CREEP BEHAVIOUR OF TYPE 310 STAINLESS STEEL. PART 2: ANALYSIS BASED ON THE STUDY OF TRANSIENTS AFTER STRESS REDUCTION EXPERIMENTS

摘要

Stress-drop experiments during secondary creep in AISI 310 stainless steel were performed at seven stress levels in the range from 70 to 210MPa, at 700°C. Incubation periods were systematically detected following the stress reductions and their duration measured. Recovery kinetics parameters and friction stress (σ_o) data were determined by extrapolation using a physical equation suggested in literature for the process. The friction stress was found to increase linearly with applied stress in the range from 70 to 120MPa and to reach a constant value σ_o≈ 66 MPa from 120 to 210 MPa. The initial friction stress level of the material was also estimated to be about 6 MPa. Secondary creep rates could be expressed by a single power law relation with n ≈ 3.5 in terms of the effective stress (σ - σ_o). Initial creep rates could also be expressed by the same power law relation in the range from 40 to 140 MPa. Assumptions are put forward for the variation of the friction stress in secondary creep stage, in the interval from 250 to 375 MPa, so that secondary creep rate in the whole interval from 70 to 375 MPa can be expressed by: ε_s=4.943x10~(-14).(σ-σ_o)~(3.85). Normalization of the effective stress by the yield stress σ_(0.05) of the material at 700°C, gives the relation: ε_s=5.86x10~(-6).[(σ-σ_o)/σ_(0.05)]~(3.85) This expression is not fully consistent with the universal equation proposed by Evans and Harrison for secondary creep in metallic materials.