A low cost heat treatment method could improve the wear resistance of austenitic manganese and aluminum steels.

Laura Bartlett and Sabra Serino, Texas State Univ., San Marcos, Texas

(Click here to see the story as it appears in the July/August issue of Modern Casting Design & Purchasing.)

High manganese and aluminum austenitic steels have been intensely investigated in recent years for their potential applications in military and transportation industries. These steels have exceptional combinations of high strength and toughness with excellent wear resistance. Adding aluminum in levels between 6% and 8.8 wt% reduces the steel’s density by 10% to 15% compared with quenched and tempered chromium and molybdenum steels. But it also decreases strain hardening and abrasive wear resistance. Metalcasting researchers have discovered that wear resistance may be improved in these steels through a low-cost case hardening heat treatment in a nitrogen atmosphere (nitriding) that produces a hard surface layer of aluminum nitride (AlN).

Low density high manganese and aluminum steels are being considered for use in tough and wear resistant automotive components and ballistic armor plating. These steels could be considered lightweight alternatives for components in the mining industry as well as ground-engaging components. For example, if Fe-30Mn-9Al-0.9C alloys with 15% lower density were substituted directly for the SAE 8620 steel track shoes of the Bradley Fighting Vehicle (BFV), the weight savings would be approximately 800 lbs. However, mechanical properties of cast Fe-Mn-Al-C alloys vary with composition, degree of age-hardening and steel cleanliness.  Age-hardening greatly increases strength in cast alloys but sharply reduces work hardening, toughness and abrasive wear resistance. A study of the abrasive wear resistance and strain hardenability of high manganese steel suggested a hard and wear resistance surface coating on the material would markedly increase the wear resistance while still allowing higher aluminum composition and thus lower density steels to be utilized.  

Surface modification often is used to greatly improve the wear, fatigue and corrosion resistance of both ferritic and austenitic steels. One of the most effective surface treatments to increase wear resistance is the nitriding process which produces a hard “white layer.”  Traditional gas and salt bath nitriding processes cause the release of toxic fumes and environmental pollution. Plasma nitriding, while much cleaner, is very costly and requires the use of expensive equipment.  

Nitriding high manganese and aluminum steel in gaseous nitrogen may be a cost effective, relatively pollution-free method for improving the wear resistance without reducing the aluminum content by producing a hard and wear resistant layer of aluminum nitride on the casting’s surface. Manganese steels are brittle in the as-cast condition, and common practice is to solution treat these steels at temperatures up to 2,012F (1,100C). This solution treatment could be performed in a nitrogen atmosphere to produce a wear resistant surface nitride layer at little additional cost. In the case of high manganese and aluminum steels, aluminum nitride forms instead of Fe3N.  The hardness of nitrides is known to increase with the amount of nitrogen, and the hardness of aluminum nitride has been reported to be 25.6 GPa, which is much higher than that of Fe3N (11.2-12.4 GPa).  

In a recent study, the effects of aluminum and silicon on the kinetics of aluminum nitride coating formation in a Fe-30Mn steel were determined.

Looking at the Results

In the study, steel test samples were solution treated to 1,922F (1,050C) for two hours and then rapidly quenched in ice water. The surface of the specimens was polished to a 0.3 μm finish and washed in ethanol directly prior to nitriding. Nitriding experiments were conducted in the temperature range of 1,652-2,012F (900-1,100C) under 99.9% pure N2. The furnace was purged for 20 minutes to eliminate residual oxygen prior to loading the specimens. The cross sections of the specimens were characterized utilizing optical metallography. A field emission scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) was used to characterize the morphology and chemical composition of the reaction layers.

The results of the chemical analysis show that the compositions of the steels vary mainly with regard to aluminum and silicon content. The microstructures of the respective steels after solution treatment for two hours at 1,922F (1,050C) and before the nitriding process are shown in Figures 1-3. The microstructures are similar and all steels were nearly 100% austenitic with only a few isolated islands of primary ferrite noted in Steel C (Fig. 3). The secondary dendrite arm spacing (SDAS) was also similar between the steels and measured between 50 and 75μm. The nitriding process was carried out for up to eight hours at 1,652, 1,832 and 2,012F (900, 1,000 and 1,100C) for the three different aluminum and silicon containing steels. Figures 4a-d show the optical micrographs of the polished cross sections of Steel B (8.8% Al and 1.6% Si) and Steel C (6% Al and 1.6% Si) after nitriding for two to six hours at 1,652F (900C).  After two hours at 1,652F (900C), Steel C is shown to develop twice the depth of aluminum nitride of Steel B.  

