Tool Steel

Part of Machine Tools

Selecting, identifying, and working with high-carbon and alloyed steels for cutting tool production.

Why This Matters

Not all steel is tool steel. Mild steel (low carbon) bends and deforms — it cannot hold a cutting edge because it is too soft. Tool steel (high carbon, often with alloying elements) can be hardened to the point where it cuts other metals, holds its edge through thousands of cuts, and maintains shape under the high stresses of machining.

In a rebuilding scenario, tool steel is often scavenged before it is made from scratch. Old files, springs, axles, and machine parts provide excellent steel if you can identify and work with it. Later, when iron smelting and refining are available, tool steel can be produced by carburization (adding carbon to iron) or by working with high-quality iron sources.

Understanding tool steel enables you to make all the cutting tools needed for further manufacture — the cornerstone capability of the machine age.

Categories of Tool Steel

Plain carbon tool steel (W-grade in modern classification): The oldest and simplest tool steel. Carbon content 0.7-1.4%. Hardened by water quench. Works well for most hand tools and low-speed machining. Sensitive to overheating during grinding (loses temper). Available from springs, old files, high-carbon bar stock.

Oil-hardening tool steel (O-grade): Slightly alloyed (manganese, chromium, silicon) for oil quench rather than water quench — less distortion, fewer cracks. Tougher than plain carbon steel. Used for dies, punches, and tools requiring toughness.

Air-hardening steel (A-grade): Highly alloyed, hardens in air cooling. Very stable dimensionally during hardening. Excellent for precision gauges and dies.

High-speed steel (HSS, M-grade): Contains tungsten, molybdenum, chromium, and vanadium. Retains hardness at red heat (600 degrees C) — can cut at much higher speeds than plain carbon steel without losing its edge. The key breakthrough of 20th-century machining. Identified by the red-hardness test: heat to red and allow to air cool — HSS remains hard; carbon steel becomes dead soft.

Stellite and cemented carbide: Even harder alloys (not steels) that enabled modern high-speed machining. Require specialized production but can extend tool life dramatically.

Identifying Tool Steel in the Field

When you cannot identify steel grade from documentation, use these tests:

Spark test: Hold the steel against a running grinding wheel. Observe the spark stream under shade:

• Low carbon (mild steel): Few long sparks, little branching, red-orange color
• Medium carbon (0.4-0.6%): More sparks, slight branching near ends
• High carbon (0.8-1.0%): Many bright yellow sparks with strong branching (the signature of carbon burnout)
• Very high carbon (1.2%+): Dense spray of fine, highly branched yellow sparks
• High-speed steel: Red sparks with very little branching (alloying elements suppress carbon sparks)

Hardening test: Quench a small sample. If it hardens (file test — file skids off), it is hardenable. If the file cuts through easily, it is low carbon.

Bend test on hardened sample: A hardened carbon steel sample snaps cleanly when bent — it is brittle. A hardened spring steel flexes slightly then snaps. This distinguishes different carbon contents.

Sources in a Rebuilding Scenario

Before tool steel can be produced from scratch, it must be sourced from existing stock:

Old files: Excellent high-carbon steel (1.0-1.2% C). The file itself is already shaped but can be re-forged at high heat (bright orange) into blanks for chisels, knives, and scrapers. Work quickly — files decarburize rapidly in the fire.

Coil springs and leaf springs: Medium to high carbon (0.6-0.85%). Good for knives, axes, springs, and chisels. Leaf springs from trucks and cars are particularly useful — large cross-section, already tested for consistent quality.

Ball bearings and roller bearings: High-carbon chrome steel (52100 equivalent) — excellent material for lathe centers, precision tools, and sharp cutters. Balls can be used as-is for burnishing; races can be forged into small tools.

Axles and gears: Usually medium carbon, some case-hardened. Test with spark test before investing effort.

Production from Scratch: Cementation

When scavenged tool steel is exhausted, it can be produced from wrought iron through cementation: packing iron pieces in charcoal in a sealed clay pot and heating to 900-1000 degrees C for 12-24 hours. Carbon diffuses into the iron surface, producing a high-carbon layer. Folding and forge-welding multiple times distributes the carbon more evenly — this is blister steel and shear steel, the traditional precursors to modern tool steel.

The result is variable in carbon content but functional for chisels, axes, and general edge tools. Consistent high-quality tool steel requires more controlled processes (crucible steel, later Bessemer and open-hearth refining), but cementation steel sufficed for all pre-industrial toolmaking and can do so again.