Scanning Electron Microscopy and X-Ray Microanalysis

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27

27.1 Case Study: Characterization of a Hard-

Facing Alloy Bearing Surface

Background: As part of a study into the in-service failure of the

bearing surface of a large water pump, characterization was

requested of the hard-facing alloy, which was observed to have

separated from the stainless steel substrate, causing the failure.

This problem illustrates the critical importance of careful

specimen preparation of a macroscopic object with centime-

ter dimensions to locate regions of microscopic interest with

micrometer dimensions. Metallographic preparation pro-

duced a polished cross section of an intact portion of the

hard-facing alloy layer as deposited onto the stainless steel

base, as shown in. Fig. 27.1. SEM-EDS analysis with

DTSA-II gave the results shown in. Fig. 27.2 for the stain-

less steel base and at one location in the hard-facing alloy.

EDS elemental mapping produced the images shown in

. Fig. 27.3. The SEM- BSE image (. Fig. 27.1) revealed the
presence of numerous voids that were predominantly located
(white arrows) at the interface between the hard-facing alloy
and the stainless steel substrate, with a smaller population of
voids located within the hard-facing alloy (yellow arrows).
The elemental maps (. Fig. 27.3) revealed that the hard-fac-
ing alloy layer had a very complex microstructure with dis-
tinct heterogeneity. The voids were found to be closely
associated with regions with that had elevated Cr, both near
the hard-facing alloy-stainless steel interface and within the
hard-facing alloy. These voids severely compromised the per-
formance of the bearing surface.
Complex fine-grained regions were also observed within
the hard-facing alloy, as shown in the elemental maps in
. Fig. 27.4 and the SEM-BSE image in. Fig. 27.5, which
shows strong atomic number contrast. SEM-EDS analysis
with DTSA-II gave the results presented in. Fig. 27.5, which
confirm the gray scale sequence as a function of composi-
tion, with the sharp rise in W (see table in. Fig. 27.5) domi-
nating backscattering.
The information provided by SEM-EDS enabled metal-
lurgists to modify the hard-facing alloy composition and
deposition conditions to eliminate the void formation during
deposition, producing satisfactory service behavior.

Stainless steel substrate

Voids

. Fig. 27.1 SEM-BSE image of the cross-section of a hard-facing alloy
deposited on a stainless steel substrate. Note the voids at the interface
between the hard-facing alloy and the stainless substrate (white arrows),
as well as a smaller population of voids entirely within the hard-facing
alloy (green arrows)

Si Cr Mn Fe Ni Mo

0.0055

±0.0001

0.1818

±0.0007 ±0.0002

0.0180 0.6507

±0.0015

0.1188

±0.0008

0.0266

±0.0002

Si Cr Mn Fe Co Ni

0.0377

±0.0002

0.0915

±0.0002

0.0006

±0.0001

0.0228

±0.0001

0.3859

±0.0005

0.4616

±0.0007

BSE MAG: 100 ́ HV: 20.0 kV WD: 11.0 mm^200 mm

. Fig. 27.2 SEM-BSE image
showing locations of SEM-EDS
analyses with NIST DTSA-II

Chapter 27 · X-Ray Microanalysis Case Studies