The depth of the aluminum nitride layer increased with nitriding time for both aluminum containing steels, and after six hours at 1,652F (900C), the depth of the aluminum nitride layer was measured to be an average of 170 μm and 230 μm for Steels B and C.  Increasing the process temperature to 1,832F (1,000C) increased the kinetics of the nitriding process, and after six hours, the average depth of the aluminum nitride layer increased to 200 μm in Steel B and 370 μm in Steel C, as shown in Figures 4 (e and f). Thus, for a constant silicon content of 1.6%, increasing the aluminum content from 6% to 8.8% lead to a decrease in the depth of the aluminum nitride layer for all times and temperatures. However, for a constant aluminum content of 8.8%, increasing silicon from 1.1% to 1.6% showed only a slight decrease in the depth of the aluminum nitride layer.

The morphology of the aluminum nitride appears similar between the respective steels, and it appears to precipitate and grow as plates along <111> crystallographic directions within the austenite. The secondary electron images of the aluminum nitride coating in Steel A (8.8% Al and 1.1% Si) and Steel B (8.8% Al and 1.6% Si) after nitriding for eight hours at 1,652F (900C) are shown in Figures 5 (a and b) and Figures 5 (c and d). The case depth and plate-like structure of the aluminum nitride are similar between the two different silicon containing steels (Figure 5).

The secondary electron micrographs of Steel B and Steel C are shown in Figure 6 after nitriding for eight hours at 2,012F (1,100C). In Steel B, the nitrided layer consists of a high density of longer and typically thinner plates with an average spacing of less than 5 μm (depending on the plane of polish) as shown in Figure 6 (a and b). In Steel C, the density of aluminum nitride in the reaction layer is much less, and the average spacing between the plates is greater than 10 μm as shown in Figure 6 (c and d). However, the plate thickness in Steel C is greater, and the case depth is almost 200 μm greater than in Steel B after nitriding for eight hours at 2,012F (1,100C).

A less lamellar arrangement of aluminum nitride is observed in Steel C as shown in Figure 6 (d). The average composition of the aluminum nitride precipitates was found to be invariant with temperature and steel composition.  The stoichiometric ratio of aluminum to nitrogen was close to one for all individual plate samples. The austenite matrix chemistry between the plates for Steels A and C after nitriding for six and eight hours at 1,832F (1,000C) and 2,012F (1,100C) were almost completely depleted of aluminum and enriched in manganese and silicon, which may also improve wear resistance. For steels nitrided at times greater than six hours, an approximately 10‑20 μm complex oxide layer was noted on the surface above the nitride layer, as shown in Figure 5d.

The presence of an oxide layer indicates that not all of the oxygen was eliminated from the furnace atmosphere. However, the oxide layer did not develop until after extended times during the nitriding process, and aluminum nitride formation was always favored because of the much higher solubility and diffusion of interstitial nitrogen in austenite over oxygen. Figure 7 shows the depth of the aluminum nitride layer as a function of time and temperature. Depending on the time and temperature, aluminum nitride coating thicknesses between 200 and 550 μm can be achieved. Steels A and B with 8.8% Al and 1.1% Si to 1.6% Si show similar coating thicknesses for all times and temperatures.

Although silicon is known to decrease the solubility of nitrogen in steels, increasing the silicon level from 1.1% to 1.6% had little effect on the depth of the aluminum nitride layer or the apparent density of the plates precipitated. However, increasing the amount of aluminum from 6% to 8.8% produced a denser array of very fine plates but sharply decreased the depth of the aluminum nitride layer. It also produced a 50% reduction in the diffusivity of nitrogen and increased the activation energy from 64 to 78 kJ/mol in the temperature range of 1,652-2,012F (900-1,100C).  

The results of the study show that high manganese and aluminum austenitic steels can be nitrided in a gaseous nitrogen atmosphere to produce a hard and wear-resistant layer of AlN at depths of up to 550 μm. This would broaden the use of low density, high manganese and aluminum steels to include some applications that require better wear resistance.

This article is based on the paper “Nitriding of Lightweight High Manganese and Aluminum Steels” (15-036), originally presented at the 119th Metalcasting Congress.