Fe-20%Cr-5%Al Alloy by Pre-Annealing Treatment
2. Introduction
C. Steve Chang, Leigh Chen, and Bijendra Jha Engineered Materials Solutions, Inc., Attleboro, MA 02703 USA
1 Abstract
Alloys of Fe-20Cr-5Al have been used extensively as the material of choice in metallic cata- lytic converter substrates. The alloy chemistry has been developed through the last decade to provide the oxidation weight gain resistance that was thought to be adequate. Mainly by the addition of rare earth and active elements, the cyclic oxidation behavior has been improved to the point to meet the regulation requirements on the durability of catalytic converters. The major uncertainty on the understanding of oxidation behaviors of thin gauge Fe-Cr-Al foil is the cause of foil length change. It has been suggested that this length change is one of the causes of buckled honeycomb observed in the fractured substrate. Analysis on the stress and strain of thin gauge Fe-Cr-Al foil under the oxidation condition has been published to account for the length changes due to the oxidation. However, the potential metallurgical factors that control the length change has not been rationalized yet.
In this paper we will present (1) the oxidation test results on Fe-Cr-Al foils with different gauge and chemistry, (2) the microstructure evolution and (3) oxide scale development during the oxidation test. The oxidation failure mechanisms will be demonstrated. A phenomenologi- cal model to incorporate the alloy chemistry, microstructure and oxide scale will be described to account for the oxidation behaviors of these Fe-Cr-Al alloys.
The alloy of choice for the metallic converter substrate has been the ferritic stainless steel with a nominal composition of 20wt% Cr, 5wt% Al and the balance of Fe. The addition of 5wt% Al provides the stable scale for the alloy to be used above 1100°C while the 20wt% Cr provides the oxidation and corrosion resistance from the ambient to where the Al oxidation becomes significant. The cyclic oxidation resistance is improved by the addition of rare earth elements such as Y, La and Ce. The addition of active elements such as Zr, Hf provides further improvement on the oxidation resistance [3].
The oxidation resistance is commonly measured by the amount of sample weight gain due to the scale formation. The oxidation weight gain is important, as it is an indication of the effec- tiveness of protective scale. The oxidation weight gain for the Fe-20Cr-5Al alloy has seen sig- nificant improvement and known to be one of the most oxidation resistant materials. Nonethe- less, Fe-Cr-Al foils for the catalytic converter applications have to be dimensionally stable to avoid rupture of the substrate during the service [4]. The source of the dimensional instability has been attributed to the stress between scale and ally, which causes the creep deformation of substrate. Heats of Fe-Cr-Al alloys having identical oxidation weight gain behaviors have shown drastically different dimension changes. This paper summarizes the effort to rationalize the oxidation length change mechanism and attempts to draw a guideline to prevent dimen- sional instability.
On the practical aspect of producing Fe-Cr-Al foils, it has been known that the conversion of Fe-20Cr-5Al alloy to thin gauge has been difficult and contributed to the high cost of the mate- rials. A commercially feasible process [5,6] to produce thin gauge Fe-Cr-Al foils has been developed to address the issue. The process starts out with roll bonding the Fe-Cr alloys (AISI 4xx type ferritic stainless steels) to proper amount of Al alloys to form a three layer compos- ites. The composite coil, which was roll bonded with proper attention so it can be cold rolled to an intermediate thickness. The intermediate thickness was selected to allow a homogenization heat treatment to be conducted at the temperature and time, which are commercially accept- able. After the heat treatment, the strip is cold rolled to provide the desirable temper and finish.
Obviously, the advantage of alloying the Al to Fe-Cr by the roll bonding process is to circum- vent the limits of Al content and the conversion difficulties[7].
In this study, oxidation tests on the oxidation weight gain and dimension stability were con- ducted on materials taken from roll bonding produced Fe-Cr-Al alloys. The effects of chemis- try and heat treatment on the oxidation behaviors, in particular the dimensional stability is ra- tionalized with a phenomenological model developed from examining the length change be- haviors of hundreds of samples. This oxidation length change model will attempt to show that from the synergetic effects of physical (e.g. density and thermal expansion) and metallurgical (e.g. scale adhesion) changes, the various length change behaviors can be accounted for.
