Manufacturing - Engineering

STAINLESS STEEL IN THE SUGAR INDUSTRY

C. G. W. Roffey* and I. A. Elsdon-Dew? ,

*Consulco (Pty) Ltd, Johannesburg, South Africa and

"f, Jackson and Co (Natal) (Pty) Ltd, Durban, South Africa

ABSTRACT The application of stainless steel is dealt with, including the general

properties of the material .with regard to wear resistance and corrosion resistance in the sugar industry. It covers the various grades of stainless steel including AlSl 41 0, 430, 304 and 31 6 and also the form or shape including rod, plate, sheet and tubing.

Discussed also are some of the failures that can occur with this material and some of the new applications which have been found to be successful, and also areas where it is considered the use of stainless steel wil l not be advantageous. A detailed investigation is given on the use of stainless steel in scrubbers on smut arrestors for bagasse and coal fired boilers. Finally, the cost of using stainless steel, as against other conventional materials is discussed, using as an example a new mill to crush a million tons of cane, and this shows the investment in stainless steel would only be approximately 0,5% extra on the whole project.

STAINLESS STEEL PROPERTIES

General < < Stainless steel" is not a single metal - it is the name given to a group of

iron based alloys containing a minimum of 11 % chromium.. .Most stainless steels also contain nickel, often in association with more exotic elements such as molybdenum, titanium, niobium - used to confer specific properties. These stainless steels are divided into other categories according to their metallurgical structures. Table 1 details these basic metallurgical structures and properties in the AISI series; Appendices A and B give the basic analyses and world wide specifications of the stainless steels referred to in this paper.

Wear resistance Martensitic grades - A I S I 400 Series (see Appendix A)

Fatigue and wear resistance increase with higher strength values. The martensitic grades can be hardened to Rc 60 and UTS values of 135 kg/mm. They find extensive use where abrasion, or torque and fatigue loading, is required. Typical examples are AISI 410, 420, 440, 431. These stainless steels can also be nitrided or hard chromed to give ex- cellent shaft bearing surfaces.

Ferritic grades - A I S I 400 Series These grades cannot be hardened and are not normally employed for

heavy wear or abrasion-resistant applications. However, they are used extensively where mild abrasion is combined with mild corrosion, which is normally sufficient to attack mild steel. Typical examples are AISI 405 and 430.

C . G. W. ROFFEY AND I. A. ELSDON-DEW 1697

TABLE 1. Basic properties. !

AISI Major Metallurgical Series alloys Basic properties

Martensitic 400 Fe + Cr + C Magnetic - Hardenable to Rc 60 Ferritic 400 Fe + Cr Magnetic - Non-hardenable Austenitic 300 Fe + Cr + Ni Non-magnetic - Slightly hardenable to Rc 30

Austenitic grades - A I S I 300 series Although these steels cannot be heat treated to the degree of hardness

of the martensitic grades, they work harden rapidly to Rc 25/30. As such they find extensive use in the more corrosive environments where mild abrasion aggravates attack. Typical examples are AISI 304 and 3 16.

Special stainless steels If the high tensile strength of the martensitic grades is required,

combined with the high corrosion resistance of the austenitic grades, then precipitation hardened or other refined stainless steels are used. These are normally very specialised applications and not usually found in the sugar industry. Typical examples are 1714 PH, 1717 PH, 2714.

Corrosion resistance Fundamentally, the higher the alloy content the greater the corrosion

resistance. Hence the basic corrosion resistances of AISI 410, 430, 304, 316 respectively increase with the austenitic grades - especially AISI 316 - having the highest resistance to pitting attack.

