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 utilised for heater tubes in chloride-containing settings because of its superior pitting resistance. When the tube is bent to a tight radius of 3 times the tube diameter (3D), the outer radius experiences 10-15% plastic tensile strain. This cold working induces residual tensile stress, increases dislocation density and can orient alpha phase grains. The pitting potential, which is the electrochemical potential at which stable pits nucleate, diminishes with increasing cold work. Stress-relief annealing at 540°C for 30 min can recover part of the strain, decrease residual stress and allow partial recrystallisation. The pitting potential of a tube that has been bent and stress relieved recovers to within 50 to 100 mV of the unbent condition, compared to a depression of 150 to 250 mV for a tube that has been bent but not stress relieved.

Mechanism of Pitting Potential Restoration and Residual Strain Recovery

Dislocations proliferate and aggregate along grain boundaries during plastic bending, which provide high-energy sites that are preferred for pit initiation. The passive film is further destabilised by tensile residual tension at the outer radius. Stress relief annealing at 540°C promotes dislocation climb and annihilation resulting in 60-80% reduction of the dislocation density. This method also reduces the residual tensile stress to 150-250 MPa to 30-60 MPa. Grade 12 molybdenum and nickel are redistributed and enriched in the surface and the passive film stability is improved during annealing. The pitting potential is recovered when the number of active spots for pit initiation decreases and the passive film becomes more homogenous.

Quantitative Pitting Potential vs Stress Relief & Bending

The pitting potentials for Grade 12 titanium tubes in 5% NaCl at 90 deg C have been determined by controlled testing over a range of situations. The pitting potential for an unbent, annealed tube (baseline) is +0.45 to +0.60 V vs. Ag/AgCl and no pitting occurs after 1000 hours. (x) A tube bent to a 3D radius in the as-bent condition with no stress relief has a pitting potential of +0.20 to +0.35 V, pitting commences in 100-300 hours, and after 500 hours the pit depth is 0.1-0.3 mm. 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 500-1500 hours and pit depths are 0.05-0.15 mm after 500 hours. For a tube bent to a 5D radius ( softer bend ) with stress relief , the pitting potential is +0.40 to +0.55 V which is quite near to the unbent baseline . Pitting potential only +0.25 to +0.40 V: limited recovery. Tube bent to 3D radius, stress alleviated at 480°C (insufficient temperature).

Effect of Bend Radius and Stress Relief Temperature on Pitting Potential

The recovery of pitting potential is affected by the bend radius and by the stress relief temperature. For a 3D bend, the pitting potential depression of the as bent is 150–250 mV below the unbent value. Stress reduction at 480 °C recovers 30–50 mV but leaves a depression of 120–200 mV. Stress relief at 540°C restores 100-150 mV, leaving a depression of 50-100 mV. Stress relief at 600C recovers 150-200 mV leaving a 20-50 mV depression, but can cause alpha-case if not done in an inert atmosphere. For a 5D bend, the as-bent depression is just 50–100 mV and stress release at 540°C fully recovers to the unbent level. For a 2D bend (tighter than 3D) the as-bent depression is 200-350 mV, and stress relief even at 540 deg C only recovers 80-120 mV leaving a considerable depression.

Grade 12 in Hot Chloride Service - Bending Radius and Stress Relief Selection Guide

The following table contains recommendations for bending Grade 12 titanium heater tubes for duty in 5% NaCl at 90°C as a function of bend radius, stress relief treatment and acceptable 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)Recommended use
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 Acceptable 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 Engineering Beyond Stress Relief Treatment

Titanium grade has an effect on the pitting potential at initiation and recovery following stress release. Grade 7 has a baseline pitting potential 100–200 mV higher than Grade 12, hence even a 3D as-bent Grade 7 tube (+0.30 to +0.45 V) could be appropriate for many applications. Pit propagation time is affected by wall thickness although pitting potential is not. For example, a 2.0 mm wall and 0.1 mm pit depth will have 95% of the wall surviving. The environment of stress release is important ; annealing in air at 540 °C produces a thin oxide which does not impair pitting resistance, but argon or vacuum annealing is preferred. Pitting potential is not greatly affected by the rate of cooling after stress reduction.

Well-informed specification making

Bend a Grade 12 titanium heater tube for duty in 5 % NaCl at 90°C. Specify stress-relief annealing at 540°C for 30 minutes in an argon environment. Cyclic polarisation testing of a sample from the bent and stress-relieved tube is required to confirm the pitting potential. The pitting potential shall be within 100 mV of the unbent baseline value of +0.45 to +0.60 V. If the bend is tighter than 3D, consider utilising Grade 7 titanium or extending the bend radius to 5D. Without stress alleviation, expect a lower service life and check the bent area quarterly for pitting. Monitor the open-circuit voltage of the heater during operation. A drop below +0.20 V vs. Ag/AgCl implies an increased risk of pitting. The engineer stress relieves Grade 12 titanium at 540°C after bending, returning the pitting potential within 50–100 mV of the unbent condition for reliable performance in hot chloride service.