What are the four main types of steel heat treatment?

The four main heat treatment processes for structural steel: (1) Annealing — heat above the upper critical temperature (A3, approximately 815-900°C for carbon steels), soak 1 hour per inch of thickness, then slow furnace cool (~20-50°C/hr). Produces soft, fully stress-relieved steel with coarse pearlite structure. Decreases strength (Fy 20-30% lower than as-rolled), increases ductility (elongation 30%+), maximizes machinability. Used after cold working to restore ductility and before machining. (2) Normalizing — heat to 870-925°C (above A3), soak, then air cool. Produces fine-grained ferrite-pearlite with higher strength and toughness than annealed. Grain size refined to ASTM 6-8. Normalizing is specified for: heavy sections that cooled slowly from rolling (coarse grain at core), impact service, and as-toughening treatment. Normalized A36 plate has 15-25% higher Charpy values than as-rolled at typical thickness. (3) Quenching — heat to austenitizing temperature (~800-900°C), soak, then rapidly cool in water, oil, or polymer. Produces martensite — very hard (HB 400-650) but brittle. Quenching alone is never a final treatment for structural steel. (4) Tempering — reheat quenched steel to a temperature below the lower critical temperature (A1, ~723°C), typically 200-675°C, hold, then air cool. Tempering temperature controls the final balance: low temperature (200-300°C) = high hardness, some toughness; high temperature (550-675°C) = good toughness, moderate hardness. Quenching + tempering (Q&T) produces A514 (yield 100 ksi) with acceptable ductility. Tempering at 600-650°C for 1 hr/inch produces the best strength-toughness balance per ASTM A514.

How does post-weld heat treatment (PWHT) work?

Post-weld heat treatment (PWHT), also called stress relief, heats the welded assembly to 600-650°C (1100-1200°F), holds for 1 hour per inch of thickness (minimum 1 hour), then controlled cools (typically ≤ 300°C/hr). PWHT serves three purposes: (1) Residual stress reduction — the elevated temperature reduces the yield strength of steel to ~10-20% of room temperature value, allowing residual stresses from welding shrinkage and fit-up to relax by creep. PWHT at 600-650°C reduces peak residual stresses by 70-90%. (2) HAZ tempering — the martensitic or bainitic microstructure in the heat-affected zone is tempered, reducing hardness from 300-400 HB (as-welded) to 200-250 HB (post-PWHT), improving toughness and reducing hydrogen cracking risk. (3) Hydrogen bake-out — hydrogen dissolved in the weld metal diffuses out at PWHT temperatures, reducing risk of delayed hydrogen-induced cracking. PWHT is required by AWS D1.1 for: (a) assemblies with weld throat > 1-1/2" in specific geometries, (b) certain P-number materials (P4, P5 alloy steels) regardless of thickness, (c) when specified by the engineer for fracture-critical or fatigue-critical applications. P-number 1 (carbon steel A36/A992/A572) typically does not require PWHT per AWS D1.1 for thickness below 1-1/2", unless specified for service conditions. Over-PWHT (too long or too hot) can reduce strength — the stress relief temperature must remain 50°F below the original tempering temperature for Q&T steels to avoid overtempering.

How is A913 steel produced by the QST process?

ASTM A913 (Grades 50, 60, 65, 70) uses the Quenching and Self-Tempering (QST) process, which is fundamentally different from traditional off-line Q&T: (1) The beam exits the rolling mill at the normal finishing temperature (~850-900°C), surfaces austenitic. (2) Water spray quench is applied to all four faces of the beam IN-LINE, immediately after the last rolling stand. The water spray intensity is computer-controlled based on the section geometry to achieve uniform cooling. (3) Cooling stops before the core has transformed — the surface forms martensite/bainite, but the core retains heat. (4) The beam exits the quench at a controlled surface temperature (~500-600°C), then the core heat conducts outward and self-tempers the surface, producing tempered martensite with excellent toughness. (5) The finished beam cools to ambient on the cooling bed under controlled conditions. QST advantages over traditional reheat Q&T: (a) Energy — uses residual rolling heat, no reheat furnace, ~30-40% energy savings. (b) Through-thickness uniformity — the gradual thermal gradient during self-tempering produces uniform properties through the thickness, unlike off-line Q&T where thick sections have a hardness gradient. (c) Toughness — A913 routinely achieves 80-150+ ft-lb CVN at 70°F, far exceeding A992 and matching or exceeding off-line Q&T A572. (d) Cost — ~5-10% premium vs A992 vs 20-40% for off-line Q&T A572 Gr 65. A913 is the preferred material for jumbo sections (W14x500+) because the QST process achieves uniform properties at sections too thick for conventional normalizing to meet CVN requirements.

What is the difference between annealing and normalizing for structural steel?

