Duplex stainless steel heat-affected zone (HAZ) microstructure analysis is a specialized metallurgical evaluation used to verify austenite–ferrite phase balance, detect detrimental precipitates such as chromium nitrides, sigma phase, and secondary austenite, and confirm the corrosion resistance and toughness of welded duplex components. This article covers the metallurgical fundamentals of the duplex HAZ, sampling practice, testing items, metallographic and instrumental methods, typical applications, and frequently asked questions, with reference to recognized standards from ASTM, ISO, EN, and GB organizations.
Technical Overview
Duplex stainless steels (DSS) are biphasic alloys engineered to contain approximately equal volume fractions of ferrite (body-centered cubic) and austenite (face-centered cubic), combining the high strength and chloride stress-corrosion-cracking resistance of ferritic grades with the toughness and formability of austenitic grades. Typical commercial grades include UNS S32304 (lean duplex 2304), UNS S32205 (standard duplex 2205), and UNS S32750 (super duplex 2507), whose pitting resistance equivalents (PREN = %Cr + 3.3 × %Mo + 16 × %N) range roughly from 24 to above 40. This dual-phase structure, however, is metastable and highly sensitive to weld thermal cycles.
The heat-affected zone (HAZ) is the portion of the base metal that is metallurgically altered by the heat of welding without melting. In duplex steels, the HAZ is conventionally divided into the high-temperature HAZ (HTHAZ), which experiences peak temperatures above approximately 1250 °C and undergoes complete ferritization followed by partial austenite reformation during cooling, and the low-temperature HAZ (LTHAZ), which is exposed to the 600–1000 °C range where intermetallic phases can nucleate. Depending on heat input, joint thickness, and process, the total HAZ width typically extends from a few hundred micrometers to several millimeters on each side of the fusion line.
The central metallurgical challenge in the duplex HAZ is kinetic: cooling that is too rapid suppresses austenite reformation and leaves nitrogen-supersaturated ferrite that precipitates chromium nitrides (Cr2N), whereas cooling that is too slow prolongs exposure to the 600–1000 °C range and promotes sigma (σ) and chi (χ) intermetallics, often accompanied by secondary austenite (γ2). Both extremes degrade pitting corrosion resistance and impact toughness. HAZ microstructure analysis therefore focuses on:
- Austenite–ferrite phase balance and its distribution across HAZ sub-zones
- Chromium nitride precipitation in ferrite, particularly in the HTHAZ and root passes
- Sigma and chi phase formation, especially in molybdenum-bearing super duplex grades
- Secondary austenite (γ2) formation and its lower alloying content
- Ferrite grain coarsening adjacent to the fusion boundary
- Consistency of the microstructure with expected mechanical and corrosion performance, evaluated against standards such as ASTM A923, ASTM E562, ASTM E1245, ISO 17639, and the applicable GB/T product standards
Testing Samples
Plate and Sheet Butt Weld Cross-Sections
Full-thickness transverse sections are extracted across butt welds in plate and sheet, capturing weld metal, both HAZ regions, and unaffected base metal in a single mount. Specimens are cut by abrasive sawing under copious coolant at a sufficient distance from the zone of interest to avoid altering the microstructure through cutting heat, and the fusion line orientation is clearly marked to guide subsequent microhardness traverses and location-specific phase measurements.
Pipe and Tube Girth Welds
Circumferential (girth) welds in duplex pipe and tube are sampled at fixed clock positions, with particular attention to the root pass, hot pass, and cap regions where thermal cycles differ most. Small-diameter tubes may be mounted as complete ring cross-sections. Chinese national product standards such as GB/T 21833 (seamless duplex tubes) and GB/T 21832 (welded duplex tubes) are frequently invoked for sampling and acceptance requirements in domestic projects.
Fillet, Repair, and Overlay Welds
Fillet welds, excavation-and-repair welds, and weld overlays receive additional scrutiny because repeat thermal cycles can either refine or further unbalance the HAZ. Multi-pass repair zones are examined section-by-section to map cumulative effects such as coarsened ferrite, increased nitride density, or intermetallic formation in reheated beads.
Procedure Qualification Coupons and Witness Samples
Welding procedure qualification coupons and production witness plates are routinely submitted to accredited third-party laboratories for HAZ microstructure verification before and during fabrication. These samples document that the qualified heat input range, interpass temperature, and shielding gas practice consistently produce an acceptable phase balance and freedom from detrimental precipitates.
