The maximum practical limit to the amount of carbide-forming elements which may be added to a steel for wear properties depends on the ability to maintain a reasonable distribution of those carbides throughout the steel’s microstructure. When steels are manufactured, they are melted in large batches, containing the desired chemical composition. The batches are poured into ingot molds, and solidify into castings which are subsequently forged or rolled into bars. During the solidification process, the carbides are formed. Under conditions of long slow solidification, these carbides form interconnected “segregated” networks, because they do not stay dissolved in the liquid steel. Large amounts of carbide particles result in more segregation, and thus more non-uniformity in the steel microstructure.
Carbide Size and Distribution
The alloying elements Cr, V, W, and Mo form hard carbide particles in tool steel microstructures. The amount and type of carbides influence wear resistance. Carbides are intended to improve wear resistance, but their non-uniform size and distribution (i.e., segregated networks) can impair toughness and grindability. Grades containing a high volume of hard carbides, like high speed steels and high vanadium cold work grades, may be particularly affected.
This carbide segregation causes two basic problems. First, areas of high concentrations of hard carbide particles may be difficult to grind, resulting in fabrication difficulties. Second, when these segregated areas are physically elongated during rolling or forging, they result in a directionally oriented microstructure, and reduce the material toughness along the transverse direction. Vanadium levels over about 3% are high enough to cause particular grinding and toughness difficulties. For this reason, despite its benefits for wear resistance, vanadium is usually limited to about 2-1/2% max. in conventionally manufactured tool steels.
The CPM Process
In order to manufacture tool steels with high wear resistance, without encountering these serious drawbacks, powder metallurgy processes are used to produce P/M tool steels having high vanadium content. Molten tool steel is atomized into fine droplets which solidify from the liquid so rapidly that the carbides are prevented from forming into large segregated networks. The solidified droplets form powder, which is then loaded into a steel can and consolidated (the individual powder particles are bonded together under high pressure), and subsequently forged or rolled into steel bars. The carbides formed during the extremely rapid solidification are fine in size (2 to 4 microns), and are uniformly distributed throughout the microstructure. Compare this to the larger carbides (up to 50 microns or more in size), and the characteristic alloy segregation or banding which results from conventional steelmaking methods. The characteristic feature of P/M tool steels is their near complete freedom from carbide segregation.
For grades with a high volume of carbides
(high wear resistance)
Effect of Carbide Content (ESP. VC)
on Wear Resistance
HRC 58-62 except as noted
Because the microstructural distribution of carbides in P/M steels is so fine and uniform, higher amounts of carbide-forming elements may be added. Thus, higher wear resistance may be developed, without the toughness and grindability limitations inherent in conventional steelmaking. The P/M process has allowed the development of grades containing 4%, 5%, 10%, and even 15% vanadium, offering far greater wear resistance than conventionally produced tool steels. Because of their high wear resistance, these high vanadium P/M grades are particularly suitable for high production operations.
In addition, the uniformity of the CPM microstructure provides improved toughness in CPM versions of conventional tool steels. The CPM versions of the same grades are more resistant to brittle failures. In fact, most CPM grades designed for metalforming tools have impact resistance comparable to the lower wear resistance grades such as D2. Thus, CPM steels may offer simultaneous improvements in both wear and toughness compared to conventional tool steels.
Toughness, CPM vs Conventional