Page 58 - MetalForming April 2014

Tooling by Design
By Peter Ulintz
Cutting and Punching Advanced High-Strength Steels
The beneficial properties of advanced high-strength steels (AHSS)—their high yield and ten- sile strength and good ductility—also can degrade tooling life and promote premature failure.
Due to the higher stresses required to penetrate AHSS materials, additional cutting clearances are required between the punch and die when com- pared to cutting mild and HSLA steels. Also, as material thickness increases so must cutting clearance (Table 1).
Exceptions to these recommenda- tions include punched holes and trim profiles subsequently subjected to edge stretching (addressed here last month, Flanging and Forming AHSS Materi- als). In these cases, cut edges subject- ed to stretching may require as much as 21-percent cutting clearance per side.
Failure Mechanisms
Tool-failure mechanisms common to AHSS include chipping, cracking and wear during punching, shearing and trimming, and galling and wear during forming operations.
Tight cutting clearances can lead to chipping and galling—a physical or chemical adhesion of the workpiece material to the tool surface. Chip- ping, the most common failure mech-
Peter Ulintz has worked in the metal stamping and tool and die indus- tries since 1978. He has been employed with the Anchor Manufacturing Group in Cleveland, OH, since 1989. His back- ground includes tool and die making, tool engi- neering, process engi-
neering, engineering management and product development. Peter speaks regularly at PMA semi- nars and conferences. He is also vice president of the North American Deep Drawing Research Group. Peter Ulintz
pete.ulintz@toolingbydesign.com www.toolingbydesign.com
Ferrous Materials—Engineered Die Clearance, Tensile Strength and Approximate Hardness Values
Clearance per Side (Δ) % Material Thickness→
Cast Iron
Mild–CRS,HRS, CDS
Bake Hardenable–
Dent Resistant
Spring Steel
Stainless–2xx, 3xx, 4xx, etc. HSLA
HSS/AHSS
DP, TWIP
CP, TRIP, Austenic Stainless UHSS
Hot Formed/Boron
MART
Tensile Strength
MPa
KSI
Hardness (approx.)
69 200 300 400 500 600 700 800 1000 1200 1400 1600 1750 10 29 44 58 73 87 102 116 145 174 203 232 255
114HB 144HB 183HB 213HB 22HRC 33HRC 38HRC 43HRC 48HRC 50HRC
Table 1—Courtesy of Dayton Progress
anism when cutting and punching AHSS materials, occurs when high process stresses cause low-cycle fatigue of the tooling material. This condition indicates that the material lacks toughness.
When cutting clearances are too large, punch cracking can sponta- neously result when the process stress- es exceed the tensile strength of the tooling material. Punch cracking is a concern with martensitic grades because of the work material’s higher strength.
Plastic deformation of the tools also can occur if process stresses exceed the compressive strength of the tool steel. Resistance to plastic deforma- tion depends primarily on tool-steel hardness, not grade.
Tool Wear
...can be characterized as either abrasive or adhesive. Abrasive wear occurs when the tool material is being
Scaling Factor— Engineered Die Clearance
HSS 1.00 1.20 1.40 AHSS 1.00 1.20 1.40 UHSS 1.00 1.30 1.40 Aluminum 1.00 1.03 1.05
worn away, typically by friction. Stam- pers can reduce abrasive wear by increasing tool-steel hardness or car- bide volume. However, this will reduce toughness and resistance to adhesive wear.
Adhesive wear results from micro- scopic welding at localized contact points between the tool- and work- material surfaces. During tool extrac- tion, these microwelds shear. If shear- ing occurs on the tool surface, small fragments will be removed, leading to a gradual loss of material.
To reduce adhesive wear, increase the toughness of the tooling material and reduce the friction between the tooling material and workpiece. This usually requires the proper selection
7-9%
9-12%
12-15%
56 MetalForming/April 2014
www.metalformingmagazine.com
15-20%
Thickness <0.060" >0.060" >0.120"
Scaling Factor (1.5 mm) (1.5 mm) (3.0 mm)