reciprocating wear in a steam environment
TRANSCRIPT
Friction vs Time
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50
Time, h
Fric
tion
Coe
ffici
ent
3000 h pre-exposure
500 h pre-exposure
No pre-exposure
Abstract:Tests to simulate the wear between sliding components in
a steam power plant have been performed. A low frequency
constant load wear apparatus, housed inside a furnace was
modifi ed to incorporate a steam environment.
The materials tested were pre-exposed in a fl owing steam
furnace at temperature for either 500 or 3000 hours to
provide some simulation of long term ageing. Tests were also
performed on as-received material for comparison purposes.
The duration of each wear test was 50 hours.
ApparatusA schematic diagram of the wear test apparatus is shown below. It has the following features:
• A split furnace
• Two arms which hold the samples
• The top arm moves relative to the fi xed bottom arm
• An LVDT measures the distance travelled
• Another LVDT measures the wear of the specimen
• Two load cells measure the normal and frictional forces
• The two arms are water-cooled
• The temperature of the furnace and the specimens are measured in-situ by sheathed thermocouples
• The top and bottom arms have been designed to rotate so that the two contacting surfaces can be made parallel
SpecimensThe samples were designed to ‘push’ fi t into the arms. The top sample is a block with a protruding stub which is rubbed against a fl at bottom sample.
MaterialsThe materials tested are listed in the table below:
SoftwareSoftware was written to control the experiment (cycle speed, water top-up, start and end times) as well as to acquire the measurement data (load, friction, wear, temperature, time, etc.) A typical screen shot showing during a test is shown to the right.
FrictionDuring testing, the horizontal and vertical loads and the horizontal and vertical displacements were logged. The vertical load is constant during a test and the horizontal load gives a measure of the friction force generated at the contact interface that can be used to calculate friction coeffi cients.
A good method for displaying this is through ‘friction loops’ (displayed on the right hand side of the fi gure above) where the friction coeffi cient is plotted against horizontal displacement.
The average friction coeffi cient for each complete friction loop was also calculated from the integral, and the trend is plotted against time. Typical examples of the friction trends are shown in the fi gures to the right for a Ni-Cr-Co alloy stub rubbed against a fl at HVOF Cr2C3 plate. Both specimens were tested in the as received condition. Initial and fi nal friction coeffi cient loops are plotted against horizontal direction, shown in (a) and (b) respectively. The average friction coeffi cient was plotted against time, shown in (c).
ResultsAll but one of the tests were performed in a steam environment. One test was carried out without steam for comparative purposes. The graph below shows that the addition of steam reduces the friction coeffi cient.
The graph below shows a comparison of friction results for different materials in the as-received condition:
The following graph shows a comparison between materials that have been pre-aged for 3000 hours in a steam environment.
Tests were performed on as-received and pre-exposed specimens to compare the effect of ‘aged’ material. Results are presented in the following fi gures for two different materials. For the Co-Ni-Cr-W alloy against stellite material, ageing increases the friction. By contrast, with the MCrAlY coating, pre-ageing markedly decreases the friction coeffi cient.
The following table shows a summary of the fi nal friction coeffi cient results with respect to material and ageing treatment:
Visual Inspection of the Wear ScarDespite the attention given to sample alignment in the design of the test system, it was found that full contact was not always achieved over the sample surfaces. The following fi gures show examples of good and bad alignment:
ConclusionsThe work presented here reveals that the addition of steam in a tribological environment clearly infl uences the friction between both as-received and aged materials.
The majority of materials experienced an increase in friction coeffi cient with age, the only exception being the MCrAlY coated stub, due to its lubricious properties which form during testing.
AcknowledgementsThe authors are grateful to Jim Banks and David Laing for their technical expertise, and would like to acknowledge funding from the Technology Strategy Board and from the Department of Business Innovation and Science for the work reported in this paper.
