4.5 Iron, Nickel and Cobalt Heat Resistant Alloys

Heat resistant alloys of iron, nickel and cobalt are used where high temperature performance, particularly creep resistance, is required. These alloys have been typically selected for gas turbine components such as blades, turbine wheels and latter stage compressor disks, which are subjected to long term rotational stresses and high temperatures. Increased understanding of the alloy systems has permitted the upgrading of forgings by mechanical and thermal treatment to satisfy requirements for high strength in applications other than creep resistance, such as low and high cycle fatigue and crack growth resistance.

Such alloys are designed to offer high strength at elevated temperatures. These characteristics, which are desirable in the end product, make forging very difficult. Furthermore, any additive that improves service temperature performance tends to decrease workability. Alloy cleanliness also has a significant effect on hot forgeability.

Alloy selection is generally directed at optimizing one or more of seven properties:

  • Creep strength
  • Tensile strength
  • Low-cycle fatigue response
  • High-cycle fatigue response
  • Fracture toughness
  • Creep rupture behavior
  • Cyclic rupture (creep-fatigue interaction) behavior.

An example of a forged iron-based heat resistance alloy is A286 (AMS 5737). This and similar alloys are forged with practices similar in many respects to those used for 18-8 austenitic stainless grades. Because they are alloyed with reactive elements such as titanium, aluminum, boron, or columbium, they respond to solution and aging cycles similar to the specialty stainless grades.

Cobalt based forging alloys such as L605, Alloy 188 and N-155 continue to be used. S-816 alloys are still used for exhaust valves on gasoline and diesel engines.

The most widely forged true heat resistant alloys are Ni-Cr-Fe-based, such as alloys 718, 706 and 625. More highly alloyed Ni-Cr-Co based materials like Waspaloy, alloy 41 and alloy 500, which are very high strength and very difficult to forge, are less widely used. Forging process for heat resistant alloys are highly refined to control temperatures, strain rate, strain and alloy condition. These controls are necessary to achieve uniform critical properties, such as grain size, and other characteristics after heat treatment.

Some "super-superalloys" such as Rene 95, IN 100, Merl 76 and low-carbon Astroloy are best forged with a more complex process that includes the initial consolidation of compacted billets of powder, followed by sintering, canning, and then hot extrusion to develop the starting billets for forging. This P/M (powder metallurgy) route precedes the use of isothermal or hot die forging process to near-net shapes. These alloys contain less cobalt and more reactive metals like titanium, aluminum, columbium, or tungsten. They tend to form stable carbides that improve creep resistance at higher service temperatures.

Heat resistant alloy forgings and processes are often computer modeled using various commercial codes. Modeling reduces costly tryout and costly inputs, such as material and die preparations, prior to tooling and process development. This practice has led to some remarkable refinements in the forgings processes and quality improvements.

Typical forging grades and nominal compositions are:

Iron Based Heat Resistant Alloy
A286 
AM5737
Nickel-Iron Based Heat Resistant Alloy
     Alloy 901
AMS 5660
Cobalt Based Heat Resistant Alloys
L605  
AMS 5758
188     
AMS 5772
N-155
AMS 5769
Nickel Based Heat Resistant Alloys
Ni-600
AMS 5665
Ni-625
AMS 5666
Ni-706
AMS 570  
Ni-718
AMS 5663
  Ni-X750
AMS 5667
  Waspaloy
AMS 5708
Alloy 41
AMS 5712

Following are nominal compositions for several heat resistant alloys.

Alloy Designation

Principal Element
Percent composition
    AL Cb Co Cr Fe Mo Ni Ti W
A286 Iron 0.2     15 54 1 26 2  
Stellite Cobalt     Bal 30 3 1 23   4
L605 Cobalt     53 20     0   15
S-816 Cobalt     Bal. 20 3 4 20    
IN 901 Nickel 0.2 4   12 36 6 43 3  
Astroloy Nickel 4   17 15   5 55 4  
IN 718 Nickel 1 5   19 18 3 52 3  
Rene 41 Nickel 2   11 19   1 55 3  
Rene 95 Nickel 4 4 8 14   3 61 3 4
Udimet 700 Nickel 4   17 15   5 55 3  
Waspaloy Nickel 1   13 19   4 58 3  
MERL 76 Nickel 5   18 12   3 59 4 v

A comprehensive listing of these alloys and corresponding compositions is given in the ASM Metals Handbook. Forging suppliers should be contacted for their experiences in forging and heat treating these classes of heat resistant alloys.

