Itroduction

Quantifying finish and wear on oversized or inaccessible parts presents a challenge in applications from machining to shipbuilding. In some cases, measurement instruments cannot access surfaces to characterize them directly. In other instances, a component or system would need to be dismantled or destroyed in order for a feature to be measured. Over the years a variety of materials have been developed to replicate surfaces, enabling remote, nondestructive analysis. Early materials served well for replicating relatively rough surfaces. More recently, materials have been developed which can replicate features down to the nanometer scale. Used in conjunction with high-resolution measurement techniques, replication now enables monitoring of critical surfaces as large as aircraft wings or as small as intercrystalline cracks.

Replication is being employed as a means of inspecting critical engineering surfaces. The technique produces an exact copy of the surface which can be peeled away and examined microscopically in the laboratory. This technique is used for detecting surface defects on the roll surface, such as success of roll grinding, cracks, fire cracks, spalling, inclusions and other surface defects on the rolls. The replicas are examined by optical or scanning electron microscopy. An additional advantage is that the replicas can be archived, for future reference.

The topgraphy of one-stage replica is a negative copy of the relief on the surface, but because of simplycity of preparation such replicas are usually used for observations. Before an examination in the SEM the replica needs only to be coated with a cunductive metal layer by sputtering or evaporation in a vacuum coater. Usually 20-30 nm thick gold layer is deposited.

Replication Materials

The term 'replication' often conjures an image of cellulose acetate, an early replicating material. They soften in acetone and can be applied to a surface which has been wet with acetone. They are stripped off when dry. Thicknesses of 22 or 35 µm are used for finer detail replication. Triafol, a thicker (100 µm) replicating sheet, is suitable for replicating rough surfaces. Triafol is an acetobutyrate film soluble in methylene chloride or acetone. The process is somewhat involved, and the solvents may also react with sample surfaces. For these reasons, acetate is now used primarily in labs for microscopic examination of metals. In its place, a variety of new materials have been developed specifically for the quick, easy replication of surface features.

Pressure-sensitive films produce fast and inexpensive replicas. In this method, the user rubs or burnishes a piece of foam-backed film to conform it to the sample surface. Primarily used for rough surfaces (approx. 20–120 microns Ra), such as grit-blasted metals, films are also useful for measuring printing rollers, sheet products, and vertical or inverted surfaces.

Vinyl polysiloxanes, originally developed for dental impressions, have excellent dimensional stability, flow easily, and have short curing times. The two-part materials are typically mixed and applied to the surface by hand. With resolution typically on the order of one micron, dental impression materials provide a mid-cost, mid-resolution replication solution.

Most recently, two-part silicone rubbers (Figure 15-1) have been developed specifically for high resolution replication. These compounds, which are typically applied with a dispensing gun, can reproduce features down to 0.1 micron or below. Some can be used over a wide temperature range and can even be applied under water for corrosion, wear and damage inspection.


Fig. 15-1: Two-part compounds have been developed for easy, high-resolution replication.
(Courtesy Struers).

Examples

Example 1: Crack on the work roll surface.


Fig. 15-2: Crack on the work roll surface.


Fig. 15-3: Replica image of the crack on the work roll surface, Mag. 32X.

Example 2: Spalling area on the roll surface.


Fig. 15-4: Spalling area on the work roll surface


Fig. 15-5: Replica image of spalling on the work roll surface, Mag. 20X.

Example 3: Crack on the roll surface.


Fig. 15-6: Crack on the roll surface.


Fig. 15-7: Replica image of the crack on the roll surface.

Field Metallography

Field Metallography describes the practice of performing microstructural analysis outside the metallurgical laboratory. It can include everything from grain sizing of forging rolls on a production floor all the way to failure analysis of a roll. The process required involves all of the specimen preparation procedures performed in the metallography lab. The environment in which field metallography is performed makes it a very challenging endeavor.

Field metallography begins with the very rough grinding of the selected area, usually with a 60 grit abrasive. The beginning surface is often in a much poorer state than any surface found in the lab. Thick scale and rust must be removed in an area larger than the ultimate spot. Each area worked should be smaller than that worked in the previous step. This reduces the opportunity of dragging large particles across a prepared area.

After rough grinding, several fine grinding steps occur. Fine grinding is performed with abrasives in the 120 through 600 mesh range. As with metallography performed in the lab, each abrasive step should remove the surface deformation produced by the previous step.

The next step is polishing of the surface. Like lab work, this is done with polishing cloths and compounds. Because of the importance of turn around time frequently found on field metallography work sites, diamond compounds are preferred over other slower working products. Again, each polishing step should remove the scratches and deformation of the previous step.

Proper etchant use after polishing is usually required to bring out the relevant microstructure. Extra care must be taken with these fluids because of the less than perfect locations requiring field metallography. An electrolytic etchent is used, utilizing either an electrolytic etcher or a 6 volt dry cell battery, alligator clips and a cotton ball.

Analysis and documentation are the final steps in field metallography. A portable microscope is used for the field analysis, but complete documentation and laboratory analysis can be achieved with cellulose acetate tape with a surface replication technique. Prepared properly and secured in the field with glass slides, field replicas can be analyzed with SEM magnifications with very good results.

The equipment used in field metallography is very diverse. For rough grinding, right angle grinders or portable belt sanders are popular. Fine grinding and polishing can be accomplished with variable speed drills or dremel tools. These tools can be found at your local hardware store. Portable electrolytic polisher/etchers are also used for the polishing and etching steps.

Field metallography, like laboratory metallography, can be properly performed with many different consumables. It is typically required on very short notice to the metallographer.

Standard: ASTM E1351-01(2006) Standard Practice for Production and Evaluation of Field Metallographic Replicas.

Example: Microstructure of the AISI steel D2 for rolls.


Fig. 15-8: Microstructure of the AISI steel D2.


Fig. 15-9: Replica of microstructure of the AISI steel D2.

References

1.) Henrik Kaker, Detecting the roll surface defects with the replica technique, unpublished report, 2009.
2.) Struers, On-site materialographic preparation and examination, brochure, 01.02/62174370.
3.) Mike Zecchino, Tom Stout, Replication Enables High Resolution Optical Metrology on Hard-to-Measure Surfaces, Veeco Instruments, 2004.
4.) Jan Hejna, Examination of Metallographic Replicas in the Scanning Electron Microscop, Practical Metallography, 39, 2002, pp. 303-320.