3 Experimental Procedure
3.1 Materials
The Fe-Cr-Al alloy foils were produced via the roll bonding process. In brief, Fe-Cr alloy (stainless steel) strips were clad with Al strips on both sides by feeding the strips into a four- high rolling mill. The cladding process was developed to apply sufficient reduction to form a well adhere three layer (Al/SS/Al) composite. The roll-bonded composites were cold rolled to
an intermediate thickness followed by heat treatment to homogenize the Al layers with the Fe- Cr alloy. A finial cold rolling was applied to reduce the alloyed strip to the finish foil thick- ness. Fe-Cr-Al alloys with Al content range between 5 to 8 wt% are readily produced by this roll bonding process. Rare earth addition in the manner of La+Ce was accomplished by casting the Fe-Cr alloy with misch-metals. Typical alloy chemistry is shown in Table 1 in weight %.
Table 1: Nominal composition of Fe-Cr-Al alloys in wt%
C Mn P S Si Cr Ni Al N O La Ce
0.02 0.2 0.02 <.002 0.2 21 0.2 6 0.02 0.008 0.01 0.03 3.2 Oxidation Test
The oxidation test was conducted on a honeycomb type sample at the cell density of approxi- mately 400 cpi (cell per square inch). The cold rolled Fe-Cr-Al alloy foils were degreased prior to corrugation and assembling. Honeycomb samples consisted of one flat and one corru- gated strip. The two strips were rolled up to form a cylinder along the rolling direction. The cylindrical honeycomb samples had the dimensions of approximately 74 mm in length and 20 mm in diameter.
Flat coupons (50 mm by 50 mm) were also used to obtain suitable samples for microstruc- ture and scale characterizations. These coupons were prepared and tested in the same condi- tions as the honeycomb samples except the dimensional change was not measured. However, oxidation weight gains of coupon and honeycomb type sample were always cross-checked to ensure that the oxidation weight gain followed the same rate.
Oxidation tests were conducted on samples in the cold-rolled and vacuum heat-treated con- ditions. Vacuum heat treatment was performed in a diffusion pump evacuated, cold wall fur- nace at 1200 °C for 30 minutes. The typical vacuum level at the soak temperature was better than 10–5 torr.
The oxidation test was conducted in a semi-cyclic manner. The samples were placed in the furnace at the ambient condition. The furnace temperature was ramp to 1100°C in two hours and held for increasing duration (5, 20 and 25 hours). The duration of hold was increased to 50 hours per cycle afterward until a total of 400 hours was accumulated. After each hold period, furnace was ramp down to ambient in 6 hours. Samples were then removed to measure the weight gain and length changes. The oxidation sample weight was measured in a precision scale with accuracy to 0.00001g. The dimensional change of oxidation sample was measured by a dial indicator on the length of the honeycomb cylinder.
Oxidation weight gain results were analyzed by applying the parabolic rate equation (1) [8]
between the weight gain (DW) and test time (t) for the alumina forming alloys:
W Dt
, = (1)
Where D is the parabolic rate constant. The rate constant was obtained by a linear curve fit on the square root of time plot. The validity of equation 1 is verified by (1) plot the weight gain against square root of time for a linear dependence and (2) plot log-log of weight gain against time for ½ slope value.
3.3 Microstructure and Oxide Scale Characterization
The microstructure of Fe-Cr-Al alloys and the morphology of surface scales were examined by the optical microscope as well as by the energy dispersive x-ray (EDX) equipped scanning electron microscope (SEM). The edge-on type samples for scale morphology examination were obtained by pulling the oxidation-tested samples in a tensile testing machine until frac- ture.
4 Oxidation Test Results
Six oxidation test samples are shown in Figure 1in the following conditions. From left to right, these samples are: (a) cold rolled, (b) vacuum heat treated, (c) 400 hours tested, normal, (d) 400 hour tested, abnormal length, (e) 400 hour test, Al depleted, no nreakaway, and (f) 400 hour tested, weight gain breakaway. The oxidation weight gain and length change results will be presented in the following sections to show the effect of the sample conditions such as heat treatment and alloy chemistry on the oxidation behavior.
