In the manufacturing of titanium heater tubes, cold drawing reduces the cross-sectional area by a certain percentage prior to final annealing. The cold work level determines the driving force for recrystallization during annealing. A low cold work level of 5% produces large recrystallized grains of ASTM 4–5 (diameter 80–120 µm). A high cold work level of 20% produces fine recrystallized grains of ASTM 8–10 (diameter 11–22 µm). In air at 300°C, the fine-grained material exhibits 3–5 times higher resistance to thermal fatigue cracking because smaller grains provide more grain boundary area to accommodate cyclic thermal strain and resist crack propagation.

The Mechanism of Grain Size Influence on Thermal Fatigue Resistance

During thermal cycling at 300°C, the titanium tube expands and contracts. The cyclic strain is accommodated by slip and grain boundary sliding. Fine-grained material has more grain boundaries per unit volume, allowing the strain to be distributed more evenly. Crack initiation occurs at slip bands and grain boundaries; finer grains reduce the slip length and the stress concentration at grain boundaries. The fatigue crack growth rate follows da/dN ∝ d^0.5, where d is grain size. Halving the grain diameter reduces the crack growth rate by 30%. Additionally, fine grains provide more barriers to crack propagation, forcing cracks to change direction and dissipate energy.

Quantitative Recrystallized Grain Size as a Function of Cold Work

Controlled annealing of Grade 2 titanium tubes at 700°C for 30 minutes, following different cold work levels, has established the following recrystallized grain sizes. At 5% cold work, the recrystallized grain size is ASTM 3–5 (diameter 50–125 µm), and the hardness after annealing is 160–180 HV. At 10% cold work, the grain size is ASTM 5–7 (32–64 µm), and the hardness is 170–190 HV. At 15% cold work, the grain size is ASTM 7–8 (22–45 µm), and the hardness is 180–200 HV. At 20% cold work, the grain size is ASTM 8–9 (16–32 µm), and the hardness is 190–210 HV. At 30% cold work, the grain size is ASTM 9–10 (11–22 µm), and the hardness is 200–220 HV. At 40% cold work, the grain size is ASTM 10–11 (8–16 µm), and the hardness is 210–230 HV.

Thermal Fatigue Life at 300°C as a Function of Grain Size

Cyclic testing in air at 300°C with a temperature swing of 250°C (50°C to 300°C) over 10,000 cycles has established the following thermal fatigue crack depths for Grade 2 titanium. For ASTM grain size 3–5 (5% cold work), the crack depth after 10,000 cycles is 150–300 µm, and the fatigue life to 1 mm crack is 30,000–50,000 cycles. For ASTM grain size 5–7 (10% cold work), the crack depth is 80–150 µm, and the life to 1 mm is 60,000–100,000 cycles. For ASTM grain size 7–8 (15% cold work), the crack depth is 40–80 µm, and the life to 1 mm is 120,000–200,000 cycles. For ASTM grain size 8–9 (20% cold work), the crack depth is 20–40 µm, and the life to 1 mm is 250,000–400,000 cycles. For ASTM grain size 9–10 (30% cold work), the crack depth is 10–20 µm, and the life to 1 mm exceeds 500,000 cycles.

Cold Work and Grain Size Selection Guide for 300°C Air Service

The following table provides recommendations for prior cold work level and resulting grain size for titanium heater tubes intended for service in air at 300°C, based on expected thermal cycles and desired fatigue life.

Engineering Beyond Cold Work Control

The titanium grade affects recrystallization behavior. Grade 7 recrystallizes at a slightly higher temperature than Grade 2 due to palladium, but the final grain size for a given cold work level is similar. Grade 5 (Ti-6Al-4V) does not fully recrystallize under these conditions. Wall thickness does not directly affect grain size but influences crack propagation time; a 2.0 mm wall with a 20 µm crack has 98% of its wall remaining. The annealing temperature and time also affect grain size; higher temperatures or longer times coarsen grains. For maximum thermal fatigue resistance, cold work above 20% is beneficial, but excessive cold work (above 40%) can cause cracking during drawing.

Making an Informed Specification

For a titanium heater tube operating in air at 300°C with significant thermal cycling, specify a prior cold work level of 20–30% before final annealing. Require a final recrystallized grain size of ASTM 8 or finer. For applications with fewer than 60,000 expected cycles, 10–15% cold work is acceptable. For severe cycling (daily start-stop for 10 years, approximately 3,650 cycles), the difference between ASTM 5 and ASTM 8 is negligible, and standard cold work is sufficient. For critical applications with rapid thermal cycling (many cycles per day), specify 30% cold work and ASTM grain size 9 or finer. By controlling the cold work level prior to annealing, the engineer tailors the grain size to achieve the required thermal fatigue resistance at 300°C.,