Method for coating heat resistant alloys

Claim 2

carrying out the cementation in a sealed container at an elevated chromizing temperature, whereby a diffusion-bonded chroMium layer is produced on the substrate of said article substantially free from internal oxidation,

Claim 3

The method of claim 1, wherein the heat resistant alloy contains by weight up to about 30 percent of a metal from the group consisting of Cr, Mo and W, the total of these metals not exceeding about 40 percent; the chromium content of the alloy ranging up to about 15 percent; up to about 10 percent by weight of a metal from the group consisting of Cb and Ta; up to about 1 percent Co; up to about 10 percent by weight of a metal from the group consisting of Ti and Al, the total amount of these metals not exceeding about 12 percent; up to about 20 percent cobalt; up to about 2% Mn; up to about 2% Si; up to about 0.1% B; up to about 1% Zr; and the balance at least about 50 percent nickel.

Claim 4

A multi-step pack cementation process for producing an oxidation and sulfidation corrosion resistant coating comprising a top coat of nickel-aluminum on an initial coating which is produced by chromizing by pack cementation in a sealed container the substrate of an article formed of a heat resistant alloy containing by weight up to about 25 percent molybdenum, about 0.5 to 10 percent aluminum as an oxidizable gamma prime forming solute metal, and the balance essentially nickel, the oxidizable solute metal having a relatively higher propensity to oxidize than the chromium coating metal in the cementation pack such that an internally oxidized structure is produced in said coating during pack cementation comprising an oxide dispersion of aluminum due to the presnce of oxygen in the pack confined in the sealed container, said process greatly inhibiting the formation of an internally oxidized structure, said process comprises,

Claim 5

embedding said coated alloy in a second cementation pack containing at least one coating metal,

Claim 6

embedding said coated alloy in a second cementation pack containing aluminum and a buffering metal for controlling the transfer and deposition of said aluminum onto the coated alloy,

Claim 7

and carrying out said second cementation at an elevated coating temperature, whereby to produce a final coating of nickel-chromium-aluminum.

Claim 8

The method of claim 3, wherein the buffering metal is chromium.

Claim 9

and carrying out said second cementation at an elevated coating temprerature to produce a final metal coating different from said chromium layer.

Claim 10

The method of claim 3, wherein the heat resistant alloy contains about 10 to 25 percent molybdenum and about 4 to 10 percent aluminum.

Claim 11

The method of claim 5, wherein the alloy contains approximately 17.5 percent molybdenum, approximately 7.75 percent aluminum and approximately 0.03 percent carbon.

Claim 12

The method of claim 3, wherein the first cementation pack contains by weight about 5 to 15 percent chromium, about 0.25 to 0.75 percent aluminUm, and about 3 to 5 percent nickel.

Claim 13

The method of claim 3, wherein the second cementation pack contains about 5 to 40 percent chromium, 1.25 to 20 percent aluminum, such that the higher range of aluminum is employed with the higher range of chromium and such that the lower range of aluminum is employed with the lower range of chromium, and the balance essentially an inert diluent containing a small but effective amount of a halide energizer.

Claim 14

The method of claim 3, wherein the final coating contains substantial amounts of nickel aluminide.