Surface hardening: carburising, nitriding, induction hardening

Why it matters

Gears, camshafts, crankshaft journals and bearing races need a surface hard enough to resist wear and contact fatigue, but a core tough enough to survive shock loads. Through-hardening cannot give both; surface hardening can. It also puts the surface into residual compression, which is why case-hardened parts have much higher fatigue strength.

Key ideas

There are two families:

• Thermochemical (diffusion) methods change the surface chemistry: carburising, nitriding, carbonitriding, cyaniding, boriding.
• Thermal (selective) methods do not change chemistry; they austenitise only the surface of a steel that already has enough carbon and quench it: flame, induction, laser and electron-beam hardening.

Carburising. A low-carbon steel (about 0.1–0.25 % C, often alloyed with Ni, Cr, Mo) is held in a carbon-rich atmosphere in the austenite range, typically 900–950 °C, where carbon solubility is high. Carbon diffuses inward to give a surface of about 0.8–1.0 % C. The part is then quenched (directly or after reheating) and tempered at 150–200 °C: the high-carbon case becomes hard martensite (about 58–62 HRC) while the low-carbon core stays tough.

• Pack carburising: parts packed in charcoal plus an energiser such as BaCO₃; simple, slow, poor control.
• Gas carburising: endothermic gas enriched with methane or propane; the industrial standard, with controlled carbon potential.
• Liquid (salt-bath) carburising: cyanide-based salts; fast but toxic.
• Case depths 0.5–2 mm; depth grows with √time, so doubling the depth needs four times the time. Areas that must stay soft are masked with copper plating or stop-off paste.

Nitriding. Nitrogen is diffused into a finished, already hardened-and-tempered alloy steel at 500–570 °C, below A1, so the steel stays ferritic and no quench is needed. Hardness comes from fine nitrides of Al, Cr, Mo and V, so special nitriding steels (e.g. containing Al and Cr) give the highest hardness (about 900–1200 HV, harder than a carburised case). Advantages: minimal distortion (finish-machined parts can be treated), excellent wear and fatigue resistance, hardness retained up to about 500 °C. Drawbacks: long cycles (20–90 h for gas nitriding in ammonia), thin cases (typically 0.1–0.6 mm) and the need for nitride-forming alloy steels. Plasma (ion) nitriding and salt-bath nitrocarburising are faster variants.

Carbonitriding and cyaniding. Both add carbon and nitrogen together. Carbonitriding uses a gas atmosphere with ammonia at about 800–870 °C; nitrogen raises hardenability, allowing an oil quench and lower distortion, but cases are thin (0.1–0.75 mm). Cyaniding uses a molten cyanide bath at a similar temperature for small parts.

Flame hardening. An oxy-acetylene flame heats the surface above A3 and a water spray quenches it. Used on medium-carbon steels (0.35–0.6 % C) and cast irons for large parts such as lathe beds and big gears. Cheap, flexible, but less controlled.

Induction hardening. A coil carrying high-frequency alternating current induces eddy currents in the part. Because current crowds toward the surface (skin effect), only a surface layer heats, in seconds, and is then spray-quenched. Higher frequency gives a shallower case. Fast, clean, repeatable and easy to automate, it suits shafts, axles, crankshaft pins and gear teeth of medium-carbon and low-alloy steels (0.35–0.55 % C). Short heating times need a slightly higher austenitising temperature.

Laser and electron-beam hardening. A scanned beam heats a very thin surface layer; the cold bulk of the part quenches it (self-quenching), giving minimal distortion and precise local hardening.

Choosing a method.

• Heavy contact loads and deep case → carburising (gears).
• Minimal distortion, finished part, high temperature in service → nitriding.
• Medium-carbon steel, selective areas, high production → induction.
• Very large parts, few off → flame.

Formulas

(Cx − C0) / (Cs − C0) = 1 − erf( x / (2·√(D·t)) ) Fick's second law for a semi-infinite solid with constant surface concentration: Cx = carbon at depth x (wt %), C0 = initial carbon, Cs = surface carbon set by the atmosphere, D = diffusivity (m²/s), t = time (s), erf = error function.

x ≈ √(D·t) (order of magnitude) ; x ∝ √t at a fixed temperature For a fixed concentration ratio, depth grows with the square root of time, so x₁² / t₁ = x₂² / t₂.

D = D0·exp(−Q / (R·T)) Diffusivity: D0 = pre-exponential factor (m²/s) and Q = activation energy (J/mol), from data; R = 8.314 J/(mol·K); T in K.

δ = 503·√(ρ / (μr·f)) Induction skin (penetration) depth δ (m); ρ = resistivity of the hot steel (Ω·m); μr = relative permeability (μr = 1 above the Curie temperature, 768 °C); f = frequency (Hz).

Worked examples

Example 1 (standard): carburising time from Fick's law. Given: steel with C0 = 0.25 % C; surface held at Cs = 1.20 % C; required 0.80 % C at x = 0.5 mm; D = 1.6 × 10⁻¹¹ m²/s at the carburising temperature (data).

1. (0.80 − 0.25)/(1.20 − 0.25) = 0.579 = 1 − erf(z), so erf(z) = 0.421.
2. From erf tables, z = 0.392.
3. z = x / (2√(Dt)) → t = x² / (4·D·z²) = (5 × 10⁻⁴)² / (4 × 1.6 × 10⁻¹¹ × 0.392²).
4. t = 2.5 × 10⁻⁷ / 9.83 × 10⁻¹² = 2.54 × 10⁴ s = 7.0 h.
5. For the same carbon level at 1.0 mm, t = 7.0 × (1.0/0.5)² = 28 h.

Example 2 (GATE level): induction frequency. Given: hot steel above the Curie temperature, ρ = 1.1 × 10⁻⁶ Ω·m, μr = 1.

1. At f = 10 kHz: δ = 503 × √(1.1 × 10⁻⁶ / 10⁴) = 503 × 1.049 × 10⁻⁵ = 5.3 mm.
2. At f = 100 kHz: δ = 503 × √(1.1 × 10⁻⁶ / 10⁵) = 1.7 mm.
3. Frequency up by 10 → depth down by √10 = 3.16. For a 1.5–2 mm case on a small shaft, about 100 kHz is suitable; deep cases on large shafts use low frequencies (1–10 kHz).

Common mistakes

• Carburising a medium- or high-carbon steel; the core would then harden and lose toughness.
• Expecting a carburised part to be hard without quenching. Carburising only adds carbon; the hardness comes from the subsequent quench.
• Quenching after nitriding; it is unnecessary and the process temperature is below A1.
• Assuming case depth doubles when time doubles; depth ∝ √t.
• Induction-hardening a low-carbon steel; there is not enough carbon to form hard martensite.
• Forgetting that higher induction frequency means a shallower case.

For GATE PI

Expect matching of processes with temperature ranges, base steels and media; comparisons of carburising, nitriding and induction hardening (distortion, case depth, quench needed or not); and numericals on case depth versus time (√t scaling), Fick's erf solution with a given erf table value, or skin depth versus frequency.

Quick check

1. Why is carburising done in the austenite range?
2. A 0.6 mm case takes 4 h. How long for a 1.2 mm case at the same temperature?
3. Which process gives the hardest surface: carburising or nitriding?
4. What happens to induction case depth if frequency is quadrupled?
5. Which steel suits induction hardening: 0.15 % C or 0.45 % C?

Answers: 1. Austenite (FCC) dissolves much more carbon than ferrite; 2. 16 h; 3. Nitriding; 4. It halves; 5. 0.45 % C.