Considerations for Materials SelectionAlloy fabrications typically end their life as a result of either attack by corrosion or a mechanical failure. Some of the following will be discussed in detail.
Common High-Temperature Failure Modes
- Metal dusting/carbon rot
- Thermal expansion
- Thermal fatigue
- Thermal shock
- Molten-metal embrittlement
It is also important to understand that the actual temperature of a muffle or radiant tube is hotter than both the temperature of the exiting parts or the furnace chamber. With this in mind, increased production through a furnace means the operating conditions for the muffle or radiant tubes have changed. The production parts may be seeing the same temperatures. To heat up the additional weight in the same time, however, more heat must be put into the system either through the muffle wall or radiant tube. So although the process appears to be the same, the muffle or radiant tube is seeing tougher conditions, and this often means decreased life. Small increases in temperature do dramatically impact the properties of high-temperature alloys.
Perhaps more important is that the creep-rupture strengths of heat-resistant alloys decrease rapidly as temperatures increase. Table 2 shows stress-to-rupture strengths for various alloys at 1700-2000°F. Increasing the operating temperature of an alloy by 100°F decreases the strength on average 30-40%.
CarburizationChromium, nickel and silicon are the three primary elements that provide an alloy resistance to the absorption of carbon. Nickel and silicon lower the maximum solubility of carbon and nitrogen. Carburization is normally an issue because highly carburized alloys become brittle. Above about 1% carbon, most wrought heat-resistant alloys have no measurable room-temperature ductility. This lack of ductility may result in the part fracturing and/or limiting the ability to repair, weld or re-straighten the carburized fixture.
Metal dusting, also known as catastrophic carburization or carbon rot, is a metal wastage, not an embrittlement phenomenon. In the right environment (carbon-rich with temperatures around 1100°F), it appears that any alloy can eventually produce metal dust. Overall, there is disagreement regarding appropriate alloy selection. Generally, nickel alloys with high chromium contents and additions of silicon and/or alumina typically offer improved performance. In the steel heat-treating industry, experience has shown that RA333 and Supertherm are two of the best for resistance. In the petrochemical industry, RA 602 CA alloy is commonly used to upgrade from 800H. Since metal dusting typically occurs in a very localized area and these alloys can be expensive, it may be most cost effective to use these alloys for only the problem area while the remainder of the component is made of the original-construction material. For example, in Fig. 6, RA333 was used for the first 2 feet of a U-type radiant tube. This area passes through the refractory and is subject to metal-dusting attack. RA330 is used for the remainder of the tube.
Molten SaltsAttack by molten salts can be a significant issue. Probably the most extreme example of this is the erratic service life of salt pots. From time to time the insufficient cleaning of spilled salt from the furnace chamber of a salt-pot furnace causes a rapid pot failure. The salt vapors that form in the chamber upon heating are very aggressive and cause a loose porous oxide scale to form on the surface. This scale is nonprotective and prone to rapid spalling.
In our experience, increasing nickel content aids resistance, whereas increased chromium levels can be detrimental. As a result, Alloy 600 (75Ni-15.5Cr) is commonly considered to have very high resistance and RA330 at 35Ni-19Cr is also considered to have good resistance. Each would still have very short life without proper cleaning prior to installation.
Thermal ExpansionOne of the leading reasons for a high-temperature alloy to fail is distortion or fracture. It is important to understand that in comparison to mild steel, stainless steels and nickel alloys are poor conductors of heat. They also have higher rates of thermal expansion. Because of their low thermal conductivity, these alloys are more prone to uneven heating or hot spots.
The burner-can shown in Fig. 8 shows what can occur with uneven heating. During low fire the burner flame impinged on the walls of the burner-can. The shiny glazed scale where the can has distorted indicates high heat exposure. The other areas of the can operated at much lower temperatures. The area with flame impingement was restrained from expanding by the cooler areas of the can. These cooler areas are also stronger because of their lower temperature. Since expansion of the hot spot was restricted, it buckled to relieve the stress.
253 MA is a registered trademark of Outokumpu Stainless. RA330 and RA333 are registered trademarks of Rolled Alloys. 602 CA is a registered trademark of ThyssenKrupp VDM. Supertherm is a registered trademark of Duraloy Technologies Inc.
For more information:Jason Wilson is technical marketing manager for Rolled Alloys, 125 West Sterns Rd., Temperance, Mich. 48182; tel: 800-928-9482; e-mail: firstname.lastname@example.org; web: www.rolledalloys.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: nickel-based alloy, stainless steel, thermal expansion, embrittlement, thermal shock, radiant tube