Heat-resistant alloy RA 602 CA provides the required scaling resistance and creep strength for calciner used to process metal oxides.

Materials used for thermal processing equipment and furnace fixtures must withstand increasingly harsh conditions including operating temperatures to 2200F (1205C). Add to that cyclical heating and aggressive environments, and traditional heat-resistant alloys often do not have the properties necessary to deliver the equipment life that users expect. RA 602 CA(tm) (UNS N06025) offers a good combination of properties at extreme temperatures to meet service life expectations.

All high temperature alloys have certain limitations. Material selection often is a result of compromises among many factors such as creep strength, maximum design temperature, environmental factors, ease of fabrication and repair, cost effectiveness and material availability. The chemical compositions of several common heat-resistant alloys are given in Table 1, and the effects of various alloying elements on material performance are shown in Table 2.

Although many of the alloys listed have been used at extreme temperatures, few have the combination of properties necessary for reliable operation at temperatures to 2200F. Three problems that commonly contribute to the premature failure of alloys components operating in the 1800 to 2200F (980 to 1205C) temperature range are:

• Thinning or perforation caused by corrosion in the form of oxidation or other forms of chemical attack
  
• Deformation resulting from insufficient high temperature strength
  
• Fracture due to loss of toughness from grain growth and/or environmental factors such as carburization

These issues were addressed in the development of the new alloy.

Fig 1 Oxidation rates for selected heat-resistant alloys at temperatures between 2000 and 2150F. Samples were cycled weekly to room temperature over a test period of 1,500 to 3,400 hours. Source: Ref 2, 3.

Oxidation

The rate of oxidation or scaling for alloy components from oxidation increases with increasing temperature. Figure 1 illustrates the behavior of several heat-resistant alloys in cyclic oxidation tests at different temperatures and test times. Heat-resisting alloys rely on the formation of a thin, tightly adherent oxide scale for oxidation resistance. This layer acts as a barrier, which dramatically reduces the susceptibility of the alloy to further corrosion in the form of scaling. Chromium is the element most commonly used to impart scaling resistance. In sufficient quantities (> 12%), chromium reacts to form a continuous chromium-oxide layer on the alloy surface. Typical chromium content of wrought heat-resistant alloys ranges from 15 to 25%.

Alloying with additional small quantities of rare-earth elements (microalloying) has enhanced the stability of this chromium oxide scale[4]. Cerium, yttrium and lanthanum are the more commonly used elements. Such alloying additions are becoming more common and are present in recent alloy developments. Aluminum and silicon additions improve scaling resistance by forming continuous alumina and silica sublayers below the chromium-oxide scale. These sublayers provide additional levels of protection between the atmosphere and the alloy.

Scaling resistance of RA 602 CA, even at extreme temperatures, is attributed to its high chromium content (25%) enhanced by additions of 2.2% aluminum and 0.1% yttrium. The aluminum addition allows for the formation of a continuous homogenous self-repairing Al2O3 subscale, and the addition of yttrium enhances the adhesion of the chromium and aluminum oxide layers.

Fig 2 High-temperature oxidation metrics; Fig 3 Surfaces of RA 602 CA (left) and Alloy 601 (right) after 3,150 hours exposure at 2100¿F.

Another measure of oxidation resistance is the amount of metal affected by internal oxidation and actual metal loss. Oxidation metrics are illustrated in Fig. 2. The appearance of RA 602 CA and Alloy 601 after 3150 hours at a temperature of 2100F (1150C) are shown in Fig. 3. Internal oxidation is prevalent in the alloy 601 sample, while only a thin oxide scale formed on the RA 602 CA surface. Freedom from internal attack is important in applications using sheet material, such as radiant tubes. Absence of internal oxidation means a greater percentage of the wall thickness is sound metal. As a result, the alloy retains a greater level of mechanical integrity. A comparison of oxide penetration in selected alloys is provided in Table 3.

