Delta Ferrite Formation in Austenitic Stainless Steel Castings
Transcrição
Delta Ferrite Formation in Austenitic Stainless Steel Castings
Materials Science Forum Vols. 730-732 (2013) pp 733-738 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.730-732.733 Delta Ferrite Formation in Austenitic Stainless Steel Castings Angelo Fernando Padilha1,a; Caio Fazzioli Tavares2,b; Marcelo Aquino Martorano1,c 1 Department of Metallurgical and Materials Engineering, University of São Paulo, Brazil 2 Açotécnica S.A., Jandira, SP, Brazil a [email protected] ; [email protected] ; [email protected] Keywords: Austenitic stainless steel; microstructure; solidification; delta ferrite Abstract. The effects of chemical composition and cooling rate on the delta ferrite formation in austenitic stainless steels have been investigated. Ferrite fractions measured by a magnetic method were in the range of 0 to 12% and were compared with those calculated by empirical formulas available in the literature. The delta ferrite formation (amount and distribution) was strongly affected by the steel chemical composition, but less affected by the cooling rate. Among several formulas used to calculate the amount of delta ferrite, the best agreement was obtained with those proposed independently by Schneider and Schoefer, the latter being recommended in the ASTM 800 standard. Introduction The properties and performance of austenitic stainless steels are strongly related to their microstructures, especially the amount and distribution of delta ferrite, which, in the case of castings, depends chiefly on chemical composition and on the cooling rate during solidification [1]. The number of relevant chemical components in austenitic stainless steels is often more than five, but published phase diagrams with more than four components are rarely available to predict the phases and their amounts on the steel microstructure. As a result, empirical maps were developed to indicate the amount and types of phases from indexes based on the alloy chemical composition. One of the best-known maps was proposed by Schaeffler [2,3], who divided the alloying elements into two groups, namely, ferrite and austenite stabilizers, whose effects could be predicted by formulas of chromium and nickel equivalent, respectively. The Schaeffler diagram was further improved by DeLong [4], Espy [5], and others and became an ASTM standard [6]. In the literature on solidification of austenitic stainless steels, there are no technical papers in which measurements of both cooling rate and the complete chemical composition are carried out. In the present work, sixteen heats of five different austenitic stainless steel types were cast and their complete chemical compositions (16 elements) were determined. The effects of the chemical composition and the cooling rate were analyzed and the experimental results compared with fractions of delta ferrite calculated using empirical formulas available in the literature. Casting and preparation of samples Stainless steel charges of approximately 100 kg were melted in a vacuum induction furnace and their chemical compositions (16 elements) were determined by optical emission spectroscopy analysis of a small portion of the melt (Table 1). All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 143.107.101.223-20/08/12,19:11:26) 734 Advanced Materials Forum VI Table 1: Chemical composition (wt. %) of the studied heats. Steel C S Mn Si Cr Ni Mo P Cu Nb N Al Ti W V Co 0.22 0.009 1.05 1.05 17.4 11.4 2.34 0.043 0.13 0.69 0.0638 0.0014 0.0069 0.011 0.050 0.077 0.21 0.009 0.891 1.06 17.4 11.0 2.05 0.003 0.11 0.57 0.0617 0.077 0.0073 0.01 0.044 0.096 0.23 0.008 1.01 1.02 18.0 10.9 2.17 0.029 0.13 0.53 0.0576 0.0011 0.0078 0.01 0.049 0.087 0.043 0.004 1.09 0.776 18.4 9.37 0.322 0.039 0.17 0.0069 0.0580 0.0036 0.007 0.037 0.048 0.249 0.038 