By
Katharine B. Small, David A. Englehart and Todd A. Christman*
Carpenter Technology Corp., Wyomissing, PA, USA
*Member
of ASM International
Growing use
of specialty alloys with high resistance to corrosion has challenged many
laboratories to refine the way they etch alloy samples for microstructural
evaluation. The big problem – the alloys resist the etchants just as they do
the corrosive conditions encountered in service.
|
| Microstructure of a stainless Type
330 sample in annealed condition @ 100x, using a tint etch consisting of a
solution of 40 ml hydrochloric acid (MCL) + distilled water (H2O) + one
gram potassium meta bisulfite (K2S2O5) + 4
grams ammonium biflouride (NH4F–HF) at room temperature. |
Such high-performance
alloys have been used increasingly for critical applications in bio-medical,
aerospace and defense industries, among others. Due to the usually demanding
material requirements, laboratory evaluation of the finished microstructure is
often desired, if not required.
The
difficulty in etching these materials has focused attention also on employing
the most effective procedures for etching a wide variety of iron-, nickel- and
cobalt-based alloy systems. Ensuing discussion, therefore, will offer guidelines
for processing stainless steels, high temperature alloys, tool and alloy
steels, and magnetic and expansion alloys.
Background
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.
Etchants
& Alloys
Table 1
lists 28 etchants commonly used in the laboratory, along with their
compositions and guidelines for use. Table 2 lists various specialty alloys,
along with the suggested etchants for each and application notes. These tables
can be used to quickly select the appropriate etching procedure for the alloys
most often used today. Additional etchants are reported in ASTM specification E-407-99
“Standard Practice for Microetching Metals and Alloys” and ASM Metals Handbook
(Volume 9, Metallography and Microstructures).
The
etchants for each alloy are presented in order of preference and successful
experience. In general, the most benign etchant is shown first, followed by
those that grow progressively stronger. If a weaker etchant is tried first and
it does not yield satisfactory results, the investigator only has to buff the
surface slightly to obtain a good polish for examination with a stronger
etchant.
If the
first etchant attempted is too strong, far more preparation is required to
restore the surface to a workable condition. If one is not familiar with the
etchants recommended, it is usually a good idea to start with the weakest
solution.
Before
etching, the examiner should first consider what s/he is looking for. If the
intent is to evaluate non-metallic inclusions, sulfide distribution or
morphology, for example, the sample is best examined in the as-polished
condition since etching can remove various inclusions and attack structures
such as ferrite stringers in an austenitic matrix.
On the
other hand, if the search is designed to examine grain size, precipitation or
cold- work deformation, the sample must be etched.
The
condition of an alloy and its heat treatment play an important part in the
selection and application of etchants. After confirming the alloy grade and
analysis, consider whether it has been annealed, aged, cold worked, tempered
and/or is in the as-hardened condition.
Highly Corrosion
Resistant Alloys
Special
techniques are required to effectively prepare highly corrosion resistant
alloys for microstructural examination. Prominent in this group are alloys such
as BioDur® CCM Plus® Alloy, MP35N Alloy* (UNS
R30035) and Custom
Age 625 Plus® Alloy (UNS N07716).
*MP35N is
a registered trademark of SPS Technologies Inc.
These nickel-base
and cobalt-base alloys, with their superior corrosion resistance, are etched
using Waterless Kalling’s, Glyceregia, Acetic Glyceregia and Ralph’s. If the
etchants typically used for highly resistant stainless and high temperature
alloys do not work, HCL + H2O2 can be used. These alloys
usually should be etched slowly.
All require
a fresh polish to avoid “flashing,” which would require a complete re-polish. Best
results are obtained by lightly re-rubbing one or two samples at a time on the
final polishing wheel (normally a 0.05 micron alumina slurry).
The
challenge with these alloys is to offset their natural tendency to self-passivate
rapidly in the presence of oxygen. To effectively process these grades, etching
must be done immediately after final polishing. Rinsing, drying and etching
should be performed without delay.
When
etching these highly resistant alloys, the etchant should be prepared in
advance of the polishing process. This advance planning will minimize the
length of time between polishing and etching, thus permitting a more effective
microstructural evaluation. Samples should be polished and etched individually
rather than processed in batches.
Microscopy
Methods
Five light
microscopy methods of illumination can be used in microstructural examination: bright-field
illumination, dark-field illumination, oblique illumination, differential
interference-contrast (DIC) and polarized light (Fig. 1). The well-equipped
laboratory should have all five because their capabilities are complementary.
Their role in microstructural evaluation cannot be overestimated.
