Appendix 5 Delta Ferrite (FN) Diagram
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About d-Ferrite and Predicting Ferrite Number of Austenitic Stainless Steel
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The standardized austenitic stainless steel grades contain more or less d-ferrite in as cast condition or in the welds. However, the d-ferrite content decreases remarkably through hotrolling and annealing of the hotrolled products. The remaining ferrite content of the coldrolled products is vanishing small after solution annealing. The welding method chosen has a significant influence on the ferrite content of the weld metal (1).
Adjusting the d-ferrite content of the cast condition may help to optimize the hotworkability. The d-ferrite may also influence the distribution of the non-metallic inclusions and segregation of the alloy elements within the cast structure.There is a well known tool to predict the d ferrite content of the weld microstructure: Schaeffler-Delong diagram which is also known by designation WRC-1988 (the latest edition) . This diagram may be succesfully applied to the cast structures including heavy section continuously cast slabs. The diagram consists of the Ni-equivalence on the vertical Y-axis and of the Cr-equivalence on the horizontal X-axis. There are several nearly parallel straight lines corresponding to constant ferrite numbers (FN i.e. approx. ferrite content). First you have to calculate the Cr and Ni equivalences of the given steel composition, then you determine the intersection of the horizontal line (corresponding to the calculated Ni-equivalence) and of the vertical line (corresponding to the calculated Cr-equivalence). The intersection point is usually located between plotted lines of the constant ferrite numbers, thus you have to interpolate the approximated FN values or you may use the nearest FN-line. Schaeffler-Delong diagram is a valuable tool in designing of new steel grade compositions. |
FNA Cr-equivalence includes Ti as an additional element. FNA Ni-equivalence contains Cu and Co as additional elements. Otherwise, the multiplication coefficients of each element are same as for Cr- and Ni-equivalences of Schaeffler-Delong. These are the formulas for FNA: Creq = Cr + 1.5 Si + Mo + 2 Ti + 0.5 Nb Nieq = Ni + 30 (C + N) + 0.5 Mn + 0.5 Cu + 0.5 Co FNA = 3.34 Creq - 2.46 Nieq - 28.6 FNA is used here on the calculation form of my website too. However, I have used another method on the Java- and Excel FN-calculators developed by myself, which corresponds better to the original Schaeffler-Delong diagram. There is an older diagram by Schaeffler too. The most apparent difference compared to Schaeffler - Delong is that the FN lines are clearly not parallel but more like lines with a common intersection point located somewhere outside on the left bottom corner. Another difference is quite important: nitrogen content is not counted in the nickel equivalence of the Schaeffler diagram. In real life the nitrogen varies a lot, thus Schaeffler - Delong taking in acount the nitrogen is propably a better alternative. However, both are missing the titanium. They use niobium (in US Cb) instead, thus one has to decide which coefficient of Ti will be used for Cr-equivalence. I have used preferably coefficient 3 but some others have used values 1 or 2 instead. The role of titanium is complicated through forming of carbides and nitrides, thus titanium is partially only in solution. The bound titanium does not promote forming of d ferrite. Prediction of the solidification mode It is frequently crucial to be able to predict or control the solidification mode during welding or continuous casting of the austenitic stainless steels. According to the research results of the effect of composition on the solidification by N. Suutala and T. Moisio can the factor CrEq/NiEq be used to predict the solidification mode. The solidification will be primary austenitic for the values CrEq/NiEq < 1.5. On the other hand, the values CrEq/NiEq > 2.0 are more likely to solidify as primary ferritic instead. In the range 1.5 to 2.0 of CrEq/NiEq will be mixed ferritic-austenitic. |
Some remarks concerning my FN-calculator.
My original formula is based on an older edition of WRC and I have made no updating according to WRC-1988 yet.
It correlates rather well to WRC-diagram within FN-range 0 to 20 but deviations increase quickly for higher FN values. There is another more accurate online FN-Calculator developed by the Oak Ridge National laboratory, especially suitable for the higher FN-values.
I have simply determined the coefficients a and b of the lines NiEq = a + b*CrEq || FN=constant as function of FN: a(FN), b(FN). Given CrEq and NiEq FN can be solved as a root of a 2nd grade polynome.
For those of you who would like to get my formula. I'm sorry, it is not available for publication. In spite of this limitation you are able to down-load the calculator to your PC and allowed to use it freely. Finally, some more elementary information for not-metallurgists. The austenite (g)-matrix has a cubic face-centered (FCC) crystal structure, i.e. there is an atom of a metallic element set on every corner of the cubic lattice and additionally another one located in the middle of every lattice face. d-ferrite has a cubic body-centered crystal (BCC) structure with an atom on every corner and in the middle point of cube additionally. A BCC crystal contains 2 metal atoms per unit cell in average while a FCC crystal contains four metal atoms per unit cell. Austenite is a non-magnetic phase but d ferrite is a magnetic one at ambient temperature. Ni, C, Mn and N stabilize in solid solution the g- phase and these elements are counted in the Ni-equivalence. Cu is also an element stabilizing g, however it is not counted within Ni-equivalence of WRC-diagram. Cr-equivalence consists of some elements promoting d ferrite: Cr, Si, Mo and Nb (Cb in US). These elements are in higher cocentration in ferrite than in austenite in solid solution. |
(1) For the influence of the welding method on the ferrite content