A New Approach for Improving the Ductility of Austenitic Stainless Steel by Austempering and Alloyin

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A New Approach for Improving the Ductility of Austenitic Stainless Steel by Austempering and Alloying with Aluminum Ahmed I. Z. Farahat Plastic Deformation Department –Central Metallurgica Research and Development Institute, CMRDI/P.O.Box 87 Helwan, Cairo, Egypt ahmedzaky61@yahoo.com

Abstract

austenitic phase undergoes a series of decomposition reactions. It has also been reported that a fine Austenitic stainless steel (ASS) has been widely used in distribution of (Fe,Mn)3AlC carbide (κ‐phase) appears different applications. The industrial demand for stainless during aging at 500‐750°C, resulting in a significant steels with improved resistance to oxidation at high temperature was the main driving force for adding improvement in mechanical strength. Alloys for aluminum as an alloying element to austenitic stainless steel. cryogenic purposes are subjected to quenching from However, the addition of aluminum increases the work‐ the homogeneous solid solution temperature field to hardening property and deteriorates the ductility of stainless avoid precipitation of κ‐phase particles, especially steel. This paper addresses a new approach to overcome the along grain boundaries that could reduce fracture deterioration of ductility by alloying (ASS) with 4wt% Al toughness. Second‐generation advanced high‐strength using vacuum induction furnace. Hot forging has been steels developed as lightweight steels for the performed at 1000˚C with a reduction ratio of 90% in cross‐ sectional area. X‐ray diffraction was used to determine the automotive industry are potential candidates for the different phases formed due to hot forging. Results showed armor application. Such steels contain high levels of that alloying with Al decreases the austenite phase and aluminium, which can lower the density by 12–18% increases the ferrite phase in (ASS), therefore, austempering relative to mild steel. The high work hardening due to process was carried out at different temperatures to increase Al content plays a dominant role during deformation the austenite phase and consequently to enhance ductility. and results in excellent mechanical properties. The Compression and hardness tests were performed at room mechanisms, responsible for this high work hardening, temperature to determine the effect of Al alloying, phase are related to the stacking fault energy (SFE) of the formation, and tempering temperatures on the strength and ductility of (ASS). austenitic phase. The SFE changes with the alloy composition and the deformation temperature. Its Keywords magnitude controls the ease of cross‐slip, and thus Al alloying; Hot Forging; Austenite Phase; Compression Strength different deformation mechanisms can be activated at different stages of deformation. As the SFE decreases Introduction the stacking faults become wider and cross‐slip more ASS has been extensively used in chemical and aeronautic difficult and mechanical twinning is favoured. Most industries, and exhibits satisfactory combinations austenitic steels, such as ASS and high manganese of mechanical strength, fracture toughness and Hadfield steels, have low‐to‐moderate SFE and microstructural stability over a wide temperature therefore tending to form extended stacking faults, range. Alloying of Austenitic stainless steels with deformation twins and planar dislocation structures.

Aluminum has been used to develop stainless steel These different lattice defects strongly influence the withstanding temperatures up to 400˚C. Above 850°C, stress strain response. Therefore, the present study has 8–10%Al and 0.8–1%C enhances the supersaturated been carried out to investigate the effect of Al on the austenitic structure. However, during isothermal mechanical behavior and different microstructures of holding within the temperature range 350–700°C, the ASS containing low Mn content.

TABLE 1 CHEMICAL COMPOSITION, wt% Alloy ASS(304)+4Al ASS (304)

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C 0.065 0.065

Si 0.39 0.39

Mn 1.5 1.5

P 0.035 0.035

S 0.028 0.028

Cr 17.6 17.6

Ni 9.28 9.28

Mo 0.28 0.28

Cu 0.20 0.20

Al 4.1 ‐‐‐‐‐

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average grain size of the as‐cast structure is 703μ m with standard deviation of 308 μm (Fig.3) as listed in Table 2b.