Although the stainless steels have excellent resistance to many acids and alkalis over a large temperature range, there are several corrosion problems which may be encountered, namely:

Chloride pitting attack All stainless steels develop a passive oxide film in contact with air or

dissolved oxygen. Chlorides tend to attack any weak areas in the passive film and penetrate at points, causing intensive localised attack - Fig. 1. There is no general loss in metal thickness, but failure is normally rapid and irreparable. The velocity of the solution in contact with the stainless steel surface plays an important part in determining the degree of attack. Stagnant and static solutions containing chlorides tend to promote heavy and rapid attack as the oxygen only reaches the metal surface at the rate of diffusion in the solution. Normally, the higher the flow rate, the less the degree of attack. I t has been shown that pitting of AISI 316 is virtually negligible in seawater, p.rovided the velocity is above 1,7 m/sec. However, in stagnant or slow movlng sea water heavy pitting can occur. I t is generally accepted that AISI 316 is the best stainless steel to combat pitting attack, but AISI 304 and 430 may be used at lower chloride con- tents and higher solution velocities. I t must be noted that the above discussion is very generalised and expert opinion must always be obtained before materials are selected to combat solutions containing chlorides.

Intergranular carbides If the austenitic stainless steels are cooled slowly through the range

450 C - 750 C, chromium carbides are precipitated. This causes the

FIGURE 1. Typical example of chloride FIGURE 2. Typical example of intergran- pitting attack. ular carbide attack (weld decay).

area around the precipitation to be depleted in chromium and lose most of its resistance to corrosion. Stainless steels in this condition are termed "sensitised" and are, subject to very rapid corrosive attack, (Fig. 2). This situation frequently arises during fabrication, both during welding (in the heat affected zone) and due to the indiscriminate use of welding torches to preheat stainless steel surfaces prior to forming.

In cases where sections heavier than 14 gauge are to be welded and subjected to a corrosive environment, it is necessary to use stainless steels which will not be sensitged in the heat affected zone. This is simply done by specifying the low carbon grades, e.g. AISI 304L and 316L, or the stabilised grades, e.g. AISI 321, 347, 316Ti.

In most cases where thinner than 14 gauge is to be welded, the rate of cooling of the heat affected zone through the critical 450 C - 750 C range of the standard grades, for example AISI 304, 316, is fast enough to prevent intergranular carbides forming, and "L" grades or stabilised stainless steels are not normally considered. Many engineers and fabricators state that the "L" grades and stabilised grades are specified for "weldability". This is erroneous. This precaution is necessary to combat a potential catastrophic corrosion situation and to ensure that the original level of corrosion resistance is maintained across the welded joint. The "welda- bility" and mechanical properties of the stainless steels are basically unaltered.

Intergranular carbides are also extensively formed if the stainless steel surface is heated in the Dresence of carbon. The usual sources of carbon in a fabricating shop are grease, oil, marking crayons, and the use of LPG burners or oxyacetylene torches to heat the stainless steel surface.

C. G . W. ROFFEY AND I. A. ELSDON-DEW 1699

FIGURE 3a. Typical example of stress FIGURE 3b. Stress corrosion and cracking. corrosion cracking.

The correct specification of the "L" grades to ensure full corrosion resistance of the weld joint is completely useless if the fabricator re-introduces the problem as outlined above. Stress corrosion cracking

This is catastrophic failure due to the combination of stresses (from operation and/or fabricating) and a corrosive environment, normally chlorides (Fig. 3). Failure would not normally occur if the stress or corrosion were present singly. The operating temperature is normally above 50 C before failure occurs. Whilst the austenitic grades are sus- ceptible to this form of failure, the ferritic ones are not. Crevice corrosion

This is due to the deposition of a porous film, wet sludge or bad design creating tight crevices. The area of stainless steel under the film or in the crevice, where the solution is not easily replenished, sets up a corrosion cell with the area in free contact with the solution. Rapid pitting attack occurs under the covered area. Fig. 4 shows the crevice attack caused by wet smuts or burnt bagasse lying on a new stainless steel tube.

APPLICATIONS

Bar stock Abrasion/ Wear resistance

These applications have been limited to the heat treatable martensitic grades. Cane shredder bars in AISI 420 and 43l'hardened to Rc 60 are giving a life of approximately 3 times that of alloy steels of the 40 - 60 ton types. Pins and bushes for intercarrier chains in these alloy steels were

1700 MANUFACTURING - ENGINEERING I

Left:

FIGURE 4. Typical example of crevice corrosion on stainless steel tube.