Reciprocating wear in a steam environmentL. J. Brown, M. G. Gee & J. W. Nunn
National Physical Laboratory, Teddington, Middlesex, TW11 0LW, UK
Stub MaterialCoating
on Stub
Plate
Material
Coating
on Plate
Pre-
exposure, h
Co-Ni-Cr-W alloy - Stellite - 0Co-Ni-Cr-W alloy - Stellite - 500Co-Ni-Cr-W alloy - Stellite - 3000Co-Ni-Cr-W alloy - Stellite HVOF Cr2C3 0Co-Ni-Cr-W alloy - Stellite HVOF Cr2C3 500Co-Ni-Cr-W alloy - Stellite HVOF Cr2C3 3000
Ni-Cr-Co alloy - Stellite - 0Ni-Cr-Co alloy - Stellite - 500Ni-Cr-Co alloy - Stellite - 3000Ni-Cr-Co alloy - Stellite HVOF Cr2C3 0Ni-Cr-Co alloy - Stellite HVOF Cr2C3 500Ni-Cr-Co alloy - Stellite HVOF Cr2C3 3000Ni-Cr-Co alloy MCrAlY Stellite HVOF Cr2C3 0Ni-Cr-Co alloy MCrAlY Stellite HVOF Cr2C3 500Ni-Cr-Co alloy MCrAlY Stellite HVOF Cr2C3 3000
Stub MaterialCoating
on Stub
Plate
Material
Coating
on Plate
Pre-
exposure, h
Final
Friction
Co-Ni-Cr-W alloy - Stellite - 0 0.5Co-Ni-Cr-W alloy - Stellite - 500 0.8Co-Ni-Cr-W alloy - Stellite - 3000 0.95Co-Ni-Cr-W alloy - Stellite HVOF Cr2C3 0 0.68Co-Ni-Cr-W alloy - Stellite HVOF Cr2C3 500 0.9Co-Ni-Cr-W alloy - Stellite HVOF Cr2C3 3000 -
Ni-Cr-Co alloy - Stellite - 0 0.65Ni-Cr-Co alloy - Stellite - 500 0.65Ni-Cr-Co alloy - Stellite - 3000 0.85Ni-Cr-Co alloy - Stellite HVOF Cr2C3 0 0.7Ni-Cr-Co alloy - Stellite HVOF Cr2C3 500 0.85Ni-Cr-Co alloy - Stellite HVOF Cr2C3 3000 1.2Ni-Cr-Co alloy MCrAlY Stellite HVOF Cr2C3 0 0.95Ni-Cr-Co alloy MCrAlY Stellite HVOF Cr2C3 500 0.95Ni-Cr-Co alloy MCrAlY Stellite HVOF Cr2C3 3000 1.0
www.npl.co.uk
Bad alignment
Good alignment
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50
Time, h
Frict
ion
Coef
ficien
t
No Steam
With Steam
0 0.2 0.4 0.6 0.8
1 1.2 1.4 1.6
0 10 20 30 40 50 Time, h
Fric
tion
Coe
ffici
ent
Ni-Cr-Co, MCrAlY/HVOF Cr2C3
Ni-Cr-Co/HVOF Cr 2 C 3
Ni-Cr-Co/Stellite Ni-Cr-Co/Stellite Co-Ni-Cr-W/HVOF Cr 2 C 3
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 10 20 30 40 50 Time, h
Fric
tion
Coe
ffici
ent
Ni-Cr-Co /HVOF Cr 2 C 3 Ni-Cr-Co MCrAlY/HVOF Cr 2 C 3
Co-Ni-Cr-W/Stellite
Ni-Cr-Co/Stellite
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 10 20 30 40 50
Time, h
Fric
tion
Coe
ffici
ent
500 h pre-exposure
No pre-exposure
3000 h pre-exposure
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-3 -2 -1 0 1 2 3
Horizontal Displacement
Fric
tion
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-3 -2 -1 0 1 2 3
Horizontal Displacement
Fric
tion
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-3 -2 -1 0 1 2 3
Horizontal Displacement
Fric
tion
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-3 -2 -1 0 1 2 3
Horizontal Displacement
Fric
tion
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0 10 20 30 40 50 60 Time, h
Fric
tion
Coe
ffici
ent
Plot of Friction Loops a) Initial Friction, b) Final Friction and c) Overall Friction vs. Time
Co-Ni-Cr-W
MCrAlY
(a)
(b)
(c)