Determination of creep curves from multiple hardening-relaxation testing

Abstract

Multiple hardening and relaxation tests have been carried out on a nickel-based superalloy at 650 8C. The stress relaxation curves were obtained at selected strain levels, from which the plastic strain rates spanning three decades were obtained. Creep curves were simulated from the relaxation test results and compared reasonably well with those obtained from the creep tests. This method may be useful in the assessment of creep resistance, particularly during the material development stage.

The stress relaxation and creep behaviour of a manganese-stabilized austenitic stainless steel

“…Stepped stress relaxation tests at 700 uC consisted in loading to a strain level for a period of time (usually 1000 s), followed by stepwise increases in the strain, with each step 0.5 per cent strain at a strain rate of about 4.3610 25 s 21 . These stepped tests are similar to the multiple hardening-relaxation tests reported in reference [5]. The second type of test, namely isostrain stress relaxation tests, at 700 uC or 538 uC involved loading at a strain rate of about 1.56 10 23 s 21 to a single strain level of 0.3 per cent, 1.0 per cent, 1.5 per cent, 2.0 per cent, or 2.5 per cent for 20-28 h. Throughout all tests, displacement and load data were recorded every second from the LVDTequipped extensometer and a load cell.…”

“…Combining this short test time with the fact that the total strain is constant and the accumulated plastic strain is minimal offers the possibility to undertake several relaxation events on one sample without any significant change in the metallurgical state. Stress relaxation results have been used to estimate the creep properties of various elevatedtemperature alloys [4][5][6][7][8]. The creep rupture life for industrial gas turbine blades can be estimated within a factor of 3 via stress relaxation testing [4].…”

“…The creep rupture life for industrial gas turbine blades can be estimated within a factor of 3 via stress relaxation testing [4]. For a nickel superalloy gas turbine disc material the creep curves were computed from stress relaxation tests [5], albeit for creep rupture lives of less than 10 h. Likewise, for a Cr-Mo-V steam turbine rotor steel, stress relaxation tests accurately predicted creep strain rates of about 10 24 h 21 at 137 MPa, but at 100 MPa the strain rate discrepancy is nearly an order of magnitude [6]. Regardless of the accuracy with which stress relaxation tests predict creep strain rates, relatively short-duration (about 1 day) stress relaxation tests revealed the deterioration of the temperature capability of one group of superalloy blades [4], a result that would probably require creep tests of much longer duration.…”

Prediction of creep properties for two nickel-base superalloys from stress relaxation testing

“…Using a procedure analogous to that in [5] stress relaxation results have been used to predict creep properties of elevated temperature alloys [4,[9][10][11][12], with strain rate discrepancies of up to an order of magnitude [11]. Creep curves have been derived from stress relaxation results, albeit for times \10 hours [10]. As previously highlighted [13] creep and stress relaxation represent different deformation histories.…”

Comparison of stress relaxation and creep strain rates for the superalloy IN738LC

“…The validity of equation (2) requires a well-defined Young's modulus at the temperature of interest. Using a procedure analogous to that in reference [5] stress relaxation results have been used to predict creep properties of elevated temperature alloys [6][7][8][9][10], with strain rate discrepancies of up to an order of magnitude [8]. Creep curves have been derived from stress relaxation results, albeit for times , 10 h [7].…”