Bottom Left:

FIGURE 5 (a). Comparison of maceration tanks in mild steel AISI 430.

~ o f i o r n Right: FIGURE 5 (b). Mild steel maceration tank after 3 seasons operation.

failing prematurely due to corrosion from the bagasse; AISI 410 and 420 have solved this problem, giving at least 3 times the life, ensuring that the chains will last the whole season without unscheduled stoppages. On average the stainless steel components used have cost double that of the alloy steel.

Problems encountered The martensitic steels in the hardened condition are very difficult

to weld, and heat treatment before and after welding is essential. The

C. G. W. ROFFEY AND I. A. ELSDON-DEW 1701

above applications do not normally involve welding, but where machining is involved the rods or hollow bars are usually supplied in the soft condition for easy machining, and heat treatment after machining is necessary to develop the full strength and hardness of the stainless steel.

Plate and sheet

Abrasion Table 2 details the uses of AISI 430 in several applications where

corrosion has been minimal and mild steel plates have worn through. I t will be noted that the anticipated life of AISI 430 is at least 5 times that of the mild steel - and in many cases a saving can be made by reducing the metal thickness. Moisture in the cane causes mild steel to corrode slightly and continuously roughens the surface, increasing the friction effect. The AISI 430 is not subject to this slight corrosion and retains its sliding surface throughout its life.

TABLE 2. Typical abrasion applications.

Previous experience Stainless steel experience

Expected Life - life -

Thickness Million AISI Thickness Million tons tons no. crushed

crushed ( a ~ ~ r o x )

Cane preparation table Mild steel 6 mm 5 million 430 6 mm 25 million ShredderICane knife disc Mild steel 20 mm 4 million 430 20 mm 15 million Bagasse intercarriers Mild steel 4,6 mm 8 million 430 3 mm 25 million Boiler feed chutes Mild steel 6 mm 8 million 430 6 mm 20 million

Corrosion There is always an inherent corrosion problem for mild steel in a

sugar mill due to the chloride content of the juice. The lives of many steel parts appear to be satisfactory, for example 5 seasons. However the use of stainless steels has completely eliminated replacements, allowed main- tenance labour to be used for other purposes, and has considerably re- duced erection and structural support costs.

Table 3 details several areas where stainless steels are being success- fully applied. Note should be taken of the drastic reduction of many metal thicknesses compared with cast iron and steel. In many cases the stainless steel was installed for the same price as the steel component.

Apart from these applications there are many others such as Donnelly chutes, juice gutters, servo bajances, juice scale tanks, etcetera.

Combined abrasion and corrosion The stainless steels are ideal materials to combat the combined effects

of wear and corrosion which completely destroy other materials and Table 4 details current experience in solving this problem. I t should be noted that lives of between 5 and 10 times more than mild steel are anticipated although in many cases the stainless steel thicknesses are drastically reduced.

1702 MANUFACTURING - ENGINEERING

TABLE 3. Typical factory applications.

Previous experience Stainless steel experience

Expected Life - life -

Thickness Million AISI Thickness Million tons no. tons

crushed crushed

Maceration tanks Mild steel 6 mm 3 seasons 430 3 mm Indefinite Juice lines Steam pipe ~6 mm 2.seasons 304 1,5 mm Indefinite Filter underpans Mild,steel ~ 4 , 5 mm 5 seasons 430 3 mm Indefinite Vapour piping Cast iyop' ~ 2 5 mm 12 seasons 430 6 mm Indefinite

1 Mild steel 12 mm 5 seasons Vacuum condensers Mild steel 12 mm 5-7 seasons 430 6 mm Indefinite

Cast iron 25 mm 12 seasons

The processes where cane is dragged over a metal surface in contact with sugar juices are ideal applications for appreciating the immediate econo7mics in the use of stainless steels.