Visual
results for the same optical field at 100x magnification are shown via
different lighting techniques.
|
|
|
| (a) Stain
etchant used to produce color |
(b) Bright
field – typical way of examining microstructure, shows grain structure and
“twins,” but some areas not clearly defined |
|
|
| ((c) Dark
field – shows negative effect with grain boundaries more distinct, but some
other areas not totally defined |
(d) Three-dimensional
effect makes grain boundaries clearer, with more sharply defined grains and
“twins.” |
Samples
(b), (c) and (d) were etched using Waterless Kalling’s Reagent.
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 de-centered 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.
|
DIC, using
bright-field microscopes that change the way light is deflected, gives a
three-dimensional image. It is the ultimate in light optical microscopy,
showing grain boundaries very clearly in highly corrosion resistant alloys. It
is the best performing means of showing relief in structure, and is capable of
identifying the effects of cold work, mechanical damage, etc. Compared with the
other methods, DIC provides increased image contrast.
Dark-field,
oblique and differential interference-contrast illumination methods can aid in
delineating grain boundaries and other microstructural features that are only
weakly visible in bright field illumination. A polarizing filter can be used in
conjunction with a stain etch to enhance color contrast.
In
dark-field technology, the grain seen is dark and the grain boundaries, light.
With the bright-field microscope, the grain is light and the boundaries, dark.
Oblique illumination brings light in on an angle, producing shadows on
microstructural features.
Stainless
Steels
When
etching austenitic stainless steels, DIC illumination will better show the
grain structure and help find cold-work deformation, when such exists.
Waterless Kalling’s reagent can be used to show the general structure of many
austenitic stainless alloys. Other agents such as Glyceregia or Acetic
Glyceregia may be required to retain ferrite, carbide precipitation.
If the only
interest is grain structure in an austenitic stainless, start etching with a
more aggressive etchant. However, start etching with Glyceregia for a shorter
time if interested in features other than grain boundaries, such as carbides,
ferrite stringers, second phases and duplex structure.
Ralph’s
reagent normally provides a good etch for general structures in ferritic
stainless steels. Waterless Kalling’s and Glyceregia also can yield good
results. Even after a good polish, scratches may still be visible. They may or
may not be a problem.
Ralph’s is
usually best for the precipitation hardenable stainless steels; however,
Vilella’s reagent will work fine if a light etch is preferred. Etching time
will vary because the alloy in the aged condition will react quicker to the
etchant. The higher the aging temperature, the quicker the response. An alloy
aged at 1100°F (590°C) will etch darker and quicker than one aged at 900°F (480°C).
The
annealed structure in PH stainless steels requires either an aggressive etchant
for a short time or a less aggressive etchant for a longer time. Here, Ralph’s
reagent could be used for a few seconds, or Vilella’s, which is less
aggressive, for a longer time.
Vilella’s
reagent is preferred for martensitic stainless steels. Etching time and
response will vary depending on whether the alloy has been annealed, hardened
or tempered. Annealed samples usually require the longest etching time because
everything is in solution, with not much to be seen. It is best to stop etching
while the specimen is still on the light side. Etching has gone too long when
the sample starts going black. A little experience will help the examiner to
stop etching at the right time. Hardened and tempered samples usually require
less etching time than annealed stock.
High
Temperature Alloys
Etching
procedures for high temperature alloys can vary greatly depending on condition of
the material and what evaluation is required. High temperature alloys, which
exhibit different aging conditions with a range of aging responses, phases,
precipitates, etc., are typically more difficult to etch than austenitic
stainless steels.
If
uncertain about which etchant to use, start with Glyceregia and increase in
severity until the desired result is obtained. This procedure requires only the
final polishing step between etchants, instead of a complete re-polish.
Glyceregia is mixed at time of use, and becomes more aggressive with the
passage of time.
Waterless
Kalling’s, another choice, works well with alloys such as Waspaloy (UNS
N07001), Pyromet®
Alloy 718 (UNS N07718) and
Pyromet Alloy A-286 (UNS K66286). It can be stored, and is thus more convenient to
use.
Stain
etchants and electrolytic etchants can be used to show specific aspects of
structure in high temperature alloys. Stain etchants, used with immersion
techniques, have been effective in highlighting structural features such as
second phases and colors.
If the
etchants typically used for high temperature alloys do not work, HCL + H2O2
can be used just as they can for stainless steels. Precipitation can be
evaluated using bright field microscopy, but DIC is better for showing grain
structure.
Other
Alloy Families
Nital (2 to
5 percent) is useful for showing carbide structure in tool and alloy steels,
but re-etching the treated sample afterward with Vilella’s darkens the matrix
and provides a clearer view of the microstructure. Many times, samples can be
etched using Nital, examined and then etched with Vilella’s without
re-polishing. Alloy condition will determine etching time.
Most
magnetic and expansion alloys can be treated as suggested for one of the
stainless steel families, or the tool and alloy steels. When examining the
Ni-Fe alloys, for example, use the procedures for austenitic stainless steels.
