High-Alloy Cast Steels

Cast high alloy steels are widely used for their corrosion resistance in aqueous media at or near room temperature and for service in hot gases and liquids at elevated and high temperatures (> 650°C). High-alloy cast steels are most often specified on the basis of composition using the designation system, which has been replaced by the Alloy Casting Institute (ACI), which formerly administered these designations.

Mechanical properties of these grades (for example, hardness and tensile strength) can be altered by suitable heat treatment. The cast high-alloy grades that contain more than 20 to 30% Cr+Ni, however, do not show the phase changes observed in plain carbon and low-alloy steels during heating or cooling between room temperature and the melting point. These materials are therefore non hardenable, and their properties depend on composition rather than heat treatment. Therefore, special consideration must be given to each grade of high-alloy cast steel with regard to casting design, foundry practice, and subsequent thermal processing.

Corrosion-resistant high-alloy cast steels, more commonly referred to as cast stainless steels, have grown steadily in technological and commercial importance during the past 40 years. The principal applications for these steels are for chemical-processing and power-generating equipment involving corrosion service in aqueous or liquid-vapor environments at temperatures normally below 315°C. These alloys are also used for special services at temperatures up to 650°C.

Cast stainless steels are defined as ferrous alloys that contain a minimum of 17% Cr for corrosion resistance. Most cast stainless steels are of course considerably more complex compositionally than this simple definition implies. Stainless steels typically contain one or more alloying elements in addition to chromium (for example, nickel, molybdenum, copper, niobium, and nitrogen) to produce a specific microstructure, corrosion resistance, or mechanical properties for particular service requirements.

Corrosion-resistant high-alloy cast steels are usually classified on the basis of composition or microstructure. It should be recognized that these bases for classification are not completely independent in most cases; that is, classification by composition also often involves microstructural distinctions.

Alloys are grouped as chromium steels, chromium-nickel steels in which chromium is the predominant alloying element, and nickel-chromium steels in which nickel is the predominant alloying element. The serviceability of cast corrosion-resistant steels depends greatly on the absence of carbon, and especially precipitated carbides, in the alloy microstructure.

The high-alloy cast steels can also be classified on the basis of microstructure. Structures may be austenitic, ferritic, martensitic, or duplex; the structure of a particular grade is primarily determined by composition. Chromium, nickel, and carbon contents are particularly important in this regard. In general, straight chromium grades of high-alloy cast steel are either martensitic or ferritic, the chromium-nickel grades are either duplex or austenitic, and the nickel-chromium steels are fully austenitic.

Martensitic grades include alloys CA-15, CA-40, CA-I5M, and CA-6NM. The CA-15 alloy contains the minimum amount of chromium necessary to make it essentially rustproof. It has good resistance to atmospheric corrosion as well as to many organic media in relatively mild service. A higher-carbon modification of CA-15, CA-40 can be heat treated to higher strength and hardness levels. Alloy CA-15M is a molybdenum-containing modification of CA-15 that provides improved elevated-temperature strength. Alloy CA-6NM is an iron-chromium-nickel-molybdenum alloy of low carbon content.

Austenitic grades include CH-20, CK-20, and CN-7M. The CH-20 and CK-20 alloys are high-chromium, high-carbon, wholly austenitic compositions in which the chromium exceeds the nickel content. The more highly alloyed CN-7M has excellent corrosion resistance in many environments and is often used in sulfuric acid service.

Ferritic grades are designated CB-30 and CC-50. Alloy CB-30 is practically nonhardenable by heat treatment. As this alloy is normally made, the balance among the elements in the composition results in a wholly ferritic structure similar to wrought AISI type 442 stainless steel. Alloy CC-50 has substantially more chromium than CB-30 and has relatively high resistance to localized corrosion in many environments.

Austenitic-ferritic alloys include CE-30, CF-3, CF-3A, CF-8, CF-SA, CF-20, CF-3M, CF-3MA, CF-8M, CF-8C, CF-16F, and CG-8M. The microstructures of these alloys usually contain 5 to 40% ferrite, depending on the particular grade and the balance among the ferrite-promoting and austenite-promoting elements in the chemical composition.

