Metallife compression curves for extending die casting die life and improving performance of die casting dies and other molds.

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Compression Curves

Compression curves for the various processes are shown at the right. Compression has the highest level just below the surface. We strive to create our compression curves with these properties in mind. Polishing can therefore be done without losing the compressive stress benefit. Doing so, reveals the higher level of compression. When the entire processed surface is polished, however, the turbulent flow improvement is no longer present.

For this reason, cosmetic critical castings are only polished, after MetaLL ife, in the areas where subsequent casting polishing is difficult such as in fillets and radius/radii.

Composite Compression Curves
show overlay of differences in each process.

How the graphed curves were plotted.

MetaLL ife processing is monitored on a production basis by using Almen intensity strips. These strips are placed in strategic locations on the die or perishable tooling being processed. The Almen strip will bend upward when processed and by measuring the height of the center curvature, the approximate amount of induced surface compression can be determined. Various strip thicknesses are used (C,A,N strips) dependent on the intensity of the process. i.e. For example, a T-21 process would use a thinner Almen strip than a more aggressive T-41H.

While the Almen strip method is useful, it only provides a relative basis for measuring surface induced compression. Surface compression is important, but the depth and shape of the curve is what ultimately determines how well the process variables are being applied. The only accurate means for determining this residual stress distribution, is by measuring the surface and subsurface distribution of the compression.

X-ray diffraction (XRD) is the most accurate and proven method of quantifying the subsurface residual stress distributions that develop during MetaLL ife or other mechanical compression processing. These XRD methods use known standards established by ASTM and SAE associations. Prior to taking these measurements exacting criteria and specifications for processing methods have to be established and implemented. These specifications are applied to multiple coupon samples which are then used in the XRD measurement phase.

Subsurface readings are then determined by successive combination measurements using x-ray diffraction instruments and then electro-polishing the surface to remove layers of material. Electro-polishing removes material without inducing or changing the residual stress values. This method is costly and destructive in nature so it often is skipped by other competitive companies because it involves the removal of the specimen coupon from the diffractometer each time to perform the electro-polishing. This increases the amount of time required to take the readings which is very labor intensive.

Badger uses X-ray diffraction for quantifying residual stress distribution. The equipment allows the residual stress in a coupon specimen to be measured. Layers of material are electrochemically removed from the specimen. The resulting residual stress layer is defined and recorded by a computer. All the data obtained is properly corrected based on the penetration of the X-ray beam and the amount of layer removal. By connecting the resulting data points it is possible to accurately draw curves showing both surface and subsurface compressive residual stress values.

Specifications - Compression and Surface

Compressive values (fatigue resistance)
46-48 Rc H-13 material - oil quenched coupons
Max compressive value			Max compressive depth
Process	Ksi rounded	MPa	Process	Inches	mm
T-21	150	1034	T-21	.006	0.1524
T-31	152	1048	T-31	.009	0.2286
T-41	155	1069	T-41	.012	0.3048
T-40	155	1069	T-40	.012	0.3048
T-41H	160	1103	T-41H	.018	0.4572
T-40H	160	1103	T-40H	.018	0.4572
T-60	150	1034	T-60	.014	0.3556
T-61	150	1034	T-61	.014	0.3556
T-70	125	862	T-70	.019	0.4826
T-71	125	862	T-71	.019	0.4826