Carpenter specialty alloys

Carpenter specialty alloys

Material Name: Carpenter specialty alloys
Recipe No.: 12062
Primary Chemical Element in Material: Fe, Ni
Sample Type: Bulk
Uses: General microstructure
Etchant Name: No data
Type (Macro/Micro): Micro
Etching Method: Chemical, electrolytic
Etchant (Electrolyte) Composition: See the tables 1 and 2.
Procedure (Condition): See the tables 1 and 2.
Note: Light microscopy to evaluate the microstructure of metals is employed extensively by quality control and failure analysis laboratories. Selection of the proper etchant depends largely upon alloy composition, heat treatment and processing. The etchants used in metallographic examination are solutions of acid and chemicals and are applied selectively to attack a highly polished surface, thus permitting microstructural examination.
There are three basic methods of etching alloy samples – immersion, swabbing and electrolytic. In the first method, the sample is immersed in the etching solution until the desired structure is developed. Samples may be immersed in stain etchants to highlight specific microstructural features.
In the second, the sample is swabbed with cotton that has been immersed in the etchant. In electrolytic etching, a D.C. source or a rectifier serves as the power supply, and the specimen is the anode in the electrolytic cell. Power requirements can be adjusted as needed, depending on sample size, anode-to-cathode spacing, electrolyte, etc.
The following rules should be observed to obtain a true and representative microstructure:
1. If a mounted specimen is used, an adherent mount is very important. Separation between the specimen and the mounting compound can result in “bleeding” of the residual etchant or water, and subsequent staining.
2. A good metallographic polish is a must. The sample must be free of scratches, disturbed metal and any kind of embedded contaminants.
3. The specimen must be thoroughly cleaned before etching. Oil or any polishing compounds must be removed.
4. After etching, the sample should be rinsed in HOT water, followed by an alcohol rinse and dried under HOT air. (Note: an alcohol rinse can dull/wash out a stain etch.) Samples with cracks must be thoroughly dried to prevent bleeding.
5. If additional etching time is required, the specimen should be re-rubbed a few seconds.
on a final polishing wheel. If this is not done, “flashing” can occur in certain alloys, necessitating a complete re-polish. Flashing will not occur and re-rubbing is not required if the alloy being etched forms no passive oxide layer, or if etching is performed electrolytically (rerubbing is usually mandatory only when using Ralph’s, Glyceregia, Acetic Glyceregia and HCL + H2O2 etchants). More complete specimen preparation instructions are readily available in literature.
Reference: Website Carpenter Technology, https://www.carpentertechnology.com/en/alloy-techzone/technical-information/manufacturing-guides/a-guide-to-etching-specialty-alloys-for-microstructural-evaluation, 2019.

Table 1: List of etchants.

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Table 2: List of alloys.

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