Duplex Alloys. Two duplex alloys CD-4MCu and Ferralium are currently of interest. Alloy CD-4MCu is the most highly alloyed duplex alloy. Ferralium was developed by Langley Alloys and is essentially CD-4MCu with about 0.15% N added. With high levels of ferrite (about 40 to 50%) and low nickel, the duplex alloys have better resistance to stress-corrosion cracking (SCC) than CF-3M. Alloy CD-4MCu, which contains no nitrogen and has relatively low molybdenum content, has only slightly better resistance to localized corrosion than CF-3M. Ferralium, which has nitrogen and slightly higher molybdenum than CD-4MCu, exhibits better-localized corrosion resistance than either CF-3M or CD-4MCu.

Improvements in stainless steel production practices (for example, electron beam refining, vacuum and argon-oxygen decarburization, and vacuum induction melting) have created a second generation of duplex stainless steels. These steels offer excellent resistance to pitting and crevice corrosion, significantly better resistance to chloride SCC than the austenitic stainless steels, good toughness, and yield strengths two to three times higher than those of type 304 or 316 stainless steels.

First generation duplex stainless steels, for example, AISI type 329 and CD-4MCu, have been in use for many years. The need for improvement in the weldability and corrosion resistance of these alloys resulted in the second-generation alloys, which are characterized by the addition of nitrogen as an alloying element.

Second generation duplex stainless steels are usually about a 50-50 blend of ferrite and austenite. The new duplex alloys combine the near immunity to chloride SCC of the ferritic grades with the toughness and ease of fabrication of the austenitics. Among the second-generation duplexes, Alloy 2205 seems to have become the general-purpose stainless.

Precipitation-Hardening Grades. The alloys in this group are CB-7Cu and CD-4MCu. Alloy CB-7Cu is a low-carbon martensitic alloy that may contain minor amounts of retained austenite or ferrite. The copper precipitates in the martensite when the alloy is heat treated to the hardened (aged) condition.

Heat-resistant high-alloy steel castings are extensively used for applications involving service temperatures in excess of 650°C. Strength at these elevated temperatures is only one of the criteria by which these materials are selected, because applications often involve aggressive environments to which the steel must be resistant. The atmospheres most commonly encountered are air, flue gases, or process gases; such atmospheres may be either oxidizing or reducing and may be sulfidizing or carburizing if sulfur or carbon are present.

Carbon and low-alloy steels seldom have adequate strength and corrosion resistance at elevated temperatures in the environments for which heat-resistant cast steels are normally selected. Only heat-resistant steels exhibit the required mechanical properties and corrosion resistance over long periods of time without excessive or unpredictable degradation. In addition to long-term strength and corrosion resistance, some cast heat-resistant steels exhibit special resistance to the effects of cyclic temperatures and changes in the nature of the operating environment.

These alloy types resemble high-alloy corrosion-resistant steels except for their higher carbon contents, which impart greater strength at elevated temperature. The higher carbon content and, to a lesser extent, alloy composition ranges distinguish cast heat-resistant steel grades from their wrought counterparts.

Iron-chromium alloys contain 8 to 30% Cr and little or no nickel. They are ferritic in structure and exhibit low ductility at ambient temperatures. Iron-chromium alloys are primarily used where resistance to gaseous corrosion is the predominant consideration because they possess relatively low strength at elevated temperatures.

Iron-chromium-nickel alloys contain more than 18% Cr and more than 8% Ni, with the chromium content always exceeding that of nickel. They exhibit an austenitic matrix, although several grades also contain some ferrite. These alloys exhibit greater strength and ductility at elevated temperatures than those in the iron-chromium group and withstand moderate thermal cycling. Examples of these alloys are the HE, HF, HH, HI, HK, and HL grades.

Iron-nickel-chromium alloys contain more than 10% Cr and more than 23% Ni, with the nickel content always exceeding that of chromium. These alloys are wholly austenitic and exhibit high strength at elevated temperatures. They can withstand considerable temperature cycling and severe thermal gradients and are well suited to many reducing, as well as oxidizing, environments. Examples of iron-nickel-chromium alloys are the HN, HP, HT, HU, HW, and HX grades. Even though nickel is the major element in the HW and HX grades, these grades are ordinarily referred to as high-alloy steels rather than nickel-base alloys.