TEHNIČKA DIJAGNOSTIKA (BROJ 3 • 2010) 3

DIJAGNOSTIKA STANJA POVRŠINE MAŠINSKIH KOMPONENTI ILI KONSTRUKCIJE PRIMENOM METODE REPLIKA DIAGNOSTIC OF SURFACE STATE OF MACHINE’S COMPONENT OR CONSTRUCTION BY REPLICA METHOD Z. Karastojković1, Z. Kovačević2, A. Raković1 1Visoka tehnička škola strukovnih studija, bulevar Dr Z. Đinđića 152a, 11070 Novi Beograd 2Institut za ispitivanje materijala, bulevar vojvode Mišića 43, 11000 Beograd

ABSTRACT Many testing methods, which are customary used in verification during production processes of machine parts, tools or assemblies, simply could not be applied after years in servicing of an assembly or equipment. For testing of any equipment, the non-destructive method always is preferable. For topographic analysis of the metal surface characteristics are developed numerous methods, but for microstructural evaluation there are only a few. If such analysis has to be provided on situ than replica method is irreplaceable. The replica should retain the details from the inspected surface and in anyway represent the negative of the microstructure. When replica is taken in field, it can be metallo-graphicaly inspected or photographed in laboratory at anytime. So, many processes which are present at inspected surface can be monitored by replica technique. The replica also is obtainable from the fractured surface and only by this technique is possible to assccess the reason(s) for damage. In this paper are presented some examples when replicas were taken from ferrous materials, steels, but also from a non-ferrous metals. Key words: replica, preparation, surface analysis, kinds of damages

IZVOD Mnoge metode ispitivanja, koje se obično koriste u ispitivanju tokom proizvodnih procesa mašinskih delova, alata ili sklopova, jednostavno se ne mogu primeniti nakon više godina rada takvog sklopa ili opreme. Za ispitivanje bilo koje opreme, metode ispitivanja bez razaranja uvek imaju prednost. Za topografsku analizu karakteristika površine metala razvijene su brojne metode, ali za otkrivanje strukture postoje samo nekoliko. Ako takvu analizu treba izvršiti na licu mesta onda je metoda replika nezamenljiva. Replika treba da zadrži detalje sa ispitivane površine i ona svakako predstavlja nega-tiv mikrostrukture. Kada je replika uzeta na terenu, ona se može metalografski ispitivati i fotografisati u laboratoriji u bilo koje vreme. Tako, mnogi procesi koji su prisutni na ispitivanoj površini mogu se ispitivati primenom tehnike replika. Replika je, tako-đe, primenljiva na površinama lomova i samo ovom tehnikom je moguće dati ocenu razloga pojave loma. U ovom radu su prikazani primeri kada su replike uzete sa železnih materijala, čelika, ali i sa obojenih metala. Ključne reči: replika, priprema, analiza površine, vrste oštećenja

1. INTRODUCTION For metal surface analysis of machine, tool part

or construction, the replica method as a non-destructive method could be used for microstructural examination(s). The surface condition as well as various surface damages, occurring frequently due to overheating or overloading, even crack(s) appea-rance, are able for metallographic analysis in situ.

The surface preparation of monitored component/assembly for replica metallography should be in fact done on the same way as for the usual surface preparation when the optical (light) microscopy is used [1,8÷12]. The area of the inspected machine part/construction first is mechanically grinded, polished, if it′s possible an electropolishing also could be used, and than etched

UDC: 620.17:621.791.1/972.520 PRETHODNO SAOPŠTENJE

TEHNIČKA DIJAGNOSTIKA (BROJ 3 • 2010) 4

for revealing the microstructure of material. The replica does not damages the inspected machine part/construction, and the monitored component can further operates.

In thermopower generator, petrochemical plan-ts, bridges, etc., there are numerous machine parts which are exposed to both high temperature and high stresses [1,4÷6]. Production of electric current or petrol/gas both for citizens and industry constan-

tly has a rising level. In condition monitoring or cer-tification of those plants the testing of components preferably should be non destructive. If in common metallographic practice is obvious to cut off a spe-cimen for further microscopic evaluation, either for light or electron microscopy, than in named plants taking a sample by cutting is just forbidden [11÷13]. Any thermopower generation plant really represents a complex structure, as can be seen from Fig. 1.

Fig. 1. Vertical cross section of one boiler plant

The role of each part in such complex structure

is of great importance, so no part could be neglected [7÷12]. How can be the main component from Fig. 1. transported to the laboratory only for inspection? In many situations those components must be inspected on-site. The benefit of field metallography is that such analisys can be carried out on site and on parts – which are difficult or imposible to transport – as like the permanent structures, for example the bridge, etc. Typical applications are on-site as a non-destructive testing in connection with quality control, inspection and maintenance of power plants, oil platforms, etc.

