Delta Ferrite Formation in Austenitic Stainless Steel Castings

Angelo Fernando Padilha1,a; Caio Fazzioli Tavares2,b;

Marcelo Aquino Martorano1,c 1Department of Metallurgical and Materials Engineering, University of São Paulo, Brazil

2Açotécnica S.A., Jandira, SP, Brazil

apadilha@usp.br ; bfazzioli@yahoo.com ; cmartoran@usp.br

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).

Materials Science Forum Vols. 730-732 (2013) pp 733-738© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.730-732.733

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Table 1: Chemical composition (wt. %) of the studied heats.

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.

Steel C S Mn Si Cr Ni Mo P Cu Nb N Al Ti W V Co Heat

AISI

302

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 1

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 2

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 3

AISI

304

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 4

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 5

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 6

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 7

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 8

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 9

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 10

AISI

316 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

11

DIN

1.4581

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 12

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 13

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 14

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 15

DIN

XG20 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

16

734 Advanced Materials Forum VI

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

Materials Science Forum Vols. 730-732 735

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.

Heat

Thickness (mm)

Average Standard

Deviation

10 20 30 40

Average Standard

Deviation Average

Standard

Deviation Average

Standard

Deviation Average

Standard

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.

736 Advanced Materials Forum VI

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 Thickness (mm) Siδδδδ/Siγγγγ Moδδδδ/Moγγγγ Crδδδδ/Crγγγγ Feδδδδ/Feγγγγ Niδδδδ/Niγγγγ

8

(AISI 304)

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

13

(DIN 1.4581)

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

6

(AISI 304)

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

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].

(a) (b) (c)

Materials Science Forum Vols. 730-732 737

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

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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

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[5] R. H. Espy, Weldability of nitrogen-strengthened stainless steels, Welding Journal Res. Suppl.

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