John P. Shingledecker

U.S. program on materials technology for ultra-supercritical coal power plants

The efficiency of conventional fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO2 emissions has recently provided an additional incentive to increase efficiency. More recently, interest has been evinced in advanced combustion technologies utilizing oxygen instead of air for combustion. The main enabling technology in achieving the above goals is the development of stronger high temperature materials. Extensive research-and-development programs have resulted in numerous high-strength alloys for heavy section piping and for tubing needed to build boilers. The study reported on here is aimed at identifying, evaluating, and qualifying the materials needed for the construction of the critical components of coal-fired boilers that are capable of operating with steam at temperatures of 760 °C (1400 °F) and pressures of 35 MPa (5000 psi). The economic viability of such a plant has been explored. Candidate alloys applicable to various ranges of temperatures have been identified. Stress rupture tests have been completed on the base metal and on welds to a number of alloys. Steamside oxidation tests in an autoclave at 650 °C (1200 °F) and 800 °C (1475 °F) have been completed. Fireside corrosion tests have been conducted under conditions simulating those of waterwalls and superheater/reheater tubes. The weldability and fabricability of the alloys have been investigated. The capabilities of various overlay coatings and diffusion coatings have been examined. This article provides a status report on the progress achieved to date on this project.

CF8C plus: a new high temperature austenitic casting alloy for advanced power systems

Abstract A new cast austenitic stainless steel, CF8C plus, has been developed by Oak Ridge National Laboratory and Caterpillar for a wide range of transportation and energy applications. CF8C plus steel has improved high temperature tensile, creep, fatigue, and creep–fatigue properties compared with standard CF8C steel. Changes to the CF8C steel composition, including additions of Mn and N, result in changes to the solidification behaviour and final microstructure of the alloy, which directly relate to the improved mechanical properties. Additionally, CF8C plus is a relatively inexpensive steel which exhibits good castability. The mechanical properties of the alloy have generated significant interest for the production/design of cast components for diesel engine turbochargers and other exhaust components, natural gas reciprocating engines for distributed power, and turbine end covers and casings for land based turbines. In the present paper, the microstructural evolution of CF8C and CF8C plus are presented in more detail, and the mechanical properties of the alloys are compared with each other and other engineering alloys.

Overview of Creep Strength and Oxidation of Heat-Resistant Alloy Sheets and Foils for Compact Heat Exchangers

The Oak Ridge National Laboratory (ORNL) has been involved in research and development related to improved performance of recuperators for industrial gas turbines since about 1996, and in improving recuperators for advanced microturbines since 2000. Recuperators are compact, high efficiency heat-exchangers that improve the efficiency of smaller gas turbines and microturbines. Recuperators were traditionally made from 347 stainless steel and operated below or close to 650 C, but today are being designed for reliable operation above 700 C. The Department of Energy (DOE) sponsored programs at ORNL have helped defined the failure mechanisms in stainless steel foils, including creep due to fine grain size, accelerated oxidation due to moisture in the hot exhaust gas, and loss of ductility due to aging. ORNL has also been involved in selecting and characterizing commercial heatresistant stainless alloys, like HR120 or the new AL20-25+Nb, that should offer dramatically improved recuperator capability and performance at a reasonable cost. This paper summarizes research on sheets and foils of such alloys over the last few years, and suggests the next likely stages for manufacturing recuperators with upgraded performance for the next generation of larger 200-250 kW advanced microturbines.

Austenitic Stainless Steels and Alloys With Improved High-Temperature Performance for Advanced Microturbine Recuperators

Compact recuperators/heat-exchangers increase the efficiency of both microturbines and smaller industrial gas turbines. Most recuperators today are made from 347 stainless steel and operate well below 700°C. Larger engine sizes, higher exhaust temperatures and alternate fuels all demand recuperator materials with greater performance (creep strength, corrosion resistance) and reliability than 347 steel, especially for temperatures of 700–750°C. The Department of Energy (DOE) sponsors programs at the Oak Ridge National Laboratory (ORNL) to produce and evaluate cost-effective high-temperature recuperator alloys. This paper summarizes the latest high-temperature creep and corrosion data for a commercial 347 steel with modified processing for better creep resistanc, and for advanced commercial alloys with significantly better creep and corrosion resistance, including alloys NF709, HR120. Similar data are also provided on small lab heats of several new ORNL modified stainless steels.Copyright

Alumina-Forming Austenitic Alloys for Advanced Recuperators

Materials selection for thin-walled recuperators has been extensively investigated over the past decade. In the latest generation of recuperated turbine engines, type 347 stainless steel has been replaced by higher alloyed steels and Ni-base chromia-forming alloys. However, high (linear) rates of chromia evaporation in exhaust gas fundamentally limits the oxidation lifetime of these chromia-forming alloys. One solution is to use alumina-forming alloys that are more resistant to this environment. The lower scale growth kinetics and resistance to evaporation in the presence of water vapor suggests an order of magnitude increase in lifetime for alumina-forming alloys. A significant problem with this strategy was the large drop in creep strength with the addition of sufficient Al to form an external alumina scale. However, new Fe-base austenitic compositions have been developed with sufficient strength for this application above 700 C.

