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Metallographic Services
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Metallography is the study of the structure of
metals and alloys using various techniques, especially optical and
electron microscopy.(1) Metallography is used to reveal the
microstructure of metals, which is affected by composition, processing
conditions including heat treatment and cold working, and post-processing
variables such as machining, loading and welding. Service environment
also affects microstructure and problems such as corrosion, stress
corrosion cracking, carburization, sulfidation and a host of other effects
can be determined from a material’s microstructure. Metallography is a
key element in many failure investigations and often provides the critical
evidence needed to make a definitive determination of cause of failure. |
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Microstructure of a weld used in duplex stainless steel, 2205, 250X
original magnification. A color mixture of austenite, ferrite and sigma
phases. |
Microstructure of base metal of the same duplex stainless steel, 2205,
250X original magnification. Austenite phases are present as white
island in blue sea of ferrite. |
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Metallurgical mounts are generally made of a cross section through a
material. The area in question is selected, Figure 1. The piece is cut
in cross section and mounted in a suitable medium. The mount is then
polished to a mirror finish, Figure 2. The sample can then be viewed via
optical or electron microscopy. Often it is necessary to etch the sample
in order to reveal its microstructure, Figure 3. At CTL, our
metallurgical mounts are made in conformance to ASTM specifications.(2) |
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Figure 1. An area of a sample is selected for mounting. Arrows
inserted for reference orientation, see Figure 2. |
Figure 2. The specimen is mounted in a solid mount and polished to a
mirror finish. The sample is positioned in the mount so that the area of
interest is revealed in cross section. Arrows indicate relationship between
original sample (Figure 1) and mounted specimen (Figure 2.) |
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Figure 3. Corrosion at the interface between a weld and the HAZ
(Heat Affected Zone) in Alloy 600. Etching reveals the grains and
identifies the HAZ and the weld. In this photo, corrosion is occurring in
the HAZ. The weld is not corroding. 63x original magnification. Etched by
immersion for 30 seconds in a solution of 30% H2O2,
HCl, and H2O mixed in a
ratio of 1:2:3. |
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Case Study: |
| A failure
was discovered in the form of a through wall penetration in an area near a
weld in Alloy 600. A metallurgical mount was made of the cross section
through the weld, Figures 1 and 2. The specimen was etched in order to
reveal the microstructure of the specimen in the area of the failure, Figure
3. The HAZ (Heat Affected Zone) adjacent to the weld was corroding and
the
weld was not corroding. OES (optical emission spectroscopy) revealed that
the weld was made from a filler material that was not in conformance to the
customer’s specification. The use of improper filler material created the
conditions for galvanic corrosion. Metallography confirmed that the
corrosion was consistent with a galvanic corrosion mechanism – with the
material in the HAZ being preferentially attacked adjacent to the weld. |
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| (1) ASM Metals Reference Book, 3rd Edition, ASM
International, Materials Park, OH, 1993. |
| (2) ASTM E
3 – 01 “Standard Guide for Preparation of Metallographic Specimens”, ASTM
International, West Conshohocken, PA 2001. |
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