Experimental Work 100 kg of ASS containing 4wt%Al was fabricated using induction vacuum furnace. The chemical composition is listed in Table 1. Dilatation test was performed to determine the critical phase transformation temperature after adding Al. Hot forging (90% reduction of cross‐ sectional area) was done at 1000˚C after heating for 30min.

The samples after hot forging were machined according to the compression test standard ASTM A370 and the compression test was performed at room temperature. Austempering process was done starting at intercritical annealing temperatures between AC1 and AC3 followed by quench tempering at 400˚C.

Results and Discussions Dilatation The dilatation behavior of the stainless steel containing 4 wt%Al compared with ASS. It was noticed that ASS containing 4%Al exhibits AC1 and AC3 transformation temperatures (660 and 970°C respectively) due to Al content. It is well known that the most important elements in ferrite forming elements group are Cr, Si, Mo, Wnd Al. Fe‐Cr alloys containing more than 13% Cr exhibit ferritic structure.

FIG. 2 MICROSTRUCTURE AND GRAIN SIZE DISTRIBUTION OF AS CAST ASS

FIG. 1 DILATATION CURVE OF STAINLESS STEEL CONTAINING AL

As‐cast Structure The as‐cast microstructure of ASS with and without Al content is shown Figure 2. It seems that alloying of ASS with Aluminum converted the dendrite microstructure of ASS into equiaxed coarse grains containing (Fe,Mn)3AlC carbide (κ‐phase). From statistical analysis, the average grain size of the as‐cast ASS is 252 μm as with standard deviation of 46 μm as illustrated in the histogram of Fig.2 and as listed in Table 2a while the stainless steel containing 4%Al, the

FIG. 3 MICROSTRUCTURE AND GRAIN SIZE DISTRIBUTION OF AS CAST ASS ALLOY CONTAINING 4.1wt%Al

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TABLE 2 GRAIN SIZE DISTRIBUTION OF THE AS CAST ASS

Parameter Mean Standard Error Median Mode Standard Deviation Confidence Level (95.0%)

a‐ASS grain size, μm 252 9 253 324 46 19

TABLE 4 GRAIN SIZE STATISTICAL ANALYSIS OF HOT FORGED ASS

b‐as cast ASS containing Al, μm 703 21 635 836 308 42

Parameter Mean Standard Error Median Mode Standard Deviation Sample Variance Confidence Level (95.0%)

The non‐metallic inclusions of stainless steel containing Al are mainly Alumina type due to alloying with Aluminum as shown in Fig.4 with average size of 110 μm as illustrated in the histogram of Fig.4.

a‐Grain size of hot b‐Grain size of ASS forged ASS, μm containing 4.1%Al 113 209 13 18 99 194 235 133 60 115 3555 43 26

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a‐Hot forged ASS without Al

a‐Non‐metallic inclusion

b‐Hot forged ASS containing 4wt%Al FIG. 5 GRAIN SIZE DISTRIBUTION OF HOT FORGED ASS

2) Effect of Hot Forging and Al Content on Hardness

b‐Non‐metallic size distribution

Figure 6 shows the hardness distribution of the as‐ cast and hot forged ASS. It seems that the hot forging increases the average hardness value from 48 to 58 HRA (~ 21% increase) as demonstrated in Table 5a and b.

FIG. 4 NON‐METALLIC OF ASS CONTAINING 4.1wt%Al

1) Effect of Hot Forging on the Average Grain Size The average grain size is approximately 113 μm as calculated in Table 4a and shown in Fig.5a. Hot forging process decreases the grains size to 55%.

On the other hand, the average grain size of hot forged ASS containing 4%wtAl is 209 μm as shown in Fig.5b and listed in Table 4b. These results have proved that the hot forging process decreases the grain size of ASS containing Al to approximately 30%. Moreover, the refinement of grain size decreases due to Al content.