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Heat resistant alloys of iron, nickel and cobalt are used where high temperature performance, particularly creep resistance, is required. These alloys have been typically selected for gas turbine components such as blades, turbine wheels and latter stage compressor disks, which are subjected to long term rotational stresses and high temperatures. Increased understanding of the alloy systems has permitted the upgrading of forgings by mechanical and thermal treatment to satisfy requirements for high strength in applications other than creep resistance, such as low and high cycle fatigue and crack growth resistance.

Such alloys are designed to offer high strength at elevated temperatures. These characteristics, which are desirable in the end product, make forging very difficult. Furthermore, any additive that improves service temperature performance tends to decrease workability. Alloy cleanliness also has a significant effect on hot forgeability.

Alloy selection is generally directed at optimizing one or more of seven properties:

  • Creep strength
  • Tensile strength
  • Low-cycle fatigue response
  • High-cycle fatigue response
  • Fracture toughness
  • Creep rupture behavior
  • Cyclic rupture (creep-fatigue interaction) behavior.

An example of a forged iron-based heat resistance alloy is A286 (AMS 5737). This and similar alloys are forged with practices similar in many respects to those used for 18-8 austenitic stainless grades. Because they are alloyed with reactive elements such as titanium, aluminum, boron, or columbium, they respond to solution and aging cycles similar to the specialty stainless grades.

Cobalt based forging alloys such as L605, Alloy 188 and N-155 continue to be used. S-816 alloys are still used for exhaust valves on gasoline and diesel engines.

The most widely forged true heat resistant alloys are Ni-Cr-Fe-based, such as alloys 718, 706 and 625. More highly alloyed Ni-Cr-Co based materials like Waspaloy, alloy 41 and alloy 500, which are very high strength and very difficult to forge, are less widely used. Forging process for heat resistant alloys are highly refined to control temperatures, strain rate, strain and alloy condition. These controls are necessary to achieve uniform critical properties, such as grain size, and other characteristics after heat treatment.

Some "super-superalloys" such as Rene 95, IN 100, Merl 76 and low-carbon Astroloy are best forged with a more complex process that includes the initial consolidation of compacted billets of powder, followed by sintering, canning, and then hot extrusion to develop the starting billets for forging. This P/M (powder metallurgy) route precedes the use of isothermal or hot die forging process to near-net shapes. These alloys contain less cobalt and more reactive metals like titanium, aluminum, columbium, or tungsten. They tend to form stable carbides that improve creep resistance at higher service temperatures.

Heat resistant alloy forgings and processes are often computer modeled using various commercial codes. Modeling reduces costly tryout and costly inputs, such as material and die preparations, prior to tooling and process development. This practice has led to some remarkable refinements in the forgings processes and quality improvements.

Typical forging grades and nominal compositions are:

Iron Based Heat Resistant Alloy
A286 
AM5737
Nickel-Iron Based Heat Resistant Alloy
     Alloy 901
AMS 5660
Cobalt Based Heat Resistant Alloys
L605  
AMS 5758
188     
AMS 5772
N-155
AMS 5769
Nickel Based Heat Resistant Alloys
Ni-600
AMS 5665
Ni-625
AMS 5666
Ni-706
AMS 570  
Ni-718
AMS 5663
  Ni-X750
AMS 5667
  Waspaloy
AMS 5708
Alloy 41
AMS 5712

Following are nominal compositions for several heat resistant alloys.

Alloy Designation

Principal Element
Percent composition
    AL Cb Co Cr Fe Mo Ni Ti W
A286 Iron 0.2     15 54 1 26 2  
Stellite Cobalt     Bal 30 3 1 23   4
L605 Cobalt     53 20     0   15
S-816 Cobalt     Bal. 20 3 4 20    
IN 901 Nickel 0.2 4   12 36 6 43 3  
Astroloy Nickel 4   17 15   5 55 4  
IN 718 Nickel 1 5   19 18 3 52 3  
Rene 41 Nickel 2   11 19   1 55 3  
Rene 95 Nickel 4 4 8 14   3 61 3 4
Udimet 700 Nickel 4   17 15   5 55 3  
Waspaloy Nickel 1   13 19   4 58 3  
MERL 76 Nickel 5   18 12   3 59 4 v

A comprehensive listing of these alloys and corresponding compositions is given in the ASM Metals Handbook. Forging suppliers should be contacted for their experiences in forging and heat treating these classes of heat resistant alloys.

