Tempering

Part of Metalworking

Controlled reheating of hardened steel to reduce brittleness while retaining useful hardness.

Why This Matters

A freshly hardened steel blade is as hard as glass — and just as fragile. Drop it on a stone floor and it shatters. Strike it against a hard knot in wood and the edge chips. Use it as a pry bar and it snaps. Hardened steel without tempering is dangerous to the user and useless for any task involving impact, flexing, or vibration.

Tempering is the essential second step of heat treatment that transforms glass-hard, glass-brittle martensite into a material that is still hard enough to hold an edge but tough enough to survive real-world use. Without tempering, every knife, axe, chisel, spring, and saw you harden will fail in service — often violently, sending sharp fragments at high speed.

The process is simple: gently reheat the hardened steel to a specific temperature (well below the hardening temperature), hold it there briefly, then cool it. The temperature you choose determines the final balance between hardness and toughness. This makes tempering the most critical skill in the blacksmith’s repertoire — it is where you dial in the exact properties needed for each specific tool.

The Science of Tempering

What Happens Inside the Steel

When steel is quench-hardened, the carbon atoms are trapped in a crystal structure (martensite) that is extremely hard but under enormous internal stress. Every carbon atom is wedged into a space slightly too small for it, distorting the surrounding iron lattice.

When you reheat this steel gently:

• At 100–200°C: Internal stresses begin to relax. Very slight toughness improvement, minimal hardness loss.
• At 200–300°C: Carbon atoms begin migrating short distances, forming tiny clusters of iron carbide (cementite) within the martensite. This relieves more stress, significantly improving toughness while gradually reducing hardness.
• At 300–400°C: Substantial carbide formation. The steel becomes noticeably tougher and more flexible but loses significant hardness.
• At 400–600°C: The martensite transforms back toward pearlite. The steel becomes very tough but approaches the hardness of un-heat-treated steel.
• At above 600°C: Essentially fully annealed — all hardening is lost.

The art of tempering is choosing the exact temperature that gives you the ideal compromise for each application.

Tempering by Oxide Colors

Before thermometers, blacksmiths judged tempering temperature by the color of the thin oxide film that forms on polished steel as it heats. This method is remarkably accurate and remains the primary technique for field tempering.

Procedure

1. Start with a fully hardened workpiece — file-hard after quenching
2. Polish a section to bare metal — sand or file a flat area until it is bright, mirror-like. The oxide colors are only visible on polished steel.
3. Heat slowly and evenly. Methods:
   • Hold the workpiece above a bed of coals (not in them — you need gentle, controllable heat)
   • Heat the thick spine/body and let heat conduct to the edge
   • Place on a thick steel plate heated from below
   • Use an oven if available
4. Watch the polished surface for oxide colors appearing and traveling across the steel
5. Quench immediately when the desired color reaches the working surface (the edge, for a blade)

The Oxide Color Chart

Color	Temperature	Hardness (HRC)	Applications
Very pale straw	200°C	62–64	Razors, engraving tools, surgical scalpels
Light straw	220°C	60–62	Woodworking chisels, plane irons, drill bits
Dark straw / gold	240°C	58–60	Knives, wood chisels, scissors
Brown	255°C	56–58	Axes, cold chisels, large shears
Purple-brown	265°C	54–56	Punches, drift pins, stone-cutting tools
Purple	275°C	52–54	Sword blades, springs, saw blades
Dark purple / blue-purple	285°C	50–52	Clock springs, flexible rules
Blue	300°C	48–50	Screwdrivers, wrenches, spring clamps
Dark blue	310°C	46–48	Too soft for most cutting — nearing annealed
Grey-blue	320°C	44–46	Not useful — over-tempered

Lighting Matters

Oxide colors are subtle and easily misjudged in poor light, firelight, or direct sunlight. Work in diffused daylight or consistent artificial light. Firelight is especially deceptive — it masks the yellow-to-brown transition where most tool tempering happens.

Reading the Colors in Practice

The colors appear as bands that travel across the steel from the heat source. If you are heating a knife blade from the spine:

1. The spine turns straw first
2. The color band moves toward the edge as heat conducts through the steel
3. When the straw/gold color reaches the edge — quench

This means the spine ends up tempered to a higher temperature (softer, tougher) while the edge is tempered to a lower temperature (harder) — which is exactly what you want in a knife blade.

Differential Tempering

Deliberately heat from the spine to temper differentially. By the time the edge reaches straw (240°C), the spine may be at purple or blue (275–300°C). This gives you a hard edge with a tough, flexible spine — a single operation that optimizes the entire blade.

Tempering Methods

Open Forge Tempering

The traditional method. Hold the workpiece above coals or in the edge of a fire, rotating slowly for even heating. This requires skill and attention but works well for experienced smiths.