(a) (b) (c) (d) (e) (f)
Figure 1: Oxidation samples (a) cold rolled; (b) vacuum heat treated; (c) 400 hr tested, no breakaway; (d) 400 hr tested, length change abnormal; (e) 400 hr tested, Al depleted; (f) 400 hr tested, weight gain breakaway
4.1 Effect of heat treatment
The oxidation weight gains and length changes for the representative cold rolled and vacuum annealed samples with normal length change are shown in Figure 2 (a). The square root of time plat is shown in Figure 2 (b). The samples were 50 micron thick and the Al content of 6 wt%.
Length change results were shown together with the oxidation weight gain to delineate the relationship between the two.
The rapid oxidation weight gain (open square markers) of cold rolled sample quickly de- pleted the Al in the alloy (at ~150 hours). The oxidation weight gain curve of cold rolled sam- ple in Figure 2 (b) changed its slope at this point.
Figure 2: Typical oxidation weight gains and length changes for Fe-Cr-Al foils at 1100°C, room air; (a) linear time scale, (b) square root of time scale
Meanwhile, the length change (solid square marker) of cold rolled sample started to show rapid increase at 150 hours as well. However, the breakaway of oxidation weight gain did not start until 300 hours of oxidation time. In contrast, the vacuum annealed sample showed much lower oxidation weight gain. The parabolic rate constants (kp) are 160 and 49.6 (10–14 g2/cm4- sec) for the cold rolled and vacuum annealed samples, respectively. The parabolic rate for the cold rolled sample was obtained by linear curve fit on the weight gain between 5 and 150 hours while that for the vacuum annealed sample was obtained between 5 and 400 hours.
0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014
0 100 200 300 400
Time, hours Weight Gain/area (g/cm2)
-1 0 1 2 3 4 5 6
Length Change (%)
w eight vacuum annealed w eight cold rolled length vacuum annealed length cold rolled
0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014
0 200 400 600 800 1000 1200
Square Root of Time (sec1/2) Weight Gain/area (g/cm2)
-1 0 1 2 3 4 5 6
Length Change (%)
50 micron vacuum annealed 50 micron cold rolled length vacuum annealed length cold rolled a)
b)
The depletion of Al in the cold rolled sample was verified by SEM/EDX analysis on the coupon samples being oxidized along with the honeycomb samples. The Al content in cold rolled sample being oxidized for 150 hours was less than 0.5 wt%. The calculated oxidation weight gain of Fe-Cr-Al alloy, assuming that Al was oxidized to form aluminum oxide, for 50 micron gauge and 6% Al composition, would yield weight gain of 0.00095 g/cm2. This value matches very well with the change of weight gain rate (at weight gain of approximately 0.001 g/cm2) as well as the length change breakaway. Based on the experimental results of weight gain and length change, it is concluded that the length change breakaway occurred when the Al is depleted in the cold rolled samples. The Al content and time to reach length change breakaway is always related for the cold rolled samples.
The vacuum annealed sample gained approximately 0.0009 g/cm2 of weight at the end of 400-hour test cycle and this is less than the Al depletion point. The vacuum annealing heat treatment reduced the oxidation weight gain rate and thus delayed the onset of length change breakaway. For the vacuum annealed samples, the correlation between the Al content and time to length change breakaway is not as predictable as that for the cold rolled sample. The unpre- dictable length change breakaway for vacuum annealed samples will be shown in the next sec- tion when oxidation behaviors for foils at different gauges are presented.
4.2 Effect of Foil Thickness
The oxidation weight gains for 6wt% Al, Fe-Cr-Al alloys tested at the thickness of 30, 40 and 50 micron are shown in Figure 3 for the vacuum annealed samples. The results are presented as weight gain and length change against square root of time.
Figure 3: Oxidation weight gain and length change for materials show abnormal length change behaviors at 30, 40 and 50 mm.
Samples were originally from the same master coil except being rolled to different thickness.
The weight gain results are shown in open symbols while length changes are shown in solid