The main strengthening mechanism of conventional heat-resistant alloys (HRAs) in operation during the steady-state creep process is carbide dispersion strengthening. To better understand alloy behavior at operating temperatures, it is helpful to look at a carbide-dispersion model as a function of temperature and time. Increases in temperature and/or long exposure to high temperature affect the growth and morphology of carbide (and sometimes nitride) precipitates in these alloys. Precipitation of secondary carbides does not start until a temperature of around 1100 F (595 C), but carbide morphology is greatly affected by subsequent increases in temperature.
The austenitic matrix, having a wide solubility range for carbon, becomes a field of carbide precipitates at high temperature. Carbon reacts readily with the chemical elements that are strong carbide formers such as chromium and in other instances tungsten. Carbide precipitates are very effective in their interaction with dislocations as long as their critical size is not reached. The effectiveness of this interaction gives these alloys their strength and resistance to creep and plastic deformation at high temperature. However, once carbides grow past a certain critical size, their interaction with dislocations becomes less effective and consequently the alloys start to lose their strength. HRA microstructures also are discussed in references 2 and 3.
To illustrate the carbide-dispersion stages, samples taken from as-cast HRAs were exposed to high temperatures and their microstructures compared with those of samples taken from furnace components having different service lives.
Microstructure evolution in neutral atmosphere
Standard cast HRAs basically are composed of nickel, chromium and iron. Compared with the amount of these basic elements, carbon is present only in small amounts, but the carbon content still is much greater than that present in wrought alloy counterparts. Chromium, a strong carbide former, has a strong affinity for carbon, and reacts (along with some other carbide forming elements) with carbon to form different carbides. The carbide microstructure changes with furnace heat-treating atmospheres, such as a neutral atmosphere (no excess oxygen or excess carbon) in a heat-treating operation or an atmosphere containing excess carbon.
Immediately after pouring the molten HRA into a mold, it starts its solidification process. Molten metal starts cooling upon its first contact with the mold walls and begins the process of grain nucleation. Grains start forming from spikes (dendrites) of already solidifying metal. These first spikes become trunks to subsequent spikes. The process of grain formation continues until the limiting parameters of grain formation, including temperature and changes in the chemical composition of the remaining molten metal present at the interfaces of these evolving grains, cease to be favorable.
During the formation and growth of dendrites, the molten metal rich in heavy chemical elements solidify first. The remaining molten metal, which is rich in lighter chemical elements, is pushed out and solidifies last. When the dendrite formation and growth for any grain are complete, the dendrites are separated and individually surrounded with pools of solidified metal rich in lighter chemical elements. These pools are called interdendritic regions, which are rich in carbon. Chromium and other elements bond with carbon in these regions and form networks of carbides called primary carbides.
Generally with proper casting procedures, the morphology of the primary carbides has fine, sharp edges. A photomicrograph of the microstructure of an actual as-cast HRA is shown in Figure 1. The primary carbide network is more or less outlined depending on the carbon content in the casting. The matrices of the dendrites (the interior of the dendrites) don't show any carbide. Because these alloys are austenitic, they have a large solubility of carbon. Although the carbides are not precipitated within the dendrite matrices, carbon in plentiful amounts is uniformly distributed within them.
HRAs are designed for use in high temperature applications. Their carbide microstructure does not show any noticeable change from room temperature until a temperature around 1100 F. In the temperature range of 1100 to 1200 F (595 to 650 C), extremely fine particles start precipitating in the dendrite matrices. The concentration of precipitates is higher in the regions of the dendritic matrices adjacent to the interdendritic boundaries than in the middle areas of the dendrites. These precipitates are a new category of carbides called secondary carbides.
The precipitation of the secondary carbides is temperature and time dependent. The longer the exposure of the casting to this temperature range, the higher the concentration of the secondary carbide becomes. When the concentration of excess carbon in the regions adjacent to the interdendritic boundaries is reduced, the precipitation of secondary carbides also is reduced. A modeled microstructure of this condition is shown in Fig. 2a. Note that primary carbides have not exhibited any change in their morphologies. The beginning of the secondary carbide dispersion in an actual casting is shown in Fig. 2b.
Precipitation of secondary carbides increases with increasing temperature due to the thermal energy that builds up in the casting as shown in Fig. 3a. At a temperature of 1400 F (760 C), secondary carbides are no longer distributed only in the vicinity of the interdendritic regions but also throughout the matrices of the dendrites. The amount of precipitates also is affected by time at temperature. Eventually, the carbon concentration in the dendrites and in the grains decreases. Secondary-carbide precipitation stops when it reaches a critical level in conjunction with unfavorable precipitation factors.
The coalescence of carbides is activated at 1400 F, although at a very small scale. Narrow strips depleted of secondary carbide precipitates adjacent to the primary carbides become apparent. This is a consequence of coalescence of secondary carbides with themselves and with primary carbides. Figure 3b shows the microstructure of a casting at 1400 F, in which secondary carbide precipitation has not reached completion.
