Plain carbon and alloy carburizing steels may be obtained in S. A. E. grades, and in grades very nearly conforming to S. A. E. analyses, but sold under trade names. These steels are used for many kinds of tool and fabrication uses besides plastics mold requirements. Therefore, they are available in many stock sizes and shapes.
Such materials usually are furnished as hot rolled stocks, with the exception of shafting, drill rod, and a limited range of rounds, hexagons, and squares. Many molders prefer to use a carburizing steel for mold impressions or mold operating members. Their reasons may be any or all of the following: ease of machining the hard surface obtained with high carbon content, good mold polish, availability of experienced heat treating operators and proper equipment, and preference due to long experience with a specific brand of steel.
The alloy carburizing steels usually have center core properties which are better than can be obtained with plain carbon carburizing steels. Much more can be done with grain structure and the resultant physical properties if the parent metal is responsive to heat treatment, as are the alloy carburizing steels.
The alloying elements may be varied to some extent to produce the desired structure in the core metal when the carburized piece is subjected to the heat treatment desired for the surface portion. Perhaps the simplest stock system for having carburizing steels available in the mold shop tool room is to standardize on one kind of plain carbon steel, and one kind of alloy steel, and maintain sizes commonly used. If other alloys are desired, they may be obtained quickly from warehouse stocks, from the same steelmaker if desired, for consistent results due to familiar methods of handling.
High Alloy Tool Steels
The mold shop tool room has use for oil hardening, air hardening, and water hardening tool steels. Mold cavities and forces, special cores, loose bars or mold cheeks, preforming dies, cams, hobs, and other critical mold parts usually require the use of a tool steel or alloy mold steel which may be heat treated directly to the hardness required without carburizing, cyaniding, or nitriding.
For mold cavities and other precision parts, many molders prefer to use an alloy mold tool steel because of the decreased possibilities of distortion. The prolonged heating during carburization has some effect upon the size of the piece of steel. For this reason, much experience and careful consideration must be employed to make shrinkage and growth allowances.
It is difficult for example, to translate the steel manufacturers’ data on shrinkage and growth during carburizing and heat treating 1″ rounds into figures which will allow for every part of a mold for a typewriter housing. By using a hardening steel of the high alloy type, the long heating time for carburizing is eliminated, and some of the factors which affect distortion are thereby eliminated. Of course, the tool steels require careful handling, and some high speed steels require a considerable amount of heating time.
In such cases, the advantage of minimum time in the furnace is not always clear cut. Some mold tooling applications require the use of tool steels because carburizing steels are not applicable. Hobs, for example, must be extremely hard at the surface, and the hardness should extend rather deeply into the center in order to make the hob resistant to the severe crushing which it must stand, and to have high tensile strength, as well. Hobs may be made of air hardening, water hardening, or oil hardening tool steels. Oil hardening is usually preferred, with water hardening steel the least used, and then generally only for small, simple pieces not critically subject to cracking in heat treatment, nor subject to heavy nobbing pressures