Page 59 - MetalForming April 2014
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Comparing Sheet Material and Tool-Steel Strength
Tooling by Design
Material
Shear Strength (PSI)
Tool Steel
Compressive Strength (PSI)
60% PSI
Aluminum
12,000
A2
305,000 (60 Rc)
183,000
1010 CRS
45,000
M2
375,000 (62 Rc)
225,000
HSLA
59,500
M4
425,000 (63 Rc)
255,000
Stainless
85,000
M4
455,000 (65 Rc)
273,000
AHSS
145,000
CPM 10V
400,000 (63 Rc)
240,000
Table 2—Courtesy of Dayton Progress
and use of lubricants, tool steel and tooling surface treatments.
Tool-Steel Selection
The most common tooling material for cold-working dies is D2. When used in AHSS applications, D2 can exhibit premature chipping and wear. To increase resistance to chipping and wear, consider a high-speed tool steel; in severe applications, a powder-met- allurgy (PM) tool steel may produce the best results.
PM tool steels have a unique microstructure characterized by small metal carbide particles uniformly dis- persed in the steel matrix. PM tool steels, while more costly to purchase, often prove to be an economical choice due to their superior toughness and lower wear rates.
As sheetmetal strength and thick- ness increase, so will tool wear. This makes tooling hardness a prime con- cern. Hardness must be sufficient to resist plastic deformation, but not so
high as to cause premature chipping or cracking. PM tool steels can achieve higher hardness after heattreat than conventional tools steels.
AHSS materials will subject punch heads and points to considerably high- er loads than comparable-yield- strength HSLA steels. Table 2 compares the shear strengths for several sheet- metal grades, and compressive strengths for three tool-steel grades with various heattreatments. This table is for illustrative purpose only, since accurate shear data are difficult to obtain and a large range of shear strengths can be observed among the many different grades within each material type.
Punch Performance
For optimum punch performance, keep head pressures below 40,000 PSI. When head pressure exceeds 20,000 PSI, use a hardened backup plate heat- treated to 45-50 Rc.
Punch-point performance peaks
Round Hole in HSLA
Round Hole in AHSS
45,000
Material Shear Strength, PSI
110,000
0.065
Material Thickness, in.
0.065
0.100
Point Diameter, in.
0.100"
919
Force Required (lbs-force)
2246
0.46
Force Required (tons)
1.12
117,000
Point Pressure (PSI)
286,000
0.375
Punch Shank Diameter, in.
0.375
8,320
Head Pressure (PSI)
20,338
Table 3—Calculated head and punch-point pressures in HSLA compared to AHSS
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