1. Annealing — Maximum Ductility, Minimum Hardness

Process

Full annealing heats the steel to its austenitizing temperature (above the upper critical temperature A3 for hypoeutectoid steels), holds it long enough for complete transformation to austenite, then cools it very slowly — typically in the furnace itself at 50–100°F per hour. The slow cooling allows carbon atoms to diffuse out of the austenite lattice, forming coarse pearlite (alternating layers of ferrite and cementite) with the lowest possible hardness and highest possible ductility for that carbon content.

Temperature Ranges

Resulting Properties

After full annealing, typical A36 steel drops to approximately 130 HB (Brinell) hardness with elongation exceeding 30%. The microstructure is coarse pearlite + ferrite — large grains with clearly visible cementite lamellae. While ideal for machining and cold forming, annealed steel has the lowest yield strength of any heat treatment condition — typically 30–36 ksi for A36 base chemistry — and is not used for load-bearing structural applications without subsequent treatment.

Applications in Structural Fabrication

• Forged connection components (clevises, turnbuckles, pin plates) — annealing after forging restores machinability
• Work-hardened cold-formed sections that have lost ductility during rolling — process anneal (subcritical, 1,000–1,300°F) relieves internal stresses
• Reclaimed structural steel with unknown prior treatment — normalizing is preferred, but annealing may be used to establish a known baseline condition

2. Normalizing — Uniform Fine-Grain Structure

Process

Normalizing is similar to annealing but uses air cooling instead of furnace cooling. The steel is heated to the same austenitizing range, held, then removed from the furnace and cooled in still air. The faster cooling rate (vs. furnace cooling) produces a finer pearlite spacing and smaller grain size, yielding higher strength and better notch toughness than annealed steel while retaining good ductility.

Why Normalize Instead of Anneal?

Normalizing is the preferred heat treatment for structural steel because:

1. It refines the coarse grain structure left by hot rolling, forging, or casting
2. It produces uniform properties throughout the cross-section, unlike the as-rolled condition which varies from surface to core
3. It increases toughness — normalized A572 Gr 50 can achieve 35 ft-lb Charpy V-notch energy at 40°F vs. 15–20 ft-lb in the as-rolled condition
4. It is faster and cheaper than full annealing (air cool vs. 12–24 hour furnace cool)

Normalizing Temperature Table

*Quenched and tempered or QST steels should NEVER be normalized because normalizing erases the controlled microstructure and reverts the steel to a lower-strength condition. Refer to the specific ASTM specification for permissible heat treatments.

Normalizing for Bridge Steel

AASHTO/AWS D1.5 Bridge Welding Code requires normalized steel for fracture-critical members (FCMs) in certain applications. A709 Grade 50W (weathering bridge steel) is commonly supplied in the normalized condition for main tension members. Normalized A709 achieves Charpy V-notch values of 25 ft-lb minimum at -10°F for Zone II applications, vs. 15 ft-lb at 70°F for as-rolled A36.

3. Quenching — Maximum Hardness Through Rapid Cooling

Process

Quenching heats steel to the austenitizing temperature, then cools it rapidly by immersion in water, oil, brine, polymer solution, or forced air. At cooling rates exceeding the critical cooling rate for that steel chemistry, austenite transforms to martensite — a supersaturated solution of carbon in body-centered tetragonal iron with extreme hardness (up to 65 HRC for high-carbon steels) but very low toughness and ductility in the as-quenched state.

Quench Media Comparison

Critical Cooling Rate and Hardenability

The critical cooling rate is the minimum cooling rate that produces a fully (or mostly) martensitic structure. Low-carbon steels (A36, 0.25% C) have very high critical cooling rates (>500°F/sec at 1,300°F), meaning they require extreme quench severity to achieve significant hardening — and even then, the maximum achievable hardness is limited by the low carbon content (approximately 40–45 HRC maximum). Alloying elements (Mn, Cr, Mo, Ni) shift the time-temperature-transformation (TTT) curve to the right, reducing the critical cooling rate and improving hardenability — which is why AISI 4140 (Cr-Mo) can be oil-quenched while equivalent carbon plain carbon steels require water.

