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Manufacturers of centrifugal separators and decanters want to maximize the separation efficiency of their machines, so the fabrication materials of

critical components, such as the rotation cylinder or the conveyor hub have to exhibit very high tensile strength and corrosive resistance, but at

the same time remain cost effective. Not an easy goal to achieve. However, Dr Frank Wischnowskii & Dipl.-Ing. Andre Kuhn of Klaus Kuhn Edelstahlgiesserei GmbH, Germany, believe their duplex centrifugal cast

stainless steel grades may well be the answer.

Stainless Steel Alloys Strengthen Critical Centrifuge Components

C entrifugal separators and decanters mechanically separate liquid from liquids and solids from liquids, and are used in a variety of industries (chemical,

pharmaceutical, wastewater treatment, etc) to clarify liquids, separate liquid mixtures, concentrate and dewater solids, etc. Their main advantage is their ability to handle continuous process streams that have highly variable levels of solids.

Higher demands on the volume and/or the extraction capabilities of these machines mean higher material requirements, especially for strength under full load and top-gravitational force and corrosion resistance and ductility for safety and durability. Enhanced duplex steel materials have now been developed that can meet these higher requirements.

M a t e r i a l Requ i rements

The material used for the wet surfaces of centrifuges and decanters, i.e. the surfaces that come in contatct with products, are primarily made from stainless steel. It offers a unique blend of high strength, corrosion resistance, ductility and weldability, contributing to reliable, long-term performance, low mainten- ance and easy serviceability of the centrifuge.

In decanters, the conical and cylindrical components of the rotation cylinder and the conveyor hub are generally fabricated

from centrifugal cast stainless steel. Depending on the chemical resistance requirements, either austenitic or ferrite-austenitic steel is used. The choice of material depends on the corrosiveness of the treated fluid and the separation process.

Because of the high rotating speed, all critical centrifugal parts of vertical separators, such as the upper and lower part of the cylinder, the piston slide and the locking ring have to satisfy higher strength requirements, so they are commonly made of forged stainless steel. Table 1 shows the chemical composition of austenitic and duplex stainless steels commonly used to make such components. In comparison, ferrite-austenitic steels have a higher tensile strength and better corrosion resistance. Thus, their high corrosion resistance and excellent mechanical properties, make duplex stainless steels the material of choice in the design of modern separators and decanters.

The new European standard EN 12547 defines the minimum impact work energy, with ISO-V-samples at room temperature, as 27 J, and the elongation by fracture as >14%. For the design of centrifugal decanters and separators, the required mechanical properties of cast and forged duplex stainless steels are much higher, as shown in Table 2.

The smaller grain size of forged duplex stainless steel grades' microstructure in comparison to conventionally cast steel grades, means it can exhibits higher strength and higher ductilit)~

DIN-code Cma x Cr Ni Mo Cu W N PREn

Cast Austenitic Stainless Steels 1.4308 0.08 18.O-21.0 8.0-11.0 0.5 18.5 1,4408 0.08 18,0-21.0 90 -12 .0 2.0-3.0 26.5 1.4412 0.08 18.0-22.0 9.0-13.0 3.0-4.0 29.5

Cast Duplex Stainless Steels 1.4460 0.06 24.0-27.0 4.0-6.0 1.75-2.5 0.10-O.25 33 1.4470 0.03 21.0-23.0 4.5-6.5 2.5-3.5 O. 113-O.20 34 1.4463 0.07 23.0-25.0 7.0-8.5 2.0-2.5 <O.13 32 1.4468 0.03 24.5-26.5 5.5-7.5 2.5- 3.5 O, 12~).25 37.5

Forged Duplex Stainless Steels 1.4462 0.03 21.0-23.0 4.5-6.5 2.5-3.5 O. 10-0.20 34 1.4410 0.03 24.0-26.0 6.0-8.0 3.0-4.5 O. 1OO.20 >40 1.4501 0.03 24.0-26.0 6.0-8.0 3.0-4.0 0.5-1.0 0.5-1.0 0 . 2 0 0 . 3 5 >40

3 6 November 2002 www, filtsep.com

article

DIN-code

Cast Duplex Stainless Steels

Forged Duplex Stainless Steels

1.4460 1.4470 1.4463 1.4468 1.4462 1.4501 1.4410

Rp0.2 min Rmmin ‘5 min Avtnin

IMPal tMPa1 WI tJ1

450 655 25 70 450 660-860 25 80 420 630 20 80 520 700 22 60 500 700800 25 100 550 2770 25 100 550 7701000 25 80

RpO.i? min [IOOT.]

