Exploring the Benefits of Alumina-Forming Alloys for Furnace Application

Finding ways to extend the service life of parts and fixtures and reducing the energy associated with these processes are goals of the Alloy Life Extension Project, a research program currently under way at the Center for Heat Treating Excellence (CHTE) at Worcester Polytechnic Institute (WPI) in Massachusetts.

Recently, much of the center’s focus has been on assessing the benefits of alumina-forming alloys, which excel in high-temperature applications because they oxidize more slowly than chromia formers and are a barrier to carbon uptake.

Project Overview

More specifically, this research project:

Identifies coatings or diffusion treatments that will improve the oxidation, carburizing and sulfidization resistance (or creep resistance) of alloy components used in radiant tubes, rails, rollers, hearths or trays, and baskets used to fixture parts being treated.
Identifies materials with lower heat capacity for energy savings and analyzing the cost savings with these materials.
Conducts failure analysis on failed parts for CHTE members.

According to study director Rick Sisson, George F. Fuller professor of mechanical engineering at WPI and technical director of CHTE, “Manufacturers are spending lots of money for alloy fixtures that go into carburizing furnaces. It is well known that alumina-forming alloys exhibit improved oxidation and carburization resistance as compared to chromia formers. As we do research on alumina formers, we are looking at ways to allow industry to work more efficiently by lessening fixture replacement costs, reducing energy consumption and improving product quality.”

Research Process

Working closely with Sisson is PhD candidate Anbo Wang. Together they have developed a research project that includes failure analysis of selected alloys and characterization of the alloys after treatment. More detail can be found in Figure 1.

Alloys Tested

Table 1 shows the different alloys tested in the project.

RA330 is a widely used alloy in carburizing furnaces. RA330 test samples were pack aluminized to form an alumina-rich coating at the surface. After exposure in the furnace for 3, 6, 12 and 18 months, aluminized RA330 samples were removed. The photomicrograph after three months service in the carburizing furnace is seen in Figure 2. Samples were electrolytic etched with 10% oxalic acid at 10V for 30 seconds.

Figure 2 shows the aluminized coating at the surface of RA330, which is composed of two layers. The XRD pattern revealed that the outer layer is mainly nickel aluminides (β-NiAl) with some aluminum oxides (Corundum: α-Al₂O₃. And the EDS analysis presents that the inner layer is discontinuous nickel aluminide (β-NiAl) in the austenite (FeNi) matrix. It was seen that the aluminized RA330 had better carburization resistance than the uncoated RA330.

Alloy 625 is a solid-solution, matrix-strengthened, face-centered-cubic alloy. At higher magnification, Ni₃Nb precipitate could be clearly identified (Fig. 3). According to the XRD data, Ni-rich intermetallic compounds (β-NiAl) are also found in the matrix.

The γ' phase typically consists of Al and Ti as the major alloying elements (i.e., β-NiAl), while the γ'' phase (Ni₃Nb) precipitates tend to be more efficient strengtheners than γ'-phase precipitates. SEM photomicrographs were also conducted on Alloy 625. At higher magnification, Figure 2 shows NiNb precipitate, which could be clearly identified. According to Figure 3, Ni-rich intermetallic compounds, β-NiAl are also found in the matrix.

The aluminized Inconel 625 also displayed better carburization resistance than the uncoated alloy.

RA 602 CA is a nickel-based alloy that employs high chromium content along with aluminum and yttrium additions that produce a tightly adherent oxide, allowing the alloy to operate at temperatures in excess of 1230˚C (2246˚F).

RA 602 CA has more than 24% Cr, which is sufficient for continuous Cr₂O₃ protective layer. RA 602 CA also has about 2% aluminum that formed self-repairing alumina scale at the surface. Figure 4 shows Al₂O₃ scale on the surface of aluminized RA 602 CA. Also shown is a 20 µm nickel-aluminide intermetallic-compound layer. Beneath the intermetallic layer, the interior matrix of RA 602 CA is uniform austenite iron-nickel phase.

Due to the exposure in a gas carburization furnace, chromium carbides Cr₂₃C₆ and Cr₇C₃ precipitated in the grain boundaries of the austenite matrix, which is shown in Figure 5.

304 Stainless Steel

Peterson Steel Corporation of Worcester, Mass., provided samples of 304 to be tested. Stainless steel 304 is the original 18-8 stainless steel, which is less expensive compared to Fe-Ni-Cr based alloys. The composition of 304 is listed in Table 5.

The aluminized 304 also displayed better carburization resistance than the uncoated 304.

Key Findings to Date

The project has already produced some interesting findings, which are summarized here.

The work to date reveals that alloys with stable alumina-layer protection on the surface have better anti-carburization properties than other alloys.
Preliminary characterization of 304/316 stainless steel indicates that aluminizing (the process of diffusing Al into the surface to form β-NiAl) could effectively eliminate the formation of carbides in the alloys.
APMT is found to be resistant to oxidation and carburization. Characterization work was conducted on the Sandvik alloy APMT after exposure in a commercial carburization furnace. APMT was designed with a high Al content to form Al₂O₃ in oxidizing environments.
Preliminary metallographical analysis indicates that the primary reason for failure of RA330 was the excessive carburization that leads to carbide formation at grain boundaries.

Energy Efficiency

Heat-treating companies spend considerable time and money replacing furnace parts and the fixtures used in those furnaces. Finding ways to extend the service life of parts and fixtures and reducing the amount of time it takes to heat them up and cool them down could produce considerable savings. That is why CHTE has undertaken its all-encompassing Alloy Life Extension Project.

In addition to the alumina-former work described previously, the project is also analyzing fixture design and material selection with an eye toward reducing the energy needed to repeatedly heat fixtures, which can be costly.

For more information: If you are interested in learning more about this research study or about CHTE and its other projects, please visit www.wpi.edu/+chte, call 508-831-5592 or e-mail Richard Sisson at sisson@wpi.edu or Diran Apelian at dapelian@wpi.edu.

About the CHTE Collaborative at WPI

The collaborative is an alliance between the industrial sector and university researchers that addresses short- and long-term needs of the heat-treating industry. Membership in CHTE is unique because members have a voice in selecting quality research projects that help them solve today’s business challenges.

Member Research Process

Research projects are member-driven. Each research project has a focus group comprised of members who provide an industrial perspective.
Members submit and vote on proposed projects.
Three to four projects are funded yearly.
Members have royalty-free intellectual-property rights to pre-competitive research.
Members have the option of paying to sponsor proprietary projects.
CHTE periodically does large-scale projects funded by the federal government or foundations. These projects keep members informed about leading-edge technology.
Members are trained on all research technology and software updates.

Other projects that CHTE is currently working on include: