Etching of Stainless Steels and Special Alloys for Optical Microscopic Study

A colleague recently asked what etching agent or agents should be used in order to identify which phases are present in an unspecified commercial grade stainless steel (SS) and presumably study theses phases?

Two main types come to mind:
1 Austenitic typically 18Cr/10Ni room temperature phase austenite a face centered cubic crystal structure. Austenitic SS are non-magnetic.
2 Ferritic typically Fe-12%Cr little or no Ni, room temperature phase is ferrite, a body centered cubic structure. Ferritic SS is magnetic.

1. So to tell which is which, use a magnet.
(NB slight magnetism may occur in austenitic SS)
2. Use etch agents (Table 1 &2) which are not suitable for both categories cf. below for Tables, references and full explanation.

Etched Microstructure of austenitic stainless steel
By Katharine B. Small, David A. Englehart and Todd A. Christman*Carpenter Technology Corp., Wyomissing, PA, USA
*Member of ASM International, Microstructure of a stainless Type 330 sample in annealed condition at 100x, using a tint etch consisting of a solution of 40 ml hydrochloric acid (MCL) + distilled water (H2O) + one gram potassium meta bisulfite (KS2O5) + 4gms ammonium biflouride (NH4F–HF) at room temperature.

LIGHT OPTICAL MICROSCOPIC METHODS

Carpenter Tech gives a useful list of light optical microscopic methods in  Fig 1: Light optical microscopy methods of illumination used in microstructural examination.
Light Optical Microscopy Methods

Bright-Field Illumination

The most commonly encountered method of illumination in which the light reflection is perpendicular to the specimen being viewed. Generally, microstructural features such as grain boundaries are dark and matrix regions are bright.

Dark-Field Illumination

The light is obliquely reflected back through the objective so that what appears bright and dark in bright-field illumination is reversed in dark-field illumination.

Oblique Illumination

The illustration source is decentered at an oblique angle producing shadows on microstructural features. This method is extremely helpful if the operator knows the illumination direction; thereby, knowing which features are raised and which are recessed by the shadow orientation.

Differential Interference-Contrast (DIC)

A beam-splitting prism, polarizer and analyzer are inserted into the light path producing shadowing variations that reveal height differences in the microstructural features.

Polarized Light

The light is passed through a polarizing filter and can be adjusted to enhance the color contrast obtained with stain etchants.

ETCHING

A comprehensive list of etching agents (Table 1) together with those recommended for different grades of stainless steels and special alloys. (Table 2) A careful examination of these data shows that some etching agents are common to both austenitic (fcc) and ferritic steels (bcc). Let me suggest that by using agents applicable to one type but not the other will distinguish one type from the other.

cf. Tables 1 and 2. _Carpenter Tech Corp
As we have now see (above) the notion of commercial grade is rather vague when used for a large markets such as stainless steels (and special corrosion resistant alloys

A full list of stainless steel families based on their phase-crystalline structure are given below together with references for further reading and information.


MORE_ALL STAINLESS STEEL TYPES:

Austenitic stainless steels :
have an austenitic, face centered cubic (fcc) crystal structure. Austenite is formed through the generous use of austenitizing elements such as nickel, manganese, and nitrogen. Austenitic stainless steels are effectively nonmagnetic in the annealed condition and can be hardened only by cold working. Some ferromagnetism may be noticed due to cold working or welding. They typically have reasonable cryogenic and high temperature strength properties. Chromium content typically is in the range of 16 to 26%; nickel content is commonly less than 35%.

Ferritic stainless steels:
Ferritic stainless steels are chromium containing alloys with Ferritic, body centered cubic (bcc) crystal structures. Chromium content is typically less than 30%. The ferritic stainless steels are ferromagnetic. They may have good ductility and formability, but high-temperature mechanical properties are relatively inferior to the austenitic stainless steels. Toughness is limited at low temperatures and in heavy sections.

But also
Martensitic stainless steels
Duplex (ferritic-austenitic) stainless steels
Precipitation-hardening stainless steels.

Refs:
1. Main reference due to Carpenter Technology Corp

2. Microstructures of Stainless steels
Umist.ac.uk Internet Microscope_micrographs-microstructures_micrographs_stainless-steel

3. Stainless-Steel Main Types from Materials Engineer

Other references on etching :

4. Technical Information_Etching from Buehler Book [pdf]

5. Struers_Application Notes Stainless Steel English [pdf]