How Does the Tube Drawing Reduction Ratio (15% vs. 30%) of a Seamless Titanium Heater Influence the Threshold Stress for Hydrogen-Induced Cracking in Sour Service at 60°C?

Heater tubes of seamless titanium are produced by drawing through dies, with reductions usually of the order of 15 to 30 percent per pass. The reduction ratio determines the amount of cold work , dislocation density and crystallographic texture in the final tube . Hydrogen-induced cracking (HIC) occurs in sour service environments including hydrogen sulfide (H2S) at 60°C when the local hydrogen concentration surpasses the terminal solid solubility. The HIC threshold stress, i.e. the lowest tensile stress at which cracking occurs at a given hydrogen concentration, is directly dependent on the tube microstructure. Increased reduction ratio (30%) leads to finer grain size and higher dislocation density which increases the hydrogen trapping sites and increases the threshold stress. The lower reduction ratio of 15% results in larger grains and fewer traps and hence a reduction of the threshold stress of 40–70 MPa.

Hydrogen Trapping and Crack Initiation Mechanism in Drawn Tube

In titanium, dislocations, grain boundaries and vacancies behave as hydrogen traps . Finer grains from higher reduction ratios provide more grain boundary area per unit volume which distributes hydrogen more evenly and delays hydride precipitation. The threshold stress for HIC is given by the relation σ_th = σ_0 + k × ρ_dislocations^{0.5}, where σ_0 is the intrinsic resistance and ρ_dislocations is the dislocation density. Dislocation densities are \sim 10^{14}-10^{15} m^{-2} for 30% reduction ratio vs \sim 10^{13}-10^{14} m^{-2} for 15% reduction. This difference raises σ_th by 30–50 MPa. Also, the crystallographic structure from higher reduction ratios aligns grains to resist hydrogen diffusion along basal planes.

Threshold Stress for Different Reduction Ratios - Quantitative

HIC threshold stresses for Grade 2 titanium tubes have been determined by controlled testing in sour brine (5% NaCl, 0.5% acetic acid, saturated with H2S, pH 3.5) at 60°C with cathodic charging. With the reduction ratio of 15% (low cold work), the threshold stress is 120-150 MPa, the hydrogen concentration at failure is 250-350 ppm and the grain size is ASTM 6-7. With 20% reduction, threshold stress increases to 150~180 MPa, hydrogen concentration at failure climbs to 300~400 ppm and grain size is ASTM 7~8. Threshold stress at 25% reduction: 170–200 MPa; concentration at failure: 350–450 ppm H; grain size: ASTM 8–9. Threshold stress at 30% reduction (high cold work) = 190-220 MPa Hydrogen concentration at failure = 400-500 ppm Grain size = ASTM 9-10 In contrast, fully annealed tubes with 0% cold work have a threshold stress of just 80 to 110 MPa and a grain size of ASTM 4 to 5.

Effect of applied stress and hydrogen concentration on cracking

The relationships between the applied time to failure and the stress-hydrogen concentration are predicted as a function of reduction ratio. At an applied stress of 100 MPa, a tube with 15% reduction reaches a critical hydrogen concentration of 300 ppm, which takes around 1 year at a hydrogen absorption rate of 1 ppm/day. That same tube with a 30% reduction has a critical concentration of 450 ppm and life of 1.8 years. The 15 % reduction tube fails at 200 ppm hydrogen level at 150 MPa applied stress in 7 months, while 30 % reduction tube fails at 350 ppm hydrogen level in 14 months. The tube with 15% reduction fails quickly with an applied stress of 200 MPa and hydrogen concentration below 100 ppm, whereas the tube with 30% reduction requires 280 ppm of hydrogen and lasts for 11 months.

Reduction Ratio Selection Guide Sour Service at 140°F

The table below provides guidance on the choice of tube drawing reduction ratio for Grade 2 titanium seamless heaters in sour service considering H 2 S partial pressure, applied stress and expected service life.

H2S Partial Pressure (bar) Solution pH Maximum Applied Stress (MPa) Recommended Reduction Ratio Minimum Grain Size (ASTM) Expected HIC Threshold Stress (MPa) <0.05 >4.0 <100 15–20% 6 >150 0.05–0.10 3.5–4.0 100–150 20–25% 7 >180 0.10–0.20 3.0–3.5 150–200 25–30% 8 >200 0.20–0.50 2.5–3.0 150–180 30% plus Grade 7 9 >220 >0.50 <2.5 >150 Not recommended for Grade 2 N/A N/A
Engineering Beyond the Reduction Ratio Choice

Titanium grade has a substantial effect on HIC resistance irrespective of reduction ratio. Palladium-stabilized Grade 7 has threshold stresses 50–100 MPa higher than Grade 2 at the same reduction ratio, attributed to palladium-catalyzed hydrogen recombination. Grade 12 has moderate improvement of 30-50 MPa. Annealing after drawing (stress alleviation at 540°C for 30 min.) reduces dislocation density and also decreases HIC threshold. Cold drawn, without subsequent annealing, is therefore the recommended condition of tube for sour service. The threshold stress is independent of wall thickness, although it does alter the duration for fracture propagation: a 2.0 mm wall containing a crack may live longer until perforation than a 1.2 mm wall.

Specify with Confidence

• For seamless titanium heater tube in sour service at 60°C specify cold drawn reduction ratio of 25-30% without additional annealing. A minimum grain size of ASTM 8 is required, and certification of dislocation density by X-ray diffraction peak broadening analysis is required. For extreme sour service with H₂S partial pressure > 0.2 bar, specify Grade 7 titanium with 30% reduction ratio. Hydrogen charging test on sample section during incoming inspection: charge to 400 ppm hydrogen, then apply 80% of specified yield stress, reject lot if cracking occurs within 100 hours. By choosing a greater reduction ratio of 30% instead of 15%, the engineer improves the threshold stress for hydrogen-induced cracking by 40–70 MPa, adding a significant safety margin in sour service situations.