Feb_March_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 8 2 7 LIFE CYCLE IMPROVEMENT EXAMPLES Fossil fuel power generation effi- ciency can be improved by working at a very high steam-turbine inlet tem- perature and pressure, much beyond the critical point of water. Supercrit- ical power generation units feature once-through boilers designed to oper- ate at pressures from 24-28 MPa versus 12-17 MPa (3500-4000 psi versus 1700- 2500 psi) for subcritical boilers at operating temperatures of 565° and 540°C (1050° and 1000°F), respective- ly. Ultra-supercritical boilers operate at 700°C (1290°F) and higher with an efficiency of 55% compared with that of 27% for conventional subcritical boilers. Development of materials and coatings with improved creep and high-temperature corrosion resistan- ce enable operation in these condi- tions. Boiler-tube materials include 9Cr ferritic/martensitic steels P91 (1C- 9Cr-0.95Mo-0.2V-0.8Nb-0.05N) and P92 (0.11C-9Cr-0.45Mo-1.75W-0.2V-0.06Nb- 0.004B-0.05N). Corrosion and wear re- sistant coating materials including Al 2 O 3 -forming iron aluminide, SiO 2 - forming Mo-Si-B, Cr 2 O 3 -forming Ni-50% applied by thermal spray, and high ve- locity laser-accelerated deposition (HVLAD) [3] of Ni-50Cr can further im- prove tube life cycle compared with other coatings. Major clean power generating pro- cesses include hydro and wind pow- er. Hydroturbines in the Himalayan regions face severe erosion due to high silt in the running river water. Silt par- ticles (mainly quartz) impact runner blades causing rapid growth of rip- ples. These rough rippled surfaces re- sist the flow of slurry, leading to loss of input energy and heavy abrasive wear followed by thinning of leading edges and fracture. Excessive wear removes half of the runner blades in six months. Re-blading takes three months, and the loss in power generation can be as high as 50%. Long-term wear simula- tion test results show improvement in life cycle after weld surfacing with Stellite 6 (Co-1C-28Cr-4W), 15Cr-15Mn steel, and 316L stainless steel over a 410 stainless steel base material (equiv- alent to the original cast CA6NM base material) [4] . Wind power is one of the fastest growing renewable energy technolo- gies. A major tribological challenge is frequent premature failure of the gear- box, which is central to transferring the wind energy captured by the blades to the electrical generator that converts it into power. Average gearbox life is five years compared with a 20-year design life [5] . Surface engineering processes are used to increase reliability of drivetrain components, including ultrafast (UF) boriding, diamond-like coatings (DLCs), and nano-boron lubrication technol- ogy [1] . For example, UF boriding more than doubles gear and bearing surface hardness over conventional carburizing (1800 versus 600 HK). Borided surface sliding wear performance is improved by an order of magnitude compared with that of carburized gears [1] . Automotive components suffer high efficiency losses due to wear and friction. For example, engine valves are subjected to combined thermal fa- tigue, erosion, and corrosion. Wear on the sealing face of the valve ob- structs throughflow and thus decreas- es the effective cylinder output and ultimately reduces performance. Valve materials are mainly austenitic stain- less steels, such as 21-4N and 21-2N. Coating the face with Stellite 6 mini- mizes erosion-corrosion wear and im- proves thermal fatigue life by tenfold [6] . Alternative coatings include Stellite 32 (Co-1.75C-25Cr-11.8W-22Ni) and Ni-11 (Ni-0.45C-3.5Si-16.7Cr-12.5Fe). Plasma transferred arc welding using powder alloy is the preferred coating process due to low dilution deposits [7] . Railroad frogs, crossings where train wheels are guided to change track, undergo severe surface fatigue wear to the front triangular nose por- tion and part of the side rails due to re- petitive impact loading as the wheels jump to another track. Failure occurs due to rolling contact fatigue result- ing in crack initiation and growth in the subsurface region, causing material re- moval by flaking. Frogs require regu- lar repair and resurfacing to build up worn areas for safe changeover with- out derailment. The life cycle of a frog is expressed in terms of gross million tons (GMT) carried over a year until the nose portion height decreases to a specified limit. Results of wear simu- lation tests on a 15Cr-15Mn austenit- ic manganese steel weld overlay and a weld deposit of modified AISI 410 in- crease the life span to 19 and 42 GMT, respectively, compared with that of rail steel (10 GMT) [8, 9] . A life span of 42 GMT means that the frog is capable of carry- ing 4.2 times the load in one year com- pared with a rail steel frog, or carrying 10 GMT per year for 4.2 years. Increasing aerospace industry de- mand for higher power and improved specific fuel consumption in aircraft jet engines requires a higher turbine entry temperature (TET) of hot gas af- ter combustion, leading to a new gen- eration of aircraft with TET exceed- ing 1800 K (1530°C or 2780°F ) during Fig. 3 — Properties of surface-modified gear teeth. (1) Compressive residual stress generated in (2) high-carbon case depth of sufficient thickness with (3) martensitic structure and (4) suitable surface finish. The high hardness (5) of the martensitic structure plus smooth surface finish enable gear teeth surfaces to resist wear, friction, scuffing, and fatigue failure in service [1] .