Cast stainless steels and nickel-base alloys

Cast stainless steels and nickel-base alloys

(Parte 1 de 6)

Stainless Steels2
Nickel-Base Alloys2
Role of Alloying Elements3
C hromium3
Nic kel3
M olybdenum4
Minor Elements4
Designations for Cast Stainless Steel and Nickel-Base Alloys5
Corrosion Resistant Castings6
S tainless Steels6
M artensitic13
Au stenitic16
Du plex3
Nickel-Base Alloy Castings36
Heat Resistant Castings46
S tainless Steels46
Nic kel-Base Alloys59
C asting Methods74
M achining78
We lding79
Purchasing Considerations80
Works Cited81
Suggested Additional Reading82
Appendix A83
Appendix B87

Cr-Ni Alloy and High Performance Nickel-Base Alloy Castings for Heat-Resisting and Elevated Temperature Corrosion Appendix C...................................................................................................8

This brochure will assist end users, specifiers, and designers in the selection of corrosion and heat resistant nickel-base ally castings. Information on the alloys, casting methods, properties and fabrication and design considerations are provided to aid in evaluation and selection. Typical applications have been provided for reference purposes but, because service conditions and performance requirements vary, the user is encouraged to obtain more information from the supplier before making a final decision.

Castings • 1

Stainless steels are distinguished from other steels by a minimum chromium content of 10.5%, which makes them more resistant to corrosive aqueous environments and to oxidation. Although there are exceptions, stainless steel castings are classified as "corrosion resistant" when used in aqueous environments and vapors below 1200°F (650°C) and "heat resistant" when used above this temperature.

The usual distinction between the heat and corrosion resistant casting grades is carbon content. For a stainless steel casting to perform well in a corrosive environment, the carbon content must be low. Heat resistant grades have higher carbon contents to improve elevated temperature strength.

The chemical composition and microstructure differences between the wrought and cast versions of stainless steels can affect performance. (See Role of Alloying Elements.) Some stainless steel casting grades can be precipitation hardened by heat treatment, but the mechanical properties of most rely on their chemical composition. The yield and tensile strengths of castings are comparable to their wrought equivalents.

Cast stainless steels generally have equivalent corrosion resistance to their wrought equivalents, but they can become less corrosion resistant due to localized contamination, microsegregation, or lack of homogeneity. For example, mold quality may cause superficial compositional changes that influence performance, and carbon pick-up from mold release agents can affect corrosion resistance. Heat treatment and weld repair procedures can influence the performance of some cast grades and should be taken into consideration during grade selection.

Additional information about the characteristics, properties and applications of specific cast stainless steel grades can be found in the following corrosion and heat resistant sections.

Except for some of the high silicon and proprietary grades, cast nickel-base alloys generally have wrought approximate equivalents. Although the cast and wrought versions of nickel-base alloys are commonly used in combination because they provide similar performance, there are some chemistry differences, primarily to improve castability and soundness.

Like stainless steels, nickel-base castings are categorized as corrosion resistant if they are used in aqueous environments and vapors below 1200°F (650°C) and heat resistant if they are capable of continuous or intermittent use for sustained times above this temperature. Carbon content is usually a distinguishing factor between the heat and corrosion resistant alloys, but this dividing line can be vague, particularly for alloys used in the 900-1200°F (480 to 650°C) range.

Additional information about the characteristics, properties and applications of specific cast nickel-base alloys can be found in the following corrosion and heat resistant

2 • Castings

Chromium, nickel, and molybdenum are the primary alloying elements that determine the structure, mechanical properties, and corrosion resistance of stainless steel and nickel-base alloy castings.

Nickel and chromium have the greatest influence on heat resistant castings. Intentional additions of less than 1 %carbon, nitrogen, niobium, tantalum, titanium, sulfur, and slightly larger additions of copper, manganese, silicon, and aluminum are used to modify properties. Some minor elements can have a positive or negative effect on properties depending on the application.

A stainless steel contains a minimum of 10.5% chromium because this level of chromium causes the spontaneous formation of a stable, transparent, passive, protective film. Increasing the level of chromium enhances corrosion resistance.

At elevated temperatures, chromium provides resistance to oxidation and sulfur-containing and other corrosive atmospheres; contributes to high temperature creep and rupture strength; and, in some alloys, increases resistance to carburization.

Nickel in stainless steels promotes the stability of austenite. Austenite is stronger and more stable at higher temperatures then ferrite. Less nickel is needed to retain an austenitic structure as the nitrogen or carbon levels increase. When sufficient nickel is added to a chromium stainless steel, the structure changes from ferritic to austenitic. Adding nickel improves toughness, ductility, and weldability.

Nickel increases resistance to oxidation, carburization, nitriding, thermal fatigue, and strong acids, particularly reducing acids. It is an important alloying element in stainless steel and nickel-base alloys used for corrosive and high temperature applications.

Wollaston Alloys, Inc., Braintree, Massachusetts

This 1,500 pound (675 kg) main feed booster pump and a 625 pound (281 kg) adaptor are used on aircraft

Castings • 3

Atlas Foundry & Machine Company, Tacoma, Washington

Stainless steel pump casings are produced in a variety of sizes and shapes for pipeline, refining, and boiler feed applications.