2. REPLICA′S METALLOGRAPHY A variety of non-destructive testing methods

were developed in past for detecting the imperfections, cracks or similar irregularities in a material from which were made different machine parts, device or construction [1÷7]. One of the useful non destructive method for micro-structural observations of the metal (surface) state is the replica technique, also reffered as a field metallography. Replica is real copy of the microstructure from the inspected surface. By taking a replica from the machine part, tool or construction, the surface shall not be damaged and can farther operates, as earlier does.

Using the replica technique, as a non destruc-tive method, has developed originaly from prepa-ration the samples for transmission electron micro-scope monitoring. Now, the non destructive metal-lography using replicas is a quite indispensable technique for monitoring the metal (material) con-ditioning during (thermo)mechanical loading either of machine parts or construction [8÷15].

The monitoring of inspected surface can be made on-site, but in such circumatances frequently is made just an overview. Full judgement often is more efficient when the replicas were brought to the laboratory and can be examined later under the more favorable conditions [11÷16]. One of the great adva-tage of the replica method lies in fact that replicas can be taken on site by skilled personal but the evaluation of microstructure can be done in the laboratory by qualified metallurgist. Further, it me-ans that results can be photographed and stored. This is of crucial importance for the future periodic retes-ting, particullary for comparison of obtained results.

Other applications of the replica technique are used for analyzing the different damages, failure analyses and other microstrutural changes caused by welding, heat-treating, etc.

2.1. Surface preparation for replica taking

The inspected area must be prepared for repli-cating on the same way as for optical microscopy. For surface analyzing of a machine/construction

1. firebox; 2. boiler drum; 3. vapor overheating; 4. economizer; 5. air heater; 6. fan; 7. filter for gases; 8. chimney; 9. ash remover; 10. coal mill; 11. mill’s fan; 12. coal stock

TEHNIČKA DIJAGNOSTIKA (BROJ 3 • 2010) 5

part, the tested area firstly should be cleaned-up and degreased by one of the chemical treatments. After that, the inspected surface must be mechanically (ro-ugh and moderate) grinded, than high polished, and finally this area needs etching as for microstructural analysis by light microscopy [11÷16].

Grinding and polishing are provided by high-powered hand drills, with attachments of small dis-ks. Those disks are fitted with grinding papers: firs-tly with rough one and further with disk of fine gra-ins, for exapmple 360 grits at the beggining up to 1000 (1200) grits at the end of mechanical treating. SiC is a common grinding material. Polishing is pro-vide by using diamond pastes or alumina powder mixed with water. Insted of pastes another shapes of polishing materials can be used as spray or suspen-sion.

Etching is usually carried out with a cotton swab saturated with the etchant. For the most structural steels as etchant also is used a nital (solution of nitric acid into the alcohol). For etching on situ usually little more concentrated solution (3–6% of nitric acid) has to be used than in a laboratory (1 to 3%).

When working with such chemicals, the safety precautions must be provided. The surrounding area of inspected machine part or construction has to be protected, indeed, especially from the acid solution influence.

2.2. Replica techniques

Replicating today represents a job, which has

never been so easy [11÷13]. In modern field me-

tallography are used three different replica techni-ques:

• Foil • Two-component polymer • Drops of lacquer 1) In this technique a softened foil is placed

onto the prepared (grinded, polished and electro-polished) sample surface. The foil is made of an ethyl acetate basis, with thickness less than 0,1 mm. The sensuation of the foil is achieved by immersion into the acethone for couple of seconds. The time of replicating at a surface usually is about 15–20 s. After removing from the inspected surface, the foil is put on the flat glass slide (plate).

It seems that metal-coated plastic foils are now dominant for replicating of different samples. The back-metal provides a higher quality of the surface image, and easier or safer handling. For improving the contrast, the plastic foil is sputtered (by plasma technique) with aluminium or gold metal.

As a standard dimensions of transparent or me-tal coated foils could be regarded 20x30 mm, while in the second type of replica the dimensions may be greater, depending from the occasions [11÷13,16].

2) Another technique uses a two-component

cold-setting polymer. The polymer is squeezed from gun with a piston to the area to be examined. Compounds are supplied in cartridges and are dis-pensed using a hand-operating gun, as it was shown in Fig. 2a). The cartridges contain both polymer and curing agent, which are mixed in a disposable nozzle during the application to the surface [11,16].

a) b)

Fig. 2. Replicating by using a hand-operated gun a) and peels of the cured replica b)

Removing the polymer from the sampled surface is done after it has hardened (cold-setting is over), Fig. 2b), without leaving any residues on the inspected surface. The curing time is obout 5 min. or less. Than, a highly accurate and stable replica will

be obtained. The cured replica is tough and dimensionaly stable. The thickness of such replica is about a few millimetres. This replica technique allows an inspection at locations where access is difficult, such as inside the pipes. In comparison

TEHNIČKA DIJAGNOSTIKA (BROJ 3 • 2010) 6

with previous technique, this technique with polymer enables the replicating the pitting: such

obtained replica further can be examined by scaning electron microscope, see Fig. 3.

a) b) Fig. 3. View of pitting at the surface a) and polymer replica, examined by SEM b)

Example from Fig. 3. offers many advantages

in using of this technique [16]. Usualy, no significant health risk is present when working with such polymer. This type of replica is usefull for applications under different conditions (as sligtly increased temperature) and on horizontal as well as vertical or overhead surfaces.

