Met-Tips Ideas for Metallographic Procedures

No. 3

In-Situ MetallographyMany times a metallographer needs to examinethe microstructure of a material withoutphysically destroying the piece. At first glancethis seems to be an impossible feat; however,with the techniques involving in-situ, or in-placemetallography, this task can become a reality. Infact, as one becomes highly proficient in thistechnique it is very difficult to differentiate amicrostructure that is the result of carefulpreparation in the laboratory from amicrostructure result ing from in-situmetallography.

In-situ metallography is the metallographicpreparation of an area located on large piecesthat cannot be destroyed by sectioning or takento a laboratory. Large rolls, shafts, tubes, pipes,inside surfaces, outside surfaces, curvedsurfaces, vertical, or horizontal surfaces can besuccessfully examined by replicating theprepared surface with softened celluloseacetate and then inspecting the replica undernormal microscopic conditions back in thelaboratory.

There are no special tools required to performin-situ metallography. All the necessary itemsare found in the laboratory: abrasive papers forgrinding, polishing cloths and compounds,etchants to reveal the microstructure, celluloseacetate and acetone, containers to squirt wateror alcohol on the prepared surface to flush awaygrinding debris or an etchant, containers tohold the removed replicas, scissors, tweezers,and blocks of wood to wrap the grinding papersor polishing cloths around. As one becomesmore involved with this technique, the necessityfor a portable kit containing all the above itemssoon becomes evident.

When in-situ metallography is required, the firstthing to determine is how uncomfortable it willbe for the person doing the preparation. If thepart to be examined is in a cramped area likethe inside of a roll or on a vertical surface high inthe air, then a rapid removal of the initialsurface is desired. An electric sander (orbital orotherwise), or even a portable drill that hassanding attachments can be used. If these arenot available, or if the piece is in an area whereelectricity cannot be furnished, then smallpieces of abrasive paper wrapped around ablock of wood can be used to manually grindthe surface.

The grinding procedure should always start withthe finest silicon carbide grit that will remove theoriginal surface. A finely machined surface canusually start with a 120 grit SiC paper, but if thesurface is heavily oxidized or extremely rough,then one should start with a coarser grade ofsilicon carbide paper or equivalent grindingdisc. Grinding steps should follow the sameprocedures as used in the laboratory, (i.e. 600grit grind). Water should be used as a lubricant,or if water is harmful to the part(s), other liquidmedia can be used. Each grinding step shouldbe 90 degrees to the previous grind to enableone to tell when all the previous grindingscratches are effectively removed. Also, at theend of each grinding step, the area should beflushed with water (or other liquid media) toflush away loose abrasive particles.

Once the grinding steps are completed, thearea is ready for polishing. Polishing will usuallyconsist of three steps—polishing with 6-microndiamond compound, followed by 1-microndiamond compound polish, and the final polishwith 0.05-micron gamma alumina.

Procedure

The polishing cloth used for the diamondpolishing should be red felt, billiard, or canvasin which the diamond compound isimpregnated. Although other polishing clothscan be used, these will have greater durabilityand longer life. A water soluble oil isrecommended as a lubricant. The finalpolishing step will consist of a medium nappedcloth using 0.05-micron gamma alumina as thepolishing abrasive. Water is usually preferred asa lubricant. As in the case of the grinding steps,a small piece of wood wrapped with thepolishing cloths is used. Although one’s fingerscan be used, more pressure can be appliedusing the cloth-covered blocks of wood.

Once the grinding and polishing procedures arecompleted, the area is ready for etching.Etching is the most difficult part of in-situmetallography. Where the effects of grindingand the replication technique can be visiblyseen, the correct degree of etching is difficult toascertain with the eye. When in doubt as to howlong to etch, the best method is to etch indegrees. Etch until a frosty appearance isobserved, replicate this result, etch heavier andreplicate again. The metallographer may wantto do this several times, particularly if he or shehas no previous experience. After each etchinginterval the area must be flushed with water oralcohol and dried before replicating.

As the etching step is the most difficultprocedure, the replication technique is theeasiest. A small piece of cellulose acetate, 3 to 5mils thick and approximately three times thesize of the area to be replicated, is held bytweezers and softened on one side only. Applyacetone with an eyedropper, allow to set forapproximately 30 seconds, then shake toremove excess acetone. The softened side isthen pressed firmly against the area with theeraser end of a pencil or the tip of the fingersand held several seconds. Care must be takenthat the acetate is not allowed to slip or asmudged replica will be obtained. Let thereplica remain in this position until it hardens,usually 10 to 15 minutes, depending upon thesize of the replica. Once it has hardened it canbe stripped with tweezers. At this point, thestripped replica should be put into a separatecontainer (usually a small plastic box), or tapedto a glass slide with double-sided tape. Several

replicas should be made of each area for abetter chance of obtaining a good replica. Whileone replica is hardening, other replicas can bemade at different locations. Needless to say, allreplicas should be labeled appropriately,particularly if several replicas are taken withdifferent degrees of etching.

Examination of a replica is carried out withnormal microscopic procedures. However, thereare several things that can be done to enhancethe viewing of the replica. If possible, the replicashould be shadowed at 45 to 60 degrees withcarbon in an evaporator. While this is the idealsituation, some laboratories do not have anevaporator and must view the replicas as theyare. Placing the replica (using double-sidedtape) on a glass slide that has been spraypainted black will greatly enhance the contrast.Insuring that all points of the replicated surfaceare flat will increase the viewing field. Excessacetate can be trimmed off before shadowingwith carbon or affixing to a glass slide. Anotherway to increase contrast is to very lightly spraythe replica with flat black paint, however,extreme care must be exercised so there will beno build-up of paint that will obliterate features.

The replica is examined in the same manner asa specimen prepared in the laboratory.Photographs can be taken and semi-quantitative evaluations can be made. Onething to remember is that the replica is just that,and features that are up or down on the surfacewill be reversed on the replica. For example,porosity will show up as elevations on thereplica and hard intermetallics that have atendency to polish in relief will be recessed onthe replica. Also, scratches will show up slightlyraised. Keeping these things in mind, one willhave no trouble in interpreting the micro-structure.

The following photomicrographs have beentaken (1) on laboratory prepared samples and(2) by the in-situ technique. Can you tell thedifference between the two?

Replication

Examination of Replica

(a) 400X

(c) 400X

(b) 400X

(d) 400X

As-Cast Brazing Alloy

Quenched and Tempered SAE 8620 Steel

Photomicrographs (a) and (d) are from replicated surfaces.

In-situ metallography is ideal for round material where the curved surface can be replicatedand the replica placed flat onto a glass slide to give a large, flat surface to be examined.

As-Received 50X

As-Received 50X

Machining Grooves on 0.25 inch rod

Note limited viewing area

Replica of Machining Grooves

Note large viewing area

Form No. 200-976-003 1/08-REV4 © 2008 LECO Corporation

LECO Corporation would like to thankDr. Lee Dillinger for his contributions to this project.

LECO Corporation3000 Lakeview Avenue • St. Joseph, MI 49085 • Phone: 800-292-6141 • Fax: 269-982-8977

info@leco.com • www.leco.com • ISO-9001:2000 • No. FM 24045 • LECO is a registered trademark of LECO Corporation.

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