Materials for Trimming and Punching Dies in Hot Forging

What the two operations do

Trimming removes the flash that the forging operation produced, and it can also take off some of the draft material so the part ends up with straight sidewalls. It is normally done with a top die and a bottom die shaped to the contour of the part. The top die acts as a punch and pushes the part down through the cutting edge in the lower die. If the top die does not follow the contour of the part, the part can be deformed while it is trimmed.

Punching is the same idea applied to excess material on an internal surface. To keep the cuts accurate, punching and trimming are often performed at the same time.

What decides the material

The published selection rests on two things: what is being trimmed, and whether the part is trimmed while hot or while cold. Punches are normally made from proprietary tool steels when carbon and stainless steels are to be trimmed, and from 1020 steel that has been hard faced when nonferrous alloys are to be trimmed. The trimming die, or bottom die, is made from D2 tool steel, or from cold rolled steel with a high strength alloy hard facing applied to the cutting edge.

Trimming a part straight off the press saves a second heating operation, but the two cold trim columns in the table below are not always available. For stainless steels and heat-resisting alloys both of them read “generally hot trim”, and where a close trim is wanted on carbon and alloy steels the table falls back on hot trimming as well.

Typical materials for trimming and punching dies

The table gives the published selection for the three groups of work metal, split by the type of trim. Where more than one grade appears in a cell, the source lists them without an order of preference.

Typical materials for trimming and punching dies. Source, Table 3 of Dies and Die Materials for Hot Forging, in ASM Handbook, Volume 14, Forming and Forging. Hardness figures are working hardness as published. (a) Both normal and close trimming. (b) Hard facing alloy 4A has a nominal composition of Co-1C-30Cr-4.5W-3Ni-1.5Fe. The source prints the third row label as “Aluminum, magnesium, and”; only aluminum and magnesium are named in that row.

How to read the table

For carbon and alloy steels trimmed cold, the punch in the table is a low-alloy block steel at a bulk hardness of 341 to 375 HB, and the blade is D2 at 54 to 56 HRC. That split puts the load into a steel that is tough rather than hard, and the hardness where the cut is actually made.

Once the part is trimmed hot, or once the work metal is stainless or heat-resisting, the punch moves to 6F2 or 6G at 341 to 375 HB for carbon and alloy steels, and to 6F2 to 6G at 388 to 429 HB for the stainless and heat-resisting group. The blade goes to D2 at 58 to 60 HRC, or to hard facing alloy 4A laid on 1035 steel where the edge has to be rebuilt rather than replaced.

The nonferrous row works the other way round. Cold normal trimming uses a 6150 punch at 461 to 477 HB against a blade of hard faced 1020 steel or O1, and close trimming puts D2 on both sides at 58 to 60 HRC. Hot trimming of the same group uses a soft 1020 punch and a hard faced blade, because the punch is there to push the part through and not to cut it.

The block steels named in the table

The 6F2 and 6G designations are the old ASM numbers for a group of low-alloy nickel-chromium-molybdenum die steels. They were never given AISI, SAE or UNS designations, so the ASM numbers from the 1948 edition of Metals Handbook are still the ones in use. Their value in a trimming die is toughness and shock resistance rather than hot hardness: they are tempered at lower temperatures than the H series and are chosen for holders, die blocks and punches where the surface does not get very hot.

Where the edge itself has to keep its hardness at temperature, the table uses D2, and where the edge has to survive impact it uses hard facing alloy 4A, a cobalt-base deposit with a nominal composition of Co-1C-30Cr-4.5W-3Ni-1.5Fe.

Grades and stock

The cold trimming side of the table is cold work tool steel, and the two grades that carry it are on the site: the D2 tool steel heat treatment guide covers the 54 to 60 HRC working range used here, and the O1 page covers the oil-hardening grade named for nonferrous trimming.

For the selection behind the table, the trimming die selection guide sets out the cold trimming choices, and the hot trimming selection page takes the same question for parts trimmed while hot. The forgings themselves are covered on the hot forging, stamping and trimming die page.

Hard faced edges are a repair and rebuild question as much as a grade question. The deposits and the procedure are set out on the tool steel weld filler metal page.

Source: ASM Handbook, Volume 14, Forming and Forging (9th Edition Metals Handbook), ASM International, 1988.