A prefix letter is used in the alloy identification system to show use category, and the specific alloy in a particular category is identified by one or two digits. L6 One problem that exists in discussing the metallurgy of tool steels is that, since there are six major categories of tool steels, it is difficult to make statements that apply to all these steels. The reason for this is that alloy elements are usually combined with carbon to form carbides. The most significant metallurgical difference between tool steels and the other steels is their microstructure. A fully hardened carbon steel or alloy steel would have only martensite as the predominant phase.
Most tool steels have a hardened structure of martensite and alloy carbides.
Cold work tool steels are used for gages, blanking, drawing and piercing dies, shears, forming and banding rolls, lathe centers, mandrels, broaches, reamers, taps, threading dies, plastic molds, knurling tools. Lowest cost tool steels. Use of w-series steels is declining. They contain graphite in the hardened structure along with martensite. Tungsten forms tungsten carbide which improves the abrasion resistance and edge retention in cutting devices.