Dec 18 2012
Topics Covered
IntroductionChemical CompositionPhysical PropertiesFabrication and Heat Treatment Machinability Forming Welding Heat Treatment Cold Working AnnealingApplications
Introduction
Super alloys or high performance alloys have the capacity to function at very high temperatures and mechanical stress, and also where high surface stability is required. They can be strengthened by solid-solution hardening, work hardening, and precipitation hardening. They comprise a number of elements in a variety of combinations to achieve the desired result. There are three groups of alloys, namely cobalt-based, nickel-based, and iron-based alloys. The cobalt-based super alloys have higher melting point than that of nickel or iron based alloys.
Super alloy MAR M 918 is a cobalt-based alloy. It comprises chromium and nickel as the major alloying elements. The following datasheet gives an overview of super alloy MAR M 918.
Chemical Composition
The following table shows the chemical composition of super alloy MAR M 918.
| Element |
Content (%) |
| Cobalt, Co |
52 |
| Nickel, Ni |
20 |
| Chromium, Cr |
20 |
| Tantalum, Ta |
4.5 |
| Carbon, C |
0.05 |
| Zirconium, Zr |
0.01 |
Physical Properties
The physical properties of super alloy MAR M 918 are given in the following table.
| Properties |
Metric |
Imperial |
| Density |
8.8 g/cm³ |
0.32 lb/in³ |
| Melting point |
1399°C |
2550°F |
Fabrication and Heat Treatment
Machinability
Super alloy MAR M 918 can be machined using conventional machining methods which are used for iron-based alloys. Commercial coolants are required for performing machining operations. High-speed operations such as grinding, milling or turning, are performed using water-based coolants.
Forming
Super alloy MAR M 918 can be formed using all conventional techniques. Heavy-duty lubricants are required for cold forming.
Welding
Super alloy MAR M 918 is welded using submerged-arc welding, gas-tungsten arc welding, shielded metal-arc welding, and gas metal-arc welding methods.
Heat Treatment
Super alloy MAR M 918 does not respond to heat treatment.
Cold Working
Super alloy MAR M 918 can be cold worked using standard tooling. Soft die materials are used for producing good finish and reducing galling.
Annealing
Super alloy MAR M 918 is annealed at 1191°C (2175°F) for 4 h followed by cooling.
Applications
Super alloy MAR M 918 is mainly used in gas turbine hot section components.