RA 253 – A Microalloyed Solution for High-Temperature Applications | 2013-09-10

Alloy development has resulted in a material – RA 253 MA® – with the price of 310 stainless and the high-temperature properties of the higher-nickel Inconel® 600.

RA 253 MA® is a versatile alloy that can be used in a range of thermal applications for equipment construction. The drive for the development of this alloy was the rapid rise in nickel prices. With low Cr and low Ni, RA 253 MA is a good, low-cost alternative to other more-expensive nickel-based materials. With the advent of the microalloying control, this alloy is comparably priced with 310 stainless steel, but it delivers high strength properties comparable to the higher nickel Inconel® 600-series materials.

Chemically similar to 309 stainless, RA 253 MA offers significantly higher creep resistance and rupture strength than 310. Its benefits include excellent oxidation resistance up to 2000˚F (1093˚C), good hot tensile strength comparable to the Inconel 600-series of materials and excellent creep and rupture properties.

RA 253 MA is a lean austenitic stainless steel that uses cerium and silicon to create a very adhesive oxide, resulting in excellent oxidation resistance. The combination of nitrogen and carbon provide creep-rupture strength that is double that of type 310 and 309 stainless at 1600˚F (871˚C).

Chemistry

RA 253 MA has a specified chemistry as indicated in Table 1.

High-Temperature Properties

Typical hot tensile strengths of several materials are shown in Figure 1.[1] Over most of the temperature ranges, it is comparable to Alloy 600, superior to 310 stainless and RA 330® but lower than RA 602 CA®. Even though hot tensile strength is reported up to 2200˚F (1204˚C), the loss of oxidation resistance at 2000˚F makes its practical use limit 2000˚F (1093˚C) in oxidizing environments.

ASME 2010 (2011 revision) Section II-D allowable design stresses for pressure-vessel plate are shown in Figure 2. The allowable stresses for RA 253 MA are higher than both 310 stainless and RA 330 but not as high as Alloy 601. ASME has allowable design stresses for this alloy up to 1600˚F (871˚C), but RA 253 MA is used at higher temperatures for many different applications because this temperature limit is only for pressure vessels.

Figures 3 and 4 show the actual 10,000 and extrapolated 100,000 hour rupture strengths of various high-temperature alloys.[1] The data shows that RA 253 MA has high creep- and rupture-stress values approaching Inconel 601 and RA 602 CA and superior to RA 330 and Alloy 600.

Figure 5 shows data for 0.0001% per hour minimum creep rate.[1] Once again, the RA 253 MA is superior to all other materials above 1300˚F (704˚C). Creep is the rate or speed at which metal is stretching and is typically reported in percent per hour. There is a period of time where creep rate is essentially constant, known as the secondary creep rate. This rate is one major basis of design at high temperature. One must assume that metal is going to creep even in light loads because the effects of creep can be seen in material that has no load other than its own weight. Therefore, in practice some creep criterion is used for design.

The furnace industry historically has used a design criterion as the stress required for a minimum creep rate of 1% in 10,000 hours or 0.0001% per hour. The design stress is set at some fraction of this number. ASME uses for one of its criteria, 100% of the extrapolated stress for 1% in 100,000 hours or 0.00001% per hour.[2] Extrapolation of stress-rupture and creep data to 100,000 hours above 1800˚F (982˚C) is not recommended. The comparison is presented here for general comparison only.

Rupture strength is reported as a stress and number of hours. It is the stress required at a specific temperature to completely break a specimen within a given amount of time. In the furnace industry, a common criterion for setting design stresses is to use some fraction of the stress that would result in rupture at 10,000 hours. ASME uses whichever is lower: 67% of the extrapolated 100,000 rupture stress or 100% of the extrapolated 1% in 100,000 hours minimum creep rate.[2]

Strengths and Limitations

RA 253 MA exhibits equal or superior oxidation resistance to many other alloys tested, including 309 and 310.[3,4] At 2000˚F (1093˚C), RA 253 MA shows excellent oxidation resistance, which is equal to the limit for 310 stainless steel and superior to 309. While short furnace excursions up to 2100˚F (1149˚C) can be tolerated, consistent oxidizing temperatures above 2000˚F can rapidly degrade the material. In general, best practices are to avoid any excursions above suggested temperature limits of any alloy.

