How Does the Cold Work Level (5% vs. 20% Reduction) of a Titanium Heater Tube Prior to Annealing Change the Recrystallized Grain Size and the Subsequent Resistance to Thermal Fatigue Cracking in Air at 300°C?

The cross section of titanium heater tubes is reduced by a certain percentage by cold drawing before final annealing in the production of titanium heater tubes. The driving factor for recrystallization during annealing is the extent of cold work. Large recrystallized grains of ASTM 4–5 (diameter 80–120 μm) are formed at low cold work level of 5%. At a high level of cold work of 20% fine recrystallized grains of ASTM 8-10 (diameter 11-22 µm) are obtained. The fine-grained material in air at 300°C shows 3-5 times stronger resistance to thermal fatigue cracking due to the larger grain boundary area available for the accommodation of cyclic thermal strain and resistance to crack propagation in the smaller grains.

Thermal Fatigue Resistance and Grain Size Effect Mechanism

The titanium tube swells and contracts throughout the thermal cycling at 300°C. Slip and grain boundary sliding accommodate the cyclic strain. Fine-grained material has more grain boundaries per unit volume therefore the strain is more uniform. Crack initiation takes place at slip bands and grain boundaries, the smaller grains decrease the slip length and the stress concentration at the grain boundaries. The rate of fatigue fracture propagation follows da/dN ∝ d^0.5, where d is the grain size. Halving the grain diameter gave a 30% reduction in fracture propagation rate. Besides, fine grains are greater obstacles to crack growth since cracks have to shift direction and dissipate energy.

Quantitative Cold Work versus Recrystallized Grain Size

The following recrystallized grain sizes were determined by controlled annealing of Grade 2 titanium tubes at 700°C for 30 minutes after various degrees of cold work. Recrystallized grain size at 5% cold work is ASTM 3–5 (diameter 50–125 µm) and hardness after annealing is 160–180 HV. The grain size is ASTM 5-7 (32-64 microns) and the hardness is 170-190 HV at 10% cold work. The grain size is ASTM 7-8 (22-45 m) after 15% cold work 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. The grain size at 30% cold work is ASTM 9–10 (11–22 µm) and the hardness is 200–220 HV. Grain size is ASTM 10-11 (8-16 µm) and hardness is 210-230 HV at 40% cold work.

300 °C Thermal Fatigue Life Dependence on Grain Size

Thermal fatigue crack depths for Grade 2 titanium have been determined by cyclic testing in air at 300°C with a temperature swing of 250°C (50°C to 300°C) for 10,000 cycles. The crack depth after 10,000 cycles is 150–300 μm for ASTM grain size 3–5 (5% cold work) and the fatigue life to 1 mm crack is 30,000–50,000 cycles. ASTM grain size 5–7 (10 % cold work) gives crack depth of 80–150 µm and life to 1 mm of 60,000–100,000 cycles. ASTM grain size 7-8 (15% cold work) Crack depth: 40-80 um Life to 1 mm: 120,000-200,000 cycles. For ASTM grain size 8-9 (20% cold work) crack depth is 20-40 micron Life to 1mm is 250,000-400,000 cycles. Crack depth is 10-20 µm for ASTM grain size 9-10 (30% cold work). Life to 1 mm is greater than 500,000 cycles.

Chart of Cold Work and Grain Size Selection for Air Service at 300°C

The table below gives the recommended amount of preceding cold work and the resulting grain size for titanium heater tubes for service in air at 300°C, depending on the estimated number of thermal cycles and the acceptable fatigue life.

Desired Fatigue Life (cycles to 1 mm crack) Expected Thermal Cycles Over Service LifeRecommended Prior Cold Work (%) Resulting Grain Size (ASTM) Expected Crack Depth after 10,000 cycles (µm) <30,000 >30,000 5 3-5 150-300 30,000-60,000 >60,000 10 5-7 80-150 60,000-120,000 >120,000 15 7-8 40-80 120,000-250,000 >250,000 20 8-9 20-40 >250,000 >500,000 30 9-10 10-20 Engineering Beyond Cold Work Control

The recrystallization behavior is dependent on the titanium grade. Grade 7 recrystallizes at a slightly higher temperature than Grade 2 due to the presence of palladium, but the final grain size for a given amount of cold work is similar. Grade 5 (Ti-6Al-4V) does not completely recrystallize under these conditions. Wall thickness does not directly affect grain size, but does affect crack propagation time. A 2.0 mm wall with a 20 µm crack has 98% of its wall remaining. The grain size is also affected by the annealing temperature and duration . A higher temperature or longer time coarsens the grains . Cold work > 20% is excellent for optimal thermal fatigue resistance. Too much cold labor (> 40%) can lead to cracking during drawing.

Writing a Smart Specification

Specify a prior cold work level of 20–30% prior to final annealing for a titanium heater tube operating in air at 300°C with extensive thermal cycling. ASTM 8 or finer ultimate recrystallized grain size required. Cold working of 10 to 15% is acceptable for applications forecast to have less than 60,000 cycles. For severe cycling (daily start-stop for 10 years, ~3,650 cycles), the difference between ASTM 5 and ASTM 8 is negligible and standard cold work is sufficient. • For critical applications where there will be rapid thermal cycling (many cycles per day), specify 30% cold work and ASTM grain size 9 or finer . The engineer controls the amount of cold work before annealing so as to manage the grain size to get the appropriate thermal fatigue resistance at 300°C.