3) The third technique uses a lacquer: only a few drops of such substance onto the inspected surface is enough [12]. After reinforcing, when the curing process of polymer is finished at room temperature, the replica is ready for microscopical analyzing. But, this kind of replica is exposed to bend during peels of from the surface. Generaly, this technique has not found a wide application but for rounded samples such replicating might has an advantage.

2.3. Metallographic examination

The application of those systems offers supe-rior and fast results. For applying the replica tec-hnique an expensive operator experience is not necessary, so one can be sure to return to the labo-ratory with good replicas every time. The replica will reflects light as like a polished metal. This fact makes replica well suited for microstructural examination with a conventional microscope, by using bright or dark field. The replica can be examined up to a magnification of 500 x times by using a light microscope or up to 5000 x with a scanning electron microscope. In metallographic analysis by replica technique a high resolution may be acheived: down to 0,1 μm.

2.4. Disadvantage of replica technique

Although there are numerous advantages there are just a few limitations in using of replica

technique. All replica techniques are not adequate means for inspection of the cavities or inner holes. If such measurements have to be provided, than 3D examination can be carried out by using a non-contact measuring instruments, such as laser measuring equipment or measuring projectors with 2D or 3D facilities [13÷16].

Image of the surface obtained by replica is reffered only at the examined layer. It means that the bulk material may has quite different structure. Than, the replicating must be repeated after regrinding and repolishing.

If the residuals from etching process are not fully removed than a kind of the corrosion process may takes place in the future, which represents a disadvantage in the hole replicating technique.

3. SOME ENGINEERING INSPECTION APPLICATIONS & DISCUSSION

The replicating represent very versatille system

for non-destructuve testing of machine parts, components or constructions [11÷13]. In Fig. 1. is represented one boiler plant. In this paper a part of it will be shortly described for replicating. The boiler drum, pos. 2. at Fig. 1, is closed cylindrical vesel. In this case, boiler drum has outer diameter about Ø1800 mm, lenght 17500 mm.

The boiler drum belongs to a structures with a heavy wall thickness, sometimes over 80mm. The importance of boiler drum function demands periodically inspection of material state, and for that purpose usually the replica method is used. The distribution of places where replicas were taken is shown in Fig. 4.

TEHNIČKA DIJAGNOSTIKA (BROJ 3 • 2010) 7

Fig. 4. Scetch of boiler drum and places for replica monitoring LDB – left bottom side, DDB – right bottom side of the boiler drum

R 1÷13 places for replica monitoring

Typical microstructures are shown in figures as follows. The slightly different microstructure is found at the cylinder part and the bottom side of the boiler drum, as is shown in Fig. 5). A pores, dark

areas in Fig. 5a), easily could be registered by using this technique. Any kind of ferritization in structure, Fig. 5b), can be also discovered by this technique.

Fig. 5. Microstructure of boiler drum at: a) cylinder body and b) boiler side

The initial structure, represented in Fig. 6a), might be changed during welding process, see Fig. 6b).Those weldments should be the matter of periodical inspection, indeed [14,15]. Instead of changes into the weldments another types of changes (creep or crack damage) in material could be distinguished, see Fig. 6 c), d).

Monitoring the microstructure by using a replica method could give us more informations about the diffusion along the grain boundaries, corrosion, precipitation or enlarging the grain size.

So, using replica method for monitoring the microstructure will make sense only by periodical investigation and comparison of obtained with previous results. Coalescence of micropores also could be registered by replica method [11,15].

It′s far away that only components from a thermopower station are able for replicating. One example, pretty known from a common life, is a crankshaft from a vehicle, in Fig. 7. is represented such examination.

100µm

100µm a) b)

TEHNIČKA DIJAGNOSTIKA (BROJ 3 • 2010) 8

Fig. 6. Microstructure of: a) base material near weldment, b) in weldment; c) beginning of creep, and

d) appearing of one crack

Fig. 7. View of crankshaft a) and cracks at the surface, registered by replica method b) The crankshafts generally are produced by cas-

ting, forging, machining and heat-treating. After re-plica is taken off, an analysis could be provided for discovering the reason(s) of cracking. But, the repli-ca technique is available for monitoring the surface

layer, for example the cracks at hard chromium layer obtained by electroplating [11], as it was shown in Fig. 8. This example certainly shows wider appli-cation of replica technique.

