How Does the Longitudinal Residual Stress Pattern from Tube Drawing Affect the Susceptibility to Sulfide Stress Cracking in Sour Service Environments?

Sour service conditions containing hydrogen sulfide (H 2 S) are among of the most challenging applications for metallic components. Generally, titanium alloys are resistant to sulfide stress cracking (SSC) as opposed to carbon and low-alloy steels. However, titanium can be vulnerable under certain conditions – low pH, high chloride concentrations, temperatures exceeding 80°C and the presence of tensile stresses. The pattern of longitudinal residual stress induced during tube drawing affects the susceptibility to SSC considerably . A mandrel pulled through a hollow billet reduces the diameter and wall thickness in tube drawing. The method produces a characteristic pattern of residual stresses with tensile stresses in the outer surface and compressive stresses in the inner surface or vice versa, depending on whether the tube is drawn without an internal mandrel (sunk drawing) or over a fixed mandrel. The surface under tensile residual stress is at significantly increased risk for SSC initiation.

Mechanism of SSC Initiation at Residual Stress Sites

Hydrogen embrittlement, which occurs in H₂S containing settings, is called sulfide stress cracking. Hydrogen sulfide poisons the hydrogen recombination process and drives atomic hydrogen into the metal lattice. Hydrogen absorbed under tensile stress diffuses to locations of triaxial stress concentration, such as grain boundaries, inclusions, or surface defects. Hydrogen reduces the cohesive strength of titanium grain boundaries and causes inter- or transgranular crack propagation. Any stresses exerted in service (pressure, thermal expansion, restrictions of mounting) are summed with residual tensile stresses from tube drawing. SSC initiation occurs when the sum of the tensile stress reaches a threshold value (about 40–60% of yield strength for titanium in sour service). The longitudinal direction is of special concern because cracks in the axial direction will propagate in the tube direction resulting in catastrophic splitting.

Quantitative Residual Stress Values for Different Wire Drawing Methods

Residual stresses measured by X-ray diffraction on Grade 2 titanium tubes revealed typical longitudinal surface stress patterns:

Sunk drawing (without internal mandrel, tube reduced by external dies only) Outer surface develops tensile residual stress of 80 to 120 MPa. Compressive residual stresses (50 to 80 MPa) arise on inner surface. External H2S exposure can cause SSC in the tensile outer surface.

Mandrel drawing (tube drawn over fixed internal mandrel): Outer surface develops compressive residual stress of 40–70 MPa. It generates tensile residual stress of 100-150 MPa at the inner surface. The inner tensile surface is susceptible to SSC by internal H2S exposure.

Plug drawing (floating mandrel, moderate reduction) Balanced residual stress pattern with both sides at 20 to 50 MPa (tensile or compressive depending on reduction ratio). Best for sour service.

Cold drawing + stress relief annealing (540 °C, 30 min): residual strains decreased to < 15 MPa on both surfaces independent of drawing process. Good for severe sour service.

Residual stress pattern based SSC susceptibility

Following controlled testing in NACE TM0177 Solution A (5% NaCl, 0.5% acetic acid, saturated with H₂S, pH 2.7, 25°C), the following SSC thresholds have been determined:

Surface tensile stress <30 MPa: No SSC initiation after 1000 hours. Safe for any sour service.

30-60MPa surface tensile stress: 500-1,000 h SSC onset time. Suitable for non-critical service with periodic inspection.

Surface tensile stress 60-100 MPa: SSC start time 100-500 hours. Crack propagation rates are 0.1 to 0.5 mm/hr once established.

SSC initiation at surface tensile stress > 100 MPa in < 24-100 h. Tube failure occurs in a few of weeks due to rapid crack propagation.

Sour Service Drawing Method Selection Guide

The table below is a decision matrix for tube drawing procedures based on the severity of sour service and the direction of stress exposure.

Sour Service Condition & H₂S Exposure Location Recommended Drawing Method Residual Stress Pattern & SSC Risk Mitigation
External H₂S exposure only (tube immersed in sour brine, internal dry or inert gas)Plug drawing with stress relief annealing Outer surface with compression or low tension. Residual stress <30 MPa. Max SSC resist.
Internal H2S exposure only (sour gas inside tube, clean environment external)Sunk drawing (inside surface compressive)Compressive stress on the inner surface (50 to 80 MPa). Tensile outside surface, not exposed. That's all right.
Both surfaces contacted by H2S (tube immersed in sour liquid, internal sour gas)Plus plug draw, full tension reliefResidual stress on both surfaces <15 MPa. Needed for double exposure. More expensive, but necessary.
Low H2S partial pressure (<0.1 psi), high pH (>4.5)Standard mandrel draw no stress alleviationInner surface tensile stress is allowed due to lesser danger of SSC. Quarterly pH monitoring.
High H2S partial pressure (>1 psi), low pH (<4), Temperature >80oC Plug drawing, Stress release plus surface shot peeningShot peening creates a layer of compression (150-200 MPa) irrespective of the drawing procedure. Most dependable.
Beyond Residual Stress Engineering -

SSC propagation time depends on wall thickness, not on the onset of SSC. A wall that is thicker (1.5 mm) and has a large residual tensile stress (100 MPa) can also crack, but it takes longer for the crack to get through. The tube diameter is also important: for the same drawing reduction, bigger diameters (50 mm versus 20 mm) show higher magnitudes of residual stress, due to higher differential deformation. The threshold H2S concentration for SSC in titanium is around 0.01 psi partial pressure at 25°C and decreases to 0.001 psi at 80°C. Alloying additions in Grade 7 and Grade 12 titanium alloys result in far stronger SSC resistance than in Grade 2, with 50–100% higher residual stress tolerance.

Making a knowledgeable specification

For sour service, when specifying a titanium heater tube, include a need for measuring of residual stress in a representative sample by X-ray diffraction or hole-drilling method. Manufacturing procedure : - plug drawing - stress relief annealing (540°C, 30 minutes, argon atmosphere) Request confirmation that the longitudinal surface residual stress is ≤ 30 MPa (tensile) on any surface exposed to H2S. Specify Grade 7 titanium for severe sour service (pH <3.5, H2S >0.1 psi, temperature >60°C) independent of residual stress, with a surface shot peening treatment to provide a compressive layer. Inspect the heater tube quarterly using the electrical resistance technique in service . An increase of 5 to 10 percent indicates likely SSC fracture beginning and requires rapid ultrasonic inspection . The engineer uses drawing method selection and post-processing to regulate residual stress patterns and prevent sulfide stress cracking, which is one of the most difficult failure mechanisms for titanium sour service equipment.