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Shot peen hardness with nanoindentation: Jorge Ramirez of Nanovea tells finishing how how shot peening can alter the mechanical properties of a surface.

The intention of shot peening is ultimately to alter the mechanical properties of a given surface. By hitting the surface with controlled shot the surface will deform plastically. Two of the more common techniques include cast sheet shot and cut wire shot. Cut wire shot is created by cutting drawn steel wire to lengths approximately the same size as the width of the wire. The resulting wire is then rounded to remove the sharp edges. Cast steel shot is created by atomizing molten steel, then heat treating and sieving the resulting material. In both cases, the surface of the material is hit with a controlled energy of shot to create the desired effect. Understanding the mechanical properties of the surface reactions created by these techniques is becoming increasingly important in various applications including medical aerospace and automotive industries. Proper Instrumentation will play a vital factor in achieving reliable and intended results.

Importance of nanoindentation for peened surfaces

Traditionally, the Rockwell hardness test has been used to evaluate peened surfaces. Unfortunately, because of the size of the indenter used and the high load applied, the data is very unreliable and has little to do with the actual surface affected by the peening process. This is because Rockwell indents easily exceed 100's of microns in depth while the peened depth is only in the range 25microns or so. Using Nanoindentation, which provides precise depth versus load data, hardness and elastic modulus at depths well under 4 to 5 microns can be directly measured. This shallow test is required to study this effect of shot peened without the influence of untreated zones.

Measurement objective

In this application, the Nanovea Mechanical Tester, in Nanoindentation mode, is used to study the mechanical properties of two separately peened surfaces versus an untreated surface for comparative review. The sample was designed as a single piece of steel with three specific zones: two peened treated areas with one done under cut wire technique and the other done under cast steel technique. The third zone was kept untreated for reference. For Nanoindentation on rough surfaces, it is necessary to find and position the indenter directly on a relatively smooth area. In our case here, smooth areas could be found on the crests of the shot bump created.

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Measurement principal

Nanoindentation is based on the standards for instrumented indentation, ASTM E2546 and ISO 14577 It uses an already established method where an indenter tip with a known geometry is driven into a specific site of the material to be tested, by applying an increasing normal load. When reaching a preset maximum value, the normal load is reduced until complete relaxation occurs. The load is applied by a piezo actuator and the load is measured in a controlled loop with a high sensitivity load cell. During the experiment the position of the indenter relative to the sample surface is precisely monitored with high precision capacitive sensor. The resulting load/displacement curves provide data specific, to the mechanical nature of the material under examination. Established models are used to calculate quantitative hardness and modulus values for such data. Nanoindentation is especially suited to load and penetration depth measurements at nanometer scales and has the following specifications: Maximum displacement (Dual Range): 50nm or 250[micro]m

Depth Resolution (Theoretical): 0.003 nm

Depth Resolution (Noise Level): 0.05 nm

Maximum force: 400 mN

Load Resolution (Theoretical): 0.03 [micro]N

Load Resolution (Noise Floor): 1.5 [micro]N

Analysis of indentation curve

Following the ASTM E2546 (ISO 14577), hardness and elastic modulus are determined through load/displacement curve as for the example below.

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Hardness

The hardness is determined from the maximum load, Prnax, divided by the projected contact areaf Ac:

H = [P.sub.max]/[A.sub.c]

Young's Modulus

The reduced modulus, Er, is given by:

[E.sub.r] = [square root of (term)]pi/2 S/[square root of (term)][A.sub.c]

Which can be calculated having derived S and AC from the indentation curve using the area function, AC being the projected contact area. The Young's modulus, E, can then be obtained from:

1/[E.sub.r] = 1 - [v.sup.2]/E + 1 - [v.sub.i.sup.2]/[E.sub.i]

Where Ei and i are the Young's modulus and Poisson coefficient of the indenter and the Poisson coefficient of the tested sample.

How are these calculated?

