POWER September 2021 - 20

FOCUS ON O&M
4. Tie rod bolts that connect the stages of
gas turbines are susceptible to fretting fatigue
at the edge of the contact region between the
turbine rotors and the tie rod landings. Courtesy:
Lambda Technologies Group
Oxide particles can become embedded
in the surfaces at these locations, creating
local stress concentrations. Shallow
shear cracks formed at the edge of fretting
zones can initiate fatigue cracking,
potentially leading to failure within the
first few years of service.
Plant operators frequently inspect
the tie rod bolts to address this problem,
replacing bolts that have signs of
damage. Replacing a set of 12 bolts
can cost upward of $400,000, not
including
the
substantial downtime
that impacts operating efficiency and
costs of power generation. A leading
company in the transmission and generation
of power approached Lambda
Technologies with an important project:
assist them in devising a solution
to increase the fatigue strength of the
tie rod bolts (Figure 4) without changing
either the material or the design.
The Benefits of Compressive
Stress
Lambda proposed introducing residual
compression into the edge of the contact
region of the tie rod bolts. The benefits
of compressive residual stress are
well-known in engineering circles, because
it reduces the effects of applied
tensile stresses. Beneficial residual compression
can be introduced through surface
treatments such as shot peening,
laser shock peening, deep rolling, and
low plasticity burnishing (LPB).
In this instance, LPB was chosen for
its repeatability and ease of application
using conventional computer numerical
control (CNC) machine tools. The highly
controlled nature of the process allowed
Lambda engineers to sculpt the compression
to the optimum levels for the
tie rod bolts.
LPB is a mechanical process that applies
a designed residual stress field
to the surface of metal components
20
to produce a deep, stable layer of beneficial
residual compression. The depth,
magnitude, and distribution of the compression
are designed specifically for
each application to provide maximum
resistance to foreign objects, corrosion
fatigue, fretting, stress corrosion cracking,
and other common damage mechanisms.
The process is well-recognized in
aerospace applications, but not as commonly
used in the power industry.
The military has used LPB for repairs
on aircraft such as the AV-8B Harrier
pass. Limiting the number of instances
where the tool comes in contact with
the surface keeps cold work low, allowing
the imparted stresses in the bolts to
remain stable in high temperatures. In
this application, LPB was applied using a
Haas CNC lathe.
Fatigue tests (Figure 5) were conducted
on the LPB-treated and untreated tie
rod bolts with simulated conditions of
fretting and embedded oxide particles.
The tie rod material deteriorated by a factor
of five in the untreated rods. The LPBWhen
damaged mechanisms are impacting
component performance, surface enhancement
techniques to impart designed compression
are a cost-effective solution to
ensure reliable performance in gas turbines.
II and the P-3 Orion. The LPB process
also has an Alternative Method of
Compliance (AMoC) certification from
the Federal Aviation Administration,
and Delta
Lambda to treat shock-strut cylinders
for its MD-88 aircraft.
Putting Designed Compression to
the Test
Every job requires an engineering phase,
analyzing the material, then measuring
the residual stress. LPB is performed using
a wheel or ball tool to roll across the
surface of the tie rod bolt, applying compression
in a single, highly controlled
Airlines contracted with
treated rods saw more than a six-fold
improvement in fatigue strength even
with oxide damage and fretting.
Additional combined thermal-mechanical
tests for years of thermal exposure
with nominal operating stresses proved
that the LPB-induced compressive residual
stresses remained stable during
operation. The testing results confirmed
that the benefits of LPB treatment would
allow the tie rod bolts to resist fretting fatigue
for the duration of the designed life
of the gas turbines they hold together.
Dollars and Sense
More than 160 tie rod bolts have been
processed with LPB at Lambda Technologies'
Cincinnati, Ohio, facility, resulting
in 14 sets of bolts that will never need
replacing and a savings of $5.6 million
to the customer in materials alone. Adding
the cost of taking the gas turbines
out of service for repairs, estimated at
about $1 million per shutdown, yields
nearly $20 million in savings at the time
of this writing.
When damaged mechanisms are impacting
component performance, surface
enhancement techniques to impart designed
compression are a cost-effective
5. This chart shows the typical benefits derived
from low plasticity burnishing (LPB) surface
treatment of tie rod bolts, compared to
untreated bolts. Courtesy: Lambda Technologies
Group
www.powermag.com
solution to ensure reliable performance. ■
-N. Jayaraman, PhD is a Fellow Emeritus
at Lambda Technologies Group, where he
previously served as Director of Materials
Research.
POWER | September 2021
http://www.powermag.com
POWER September 2021

Table of Contents for the Digital Edition of POWER September 2021