The most dominant process at a temperature of 1600 F (870 C) is the coalescence of carbides. Because coalescence involves the joining together of carbides, the amount of precipitates decreases. Also, there is no noticeable precipitation of new secondary carbides. There is, however, a noticeable change in the sizes of the secondary carbides. Although the primary carbides are increasing in volume, it is not as apparent at this temperature. This condition is depicted in Fig. 4a, while the microstructure of a casting in service at 1600 F is shown in Fig. 4b.
Changes in the HRA microstructure are more pronounced at a temperature of 1800 F (980 C), as depicted in Fig. 5a. There are no signs of new precipitation of secondary carbides, and carbide coalescence is so active at this temperature that the amount of secondary carbides drastically drops. Also, the concentration of noncoalesced secondary carbides diminishes at a high rate, and coarsening of primary carbides becomes very apparent. The carbides are no longer well defined with fine, sharp edges, but instead are rounded. Carbide-depleted strips adjacent to the primary carbide network widen. The microstructure of a casting at this temperature is shown in Fig. 5b.
A typical microstructure for a standard HRA at a temperature of 2000 F (1095 C) is modeled in Figure 6a. Note that the carbides become blocky and huge compared with their shape at 1800 F. Primary-carbide coarsening occurs by advanced coalescence of secondary carbides, eventually depleting them from the matrices of the grains and leaving only a few within the matrices of the dendrites. Figure 6b shows the microstructure of a casting at 2000 F.
At temperatures higher than 2000 F, the carbide coarsening process starts reversing itself. At a temperature of 2150 F (1180 C), the dendrite regions in particular and the grain matrices in general become free of any precipitates. Even the primary carbides start dissolving back into solution. This condition is depicted in Fig. 7a, and the microstructure of a casting at this temperature is shown in Fig. 7b. The carbon diffuses back into the austenitic matrix to homogenize its distribution.
Why HRAs eventually fail
The evolution of casting microstructure at time and temperature helps explain how furnace parts made of HRAs fail in carburizing (discussed below) and oxidizing atmospheres. In a furnace where carbon is added to carburize parts, carbon not only diffuses into the heat-treated parts, but also it diffuses into the HRA furnace components such as trays, chain guides, roller rails, rollers, radiant tubes, fixtures, rolls, belts and other parts. Carbon diffusion is a function of temperature and time. The higher the temperature, the more accelerated the carburization process becomes. Therefore, a casting exposed to a temperature of 1800 F (980 C) has a much coarser primary and secondary carbide network than what is shown in Fig. 5. This comparison is illustrated in the microstructure shown in Fig. 8. Because carbon diffuses from the carbon-rich furnace atmosphere into the casting, the surface of the casting will have a higher carbon concentration than will the interior of the casting. The outer sections of a casting wall will have larger carbides present than will its interior.
Carbides, being ceramic compounds, are brittle. The more accelerated the diffusion of carbon into the casting, the coarser the carbides become and the more strains and stresses they introduce within a casting. Upon reaching a certain critical size of carbides (which could vary from one HRA to another depending on the ductility of the alloy), the ductility of the matrix of a grain can no longer support the built-in stresses and strains imposed by the changes in carbide size. This initiates the failure process. Microcracks start to develop around these oversized carbides and they begin branching into one another and subsequently form large macrocracks. The macrocracks eventually end the service life of the casting.
Another case of failure caused by the oversized carbides occurs during the thermal cycling of furnace parts. The coarser the carbides get, the more influence they impose on the failure of a casting. Oversized carbides do not resist thermal fluctuations. Because carbides are ceramic compounds, they start cracking when subjected to temperature spikes or fast ramp up and ramp down of a furnace. As previously shown in the photomicrographs, carbides-especial primary carbides-form continuous networks, and because of their brittle nature, once a carbide segment of a network starts cracking, crack propagation is very fast. Macrocracks are, therefore, produced readily in heavily carburized castings, which causes shorter service life for furnace parts.
Also, most HRAs with higher contents of chromium, tungsten or other carbide formers and no carbide-refining additives are more prone to carbide coarsening than are alloys made using carbide-refining technology. Heading off premature failures
The service lives of HRA furnace hardware can be extended and furnace operating time can be maximized via metallurgical treatment of the alloy microstructure and by following basic maintenance recommendations. An example of metallurgical treatment is using carbide-refinement technologies (such as that used to manufacture Steeltech's Advanced Alloys-see August 2002 IH, p 40) and carbon-diffusion barriers to avoid coarsening of carbides.
Some maintenance practices include:
- Avoiding thermal shocking of furnace hardware other than that intended for use in a quenching operation
- Adopting progressive furnace heating and cooling
- Calibrating burners periodically on a regular schedule
- Monitoring furnace temperatures and using tighter temperature tolerances at the operating temperature to minimize surges
- Using neutral flames for longer radiant-tube service life
- Eliminating bursting flames inside radiant tubes, which reduce radiant-tube service life and result in inefficient furnace heating
- Using the HRA recommended for a specific application because different HRAs are designed for different temperature ranges, atmospheres and cooling processes