Creep-rupture strength

Metals behave differently at high temperatures than they do near room temperature. Loading a metal bar to just below its yield strength at room temperature will not lead to failure or deformation regardless of the amount of time the stress is applied. At temperatures above 1000F (540C) and higher, mechanical strength is no longer independent of time. A metal component stressed to just below its yield point at red heat (1500F, or 815C, for example) will creep (stretch) slowly over time. Depending on the alloy, the stress level and the temperature involved, the component could last for hours, weeks or years until the metal finally fractures or ruptures. Because of creep, an alloy component can deform even under the stresses imposed by supporting its own weight. For this reason, the creep rupture strength is an important design criterion for any alloy intended for use above 1000F.

The creep rupture properties of heat resisting alloys are heavily dependent on alloy content and grain size. In the case of RA 602 CA, its high creep-rupture strength primarily is a result of alloy content[5]. The relatively high carbon level ensures the precipitation of homogeneously distributed bulky carbides. Additions of titanium and zirconium ensure that these carbides and also carbonitrides are finely dispersed. Even solution annealing at 2230F (1220C) does not dissolve the carbides completely. Thus, the alloy achieves its high creep rupture properties from a combination of solid-solution hardening and carbide strengthening. The result is an alloy having greater creep-rupture properties than traditional alloys such as RA330 and alloy 601 (Tables 4 and 5). Further comparison indicates that at extreme temperatures, RA 602 CA compares favorably to other heat-resistant superalloys such as Alloy 214 and Alloy 230 (Table 6).

Fig 4 Fracture surface of a failed Alloy 600 thermocouple sheath taken from a molten-salt pot operating at temperatures in excess of 2100F. Extensive grain growth has occurred due to overheating.

Grain growth

Brittle fracture of components exposed to extreme temperatures is a common occurrence. Operating at temperatures near or exceeding 1800F (980C) will lead to grain growth. A very coarse grain structure negatively affects alloy performance in several ways. Reduced resistance to thermal fatigue under cyclic conditions can cause cracking and fracturing. Susceptibility to corrosive mechanisms that preferentially attack at the grain boundaries also is increased. Such mechanisms include carburization, molten chloride-salt attack and corrosion by halogens. The attack is accelerated due to the reduced grain boundary volume and the more direct path into the alloy from the surface. Such types of attack reduce ductility. Often, the result is a component prone to brittle fracture. An example of such a failure is shown in Figure 4.

A small addition (approximately 0.08%) of zirconium in RA 602 CA is effective in pinning grain boundaries, thus greatly slowing the rate of grain growth[9]. The results of a recent grain growth study are detailed in Table 7. The information compares the grain growth of several heat-resistant alloys that are commonly used at temperatures above 1900F (1040C). The data was compiled from intermittent exposure of mill-annealed sample coupons to a temperature of 2050F (1120C) for a total of 990 hours.

Further evidence of the grain-growth resistance of RA 602 CA came from the examination of samples used in earlier cyclic oxidation testing. After exposure to 2100F (1150C) for 3,400 hours and 2150F (1180C) for more than 1,500 hours, the grain size only changed from ASTM 7 to ASTM 5.5 in both cases. In contrast, the grain size of alloy 601 exposed in the same tests increased to greater than ASTM 00.

The pick up of carbon from high temperature atmospheres is another potential reason for an alloy to lose its ductility during service. Table 8 compares the carburization resistance of several heat resistant alloys.

Conclusions

Higher operating temperatures can optimize many processes by increasing production, enhancing properties and improving the quality of parts being heat treated. The properties of traditional heat-resistant materials have limited furnace operators to the choices of living with erratic alloy component life or reducing process temperatures. The new alloy offers the possibility of long-term operation at temperatures to 2200F (1205C) without compromise.

Trademarks: 602 CA is trademark of Krupp VDM Technologies; 253 MA and 353 MA are trademarks of Avesta-Polarit; RA330 and RA333 are registered trademarks of Rolled Alloys; 800HT is a registered trademark of Special Metals Corp.; 230 is a registered trademark of Haynes International and 214 is a trademark of Haynes International.