Welding metallurgy is the science that governs what happens inside a metal when it is joined by welding. At its core, it concerns the relationship between a metal’s internal structure — its atomic arrangement, grain morphology, and phase composition — and the mechanical properties it exhibits at the macroscopic level. For welding inspectors, engineers, and fabricators working in the pressure vessel, piping, structural, and offshore industries, welding metallurgy provides the scientific foundation for virtually every fabrication requirement they encounter.

Understanding welding metallurgy is not simply academic. It explains why a welder must apply preheat to a thick carbon steel pipe, why a P91 component requires a mandatory postweld heat treatment hold, why an austenitic stainless steel weld must be kept below a maximum interpass temperature, and why hard zones adjacent to a weld are a leading cause of hydrogen-induced cracking in sour service environments. Without this understanding, an inspector can enforce procedures but cannot truly evaluate whether those procedures are producing the correct outcome.

This guide covers the essential concepts of welding metallurgy as they apply to practical inspection and fabrication work, with particular emphasis on ferrous alloys — carbon steels, low alloy steels, and stainless steels — which form the overwhelming majority of structural weldments in the oil and gas, petrochemical, and power generation industries.

Why Welding Metallurgy Matters to Inspectors

Mechanical properties of metals are not fixed by chemistry alone. They are determined by the interaction of chemical composition, thermal history, and mechanical treatment. A steel plate with a given chemical analysis can exhibit vastly different strength, hardness, toughness, and ductility depending on whether it has been normalised, quenched and tempered, cold-worked, or welded. The welding process introduces a complex, localised thermal cycle that modifies each of these factors simultaneously — and not uniformly.

The mechanical properties of primary concern in welded structures include:

• Tensile strength and yield strength — The ability to resist loading without permanent deformation or fracture
• Ductility — The capacity for plastic deformation before failure, critical for damage tolerance
• Toughness (notch toughness) — The energy absorbed during fracture, typically assessed by Charpy impact testing in accordance with ASME and AWS mechanical testing requirements
• Hardness — A measure of resistance to penetration, directly related to microstructure and carbon content
• Fatigue strength — Resistance to crack initiation and propagation under cyclic loading
• Creep resistance — The ability to resist slow plastic deformation at elevated temperatures, critical for P91 and similar creep-resisting steels

All of these properties are influenced — sometimes profoundly — by the thermal cycles introduced during welding. The inspector who understands welding metallurgy can recognise when a deviation from procedure is likely to compromise any of these properties and can take appropriate corrective action.

Two Key Categories of Metallurgical Change

Every change that occurs in a metal during and after welding can be traced to one of two fundamental mechanisms: the effect of increasing temperature, and the effect of cooling rate. These two categories are not independent — they are sequential stages of the same thermal event — but they produce different types of microstructural change and respond to different control measures.

Changes Due to Temperature Increase

As a metal is heated, its internal energy increases. For steel, a series of well-defined transformations occur at specific temperature thresholds:

• Below approximately 723 degrees C (the lower critical temperature, Ac1): The steel exists as a mixture of ferrite and pearlite (or ferrite and cementite, depending on carbon content). No phase change occurs at this stage, but the steel softens slightly and thermal stresses begin to develop.
• Between 723 degrees C and 912 degrees C (between Ac1 and Ac3): The steel partially transforms to austenite. This is the inter-critical region, where ferrite and austenite coexist.
• Above the Ac3 temperature: The steel is fully austenitic. Grain growth accelerates, particularly in regions closest to the fusion line where temperatures approach the solidus.
• Above the solidus temperature: The metal melts and becomes the weld pool. On solidification, a cast-type columnar grain structure forms in the weld metal.

Changes Due to Cooling Rate

On cooling, the austenite formed during heating transforms back into lower-temperature phases. The specific phases that form — and therefore the final mechanical properties — depend entirely on how rapidly the steel cools through the transformation range. This is the foundation of hardenability and is the reason why weld cooling rate is one of the most important variables in fabrication metallurgy.

The Heat-Affected Zone (HAZ)

One of the most important concepts in welding metallurgy — and one with direct relevance to inspection — is the heat-affected zone. The HAZ is the region of base metal adjacent to the weld that has not melted but has undergone microstructural changes as a result of the welding heat. Unlike the weld metal itself, the HAZ is made from the base material and inherits whatever residual elements, inclusions, or hydrogen that material contains.

