Inconel MA758 Introduction

Dispersion
Strengthened Alloy

Nickel

Chromium

Corrosion
Resistance

93

Mechanical
Behavior

94

Purity

100

Overview

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INCONEL® alloy MA758 is an oxide-dispersionstrengthened (ODS) nickel-chromium superalloy, made by mechanical alloying. It is a development of the wellestablished aerospace superalloy, INCONEL alloy MA 754, with its chromium content increased to 30%, to improve its performance in corrosive environments at high temperatures. The alloy’s high-temperature strength is enhanced by the use of yttrium oxide for dispersion strengthening.

In addition to its strength and corrosion resistance at high service temperatures, it is readily fabricated and can be hot formed into a range of shaped components for use in thermal processing applications.

INCONEL alloy MA 758 is used in some of the most demanding applications for high-temperature metals and alloys. For example, for furnace skid rails to transport heavy steel billets and slabs, at 1260°C (2300°F) and above, and for the bowl-shaped components used to spin molten glass, at around 1200°C (2200°F), to produce fiberglass for thermal insulation.

It is used in a range of thermal processing applications, from furnace components, such as hearth rollers, to jigs, tools, and other fabrications for supporting workpieces through heat-treatment processes. It has been specified for a fuel atomizer application in diesel engines.

Made by Mechanical Alloying

Mechanical alloying, the process used to produce INCONEL alloy MA 758, is a dry, high-energy ball milling operation that produces composite metal powders with controlled, extremely fine, microstructures. Typically, a mixture of commercially available metal powders, master alloy powders, and a very fine refractory oxide powder (Y2O3 for INCONEL alloy MA 758) is charged into the mill.

During the process, metal powder particles are trapped between the rapidly colliding grinding balls and, under the controlled conditions of mechanical alloying, are cold-welded together. The oxide powder particles are trapped within the composite powders along the weld interfaces between the different composite constituents. The cold-welding leads to the build-up of larger powder particles. This is followed by a fracturing stage in which the larger composite powder particles are broken down. A balance is achieved between the welding and the fracturing so that the overall particle size of the composite powder remains nearly constant. The interplay between the welding and fracturing subdivides and kneads the ingredients to create a uniform distribution of the oxide and other metallic components.

Mechanically alloyed powders are consolidated by placing them in sealed cans for extrusion or hot pressing, followed by conventional hot- and coldworking processes. A final anneal at very high temperatures is required to develop the stable, coarse, grain structure suitable for the most demanding stressrupture applications.

Mechanical Properties

The mechanically alloyed ODS superalloys are well known for their exceptionally high strength at high service temperatures. At high temperatures, the three superalloys are all significantly stronger than the cast alloy.

Corrosion Resistance

Because of its high chromium content and the presence of yttrium particles in the matrix, INCONEL alloy MA 758 has outstanding oxidation resistance.

Environmental degradation at high temperatures involves the destruction of an alloy’s protective scale, followed by penetration by harmful elements; sulfur, oxygen, nitrogen, etc. The exposure of an alloy to molten glass accentuates these problems because of the continuous fluxing of the alloy’s protective scale. INCONEL alloy MA 758 has shown excellent resistance to this type of attack. In a 5-day test at 1200°C (2190°F), the alloy was immersed in a “C” glass (65% SiO2, 8.5% Na2O, 14% CaO, 4% Al2O3, 5% B2O3, 3% MgO). It showed only 0.03 mm (0.001 inches) of attack after 5 days in this extremely aggressive environment.

Physical Properties

Mechanical Properties