Problems Fabrication

Problems were initially experienced in the bending and forming of AISI 430 as many mills did not have suitable equipment: However, close co-operation between the stainless steel suppliers and sugar mill engineers has now enabled all mill workshops to handle plates up to 10 mm without problems.

Welding AISI 430 welding is problematic as the weld and heat affected

zone tend to be brittle particularly with excess heat in parts. All applications of AISI 430 are limited to statical~y'loaded components - it is not advisable to use "as welded" AISI 430 on dynamically loaded components, for example fans, or where heavy vibration is present. To obtain the best structural weld with AISI 430 it is essential to ensure full penetration with the absolute minimum heat input. The normal welding rods AISI 430/310 (25% Cr/20% Nickel) have been found to be very successful and this rod can be used for joining AISI 430 to mild steel.

Corrosion Mild steel is subject to heavy pitting attack, especially "tramline"

attack at the welds in pipework. Where AISI 430 is in contact with sugar juices or derivatives,

virtually no corrosion problems have been encountered.

Abrasion Where impact is the prime form of attack, then AISI 430 is not

suitable due to the verv low i m ~ a c t resistance of the material. Where heavy abrasion, combined with impact occurs, e.g. induced draft fans, then the stainless steels have little to offer over mild steel.

Where metal to metal abrasion occurs in mud mixers and some

C. G. W. ROFFEY AND I. A. ELSDON-DEW 1703

TABLE 4. Typical mill plant application.

Previous experience Stainless steel experience

Life - Type Thickness Million AISI Thickness

tons no. crushed

Intercarrier boots Mild steel 12 mm 2 million 430 6 mm Intercarrier decks Mild steel 10 mm 2 million 430 6 mm Clarifiers Mild steel 6 mm 12 million 430 6 mm Mud mixers Mild steel 6 mm 3 million 430 6 mm Diffuser plates Mild steel 6 mm 2,5 million 430 6 mm

Expected life -

Million tons I

crushed

22 million 22 million Indefinite 15 million 25 million

clarifiers, lubrication is the only answer to prevent heavy wear and galling.

Tubing The major application of stainless steel in the South African sugar industry

to date has been that of tubing for juice heaters, juice evaporators, pan con- densers.

Materials Over the last 10 years stainless steel has replaced the 70130 brass

tubes in the industry to the degree that now in South Africa over 80% of the tubes in operation are in stainless steel. Up to 4 years ago AISI 304 was the only grade used, but in the last 4 years this has given way to the slightly cheaper ferritic quality Vallinox 430, which is not susceptible to stress corrosion cracking.

TABLE 5. Tubing specification.

Subject to Weight (1) Hardness UTS Expected stress Cost

Product kg/m for 2" Brinnell kg/mm2 life corrosion per kg OD x 16 SWG in years cracking R

Mild steel 2,lO 120 36,5 3 No 1 Brass 70130 2,25 120 35 3-7 Yes 7,75 AISI 304 S/S 1,85 160 60 12 Plus Yes 4,5 AISI 316 S/S 1,85 160 63,5 12 Plus Yes 595 Vallinox 430 S/S 1,79 180 58,5 12 Plus No 4

costs Spiralling copper prices, combined with an uncertain supply position,

made stainless steel a decidedly cheaper material, particularly when tube wall thickness is reduced from 2 mm or 1,5 mm to 1,2 mm. The Vallinox 430 tube offers a further 10% saving over AISI 304 tubes.

Fabrication As both Vallinox 430 and AISI 304 tubes are supplied in the fully

annealed condition, there are no problems in expanding the tube ends. I t is not necessary to attempt to anneal the tube ends on site - in fact any heating of the Vallinox 430 will destroy the ductility.

MANUFACTURING - ENGINEERING

Heat transfer Lengthy discussions of the relative merits of stainless steel and brass

heat transfer rates are irrelevant since the film formation and thinner wall of the stainless steel even out any original advantages claimed for brass.

