Heat Affected Zone (HAZ) Metallurgy: Subzones, Grain Growth and Properties

The heat-affected zone (HAZ) is the region of base metal adjacent to a weld’s fusion boundary that experiences a thermal cycle severe enough to alter its microstructure, yet never reaches the liquidus. Because this thermal cycle varies continuously with distance from the fusion line, the HAZ is not metallurgically uniform – it splits into distinct subzones, each defined by its peak temperature relative to the iron-carbon phase diagram, and each carrying its own grain size, transformation product, and fracture risk. This guide classifies the HAZ subzones in steel welds, explains the grain growth and transformation mechanisms that produce them, and connects each zone to the properties and qualification testing welding engineers must control.

What Is the Heat-Affected Zone?

During arc welding, heat conducted away from the weld pool establishes a steep, transient temperature gradient in the surrounding base metal. Any location whose peak temperature exceeds the threshold at which solid-state transformation begins, but stays below the local solidus, belongs to the HAZ. Arc energy, travel speed, plate thickness (which sets two- versus three-dimensional heat flow), and preheat all shift the position of every isotherm and therefore the width of each subzone described below. The same temperature thresholds – A_c1, A_c3, and the solidus – that define equilibrium phase fields on the iron-carbon phase diagram separate one HAZ subzone from the next; the difference is that welding reaches and leaves these temperatures within seconds rather than hours, so transformation products differ sharply from those produced by slow furnace heat treatment.

HAZ Subzones in Steel Welds

Moving outward from the fusion boundary, peak temperature falls continuously, and the HAZ passes through five metallurgically distinct subzones before reaching unaffected base metal.

Partially Melted Zone (PMZ)

The PMZ is a narrow band, often under 100 micrometres wide, immediately outside the fusion boundary where peak temperature falls between the alloy’s solidus and liquidus. Localized melting occurs at grain boundaries and segregated regions – sulfide- or phosphorus-rich films, for example – and rapid resolidification can leave low-melting-point eutectic films decorating the prior grain boundaries. In crack-sensitive alloys this is the preferred nucleation site for liquation cracking.

Coarse-Grained HAZ (CGHAZ)

The CGHAZ extends from the PMZ inward to the point where peak temperature falls below the grain-coarsening threshold, roughly 1100–1200°C for most structural steels. Austenite grain boundary mobility increases sharply with temperature, and the microalloy carbonitride particles that normally pin grain boundaries dissolve above this threshold, so austenite grains here grow rapidly, often reaching several times the base-metal grain size. On cooling, this coarse austenite transforms under the rapid cooling rates typical of welding into upper or lower bainite in lower-hardenability steels, or coarse, plate martensite in higher-hardenability steels. Coarse prior-austenite grain size combined with a hard transformation product makes the CGHAZ the most frequent location for both hydrogen-induced cold cracking and reduced Charpy toughness.

Fine-Grained HAZ (FGHAZ)

Between the grain-coarsening threshold and A_c3, the steel fully reaustenitizes, but pinning particles remain stable enough to restrict grain growth. The resulting austenite, and the ferrite-pearlite or fine bainite it transforms to on cooling, is fine-grained – similar to an normalized structure. The FGHAZ generally shows the best combination of strength and toughness of any HAZ subzone, which is why it is sometimes called the normalized region.

Intercritical HAZ (ICHAZ)

Between A_c3 and A_c1, peak temperature reaustenitizes only the carbon-rich constituents of the original microstructure – pearlite colonies, for instance – while primary ferrite remains untransformed. On cooling, these small, carbon-enriched austenite islands transform to martensite or partially retain as austenite, producing martensite-austenite (M-A) constituents within an otherwise unchanged ferritic matrix. The ICHAZ is the most metallurgically heterogeneous subzone and, particularly when reheated by a later weld pass, a principal site of local brittle zones.

Subcritical HAZ (SCHAZ)

Below A_c1, no reaustenitization occurs, but the steel still experiences a thermal spike of roughly 300–700°C. In quenched-and-tempered base metal, this is an additional, uncontrolled tempering cycle that can overtemper martensite and coarsen carbides, locally reducing strength and hardness below the base-metal specification – the HAZ soft zone characteristic of high-strength quenched and tempered steels.

