SOME CREEP-RUPTURE DATA
ANALYTICS AS APPLIED TO AISI 310S
SUPERALLOY AUSTENITIC
STAINLESS STEEL SHEETS AT
ELEVATED TEMPERATURES AND
HIGH APPLIED STRESSES
A.KANNI RAJ
School of Advanced Sciences, Vel Tech Rangarajan Dr.Sagunthala R&D Institute of Science & Technology 400feet Outer Ring Road, Avadi, Chennai-600062, Tamil Nadu, India
Email : [email protected], Web site : https://www.veltech.edu.in
Abstract : Creep-rupture of AISI 301S stainless steel is analyzed at 973-1073K and 40-150MPa. Various creep curves and creep rate curves are compared to understand the effect of temperature and applied load. Creep deformation and creep fracture mechanisms are manipulated from concerned mechanism maps. Creep deformation follows high temperature climb. Creep activation energy so obtained is 345KJ/mol. It is larger than activation energy for self-diffusion in pure FCC iron (that is 270-311kJ/mol). So, it indicates involvement of alloying elements in dislocation network recovery grain coarsening during deformation. Optical micrographs show wedge cracks and triple-point cracks are fracture mechanisms.
Keywords: Creep-Rupture, AISI 310S Stainless Steel, activation energy, high temperature climb, wedge and
triple-point cracking
1. Introduction
Creep is a slow time-dependent plastic deformation that ultimately causes fracture. It occurs in components that are operating at elevated temperatures under heavy loads. However, it occurs even if applied load is less than yield strength and is possible at all temperature. Temperature need not be above half of the melting point. It is expected to occur at temperature above half of the melting point. Challenge posed by creep is severe in electric power generation equipment, aircraft gas turbines, chemical process plants, supersonic transport, space vehicle, etc. So, materials used in elevated temperatures have to be assessed for creep. As machinery component life is fixed at 10000-100000hours, laboratory tests are conducted for 10000-100000hours by creep test or stress-relaxation. It is time consuming and not cost effective. Hence, stress-rupture or creep-rupture tests are conducted for 100-500hours and data so obtained are extrapolated to 10000-100000hours with the help of Larson-Miller parameter [Rishiraj (2018), Kanniraj (2011), Voicu et al (2009), Kassner et al (2000), and Kanniraj (2013)].
Superalloys and austenitic stainless steels are the most frequently used in structures which operate in creep regime. Preferring an austenitic matrix rather than ferritic matrix is mainly due to higher creep resistance of the former than that of the latter. Main reason for this difference is due to diffusivity of iron which is two orders of magnitude smaller in austenitic matrix than in ferritic matrix. Among various austenitic stainless steels, AISI 310S stainless steel is very important for creep regime operation. It possesses good creep resistance, excellent high temperature strength, corrosion resistance, ductility and micro-structural stability [Kanniraj (2013), Maruyama et al (2007), Monteiro et al (2017), Decicco et al (2005), and Taveres (2009)].
2. Materials & Methods
Creep in ambient air is tested in Mayes TC high sensitivity constant load mechatronics creep testing machines (equipped with a microprocessor control and self-adaptive temperature control). Load is applied to ASTM E8M subsize specimen through the lever beam with lever ratio of 15. Strain measurement is done by attaching linear variable differential transformers to projections specially made in gauge portion of ASTM E8M subsize standard specimens. Time is noted from inbuilt elapsed timer of machine (breaking of specimen stops timer by mechatronics action with help of mercury level switches). Accuracy in measurements are: load ±1N, temperature ±1K, creep strain ±0.002% and time ± 0.1h. Also, chemistry and metallography are not shown for discussion as they are available elsewhere [Kanniraj (2013), Kanniraj (2007), Kanniraj (2018), and Suriyanarayanan et al (1999)]
3. Results & Discussion
Effect of applied stress on creep curves is presented in Figs.1-3, respectively for 973K, 1023K and 1073K, whilst effect of test temperature on creep curves is presented in Figs.4&5 respectively for 75.8MPa and 86.5MPa. Creep curves show all three stages (creep regimes), ie, primary (transient), secondary (steady state) and tertiary (void growth) regimes, except for high stresses. Secondary or steady state creep is not observed for 94.6MPa at 1023K showing minimum creep rate, as opposed to steady state creep rate (for calculation of activation energy and stress exponent). Similarly, steady state creep is not observed for 75.8MPa and 86.5MPa at 1073K. It is conceivable that extended tertiary creep observed in these cases could result primarily from damage associated with grain boundary cavitation. A severe localised deformation is observed at these stress levels. Also, primary creep is absent for low stress levels at all test temperatures.