Many of the alloys used for their D.C. magnetic properties (such as stainless Type
430F) can be etched using procedures for ferritic stainless steels. Others,
such as Fe and Si Core Irons, can be etched like low alloy steels using Nital.
Valuable
Tips
Some
etchants, like Waterless Kalling’s, can be made in bulk and stored for use as
needed. This is convenient for the examiner. Other reagents, like Glyceregia
and Acetic Glyceregia, must be prepared each time they are required. Both Glyceregia
solutions undergo a continuous chemical reaction as they age, becoming
aggressive in less than one hour. Special care must be taken then because the
aggressive solution can affect etching time and procedure. It is a good idea,
therefore, for the examiner to use the solution as soon as possible after
mixing it.
As shown in
Tables 1 and 2, many different etchants can be used successfully for
specific alloys and special circumstances when examining basic microstructure.
The choice of which reagent to use, however, does not have to be difficult.
Table 1
List of Etchants
|
Etchant No.
|
Etchant Name
|
Composition
|
Remarks
|
ASTM E 407 Designation
|
|
1
|
Nital (2%)
|
2cc HNO3 + 98cc Ethyl alcohol
|
Immersion
|
74 Nital
|
|
3
|
Picral (5%)
|
5gr Picric acid + 100cc Ethyl
alcohol
|
Immersion
|
76 Picral
|
|
4
|
Oxalic acid
|
10gr oxalic acid + 10cc H2O
|
Electrolytic at 200/400 Ma.
|
|
|
6
|
Nital (5%)
|
5cc HNO3 + 95cc Ethyl alcohol
|
Immersion - Do Not Store
|
74 Nital
|
|
7
|
HCl in alcohol
|
15cc HCl + 100cc Ethyl alcohol
|
Immersion
|
|
|
8
|
Ferric Chloride
|
5g Ferric Chloride + 50cc HCl
+100cc H2O
|
Use Fresh Swab - Use Under Hood -
Do Not Store
|
|
|
9
|
Marble's Reagent
|
4g CuSO4 + 20cc HCl + 20cc H2O
|
Immersion or Swab
|
25 Marble's
|
|
10
|
Viella's
|
5cc HCl + 2gr Picric acid + 100cc
Ethyl alcohol
|
Immersion or Swab
|
80 Vilella's (ASTM contains 1gr
Picric)
|
|
11
|
Aqua Regia in alcohol
|
100cc HCl + 3cc HNO3 + 100cc Methyl
alcohol
|
Immersion
|
12 Aqua Regia
|
|
12
|
Chromic acid
|
10gr CRO3 + H2O
|
Electrolytic at 200/400 Ma.
|
|
|
13
|
2% H2SO4
|
2cc H2SO4 + 98cc H2O
|
Use Electrolytic - Under Hood -
200/400 Ma.
|
|
|
15
|
G
|
12cc H3PO4 + 41cc HNO3 + 47cc H2SO4
|
Use Electrolytic - Under Hood -
200/400 Ma.
|
|
|
18
|
Acetic Glyceregia (Mixed Acids)
|
15cc HCl + 10cc HNO3 + 10cc Acetic
Acid + 2/3 Drops Glycerine
|
Use Fresh - Under Hood - Swab - Do
Not Store
|
|
|
19
|
Waterless Kalling's
|
5gr CuCl2 + 100cc HCl + 100cc Ethyl
alcohol
|
Immersion or Swab
|
95 Kalling's 2
|
|
22
|
HF + HNO3
|
1 to 3cc HF + 2 to 6cc HNO3 + 100cc
H20
|
Swab - Handle with care - HF cause
serious burns - Use in plastic container HF attacks glass
|
|
|
23
|
HNO3 + H2O
|
75cc HNO3 + 25cc H20
|
Use Under Hood - Electrolytic 5 to
7 amps
|
|
|
26
|
Glyceregia
|
15cc HCl +10cc Glycerol + 5cc HNO3
|
Use Fresh - Under Hood - Swab - Do
Not Store
|
87 Glyceregia
|
|
28
|
Ralph's
|
100cc H2O + 200cc methyl alcohol +
100cc HCl + 2gr CuCl2 + 7gr FeCl2 + 5cc HNO3
|
Swab
|
|
|
29
|
Special #4
|
10% Sodium meta-Bisulfate in
distilled water
|
Immersion
|
|
|
30
|
Special #5
|
20ml HCl + 4ml H2O2 (3%)
|
Swab
|
|
Note:
Please see ASTM for proper handling of all chemicals.