Because the temperature gradient across the HAZ is steep — falling from near the liquidus at the fusion line to ambient temperature at the boundary with unaffected base metal — a single welding pass creates multiple microstructural sub-zones within the HAZ, each with different peak temperatures and therefore different final microstructures.

Sub-Zones of the HAZ

The Iron-Carbon Phase Diagram

The iron-carbon phase diagram is the single most important reference in ferrous welding metallurgy. It maps the stable phases that exist in iron-carbon alloys as a function of temperature and carbon content, and it directly explains the transformations that occur in the HAZ during welding. While a detailed treatment is given in the dedicated Iron-Carbon Phase Diagram article, the key points relevant to inspection are summarised here.

Key Critical Temperatures for Inspectors

Cooling Rate and Microstructure Formation

The transformation of austenite to its various daughter phases — ferrite, pearlite, bainite, or martensite — is governed by the competition between thermodynamic driving force (which increases with cooling rate) and diffusion kinetics (which decrease with cooling rate). This competition is captured in the Continuous Cooling Transformation (CCT) diagram, which maps the microstructure produced as a function of cooling rate for a specific steel composition.

Effect of Cooling Rate on Microstructure

This is why carbon equivalent (CE) calculations are used to pre-assess weldability before welding begins. The CE value quantifies the susceptibility of a steel to martensite formation, allowing the engineer to prescribe the appropriate preheat temperature to slow the cooling rate sufficiently.

Preheat, Interpass Temperature, and Postweld Heat Treatment

Three thermal control measures allow fabricators and inspectors to manage the metallurgical consequences of welding heat. Each addresses a different stage of the welding thermal cycle and a different metallurgical risk.

Preheat

Preheat is the deliberate application of heat to the base material before welding begins. Its primary purpose is to slow the cooling rate in the weld and HAZ, reducing the risk of martensite formation and hydrogen-assisted cold cracking. Secondary benefits include reducing thermal shock and lowering residual stress levels.

Preheat requirements depend on:

• Carbon equivalent of the base material — higher CE requires higher preheat
• Section thickness — thicker sections act as a greater heat sink, increasing cooling rate
• Hydrogen level of the welding consumable — higher diffusible hydrogen requires more aggressive preheat
• Degree of restraint — highly restrained joints require higher preheat to prevent cracking

Interpass Temperature Control

Interpass temperature is the temperature of the weld area immediately before each subsequent pass is deposited. For most carbon and low alloy steels, a minimum interpass temperature equal to the preheat temperature must be maintained throughout the weld. However, a maximum interpass temperature is also specified for:

• Austenitic stainless steels — to prevent sensitisation and limit distortion (typically 150 degrees C maximum)
• Duplex stainless steels — to control phase balance and prevent sigma phase formation (typically 150 degrees C maximum)
• High heat input processes — where excessive heat can cause grain coarsening and loss of toughness
• TMCP (thermomechanically controlled process) steels — where overheating can destroy the fine-grained microstructure produced by controlled rolling

Postweld Heat Treatment (PWHT)

PWHT is the controlled heating and holding of a completed weldment at a temperature below the lower critical temperature (A1), followed by controlled cooling. Its purposes are:

• Stress relief — reducing residual stresses that would otherwise promote stress corrosion cracking or brittle fracture
• Tempering — softening hard HAZ microstructures to improve ductility and toughness
• Hydrogen release — allowing diffusible hydrogen to escape from the weld metal and HAZ
• Dimensional stability — important for precision components subjected to subsequent machining

Ferrous Metallurgy for Inspectors

Ferrous alloys — those based on iron — form the foundation of most welded construction in the energy, petrochemical, and structural industries. Within the broad category of ferrous alloys, the inspector will most frequently encounter carbon steels, low alloy steels, and stainless steels. Each presents distinct metallurgical characteristics and inspection requirements.

Pure Iron and the Allotropic Transformations

Iron is allotropic, meaning it exists in different crystal structures at different temperatures. These crystal structures are designated using Greek letters:

• Alpha iron (ferrite) — Body-centred cubic (BCC) structure at room temperature up to 912 degrees C. Magnetic. Low carbon solubility (< 0.02 wt%).
• Gamma iron (austenite) — Face-centred cubic (FCC) structure from 912 to 1394 degrees C. Non-magnetic. High carbon solubility (up to 2.14 wt%). This is the phase present in the weld pool and CGHAZ during welding.
• Delta iron (delta ferrite) — BCC structure from 1394 to 1538 degrees C (the melting point). In stainless steel welding, delta ferrite retained at room temperature plays a critical role in preventing hot cracking.

