Tempering

Part of Machine Tools

Reducing brittleness in hardened steel by controlled reheating — the essential second step of heat treatment.

Why This Matters

A freshly hardened steel tool is as hard as it will ever be — and also as brittle. Glass-hard martensite has very little ductility. A hardened chisel struck firmly on stone chips; a hardened drill dropped on concrete shatters; a hardened lathe tool catches an interrupted cut and snaps. Tempering solves this by sacrificing a small amount of hardness in exchange for a large improvement in toughness (resistance to fracture).

The tempering step is what distinguishes a useful tool from a dangerous one. Every hardened steel tool must be tempered immediately after quenching. The exact tempering temperature determines the final hardness-toughness balance — and the correct temperature depends on the tool’s application, its cross-section, and the steel’s carbon content.

Understanding tempering enables you to make tools that hold up in real use rather than tools that look right but break at first hard application.

The Tempering Mechanism

During hardening, carbon atoms are trapped in a stressed, supersaturated crystal lattice — this is martensite. It is hard because the carbon prevents dislocation movement (the mechanism of plastic deformation). But it is brittle because the lattice stress leaves it with no ability to absorb impact without fracture.

Tempering heats the martensite moderately (150-600 degrees C), which allows carbon to diffuse slightly out of the martensite lattice and form small carbide particles. The result — tempered martensite — retains most of the hardness but develops significant toughness because the lattice stress is relieved and the fine carbides actually impede fracture propagation.

Higher tempering temperatures produce softer, tougher steel. Lower temperatures produce harder but more brittle steel. The correct temperature is a compromise appropriate to each tool’s service conditions.

Temperature Colors on Bright Steel

Before pyrometers, smiths judged tempering temperature by the oxide color that develops on a bright steel surface as it heats. This is highly practical and accurate enough for tool making. Polish the hardened tool to bright metal (file or sand off any scale), then apply gentle heat:

• 220 degrees C: Faint straw yellow — for scrapers, razors (maximum hardness)
• 240 degrees C: Straw yellow — for engravers, fine gauges
• 250 degrees C: Dark straw — for plane irons, chisels, cold chisels
• 260 degrees C: Bronze/brown — for drill bits, wood chisels
• 270 degrees C: Purple-brown — for lathe tools, center punches
• 280 degrees C: Purple — for twist drills, springs
• 300 degrees C: Blue — for springs, flexible tools
• 350+ degrees C: Gray-blue, then gray — approaching annealed

The color progresses uniformly from the heated end toward the edge of the tool. The moment the correct color reaches the cutting edge, quench the tool in water or oil. The quench stops the progression and locks in that temper color.

Tempering Methods

The traditional oxide-color method: Polish the tool to bright metal immediately after hardening (while still warm, before full scale forms). Apply heat to the body of the tool, away from the edge, using a rod heated in a forge or a gas torch. Watch the colors travel toward the cutting edge. Quench when the desired color reaches the edge.

This works best on small tools — chisels, plane irons, small drills. For large tools, the gradient is steeper and control is easier.

Oven tempering: Place the hardened tool in an oven or furnace at the target temperature for 30-60 minutes, then air cool. This produces the most uniform temper because the entire tool reaches the same temperature. Requires an oven with reasonably accurate temperature control — a wood-fired kiln with a pyrometer, or a modern kitchen oven (typically accurate to plus-or-minus 15 degrees C, adequate for tempering).

Oil bath tempering: Immerse the tool in a bath of oil heated to the tempering temperature. Oil has high specific heat and transfers heat to the tool evenly. Traditional smiths used lead or tin baths for more accurate temperature control (lead melts at 327 degrees C, so a lead bath cannot exceed this temperature — convenient for blue tempering).

Specific Temperatures by Tool Type

The following temperatures produce good results with high-carbon tool steels (0.8-1.0% carbon):

Lathe and planer tools: 200-220 degrees C (straw) for HSS tools that need maximum hardness; 250-260 degrees C (bronze) for carbon steel lathe tools that will see interrupted cuts.

Cold chisels and punches: 260-280 degrees C (purple) — need toughness to absorb hammer blows without chipping.

Woodworking chisels and plane irons: 230-250 degrees C (dark straw to bronze) — need a hard edge but not extreme toughness.

Drills: 260-280 degrees C (purple to purple-blue) — must resist both cutting forces and occasional breakage.

Springs: 300-350 degrees C (blue) — maximum flexibility, minimum brittleness.

Knives and axes: 220-240 degrees C for fine knives (straw, holds a keen edge); 260-280 degrees C for axes (purple, more impact-resistant).

Double Tempering

For critical tools — especially complex shapes or high-alloy steels — double tempering is recommended. Temper at the target temperature, allow to cool completely to room temperature, then temper again at the same temperature. The second cycle relieves stresses introduced during the first cooling and ensures any retained austenite (which may transform to martensite during the first cooling) is also tempered.

Double tempering adds little extra time and significantly improves reliability in service. All cutting tools that will be used for precision metalwork should be double-tempered.