MANUFACTURING HEAT TREAT

This is the first of three articles by metallurgist Marc Glasser on three individual heat resistant alloys. This article will feature RA 253 MA. Please submit your questions about heat-resistant alloys for Marc to editor@heattreattoday.com.

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Alloy 253MA®, marketed in the United States as RA 253 MA®, is a unique stainless steel. It exhibits oxidation resistance to 2000°F. It has shown useful creep resistance in some high-temperature vacuum applications up to 2100°F. Since it is a stainless steel, it is more economical than heat-resistant alloys with higher nickel content. In addition, RA 253 MA exhibits higher creep strength than most heat-resistant alloys with higher nickel content. This alloy is one of the few alloys with measured creep strength up to and above 2000°F.

The Chemistry of RA 253 MA

The chemistry of RA 253 MA is shown in Table 1. The alloy contains additions of silicon and the rare earth metal, cerium, which together create a very adherent oxide up to temperatures between 1950°F and 2000°F. Furthermore, the nitrogen addition enhances the creep strength.

 

Table 1: RA 253 MA Chemistry

At first glance, RA 253 MA is similar to 309, in terms of chromium and nickel content. However, the silicon and cerium additions enhance the oxidation resistance and the nitrogen boosts the creep strength to more than triple that of 309 and 310 stainless steels at 1800°F. Above 1800°F, 309, 310, RA330, and 600 no longer exhibit usable creep strength, whereas RA 253 MA continues to exhibit usable creep strength up to temperatures of between 2000°F and 2100°F. Table 2 shows the creep properties (1% in 10,000 hours or 0.0001%) of RA 253 MA and other heat resistant materials.

 

Table 2: Creep Rates for RA 253 MA and Other Heat Resistant Materials

Average Stress, ksi, for 0.0001% per hour Minimum Creep Rate

 

The Implications in Light of the Performance

In practical terms, the implications of this performance include:

1.  The ability to design parts and fixtures from thinner sections, thus reducing weights significantly, through proper engineering and design.
2.  The ability to design and fabricate fixtures that can hold more weight per furnace load compared to a fixture of the same dimensions with a lesser alloy.
3.  The relatively low nickel content of the alloy, allowing the material to be used successfully in OXIDIZING sulfur atmospheres.

RA 253 MA is best suited for high-temperature structural parts that will see oxidizing, inert, or vacuum environments. Other factors to be cognizant of when considering RA 253 AM include:

1.  The alloy is a stainless steel and therefore subject to sigma phase embrittlement in the temperature range of 1150°F to 1600°F. This means that, over time, the intermetallic sigma phase can form. Sigma phase is quite brittle at room temperature. At operating temperature, the material is still ductile and usable. However, if sigma forms and the material cools to room temperature, care must be taken not to allow any shock impact. A sudden, hard impact from a forklift would be an example of such a shock impact that could break an embrittled basket. Once reheated to operating temperature, the brittleness is not a concern.
2.  The oxidation resistance in wet (water vapor) environments decreases.
3.  The alloy is not resistant to carburization or nitriding.
4.  The alloy does not hold up in reducing sulfur environments.

Conclusion

In summary, RA 253 MA is an excellent choice for environments where a combination of oxidation resistance and superior creep strength are required. Its excellent creep strength allows for the fabrication of either lighter weight or higher weight capacity fixtures and components in high heat applications. Its high strength and higher nickel content compared to ferritic stainless steels make this grade worthy of consideration for automotive exhaust applications.

Even though RA 253 MA has a significantly higher price per pound than the current ferritic chromium-iron alloys, the high creep strength allows for lighter, thinner components, while nominal 11% nickel addition will provide for a more corrosion resistance than a ferritic alloy. Conversely, when RA 253 MA replaces a ferritic steel without making dimensional changes, the additional creep strength should result in a part with a longer life, which could reduce warranty costs. Finally, the higher oxidation limits can be utilized by design engineers to make a more efficient system, which can operate at higher temperatures.

253MA is a trademark material of Outokumpu.

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Marc Glasser is Director of Metallurgical Services at Rolled Alloys and is Heat Treat Today‘s resident expert in process metallurgy, heat treatment, materials of construction, and materials science and testing.