4.2.4 Special Stainless Grades

There are many special grades of stainless steels that are not classified exclusive to any one of the above three families. For example, there is a series of forging alloys that include grades such as 17-4PH, 15-5PH and PH13-8Mo, which are precipitation hardenable (PH) grades. These grades include Cr-Ni additions similar to the austenitic grades but with additions of copper, titanium, columbium and/or aluminum. These additions permit a solution treatment, followed by a suitable aging cycle, to promote a compound strengthening effect.

The alloys are usually "soft martensitic" in the annealed condition and are then transformed to a hard martensitic condition either by an aging cycle or by a sub-zero cooling cycle. Subsequent intermediate temperature treatments or aging cycles then produce the final properties. These alloys are typically capable of achieving tensile and yield strengths higher than the martensitic grades of stainless, yet exhibit resistance to corrosion similar to the 18-8 family of austenitic steels.

There are also a few special purpose stainless grades that are approximately twice as strong as the 304-type austenitic grades and are as corrosion resistant. These are called "duplex" stainless grades. The most common applications are nuclear components. They have higher chromium contents than the PH grades, similar nickel and lower carbon. Other elements are used to enhance strength. The resulting microstructure is a mixture of austenite and ferrite with very little carbide; hence the term "duplex". UNS designatons for the more common grades are S32550, S31803 and S32304.

Major alloy contents for some commonly forged special stainless grades are:

Alloy AMS
Percent Composition
AL
Cb
Cr
Cu
Mo
Ni
17-4PH 5643
 
0.3
17
    
4
17-7PH A7051
  
0.3
15
3.5
    
5
15-7PH A7051
1
 
15
   
3
7
15-5PH 5659
1
 
15
3
   
5
PH13-8Mo 5629
1
 
13
 
 2
8
22052 S318033
 
 
22
  
3
5.5

1ASTM specification
2Also contains 0.2% N
3UNS specification (duplex-stainless)

There is a grade in this family called AM-355; it contains 16% Cr, 5% Ni, 3% Mo, and is heat treated with slightly different cycles from the PH grades. This grade has continued to be offered as a forging grade. SAE and AISI guides and UNS designations list several other grades that have special purposes and, while not commonly forged, are just as forgeable as these grades.

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There are many special grades of stainless steels that are not classified exclusive to any one of the above three families. For example, there is a series of forging alloys that include grades such as 17-4PH, 15-5PH and PH13-8Mo, which are precipitation hardenable (PH) grades. These grades include Cr-Ni additions similar to the austenitic grades but with additions of copper, titanium, columbium and/or aluminum. These additions permit a solution treatment, followed by a suitable aging cycle, to promote a compound strengthening effect.

The alloys are usually "soft martensitic" in the annealed condition and are then transformed to a hard martensitic condition either by an aging cycle or by a sub-zero cooling cycle. Subsequent intermediate temperature treatments or aging cycles then produce the final properties. These alloys are typically capable of achieving tensile and yield strengths higher than the martensitic grades of stainless, yet exhibit resistance to corrosion similar to the 18-8 family of austenitic steels.

There are also a few special purpose stainless grades that are approximately twice as strong as the 304-type austenitic grades and are as corrosion resistant. These are called "duplex" stainless grades. The most common applications are nuclear components. They have higher chromium contents than the PH grades, similar nickel and lower carbon. Other elements are used to enhance strength. The resulting microstructure is a mixture of austenite and ferrite with very little carbide; hence the term "duplex". UNS designatons for the more common grades are S32550, S31803 and S32304.

Major alloy contents for some commonly forged special stainless grades are:

Alloy AMS
Percent Composition
AL
Cb
Cr
Cu
Mo
Ni
17-4PH 5643
 
0.3
17
    
4
17-7PH A7051
  
0.3
15
3.5
    
5
15-7PH A7051
1
 
15
   
3
7
15-5PH 5659
1
 
15
3
   
5
PH13-8Mo 5629
1
 
13
 
 2
8
22052 S318033
 
 
22
  
3
5.5

1ASTM specification
2Also contains 0.2% N
3UNS specification (duplex-stainless)

There is a grade in this family called AM-355; it contains 16% Cr, 5% Ni, 3% Mo, and is heat treated with slightly different cycles from the PH grades. This grade has continued to be offered as a forging grade. SAE and AISI guides and UNS designations list several other grades that have special purposes and, while not commonly forged, are just as forgeable as these grades.

Return to Table of Contents

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There are many special grades of stainless steels that are not classified exclusive to any one of the above three families. For example, there is a series of forging alloys that include grades such as 17-4PH, 15-5PH and PH13-8Mo, which are precipitation hardenable (PH) grades. These grades include Cr-Ni additions similar to the austenitic grades but with additions of copper, titanium, columbium and/or aluminum. These additions permit a solution treatment, followed by a suitable aging cycle, to promote a compound strengthening effect.

The alloys are usually "soft martensitic" in the annealed condition and are then transformed to a hard martensitic condition either by an aging cycle or by a sub-zero cooling cycle. Subsequent intermediate temperature treatments or aging cycles then produce the final properties. These alloys are typically capable of achieving tensile and yield strengths higher than the martensitic grades of stainless, yet exhibit resistance to corrosion similar to the 18-8 family of austenitic steels.

There are also a few special purpose stainless grades that are approximately twice as strong as the 304-type austenitic grades and are as corrosion resistant. These are called "duplex" stainless grades. The most common applications are nuclear components. They have higher chromium contents than the PH grades, similar nickel and lower carbon. Other elements are used to enhance strength. The resulting microstructure is a mixture of austenite and ferrite with very little carbide; hence the term "duplex". UNS designatons for the more common grades are S32550, S31803 and S32304.

Major alloy contents for some commonly forged special stainless grades are:

Alloy AMS
Percent Composition
AL
Cb
Cr
Cu
Mo
Ni
17-4PH 5643
 
0.3
17
    
4
17-7PH A7051
  
0.3
15
3.5
    
5
15-7PH A7051
1
 
15
   
3
7
15-5PH 5659
1
 
15
3
   
5
PH13-8Mo 5629
1
 
13
 
 2
8
22052 S318033
 
 
22
  
3
5.5

1ASTM specification
2Also contains 0.2% N
3UNS specification (duplex-stainless)

There is a grade in this family called AM-355; it contains 16% Cr, 5% Ni, 3% Mo, and is heat treated with slightly different cycles from the PH grades. This grade has continued to be offered as a forging grade. SAE and AISI guides and UNS designations list several other grades that have special purposes and, while not commonly forged, are just as forgeable as these grades.

Return to Table of Contents

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