). If the "U-shape" hardness profile exists, the mill's stress relief process was insufficient.
What is the minimum material removal required to eliminate the cold-worked skin?
While variable based on the reduction ratio, a conservative engineering rule of thumb is that the high-hardness skin extends 0.010" to 0.020" (0.25mm to 0.5mm) into the wall. If the tubing is purchased as "Machining Stock" (e.g., for liners), machining 1.0mm off the ID/OD effectively removes the non-compliant layer, rendering the bulk material properties relevant again.
Does "Peel and Polish" processing guarantee NACE compliance for cold drawn tube?
Not inherently. Peeling removes the OD defects and some of the work-hardened layer, but it does not address the ID (Inner Diameter). In tubular applications where the process fluid is internal (e.g., flowlines, injection quills), a peeled OD is irrelevant to the corrosion mechanism occurring on the ID. The ID must be honed or chemically treated, or the tube must be thermally normalized.
Why is ASTM A519 often forbidden in offshore structural applications despite high strength?
ASTM A519 lacks the Charpy V-Notch (CVN) impact toughness requirements standard in structural pipe specs like API 5L or EN 10225. Cold drawing increases yield strength but significantly reduces ductility and toughness. In low-temperature offshore environments, this creates a risk of brittle fracture, independent of the sour service compliance issues.
