Creep Properties of Heat Resistant Steels and Superalloys

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Another class of stainless steel alloys is described in U. Patent 4,, issued to Douthett et al. In the alloys described in the ' patent, the concentration of carbon is restricted to 0. The alloys described in the ' patent rely on post casting stress relief heat treatments to improve their mechanical properties. The disclosed system is directed to overcoming one or more of the problems set forth above. The alloy has a creep rupture life exceeding 3, hrs and a minimum creep rate of less than 1 x 10 "3 at a stress of MPa and a temperature of 0 C, when creep tested in the as-cast state under ASTM El 39 test conditions.

The alloy also has a 0. The article also shows no detectable ferromagnetic phases like ferrite or martensite when measured with a measurement device after casting and after high temperature aging for hrs at 0 C.

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The article has a creep rupture life exceeding 20, hrs at a stress of 35 MPa and a temperature of 85O 0 C, when creep tested in the as-cast state under ASTM El 39 test conditions and a creep rupture life exceeding hours and a minimum creep rate less than 5 x 10 3 at a stress of MPa and a temperature of 75O 0 C, when creep tested in the as-cast state under ASTM El 39 test conditions. The present disclosure also discloses a heat and corrosion resistant cast austenitic stainless steel alloy which has a completely austenitic microstructure in the as-cast state.

The alloy includes about 0. Detailed Description. CF8C is the traditional cast equivalent of type stainless steel. The chemistry of CF8C-Plus is based on the composition of CF8C with precise additions of nickel Ni , manganese Mn , and nitrogen N combined with a reduction in silicon Si and adjustments of other minor alloying elements. These alloy modifications were made to improve the high-temperature mechanical properties and the casting characteristics of the CF8C steel using inexpensive alloying elements without the need for post casting heat treatments.

Table I is directed towards the maximum and minimum ranges of the compositional elements made in accordance with the present disclosure. Table I also includes in column labeled "Example alloy" an example of an embodiment of an alloy made in accordance with the present disclosure.

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  • Embodiments covered by the present disclosure include alloys with any subset of compositional ranges falling within the minimum and maximum ranges shown in Table I. It should be noted that allowable ranges of cobalt Co , vanadium V , and titanium Ti may not significantly alter the performance of the resulting material. Specifically, based upon current information, Co may range from 0 to about 5 weight percent, V may range from 0 to about 3 weight percent, and Ti may range from 0 to about 0. To study the effect of these modifications on the mechanical properties and creep behavior of the materials, mechanical testing was carried out, and the test results of the modified alloy named CF8C-Plus samples are compared with those of the traditional CF8C steel alloy.

    Ib shows the microstructure of an exemplary polished and etched as-cast CF8C-Plus alloy. The microstructure of the as-cast CF8C alloy includes an austenite matrix with delta ferrite 10 pools in the interdentrite core regions, and niobium carbide NbC 12 in the interdentritic regions.


    In contrast, the microstructure of the as-cast CF8C-Plus alloy does not show any delta ferrite The CF8C had a ferrite number of about Both these macroscopic measurements and microscopic studies indicate that the CF8C-Plus material in the as-cast state is substantially free of delta ferrite 10 in the as-cast state. To investigate the microstructural evolution of CF8C and CF8C- Plus steel during aging, sand cast keel bars were encapsulated in quartz tubes, evacuated, and backfilled with argon.

    These were aged in an air box furnace at 0 C for 3, hours. These specimens were polished and etched for optical microscopy using an etchant composed of glycerin, hydrochloric acid, nitric acid, and acetic acid having a volumetric ratio of : 1 : 1.

    Comparison of FIGS. XEDS analysis of these regions in the aged material indicates that they are enriched in silicon Si and chromium Cr compared to the delta- ferrite 10 found in the as-cast structure. Based on the chemical composition of the phase and the knowledge that delta-ferrite 10 can transform rapidly to sigma phase 14 in stainless steels, it is concluded that aging for 3, hours at 0 C transforms a majority of the delta ferrite 10 in CF8C steel into sigma phase Electron diffraction patterns from these regions were studied using transmission electron microscopy TEM and confirmed the presence of body centered tetragonal bet sigma phase.

    The structure of CF8C-Plus alloy before and after high temperature aging samples is austenitic with interdendritic carbides No obvious change was observed in carbide size or morphology after aging. These studies indicate that in contrast to CF8C alloy, the CF8C-Plus alloy is substantially free of delta ferrite 10 or sigma phases 14 of steel after high temperature aging at 0 C for 3, hours.

    From the centrifugal castings, tensile, creep, and fatigue specimens were machined in both the hoop and longitudinal orientations. Air creep testing was performed in accordance with ASTM El 39 at constant load in lever-arm type creep machines with extensometers attached to the shoulders of the specimens to measure creep deformation. For the creep-fatigue tests, a strain hold was imposed at maximum tensile strain during the cycle. Table II compares the average tensile properties, namely 0. As seen in the table, the creep rupture life of CF8C-Plus steel is over an order of magnitude higher than that of CF8C steel in all cases.

    The creep ductility of CF8C-Plus steel, both as measured as a percentage change in elongation and percentage change in area, also shows a significant improvement over that of CF8C steel.

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    In most cases, this decrease in minimum creep rate is over an order of magnitude lower than that of CF8C. Table III. At 0 C both materials show similar behavior at high strains, but CF8C-Plus alloys show significant improvement in cycles to failure for the lowest strain ranges. A similar result is found at 0 C. Additionally, low cycle fatigue tests were run at 0 C with an R- ratio of 0 to 0. For these creep-fatigue experiments, a second hold time at the maximum strain 0.

    Table V shows the results for these tests. The effects of further alloying elements in CF8C-Plus material was also studied.

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    Fifteen pound lab-scale heats of CF8C-Plus with minor alloy additions were produced by induction melting with an argon cover gas and cast into graphite blocks mm mm X One heat was cast to the CF8C-Plus composition, and four other heats contained a single alloy addition each. This alloy is used as the baseline to compare the effect of additional alloying elements in the alloy. No post-casting stress-relief or solution annealing treatment was given to these castings. Sign In. Advanced Search. Proceedings Papers Select Year Previous Volume. Close mobile search navigation In This Volume.

    Advanced Heat-Resistant Alloys 9. Computational Modeling of Creep Damage and Fracture 8. Fatigue and Creep-Fatigue Interaction 8. Flaw Assessment at Elevated Temperature 8. Life Assessment of Components 4. Materials Performance in Nuclear Applications 5. Microstructural Changes and Damage Mechanisms Nondestructive Inspection for Creep Damage 6. Parametric Analysis, Statistics and Property Prediction 5. Weldment Behavior, Testing and Modeling 3. Conference Volume Navigation.

    Proceedings Papers

    View Article. Pankiw , D. Topics: Alloys , Creep , Design , Superalloys. Jablonski , Karol K. Viswanathan , Robert Purgert. Maziasz , John P. Shingledecker , P. Carter , R. Gandy , Jonathan Parker. Yokobori, Jr. Sugiura , D. Yoshino , M. Tabuchi , Y. Penny , W. Boswell , Bobby Wright. Douglas , M. Spindler , R. Topics: Copper , Creep , Stress. Swindeman , Sam Y. Zamrik , Phillip J. Yoneyama , K. Kubushiro , H. Topics: Creep , Damage , High temperature. Kelly , Katherine J. Mathew , C. Girish Shastry , S. Latha , K. Bhanu Sankara Rao.