RA 253 MA has performed well in some mildly carburizing environments, in spite of its lower alloy content. Experience has shown that it takes only traces of oxygen in the gas (e.g., in the form of carbon dioxide or steam) to produce a thin and tough oxide layer on 253 MA, leading to a good protection against pickup of both carbon and nitrogen. Under reducing conditions, however, the use of RA 253 MA in carburizing environments should be avoided. Due to its lower nickel content, it is less resistant to carburization than higher-nickel alloys such as RA 330. In a simulation where coupons were exposed for 15 weeks of simulated bake cycles between 1700-1950˚F (930-1065˚C) in “green mix” used for production of carbon electrodes, room-temperature tensile tests showed the ductility given in Table 2.

Table 2. Ductility based on room-temperature tensile tests

Alloy UNS Retention of ductility (% reduction of area)

RA 253 MA S30815 0.5

302B S30215 nil

800H N08810 1.4

RA 330® N08330 16.6

The onset of sigma-phase formation in RA 253 MA is significantly slower than in 310S (Fig. 6). There is also a ferritic-embrittlement range of 750-1050˚F (399-566˚C). Neither of these phases will have an effect at operating temperatures. Upon cooling to room temperature, the material becomes very brittle, making it less resistant to thermal cycling. During subsequent ramp-up, if the material is highly constrained and cannot expand freely, the potential for cracking exists.

Corrosion Resistance in Salt-Bath Applications

Exposure to sodium and potassium salts for heat treating high-speed steel indicates that RA 253 MA may be comparable to Alloy 600 (Table 3).

Table 3. Intergranular attack based on exposure to sodium and potassium salts

Grade Nickel weight % Depth of intergranular attack mils (mm)

RA 253 MA 11 6.9 (0.18)

RA600 76 7.5 (0.19)

RA309 13 12.5 (0.32)

RA330 35 13.8 (0.35)

In this trial, plate samples were exposed to 210-252 cycles in preheat salts at 1300-1500˚F (704-816˚C), high-heat salt 2200˚F (1200˚C) and then quenched in 1100˚F (593˚C) salt. Table 3 illustrates that RA 253 MA has the potential to perform well in a salt-bath environment, attributable to its high silicon and chromium levels. While alloy selection is important, the most important factor is regularly scheduled maintenance and cleaning of the salt bath and surrounding areas.

In salt-bath heat treating, the service life of the pot is primarily determined by maintenance, not alloy. Pots must be desludged regularly. When changing pots, every bit of old spilled salt must be removed from the furnace refractory.

Corrosion Resistance

In the presence of oxygen, RA 253 MA has good resistance to hot sulfur-bearing environments. However, RA 253 MA is not resistant to reducing, sulfur-bearing atmospheres. Even though the atmosphere may be oxidizing, the partial pressure of oxygen can be extremely low while a stainless steel is in service. Due to this low pressure, local sulfidation attack can occur even when the atmosphere is considered oxidizing.

Test samples exposed to an atmosphere containing 13.6% SO2 at 1850˚F (1010˚C) for 1,860 hours exhibited the depth of intergranular oxidation and sulfidation shown in Table 4.

Table 4. Sufidation attack after exposure to an atmosphere containing 13.6% SO2 at 1850˚F (1010˚C) for 1,860 hours

Alloy Depth of attack in mils (mm)

RA 253 MA 8 (0.20)

RA 333 8 (0.20)

RA 309 18 (0.46)

RA 310 20 (0.51)

RA 330 24 (0.61)

Microstructure

RA 253 MA exhibits a microstructure as shown in Fig. 7. This table of microstructures also shows the effect of time and temperature on the alloy. The formation of sigma phase at temperatures of 1292, 1472 and 1652˚F (700, 800 and 900˚C) are shown in the appendix as well. Clearly, sigma-phase precipitation is almost nonexistent at 1650˚F, as evidenced by microstructure and Charpy impact data. In addition, a TTT curve (Fig. 6) of sigma-phase formation shows RA 253 MA to require up to an order of magnitude more time to begin sigma precipitation compared to 310 and 310S stainless.[4]

Applications for Use

RA 253 MA has been successfully used in a myriad of applications including but not limited to bell annealing-furnace covers, muffle belt conveyors, car exhaust manifolds, exhaust-gas flexible tubes, hot-air ducts, cooling-tower tubes in sulfite process pulp mill, and heat-treatment trays for neutral hardening. IH

For more information: Contact Marc Glasser, Tyler Reno or Paul Whitcraft at Rolled Alloys, 125 West Sterns Road, Temperance, MI; tel: 800-521-0332, e-mail: metallurgical-help@rolledalloys.com; web: www.rolledalloys.com. RA 330 and RA 333 are registered trademarks of Rolled Alloys. 602 CA and 253 MA are registered trademarks of Outokumpu VDM. Inconel is a registered trademark of Special Metals.