Scale remoual Where scale is soft, a brush is usually employed, but on harder scales

a skatoskalo is used. As the stainless steel has a harder surface than brass, the scoring of the tube is reduced and thus there is less "key" for the scale to adhere to. Acid cleaning is being investigated, but results are not avail-

<.> <? able at this stage. ?, +

Corrosion The increasing use of nitrogenous fertilizers has resulted in an in-

creasing amount of ammonia being evolved in the effect system and whereas the stainless steels are fully resistant, the brasses (particularly in the hard drawn condition) are very susceptible to season cracking.

There, have been some failures of AISI 304 tubing in South Africa due to stress corrosion cracking. This has normally occurred adjacent to the tube plate, ,ahd in one instance was induced by heavy fatigue stresses due to the tube not being supported over a 7 m length.

One of the major factors (apart from the price) for the swing to Vallinox 430 tubing in South Africa is that this ferritic stainless steel is not susceptible to stress corrosion cracking. There has been no evidence of pitting attack in either Vallinox 430 or AISI 304 tubes in evaporator effects even though chlorides of up to 6 000 ppm are common.

POLLUTION CONTROL

Due to environmental problems, there has been a concerted effort in South Africa over the past 5 years to control atmosphere and water pollution. The sugar industry has been involved in this problem - particulary in control- ling atmospheric pollution from the boiler flue gases.

The main control systems have involved wet scrubbers and many engineers, because of previous satisfactory experience with excellent stainless steels in the mill, turned to these materials for the construction of the scrubbers - with very mixed results.

There are several types of wet scrubbers, involving the following principles: packed tower system; impingement plate systems; cyclonic spray systems; fluidised beds systems; venturi systems; self-induced spray systems; flooded disc systems.

In every case water is injected into the gas stream, converting the relatively inactive gases into their extremely reactive hot acids, resulting in major corrosion problems. The various boiler feed stocks, their corrosive products, and attendant corrosion problcms for stainless steel are detailed below.

The above corrosion problems are further severely aggravated by: a) the type of water used for scrubbing; b) the type of water system; c) the degree of solids in the flue gases. If the scrubbing water comes from another part of the plant, it invariably

contains chlorides. There are several cases where lagoon or river water is used,

C . G. W. ROFFEY AND I. A. ELSDON-DEW 1705

TABLE 6. Typical corrosion problems encountered in wet scrubbing coal and bagasse fired boilers.

Corrosive products Fuel stock in wet scrubbers Corrosion problems

Bagasse only Hot chlorides 1) Pitting attack (Probably 100-500 ppm) 2) Crevice corrosion under flue dust deposits

3) Possibility of stress corrosion at higher chloride levels

Coal only Hot sulphuric acid 1) Heavy general corrosion 2) Crevice corrosion 3) Weld decay

Mixed or Hot chlorides 1) Heavy pitting attack alternating (Up to 6 000 ppm) 2) Heavy general corrosion coal and and 3) Heavy crevice corrosion bagasse Hot sulphuric acid 4) Possibility of stress corrosion

5) Weld decay

and these waters contain other impurities apart from chlorides. Most of the water systems are closed cycle operations which make pH

control almost impossible and lead to heavy build-up of the corrosive elements I

in the scrubbing water. All these factors seriously aggravate the problem. I I t is apparent from the above discussion that the specification of materials

to handle these arduous conditions is very different from the mild corrosion encountered with sugar juices. From our experiences as consultants in this 1 field, we believe that the following information is absolutely essential before any materials can be specified:

I a) Actual firing system, for example, coal, bagasse, or mixed; b) pH and chloride analysis of the water to be used for scrubbing; c) continuous chloride and pH checks on the recycling water; d) pH and chloride analysis of the make-up water; e) fuel analysis of the flue gases, with particular attention to chlorides,

sulphates and solids content. This information is required for every boiler, and it is dangerous to take

one set of results and apply it to another boiler system or mill. We have found

I that AISI 430 has given satisfactory service as scrubber shells, vanes, droplet I

eliminators when operating on bagasse fired boilers where there is no sand con- ,

tent in the flue gas, the pH does not fall below 5,5, and the chloride content of the water does not exceed 500 ppm. None of the stainless steels will give a guaranteed long life in wet scrubbers handling gases from coal or mixed flue fired boilers - particularly if the scrubbing water contains chlorides and oper- ates on a closed cycle system allowing heavy build-up of the corrosive elements.

