What is the heat affected zone in a structural weld and why does it matter for WPS control?

The heat affected zone (HAZ) is the base metal adjacent to the weld fusion boundary that reaches temperatures high enough to alter its microstructure but does not melt. For high-strength steels and low-alloy steels, the HAZ is often the most vulnerable location for hydrogen-induced cracking and toughness degradation — which is why AWS D1.1:2025 requires WPS parameter control, not just weld metal property control.

Does AWS D1.1:2025 specify maximum heat input limits to protect the HAZ?

AWS D1.1:2025 does not set universal heat input maximums — that is left to the WPS and PQR qualification envelope. However, when CVN toughness is specified (Table 6.8), the maximum heat input used during PQR testing becomes an essential variable, meaning production welding cannot exceed it without requalification.

How does preheat affect heat affected zone microstructure in structural steel?

Preheat slows the cooling rate through the HAZ, reducing the risk of hard, brittle martensite formation — especially in steels with higher carbon equivalent values. However, excessive preheat combined with high heat input can coarsen the austenite grain structure, reducing toughness. The WPS preheat range must balance hydrogen cracking prevention against over-softening or grain growth.

Which base metals are most sensitive to HAZ toughness degradation in structural applications?

High-strength quenched-and-tempered steels (A514, A517, A709 Grade HPS 100W) are most sensitive — their through-thickness strength depends on the tempered martensite structure that excess heat can anneal. A572 Grade 50 and A36 are far more tolerant, but are still susceptible to HAZ hydrogen cracking when the carbon equivalent is high and preheat is inadequate.

Jul 29, 20267 min read

HAZ Control: WPS Parameters That Protect Structural Steel Toughness

Heat affected zone toughness failures are preventable. Learn which WPS parameters CWIs and QC managers must control to protect HAZ integrity in structural steel welds.

WPS HAZ heat affected zone AWS D1.1 toughness heat input preheat CWI

New to procedures? Start with WPS welding: what a welding procedure specification is, with a filled-in example.

Why the HAZ Is Often the Weakest Link

When a structural weld fails in service, investigators frequently find the fracture origin in the heat affected zone rather than the weld metal itself. The weld metal is purpose-manufactured: filler metal classifications carry minimum toughness ratings, and the welding engineer selects them to overmatching strength. The base metal in the HAZ has no such design intent — it is simply the structural steel that happened to be adjacent to the arc.

During welding, the HAZ cycles through extreme temperatures — from below preheat to near-melting at the fusion boundary — in a matter of seconds. For carbon and low-alloy structural steels, this thermal cycle drives grain growth, carbide redistribution, and in some cases, partial austenitization followed by rapid quench. The result can be a narrow band of base metal with hardness and brittleness that the structural design never assumed.

The WPS is the primary tool for managing what happens to the HAZ. The parameters the CWI verifies — heat input, preheat, interpass temperature, travel speed — all directly control the thermal history that the HAZ experiences.

Heat Input: The Master Variable

Heat input — the energy deposited per unit length of weld — drives HAZ width, peak temperature, and most importantly, the cooling rate. The formula in AWS D1.1:2025 is:

Heat Input (kJ/in) = (Amps × Volts × 60) / (Travel Speed in in/min × 1000)

For a given base metal, there are both upper and lower limits that matter:

Too high: Slow cooling through the critical transformation range (approximately 1472°F to 932°F / 800°C to 500°C) coarsens prior austenite grain structure in the HAZ, degrading toughness. On quenched-and-tempered steels like A514, excessive heat input anneals the tempered martensite structure and destroys the mechanical properties the steel manufacturer created through controlled thermal processing.

Too low: Rapid cooling produces hard martensite with high hydrogen susceptibility. The HAZ becomes prone to hydrogen-induced cold cracking — particularly at the toe of fillet welds and root of groove welds where stress concentration and hydrogen diffusion paths converge.

The WPS heat input range, established during PQR testing, defines the safe operating window for a specific process-material combination. When CVN toughness is specified (AWS D1.1:2025 Table 6.8 Row 7), the maximum heat input from the PQR becomes an essential variable — production welding above that maximum invalidates the CVN qualification.

Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or earlier).

Preheat: Managing the Lower Bound

Preheat controls the minimum temperature of the base metal immediately before and during welding. Its primary purpose is slowing the HAZ cooling rate to prevent hard martensite formation and giving diffusible hydrogen time to escape before the steel is cold enough to crack.

AWS D1.1:2025 Table 5.3 (prequalified WPS) and the general qualification route both require minimum preheat values based on:

Base metal carbon equivalent (CE)
Base metal group classification (Table 6.9)
Thickness of the thicker member
Welding process

For the CWI, preheat verification means more than touching a temperature stick to the steel before the first arc strike. It means:

Measuring at the correct location — typically 3 in [75 mm] from the weld joint on each member
Verifying both members, not just the thicker one
Monitoring through the weld — preheat requirements extend through the welding operation, not just at start
Using calibrated equipment — contact pyrometers or temperature-indicating crayons with proper range ratings

The method of measurement must match the surface condition. Temperature-indicating crayons can be affected by mill scale, moisture, and paint. Infrared thermometers need emissivity calibration for bare steel. The WPS should specify acceptable measurement methods.

Interpass Temperature: The Upper Bound

While preheat controls the minimum, interpass temperature caps the maximum. The maximum interpass temperature on the WPS prevents excessive heat buildup in multi-pass welds that can:

Coarsen the HAZ grain structure in high-strength steels
Reduce the toughness of previously deposited passes through reheating above the critical temperature
Produce slow cooling rates that generate unacceptable microstructures in Q&T steels

The WPS maximum interpass temperature must be enforced, not just recorded. In high-production environments, welders may try to run back-to-back passes without allowing the joint to cool. The CWI must verify with an actual temperature measurement before each pass, not estimate by feel or elapsed time.