Fig.1 Creep curves for AISI 310S austenitic stainless steel sheet at 973K.
Fig.2 Creep curves for AISI 310S austenitic stainless steel sheet at 1023K.
Fig.3 Creep curves for AISI 310S austenitic stainless steel sheet at 1073K.
Fig.4 Creep curves for AISI 310S austenitic stainless steel sheet at 75.8MPa.
Fig.5 Creep curves for AISI 310S austenitic stainless steel sheet at 86.5MPa.
Fig.6 Creep curve and creep rate curve of AISI 310S at 1023K and 75.8MPa.
Fig.7 Creep rate curves for AISI 310S stainless steel sheet at 1023K.
Fig.8 Creep rate curves for AISI 310S stainless steel sheet at 75.8MPa.
Creep curves and creep-rate curves provide steady state creep rate (minimum creep rate) and time at fracture (rupture life). Creep rate (minimum or steady state) and rupture life (time to fracture) are related to temperature and stress as per following two Arrhenius-type equations from which the creep activation energy (Qc) and stress exponent (n) can be evaluated. They are ἐs=kσne(-Qc/RT) and tr=k’σ-ne(Qc/RT) where k and k’ are
constants and R is the universal gas constant (R=8.314J/K/mol).
predicted that creep followed high temperature climb mechanism. Monkman-Grant relation (ἐsmtr=C, where ἐs is
steady state creep rate, tr is the rupture life and m and C are constants) is found to be obeyed at all test
temperature with m=0.95 and C=0.05. (Literature range of m and C for austenitic stainless steels are m=0.77-0.93 and C=0.04-0.08.)
Remnant life assessment is done using Larson-Miller parameter master curve. It is evaluated for AISI 310S stainless steel sheet is Larson-Miller Parameter=T[48.7+ln(tr)]= -8683 ln(σ)+99630 where T is the absolute
temperature, tr is rupture life in seconds and s is the stress in MPa. Rupture ductility (er) at an applied stress
increased when the test temperature is increased. Also, rupture ductility at a constant temperature increased when the applied stress (s) is increased. As far as creep-fracture is concerned, it started long before rupture ductility (er) is reached. The reason for the slow occurrence is the continuous degradation of the material which
occurs in three steps: nucleation of cracks, stable growth of cracks and unstable crack growth leading to final fracture. It is generally accepted that intergranular creep fracture occurs by the formation of three alternative types of crack or void. They are intergranualr cavities, intergranular wedge cracks and plastic growth of holes. An optical microscopic examination supports intergranular fracture, branching internal cracks and wedge cracks. But wedge cracks are mainly seen at all test temperatures.
Substructural changes in creep are insensitive to the temperature, and depends on the time and stress. Both initiation and growth of wedge cracks result from grain-boundary sliding. The wedge cracks have formed approximately at 90º to the loading direction as have been reported earlier in stainless steels. Thus, the extensive grain-boundary sliding was responsible for a rapid increase in void/crack formation. The wedge type cracks were observed mainly at triple points. In general, the triple point cracks are nucleated at low strains during creep at relatively high stress levels.
4. Conclusions
Creep curves obtained show all creep regimes when tested at 973K, 1023K and 1073K under 40-150MPa excepting very high stress levels. Monkman-Grant relation is found to be obeyed at all test temperatures. Stress exponent (n) decreases when temperature increases. From observed value of stress exponent (average value of n=6.5), it can be predicted that creep takesplace via high temperature climb mechanism.
Activation energy of creep (Qc=345kJ/mol) is larger than that for self-diffusion (270-311kJ/mol) in pure FCC iron. This indicates that alloying elements are involved in dislocation network recovery grain coarsening during creep. Optical microscopic examination supports presence of intergranular creep fracture, branching cracks and wedge cracks.
References
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