Shot peen hardness with nanoindentation: Jorge Ramirez of Nanovea tells finishing how how shot peening can alter the mechanical properties of a surface.
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.
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.
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.
The hardness is determined from the maximum load, Prnax, divided by the projected contact areaf Ac:
H = [P.sub.max]/[A.sub.c]
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.
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.
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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|Date:||Mar 1, 2012|
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