Steam Treatment Tool Life Improvement for M2, M7 and A6 Tools
Steam treatment puts a thin black oxide layer on a finished tool, and the layer changes what happens at the cutting edge and on the working face of a punch. It is bought for tool life, not for appearance. The two tables below record what that layer was worth in production, tool by tool, for high-speed steel cutting tools and for the punches and trim dies used in nut and bolt forming. This is a reference summary of published practice and not a specification of ours.
What steam treatment does
Steam treatment is an oxide coating formed out of the steel itself rather than a layer deposited on top of it. The finished tool is held in steam at about 565 °C (1050 °F), and a black oxide layer builds on the surface, typically less than 5 micrometres (0.2 mil) thick. The layer is hard, it stays on the tool under service conditions, and it is the reason the tool life values below move.
Hot work dies reach a similar result on their own. An adherent black oxide scale forms during heat treatment, and that scale resists abrasion and holds die lubricant on the working face. If the scale is ground or polished off after heat treatment, the die can be reheated in air to a temperature below its final tempering temperature so the oxide forms again.
The same type of layer can be produced from salt instead of steam. Heating in liquid sodium hydroxide and sodium nitrate salts for 5 to 20 minutes at about 140 °C (285 °F) also builds a hard oxide layer on high-speed steel cutting tools. Both routes are applied to high-speed steels and to hot work tool steels, and both are judged on the tool life they return.
Tool life gains on cutting tools
The first table covers cutting operations on M2, M7 and A6 tools. The operations are production jobs, not laboratory tests, and they run from broaching and drilling through hobbing, milling, sawing and tapping. The gains recorded in the source run from a little under twice the previous life to about six times, with one entry reported as an endurance percentage rather than a count of parts.
Results by tool and workpiece material
| Tool | Application | Tool life before steam treating | Tool life after steam treating |
|---|---|---|---|
| M2 broachers | Cutting AISI 1010 latch | 20 h per grind | 70 h per grind |
| M2 drills | Drilling Bakelite plastic insulating blocks | 10 holes | 25 holes |
| M2 drills | Phenolic terminal plates | 1700 holes per grind | 8500 holes per grind |
| M2 drills | Drilling AISI 4030 steel 25 mm (1 in.) thick | 17 holes | 81 holes |
| M7 end mill tools | Cutting 8740 steel forgings | 30 pieces | 200 pieces |
| A6 hobs | Cutting teeth on AISI 3140 forged gear | not stated in the source | 62.2% increased life |
| M2 milling cutters | Two slots in 1020 steel | 150 cuts per grind | 306 cuts per grind |
| M2 milling cutters | Slotting 1020 steel bars | 2000 per grind | 7000 per grind |
| M2 saw blades | Cutting 75 mm (3 in.) rods, austenitic steel | 100% endurance at 0.52 m/s (102 sfm) | 120% endurance at 0.57 m/s (112 sfm) |
| M2 taps | Cutting SAE 52100 steel | 1800 pieces | 3000 pieces |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 16-1, Machining tool life improvements due to steam oxidation, printed page 307. The source prints the three M2 drilling applications on separate lines, each with its own tool life values, and gives the A6 hob entry as a percentage gain with no baseline life. Written as printed, including the spelling broachers. This is a reference summary of published practice and not an Aobo Steel specification.
Tool life gains on forming punches and trim dies
The second table is a different family of tooling. These are M2 punches and a trim die run in nut and bolt forming, where the failure mode is wear and galling on the working face rather than cutting edge breakdown, and the count of parts per tool is the only number the shop tracks.
Results in nut and bolt forming
| M2 tool | Application (steel type) | Tool life before steam treating (a) | Tool life after steam treating (b) |
|---|---|---|---|
| 4th station punch | Castle nut (1030) | 21,000 nuts | 42,000 nuts |
| 4th station punch | Slotted insert nut (1030) | 22,000 nuts | 38,000 nuts |
| 4th station punch | Castle nut (1030) | 29,000 nuts | 80,000 nuts |
| 3rd station punch | Castle nut (1110) | 20,000 nuts | 35,000 nuts |
| 4th station punch | Castle nut (1110) | 15,000 nuts | 35,000 nuts |
| Trim die | Bolt head (1335) | 7,000 bolts | 16,000 bolts |
Source, Tool Steels (G. A. Roberts and G. Krauss), Table 16-2, Effect of steam oxidation on tool life in forming various carbon steel nuts and bolts, printed page 307. In the source, (a) hardened and triple tempered and (b) hardened, triple tempered and steam treated. All punches in the table are M2. This is a reference summary of published practice and not an Aobo Steel specification.
How the two tables are compared
Both tables report the same comparison. Each tool was run to failure in the condition the shop normally used, then re-run after the oxide layer had been applied, and the counts are parts, holes, cuts or hours per tool between regrinds or replacements. Nothing else was changed between the two runs, so the difference in the two columns is the effect of the treatment and not the effect of a change in speed, feed or tool material.
Two entries need a word of caution before they are used as a planning figure. The A6 hob entry is printed as a percentage gain only, so there is no baseline tool life to work from. The M2 saw blade entry is reported as endurance at a given surface speed, with the speed itself changing between the two runs, which makes it a statement about performance at the higher speed rather than a like for like count.
Related reference data
M2 is the grade that carries almost all of this data. Its composition, its heat treatment and the hardness it reaches are on M2 tool steel, and the standards it is bought against are listed on M2 high speed steel specifications. The cutting tools in the first table are described by operation on high speed steel for cutting tools, hobs and broaches, and the punches and trim dies in the second table sit with the rest of the forming tooling on tool steel for forming, drawing and bending dies.
Compiled from Tool Steels (G. A. Roberts and G. Krauss), Table 16-1 and Table 16-2, on printed page 307, in the surface modification chapter. Both tables were read from the page image and cross-checked against the searchable text of the file. The tables are reproduced as published and are a reference summary rather than an Aobo Steel specification, so confirm the treatment cycle for the grade and the application before it is written into a process sheet.