Quenching is rarely applied to structural steel sections, but is used extensively for:

• High-strength bolts (A325, A490, F3125) — quenched and tempered to achieve 120–150 ksi tensile strength
• Anchor rods (F1554 Gr 105) — quenched and tempered from 1,550°F
• Pin connections, bearing plates, and high-wear components in expansion bearings

4. Tempering — Restoring Toughness After Quenching

Process

As-quenched martensite is too brittle for structural use. Tempering reheats the quenched steel to a temperature below the lower critical temperature (A1, approximately 1,340°F or 725°C), holds it, then cools in air. This allows carbon atoms to precipitate as fine carbides, relieving the lattice strain in martensite and converting it to tempered martensite — an engineered combination of high strength (from the fine carbide dispersion) and good toughness (from the stress-relieved matrix).

Tempering Temperature vs. Properties

Temper Embrittlement Warning

Carbon and low-alloy steels tempered in the range of 500–1,100°F (260–595°C) can suffer temper embrittlement — a loss of notch toughness caused by segregation of impurity elements (P, Sn, Sb, As) to prior austenite grain boundaries. This is reversible by re-austenitizing and re-tempering, and is mitigated by:

• Using vacuum-degassed steels with low tramp element content (P ≤ 0.010%, S ≤ 0.005%)
• Adding 0.15–0.25% Mo to scavenge impurities
• Cooling rapidly from the tempering temperature through the embrittlement range
• Avoiding the critical temperature range altogether for toughness-critical components

A325 and F3125 Grade A325 bolts (120 ksi minimum tensile) are tempered at 800°F minimum. A490 bolts (150 ksi minimum tensile) are tempered at a minimum of 800°F. The minimum tempering temperature is verified by the manufacturer and recorded on the certification.

5. Stress Relieving (Post-Weld Heat Treatment — PWHT)

Process and Purpose

Stress relieving reduces residual stresses from welding, cold forming, flame cutting, and machining by heating the steel to a temperature below the transformation range, holding, and slowly cooling. Unlike annealing, no phase transformation occurs — the elevated temperature simply allows dislocations to rearrange and residual elastic stresses to relax through localized plastic flow (creep).

For carbon and low-alloy structural steels, the stress-relieving temperature range is 1,100–1,200°F (595–650°C), held for 1 hour per inch of thickness (minimum 1 hour), with controlled heating and cooling rates (typically 400°F/hr maximum for sections up to 2 inches thick, decreasing for heavier sections).

When PWHT Is Required

Per AWS D1.1 Structural Welding Code — Steel:

• Thickness > 1.5 in (38 mm) in restrained joints — PWHT may be specified by the engineer
• Thickness > 2 in (50 mm) — PWHT is strongly recommended for restraint conditions
• All thicknesses for certain service conditions (hydrogen service, impact loading, cyclic loading where fatigue improvement is needed)

AISC 360 does not generally require PWHT for building structures, but project specifications from the engineer of record may invoke it. Bridge structures per AASHTO/AWS D1.5 require PWHT for electroslag and electrogas welds and for certain repair welds.

PWHT Temperature Ranges

*Austenitic stainless steels do not undergo transformation stress relief; they require solution annealing at 1,900–2,050°F followed by rapid cooling. For dimensional stability, low-temperature stress relief at 750–850°F may be used but does not relieve welding stresses as effectively.

Local PWHT Methods

When full-furnace stress relieving is impractical, localized heating methods are used:

• Electric resistance heating pads — flexible ceramic pad heaters strapped to the weld zone, controlled by thermocouples. Most common method for field welds.
• Induction heating — electromagnetic coils induce eddy currents in the steel surface. Faster than resistance but requires specialized equipment.
• Gas flame (torch) heating — least controlled; only acceptable for non-critical applications with continuous temperature monitoring.

The heated band must extend a minimum of 3× the weld thickness on each side of the weld (typically 6 inches minimum), and the entire heated zone must be insulated to control the cooling rate.

6. Case Hardening — Surface Hardness with Tough Core

Case hardening (carburizing, nitriding, carbonitriding, or induction hardening) diffuses carbon and/or nitrogen into the steel surface at elevated temperatures, creating a hard, wear-resistant case while maintaining a tough, ductile core. used for bearing surfaces and wear plates in structural applications. This is less common in general structural fabrication and more relevant to mechanical components incorporated into structural assemblies — expansion bearing rocker surfaces, pin holes in bridge bearings, and guide rollers.

