Heat-resistant alloys (cast)

Cast alloys suitable for use at service temperatures to at least 538°C and, for some alloys, to 1093°C are classed as "heat-resistant" or "high-temperature" alloys. They have the characteristic of corroding at very slow rates compared with unalloyed, or low-alloy cast iron or steel in the atmospheres to which they are exposed, and they offer sufficient strength at operating temperature to be useful as load-carrying engineering structures. Iron-base and nickel-base alloys comprise the bulk of production, but cobalt-base, chromium-base, molybdenum-base, and columbium-base alloys are also made.

Although some cast heat-resistant alloys are available in compositions similar to wrought alloys, it is necessary to differentiate between them. Cast alloys are made to somewhat different chemical specifications than wrought alloys; physical and mechanical properties for each group are also somewhat different.

For these reasons, it is advisable to follow the alloys designated as the H-Series by the Alloy Casting Institute of the American Steel

Founders Society as well as nickel-base alloys and cobalt-base alloys. Most of the nickel-base and cobalt-base alloys are also known as super-alloys because of their exceptional high-temperature stress-rupture strength and creep resistance as well as corrosion and oxidation resistance.

There are, moreover, a number of heat-resistant cast alloys that are not available in wrought form; this is frequently of advantage in meeting special conditions of high-temperature service. In addition to the grades HA to HX discussed below, the industry produces special heat-resistant compositions. Many of these are modifications of the standard types, but some are wholly different and are designed to meet unique service conditions.

Selection of a particular alloy, of course, is dependent upon the application, and in this article composition, structure, and properties of the various cast heat-resistant alloys are discussed from this point of view.

Proper selection of an alloy for a specific high-temperature service involves consideration of some or all of the following factors: (1) required life of the part, (2) range and speed of temperature cycling, (3) the atmosphere and its contaminants, (4) complexity of casting design, and (5) further fabrication of the casting. The criteria that should be used to compare alloys depend on the factors enumerated, and the designer will be aided in the choice by providing the foundry with as much pertinent information as possible on intended operating conditions before reaching a definite decision to use a particular alloy type.

Physical and Mechanical Properties

For high-temperature design purposes a frequently used design stress is 50% of the stress that will produce a creep rate of 0.0001%/h maximum operating temperature. Such a value should be applied only under conditions of direct axial static loading and essentially uniform temperature or slow temperature variation. When impact loading or rapid temperature cycles are involved, a considerably lower percentage of the limiting creep stress should be used. In the selection of design stresses, safety factors should be higher if the parts are inaccessible, nonuniformly loaded, or of complex design; they may be lower if the parts are accessible for replacement, fully supported or rotating, and of simple design with little or no thermal gradient.

H-Series Cast Alloys

The H-Series cast alloys include iron-chromium, iron-chromium-nickel, and iron-nickel-chromium alloys also containing 0.20 to 0.75% carbon, 1 to 2.5% silicon, and 0.35 to 2% manganese. A letter (A to X) following the H is used to distinguish alloy compositions more closely. The iron-chromium cast alloys (HA, HC, and HD) contain as much as 30% chromium and under 7% nickel. The iron-chromium-nickel cast alloys (HE, HF, HH, HI, HK, and HL) contain as much as 32% chromium and 22% nickel. And the iron-nickel-chromium cast alloys (HN, HP, HP-50WZ, HT, HU, HW, and HX) contain as much as 68% nickel (HX) and 32% chromium (HN) so that some of these alloys are actually nickel-base instead of iron-base alloys.

In selecting alloys from this group, consider the following factors:

1. Increasing nickel content increases resistance to carburization, decreases hot strength somewhat, and increases resistance to thermal shock.

2. Increasing chromium content increases resistance to corrosion and oxidation.

3. Increasing carbon content increases hot strength.

4. Increasing silicon content increases resistance to carburization, but decreases hot strength.

All are noted primarily for their oxidation resistance and ability to withstand moderate to severe temperature changes. Most are heat-treatable by aging room-temperature tensile properties in the aged condition ranging from 503 to 793 MPa in terms of ultimate strength, 297 to 552 MPa in yield strength, and 4 to 25% in elongation. Hardness of the aged alloys ranges from Brinell 185 to 270. Applications include heat-treating fixtures, furnace parts, oil-refinery and chemical processing equipment, gas-turbine components, and equipment used in manufacturing steel, glass, and rubber.

Both the nickel-base and cobalt-base alloys are probably best known for their use in aircraft turbine engines for disks, blades, vanes, and other components. The nickel alloys contain 50 to 75% nickel and usually 10 to 20% chromium and substantial amounts of cobalt, molybdenum, aluminum, and titanium, and small amounts of zirconium, boron, and, in some cases, hafnium. Carbon content ranges from less than 0.1 to 0.20%. Because of their complex compositions, they are best known by trade names, such as B-1900; Hastelloy X; IN-100, -738X, -792; Rene 77, 80, 100; Inconel 713C, 713LC, 718, X-750; MAR-M 200, 246, 247; Udimet 500, 700, 710; and Waspaloy. The high-temperature strength of most of these alloys is attributed to the presence of refractory metals, which provide solid-solution strengthening; the presence of grain-boundary-strengthening elements, such as carbon, boron, hafnium, and zirconium; and, because of the presence of aluminum and titanium, strengthening by precipitation of an Ni3(Al,Ti) compound known as "gamma prime" during age hardening. Many of these alloys provide 1000-h stress-rupture strengths in the range of 690 to 759 MPa at 649°C, and 55 to 124 MPa at 982°C.

The cobalt alloys contain 36 to 65% cobalt, usually more than 50%, and usually about 20% chromium and substantial amounts of nickel, tungsten, tantalum, molybdenum, iron and/or aluminum, and small amounts of still other ingredients. Carbon content is 0.05 to 1%. Although not generally as strong as the nickel alloys, some may provide better corrosion and oxidation resistance at high temperatures. These alloys include L-605; S-816; V-36; WI-52; X-40; J-1650; Haynes 21, 151; AiResist 13, 213, 215; and MAR-M 302, 322, 918. Their 1000-h stress-rupture strengths range from about 276 to 483 MPa at 649°C and from about 28 to 103 MPa at 982°C.

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