Heat treatment: annealing, normalising, hardening, tempering

Why it matters

The same bar of 0.45 % carbon steel can be soft enough to machine easily, tough enough for a crankshaft, or hard enough to cut other metals — the only difference is heat treatment. Choosing the right temperature, soaking time and cooling medium is a daily production decision, and heat-treatment faults (soft spots, quench cracks, distortion, brittleness) are among the most common causes of component rejection.

Key ideas

Every heat treatment is a cycle of heating to a set temperature, soaking long enough for the whole section to reach it and transform (a common rule is about 1 h per 25 mm of thickness), and cooling at a controlled rate. The temperatures come from the Fe–Fe₃C diagram (A1, A3, Acm); the cooling rate decides the product through the CCT diagram.

Annealing (slow furnace cooling).

• Full annealing: heat hypoeutectoid steels 30–50 °C above A3 (hypereutectoid steels 30–50 °C above A1), soak, cool in the furnace. Gives coarse pearlite and ferrite (or cementite): the softest, most ductile condition, used before machining or cold forming and to remove a coarse cast or welded structure. Hypereutectoid steels are not heated above Acm because slow cooling would then form a brittle grain-boundary network of cementite.
• Process (subcritical) annealing: heat below A1 (about 550–650 °C) between cold-working stages to recrystallise ferrite and restore ductility, e.g. for wire and sheet.
• Spheroidise annealing: long holding just below A1 (or cycling around it) turns cementite lamellae into spheres in a ferrite matrix; gives best machinability and formability of high-carbon steels such as bearing steels.
• Stress-relief annealing: heat to about 500–650 °C, below A1, and cool slowly to remove residual stresses from welding, casting or machining, with little change in microstructure.

Normalising (air cooling). Heat 40–60 °C above A3 (or Acm for hypereutectoid steels — here going above Acm is intended so that cooling in air breaks up the cementite network) and cool in still air. The faster cooling gives finer pearlite, a smaller ferrite grain size and a more uniform structure, so normalised steel is somewhat stronger and harder than annealed steel, with good toughness. It is cheaper than annealing (furnace is freed sooner) and is used for forgings, castings and structural steel.

Hardening (quenching). Austenitise (hypoeutectoid: 30–50 °C above A3; hypereutectoid: 30–50 °C above A1, so that undissolved cementite particles remain and add wear resistance) and quench faster than the critical cooling rate to form martensite.

• Quenchants in decreasing severity: agitated brine, water, oil, polymer solutions, molten salt, air. A more severe quench hardens deeper but raises the risk of distortion and quench cracking.
• Achievable hardness depends on carbon content (martensite hardness rises with carbon up to about 0.6 %); steels below about 0.25 % C cannot be usefully through-hardened.
• Hardenability is the depth to which a steel can be hardened, set by alloy content and grain size, not by its maximum hardness. It is measured by the Jominy end-quench test — a 25 mm bar quenched from one end by a water jet, hardness measured along its length — and expressed by Grossmann's ideal critical diameter.

Tempering. Hardened (as-quenched) martensite is hard, brittle and highly stressed. Reheating below A1 and cooling relieves stress and lets carbon precipitate as carbides, trading hardness for toughness:

• 150–250 °C: low-temperature tempering for tools, gauges and case-hardened parts (hardness barely drops, stresses relieved).
• 350–500 °C: springs (high elastic limit).
• 500–650 °C: high-temperature tempering of shafts, gears, bolts; hardening plus this tempering is called hardening and tempering (quench and temper), giving tempered martensite with the best combination of strength and toughness.
• Temper embrittlement (tempered martensite embrittlement near 250–400 °C, and temper brittleness in some alloy steels held at 375–575 °C) reduces impact toughness; avoid those ranges or add Mo. Tempering temperature and time trade off; the Hollomon–Jaffe parameter puts them on one scale.

Isothermal treatments.

• Austempering: quench into a salt bath above Ms, hold until austenite transforms fully to lower bainite. Less distortion than quench-and-temper, good toughness.
• Martempering (marquenching): quench into a bath just above Ms, hold only until temperature equalises through the section, then air-cool through the martensite range; then temper. Reduces distortion and cracking.

Formulas

T_anneal, T_harden ≈ A3 + (30 to 50) °C (hypoeutectoid) ; ≈ A1 + (30 to 50) °C (hypereutectoid hardening) T_normalise ≈ A3 (or Acm) + (40 to 60) °C Rule-of-thumb temperatures in °C; the exact ranges come from the steel specification or data book.

A3 ≈ 910 − 203·√C Empirical estimate of A3 (°C) for a plain carbon steel, C in wt %; alloying terms are added for alloy steels (from your data book).

P = T·(C_HJ + log10 t) Hollomon–Jaffe tempering parameter: T = tempering temperature (K), t = time (h), C_HJ ≈ 20 for plain carbon and low-alloy steels (material constant). Equal P gives equal tempered hardness.

Worked examples

Example 1 (standard): choosing temperatures for C45 (0.45 % C) steel.

1. A3 ≈ 910 − 203 × √0.45 = 910 − 203 × 0.671 = 774 °C; A1 = 727 °C.
2. Full anneal or harden: A3 + 30 to 50 → about 805–825 °C, then furnace cool (anneal) or water/oil quench (harden).
3. Normalise: A3 + 40 to 60 → about 815–835 °C, cool in air.
4. Expected annealed structure (lever rule just below 727 °C): proeutectoid ferrite = (0.76 − 0.45)/(0.76 − 0.022) = 42 %, pearlite 58 %.

Example 2 (GATE level): equivalent tempering time. Given: a part is specified to be tempered at 550 °C for 2 h. The furnace is available only at 600 °C. Find the time at 600 °C that gives the same hardness (C_HJ = 20).

1. P = (550 + 273.15) × (20 + log10 2) = 823.15 × 20.301 = 16 711.
2. At 600 °C: log10 t = 16 711 / 873.15 − 20 = 19.139 − 20 = −0.861.
3. t = 10^(−0.861) = 0.138 h = 8.3 min. A 50 °C increase cuts the time by a factor of about 15, showing how sensitive tempering is to temperature.

Common mistakes

• Heating a hypereutectoid steel above Acm for hardening: coarse austenite, more retained austenite, more cracking.
• Confusing hardness with hardenability; a low-alloy steel and a plain carbon steel of the same carbon reach the same maximum hardness, but the alloy steel hardens deeper.
• Skipping or delaying tempering after quenching, which risks cracking.
• Saying normalising gives a coarser structure than annealing; it is finer.
• Tempering in the 250–400 °C range and expecting improved toughness.
• Using °C instead of kelvin in the Hollomon–Jaffe parameter.

For GATE PI

Expect matching questions (process ↔ temperature range ↔ cooling medium ↔ resulting structure), why hypereutectoid steels are hardened from above A1 only, the purpose of each annealing variant, hardness versus hardenability and the Jominy test, and the differences between austempering and martempering. Numericals are less common but can involve lever-rule microstructures or tempering-parameter equivalence.

Quick check

1. Which treatment gives the softest condition of a medium-carbon steel?
2. Why is normalised steel stronger than annealed steel of the same composition?
3. What test measures hardenability?
4. Which isothermal treatment produces bainite?
5. Typical tempering range for springs?

Answers: 1. Full annealing (or spheroidise annealing for high-carbon steels); 2. Faster air cooling gives finer pearlite and smaller grains; 3. Jominy end-quench test; 4. Austempering; 5. About 350–500 °C.