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Metal Casting Technologies : Whos who September 2011
METAL Casting Technologies September 2011 43 Microstructural observations and transformation sequence Figure 1(a) and (b) show the microstructure in the as-cast and solution treated conditions respectively. Atomic number contrast reveals the presence of sigma (σ) chi phase (x) and fine carbides at the austenite-ferrite (γ1 - δ) interfaces in the as-cast microstructure (view a). Solution treatment decomposed all the secondary phases and the structure consisted of austenite and ferrite (view b). The dark isolated particles are non-metallic inclusions. The sample keel block was poured from the last metal from the ladle and hence it contains a higher inclusion count. It was found that holding in the range 500-700oC resulted in the formation of four phases: carbide, σ-phase, x-phase and secondary austenite (γ2). These can be seen in Figure 1(c) which shows microstructure of the sample held for 32 hours at 700oC. The sequence of transformation can be described as follows. Fine carbides formed first at the γ1 - δ interfaces. The carbides gave the darkest contrast in BSE images. They were too small for reliable EPMA analysis so they were examined by SEM-EDS. It is believed the carbides are M23C6 or M6C but this is yet to be confirmed by transmission electron microscopy (TEM). It is noted that the C content of the steel examined was 0.037wt% which is above the normal maximum specified limit of 0.03wt% for cast duplex grades. This is likely to give more precipitated carbide than would be expected for within C specification material. Micro-analytical results for all the phases are listed in Table 2. After the carbides formed an aggregate of (γ2 + x) formed at the γ1 - δ boundaries whereas a globular form of x phase formed within the δ ferrite. The x phase gives the brightest contrast in BSE images due to its enrichment in Mo. Next, within the remaining δ ferrite an aggregate of (γ2 - σ) formed possibly by eutectoid reaction but some of the γ2 had a plate-like Widmanstatten form. The σ phase giving grey BSE contrast compared the whiter x phase since although σ has higher Cr and Fe contents than x it has a much lower level of Mo (about one-third that of x). Table 2 also shows that secondary austenite (γ2) contains higher Ni and Fe levels but less Cr, Mo, Mn and Si than γ1. This is in agreement with work on γ2 formation in wrought material . At 800oC the sequence of transformation was similar to that outlined above but within the δ ferrite the amount of σ phase formed was initially much higher than that of the x phase (Figure 1(d)). After 32 hours this structure became coarser and some of the σ phase had transformed to x phase, the x phase consuming Mo while rejecting Ni into the γ2. In Figure 1(c) and (d) it is seen that the initial precipitated carbides show the position of the original austenite (γ1) -- δ ferrite interphase boundaries. On holding these boundaries have moved into the original δ ferrite regions due to growth of austenite at the expense of the ferrite. After holding at 900oC no carbides were observed. The δ ferrite transformed to coarse eutectoid (γ + σ) which was followed by transformation of some σ phase to irregular shaped x phase. The structure formed after holding for 16 hours is shown in Figure 1(e) which clearly shows these four phases. At 1000oC no transformation was observed even after 32 hours holding time. Phases wt% Si Cr Mn Fe Ni Mo C Before isothermal heat treatment Ferrite (δ)* 1.12 23.20 1.28 64.07 5.47 4.85 N/A Primary austenite (γ1) 1.10 21.90 1.38 65.62 6.79 3.21 N/A After isothermal heat treatment Ferrite (δ) 0.92 25.51 0.82 68.32 3.05 1.39 N/A Primary austenite (γ1)* 0.84 21.85 1.14 65.92 7.70 2.55 N/A Secondary austenite (γ2) 0.78 19.27 1.22 67.61 9.15 1.97 N/A Sigma (σ) 1.34 31.78 0.95 57.08 2.81 6.05 N/A Chi (χ) 1.57 25.21 1.02 51.74 3.29 17.18 N/A Carbide* 0.38 43.09 0.48 26.87 2.37 4.72 22.10 *The results are obtained from SEM-EDS Table 2. Normalised chemical composition (wt%) of phases in the cast duplex stainless steel from EPMA and SEM-EDS
Whos who September 2012