The stress-strain behaviour of fuel-cell reformer tubes used in a creep-fatigue condition was analyzed by a finite element method. By applying Manson's strain-range-partitioning method to the stress-strain hysteresis loop obtained in this analysis, the fatigue life was estimated for the tubes made of low C-HP-Nb (0.15C-25Cr-35Ni-Nb steel) and HP-BST-M (0.49C-25Cr-35Ni-Nb-Ti steel), which have excellent creep-fatigue properties. Also, the cost efficiency (life/cost) was compared between these materials and a low cost alloy, HK40 (25Cr-20Ni steel). The main results obtained are as follows;
(1) The plastic strain range, Δε
p, is negligibly small in these tubes. Therefore, the total strain range, Δε
t, is nearly equal to the sum of the elastic and creep strain ranges, Δε
e+Δε
c.
(2) The total strain range, Δε
t, is almost constant for a variety of creep constitutive equations. The creep strain range, Δε
c, therefore, can be decreased by using materials with high creep strength, such as HP-BST-M.
(3) A longer tube life can be expected by using HP-BST-M because of its smaller Δε
c and better Δε
ij-
Nij properties than low C-HP-Nb.
(4) The HP-BST-M is more cost-efficient than low cost HK40.
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