Super Alloy Nimonic 81™

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.

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