Annealing and normalizing differ fundamentally in cooling rate and resulting microstructure: Annealing — furnace cool (20-50°C/hr), slowest practical cooling. Produces coarse pearlite (thick carbide lamellae in ferrite matrix) that is soft (HB 120-150 for A36), with maximum ductility (elongation 30-35%) but low strength (Fy ~28-32 ksi for A36 composition). Grain size: ASTM 3-5 (coarse). Normalizing — air cool (~100-500°C/hr depending on section). Produces fine pearlite (thin carbide lamellae, closely spaced) that is stronger (HB 140-170 for A36, Fy ~35-40 ksi) but less ductile (elongation 22-25%). Grain size: ASTM 6-8 (fine). When to specify normalizing over annealing: (1) Impact service — fine grain size of normalized steel raises Charpy upper shelf and lowers transition temperature 20-40°F. (2) Heavy sections — plates/strips thicker than ~1.5" cool slowly from hot rolling, producing a coarse microstructure at the core; normalizing refines this. (3) Multi-pass welded joints — normalizing between passes (in vessel fabrication) refines the coarse HAZ from previous passes. When to specify annealing: (1) Before extensive machining — annealed steel machines with less tool wear and better surface finish. (2) After cold forming (bending, punching) to relieve work hardening before welding. (3) Maximum ductility applications — e.g., expansion joints, energy-absorbing devices. Cost: normalizing adds ~$0.03-0.06/lb vs as-rolled; annealing adds ~$0.05-0.10/lb (furnace time is longer).

How does heat treatment affect the mechanical properties of A514 (Q&T 100 ksi) steel?

ASTM A514 (quenched and tempered, Fy ≥ 100 ksi, Fu 110-130 ksi) achieves its properties through precise heat treatment control: (1) Austenitizing at 900°C followed by water or oil quench to fully martensitic microstructure. (2) Tempering at 540-650°C to precipitate fine alloy carbides (chromium, molybdenum, vanadium carbides) that provide secondary hardening while tempering the martensite for toughness. Tempering below 540°C leaves the steel too brittle; above 650°C overtempers and strength falls below 100 ksi. The precise tempering temperature is grade-specific (A514 has 8 subgrades A-H with varying chemistry). Key mechanical properties: hardness 235-293 HB (HRC 20-31), elongation minimum 18% in 2" (ASTM A6), Charpy V-notch — typically 20-50 ft-lb at 70°F but can drop to 10-25 ft-lb at -20°F, so A514 is NOT recommended for fracture-critical applications in cold climates without supplemental CVN qualification. Welding A514: (a) Preheat 150-200°F minimum, (b) low-hydrogen electrodes (H4 or H2 classification) mandatory, (c) interpass temperature ≤ 400°F to avoid overtempering the base metal HAZ, (d) heat input controlled to 35-55 kJ/in to balance cooling rate (too fast = HAZ martensite, too slow = overtempered soft zone). PWHT is generally NOT applied to A514 because it overtempers the base metal, reducing strength below 100 ksi. If PWHT is unavoidable (e.g., code requirement for thick sections), a qualification test coupon with the PWHT cycle must demonstrate tensile properties are maintained, and the PWHT temperature must be at least 50°F below the mill tempering temperature.

Steel Heat Treatment — Annealing, Quenching & Tempering

Heat treatment alters the mechanical properties of steel by controlling the heating and cooling process. For structural steel, heat treatment affects strength, ductility, toughness, and weldability. This page covers the major heat treatment methods, their effects on properties, and when they are specified for structural applications.

Why Heat Treatment Matters for Structural Steel

Property	Affected By	Structural Impact
Yield strength	Cooling rate, carbon content	Member capacity
Tensile strength	Quenching, tempering	Fracture resistance
Ductility	Annealing, normalizing	Seismic performance, deformation capacity
Toughness	Normalizing, tempering	Brittle fracture resistance (Charpy)
Weldability	Carbon equivalent, cooling	Heat-affected zone cracking
Hardness	Quenching	Wear resistance, tool applications

Heat Treatment Methods

Annealing

Annealing heats steel above the critical temperature and cools it slowly (in the furnace). This softens the steel, relieves internal stresses, and improves machinability.

Parameter	Value
Temperature	1500-1650°F (815-900°C)
Soak time	1 hour per inch of thickness
Cooling	Furnace cool (very slow)
Effect on strength	Decreases
Effect on ductility	Increases
Effect on toughness	Increases

When specified: After cold working, to restore ductility. Before machining, to improve workability. To relieve residual stresses from welding.

Normalizing

Normalizing heats steel above the critical temperature and cools it in still air. This produces a uniform, fine-grained microstructure with better toughness than the as-rolled condition.

Parameter	Value
Temperature	1600-1700°F (870-925°C)
Soak time	1 hour per inch of thickness
Cooling	Still air (faster than anneal)
Effect on strength	Moderate increase
Effect on ductility	Maintains or slightly decreases
Effect on toughness	Significant increase

When specified: For thick plates (over 2 in) where Charpy toughness is required. For pressure vessel steel. For forged components. Normalizing is the most common heat treatment specified for structural steel.

Quenching and Tempering (Q&T)

Quenching rapidly cools steel from above the critical temperature using water, oil, or polymer. This produces a hard, brittle martensite structure. Tempering then reheats to a lower temperature to restore some ductility while maintaining high strength.

