Page 29 - MetalForming August 2010

Enhanced PM Alloys
C
Cr
Mo
V
W
S
A11 (K294)
2.45
5.2
1.3
9.7
–
0.09 max
M4 (S693)
1.33
4.1
5.0
4.1
5.9
0.02 max
gap between the 5- and 12-percent chromium steels. Their well-balanced chemical composition and the ESR process afford a homogeneous tem- pered martensitic matrix, largely responsible for their relatively good toughness.
Adhesive wear is one of the primary types of damage encountered during the use of cold-work tools. Contact between the tooling and the workpiece leads to local material adhesion. The subsequent separation of the workpiece from the tool involves micro-crack for- mation, which does not always take place exactly in the original contact area and thus causes additional adhesive wear. Both of these new 8 Cr alloys exhibit very high resistance to adhesive wear.
To evaluate the resistance to abrasive wear of these alloys, the pin-on-disc method was used and weight loss after certain time intervals was measured. The modified 8 Cr alloy performed similarly to the 12 Cr grade, while the 8 Cr + V grade exhibited improved abra- sive-wear resistance compared to the 12 Cr steels.
High-Performance PM Steels
Highly alloyed steels are difficult materials to cast, because of the presence of high amounts of alloying elements (chromium, tungsten, molybdenum and vanadium) and carbon in these alloys. These elements segregate during solidification and form a carbide net- work, which makes the cast steel very brittle.
During hot working, the network is elongated and individual large carbide particles may be reduced to smaller sizes, but the final product will still have a rather coarse and nonuniform microstructure. Laws of nature provide the driving force for segregation, there- fore the only remedy is to not give the law of nature enough time to act. If an ingot is small enough, it can be made to solidify so quickly that segregation is reduced to negligible levels—this is the basic idea behind producing highly alloyed tool-steel powders by atomiza-
tion. A standard
cast ingot needs
hours to solidify, while a powder parti- cle needs only fractions of a second for complete solidification. That means a powder particle is a very small ingot.
Solidification time greatly affects alloy microstructure and therefore the PM alloys’ metallurgical and mechani- cal properties. Uniformity of structure is exhibited over the entire cross section; the tool steels produced exhibit an increase in toughness and fatigue strength by as much as 20 percent.
PM Selection
Two new tool-steel grades that have resulted from enhancements made to the PM process are depicted in the table above, the first an AISI A11 cold-work tool steel (K294) and the second an AISI M4 high-speed PM tool steel (S693).
The success of these alloys is well recorded. The A11 cold-work tool steel
serves in applications requiring a high degree of compressive strength and wear resistance, such as blanking and trimming operations where abrasive wear and edge retention are critical suc- cess factors. And, M4—a universal high- speed steel that can achieve hardness levels of 64 Rc—gets the call in high- pressure applications. The high-tem- perature tempering of both of these PM alloys provides a stable substrate for coatings.
Advancements to the powder-met- allurgy production process now permit finer distribution of the sulfides with- in these alloys, which has lead to improved uniformity and enhanced mechanical properties. Their more uniform and homogeneous structure minimizes the negative impact of sul- fide additions into the matrix, while still providing improved machining characteristics. MF
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METALFORMING / AUGUST 2010 27