Heat-affected zones

With assumed steady arc values of 24 V, 200 A, 300 mm/min travel and 0.8 thermal efficiency, the example gives 0.960 kJ/mm arc energy and 0.768 kJ/mm heat input. At 600 mm/min, heat input falls to 0.384 kJ/mm with the other inputs fixed. This calculation does not establish HAZ width, fusion or properties. Compare the actual thermal cycle and original material condition. SSAB heat-input formula.

The HAZ is unmelted parent material altered by the thermal cycle between the fusion region and unaffected material. CWB Group, heat-affected-zone explanation

Check alloy, initial condition, thickness, joint and thermal history for hardness, toughness and corrosion behavior.

Separate fusion, thermal alteration and appearance

In a fusion weld, weld metal has melted and resolidified; the adjacent HAZ has experienced different peak temperatures without becoming the bulk molten weld. A section through the joint can therefore contain several microstructural regions. Thermal cutting also changes material beside the removed edge, so a welded-joint acceptance rule should not be transferred to a cut edge without checking its purpose.

Visible discoloration is a surface observation, not a direct map of every metallurgical change. TWI notes that heat tint can vary with contamination and surface conditions without defining the HAZ extent. Its discussion also covers property changes beyond appearance. Removing color cannot by itself demonstrate restoration of the original bulk material condition. TWI HAZ overview

“HAZ width” may mean an etched microstructural boundary, a hardness-defined region or another specified criterion. The section location and side of the joint also affect interpretation. Widths measured by different criteria are not necessarily the same quantity.

Steel: neither always hard nor always soft

SSAB's Strenx welding guide, printed V1 2025, describes HAZ hardness distribution as dependent on steel grade, plate thickness and heat input. Its Strenx 1100 E illustration shows different softening profiles at different inputs. That is a named product example, not a hardness profile for all high-strength steels or all thicknesses. The guide also distinguishes weld-metal hardness, which depends on the selected weld metal. SSAB, Welding of Strenx, hardness distribution

A thermal-cutting example illustrates the opposite effects within one neighborhood. SSAB describes re-quenched and tempered regions beside thermally cut quenchable steels. One region can harden while another softens. Its schematic distances and temperatures are explanatory examples, not universal removal allowances. SSAB thermal-cutting effects

The engineering question is which properties are required where. A narrow hard region, a softened region and a toughness change need different evidence. Nominal parent-plate strength does not establish the properties of every location after fabrication. Likewise, selecting a stronger filler does not directly restore heat-altered parent material.

Aluminum: original temper matters

European Aluminium's automotive manual, explicitly Version 2015, describes local softening in fusion-welded aluminum. Work-hardened non-heat-treatable tempers can lose strengthening through recovery and recrystallization; heat-treatable alloys can change through their precipitation response. Annealed starting conditions need not show the same strength loss as strengthened tempers. The manual also warns that post-weld solution treatment and quenching can introduce distortion. European Aluminium, Aluminium Fusion Welding, §2.7

Consequently, alloy, temper and final fabrication condition jointly determine applicability. “6061” or “5052” alone leaves an important part of the assessment unresolved. Do not assume that natural aging, artificial aging or an unspecified heat treatment restores all original properties. The approved assembly route and required final tests must support any recovery claim.

Control the complete thermal cycle

SSAB's Welding of Hardox, printed V5-2024, explains that excessive heat input can widen the HAZ and impair relevant properties, but very low input can also be detrimental. Its process guidance therefore should not be reduced to “use the lowest possible number.” Cooling behavior, preheat/interpass conditions and the particular grade remain part of procedure selection. SSAB Hardox welding guide, heat input

With repeated passes, sequence and reheating contribute to the thermal history beyond the last pass alone. Joint geometry, thickness, restraint, consumable and travel conditions also matter. A revised pass sequence or repair weld can alter that history; a similar-looking bead is insufficient evidence of equivalence.

Original heat-input calculation

For an invented steady arc-welding example, take voltage 24 V, current 200 A, travel speed 300 mm/min, and assumed thermal efficiency 0.8. Using Q = k × U × I × 60 / (1000 × v), with Q in kJ/mm, gives:

Q = 0.8 × 24 × 200 × 60 / (1000 × 300) = 0.768 kJ/mm.

The corresponding arc energy before efficiency adjustment is 0.960 kJ/mm. At 600 mm/min with the same other inputs, calculated Q becomes 0.384 kJ/mm, half the original value. The formula and distinction between delivered energy and efficiency follow the Hardox guide; the operating values are teaching assumptions, not a welding recommendation. SSAB heat-input formula

This arithmetic does not predict a HAZ width or prove fusion, penetration, cooling time or acceptable properties. It also assumes the stated steady electrical values represent the process; pulsed-waveform evaluation may require an appropriate energy measurement rather than multiplying convenient average readings. Compare qualified procedures on a consistent calculation basis.

Inspection and acceptance

ISO 9015-1:2001, published April 2001 and confirmed 2022, specifies hardness testing on transverse sections of arc-welded metallic joints. Its public scope identifies normal Vickers HV5/HV10 testing and explicitly excludes test welds in austenitic stainless steels. It is a test-method scope, not a universal allowable-hardness table. ISO 9015-1:2001

Different evidence addresses different properties: metallographic sections and hardness traverses describe local variation, mechanical tests characterize relevant joint properties, and corrosion tests address specific exposures. A hardness result alone does not establish toughness or corrosion performance. Sampling position, load and fabrication condition limit interpretation; acceptance depends on the governing requirements.

The cited online explanatory pages are undated. The aluminum manual and Hardox guide dates above are printed source identifiers. The Strenx guide's final page prints 420-UK, V1 2025, although the URL filename says v1-2026; the filename does not establish the printed edition. Sources were reviewed on 2026-11-02 PST.

Related reference

Welding methods · Quench hardening