When a Grade 12 Titanium Heater Tube Is Bent to a 3D Radius and Then Stress-Relieved at 540°C, How Does the Residual Strain Recovery in the Outer Radius Change Its Pitting Potential in 5% NaCl at 90°C?

Grade 12 titanium (Ti-0.3Mo-0.8Ni) is utilized for heater tubes in chloride containing situations due to its superior pitting resistance. When the tube is bent to a tight radius of 3 times the tube diameter (3D), plastic tensile strain of 10 – 15% occurs at the outer radius. This cold work leads to residual tensile stress, increased dislocation density and can lead to orientation of the alpha-phase grains. The pitting potential is the electrochemical potential at which stable pits are initiated . The pitting potential reduces with increased cold work . Stress relief annealing for 30 min at 540°C recovers some of the strain, decreases residual stress and allows some recrystallization. Stress-relief of a bent tube depresses the pitting potential only by 50-100 mV compared to the unbent condition, instead of 150-250 mV depression for an as-bent tube without stress-relief.

Mechanism of residual strain recovery and pitting potential restoration

Plastic deformation by bending causes dislocations to grow and pile up at grain boundaries creating high-energy spots that are ideal sites for pit initiation. Tensile residual stress at the outer radius further destabilizes the passive layer. Stress-relief annealing at 540°C enables dislocation climb and annihilation, lowering the dislocation density by 60-80%. This procedure also reduces the residual tensile stress from 150–250 MPa to 30–60 MPa. During annealing, the molybdenum and nickel in Grade 12 redistribute to enrich the surface and improve the stability of the passive film. The passive film becomes more uniform, the number of active sites for pit initiation is reduced and the pitting potential is restored.

Quantitative Pitting Potential vs. Bending and Stress Relief

Pitting potentials of Grade 12 titanium tubes under controlled tests in 5% NaCl at 90°C have been established for various situations. For an unbent, annealed tube (baseline), the pitting potential is +0.45 to +0.60 V vs. Ag/AgCl, no pitting seen after 1,000 hours. For a tube bent to a 3D radius in the as-bent condition and no stress relief, the pitting potential reduces to +0.20 to +0.35 V and pitting starts after 100–300 hours, with a pit depth of 0.1–0.3 mm after 500 hours. For a tube bent to a 3D radius and stress-relieved at 540 °C for 30 min, the pitting potential recovers to +0.35 to +0.50 V, pitting occurs after 500–1,500 h, and pit depths are 0.05–0.15 mm after 500 h. For a 5D radius bent tube (gentler bend) with stress relief, the pitting potential is +0.40 to +0.55 V, very near to the unbent baseline. The tube was bent to a 3D radius and stress alleviated at 480°C (not hot enough). Partial recovery was observed with the pitting potential of +0.25 to +0.40 V.

Effect of bend radius and stress-relief temperature on pitting potential

The recovery of pitting potential is shown to be dependent on both the bend radius and the stress relief temperature. The 3D bend as bent pitting potential depression is 150-250 mV below the unbent value. Stress relief at 480 °C recovers 30–50 mV, leaving a depression of 120–200 mV. Stress relief at 540 °C recovers 100 to 150 mV, leaving a depression of 50 to 100 mV. Stress relief at 600°C recovers 150–200 mV but leaves a depression of 20–50 mV, yet the process may result in alpha-case production if not performed in an inert atmosphere. In the case of a 5D bend, the as-bent depression is only 50-100 mV, and stress relief at 540°C fully recovers the unbent level. For a 2D bend (tighter than 3D) the depression as-bent is 200-350 mV. Even stress reduction at 540°C, only recovers 80-120 mV leaving a huge depression.

Grade 12 Hot Chloride Service Bend Radius and Stress Relief Selection Guide

Recommendations for bending Grade 12 titanium heater tubes for service in 5% NaCl at 90°C are shown in the following table, as a function of bend radius, stress relief treatment, and desired pitting resistance.

Bend Radius (R/D) As-Bent Pitting Potential (V vs. Ag/AgCl) Stress Relief Temperature (°C) Post-Relief Pitting Potential (V vs. Ag/AgCl)Suggested Application
5D +0.30 to +0.45 540°C +0.45 to +0.60 High dependability service
5D +0.30 to +0.45 None +0.30 to +0.45 Moderate service
3D +0.20 to +0.35 540°C +0.35 to +0.50 Adequate with inspection 3D +0.20 to +0.35 None +0.20 to +0.35 Not recommended for hot chloride 2D +0.05 to +0.20 540°C +0.15 to +0.30 Avoid if possible 2D +0.05 to +0.20 600°C (argon) +0.25 to +0.40 Marginal with Grade 7 Stress Relief Treatment Engineering

The pitting potential and the stress release recovery are titanium grade dependent. Grade 7 has a baseline pitting potential 100 – 200 mV higher than Grade 12 hence even a 3D as bent Grade 7 tube may be appropriate to many applications (+ 0.30 to + 0.45 V). Wall thickness does not influence the pitting potential but affects the time for pit propagation; 95% of a 2.0 mm wall remains with 0.1 mm pit depth. The environment is important for stress relief. Annealing in air at 540°C produces a thin oxide, which does not degrade pitting resistance but annealing in argon or vacuum is preferred. The cooling rate after stress relief has no significant influence on the pitting potential.

Develop an Informed Specification

Specify stress-relief annealing at 540°C for 30 minutes in an argon environment after bending, for a Grade 12 titanium heater tube requiring a 3D bend for service in 5% NaCl at 90°C. Perform cyclic polarization testing on a sample of the bent and stress relieved tube to confirm pitting potential; the pitting potential should be within 100mV of the unbent baseline value of +0.45 to +0.60V. For bends tighter than 3D use Grade 7 titanium or increase the bend radius to 5D. If stress relief is not achievable, accept a reduced service life and examine the bent area periodically for pitting. The open-circuit potential of the heater should be monitored in operation, with a potential below +0.20 V vs. Ag/AgCl increasing the risk of pitting. The engineer stress-relieves at 540°C after bending to get the pitting potential of the Grade 12 titanium back to within 50–100 mV of the unbent condition, so the titanium may operate reliably in hot chloride service.