Ken Harris

Development and Turbine Engine Performance of Three Advanced Rhenium Containing Superalloys for Single Crystal and Directionally Solidified Blades and Vanes

Turbine inlet temperatures over the next few years will approach 1650°C (3000°F) at maximum power for the latest large commercial turbo fan engines, resulting in high fuel efficiency and thrust levels approaching 445 kN (100,000 lbs). High reliability and durability must be intrinsically designed into these turbine engines to meet operating economic targets and ETOPS certification requirements.This level of performance has been brought about by a combination of advances in air cooling for turbine blades and vanes, design technology for stresses and airflow, single crystal and directionally solidified casting process improvements and the development and use of rhenium (Re) containing high γ′ volume fraction nickel-base superalloys with advanced coatings, including full-airfoil ceramic thermal barrier coatings. Re additions to cast airfoil superalloys not only improve creep and thermo-mechanical fatigue strength but also environmental properties, including coating performance. Re dramatically slows down diffusion in these alloys at high operating temperatures.A team approach has been used to develop a family of two nickel-base single crystal alloys (CMSX-4® containing 3% Re and CMSX®−10 containing 6% Re) and a directionally solidified, columnar grain nickel-base alloy (CM 186 LC® containing 3% Re) for a variety of turbine engine applications. A range of critical properties of these alloys is reviewed in relation to turbine component engineering performance through engine certification testing and service experience.Industrial turbines are now commencing to use this aero developed turbine technology in both small and large frame units in addition to aero-derivative industrial engines. These applications are demanding, with high reliability required for turbine airfoils out to 25,000 hours, with perhaps greater than 50% of the time spent at maximum power. Combined cycle efficiencies of large frame industrial engines is scheduled to reach 60% in the U.S. ATS programme. Application experience to a total 1.3 million engine hours and 28,000 hours individual blade set service for CMSX-4 first stage turbine blades is reviewed for a small frame industrial engine.© 1997 ASME

CM 186 LC® Alloy Single Crystal Turbine Vanes

There is a need to introduce advanced turbine technology at reduced cost. SX superalloy vanes demonstrate excellent engine performance and durability benefits compared to their polycrystalline counterparts. However, their manufacturing cost can be prohibitive due to low casting and solution heat treatment yields due to rejectable grain defects. High purity (carbon and boron free), ultra high creep and fatigue strength SX alloys are limited to low angle boundaries (LABs) normally not exceeding 6° in critical airfoil locations. Carbon (C) and boron (B) containing SX superalloys (Ross, et al., 1996) can accommodate low angle boundaries in the 9°–12° range with an overall sacrifice in creep and fatigue properties. Aero engine vane segments with complex configurations, can result in not only LAB defects exceeding 9°–12° but also high angle grain boundary (HAB) defects ≥ 15° occurring during the SX solidification process. This is further excaberated by recrystallised grains occurring during solution heat treatment from residual casting stresses and associated strain.CM 186 LC® is a hafnium (Hf) containing nickel-base superalloy developed for directionally solidified (DS) columnar grain turbine airfoils. SX casting experience — development and production — has shown the alloy can be readily cast into aero turbine multi-airfoil segments. Mechanical property and turbine engine testing show the alloy can accommodate grain boundaries at least up to 30° resulting in high SX casting yields. The SX vane components are either used as-cast or approximately 50% partial solutioned which avoid any recrystallisation (Rx) problems. Component costs can be < 50% of that of a conventional high purity SX alloy.Mechanical property, oxidation and coating performance characterisation studies on SX CM 186 LC (including DS test pieces) and turbine engine test and application experience show a 72°F (40°C) metal temperature capability improvement (thin wall) over DS MAR M 002 alloy.Copyright

Extensive Industrial Gas Turbine Experience With Second Generation Single Crystal Alloy Turbine Blades

The rhenium containing second generation single crystal alloy CMSX-4® was introduced for turbine blading in the Solar® Turbines Incorporated Mars® T-14000 engine in 1990. Based on the initial success with the first 4,000 hour engine test, the alloy was confirmed as bill of material for the stage 1 turbine blades in the T-14000 engine. Field experience has been excellent and there are now about 1,500,000 hours total accumulated engine time with several engines having completed their first overhaul cycle (Typically 30,000 hours). Use of this alloy is now being considered by Solar for other turbine airfoil components.This update provides the results of metallurgical evaluations on high time blades. A detailed analysis of the substrate and coating, microstructure following the 25,000 hours field exposure was conducted and the results reported herein. A brief discussion of the effect of PtAl coatings on fatigue properties is also included.Copyright

