Molybdenum and its applications

Nickel-based alloys

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
Alloy	UNS No	EN No	Ni	Cr	Fe	Mo	W	C	Cu
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
Alloy	UNS No	EN No	Ni	Co	Fe	Cr	Mo	W	Al	Ti	C
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 plants make use of Moly grade nickel base alloys (up to 28%Mo) for their unique corrosion resistance.
(Courtesy of Haynes International)