I

MARKET STATISTICS

Approximately 400 tons per annum of stainless steel plate - 95% of which is AISI 430 - is being used in the South African sugar industry. The tube market is very variable, but averages approximately 450 tons per annum. Although stainless steel tubes have traditionally had the major portion of the total stainless steel market, we estimate that the rapidly increasing plate tonnage will outstrip the tube tonnage by 1976. Most applications for stainless steel are replacements for other materials when they fail. Approximately

1706 IvIANUFACTURING - ENGINEERING

18 million tons of cane is crushed per annum in South Africa, giving an approxi- mate figure of 45 tons of stainless steel plate per million tons of crushed cane.

However, if a new mill were erected to crush a million tons of cane per annum. we antici~ate that 350 - 400 tons of stainless steel would be reauired- solely for proven applications. The additional capital expenditure involved on the project when using stainless steel instead of other materials would result in an overall increase of less than 0,5 %.

ACKNOWLEDGEMENT

We would like to express our sincere thanks to the South African sugar industry. I t is only through their'iwholehearted co-operation and helpfulness that this development of stainless steels in the sugar industry has been achieved.

APPENDIX "Ayy BASIC ANALYSES OF THE STAINLESS STEELS REFERRED TO IN THE PAPER

Metallurgical % Basic alloying elements AISI no. structure

C Cr Ni Mo Other

Martensitic

,

Ferritic Austenitic

9,

APPENDIX "B" WORLDWIDE SPECIFICATIONS OF STAINLESS STEELS REFERRED TO IN THE PAPER

Germany AISI French Italian Werkstoff UK Japan Swedish no. no.

ZlO.Cl3 ZlO.Cl3 Z2O.Cl3 Z3O.Cl3

- Z15.CN16.2 Z 8.C17 Z6.CN18.10 Z2.CN18.10 Z1O.CNT 18.10 Z10 CNN6 18.10 2 5 CND 18:12 22 CND 18.12 28 CNDT 18.12

sus 2 1 sus 22

SUS 23 -

SUS 24 SUS 27 SUS 28 SUS 29 sus 43 SUS 32 sus 33 -

SIS 2301 SIS 2302

SIS 2303 -

SIS 2321 SIS 2320 SIS 2332 SIS 2352 SIS 2337 SIS 2338 SIS 2343 SIS 2353 SIS 2344

C . G. W. ROFFEY AND I. A. ELSDON-DEW

ACERO INOXIDABLE EN LA INDUSTRIA AZUCARERA

C. G. W. Roffey y I. A. Elsdon-Dew

RESUMEN Este folleto trata sobre la utilizacidn del acero inoxidable, incluyendo las

propiedades del material, con relacidn a su resistencia al desgaste y a su resistencia a la corrosidn, en la industria de la caAa de azlicar. Cubre diferentes tipos de aceros inoxidables incluyendo 10s AlSl 41 0, 430, 304, y 31 6 en secciones redondas, en Isminas y en tuberias.

TambiBn se discuten algunas fallas que ocurren con este material y algunos de 10s nuevos usos en 10s cuales ha tenido Bxito; Asi como tambien fireas donde se Cree que el uso del acero inoxidable podria ser ventajoso. Se incluye una investigacidn detallada referente al uso de acero inoxidable en lavadoras de gases y eliminadores de hollin en calderas de bagazo o carbdn.

Finalmente se comparan 10s costos entre utilizar acero inoxidable y usar materiales convencionales llegando a la conclusidn de que en una ftibrica nueva, para moler un milldn de toneladas de cafia la inversidn en acero inoxidable incrementaria en un 0.5% el valor del proyecto.