Metal Dusting Causes HRA Headaches
Metal dusting is a phenomenon that typically results in premature failure of components with accompanying high-repair costs and loss of production. It is not known why one shop will find it to be a serious problem and another, even within the same corporation and supposedly carburizing using the same atmosphere, regards it as just a minor aggravation. A lack of published industrial experiences recorded under well-defined conditions makes it somewhat difficult to come up with definitive design and/or materials solutions. However, the selection of component materials needs to be made with a balance between required performance and optimal resistance to metal dusting and cost in mind.
Metal dusting (also referred to as carbon attack, carbon rot and catastrophic carburization) is a continually occurring problem in carburizing furnaces. The result of this phenomenon is that the metal disappears, turning into a black dust. The metal appears to be worm-eaten on the surface, or it is perforated. In some cases, bars that undergo metal dusting can look like a beaver chewed away on it.
There are two temperature ranges in which metal dusting can occur. In heat treat furnaces, it usually occurs at lower temperatures raging from around 800 to 1200°F (430 to 650°C). Such temperatures exist in the stagnant-atmosphere locations near the furnace walls. While the furnace may operate at a temperature of 1750°F (955°C), the temperature is lower where alloy tube hangers, atmosphere sampling tubes or electrical leads pass through the refractory wall. Less commonly seen is a higher temperature metal dusting mode in the vicinity of 1550 to 1750°F (845 to 955°C).
Alloy choices to stem metal dusting
Heat resistant alloys (HRA) that have good resistance to metal dusting contain the alloying elements chromium and silicon, and carbide-forming elements such as tungsten. For aluminum to be helpful as an alloy addition, it appears necessary that the HRA also contain about 25% chromium.
Commercially available heat resistant alloys vary greatly in susceptibility to metal dusting. The wrought grade RA333® and the cast alloys Supertherm® /NC14® and 22H®/ NC11® are consistently the more resistant to metal dusting, while RA330® and cast HT (35 Ni-17 Cr) are intermediate in resistance. Where metal dusting alone is the issue, the higher chromium, lower nickel HK (25 Ni-20 Cr) is known to outperform HT. The high-nickel alloys (such 600 alloy) and the commonly used 82 alloy weld filler metal have been the least resistant.
Selecting an alloy to resist metal dusting is not the same as selection to maximize ductility in a carburizing atmosphere. For example, 600 alloy (76 Ni-15.5 Cr) does a good job of retaining some ductility even when carburized.
Where metal dusting is concerned, high nickel alone does not suffice. Some higher level of chromium, preferably combined with carbide formers or aluminum, is needed. While 600 alloy is ductile, it is not the best choice to resist metal dusting. Even RA330 (35 Ni-19 Cr) better resists metal dusting.
It is possible that there is no alloy completely immune to metal dusting in some atmospheres. One of the best descriptions of the behavior of high-temperature materials in carbonaceous atmospheres is from a study conducted by Louis Wolfe in 1978  in the petrochemical industry. The study showed metal dusting in alloys ranging from pure iron to Alloy 400 (70 Ni-30 Cu). Table 2 lists the nominal chemical composition of commonly used wrought and cast high-temperature alloys.
Metal dusting susceptibility
Tests have been conducted to determine the susceptibility of selected alloys to metal dusting by exposing the samples for 37,000 hours at an operating temperature of 1700°F (930°C) in the metal dusting zone of an industrial carburizing furnace. The atmosphere was endothermic enriched with methane to 1.2% carbon potential. Metal dusting occurred where the samples passed through the refractory. Results are shown in Table 1 .
Metal dusting resistance is influenced by grain size. A finer grain size greatly increases grain boundary diffusion rate of chromium to the surface to form a protective film. Slightly cold working the surface (as from machining) increases the self-diffusion coefficient of chromium by increasing the density of vacancies in the crystal lattice. IH