Dec 12 2012
Topics Covered
Introduction
Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Fabrication and Heat Treatment
Machinability
Forming
Welding
Heat Treatment
Forging
Hot Working
Cold Working
Annealing
Applications
Introduction
Super alloys or high performance alloys contain a number of elements in a variety of combinations to obtain a desired result. These alloys are excellent to use in environments with high temperatures and severe mechanical stress, and also in cases where high surface stability is needed. Super alloys provide good creep and oxidation resistance.
Strengthening of super alloys is performed by work hardening, solid-solution hardening, and precipitation hardening methods.
Nimonic 81™ is a nickel-chromium austenitic high temperature alloy. Addition of aluminum and titanium helps strengthening this alloy.
The following datasheet will provide more details about Nimonic 81™.
Chemical Composition
The following table shows the chemical composition of Nimonic 81™.
|
Element |
Content (%) |
|
Nickel, Ni |
63 |
|
Iron, Fe |
1 |
|
Chromium, Cr |
30 |
|
Molybdenum, Mo |
0.30 |
|
Cobalt, Co |
2 |
|
Titanium, Ti |
1.8 |
|
Aluminum, Al |
0.9 |
|
Silicon, Si |
0.50 |
|
Copper, Cu |
0.20 |
|
Manganese, Mn |
0.50 |
|
Carbon, C |
0.050 |
|
Zirconium, Zr |
0.06 |
|
Sulfur, S |
0.015 |
|
Boron, B |
0.003 |
Physical Properties
The physical properties of Nimonic 81™ are outlined in the following table.
|
Properties |
Metric |
Imperial |
|
Density |
8.06 g/cm³ |
0.291 lb/in³ |
|
Melting point |
1305-1375°C |
2381-2507°F |
Mechanical Properties
The mechanical properties of Nimonic 81™ are provided below.
|
Properties |
Metric |
Imperial |
|
Tensile strength (precipitation hardened, value at room temperature) |
1050 MPa |
152000 psi |
|
Yield strength (@strain 0.200%, precipitation hardened, value at room temperature) |
600 MPa |
87000 psi |
|
Elongation at break (precipitation hardened) |
40% |
40% |
Thermal Properties
The thermal properties of Nimonic 81™ are given below.
|
Properties |
Metric |
Imperial |
|
Thermal expansion co-efficient (@21-100°C/69.8-212°F) |
11.1 µm/m°C |
6.17 µin/in°F |
|
Thermal conductivity |
10.9 W/mK |
75.6 BTU in/hr.ft².°F |
Fabrication and Heat Treatment
Machinability
Traditional machining methods that are used for iron-based alloys can be used for Nimonic 81™. This alloy can be work-hardened during machining. Before cutting is performed, it is recommended that heavy duty machining equipment and tooling should be used so as to reduce chatter or work-hardening of the alloy. During high speed operations such as grinding, turning, or milling, water-base coolants are suitable. The use of heavy lubricants while drilling, tapping, broaching or boring is recommended.
Forming
Nimonic 81™ has good ductility and can be easily formed using all the conventional methods. This alloy is stronger than regular steel; hence it requires a more powerful equipment to complete the forming process. It is recommended that heavy-duty lubricants should be used while cold forming. At the end of this process, all traces of lubricant has to be cleaned off to prevent embrittlement.
Welding
Nimonic 81™ can be welded using the traditional welding methods. Some of the commonly used methods are shielded metal-arc welding, gas metal-arc welding, submerged-arc welding, and gas-tungsten arc welding. It is recommended that matching alloy filler metal should be used and that the weld beads should be slightly convex. Before the welding process begins, the surface to be welded should be cleaned and should be free from oil, paint or crayon stains.
Heat Treatment
Nimonic 81™ has to be solution annealed at 1098°C (2010°F) for 8 h and then air cooled. This process is repeated at 704°C (1300°F) for 16 h and air cooled, so as to complete precipitation hardening process.
Forging
Nimonic 81™ can be forged at 1176-1010°C (2150-1850°F).
Hot Working
Nimonic 81™ can be hot worked at 1176-1010°C (2150-1850°F). It is better to avoid hot working in the range of 982-538°C (1800-1000°F).
Cold Working
Cold working can be performed using standard tooling. It is better to avoid using plain carbon tool steels for forming as they cause galling. To minimize galling and provide a neat finish, soft die materials containing bronze and zinc alloys are recommended. However, the life of the die is short. Likewise, heavy duty lubricants should be used to reduce galling in all forming operations.
Annealing
Nimonic 81™ can be solution annealed at 1098°C (2010°F) for 8 h .
Applications
Nimonic 81™ is mainly used in high temperature components of gas turbines.