A power-law fit through the upper 1/3 to1/2 of the unloading data intersects the depth axis at ht. The stiffness, S, is given by the slope of this line. The contact depth, he, is then calculated as:

[h.sub.c] = [h.sub.max] - 3[P.sub.max]/4S

The contact Area Ac is calculated by evaluating the indenter area function. This function will depend on the diamond geometry and at low loads by an area correction.

For a perfect Berkovich and Vickers indenters, the area function is Ac=24.5hc2 For Cube Corner indenter, the area function is Ac=2.60hc2 For Spherical indenter, the area function is Ac=2JiRhc where R is the radius of the indenter. The elastic components, as previously mentioned, can be modeled as springs of elastic constant E, given [sigma] = Ez is the formula: where o is the stress, E is the elastic modulus of the material and ?is the strain that occurs under the given stress, similar to Hooke's Law. The viscous components can be modeled as dashpots such that the stress-strain rate relationship can be given as,

[sigma] = [eta] dz/dt

where a is the stress, [eta] is the viscosity of the material, and d[epsilon]/dt is the time derivative of strain.

Since the analysis is very dependent on the model that is chosen. Nanovea provides the tool to gather the data of displacement versus depth during the creep time. The maximum creep displacement versus the maximum depth of indent and the average speed of creep in nm/s is given by the software.

Creep may be best studied when loading is quicker. Spherical tip might be a better choice.

Other tests possible includes the following:

Stress-Strain, Yield Strength Creep, Compression strength and Fatigue testing and many others.

Test conditions & procedures

The following indentation parameters were used:
Applied Force (mN)             200
Loading rate (mN/min)          400
Unloading rate (mN/min)        400
Indenter type            Berkovich


Discussion & conclusion

Both shot peened techniques revealed close to double the hardness measured on the untreated surface. We have measured more variation on the cast steel shot versus cut wire which is expected because of the non uniformity of beads size compared to the uniformity of the cut wire technique. Because both areas were created with the same intensity, it was expected as measured that the average hardness would be similar. The elastic modulus of cast steel zone was slightly higher than what was found on the untreated surface. However, the elastic modulus measured in the cut wire zone was almost double that of the two other zones. This increased plastic reaction may be caused by a reaction similar to forging provided by the cut wire technique. Cast materials will give isotropic properties which could explain the closeness to the untreated area. A forging process will create a surface with properties that differs in various directions.

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In conclusion, we have shown that the Nanovea Mechanical Tester, in Nanoindentation Mode, is extremely reliably tool to measure and investigate shot peened surfaces. Other test such as yield strength using a five micron flat tip (patent pending) could provide additional information on the surface using Nanoindentation, among many other measurements. Roughness is a concern with this type of surface and the low load used. However, with good microscopy and precise location it is possible to find smooth area to perform these low load tests.

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In the next issue: Powder Coating Ovens and Curing Equipment Dust and Fume Extraction
Results Untreated Steel  Hardness  Hardness  Modulus  Depth

                         Vickers     Gpa       Gpa      nm

test 1                        288      3.05      317   1697
test 2                        298      3.16      213   1695
test 3                        364      3.85      241   1542
test 4                        328      3.47      268   1610
test 5                        329      3.47      308   1601
test 6                        354      3.75      282   1550
average                       327      3.46      272   1616
stdev                          30      0.31       40     68

Results S-170  Hardness  Hardness  Modulus  Depth

               Vickers     Gpa       Gpa      nm

test 1              554      5.86      287   1272
test 2              776      8.21      287   1106
test 3              724       766      340   1124
test 4              630      6.67      283   1205
average             671       7.1      299   1177
stdev                85      0.90       24     66

Results SCW20  Hardness  Hardness  Modulus  Depth

               Vickers     Gpa       Gpa      nm

test 1              616      6.52      694   1171
test 2              577      6.11      511   1218
test 3              704      7.45      731   1101
test 4              645      6.83      498   1159
test 5              693      7.34      332   1147
average             647      6.85      553   1159
stdev                53      0.56      162     42
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Title Annotation:COATING
Publication:Finishing
Date:Mar 1, 2012
Words:1486
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