IMPal

375 370 350 430 460 490 480

Rmmin (100%) IMPal

540 525 645 680 590 710

Therefore, critical parts in high

rotating separators are commonly

made from forged duplex stainless

steels. DIN-code Rp,,,

IMPal Rm IMPal

A”

[Jl RPo.* (100 “C) Rmmin [IO0 “C]

tMPa1 IMPal

1.4460 562 744 34 156 1.4462 473 717 34 210 465 1.4463 480 727 37 180 390

673 660

/ 1.4468 556 763 33 206 491 732

Over many years we have acquired i experrtise in the production of

austenitic, as well as duplex stainless steel, and have been using

centrifugal cast stainless steel to fabricate critical parts of

centrifuges and separators.

The shape of products made by the conventional centrifugal

casting process is limited and depends on the direction of the

rotation axis of the dies. Cylinders are normally made from

horizontal castings, while conical parts, bushes or rings can be

made by vertically rotating the die and changing the rotation

speed The high rotation forces and the unidirectional

solidification from the outside to the inside in centrifugal

casting produce a very pure microstructure. Impurities and gas

inclusions are forced to the inner surface because their density

is less than steel, and can then be removed in the subsequent

machining. Thus, products made by centrifugal casting have a

better metallurgical integrity than other casting techniques such

as sand- or invest casting. This results in mechanical properties

that are superior to conventional static castings. The almost

anisotropic nature of the materials’ properties regardless of

the direction of the test sample- provides better uniform

physical properties, compared to forged products. Centrifugal

cast duplex stainless steels have superceeded previous grades in

many applications, and are now being used as the first choice

by designer of centrifuges or decanters. Table 3 gives the

average mechanical properties of commonly used centrifugal

cast duplex stainless steel. The listed materials are used to

make the rotating cylinder, as well as the conveyor hub of

decanters.

capabilities of decanters have led to higher rotational speeds for

the cylinder and higher conveyor hub torques. These in turn have

led to the development of new centrifugal cast duplex steel

grades that not only have high strength properties, but also

sufficient ductility and high corrosion resistance.

Effective hardening without losing ductility is possible by

solid solution hardening with the element, nitrogen. To combine

high tensile strength with good corrosion resistance, new duplex

alloys with high nitrogen contents have been developed. In

common cast duplex stainless steel, nitrogen is alloyed in the

range of 0.1 up to 0.25 wt %. The solubility of nitrogen in the

melt is limited and depends on the temperature and the chemical

composition of the melt. A common method of increasing the

natural solubility of nitrogen in the melt is to increase the

manganese content [l, 21. However, high manganese levels in

duplex stainless steel increase the precipitation kinetic of the

sigma phase and reduces the transition temperature [3].

Preliminary experiments and the study of the literature of

high nitrogen duplex stainless steel [4,5] indicated that a

manganese content between 2.0 and 3.5 wt % is sufficient to

produce a nitrogen content of 0.30-0.35 wt % in a duplex

stainless steel. This information was used to develop two high

nitrogen cast duplex stainless steel. Their compositions are

shown in Table 4.

The chemical composition is balanced to obtain an austenite

content of between 50-60 Vol. %. The microstructure of K 900

and K 901, can be seen in Figures 1 & 2, respectively

Mn Cr MO W Cu Co Higher demands on the volume

2.03.5 24.027.0 5.07.0 1.0-2.0 2.0-4.0 2.03.5 <0.5 and/or the extraction 2.03.5 24.0-27.0 5.0-7.0 1.0-2.0 2.04.0 2.03.5 1.5-3.0 0.30X1.35

Filtration+Separation November 2002 37

Figure 1: Microstructure of alloy K 900 Ferrite content: 50 Vol. %.

These development in centrifugal cast duplex stainless steel

gives designers of centrifuges and decanters the opportunity to

substitute parts made by forging, with more cost efficient and

highly reliable parts made by centrifugal casting.

