Alloy Fabrications

This is the second of three articles by metallurgist Marc Glasser on three individual heat resistant alloys. This article will feature RA330®. Please submit your questions about heat-resistant alloys for Marc to editor@heattreattoday.com.

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RA330® is a nickel alloy containing 35% nickel, 19% chromium, and 1.2% silicon. Over the years, it has become one of the most widely used wrought heat-resistant alloys due to the combination of its versatility, availability, properties, and cost-effectiveness.

 

The Chemistry of RA330

The chemistry of RA330 is shown below in Table 1.

There are several important benefits to this alloy including:

1.  Oxidation resistance up to 2100°F
2. Usable creep resistance up to 1850°F
3. Utility up to 2100°F when there are no loads applied and some deflection can be tolerated
4. Resistance to many heat treating atmospheres including carburizing and nitriding
5. Sufficient nickel content to prevent sigma phase formation and embrittlement

The oxidation resistance of various alloys is shown in Table 2 below¹.

 

The oxidation limit for RA330 is higher than that of any stainless steel, comparable to alloy 600, and only exceeded by nickel alloys with much higher nickel content.

RA330 Creep Strength

Table 3² shows the creep strength required to produce 1% strain in 10,000 hrs.

 

The creep strength of RA330 is better than all heat-resistant stainless steel grades except RA 253 MA. It is comparable to alloy 600 but less than the higher nickel alloys 601, RA333, and RA 602 CA. When comparing the economics of RA330 with those of the more expensive nickel alloys, RA330 often has enough creep strength for many heat treating applications and is often the most economical option. There are companies who use RA330 above 1800°F and sometimes as high as the 2100°F oxidation limit. They compensate for the very low creep strength at these temperatures by using braces such as gussets or supports. These supports may be made of ceramic or a different alloy with significantly higher creep strength at this temperature.

Strength Variables and Value

One of the excellent attributes of RA330 is its ability to resist the various atmospheres used in surface or case hardening operations. Thermodynamically, the formation of nickel carbides and nitrides are not favored. With 35% nickel, RA330 has sufficient nickel content to resist carburization, nitriding, and combinations of both. The alloy is not immune to surface hardening, just resistant. The length of resistance time is a function of the process and process variables. For example, field experience shows that 310 muffles used in carburizing atmospheres can completely carburize in as little as 1 month, especially at high temperatures. After that, the material is brittle and can rupture easily. Often, the usable life will be between 1 and 3 months depending on process temperature. A corresponding RA330 muffle under the same atmosphere will last up to 1 year.

Stainless steels are subject to sigma phase formation and embrittlement. Sigma phase is an intermetallic phase that consists of iron and chromium. It precipitates between approximately 1100 and 1600°F. Sigma phase does not embrittle materials at these relatively high temperatures, but at room temperature, sigma phase can reduce charpy impact values to single digits. One sudden impact can cause catastrophic failure. RA330, with 35% nickel, has enough nickel to prevent sigma phase formation.

Applications of RA330

RA330 is available from stock in many product forms. In addition to the traditional plate, sheet, and round bar, RA330 is also available in expanded metal, pipe, and hexagonal nuts. Round bar can quickly be turned into threaded bar. The ability to draw on all these items from stock make RA330 the ideal alloy for maintenance and repair.

RA330 is resistant to thermal fatigue. This property lends RA330 to be the wrought alloy of choice for alloy fixtures and baskets that require quenching a least once a day.

For all of these reasons, RA330 is often an excellent choice for heat treating applications. It has good oxidation resistance, good resistance to case hardening atmospheres, no sigma phase formation, and thermal fatigue resistance. It is available from stock in many forms and sizes. RA330 may not always be the best solution, but often it is the solution that works best.

One of the few atmospheres in which RA330 is not a good choice is sulfur. Like other nickel alloys, the nickel forms a nickel-sulfur intermetallic at a low temperature. In such environments, a lower nickel stainless steel such as 309 or 310 is often a better choice.

RA330® is a trademark of Rolled Alloys.

1. Glasser, Marc, “Selecting an Appropriate Heat-Resistant Alloy,” Industrial Heating. September 2014: 59-65.

2. Condensed from “High-Temperature Environments: Alloy Properties,” https://www.rolledalloys.com/technical-resources/environments/high-temperature/

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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.