0.005 0.563 0.399 17.9 10.7 0.110 0.020 0.048 0.0074 0.0270 0.001 0.0035 0.012 0.050 0.037 0.085 0.009 0.677 0.801 18.7 9.25 0.314 0.030 0.12 0.017 0.0729 0.001 0.0034 0.015 0.048 0.083 0.076 0.007 0.931 0.869 18.9 9.19 0.225 0.042 0.12 0.0064 0.0620 0.001 0.0044 0.017 0.047 0.083 0.040 0.006 0.457 1.26 20.5 10.0 0.123 0.029 0.042 0.0072 0.0340 0.076 0.004 0.013 0.050 0.034 0.033 0.007 0.415 1.34 19.5 10.0 0.116 0.027 0.042 0.007 0.0370 0.048 0.0034 0.014 0.049 0.036 0.080 0.006 0.852 0.902 18.2 9.08 0.255 0.023 0.099 0.0062 0.0660 0.0017 0.0043 0.021 0.047 0.083 0.038 0.005 0.998 1.10 17.7 9.62 2.20 0.030 0.14 0.008 0.0476 0.0013 0.0048 0.012 0.048 0.086 0.074 0.008 0.913 1.14 18.7 10.2 2.15 0.031 0.14 0.61 0.0571 0.0034 0.0064 0.013 0.051 0.075 0.070 0.007 0.968 0.940 18.7 11.1 2.23 0.031 0.14 0.72 0.0568 0.001 0.0056 0.012 0.051 0.081 0.065 0.008 0.713 0.985 18.5 10.5 2.19 0.029 0.14 0.49 0.0622 0.001 0.0061 0.013 0.053 0.076 0.036 0.006 1.08 1.15 17.7 10.1 2.18 0.029 0.18 0.011 0.0340 0.0025 0.0078 0.017 0.050 0.091 0.151 0.007 1.10 1.70 27.2 18.9 0.338 0.038 0.099 0.016 0.0523 0.002 0.0071 0.012 0.078 0.072 Heat 1 AISI 302 2 3 4 5 6 AISI 304 7 8 9 10 AISI 316 11 12 13 DIN 1.4581 14 15 DIN XG20 16 The stainless steel melt was poured into an investment casting (lost wax process) mold. The melt pouring temperature, which was measured in all heats, was in the range between 1515oC and 1633oC. The mold was made of at least eight layers of a combination of colloidal silica slurry and zircon/alumina silicate stucco. After solidification and cooling to room temperature, the specimens were extracted from the mold cavity. The specimen shape and dimensions are given in Figure 1, showing different thicknesses in each step of a "ladder" to impose an increasing cooling rate from the largest to the thinnest step. Figure 1: Test specimen geometry (“ladder”) showing four different thicknesses (10, 20, 30, and 40 mm) and the feeder (cylinder on the left) used to prevent shrinkage defects. Dimensions are in mm. Materials Science Forum Vols. 730-732 735 Type R (Pt-13%Rh, Pt) thermocouples were inserted into the mold cavity in contact with the steel to measure the cooling curves at different positions during solidification. After combining these curves with simulation results from the software SolidCast, the average cooling rates during solidification were estimated to be approximately 0.78, 0.75, 1.4, and 2.7 K/s for each of the four ladder steps (40, 30, 20, 10 mm), respectively. Using the optical emission spectroscopy analysis, the chemical composition of the ladder specimens was measured at three different locations to verify the existence of any variation in the concentration of elements (macrosegregation) within the specimens. Since measured variations were within the experimental error of the analytical technique, no significant macrosegregation was detected. The microstructures of the samples were characterized by several complementary techniques of microstructure analysis. Delta ferrite content was determined with a Fischer feritscope model MP30E and the ferrite morphology was studied by optical metalography. The chemical composition (metallic elements) of the phases (ferrite and austenite) was measured by X-ray microanalysis using an energy-dispersive spectrometer (EDS) attached to a scanning electron microscope. The metallographic preparation for both the optical and scanning electron microscopy consisted of grinding, followed by mechanical polishing with diamond paste, and finally etching with aqua regia (100 ml HCl + 3 ml HNO3 + 100 ml methyl alcohol). Results and discussion Ferrite fractions measured by the magnetic method were in the range of 0 to 12% (Table 2). Figure 2(a) shows a typical mapping of delta ferrite distribution for the specimen of a selected steel heat, with delta ferrite