Hybrid materials design to control creep in metallic pipes

• Adjusting reinforcement angle (θ) controls multiaxial creep strains in pipes.
• Hoop and longitudinal stresses are balanced with a 54.7° reinforcement angle (θN).
• Braid reinforcement orientation changes with time to approach θN.
• Tungsten braid (53°) reinforced 253MA pipe gives a 300-times creep life extension.

A hybrid material has been developed to improve creep performance in pressurized metallic pipes subjected to high-temperatures. Model materials were selected for an investigation of reinforcement design parameters in architectured materials. Brass pipes (65 wt.% Cu/35 wt.% Zn) with austenitic stainless steel reinforcement were pressurized and creep rupture tested at 673 K. Compared to unreinforced pipes of equal dimensions, a 47-times reduction in the effective strain rate was observed with a 50° reinforcement angle. A ‘neutral angle’ of 54.7 ± 1.5° was determined experimentally, where tangential (hoop) and longitudinal stresses on the pipe can be balanced and strains minimized. For initial angles below the neutral angle, creep strain was shown to facilitate a shift in orientation towards the neutral angle. For an initial angle of 42°, this shift towards the neutral angle resulted in instantaneous creep rate dropping from 170% of the mean creep rate to 60% of the mean creep rate over 820 h, when the final angle was measured to be 50°. A high-temperature prototype (tungsten braid oriented at 53° over a 253MA stainless steel pipe) was shown to give a creep life extension in excess of 300-times at 1313 K.

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