CM 939 Weldable® Alloy

IN 939 alloy, developed by the International Nickel Co. in the late 1960s, is a unique 22% Cr hot corrosion resistant γ′ strengthened, cast nickel-base superalloy. It is widely used in industrial gas turbines for equiaxed vanes, vane segments and burner nozzles and is of interest to the aero turbine industry for LP and PT integral nozzles (vane rings) and high temperature turbine casings. However, IN 939 is considered difficult to weld repair without parent metal microcracking and can exhibit marginal ductility in heavy section castings. Cannon-Muskegon has developed a proprietary chemistry modified version of IN 939 alloy designated CM 939 Weldable® . Emphasis has been directed on optimizing aim chemistry and ultra high purity manufacture using CM-developed single crystal superalloy melting and raw material technology and subsequently on obtaining superior casting microstructure for improved weldability and mechanical properties. Alloy purity and heat cleanliness will be discussed, along with a simplified two-step heat treatment cycle, resulting in attractive microstructure, mechanical properties and repair weldability. Significant market interest has resulted in extensive vacuum casting experience throughout the gas turbine industry. Excellent results in terms of fluidity, casting cleanliness and minimal microporosity have been obtained without any hot tearing or hot cracking problems.Copyright

CMSX-486® Alloy Update

Modern turbine engine performance and life cycle requirements demand single crystal (SX) superalloy turbine airfoil and seal components. However, complex SX components, such as vane segments, can result in severe manufacturing cost challenges due to low manufacturing yield. As presented at TURBO EXPO 2002 and 2006, these requirements led to the development of CMSX-486® alloy, a grain boundary strengthened SX superalloy with improved creep-rupture strength over SX CM 186 LC® alloy. This paper will review the unique properties that make this alloy desirable, with particular attention to ongoing developments. Significant market interest has resulted in additional property evaluation, including strain-controlled low cycle fatigue testing which has produced fatigue lives similar to HIP’ed and solutioned CMSX-4® SX alloy at 1038°C. This was surprising considering the non-homogeneous microstructure of CMSX-486 alloy, which is used in the as-cast + double aged heat treat condition. Also, burner rig dynamic, cyclic oxidation and Type I hot corrosion results will be presented for CMSX-486 (SLS) [La+Y] alloy in comparison to CMSX-4, CMSX-4(SLS)[La+Y] and CMSX-486 alloys. Scanning electron microscopy analysis shows residual sulfur and phosphorus in CMSX-486 (SLS) [La+Y] are tied up as Y and La carbo-sulfides and phosphides.Copyright

CMSX-486® Single Crystal Alloy: Production Experience and Development of an Improved Version

Modern turbine engine performance and life cycle requirements demand single crystal (SX) superalloy turbine airfoil and seal components. However, complex SX components, such as vane segments, can result in severe manufacturing cost challenges due to low manufacturing yield. These requirements led to the development of CMSX-486® alloy, a grain boundary strengthened SX superalloy with improved creep-rupture strength over SX CM 186 LC® alloy. CMSX-486 alloy has excellent casting yield achieved through generous grain inspection criteria and is used as-cast, which minimizes post-cast processing costs and eliminates the risk of recrystallization during solution heat treatment. CMSX-486 alloy has attained production status and further improvements to the alloy are under evaluation. This paper will review the unique properties which make this alloy of serious interest, with particular attention to ongoing production experience. Discussion will also include direction and results of an improved oxidation resistant version of CMSX-486 alloy which is currently under development.Copyright

CMSX®-486: A New Grain Boundary Strengthened Single Crystal Superalloy

Modern turbine engine performance and life cycle requirements demand single crystal superalloy turbine airfoil and seal components. Complex SX vane segments can result in severe manufacturing cost challenges. This has resulted in the development of an improved creep-rapture strength SX superalloy designated CMSX® -486. This alloy is grain boundary strengthened through optimised additions of boron, carbon, hafnium and zirconium and is designed for use as-cast, to maximise complex casting yield through the use of generous grain specifications and without solution heat treatment recrystallisation problems. The paper in particular gives comprehensive low and high angle boundary creep-rupture properties of CMSX-486, enabling grain specifications to be developed depending upon component design requirements. The alloy has now entered its turbine application development phase.Copyright

