A R C H I V E S
o f
F O U N D R Y E N G I N E E R I N G
Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences
ISSN (1897-3310)
Volume 8
Issue 4/2008
61 – 64
12/4
A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 8 , I s s u e 4 / 2 0 0 8 , 6 1 - 6 4 61
The characteristics of two-phase
22Cr-6Ni-2Mo-1,5Cu cast steel
B. Kalandyk
Department of Cast Alloys and Composites Engineering, AGH Faculty of Foundry Engineering,
Reymonta St. 23, 30-059 Cracow, Poland
Corresponding author. E-mail address: bk@agh.edu.pl
Received 23.06.2008; accepted in revised form 14.07.2008
Abstract
The paper discusses the effect of the casting wall thickness on the segregation of alloying elements in two-phase (F-A) 22Cr-6Ni-2Mo-1,5Cu cast steel. The values of the distribution coefficient k0 for Cr, Mo, Si, and Ni, Mn, Cu in the cast stepped test piece walls of three different thicknesses were determined. After solutioning, the values of Rm, R0,2, A5 and Z were measured in the wall of 45 mm thickness and were compared with the results obtained for austenitic cast steel of 18Cr-9Ni grade. In the examined cast steel also the content of δ ferrite and the value of PREN were determined.
Keywords: Stainless cast steel Cr-Ni, Microstructures, Distribution coefficient, Mechanical properties
1. Introduction
F-A steels and cast steels are defined as a group of two-phase alloys resistant to corrosion. The two-phase structure is produced by properly selected, ferrite and austenite stabilising elements. Theoretically, steels in this group are characterised by an approximately equal volume fraction of both phases, in practice, however, the fraction of δ ferrite changes in a range from 35 to 65% [1-4]. Ferrite content depends on the chemical composition of steel/cast steel and on the type of the applied heat treatment. Proper control of these parameters can ensure very good mechanical and casting properties, successfully combined with high corrosion resistance and weldability. The presence of two fundamental phases in the structure of F-A steel/cast steel and the high content of alloying elements promote the effect of segregation. Differences in the content of alloying elements in both phases are usually described with a coefficient of distribution k0 = Xiα/Xiγ (Xiαconcentration of the element i in ferrite; Xiγ
concentration of the element i in austenite). The value k0>1 indicates high content of an alloying element in ferrite, while k0<1
suggests the dissolution of an element in austenite [5]. F-A cast steels form special group of alloys assigned mainly for applications which demand the yield strength higher than the one obtainable in austenitic Cr-Ni steels, combined with good resistance to corrosion of general, pitting (PREN) and crevice type, and/or good abrasion resistance [6-8]. Since they are characterised by good weldability and require lower nickel content than the austenitic cast steels (5-6% as compared to 10-13%Ni in austenitic cast steel), their cost is lower than that of the austenitic 18Cr-9N cast steel [9]. Good castability is another advantage which enables thin-wall castings to be made for, e.g., elements of pumps (steering wheels, rotors, bodies).
2. Methods of investigation
thickness of 20, 45 and 120 mm (total height of the test piece, riser included, was 47cm). Next, the heat treatment (solutioning at 10600C/water) was carried out. The examinations of the chemical composition of ferrite and austenite were carried out under the scanning electron microscope. The differences in the concentration of ferrite- and austenite-forming elements in both ferrite and austenite were determined by the method of EDS analysis. Investigations covered the six main elements, i.e. Cr, Mn, Mo, Si, Ni and Cu. The method of the fluorescent X-ray analysis in microregions was also applied. For this purpose, an ARL SEMQ X-ray microanalyser was used, making scanning
analysis step by step along the line normal to a phase boundary. The spacing between the measuring points was 5μm. As standard reference samples, pure elements of Cr, Ni, Mo, Si Mn supplied by TUSIMIS were used. The results of calculations of the chemical composition in microregions were subjected to ZAF correlation, using an ARL MAGIC IV program. The mechanical properties were determined from the results of static tensile test (Table 1). The ferrite fraction in as-cast state and after solutioning at 10600C was determined on Quantimet.
Table 1.
Chemical composition of the examined 22Cr-6Ni-2Mo-1,5Cu cast steel and mechanical properties after solutioning
Mark C Cr Ni Mo Cu N Rm
[MPa] average
R0,2
[MPa] average
A5
[%] average
Z
[%] average
F-A 0,03 21,3 5,51 1,8 1,30 0,14 785 769 781
778,3 553 523 534
536,6 24,4 25,1 24,7
24,7
49,9 57,0 50,9
55,6
A 0,14 17,9 8,53 0,04 - - 567 559 564
563,3 241 244 247
244 50 48,5 50,3
49,6
49,2 59,6 51,5
53,4
Fig. 1. The shape of cast ingot
3. Results and discussion
The microstructure of the examined cast steel is composed of ferrite and austenite. From the results of the δ ferrite content determination it has been concluded that the average content of δ ferrite increases slightly after solutioning respective of the as-cast state (from 55,7% to 56,3%). The presence of the regions with secondary γ2 austenite in as-cast state and small amounts of this constituent after solutioning heat treatment was noticed (Fig.2c,d). The PREN coefficient of the examined cast steel was 29,5%, and the equivalent values of chromium and nickel amounted to Creq = 22,78%, Nieq= 13,02%, (Creq/Nieq =1,75), respectively.
