Here is a guideline for etching low-alloy steel
GUIDELINE FOR ETCHING LOW-ALLOY STEEL
1. Scope
This document is an in-house guideline for both MICRO and MACRO Etching steel samples.
2. 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."
3. Procedure
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.
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.
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.
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.
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.
Table 1
Carbon and Low-Alloy Steel Etchants
ETCHANT COMPOSITION TIME CHARACTERISTICS REVEALED
Nital 1-5% Nitric Acid95%-99% Methanol 5-30 minutes 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 chromium-bearing low-alloy steels resistant to action of picral. Darkens pearlite. Polished surface.
Picral 4 gram Picric Acid100 ml Methanol 3 - 5 hours Carbon and Low-Alloy Steels. Macroscopic examination - Carbon SegregationMicroscopic 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.
Iodine Tincture, USP 15 to 60 minutes Macroscopic Examination - works better with steel is 180 to 200 °F. Slow but available from Pharmacy
1 part HCL Hydrochorlic Acid (Muriatic Acid) - 1 part Distilled Water 15 to 60 minutes Macroscopic Examination - Segregation, porosity, hardness penetration, cracks, inclusions, dendrites, flow lines, soft spots, structures, weld examination. - Sa-cut, machined or average ground surface.
4. 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.
5. Principal MacroEtch Observations to be Recorded
Identification to use on Photograph Description Location
A Cracks Surface or Subsurface
B Seams or Laps Surface or Subsurface
C Decarburization Surface or Subsurface
D Pinhole Surface or Subsurface
E Segregations Surface or Subsurface
a Pipe Center or Central Area
b Porosity Center or Central Area
c Bursts Center or Central Area
d Segregations Center or Central Area
" Flakes or Cooling Cracks General
$ Dendritic Pattern General
( Ingot Pattern General
* Grain Boundaries General
6. 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.
Take a look at the post under the title of "Etching", posted by "Thirdeye" on November 20, 2004 on the Technical section of this forum. There are a lot of opinions there. May be one of them might be useful for you.
Giovanni S. Crisi
Sao Paulo - Brazil
Very good procdure guidlines. I use the nital, only with distilled water (less flamible, I do most of my macro-ecth in a welder qulification lab. too close to sparks for me to be comfortable with a methanol base). As for the rough sanding (that all thats required for macro-etch) I use a ruberbacking disc and a 4-1/2 angle grinder with 100 grit sanding disc for the quick work, Just watch the pressure and time under the disc and keep a eye open for temper colors(usually if the spcimen starts to turn straw immidiately quench prior to further sanding). For the nital solution I use is 95ml distilled water(H2O) with 5ml nitric acid (HNO3) And the resolution time usually between 2 to 10 seconds. this is very simple, may it will help.
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