Grain Growth Mechanics in the Coarse-Grained HAZ

Driving Force for Austenite Grain Boundary Migration

Grain growth is driven by the reduction in total grain boundary energy as curved boundaries migrate toward their centre of curvature, shrinking small grains and enlarging neighbouring ones. Boundary mobility increases sharply with temperature, so the rate of growth accelerates rapidly as peak temperature rises through the CGHAZ range.

Grain Growth Kinetics

Particle Pinning and Microalloying

Fine carbonitride particles restrain boundary migration by exerting a retarding (Zener) drag force as a boundary attempts to sweep past them. Titanium nitride is stable to temperatures near the solidus and is the most effective pinning phase for limiting CGHAZ grain coarsening in Ti-treated steels, while aluminium nitride and niobium carbonitride dissolve at progressively lower temperatures and are more effective at restraining grain growth in the FGHAZ than directly at the fusion line.

Cooling Rate, Hardenability, and CGHAZ Transformation Products

The t8/5 Cooling Parameter

Welding metallurgists characterize HAZ cooling rate using t8/5, the time taken to cool from 800 to 500°C – the range in which austenite decomposes to ferrite, bainite, or martensite in most structural steels. t8/5 is controlled primarily by arc energy and plate geometry:

Carbon Equivalent and Cracking Susceptibility

For a given t8/5, whether the CGHAZ transforms to soft ferrite/bainite or hard martensite depends on hardenability, commonly summarized by a carbon equivalent formula:

Multi-Pass Reheating Effects and Local Brittle Zones

In multi-pass welds, each pass reheats some of the HAZ left by the previous pass. If that reheating falls in the intercritical range, it produces an intercritically reheated CGHAZ (IC-CGHAZ): carbon-enriched islands within the previously coarse-grained, already hard structure partially reaustenitize and, on cooling, form martensite-austenite constituents embedded in a coarse-grained matrix. This combination is frequently the lowest-toughness location in the entire weld and is the classic local brittle zone (LBZ) targeted by notch-location requirements in Charpy impact testing of welded joints. Reheating is not always detrimental, however: if a later pass instead tempers the CGHAZ at a subcritical temperature, it can reduce hardness and improve toughness – the basis of deliberate temper bead welding techniques used in repair welding of hardenable steels.

HAZ Hardness and Toughness Properties

A hardness traverse across a welded joint typically shows a peak at the CGHAZ in hardenable steels, a minimum (the soft zone) at the SCHAZ in quenched-and-tempered steels, and intermediate, more uniform values through the FGHAZ. Acceptance limits depend heavily on code and service: sour-service applications governed by NACE MR0175 commonly limit HAZ hardness to around 250 HV (22 HRC) to control sulfide stress cracking, while general structural fabrication codes more typically cite indicative limits in the 350–450 HV range. Hardness testing across the joint, together with Charpy specimens notched to sample the CGHAZ or fusion line, forms the core mechanical evidence in a welding procedure qualification record. In corrosive service, the same metallurgical differences across the HAZ can also create local electrochemical differences (see corrosion mechanisms) that influence preferential attack near the weld, such as knife-line attack adjacent to the fusion line in stabilized stainless steels.

Standards Governing HAZ Qualification

Industrial Significance

HAZ metallurgy is central to fabrication codes across industries. In pipeline and pressure vessel welding, carbon equivalent and preheat tables exist specifically to keep the CGHAZ below its martensite-cracking threshold given the project’s expected cooling rate. In structural and offshore fabrication, Charpy toughness requirements are notch-located to sample the CGHAZ and any local brittle zones, since a structurally adequate weld metal can still fail through low-toughness HAZ material. In repair and field welding of hardenable steels, temper bead techniques deliberately sequence weld passes so that later beads temper rather than reheat into the intercritical range. Across all of these applications, correctly prepared metallographic sections – sectioned, mounted, ground, polished, and etched to reveal each subzone – remain the primary evidence used to confirm that a welding procedure produces the intended HAZ microstructure.

Frequently Asked Questions