100µm

100µm a)

100µm d) c)

b)

100µm

TEHNIČKA DIJAGNOSTIKA (BROJ 3 • 2010) 9

a) x 50 b) x 25

Fig. 8. Cracks in hard chromium layer monitored by replica at: a) bright and b) dark field microscopy

Non-ferrous metals rarely are examined by using the replica technique, but it is also possible, as could

be seen from Fig. 9.

Fig. 9. Microstructure of pure copper obtained by: a) light microscopy and b) replica

The slightly difference is evident when the structure is monitored by light microscope or by using the replica, but such difference in colour of the picture might be neglected.

Replica technique is available for monitoring the structure changes into the metal. For further discussion of findings, a solid knowledge about metallurgical reactions is needed.

For discovering the cracks or microcracks, the replica method may offer an overview on the damage character (brittle or ductile), dimension of crack, and at the same time a look to the microstructure change of inspected material/machine component.

CONCLUSION The replica technique is used for metallo-

graphic observations on site, without sampling by cutting. In modern field metallography three different replica techniques are used: foil, two-component polymer, and drops of lacquer. The most popular is the first one.

After replica is taken off, an analysis could be provided for discovering the reason(s) of cracking at the surface. The replica technique is also available for monitoring the surface layer obtained by electroplating or any another method, especially when cracks were appeared.

Replica technique is available for monitoring the structure changes into the metal. For further discussion of findings, a solid knowledge about metallurgical reactions is needed. Monitoring the microstructure by using a replica method could give us more information about the diffusion along the grain boundaries, corrosion, precipitation or enlarging the grain size. So, the using replica method for monitoring the microstructure has the sense only by periodically investigation and comparison of obtained with previous results.

Image of the surface obtained by replica is reffered only at the examined layer. It means that the bulk material may possess quite different structure.

TEHNIČKA DIJAGNOSTIKA (BROJ 3 • 2010) 10

REFERENCE

[1] H. Thielsch: Defects and Failures in Pressure Vessels and Piping, New York 1965, Reinhold Publ. Corp, pp. 333–367.

[2] С. З. Бокштејн: Строение и својства мета-ллических сплавов, Москва 1971, Метал-лургија, пп. 379–398.

[3] R. E. Reed-Hill: Physical Metallurgy Princip-les, New York 1973, Van Nostrand, pp. 827–882.

[4] А. Н. Подгорниј, В. В. Бортовој, П. П. Гон-таровскиј, и др: Ползучест елементов маши-но-строитељних конструкциј, Киев 1984, Наукова думка, пп. 171–179.

[5] A. M. Архаров, С. И. Исаев, И. А. Кожинов, и др.: Теплотехника, Москва 1986, Машино-строение, пп. 149–167.

[6] С. Тајра, Р. Отани: Теорија високотемпера-турној прочности материалов, перевод с ја-понског, Москва 1986, Металлургија, пп. 50–91.

[7] А. П. Гуљајев: Металловедение, Москва 1986, Металлургија, пп. 211–252.

[8] Z. Karastojković, Z. Đorđević: Grafitizacija u čeliku u toku rada na povišenim temperaturama i njeno otkrivanja replikama in situ, V Jugo-slovenski simpozijum o termičkoj obradi metala, Vrnjačka Banja, Zbornik radova, in Serbian, pp. 284–288.

[9] Guideline for the Assessment of Microstructure and Damage Development of Creep Exposed Materials for Pipes and Boiler Components, Essen 1992, pp. 8–88.

[10] З. Карастојковић, З. Ковачевић: Метало-графско испитивање деловања котловске воде и водене паре на корозију котловских цеви, Међународно саветовање “Индустриј-ске воде”, Панчево 1995, Зборник радова, in Serbian, pp. 173–178.

[11] J. Vagn Hansen: Non-Destructive Metallo-graphy Used On-site, Possibilities and Expe-riences, NDT, 3/1998/10

[12] G. Petzow, V. Carle: Metallographic Etching, Ohio 1999, ASM, pp. 50–53.

[13] T. L. de Silveira, I. Le May: The Arabian J. Sci. and Eng., 31/2006/2C, pp. 99–118.

[14] Z. Karastojković, Z. Janjušević, Z. Kovačević, i dr.: Korozija toplovoda, Integritet i vek konstrukcija, 7/2007/2, in Serbian, pp. 129–132.

[15] З. Карастојковић, З. Ковачевић: Микростру-ктурно праћење квалитета током дуготрај-ног рада на повишеним температурама че-лика ČSN 15223 методом реплика, Саве-товање „Фестивал квалитет“, Крагујевац 2009, in Serbian, CD issue, paper 61.

[16] Prospekt firme STRUERS – Repliset.