content (specimen average) of 7.5% (Table 2). Although the influence of the cooling rate was weak (Figure 2(b)), a slight decrease in delta ferrite fraction with increasing cooling rate can be observed. The metallographic analysis showed that the steel sample with approximately 10% ferrite presented an almost continuous ferrite network microstructure (Figure 3a), which deteriorates toughness when this ferrite suffers embrittlement caused by high temperature exposition [7,8]. In the samples with delta ferrite fractions around 5%, the ferrite network was semi-continuous (Figure 3b), while for lower fractions (around 2%) the ferrite was arranged in isolated cores (Figure 3c). El Nayal and Beech [9] and Suutala and Moisio [10] defined a criterion based on the ratio of chromium to nickel equivalents (Creq/Nieq) to determine which of the following solidification modes is observed in austenitic stainless steels: mode A (L→L + γ→ γ), mode AF (L→L + γ→L + γ + δ→γ + δ), or mode FA (L→L + δ→L + δ + γ →γ + δ). In this sequence, L is the liquid phase, γ is austenite, and δ is ferrite. In agreement with this criterion, heats 1, 2, 3, and 16 solidified in mode A and their microstructures did not have any ferrite. On the other hand, samples from heats 4 to 15, which showed some delta ferrite in the microstructure, solidified in mode FA, i.e., with ferrite as the leading phase and formation of interdendritic austenite at the expense of ferrite dendrites during solidification. After solidification, during cooling to room temperature, further austenite might form, consuming ferrite. The microanalyses with EDS were used to calculate the solute partition ratios of Si, Mo, Cr, Fe, and Ni contents in austenite to those in delta ferrite. These ratios were obtained for all steps of the ladder samples from three experiments, namely, heats 8 (AISI 304), 13 (DIN 1.4581), and 6 (AISI 304), as shown in Table 3. In the report published by the Jernkontoret [12], the partition ratios for 736 Advanced Materials Forum VI chromium and nickel were: Crδ/Crγ = 1.2 and Niδ/Niγ = 0.5 (steel AISI 304), and Crδ/Crγ = 1.3 and Niδ/Niγ = 0.6 (steel DIN 1.4583). These values are in reasonable agreement with those reported in Table 3 for the present work samples. Table 2: Delta ferrite content measured with the feritscope in the ladder steps of different thicknesses from samples of different heats. The average value and standard deviation are shown for each step and an average for all steps of the same ladder sample (same heat) is also given. Thickness (mm) 10 Heat 20 Standard 30 Standard Average Average Deviation Standard Average Deviation Standard 40 Average Deviation Standard Average Deviation Deviation 1 0 0 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 0 0 0 4 4.2 0.3 4.5 1.0 4.6 1.1 4.6 0.7 4.5 0.9 5 1.8 0.4 1.8 0.5 1.8 0.7 1.5 0.6 1.7 0.6 6 1.5 0.3 1.8 0.5 2.2 0.6 2.4 0.9 2.1 0.7 7 2.8 0.3 3.0 0.8 3.4 0.9 3.1 0.9 3.1 0.9 8 10.6 0.6 11.2 1.2 11.6 1.1 11.3 1.7 11.3 1.3 9 6.5 0.4 7.2 0.6 7.5 0.9 7.8 1.0 7.5 0.9 10 1.8 0.6 1.7 0.8 1.8 0.8 1.7 0.6 1.8 0.7 11 8.1 0.5 8.8 0.7 9.1 1.1 10.1 1.4 9.2 1.3 12 7.1 0.4 8.1 0.5 8.1 0.9 8.3 0.9 8.1 0.9 13 3.6 1.1 4.7 0.8 5.0 0.8 5.3 0.7 4.9 0.9 14 6.2 0.7 7.1 0.9 7.3 1.1 6.5 2.1 6.8 1.5 15 8.1 0.6 8.8 0.6 9.7 1.3 9.6 1.4 9.3 1.3 16 0 0 0 0 0 0 0 0 0 0 (a) (b) Figure 2: Measurements of delta ferrite fraction: (a) on the central longitudinal section of specimen obtained from heat 9 (AISI 304); (b) as a function of cooling rate (R) for heats in Table 1. Materials Science Forum Vols. 730-732 (a) (b) 737 (c) Figure 3: Morphology of delta ferrite (cooling rate 2.7 K/s): (a) continuous network (heat 8 AISI 304); (b) semi-continuous network (heat 13 DIN 1.4581); and (c) isolated cores (heat 6 AISI 304). Table 3: Solute partition ratios of Si, Mo, Cr, Fe, and Ni concentrations in ferrite (δ) to those