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TABLE 5 HARDNESS OF ASS

Parameters Mean Standard Error Median Mode Standard Deviation Confidence Level (95.0%)

a‐Hardness of as b‐Hardness of cast ASS, HRA hot forged ASS 47.5 58.2 0.5 0.8 47.3 59.7 —— 51.4 1.7 3.6 1.2 1.6

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TABLE 6 HARDNESS OF AS‐CAST ASS CONTAINING 4Al

Parameters Mean Standard Error Median Mode Standard Deviation Confidence Level (95.0%)

a‐Hardness of b‐Hardness of as‐cast, HRA hot forged, HRA 66.7 65.3 0.1 0.3 66.7 65.3 66.7 65.3 0.5 1.3 0.2 0.7

The hardness distribution of the as‐cast and hot forged ASS containing Al is shown Fig.7. It was found that hot forging has no effect on the average hardness value where it is approximately constant (67HRA) as demonstrated in Table 6a and b.

Conversely, alloying of ASS with Aluminum and hot forging increase the average hardness of ASS from 48 HRA to 67HRA (~ 40&16%). This can be attributed to the formation of Fe‐Al.

a‐As‐cast ASS

3) X‐ray Diffraction After casting of stainless steel with Al content, the expected microstructure changes and the volume fraction of austenite and ferrite are different than fully ASS. Therefore, the Cr and Ni equivalent equations were used to expect the volume fraction of ferrite and austenite as shown in Fig.8. It is clear that austenite phase decreases to approximately 30% while the ferrite becomes 70% as expected from the modified Schaeffler diagram.

b‐Hot ro led ASS FIG. 6 HARDNESS DISTRIBUTION OF AS CAST AND HOT FORGED ASS

a‐As‐cast ASS containing 4wt%Al

FIG. 8 MODIFIED SCHAEFFLER DIAGRAM FOR THE AS CAST STAINLESS STEEL CONTAINING AL (ABOUT 70% FERRITE)

Figure 9 shows the x‐ray diffraction plateau of different cases for the ASS containing 4%wtAl after austempering process. It was found that the austenite phase appears at 2θ equal to 43˚ before the ferrite phase which appears at 44˚. To enhance the ductility of the steel via increasing the austenite phase volume fraction, intercritical annealing was done at different temperature between AC1 and AC3 followed by quench tempering at 400˚C for 30min. The austenite phase increases with the increasing tempering temperature as shown in Fig.10. It was proven that austempering

b‐As‐cast and hot forged ASS containing 4wt%Al FIG. 7 HARDNESS DISTRIBUTION OF THE AS CAST AND HOT FORGED ASS CONTAINING AL

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temperature at 900˚C exhibits the best combination of austenite‐ferrite (approximately 50‐50%).

Conversely, the austenite phase abruptly decreases at 950˚C. Therefore, the effect of austenite and ferrite can be directly reflected on the maximum compression strength and hardness as shown in Figures 11 and 12. Figure 11 confirms that increasing the austempering temperature increases the compression strength and strains due to austenite transformation into martensite during the test of compression. At 750°C o,nly sigma phase appears at 2θ equal to 41.85˚and disappears with the increasing austempering temperature, [13].

FIG. 12 EFFECT OF AUSTENITE VOLUME FRACTION ON THE HARDNESS

Conclusion Alloying of ASS with Aluminum coarsens the as‐cast grains of stainless steel but Al converts grains into equiaxed structure showing not a dendrite structure.

Aluminum and (Fe,Mn)3AlC carbide forming (κ‐phase) which increases hardness and decreases the volume fraction of austenite. Austempering at 900˚C exhibits the best combination of austenite‐ferrite and enhances the ductility. On the contrary, the austenite phase sharply decreases at 950˚C.

FIG. 9 X‐RAY OF THE AS CAST, HOT FORGED AND AUSTEMPERED ASS CONTAINING 4wt%Al

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FIG. 10 EFFECT OF AUSTEMPERING TEMPERATURE ON THE AUSTENITE VOLUME FRACTION

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FIG. 11 EFFECT OF AUSTENITE VOLUME FRACTION ON THE MAXIMUM COMPRESSION STRENGTH

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