Figure 2: Microstructure of alloy K 901 Ferrite content: 40 Vol. %.

New super duplex cast stainless steel grades have been

developed that are ideal for the fabrication of critical centrifuge

components made by centrifugal casting. The new grades have

superior strength, good toughness and match the strong

K 900 RT 104/104/101

155/178/156

Heat RPcl.2 RP,.cl Rm A, [%I Av. [J] treatment [MPal [MPal IMPal

K 900 400 “C/ 1 h 618 741 897 26 36 25/20,‘30

K 901 400 “C/l h 642 734 886 38 41 73/60/65 350 “C/l h 628 743 856 32 40 98/105/100 350 “C/5h 643 732 861 34 38 88/86/90

mechanical properties of

forged duplex stainless steel

grades. The centrifugal

casting method has been

shown to have a number of

advantages over conventional

casting methods, and is

therefore the preferable

manufacturing process for

these new duplex steel

grades.

In future, the mechanical

behaviour of these new

grades under cyclic loads

will be investigated. Also,

additional data for

comparison with other

developments in duplex

stainless steels will be

38 November 2002

To reduce the formation kinetic of the sigma-phase the References molybdenum (MO) is substituted by a double amout of tungston

(W) to reach the same PRE level as a higher molybdeum, but

tungscon-free duplex steel [6,7]. The second advantage of the

low molybdenum content is that a lower solution heat treatment

temperature is required. Therefore, the possibility of nirtide

precipitation, which occurs as a consequence of a high solution

heat treatment temperature, is reduced.

P Giimpel 81 F H Strom. 1988. Thyssen Ed&t. Techn. Brr., 14, 1,

p.lOXff.

2.

3.

G Stein. 1985. VCB-Kraftwerkstechnik, 65, 11, p.1069ff.

M Pohl, F Wschnowski & A Ibach. 1995. Pmk. Met., Sonderh, 27,

p.363ff.

4. A Gregori & J-O Nilsson. 1999. Proc. Conf Stainless Steel, Sardinia,

Italy, Vol. 2, p.23ff. In alloy K 901, the cobalt (Co) content has been increased to

between 2.0 and 3.5 wt %, which improves the mechanical

properties in application temperatures of 100 “C (Table 5).

Overall, the mechanical properties of the cobalt-containing

alloy K 901 at room temperature (RT) and 100 “C are slightly

better than alloy K900. In comparison to the essentially

cobalt-free alloy, the alloying with cobalt produces a reduction

in the yield stress, when raising the temperature from RT to

100 “C.

Precipitaion hardening can be used to improve the strength of

these steels’ properties. With heat treatment in the temperature

range of 350-500 “C, the formation of chromium rich al-phase

and copper rich E-phase in the ferrite is possible [S]. Mechanical

tests show that the precipitation hardening at 400 “C is extremely

rapid, as is the reduction in toughness. By decreasing the heat

treatment temperature to 350 “C the kinetics of the precipitation

is reduced and the heat treatment can be used for additional

optimization of the alloy’s strength (Table 6).

6.

8.

9.

collected and presented. 0

G HochGrtler, P J Uggowitzer & M 0 Speidel. 2000. Proc. Conf:

Duplex America, Houston, USA, p.477ff.

H Okamoto. 1992. I’roc. Coni Appl. of Stainless Stwls, Stockholm,

Sweden, p.36Off.

K T Lee & Y H Lee. 1997. Proc. Con/I Duplex Stainkss Steel,

Mastrich, The Netherlands, p.903ff.

C J Park & J-S Kim. 1999. l’roc. Conf; Stainless Steel, Sardinia, Italy,

Vol. 3, p.475ff.

J Charles. 2000. Proc. Colzf: Duplex America, Houston, USA,

p.395ff.

Contact: Dr Frank Wlschnowskii 6 DipI.-lng. Andre Kuhn,

Klaus Kuhn Edalstahlgiassarai GmbH, Otto-Hahn-StraBa 12-14, D-42477 Radavormwald,

Germany. Tel. +49 21 95 671 0:

Fax +49 21 95 671 49; Wabsita: www.kuhn-edalstahl.com

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