Under AWS D1.1:2025 Table 6.8, when CVN toughness is specified, an increase in the maximum interpass temperature above the PQR-qualified value is a supplementary essential variable requiring requalification. This change in the 2025 edition (which removed preheat from this scope) means that interpass monitoring is now the primary CVN-related thermal control point in the Table 6.8 framework.

Travel Speed and Its Indirect Role

Travel speed is not just a productivity parameter — it directly controls heat input through the formula above. A welder who slows travel speed to improve fusion on a difficult joint position may inadvertently push heat input beyond the WPS maximum, degrading HAZ toughness even if amperage and voltage stay within range.

The WPS should specify travel speed ranges, not just amperage and voltage. For semi-automatic and automatic processes (FCAW-G, SAW, GMAW), travel speed is a recordable production variable. For manual SMAW, electrode diameter and bead length per electrode are the practical proxies.

CWIs should periodically spot-check travel speed on critical work — particularly on the first pass from each production welder running a CVN-qualified WPS.

Base Metal Considerations by Steel Group

A36 and A572 Grade 50

These workhorses of structural fabrication have low carbon equivalents and high tolerance for normal heat inputs and preheat variations. HAZ concerns are primarily limited to:

Hydrogen cracking on thick sections (over 1 in) at marginal preheat
Lamellar tearing in heavily restrained T-joints on plate with low through-thickness ductility

See the related discussion in our lamellar tearing risk guide and the A572 Grade 50 GMAW WPS walkthrough.

A913 and HSLA Steels

A913 (quenched and self-tempered) and similar thermomechanically controlled process (TMCP) steels maintain their properties through a proprietary thermal history. Excessive heat input disrupts the self-tempered martensite near the surface and can reduce strength in the HAZ below the base metal minimum. The steel manufacturer's welding guidelines should supplement the AWS D1.1 requirements.

A514 and A517 (HY-80 equivalent)

These quenched-and-tempered steels carry the most stringent HAZ control requirements. AWS D1.1:2025 restricts interpass temperature to a maximum of 400°F [200°C] for A514/A517 — lower than most other prequalified base metals. PWHT above 1100°F [593°C] is prohibited because it reverses the quench-and-temper treatment. Any repair or weld added after shop heat treatment requires a dedicated WPS reviewed against the manufacturer's technical guidance.

CWI Verification Points for HAZ Control

Effective HAZ protection is a surveillance function, not a paperwork exercise. The CWI must verify these parameters in real time:

Parameter	When to Check	Acceptable Method
Preheat	Before every weld, after any interruption	Contact pyrometer, temp crayon
Interpass temperature	Before each pass in multi-pass joints	Contact pyrometer, temp-indicating label
Amperage	During welding	Calibrated clamp meter on work lead
Voltage	During welding	Calibrated voltmeter at wire feeder/power source output
Travel speed	During welding	Timed length of bead (in/min)
Heat input calculation	After each pass sample	Computed from actual A, V, IPM values

When records show heat input approaching the WPS maximum, the CWI should intervene before the next pass — not after the weld is complete. A documented parameter exceedance on a CVN-critical joint triggers a nonconformance that may require additional NDE or mechanical testing.

Documenting HAZ Control in Your WPS Library

The WPS record must explicitly state:

Heat input range (minimum and maximum, in kJ/in) — derived from the PQR
Preheat temperature (minimum) and the verification method
Maximum interpass temperature and the verification method
Travel speed range (or electrode stickout/diameter for SMAW)

If the project requires CVN toughness, annotate the WPS with the Table 6.8 variable envelope and reference the PQR by number. This ensures that any revision to the WPS triggers a check against the CVN-qualified envelope before the revision is approved.

For shops managing multiple WPS documents across different base metal groups and processes, software-based WPS management that enforces parameter ranges at the form level — flagging entries that exceed the qualified envelope — substantially reduces the risk of a non-conforming procedure reaching production. Learn more at our WPS library management guide, or see how digital tools compare to spreadsheets at WPS Welding — Pricing.

FAQ

Frequently asked.

What is the heat affected zone in a structural weld and why does it matter for WPS control?: The heat affected zone (HAZ) is the base metal adjacent to the weld fusion boundary that reaches temperatures high enough to alter its microstructure but does not melt. For high-strength steels and low-alloy steels, the HAZ is often the most vulnerable location for hydrogen-induced cracking and toughness degradation — which is why AWS D1.1:2025 requires WPS parameter control, not just weld metal property control.
Does AWS D1.1:2025 specify maximum heat input limits to protect the HAZ?: AWS D1.1:2025 does not set universal heat input maximums — that is left to the WPS and PQR qualification envelope. However, when CVN toughness is specified (Table 6.8), the maximum heat input used during PQR testing becomes an essential variable, meaning production welding cannot exceed it without requalification.
How does preheat affect heat affected zone microstructure in structural steel?: Preheat slows the cooling rate through the HAZ, reducing the risk of hard, brittle martensite formation — especially in steels with higher carbon equivalent values. However, excessive preheat combined with high heat input can coarsen the austenite grain structure, reducing toughness. The WPS preheat range must balance hydrogen cracking prevention against over-softening or grain growth.
Which base metals are most sensitive to HAZ toughness degradation in structural applications?: High-strength quenched-and-tempered steels (A514, A517, A709 Grade HPS 100W) are most sensitive — their through-thickness strength depends on the tempered martensite structure that excess heat can anneal. A572 Grade 50 and A36 are far more tolerant, but are still susceptible to HAZ hydrogen cracking when the carbon equivalent is high and preheat is inadequate.