Parameter	Quenching	Tempering
Temperature	1500-1650°F	800-1200°F
Cooling	Water/oil (rapid)	Air (controlled)
Effect on strength	Very high increase	Decreases from quench
Effect on ductility	Very low (brittle)	Restores ductility
Effect on toughness	Low (without temper)	Good (with temper)

When specified: High-strength plates (A514, A852). Pressure vessels. Military and bridge applications. Quenched and tempered plates can achieve Fy = 100 ksi.

Stress Relief

Stress relief heats steel to a moderate temperature below the critical range to reduce residual stresses from welding or forming without significantly changing mechanical properties.

Parameter	Value
Temperature	1100-1200°F (595-650°C)
Soak time	1 hour per inch of thickness
Cooling	Slow (furnace or still air)
Effect on strength	Minimal change
Effect on stress	Reduces residual 60-80%

When specified: After heavy welding. For thick-section welds (over 1.5 in). When distortion must be minimized. Required by AWS D1.1 for certain weld categories.

Heat Treatment by ASTM Specification

ASTM Spec	Grade	Condition	Fy (ksi)	Heat Treatment
A36	—	Hot-rolled	36	None (as-rolled)
A572	Gr 50	Hot-rolled/Norm.	50	Normalized (optional)
A992	50	Hot-rolled	50	None (as-rolled)
A588	—	Hot-rolled/Norm.	50	Normalized (thick plate)
A514	—	Quenched & tempered	100	Q&T required
A852	—	Quenched & tempered	70	Q&T required
A913	50-65	Quenched & self-tempered	50-65	QST process
A1065	—	Hot-rolled	50	None

Effect on Charpy V-Notch Toughness

Heat treatment significantly affects CVN toughness, which is critical for fracture-critical members and seismic applications.

Condition	CVN at 70°F (ft-lb)	CVN at 0°F (ft-lb)	CVN at -20°F (ft-lb)
As-rolled	25-50	10-25	5-15
Normalized	50-100	25-60	15-40
Q&T	40-80	20-50	10-30
QST (A913)	80-150	40-100	25-60

Values are approximate for A572/A992 chemistry. Actual values depend on specific chemistry and processing.

Heat-Affected Zone (HAZ)

Welding is a localized heat treatment. The HAZ near the weld experiences temperatures from melting (at the fusion line) down to the base metal temperature.

HAZ Zones

Zone	Temperature Range	Effect on Properties
Fusion zone	> 2700°F (melting)	Weld metal (different chemistry)
Coarse-grained HAZ	2000-2700°F	Grain growth, may be brittle
Fine-grained HAZ	1500-2000°F	Grain refinement, good toughness
Intercritical HAZ	1300-1500°F	Partial transformation
Subcritical HAZ	700-1300°F	Tempering of cold-worked regions
Base metal	< 700°F	Unaffected

HAZ Hardness and Weldability

The HAZ hardness depends on the carbon equivalent (CE) of the steel and the cooling rate:

CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

CE	Weldability	Preheat Required	HAZ Concern
< 0.35	Excellent	None	Minimal
0.35-0.40	Good	Low	Slight hardening
0.40-0.45	Fair	Moderate	Possible cracking
0.45-0.50	Poor	High	Cracking likely
> 0.50	Very poor	Very high	Severe cracking risk

A992 (CE ≤ 0.45 typical) has good weldability. A514 (CE > 0.50) requires careful preheat and interpass temperature control.

Preheat Requirements

AWS D1.5 and AWS D1.1 require preheating based on steel grade, thickness, and welding process:

Thickness (in)	A36 (°F)	A992 (°F)	A514 (°F)
≤ 3/4	50*	50*	50*
> 3/4 to 1-1/2	50	50	200
> 1-1/2 to 2-1/2	150	150	300
> 2-1/2	225	225	400

*50°F minimum means ambient temperature is acceptable if above 50°F.

Frequently Asked Questions

Does structural steel need heat treatment? Most structural steel (A36, A992, A572) is used in the as-rolled condition and does not require heat treatment. Heat treatment is specified for: (1) thick plates where toughness is critical (normalizing), (2) high-strength applications (Q&T plates like A514), and (3) post-weld stress relief for thick welds.

What is the difference between normalizing and annealing? Normalizing cools in still air, producing a fine-grained, uniform structure with good strength and toughness. Annealing cools slowly in the furnace, producing a softer structure with maximum ductility but lower strength. Normalizing is preferred for structural steel because it maintains strength while improving toughness.

Does welding count as heat treatment? Yes. The heat-affected zone (HAZ) near a weld experiences temperatures equivalent to annealing, normalizing, or quenching, depending on distance from the weld and cooling rate. This is why preheat and interpass temperature control are important: they control the effective heat treatment in the HAZ.

What is QST steel (A913)? Quenched and Self-Tempered (QST) steel is produced by a controlled online process where the hot-rolled shape is sprayed with water (quenching) and then allowed to self-temper using residual heat from the core. A913 Grade 50 and 65 provide excellent toughness and weldability without separate heat treatment. Popular for seismic applications.

Steel Grades — ASTM specifications
Steel Yield Strength — Fy by grade
Steel Chemical Composition — Elements and weldability
Welding Procedure — WPS requirements
Steel Charpy Values — Impact toughness data

Disclaimer

This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.