Carburizing Process

Pack, gas, or liquid carburizing at 1,650–1,750°F (900–955°C) for 4–8 hours produces a case depth of 0.030–0.060 inches with surface hardness of 58–62 HRC after quenching and low-temperature tempering (300–350°F). The core remains at 25–35 HRC, providing impact resistance. Low-carbon steels (AISI 1018, 1020, A36) are ideal for carburizing because the low core carbon content (0.20%) produces a tough core while the diffused surface carbon (0.80–1.00%) provides hardenability for the case.

Heat Treatment Effects on Structural Steel Properties

Practical Guidance for Fabricators

Pre-Heat Treatment Checklist

1. Verify material certification — know the exact grade, heat number, and supplied condition before specifying heat treatment
2. Check specification limits — ASTM A6 limits re-heat treatment of structural shapes; AISC 360 M2.2 requires that heat-treated steel meet the specified properties after treatment
3. Consider distortion — non-uniform heating or cooling of asymmetric sections causes permanent bowing and twisting; use fixtures or perform treatment in the horizontal position
4. Control atmosphere — heating in air above 1,200°F causes scaling (oxide formation) and decarburization (carbon loss from the surface). Use inert gas (argon, nitrogen), vacuum, or controlled endothermic atmosphere for finish-critical parts
5. Temperature measurement — use at least two thermocouples on the thickest and thinnest sections; maintain ±25°F tolerance on holding temperature
6. Document everything — record time-temperature charts, quench medium temperature, and post-treatment hardness readings for quality assurance

Maximum Allowed Reheat Cycles

Most structural steels can be re-heat treated a limited number of times before grain growth and property degradation occur:

• Normalizing: up to 3 cycles (subsequent cycles produce diminishing grain refinement)
• Annealing: up to 2 cycles (excessive grain growth risk beyond this)
• Stress relieving: unlimited for subcritical temperatures (no microstructural change occurs below A1)
• Quenching and tempering: 1 additional cycle maximum (re-austenitizing erases the prior temper)

Worked Example — Stress Relieving a Welded Box Column

Scenario: A W14×730 column (A572 Gr 50, flange thickness 4.91 in) has four 2-inch full-penetration welds for moment connections in orthogonal directions, creating a highly restrained welded assembly. The engineer specifies PWHT to reduce triaxial restraint stress.

Step 1 — Determine Temperature:

• A572 Gr 50 is a microalloyed low-carbon steel (0.23% C max)
• Stress relief temperature: 1,150°F ± 25°F (per AWS D1.1 Table 5.3)
• Maximum temperature: 1,200°F (staying ≥ 50°F below A1 to avoid transformation)

Step 2 — Hold Time:

• Governing thickness = 4.91 in (flange) → hold time = 4.91 hours minimum, round up to 5 hours

Step 3 — Heating Rate:

• Maximum heating rate per AWS D1.1 = 400°F/hr for sections up to 2 in → reduce proportionally
• For 4.91 in: heating rate ≤ 400 × (2/4.91) = 163°F/hr
• Heat from ambient (70°F) to 1,150°F: (1,150 – 70) / 163 = 6.6 hours

Step 4 — Cooling Rate:

• Maximum cooling rate = same as heating rate = 163°F/hr down to 600°F
• Cooling time: (1,150 – 600) / 163 = 3.4 hours
• Below 600°F: cool in still air (uncontrolled)

Step 5 — Total Cycle:

• Heat: 6.6 hr + Hold: 5.0 hr + Cool: 3.4 hr = 15 hours total furnace time
• Verify with two thermocouples on opposite flanges; record continuous time-temperature chart

Quick Reference — Heat Treatment Summary

References

• AWS D1.1/D1.1M:2020 — Structural Welding Code — Steel, Sections 5.8 and 7.13 (PWHT)
• ASTM A6/A6M-22 — General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet Piling
• ASTM A991/A991M — Standard Test Method for Conducting Temperature Uniformity Surveys of Furnaces
• ASM Handbook Volume 4 — Heat Treating (ASM International)
• AISC 360-22 — Specification for Structural Steel Buildings, Section M2 (Fabrication)
• AASHTO/AWS D1.5M/D1.5:2020 — Bridge Welding Code, Section 5 (PWHT requirements)
• SAE AMS 2759 — Heat Treatment of Steel Parts, General Requirements