The Role of Alloying Elements in Steel

Alloying elements are added to steel to modify one or more properties. For welding inspectors, the most relevant effects of the principal alloying elements are:

Low Alloy and High Alloy Steels

Low Alloy Steels

Low alloy steels contain less than approximately 5% total alloying elements. They are used extensively in pressure vessels, boilers, and piping systems where higher strength or elevated temperature service is required beyond what plain carbon steel can offer. Key families encountered in fabrication include:

• Chromium-Molybdenum steels (Cr-Mo) — P11 (1.25Cr-0.5Mo), P22 (2.25Cr-1Mo), and the advanced P91 (9Cr-1Mo-V) — used for elevated temperature service in power generation and petrochemical plant. The welding requirements for P91 are among the most demanding encountered in fabrication.
• Nickel steels — Used for low temperature service, including cryogenic applications. 3.5% Ni steel is common for LNG storage; 9% Ni steel extends service to -196 degrees C.
• High strength low alloy (HSLA) steels — API 5L grades X52 to X100 used in offshore pipelines. Weldability requires careful control of heat input and use of matching low-hydrogen consumables.

High Alloy Steels (Stainless Steels)

Stainless steels contain a minimum of 10.5% chromium, which forms a thin, self-repairing chromium oxide passive film that provides corrosion resistance. From a welding metallurgy perspective, the four principal families each present distinct challenges to the inspector.

Stainless Steel Metallurgy: Key Concerns for Inspectors

Sensitisation and Weld Decay

Sensitisation is the most widely encountered metallurgical issue in austenitic stainless steel welding. When austenitic stainless steel is held or cooled slowly through the temperature range of approximately 550–850 degrees C, chromium atoms preferentially migrate to grain boundaries where they combine with carbon to form chromium carbides (principally Cr₂₃C₆). This depletes chromium from the region immediately adjacent to the grain boundary to below the 10.5% minimum required for passivity, creating zones susceptible to intergranular corrosion.

The affected region typically appears as a band of corrosion attack in the HAZ parallel to and slightly removed from the weld — hence the historical term weld decay. Mitigation strategies include:

• Using extra-low carbon grades: 304L (max 0.03% C) or 316L (max 0.03% C)
• Using stabilised grades: 321 (stabilised with titanium) or 347 (stabilised with niobium), which preferentially form harmless TiC or NbC carbides rather than Cr₂₃C₆
• Solution annealing after welding (heating to 1040–1100 degrees C and quenching), which redissolves the carbides
• Controlling heat input and interpass temperature to minimise time in the sensitisation range

Hot Cracking in Austenitic Welds

Hot cracking (also called solidification cracking or liquation cracking) occurs in the weld metal or HAZ at elevated temperatures during or immediately after welding. It is caused by the segregation of low-melting-point constituents (principally sulphur and phosphorus compounds) to grain boundaries during solidification, forming thin liquid films that cannot withstand the thermal contraction stresses imposed as the weld cools.

The primary mitigation for austenitic stainless steels is the controlled presence of delta ferrite in the weld metal. Ferrite disrupts the continuous grain boundary network, preventing low-melting films from forming a connected path. The Schaeffler diagram and the WRC-1992 diagram are used by welding engineers to predict and control ferrite content in stainless steel welds.

Carbon Equivalent and Weldability Assessment

The carbon equivalent (CE) is an empirical measure of a steel’s susceptibility to martensite formation in the HAZ — and therefore its susceptibility to hydrogen-assisted cold cracking. Several formulae exist; the two most widely used in fabrication are the International Institute of Welding (IIW) formula and the Pcm formula developed for low-carbon, microalloyed steels.

Use our Carbon Equivalent Calculator to compute both IIW and Pcm values directly from your material test certificate chemistry, with automatic preheat guidance.

Mechanical Properties and Their Metallurgical Basis

Every mechanical property of a weldment has a metallurgical explanation. Inspectors who understand these connections can anticipate which properties are at risk when a fabrication deviation occurs.

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