Table 2
List of Alloys
|
Type
|
Etchant
|
Applications
|
Special Notes
|
|
403, 405, 410, 420, TrimRite®, Trinamet®
stainless
|
10, 28, 19
|
General Structure - grain size,
carbides, martensite and ferrite
|
|
|
440A, 440B, 440C
|
10, 26
|
General structure - grain
boundaries, carbides and martensite
|
|
|
416
|
10, 19
|
General Structure - grain size,
carbides, martensite and ferrite
|
|
|
28
|
Excellent sulfide retention
|
|
|
304, 304L, 309, 310, 316, 347
|
10, 26, 4, 12, 28
|
General Structure - grain
boundaries, grain size, carbide precipitation
|
|
|
20Cb-3® stainless
|
10, 26, 4
|
General Structure - grain
boundaries, grain size, carbide precipitation
|
|
|
303
|
28
|
General Structure - grain
boundaries, grain size. Excellent sulfide retention.
|
|
|
26, 10
|
Similar to #11 but sulfides
attacked
|
|
|
430F, 430FR
|
10, 19, 18
|
General Structure
|
|
|
430
|
6, 26, 10
|
General Structure - grain
boundaries and grain size
|
|
|
Chrome Core® 18-FM, Chrome Core 12-FM, Chrome Core 13-FM, Chrome Core 13-XP
|
19, 26
|
General Structure
|
|
|
Custom 450®, Custom 455®, Custom 465®, Custom 475®, Custom 630, 15Cr-5Ni, Carpenter 13-8
|
28
|
General Structure - martensite and
austenite
|
|
|
21Cr-6Ni-9Mn, 22Cr-13Ni-5Mn
|
28, 19, 26
|
General Structure - grain
boundaries, grain size, carbide precipitation
|
|
|
Pyromet® A-286, Pyromet V-57
|
19, 26, 9
|
General Structure, including size
and grain boundaries
|
|
|
13
|
Gamma prime precipitates - banding
and depletion
|
|
|
NCF 3015(1), Nickel 200/201, Thermo-Span, Pyromet 720, Carpenter
Alloy 925
|
19, 26
|
General structure including grain
size and grain boundary precipitate.
|
|
|
Pyromet 706, 718, and 901
|
19. 26
|
General structure including grain
size and grain boundary precipitate.
|
|
|
13
|
Gamma prime precipitates - banding
and depletion
|
|
|
Pyromet 860
|
19, 26, 15
|
General Structure, including size
and grain boundaries
|
|
|
13
|
Gamma prime precipitates - banding
and depletion
|
|
|
Waspaloy
|
19, 26, 15
|
General Structure, including size
and grain boundaries
|
Generally 19 is more effective for
aged material and 15 for solution treated material for general structure and
grain size.
|
|
13
|
Gamma prime precipitates - banding
and depletion
|
|
|
Pyromet® 680
|
19, 26, 18
|
General Structure
|
Use 18 in the annealed condition
|
|
Pyrowear® 675
|
10
|
General Structure
|
|
|
Ti Base Alloys
|
22, 23
|
General Structure
|
|
|
MP35N(2), Alloy 2 (AMS 5842)(3), MP35N LTi
|
30
|
General Structure
|
|
|
BioDur Carpenter CCM®, BioDur CCM Plus® alloy
|
30
|
General Structure
|
Fresh sample needed. Should
prepare one sample at a time. Use of Differential Interference Contrast (DIC)
would help.
|
|
Pyromet 625, Custom Age 625 PLUS®
|
26, 18, 19
|
General Structure
|
Fresh sample needed. Should
prepare one sample at a time. Use of Differential Interference Contrast (DIC)
would help.
|
|
BioDur® 108
|
26
|
General Structure
|
|
|
Nickel Copper 400
|
28
|
General Structure
|
|
|
Nimark® 200/250/300, Carpenter Ferrium S53(4)
|
28
|
General Structure
|
|
|
AerMet® 310/340
|
19, 29
|
General Structure
|
|
|
AerMet 100
|
19, 29
|
General Structure
|
|
|
18Ni - 200 Maraging Steel, 18Ni -
250 Maraging Steel, 18Ni - 300 Maraging Steel
|
12
|
Prior austenitic grain boundaries
|
|
|
Cr-Fe, Glass Sealing 18, Glass
Sealing 27, 430F, 446
|
11, 26, 28, 7, 8
|
General Structure, grain size
|
|
|
Kovar® alloy
|
28, 26, 19
|
General Structure
|
|
|
Core Irons
|
6, 11, 7
|
General Structure
|
|
|
Fe and Si Core Irons
|
11, 7
|
General Structure
|
|
|
Ni-Cr-Fe - 22-3, 45-5, 42-6
|
28, 26, 19
|
General Structure
|
|
|
Co-Fe Hy-Sat Alloy 27,
Co-Fe-V Remendur, Hiperco® 50
|
26, 28, 19
|
General Structure
|
