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TOOL STEEL
TECHNICAL LIBRARY

SERIES I: Introducing the Concept of Tool Steel Microstructure

SERIES II: Typical Failure Modes for Cold Work Tooling and Their Association with Microstructure

SERIES III: Basics of Heat Treatment • Part 1

SERIES III: Basics of Heat Treatment • Part 2

SERIES III: Basics of Heat Treatment • Part 3

SERIES III: Basics of Heat Treatment • Part 4

SERIES III: Basics of Heat Treatment • Part 5

SERIES III: Basics of Heat Treatment • Part 6
 

ZAPP HIGH-PERFORMANCE
STEEL GRADES:

TOOL STEELS
Z-TUFF PM
Z-WEAR PM
Z-A11 PM
Z-A11LV PM
Z-420 PM

HIGH SPEED STEELS
Z-M4 PM
Z-T15 PM
Z-M48 PM
Z-MAX PM
Z-M2
 

ZAPP PERFORMANCE PRODUCTS:
ZDM BLANKS
 

TOOL STEEL TECHNICAL TRAINING

SERIES II: Typical Failure Modes for Cold Work Tooling and Their Association with Microstructure

The first installment of our technical discussion series introduced the concept of microstructure and how it relates to material properties. In the case of tool steels, examining microstructure is also a key step in understanding specifically how and why tools fail.

Conventional AISI M2 Heat Treated Microstructures

The first photomicrograph (Figure 1) shows a 500X view of the microstructure of a forming punch made from 3" diameter M2 material which failed prematurely due to longitudinal cracking. The microstructure appears normal for standard quality M2 and it also indicates that the heat treatment process was correct. However, it is clearly evident how the crack follows along a line of banded carbides. The carbides provide wear resistance needed in heavy duty punch applications, but the banding is a natural occurrence resulting from the solidification process utilized in the production of conventionally melted high speed steel ingots. It is more pronounced in larger diameter bars and has the unintended consequence of providing a very convenient path for crack propagation under the right combination of stress and impact. The banding is even more evident when looking at the structure with lower magnification as shown in the second photomicrograph taken at 100X (Figure 2).

Figure 3

The third photomicrograph (Figure 3) shows a 500X view taken of another punch used in the same application. This punch was not found to exhibit cracking, but unfortunately it developed pronounced issues with corner chipping. The particular bar of material appears to show less carbide banding. However, examination of the microstructure revealed many large clusters of carbide. This again is a result of the solidification process, and the photo shows how the chipping can subsequently originate at the large carbides. When a carbide cluster occurs near the edge of the punch face, it provides a ready means for premature tool failure.

In both examples, the tooling did not have the opportunity to “wear out.” The punches had to be changed early resulting in unplanned down time and added cost. Both problems could be solved by the use of PM tool steels. One alternative would be to consider the use of Z-Wear PM. This grade offers less wear resistance than the M2 due to the lower actual carbide volume. However, the lower alloy content combined with the uniform PM structure would offer over 3 times the toughness of the conventional grade. This would almost certainly resolve the cracking and/or chipping issues and shift the punch performance into a more predictable wear scenario. One could also consider the use of Z M4 PM which is a higher alloy, higher cost grade. While not offering as much of an improvement in toughness (compared to Z-Wear), it would provide wear resistance that in most cases exceeds that of the conventional M2. Both grades are valid options which could improve performance and lower overall costs. A typical PM microstructure is pictured below.

Figure 3

Questions or comments may be sent to Gary Maddock at gmaddock@zapp.com.

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