Page 24 - MetalForming April 2009

Forming Higher-Strength Steels
the true-stress true-strain curve (Fig. 4).
Die-Design Considerations
In general, the beneficial properties of higher-strength materials—high yield strength, high tensile strength and high hardness—also contribute to degraded tool life and premature failures. Tool- failure mechanisms common to higher- strength materials include chipping, cracking and wear in piercing, shearing and trimming operations. Predominant mechanisms during forming operations include galling and wear.
Cutting, Punching and Trimming
The higher stresses needed to pene- trate stronger materials require addi- tional cutting clearances between the punch and die as compared to those required for mild steel. The table, Sug- gested Cutting Clearances for Several High-Strength-Steel Grades, provides some recommended cutting clearances for several high-strength grades. Exces- sively tight cutting clearances often lead
to increased galling and chip- ping. But when cutting clear- ances are too large, cracking may result. Plas- tic deformation of the tools also can occur if process stresses exceed the com- pressive strength of the tool steel.
Suggested Cutting Clearances for Several High-Strength-Steel Grades
22 METALFORMING
write no. 15
write no. 16
Tool wear becomes a major issue when stamping higher-strength steels, and this wear is characterized as either abrasive or adhesive. Abrasive wear occurs when the tool material wears away, typically by friction. Reduce abra- sive wear by increasing the tooling mate- rial’s hardness or carbide volume, although that will reduce its toughness and resistance to adhesive wear. Adhe- sive wear results from microscopic weld- ing at localized contact points between
the tool and work-material surfaces. Reduce adhesive wear by increasing the toughness of the tooling material and by reducing the friction between the tool- ing material and workpiece. This usually requires the proper selection and use of lubricants, tool steel and tooling-surface treatments.
In general, the best overall punch performance for blanking, piercing and trimming is achieved through a com- bined balance of tool-steel toughness
Thickness (mm)
2.0 1.5 1.0 0.5
Material Grade ->
Suggested Cutting Clearances (punch-to-die clearance per side)
14-16% 14% 12-14% 12%
DP800
14-16% 12-14% 12% 10-12%
DP1000
14% 12-14% 10-12% 10%
MS1200
12-14% 12% 10-12% 10%
MS1400
Drawing/Forming
Die Material
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