M50 (AMS 6191) and M50NiL (AMS 6278) High-performance VIM-VAR Melted Bearing Steels for the Aviation Industry

摘要

The history of aircraft turbine engine bearings is one of great improvements in reliability and performance. Today's aerospace engineering challenges demand materials which can operate under conditions of temperature extremes, high loads and harsh environments. Progress in steel has followed two parallel paths. One is in steel composition from low alloys to high-speed, fracture-tough, corrosion-resistant compositions. The other path is in steel quality, from electric arc furnace to vacuum and remelting methods, and forging and inspection techniques to prevent stress-raising flaws. This papers reviews some of the key parameters in the production and development of the aircraft bearing steels M50 (AMS 6191) and M50NiL (AMS 6278). The nominal chemistry of the through-hardening M50 high-speed steel type includes (by weight): 0.83% carbon, 4.20% chromium, 4.25% molybdenum and 1.0% vanadium. Careful balancing of these components combined with VIM-VAR melting procedures leads to the following material properties in operation (minimum): hardness > 61HRc, hot hardness at 300°C > 58 HRc, fracture toughness 15 - 20 Mpam{sup}(-1/2), fatigue strength ～1000MPa (3×10{sup}6 cycles). The nominal chemistry of the surface-hardening M50NiL steel type includes (by weight): 0.13% carbon, 4.10% chromium, 4.40% molybdenum, 1.15% vanadium and 3.40 nickel. M50NiL was developed from M50, and VIM-VAR melting procedures also lead to material properties in operation (minimum) of: case hardness > 61HRc, core hardness ～40-45 HRc, fracture toughness (core) > 50 MPam{sup}(-1/2), fatigue strength ～800 MPa (3×10{sup}6 cycles). Several important parameters, such as the VIM procedure and VIM melt "recipe", electrode teeming including launder tundish system, heat treatment and surface conditioning of the electrodes, VAR procedure and VAR melt recipe including the influence of ingot homogenization, must be optimized following the production process to achieve the abovementioned material properties. Results from more than 30 heats from the modern VIDP furnace at Bohler Edelstahl and more than 60 heats from the VAR are discussed. Special attention is given to the influence of the production method on the size and distribution of primary carbides in the finished product. Also, the effects of non-metallic inclusions on the finished product and methods of preventing this are included in this paper. The results of this melting practice are compared with data and details from the literature and competitor's material and results. Case study of the material in use is considered at the end of this paper.