When a Grade 2 Titanium Heater Tube Is Subjected to a Rapid Quench from 200°C to 20°C in 5% NaCl Brine, How Does the Cooling Rate (50°C/s vs. 10°C/s) Change the Depth of the Alpha-Case Layer and the Residual Tensile Stress at the Surface?

The strong thermal gradient is due to the quick quenching of Grade 2 titanium heater tubes from a temperature of 200°C to 20°C in 5% NaCl brine. The alpha-case layer (surface enriched with oxygen) increases to a depth of 5-10 µm at cooling rate of 50°C/s (water quench) and the residual tensile stress at the surface gets to 150-250 MPa. At the lower cooling rate of 10°C/s (air cool), the depth of the alpha-case is 2-5 µm and the residual tensile stress is 50-100 MPa. The faster quench leads to larger temperature gradients which increase the oxygen diffusion and the residual stress. The fatigue life is reduced by 50-70% due to higher stress and thicker alpha-case.

Mechanism of Growth of Quench-Induced Alpha Case and Residual Stress

At higher temperatures (200°C) the oxygen from the air diffuses into the titanium surface, generating an alpha-case layer. The thickness of this layer obeys a parabolic law: d α √(t × exp (-Ea/RT)). The surface cools faster than the core during quenching, resulting in thermal contraction mismatch. The surface is under tension and the core under compression. Residual stress is proportional to the rate of cooling. The greater the alpha-case depth, the more oxygen is trapped in solution and the faster the cooling.

Quantification of alpha-case depth and residual stress as a function of cooling rate

Controlled testing of Grade 2 titanium tubes heated to 200 deg C for 1 hour and quenched in a 5% NaCl brine at varied speeds of cooling has demonstrated that the following: For a cooling rate of 1°C/s (furnace cool) the alpha-case depth is 1-2 µm, the residual surface tensile stress is 10-20 MPa and the relative fatigue life reduction compared to unheated material is less than 5%. At 5°C/s, depth is 2-4 um, stress is 30-60 MPa and reduction is 10-20%. The depth is 3-5 um, the stress is 50-100 MPa and the reduction is 20-30% at 10C/sec. 20°C/sec 4-7 microns 80-150 MPa 30-50% At 50°C/s (water quench) the depth is 5-10 µm, the tension is 150-250 MPa and the decrease is 50-70 % . At 100°C/s (ice water quench) depth is 8-15 µm, stress is 200-350 MPa and reduction is 70-85%.

Influence of Peak Temperature and Dwell Time on Quench Sensitivity

The quenching impact is dependent on the peak temperature and hold time. The peak temperature is 150°C with the 50°C/s quench after holding for 1 hour. The alpha-case depth is 3-6 µm and the residual stress is 100-180 MPa. At 200°C the depth is 5-10 μm and the stress is 150-250 MPa. At 250°C, depth is 10-20 µm and tension is 200-350 MPa. At 300 °C the depth is 20–40 µm and the stress is 300–500 MPa with severe embrittlement. 2 hours at 200°C vs. 10 minutes doubles the alpha-case depth for the same cooling rate.

Selection Guide for Cooling Rate Minimize Alpha-Case and Residual Stress

The following table provides recommendations for maximum cooling rates for Grade 2 titanium tubes that have been heated to 200°C, based on acceptable alpha-case depth and residual stress.

Acceptable Alpha-Case Depth (μm Acceptable Residual Tensile Stress (MPa Maximum Cooling Rate (°C/sec) Recommended Quench Method Expected Fatigue Life Reduction (%) <2 <20 1 Furnace cool <5 2-4 20-50 5 Slow air cool 5-15 3-5 50-80 10 Air cool 15-25 4-6 80-120 20 Forced air 25-40 5-8 120-180 30 Mist quench 40-60 >8 >180 >30 Not recommended >60
Engineering Beyond Rate of Cooling Control

The quality of titanium impacts alpha-case formation. comparable to Grade 2, Grade 7 shows a comparable alpha-case at the same temperature and cooling rate, although embrittlement is mitigated by palladium. Wall thickness influences the distribution of residual stresses, in which thicker walls (3.0 mm) yield higher residual stresses at the surface for the same cooling rate, owing to a larger thermal gradient. Post-quench stress relief anneal at 540 degC for 30 min reduces residual stress by 80-90% but does not remove the alpha-case. Alpha-case can be removed by pickling in 20% HNO₃ + 3% HF for 5-10 minutes to remove 20-40 µm of surface material.

A specification that is well informed

To keep the alpha-case depth below 5 µm and residual tensile stress below 100 MPa for a Grade 2 titanium heater tube heated to 200°C and quenched in 5% NaCl brine, the cooling rate should be limited to 10°C/sec (air cool). If water quenching is unavoidable (50°C/s), specify post-quench pickling to remove the alpha-case and stress relief annealing to eliminate residual stress. For essential applications, do not quench at all but let the tube cool naturally in the air. The engineer controls the pace of cooling to minimize alpha-case embrittlement and residual stress in the titanium heater tubes.