Return to Table of Contents

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Heat resistant alloys of iron, nickel and cobalt are used where high temperature performance, particularly creep resistance, is required. These alloys have been typically selected for gas turbine components such as blades, turbine wheels and latter stage compressor disks, which are subjected to long term rotational stresses and high temperatures. Increased understanding of the alloy systems has permitted the upgrading of forgings by mechanical and thermal treatment to satisfy requirements for high strength in applications other than creep resistance, such as low and high cycle fatigue and crack growth resistance.

Such alloys are designed to offer high strength at elevated temperatures. These characteristics, which are desirable in the end product, make forging very difficult. Furthermore, any additive that improves service temperature performance tends to decrease workability. Alloy cleanliness also has a significant effect on hot forgeability.

Alloy selection is generally directed at optimizing one or more of seven properties:

  • Creep strength
  • Tensile strength
  • Low-cycle fatigue response
  • High-cycle fatigue response
  • Fracture toughness
  • Creep rupture behavior
  • Cyclic rupture (creep-fatigue interaction) behavior.

An example of a forged iron-based heat resistance alloy is A286 (AMS 5737). This and similar alloys are forged with practices similar in many respects to those used for 18-8 austenitic stainless grades. Because they are alloyed with reactive elements such as titanium, aluminum, boron, or columbium, they respond to solution and aging cycles similar to the specialty stainless grades.

Cobalt based forging alloys such as L605, Alloy 188 and N-155 continue to be used. S-816 alloys are still used for exhaust valves on gasoline and diesel engines.

The most widely forged true heat resistant alloys are Ni-Cr-Fe-based, such as alloys 718, 706 and 625. More highly alloyed Ni-Cr-Co based materials like Waspaloy, alloy 41 and alloy 500, which are very high strength and very difficult to forge, are less widely used. Forging process for heat resistant alloys are highly refined to control temperatures, strain rate, strain and alloy condition. These controls are necessary to achieve uniform critical properties, such as grain size, and other characteristics after heat treatment.

Some "super-superalloys" such as Rene 95, IN 100, Merl 76 and low-carbon Astroloy are best forged with a more complex process that includes the initial consolidation of compacted billets of powder, followed by sintering, canning, and then hot extrusion to develop the starting billets for forging. This P/M (powder metallurgy) route precedes the use of isothermal or hot die forging process to near-net shapes. These alloys contain less cobalt and more reactive metals like titanium, aluminum, columbium, or tungsten. They tend to form stable carbides that improve creep resistance at higher service temperatures.

Heat resistant alloy forgings and processes are often computer modeled using various commercial codes. Modeling reduces costly tryout and costly inputs, such as material and die preparations, prior to tooling and process development. This practice has led to some remarkable refinements in the forgings processes and quality improvements.

Typical forging grades and nominal compositions are:

Iron Based Heat Resistant Alloy
A286 
AM5737
Nickel-Iron Based Heat Resistant Alloy
     Alloy 901
AMS 5660
Cobalt Based Heat Resistant Alloys
L605  
AMS 5758
188     
AMS 5772
N-155
AMS 5769
Nickel Based Heat Resistant Alloys
Ni-600
AMS 5665
Ni-625
AMS 5666
Ni-706
AMS 570  
Ni-718
AMS 5663
  Ni-X750
AMS 5667
  Waspaloy
AMS 5708
Alloy 41
AMS 5712

Following are nominal compositions for several heat resistant alloys.

Alloy Designation

Principal Element
Percent composition
    AL Cb Co Cr Fe Mo Ni Ti W
A286 Iron 0.2     15 54 1 26 2  
Stellite Cobalt     Bal 30 3 1 23   4
L605 Cobalt     53 20     0   15
S-816 Cobalt     Bal. 20 3 4 20    
IN 901 Nickel 0.2 4   12 36 6 43 3  
Astroloy Nickel 4   17 15   5 55 4  
IN 718 Nickel 1 5   19 18 3 52 3  
Rene 41 Nickel 2   11 19   1 55 3  
Rene 95 Nickel 4 4 8 14   3 61 3 4
Udimet 700 Nickel 4   17 15   5 55 3  
Waspaloy Nickel 1   13 19   4 58 3  
MERL 76 Nickel 5   18 12   3 59 4 v

A comprehensive listing of these alloys and corresponding compositions is given in the ASM Metals Handbook. Forging suppliers should be contacted for their experiences in forging and heat treating these classes of heat resistant alloys.

Return to Table of Contents

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