Advantages: No special equipment, immediate feedback via oxide colors Disadvantages: Difficult to control precisely, uneven heating, requires constant attention

Oven Tempering

If any oven with temperature control is available (wood-fired bread oven with a thermometer, gas oven, electric oven):

1. Set to desired temperature
2. Place the workpiece inside
3. Hold at temperature for 1 hour per 25 mm of thickness (minimum 1 hour)
4. Turn off oven and allow to cool inside

Advantages: Extremely uniform, reproducible results, no skill required Disadvantages: Requires an oven with temperature control

Double Tempering

For critical tools (surgical instruments, precision cutters, springs), temper twice. After the first cycle, let the workpiece cool completely to room temperature, then temper again at the same temperature. The second cycle converts any retained austenite that the first cycle missed, producing a more stable and uniform structure.

Sand Bath Tempering

A middle ground between forge and oven:

1. Fill a metal container with clean, dry sand
2. Heat the container over a fire
3. Monitor temperature by placing a piece of polished steel on the sand surface and watching oxide colors
4. When the sand reaches the desired temperature, bury the workpiece in it
5. Hold for 30–60 minutes
6. Remove and let cool

Advantages: More uniform than direct forge heating, no oven required Disadvantages: Temperature control is approximate

Oil Bath Tempering

For precise, repeatable results:

1. Fill a deep metal container with high-temperature cooking oil (not motor oil for this application — it smokes excessively)
2. Heat the oil over a fire to the desired tempering temperature
3. Monitor with a thermometer if available, or use a polished steel test strip
4. Submerge the workpiece for 30–60 minutes
5. Remove and let cool

Advantages: Very uniform, precise temperature control, good for batch processing Disadvantages: Fire risk at higher temperatures, limited to ~300°C with most oils

Oil Fire Safety

Oil baths above 250°C approach the smoke point of most cooking oils. Use a deep container, keep a metal lid ready to smother flames, and never leave unattended. At 300°C+, use mineral oil (if available) which has a higher flash point.

Application-Specific Tempering

Knives and Blades

• Edge: 220–240°C (straw to gold) — hard enough to hold a keen edge
• Spine: 275–300°C (purple to blue) — flexible enough to absorb shock
• Method: Heat from spine, watch colors travel to edge, quench when edge reaches straw

Chisels and Punches

• Tip: 240–260°C (gold to brown) — hard enough to cut, tough enough for hammer blows
• Body: 300°C (blue) — absorbs shock from hammering
• Method: Heat the body and let color travel to tip; quench when tip reaches desired color

Springs

• Entire piece: 275–300°C (purple to blue) — uniform throughout
• Springs need to flex repeatedly without taking a permanent set. This requires toughness over hardness.
• Method: Oven temper for maximum uniformity. Double temper for critical springs.

Saws

• Teeth: 260–275°C (brown to purple) — hard enough to cut, tough enough not to chip
• Blade body: 300°C (blue) — flexible enough to bend without breaking
• Method: Set saw teeth into water or wet sand while heating the blade body. Or temper the whole blade at 300°C, then selectively harden only the teeth by heating them individually with a torch.

Drill Bits

• Tip: 220–230°C (pale to light straw) — very hard for cutting
• Shank: 300°C (blue) — tough to resist twisting forces
• Method: Heat from shank end. Quench when tip reaches pale straw.

Troubleshooting

Problem	Symptom	Cause	Solution
Edge rolls over in use	Visible deformation at edge	Tempered too high	Re-harden and temper at lower temp
Edge chips in use	Small pieces break from edge	Tempered too low	Re-temper at 10–20°C higher
Blade snaps under load	Catastrophic fracture	Not tempered at all, or tempered too low	Always temper immediately after hardening
Uneven hardness	Hard spots and soft spots	Uneven tempering heat	Use oven or sand bath for uniformity
Spring takes a set	Permanently bent after flexing	Tempered too high, or insufficient carbon	Re-harden and temper at lower temp (purple, not blue)
Tool shatters on first use	Explosive failure	Skipped tempering entirely	This is a safety failure — never skip tempering

Record Keeping

Tempering is empirical. What works for one batch of steel may not work identically for another because carbon content, grain size, and alloy composition all affect the ideal tempering temperature. Keep records:

• Steel source (spring steel, file steel, bloomery steel, etc.)
• Hardening temperature (color / magnet test result)
• Quench medium (oil, water, brine)
• Tempering color / temperature
• Performance in use (did the edge hold? Did it chip? Did it bend?)

Over time, these records become your community’s metallurgical reference — a body of empirical knowledge that lets you consistently produce tools with optimal properties from whatever steel you have available.