Overview of Creep Strength and Oxidation of Heat-Resistant Alloy Sheets and Foils for Compact Heat-Exchangers

The Oak Ridge National Laboratory (ORNL) has been involved in research and development related to improved performance of recuperators for industrial gas turbines since about 1996, and in improving recuperators for advanced microturbines since 2000. Recuperators are compact, high efficiency heat-exchangers that improve the efficiency of smaller gas turbines and microturbines. Recuperators were traditionally made from 347 stainless steel and operated below or close to 650°C, but today are being designed for reliable operation above 700°C. The Department of Energy (DOE) sponsored programs at ORNL have helped defined the failure mechanisms in stainless steel foils, including creep due to fine grain size, accelerated oxidation due to moisture in the hot exhaust gas, and loss of ductility due to aging. ORNL has also been involved in selecting and characterizing commercial heat-resistant stainless alloys, like HR120 or the new AL20-25+Nb, that should offer dramatically improved recuperator capability and performance at a reasonable cost. This paper summarizes research on sheets and foils of such alloys over the last few years, and suggests the next likely stages for manufacturing recuperators with upgraded performance for the next generation of larger 200–250 kW advanced microturbines.Copyright

INVESTIGATION OF A MODIFIED 9CR-1MO (P91) PIPE FAILURE

A modified 9Cr-1Mo feedwater (condensate) line at an Eastman Chemical Company plant failed in January 2005. The line was in continuous service since start-up December 2001 until failure. The Plant Superintendent estimated there were three thermal cycles since start-up, although there may have been as many as 25 thermal cycles during commissioning. Normal operating temperature was 325 F (163 C) and pressure was 1762 psig. The line was steam traced with the tracing activated only when ambient outdoor temperature dropped to 40 F (5 C). A modified 9Cr-1Mo steel (P91) pipe failure in a feedwater line in a chemical plant was investigated. The failure occurred in the vicinity of an elbow produced with socket welds of the pipe to the elbow. Based on metallography and hardness measurements, it was concluded that failure occurred because of an improper post-weld heat treatment of the socket weldment.

Analysis of Creep and Stress Relaxation Data for Ultra-Supercritical Steam Turbine Materials

Development efforts are underway to qualify boiler and turbine materials for the ultra-supercritical (USC) steam conditions of 720/760°C and 31/35 MPa. An evaluation of the creep-rupture and stress relaxation data was performed for the critical turbine components of casing and bolting. The casing materials were Inconel 740, Udimet 500, and CCA617. Time-temperature parametric analysis of the rupture data was performed to determine the average rupture stress for a life of 100,000 hours at temperatures from 700 to 760°C. A multiple heat analysis was used for Inconel 740, and the average stress varied from 88 MPa to 109 MPa at 760°C. For Udimet 500, data for both cast and forged materials were used, and the average rupture stress was approximately the same, greater than 150 MPa at 760°C. The maximum useful temperature for CCA617 is about 730°C based on a 100 MPa/105 hours criterion. The bolting materials were Nimonic 105 and Udimet 500. For Nimonic 105, the initial strain was 0.15% and the calculated relaxed stress at 760°C was 32 MPa (using the Larson Miller parameter). For Udimet 500, the initial strains were 0.2% and 0.3%. The calculated time to a relaxed stress of 47 MPa depended on the time-temperature parameter used, and ranged from 50,000 to 100,000 hours at 760°C for an initial strain of 0.3%.Copyright

Thermal Shock Testing and Analysis of IN617 and Super 304H Samples

The DOE/OCDO sponsored Ultrasupercritical (USC) Steam Boiler Consortium is conducting thermal shock tests on austenitic and nickel-based materials to assess their use in thick-section boiler components. This paper describes the tests on CCA617 (a controlled chemistry version of IN617) and Super 304H thick-walled tubes. Details are given of the metallurgical analyses of the observed cracking in the bore and on the OD of the samples, and of the thermal-mechanical analyses to explain the results. Elastic-plastic and elastic-plastic-creep analyses are used to calculate damage based on rupture life and creep strain accumulation. Fatigue calculations are performed. The results of the metallurgical and mechanical analyses are compared and conclusions drawn as to the accuracy and effectiveness of available high temperature life prediction techniques.© 2007 ASME

Age Induced Gamma Prime Coarsening and Hardness Behavior in Pyromet 31V

Significant increases in efficiencies of natural gas powered reciprocating engine systems (ARES) will likely require increases in engine operating temperatures and pressures. This in turn will place increased demands on the engine’s high temperature exhaust components such as valves, seats, exhaust manifold, and turbocharger housing. Although an alloy’s response to service at time and temperature can be discerned through laboratory study, it is inherently difficult to determine actual temperatures sustained by moving engine components during service. Consequently in these engine systems, service time is known, but the local operating temperature within components is uncertain. Pyrometry provides some measure of temperature, but is limited to measurements of only some surfaces of some components. Computational modeling is used to predict operating temperatures, thermal gradients, and stresses, but it must still be validated from measurements.