Molybdenum grade superalloys

Molybdenum is a very important alloying element in high performance nickel-based alloys. These alloys fall into two basic classes:

  • corrosion-resistant alloys
  • high temperature alloys. The high temperature alloys can be further subdivided into two categories:
    • solid-solution strengthened
    • age-hardenable

The nominal compositions in weight percent of some of the commercially important alloys of each type are given in Tables 1 and 2.

Corrosion-resistant nickel-based alloys
AlloyUNS NoEN NoNiCrFeMoWCCu
B-3® N10675 2.4600 65** 1.5 1.5 28.5 3* 0.01* 0.2*
C-276 N10276 2.4819 57 16 5 16.0 4 0.01* 0.5*
C-22® N06022 2.4602 56 22 3 13.0 3 0.01* 0.5*
C-2000® N06200 2.4675 59 23 3* 16.0 - 0.01* 1.6
G-30® N06030 2.4603 43 30 15 5.5 2.5 0.03* 2
G-35® N06035 2.4643 58 33.2 2* 8.1 0.6* 0.05* 0.3*
* Maximum ** Minimum

Table 1: Nominal compositions of some corrosion-resistant nickel-based alloys

High temperature alloys
AlloyUNS NoEN NoNiCoFeCrMoWAlTiC
Solid-Solution Strengthened
X N06002 2.4665 47 1.5 18 22 9 0.6 0.5* 0.15* 0.10
S N06635 - 67 2* 3* 16 15 1* 0.25 - 0.02*
625 N06625 2.4856 62 1* 5* 21 9 - 0.4* 0.4* 0.10*
617 N06617 2.4663 54 12.5 1 22 9 - 1.2 0.3 0.07
230® N06230 2.4733 57 5* 3* 22 2 14 0.5* 0.1* 0.10
Age-Hardenable
718 N07718 2.4668 52 1* 19 18 3 - 0.5 0.9 0.05
263 N07263 2.4650 52 20 0.7* 20 6 - 0.6* 2.4* 0.06
282® - - 57 10 1.5* 20 8.5 - 1.5 2.1 0.06
Waspaloy N07001 2.4654 58 13.5 2* 19 4.3 - 1.5 3 0.08
R-41 N07041 2.4973 52 11 5* 19 10 - 1.5 3.1 0.09
242®™ N10242 - 65 1* 2* 8 25 - 0.5* - 0.03*
* Maximum

Table 2: Nominal compositions of some high temperature alloys

In corrosion resistant nickel-based alloys, molybdenum imparts resistance to nonoxidizing environments such as the halide acids (HCl, HBr and HF) and sulfuric acid, for example. Accordingly, the alloy most resistant to these environments is B-3 alloy which contains 28.5% Mo. Molybdenum also acts in conjunction with chromium to provide resistance to localized corrosion attack such as pitting and crevice corrosion. Alloys such as C-22 and C-2000 are particularly resistant to this type of attack.

The corrosion-resistant nickel-based alloys find extensive use in the chemical processing, pharmaceutical, oil & gas, petrochemical and pollution control industries in which highly corrosive environments are very common.

In the case of high temperature alloys, additions of molybdenum are often used to impart resistance to damage caused by high temperature creep. For the solid-solution strengthened alloys, advantage is taken of the fact that molybdenum diffuses very slowly in nickel. Since high temperature creep is generally diffusion controlled, additions of molybdenum are quite effective in reducing creep rates. In the age-hardenable alloys which utilize the precipitation of gamma-prime, Ni 3(Al,TI), molybdenum additions strengthen the martix and reduce the lattice mismatch between the martix and the gamma-prime particles, thereby improving the stability of the precipitates. The effect of molybdenum in 242 alloy is quite unique. Its specific combination of molybdenum and chromium promote the formation of long range ordered Ni 2(Mo,Cr)-type particles which impart significant strength without significantly reducing ductility. Molybdenum additions are also very effective in reducing the coefficient of thermal expansion.

Alloys such as S and 242, which contain large amounts of molybdenum, find use as seal rings in gas turbine engines in order to exploit this effect. The high temperature alloys are extensively used in gas turbine engines for components such as turbine disks, combustors, transition ducts, turbine cases, seal rings, afterburners parts, and thrust reversers. They are also used in applications involving industrial heating, heat treating, mineral processing, heat exchangers, and waste incineration.

Chemical plant

Chemical plants make use of Moly grade nickel base alloys (up to 28%Mo) for their unique corrosion resistance.
(Courtesy of Haynes International)