Guideline Macro Micro Etching

GUIDELINE FOR MACRO and MICRO ETCHING METALLIC MATERIALS - PROCESSES AND REAGENTS

1. Scope This document is an in-house guideline for both MICRO and MACRO Etching metallic material samples.

2. General The surfaces to be etched should be smoothed by filing, machining, or grinding on metallographic papers. With different alloys and tempers, the etching period will vary from a few seconds to several minutes, and should be continued until the desired contrast is obtained. As a protection from the fumes liberated during the etching process, this work should be done under a hood. After etching, the specimens should be thoroughly rinsed and then dried with a blast of warm air. Coating the surface with a thin clear lacquer will preserve the appearance.

3. Purpose The AWS Structural Steel and Bridge Welding Codes, AWS D1.1 and D1.5, respectfully, specifies that MacroEtching of weld samples is required for procedure and welder qualifications, yet does not provide details on how to complete these tests. This guideline fills the void that exists between the codes and practical applications of them with respect to "MacroEtching."

4. Procedure 4.1 Specimen Surfaces preparation

Care must be taken to prevent overheating the specimen during polishing. Excessive pressure during polishing will form deep scratches and will increase the depth of disturbed metal on the surface of the specimen.

4.1.1 Rough polishing This operation can be performed by means of different type sand paper. For rough polishing, a adhesive sand paper should be used as a covering for the rotating polishing wheels. During rough polishing the specimen is moved in a clockwise direction around the polishing wheel to

insure equal metal removal from the entire surface by not allowing prolonged polishing in any one direction. 4.1.2 Fine Polishing Fine polishing is often done using a water coolant and silicon carbide abrasive paper mounted on a rotating disc. The two-wheel unit in the laboratory can be equipped with rough polishing wheel and a final polishing wheel. Wheel speed for final polishing has to be set at a second position (about 1150 rpm). A small stream of water is directed at the center of the rotating wheel. 4.1.3 Final Polishing Final polishing is very similar to that used for rough polishing. Aluminum oxide is the most popular final polishing abrasive for ferrous and non-ferrous materials. It is also possible to use chromium oxides for soft materials. The abrasive particles used in final polishing are generally carried on a napped or short pile cloth such as billiard cloth or "microcloth." Most polishing cloths can be obtained cut to size and coated with an adhesive backing. The adhesive back eliminates the need for mechanical clamping. CAUTION! Be sure that you supply the rotating wheel with continuous flowing water. This step is taken to insure cool surface during polishing. 4.2 Etching of specimens Temperature - 160 to 180 °F. Polished metal specimens usually show no structural characteristics. Etching of the metal surface is done to make visible the crystalline structure of the metal and to produce optical contrast between the various constituents. Etching is done by exposing the cleaned and polished specimen surface to suitable etching solution such as those described in Table 1. These etching reagents are powerful, hazard and must be handled with care. CAUTION! Do not forget: etchants are composed of organic and inorganic acids, alkalis or other complex substances.

If the specimen is not sufficiently etched after the first application the etching process may be repeated. If the specimen is overetched it must be repolished prior to re-etching. Immediately following the etching the specimen should be washed in warm water to stop the etching, then immersed in alcohol and finally dried in a blast of warm air. Rapid drying is important to prevent water spots.

Etching reveals structural characteristics by preferential etching. That means that some areas such as grain boundaries are more highly stressed than other areas and are more subject to chemical etching.

4.3 For Ferrous Metals Etching solutions suitable for carbon and low alloy steels, together with directions for their use, are suggested as follows.

CAUTION: Always pour the acid into the water. Nitric acid causes bad stains and severe burns. The solution may be used at room temperature and applied to the surface to be etched with a glass stirring rod. The specimens may also be placed in a boiling solution of the acid, but the work should be done in a well-ventilated room. The etching process should be continued for a sufficient period of time to reveal all lack of soundness that might exist at the cross-sectional surfaces of the weld.

Table 1

Carbon and Low-Alloy Steel Etchants ETCHANT Nital

Picral

COMPOSITION § 1-5% Nitric Acid § 95%-99% Methanol

§ §

4 gram Picric Acid 100 ml Methanol

TIME 5-30 minutes

3 - 5 hours

CHARACTERISTICS REVEALED Macroscopic examination - Carburization and decarburization, hardness penetration, cracks, segregation, weldd examination.- Average ground or polished surface. Microscopic Examination - Develops ferrite grain boundaries in low-carbon steel; produces maximum contrast between pearlite and a cementite or ferrite network; develop grain boundaries in 4% silicon steel; develops ferrite boundaries in structures consisting of martensite and ferrite; etches chromiumbearing low-alloy steels resistant to action of picral. Darkens pearlite. Polished surface. Carbon and Low-Alloy Steels. Macroscopic examination - Carbon Segregation

Iodine Tincture, USP Iodine and Potassium Iodide

15 to 60 minutes §

§

§

Hydrochloric Acid

§

§

Ammonium Persulfate

§ §

1 part of powdered iodine (solid form), 2 parts of powdered potassium iodide, 10 parts of water, all by weight. 1 part HCL Hydrochlor ic Acid (Muriatic Acid) 1 part Distilled Water 1 part of ammonium persulfate 9 parts of water, by weight

30 to 60 minutes

15 to 60 minutes

Microscopic Examination - Reveal s maximum detail in pearlite, untempered and tempered martensite, and bainite; reveals undissolved carbide particles in martensite; differentiates ferrite, martensite, and massive carbide by coloration; differentiates bainite and fine pearlite; reveals carbide particles in grain boundaries of low-carbon steel and wrought iron - Polished surface. Macroscopic Examination - works better with steel is 180 to 200 °F. Slow but available from Pharmacy The solution should be used at room temperature, and brushed on the surface to be etched until there is a clear definition or outline of the weld.

Macroscopic Examination - Segregation, porosity, hardness penetration, cracks, inclusions, dendrites, flow lines, soft spots, structures, weld examination. - Sacut, machined or average ground surface. Use at or near Boiling Point of Acid Solution. Applied by vigorously rubbing the surface to be etched with a piece of cotton saturated with the solution. The etching process should be continued until there is a clear definition of the structure in the weld.

5. Acceptance For acceptable qualification, the test specimen, when inspected visually, shall conform to the following requirements: Fillet Welds. Fillet welds shall have the following:

1. No cracks 2. Thorough fusion between adjacent layers of weld metals and between weld metal and base metal 3. Weld profiles conforming to intended detail, but with none of the variations prohibited in AWS D1.5 3.6 4. No undercut exceeding 1 mm. 5. Fusion to the root of the joint but not necessarily beyond. 6. Leg sizes equal to or greater than the specified leg size.

6. Principal Etch Observations to be Recorded Identification to use on Photograph A B C D E a b c d    

Description

Location

Cracks Seams or Laps Decarburization Pinhole Segregations Pipe Porosity Bursts Segregations Flakes or Cooling Cracks Dendritic Pattern Ingot Pattern Grain Boundaries

Surface or Subsurface Surface or Subsurface Surface or Subsurface Surface or Subsurface Surface or Subsurface Center or Central Area Center or Central Area Center or Central Area Center or Central Area General General General General

7. Reporting Requirements A digital photograph shall be taken of the macro/micro etched specimen for inclusion in any report or PQR. Indications on the photograph shall be marked as listed above.