20.2 An alloy tie bar in a chemical plant has been designed to withstand a stress, σ, of 25 MN m²² at 620°C. Creep tests carried out on specimens of the alloy under these conditions indicated a steady-state creep rate, &, of 3.1 x 10-12 s-¹. In service it was found that, for 30% of the running time, the stress and temperature increased to 30 MN m²² and 650°C. Calculate the average creep rate under service conditions. It may be assumed that the alloy creeps according to the equation έ = Aose-Q/RT where A and Q are constants, R is the universal gas constant, and T is the absolute temperature. Qhas a value of 160 kJ mol-¹. Answer 6:82×10-125-1