Tool Steel Weld Filler Metals
A tool steel repair seldom runs on a standard covered electrode. The deposits that match a tool steel in service are proprietary products, and the choice between one of those, a carbon or low-alloy steel wire and a nonhardenable austenitic wire is set by where the deposit sits on the tool and what the tool has to do afterwards. This page sets out the three filler metal categories, the four selection factors, and the weld deposit compositions that are used across the tool steel groups.
Four factors decide the filler metal
A filler metal for a tool or die is selected against four things. They are the composition of the base metal, the heat treated condition of the base metal, whether that is annealed or hardened and tempered, the service requirements of the weld deposit, and the post-weld heat treatment the part will receive. The heat treated condition carries as much weight as the composition, because the same grade welded annealed and welded at 52 HRC are two different jobs.
The three categories of filler metal
The first category produces weld deposits that correspond in hardenability or in composition to the basic tool steel types, the water hardening, oil hardening and air hardening cold work grades, the hot work grades and the high speed grades. These deposits come hard in the as-welded condition, and they are not covered by the standard filler metal specifications, which is why they are bought as proprietary products under a producer name rather than under an AWS classification. The table below gives the nominal composition of the deposits in this category.
| AISI steel type | Filler-metal type | C | Mn | Si | Cr | Ni | V | W | Mo |
|---|---|---|---|---|---|---|---|---|---|
| W1, W2 | Water-hardening | 0.95 | 0.30 | 0.20 | . . . | . . . | 0.20 | . . . | . . . |
| S1, S5, S7 | Hot-work | 0.33 | 0.40 | 1.00 | 5.00 | . . . | . . . | 1.25 | 1.35 |
| O1, O6 | Oil-hardening | 0.92 | 1.28 | 0.30 | 0.50 | . . . | . . . | 0.50 | . . . |
| A2, A4, D2 | Air-hardening | 0.95 | 0.40 | 0.30 | 5.25 | . . . | 0.25 | . . . | 1.10 |
| H11, H12, H13 | Hot-work | 0.33 | 0.40 | 1.00 | 5.00 | . . . | . . . | 1.25 | 1.35 |
| M1, M2, M10, T1, T2, T4 | High-speed | 0.90 | 0.30 | 0.35 | 4.00 | . . . | 1.00 | 1.50 | 8.00 |
Source, Table 19 of Welding of Low-Alloy Steels, in ASM Handbook Volume 6, Welding, Brazing and Soldering. Compositions of similar types of weld metal from different producers can vary.
Composition data is for general reference only. Actual values vary by standard, mill, and heat number. Confirm against the material test certificate (MTC) or contact Aobo Steel.
The second category runs the other way on matching. It covers carbon and low-alloy steel deposits with moderate hardness and toughness as welded or stress relieved. Some of them respond to a quench and temper cycle, and most are intended for work where the deposit does not have to match the hardened tool steel. These grades sit inside the ANSI and AWS specifications for carbon steel and low-alloy steel filler metals, so they are stocked rather than made to order.
The third category is nonhardenable. Austenitic stainless steels, the nickel-chromium-iron, nickel-copper and copper-nickel alloys all fall here. Their value is ductility. When a tool that is already hardened has to be repaired, a ductile deposit resists both weld metal cracking and base metal cracking far better than a hard deposit does, and the same wires are useful for building up worn tooling before a hard surface is put on top. A nonhardenable deposit can be overlaid with a hardfacing deposit to restore a hard, wear resistant surface over it.
Matching hardenability rather than matching composition
On many tool steel welds the requirement is that the weld metal and the base metal respond to heat treatment in a similar way, so that the weld metal finishes at the same hardness as the base metal after a quench and temper. That usually means matching the composition of the base metal, and it does not have to. A deposit carrying less carbon and more alloy can reach the same final hardness or strength, and dropping the carbon in the weld metal generally lowers the risk of weld cracking and improves the toughness of the deposit. Where the goal is a hard, wear resistant deposit that survives, the carbon content of the deposit is the number to watch.
Where the deposit sits sets how strict the match has to be
A weld that lands in a functional area of the tool, on a cutting edge or a wear surface, has to be selected for hardness. The deposit must reach the hardness or strength the surface needs either as welded or after a stress relief or temper, without a separate hardening treatment, because a finished tool often cannot be quenched again without losing its dimensions. A weld that lands away from the working area is a different problem. There the deposit only has to hold the tool together, so it can be chosen for maximum crack resistance, ductility and toughness instead of for hardness. On a repair of a worn tool or a tool built up from components, that freedom is worth taking.
Where a soft deposit is used to fill volume under a hard one, the tool steel layer above it still has to be thick enough to absorb dilution from the layer underneath and to keep the wear properties it was chosen for. Dilution is the reason a shallow deposit rarely performs the way the filler metal data sheet suggests.
Source: ASM Handbook, Volume 6, Welding, Brazing and Soldering, ASM International, 1993.