in austenite (γ) of three different steel ladder samples in the four steps of different thicknesses. Heat 8 Thickness (mm) Siδ/Siγ Moδ/Moγ Crδ/Crγ Feδ/Feγ Niδ/Niγ 10 0.95 1.07 1.33 1.00 0.38 20 1.04 4.22 1.41 0.98 0.42 30 1.16 0.68 1.42 0.96 0.43 40 1.32 3.00 1.35 0.97 0.48 Average 1.12 2.24 1.38 0.98 0.43 10 0.91 1.15 1.26 0.97 0.59 20 1.18 1.78 1.38 0.97 0.42 30 1.37 2.37 1.36 0.96 0.42 40 1.10 0.81 1.28 0.94 0.77 Average 1.14 1.53 1.32 0.96 0.55 10 1.10 1.88 1.33 0.91 0.30 20 1.09 1.28 1.39 0.96 0.35 30 1.11 0.70 1.28 0.99 0.33 40 1.50 - 1.60 0.87 0.57 Average 1.20 1.29 1.4 0.93 0.39 (AISI 304) 13 (DIN 1.4581) 6 (AISI 304) Figure 4: Delta ferrite content as a function of the ratio of chromium to nickel equivalents calculated from the formulas proposed by (a) Schaeffler [2,3] and (b) Schoefer [6]. 738 Advanced Materials Forum VI The influence of steel chemical composition on the delta ferrite content was noteworthy. Using suggested formulas for nickel and chromium equivalents, the correlation between the ferrite content and the ratio of chromium to nickel equivalents were examined (Figure 4). Among the many tested formulas for predicting delta ferrite content, the two that showed better correlation coefficients are those proposed by Schneider and Schoefer; the latter being recommended by the ASTM A800 Standard [6]. In Figure 4, examples of strong and weak correlations are given using the formulas proposed by Schaeffler [2,3] and Schoefer [6]. Conclusions The following conclusions can be drawn: (a) the amount and distribution of delta ferrite was strongly affected by the steel chemical composition, but less affected by the cooling rate; (b) among several formulas used to predict the delta ferrite content, the best agreement was obtained with those proposed independently by Schneider and Schoefer; the latter is recommended in the ASTM 800 standard; (c) In the heats 4 to 15, which probably solidified following the sequence L → L + δ → L + δ + γ → γ + δ, the solute partition ratios of Si, Mo, Cr, Fe, and Ni contents in ferrite to those in austenite are in agreement with values published in the literature for similar steel compositions. References [1] A. F. Padilha and P. R. Rios, Decomposition of austenite in austenitic stainless steels, ISIJ International 42 (2002) 325 - 337. [2] A. L. Schaeffler, Selection of austenitic electrodes for welding dissimilar metals, Welding Journal Res. Suppl. 26 (1947) 603s - 620s. [3] A. L. Schaeffler, Constitution diagram for stainless steel weld metal, Metal Progress 56 (1949) 680 - 680B. [4] W. T. DeLong, Ferrite in austenitic stainless steel weld metal, Welding Journal Res. Suppl. 53 (1974) 273s – 286s. [5] R. H. Espy, Weldability of nitrogen-strengthened stainless steels, Welding Journal Res. Suppl. 61 (1982) 149s - 156s. [6] ASTM 800/A 800M-01, Standard practice for steel casting, austenitic alloy, estimating ferrite content thereof, American Society for Testing and Material. [7] T. Yamada, S. Okano and H. Kuwano, Mechanical property and microstructural change by thermal aging of SCS14A cast duplex stainless steel, J. Nucl. Mater. 350 (2006) 47 - 55. [8] A. F. Padilha, D. M. Escriba, E. Materna-Morris, M. Rieth and M. Klimenkov, Precipitation in AISI 316L(N) during creep tests at 550 and 600 ºC up to 10 years, J. Nucl. Mater. 362 (2007) 132 - 138. [9] G. El Nayal and J. Beech, Relationship between composition, impurity content, cooling rate, and solidification in austenitic stainless-steels, Mater. Sci. Tech. 2 (1986) 603 - 610. [10] N. Suutala and T. Moisio, Use of chromium and nickel equivalents in considering solidification phenomena in austenitic stainless steels, In: Solidification Technology in the Foundry and Cast House, The Metals Society, Coventry (1980) 310 - 314. [11] A guide to the solidification of steels, Jernkontoret, Stockholm (1977) 91-101.