Improved Performance Rhenium Containing Single Crystal Alloy Turbine Blades Utilising PPM Levels of the Highly Reactive Elements Lanthanum and Yttrium

Turbine inlet temperatures have now approached 1650°C (3000°F) at maximum power for the latest large commercial turbofan engines, resulting in high fuel efficiency and thrust levels approaching or exceeding 445 kN (100,000 lbs.). High reliability and durability must be intrinsically designed into these turbine engines to meet operating economic targets and ETOPS certification requirements.This level of performance has been brought about by a combination of advances in air cooling for turbine blades and vanes, computerized design technology for stresses and airflow and the development and application of rhenium (Re) containing, high γ′ volume fraction nickel-base single crystal superalloys, with advanced coatings, including prime-reliant ceramic thermal barrier coatings (TBCs). Re additions to cast airfoil superalloys not only improve creep and thermo-mechanical fatigue strength but also environmental properties, including coating performance. Re slows down diffusion in these alloys at high operating temperatures.(1)At high gas temperatures, several issues are critical to turbine engine performance retention, blade life and integrity. These are tip oxidation in particular for shroudless blades, internal oxidation for lightly cooled turbine blades and TBC adherence to both the airfoil and tip seal liner. It is now known that sulfur (S) at levels 0.2 ppm in these alloys reduces the adherence of α alumina protective scales on these materials or their coatings by weakening the Van der Waal’s bond between the scale and the alloy substrate. A team approach has been used to develop an improvement to CMSX-4® alloy which contains 3% Re, by reducing S and phosphorus (P) levels in the alloy to 0.2 ppm residual S in the alloy as very stable Y and La sulfides and oxysulfides, thus preventing diffusion of the S atoms to the alumina scale layer under high temperature, cyclic oxidising conditions. La also forms a stable phosphide.CMSX-4 (ULS) [La + Y] HP shroudless turbine blades will commence engine testing in May 1998.Copyright

New Single Crystal Superalloys, CMSX®-8 and CMSX®-7

Single crystal (SX) superalloys have wide application in the high pressure turbine section of aero and industrial gas turbine engines due to the unique combination of properties and performance. Since introduction of single crystal casting technology, SX alloy development has focused on increased temperature capability, and major improvements in alloy performance have been associated with the introduction of new alloying elements, including rhenium (Re) and ruthenium (Ru). 3% Re-containing second generation alloys, such as CMSX-4®, PWA 1484 and Rene’ N5 have seen the greatest market utilization and have become the benchmark alloys for comparing new alloy developments. However, Re and Ru are rare elements with limited production/availability and corresponding high costs. This has resulted in significant escalation of SX alloy costs, and consequently, there has been much interest in the development of improved SX superalloys with lower Re or no Re content compared to second generation alloys.Cannon-Muskegon® has developed two new SX superalloys: 1.5% Re CMSX®−8 alloy and CMSX®−7 alloy, which contains no Re, as alternatives to first and second generation alloys for applications which require slightly less ultra high temperature capability compared to CMSX-4 alloy or the improved CMSX-4(SLS) alloy. This paper provides an overview of development and characterization of these SX alloys and alloy modifications, CMSX-8 (SLS) and CMSX-8[B/C](SLS).Copyright

An Overview of Advanced Ni-Base Superalloys for Small Turbines and Missile Engine Applications

Ni-base superalloy cast materials provide an outstanding balance of high temperature strength, fatigue resistance, oxidation resistance and coating performance and can be produced to very tight tolerances in extremely complex configurations, such as axial and centrifugal integral cast turbine wheels. As a result, cast superalloys are used in the most demanding applications of aero and industrial gas turbine engines. Use of these materials is expanding to smaller microturbine, turbojet, turbocharger and missile engine applications due to this unique combination of desirable properties. This paper will present an overview of the application of investment cast Ni-base superalloys and process capability to small turbine and missile engines. Alloy property data will be presented, including advanced equiax alloys CM 247 LC® , CM 681 LC® and CM 939 Weldable® , directionally solidified alloys CM 247 LC and CM 186 LC® and single crystal alloy CMSX-4® , along with typical or candidate applications for each.Copyright