Molybdenum additions improve resistance to pitting and crevice corrosion in chloridecontaining environments and corrosion by sulfuric, phosphoric, and hydrochloric acids. The elevated temperature mechanical properties of austenitic stainless steels and the strength and tempering resistance of martensitic stainless steels are improved by molybdenum.

The presence of small amounts of carbon and nitrogen cannot be avoided during melting. In some grades, these elements are added deliberately. Increasing the carbon content in high temperature alloys improves high temperature strength and creep resistance, but reduces ductility. Conversely, carbon can be detrimental to corrosion resistance when it combines with chromium to form chromium carbides along grain boundaries. This reduces the chromium adjacent to the grain boundary (sensitization) and can lead to corrosion of chromium-depleted areas (intergranular corrosion). Titanium, columbium, and tantalum additions preferentially combine with carbon and nitrogen to prevent sensitization and eliminate susceptibility to intergranular corrosion.

Nitrogen additions to austenitic and duplex stainless steels improve pitting resistance and retard the kinetics of sigma phase formation. Additions of sulfur, selenium, and lead in stainless steel improve machinability. Columbium additions can improve hightemperature creep strength. Copper additions improve resistance to sulfuric acid. A combination of manganese and nitrogen may be used as a partial substitute for nickel in some stainless steels.

Silicon is added to cast stainless steel grades to increase casting fluidity and improve castability. As carbon plus silicon content is increased, partial eutectic solidification improves castability and casting soundness. Silicon is generally limited to 1.5% in castings intended for service above 1500°F (815°C) because it lowers the high temperature creep and rupture properties. Silicon also improves oxidation resistance, particularly where elements with a volatile oxide such as tungsten or niobium (columbium) are used to improve high temperature strength. In carburizing atmospheres such as ethylene furnaces, silicon levels as high as 2% have been found to be beneficial. Aluminum also improves resistance to oxidation.

4 • Castings

In North America, the common designations for cast stainless steel and nickel-base alloys are descriptive of their chemistry and purpose. This designation system was established by the Alloy Casting Institute (ACI) and has been adopted by ASTM.

A designation beginning with the letter "C" indicates that the alloy is used primarily for corrosive service; if the first letter is "H", the alloy is used primarily for high temperature service at or above 1200°F (649°C). The second letter indicates the approximate nickel and chromium contents of the alloy grade on the FeCrNi ternary diagram (ASTM A 781, Appendix X1 and Figure X1.1). For C classifications, the single or double digit number following the first two letters indicates the maximum carbon content of the grade (% x 100). For H classifications, this number is the midpoint of the carbon content range in units of 0.01 % with a ±0.05% limit. Other alloying elements, if present, are represented by one or more letters following the number. For example, the designation CF8M indicates that the grade is corrosion resistant (C), contains between 17% and 21 % chromium and be tween 8% and 12% nickel (F), a maximum carbon content of 0.08% (8), and molybdenum (M); HD indicates that the grade is heat resistant (H), and contains between 26% and 30% chrom- ium and between 4% and 7% nickel (D).

This CA6NM, 5-inch (127 m) multi-bowl wellhead

Christmas tree assembly is used in North Sea sour gas production.

Ray Atkinson

The corrosion resistant, cast, stainless steel grades are grouped into families based on their microstructure (martensitic, austenitic, or duplex). General characteristics of each stainless steel family and specific information about each of the widely used grades are provided in the following sections.

Stainless steel castings are classified as "corrosion resistant" if they are used in aqueous environments and vapors below 1200°F (650°C). For a stainless steel casting to perform well in a corrosive environment, the carbon content and quantity of precipitated carbides in the microstructure must be low. The carbon content in corrosion resistant grades is usually below 0.20% and sometimes below 0.03%. Increasing the chromium content enhances corrosion resistance and nickel increases resistance to strong acids, particularly reducing acids. The influence of other alloying elements is discussed in the Role of Alloying

Elements section.

The chemical compositions, ASTIVI specifications, approximate wrought equivalents, and common end use microstructures of corrosion resistant stainless steel castings can be found in Tables 1 and 2. The ASTIVI strength and elongation requirements are shown in Table 3 and are compared in Figures 1 through 3. Typical short-term high temperature properties for several grades are shown in Table 4. Standard heat treatments are shown in Table 5. Typical hardness, impact, and physical properties are shown in Tables 6 through 8.

These are centrifugally cast

CA15 turbine combustor cases.

Wisconsin Centrifugal, Waukesha, Wisconsin, USA

6 • Castings

Castings • 7 Castings • 7

8 • Castings 8 • Castings

Castings • 9 Castings • 9

10 • Castings 10 • Castings

Castings • 1 Castings • 1

12 • Castings

Figure 1 Relative tensile strength of corrosion resistant stainless steel castings

Figure 2 Relative yield strength of corrosion resitant stainless steel castings

The most widely used martensitic grades are CA6NM, CB7Cu1, and CB7Cu2. Martensitics are resistant to moderate atmospheric corrosion and mild organic media corrosion. Their corrosion resistance is lower than that of more highly alloyed grades, limiting their use in process environments. Their strength and tempering resistance are improved by molybdenum.

(Parte 1 de 6)