a) wall thickness g= 20 mm
b) wall thickness g= 120 mm
c) wall thickness g= 20 mm
d) wall thickness g=120 mm
Fig. 2. Microstructure of the examined cast steel Cr-Ni after solutioning – a,b) magn. 25x, c,d); magn. 100x
From the investigations carried out so far it follows that there are segregations of alloying elements (Cr, Ni, Mo, Cu, Si, Mn) between ferrite and austenite. Elements like Ni, Cu or Mn segregate to austenite, while Mo, Si and Cr segregate to ferrite. The average concentration of elements in ferrite and austenite in the walls of a stepped test piece was established using a mean calculated from the ten measurements (confidence interval of 2σ). Next, the distribution coefficients k0 were calculated for the three wall thicknesses in both as-cast state and after solutioning. It has been observed that with the increasing wall thickness the values of the distribution coefficient increase in austenite (the exception is Cu) and decrease in ferrite (the exception is Mo). Changes in the distribution coefficient k0 in function of the casting wall thickness are shown in Table 2 and in Figure 3. From the obtained results it has been concluded that the distribution coefficients of ferrite-forming elements are changing in the sequence of Mo > Si > Cr.
Table 2.
Mean values of the distribution coefficient XF/XA in as-cast state and after solutioning (S:10600C) in function of the casting wall thickness
Elements
As cast Xiα /Xiγ
S:10600C Xiα /Xiγ 20 [mm]
S:10600C Xiα /Xiγ 45 [mm]
S:10600C Xiα /Xiγ 120 [mm] Mo 1,553 1,82 1,87 2,4
Cr 1,087 1,12 1,102 1,09 Ni 0,71 0,74 0,72 0,68 Mn 1,292 0,962 0,943 0,79 Cu 0,705 0,84 0,81 0,83 Si 1,176 1,28 1,26 1,20
Examined carefully, the results of the studies on the segregation of elements (Fig. 3), expressed as a distribution coefficient k0, show us that molybdenum belongs to the group of elements segregating most heavily in the examined F-A cast steel. The degree of its segregation is changing with increasing thickness of the casting wall.
0 0,5 1 1,5 2 2,5
Mn Cr Si Ni Cu Mo
Xi
α /X
i
γ
g = 20 mm
g = 45 mm
g = 120 mm
Fig. 3. Ordinary values of the distribution coefficient of alloying elements in ferrite Xiα and austenite Xiγ in the examined cast steel
after solutioning
The second element heavily segregating in ferrite is silicon. Both Si and Cr were observed to change the degree of segregation when the casting wall thickness increased, but these changes were relatively small. Therefore the conclusion is that the casting wall thickness is not so important for the Cr and Si segregation as it is for the segregation of Mo.
In austenite-forming elements, the change in the distribution coefficient proceeds according to the sequence: Ni > Cu > Mn. It has also been reported that the degree of segregation of Ni and Mn changes with the increasing casting wall thickness, though these are the changes of rather minor influence. As regards Cu, no explicit correlation has been observed to exist between the distribution coefficient and casting wall thickness.
So, examining the segregation behaviour of the alloying elements in ferrite and austenite one can conclude that Mo is the element that tends to segregate most heavily, while other elements reveal only a minor degree of segregation. It has been noticed that the values of the distribution coefficient for Mn and Ni, and for Mo, Cr and Si are changing with the increasing wall thickness of a stepped test piece. Only Cu does not follow this relationship.
After solutioning, from the wall of a stepped test piece, the specimens for testing of mechanical properties were cut out. Basing on the results of static tensile test, mean values of the parameters of mechanical properties: Rm = 778,3 MPa, R0,2 = 536,6 MPa, A5 = 24,7%, Z = 55,6% (Table 1) were determined. The obtained values are corresponding to a ferritic-austenitic cast steel of CD-4MCu grade (UNS J93370) used for castings with high Cr concentration Cr (24,5-26,5%Cr) [7].
0 200 400 600 800 1000
A F-A
Rm, R
0
,2
[MPa
]
Rm R0,2
Fig. 4. Mechanical properties (Rm i R0,2) of the analysed cast steels F-A and 18Cr-Ni9 after solutioning (Table 1)
Compared to 18Cr-9Ni cast steel (R0,2 = 244 MPa- Table 1; Fig.4) of purely austenitic structure, over double increase in the value of the yield strength amounting to 523-553 MPa was obtained with an elongation of 24,7%.
4. Conclusions
From the results of the investigations of stainless cast steel Cr-Ni the following conclusions are drawn:
− The microstructure of the examined cast steel in as-cast state is composed of ferrite, austenite and γ2 phase; after solutioning it contains ferrite, austenite and scarce precipitates of γ2 phase.
− The PREN coefficient of the examined cast steel is 29,5%. − The distribution coefficient assumes the values of k0 >1 for
Mo, Cr, Si, and k0 <1 for Mn, Cu, Ni. The highest value of this coefficient was obtained for Mo.
− For the examined F-A cast steel, the obtained mean values of the mechanical parameters are: Rm=778,3MPa, R0,2=536,6MPa, A5=24,7%, Z=55,6%. These values are corresponding to an F-A CD-4MCu steel (UNS J93370). − Over double increase in the yield strength value respective
of 18Cr-8Ni cast steel has been reported with on elongation of 24,7%.
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