Testing Items
A complete HAZ evaluation for duplex stainless steel typically comprises the following items, each tied to a defined location relative to the fusion line (for example, at the fusion boundary, and at 0.5 mm, 1 mm, and 2 mm into the HAZ):
- Ferrite and austenite volume fractions in HTHAZ, LTHAZ, and base metal (target typically near 50/50, with common acceptance windows such as 35–65% ferrite depending on the governing specification)
- HAZ width mapping and identification of sub-zones, including the extent of the fully ferritized region
- Detection, density estimation, and morphology classification of chromium nitride (Cr2N) precipitates in ferrite
- Screening for sigma (σ) and chi (χ) intermetallic phases, especially at 600–950 °C exposure locations
- Identification of secondary austenite (γ2) and assessment of its chromium, molybdenum, and nitrogen depletion
- Ferrite grain size and coarsening assessment adjacent to the fusion boundary, referenced to ASTM E112 practice
- Microhardness profiles across weld metal, HAZ, and base metal to reveal hard or softened bands
- Complementary acceptance tests where specified: Charpy V-notch impact energy and ferric chloride pitting exposure per ASTM A923 Methods B and C, or pitting tests per ASTM G48
- Ferrite content screening on actual fabrications by magnetic induction instruments in accordance with ISO 8249
Testing Methods
Specimen Preparation
Metallographic preparation follows ASTM E3. After coolant-lubricated sectioning and mounting (hot epoxy or cold resin), specimens are ground through successive silicon carbide grades and polished with 6 µm and 1 µm diamond suspensions, finishing with colloidal silica oxide polishing. Careful final polishing is essential for duplex grades, because smeared ferrite can mask fine nitrides and distort image-analysis thresholding.
Etching and Optical Metallography
Several etchants are used selectively, consistent with ASTM E407 and ASTM A923 Method A. Electrolytic etching in 10–20% sodium hydroxide solution (approximately 3–6 V for several seconds) colors ferrite tan-to-brown, leaves austenite bright, and outlines sigma phase and chromium nitrides as dark-etched constituents, making it the primary screen for detrimental phases. Kalling reagent No. 2 attacks ferrite preferentially and outlines intermetallics; Beraha-type tint etchants render ferrite and austenite in contrasting colors for phase quantification; boiling alkaline Murakami reagent selectively stains sigma phase for confirmation. Optical examination proceeds from low magnification (about 50×–100×) for zone mapping to 400×–1000× for nitride and intermetallic screening, with macro- and micro-examination of welds additionally structured in accordance with ISO 17639.
Quantitative Phase Analysis
Phase balance is measured by systematic manual point count per ASTM E562 (typically a 100-point grid applied to 10–20 fields per location, reported with statistical confidence) or by automatic image analysis per ASTM E1245, in which grey-level thresholding separates the etch-contrasted phases. Field selection must avoid weld defects, etching artifacts, and nitride clusters that bias results, and measurements are reported separately for each HAZ sub-zone rather than averaged across the joint.
Scanning Electron Microscopy, EDS, and EBSD
Scanning electron microscopy (SEM) in backscattered-electron mode resolves Cr2N particles, which are often on the order of tens of nanometers and may be ambiguous optically. Energy-dispersive X-ray spectroscopy (EDS) confirms the chromium enrichment of sigma phase and the chromium–molybdenum enrichment of chi phase, as well as the relative depletion of γ2. Electron backscatter diffraction (EBSD) provides phase and orientation maps that distinguish fcc austenite from bcc ferrite independent of etching response, quantify phase fractions with high fidelity, and reveal Widmanstätten and grain-boundary austenite morphologies formed during cooling.
Microhardness Testing
Microhardness traverses are performed per ASTM E384 or ISO 6507-1 using HV0.5 or HV1 loads, with indent spacing of at least 2.5 diagonal lengths, running from the weld metal across the fusion line into unaffected base metal. Standard 2205 HAZ values typically fall in the range of roughly 230–300 HV; localized excursions well above the base-metal level flag possible sigma formation, while systematic softening or hardening bands guide where supplementary metallography should be concentrated.
Complementary Corrosion and Toughness Testing
Where acceptance is contractually tied to performance testing, ASTM A923 Method B (Charpy V-notch at the specified sub-zero temperature, commonly −15 °C for grade 2205 plate with minimum energy on the order of 54 J) and Method C (ferric chloride exposure with a typical maximum weight loss of about 10 mg/dm²/day) are applied. ASTM G48 Method A pitting exposures and critical pitting temperature measurements per ASTM G150 may be added for super duplex components in severe chloride service, linking microstructural findings to quantitative corrosion performance.
Applications
HAZ microstructure analysis is applied wherever welded duplex stainless steel must retain its designed combination of strength and chloride corrosion resistance. Representative fields include:
- Oil and gas flowlines, risers, manifolds, and subsea hardware in standard and super duplex grades
- Chemical and petrochemical process equipment, including reactors, columns, heat exchangers, and piping handling chlorides or sour media
- Desalination plants and seawater cooling, intake, and firefighting systems
- Pulp and paper digesters, bleach plant washers, and liquor handling equipment
- Flue gas desulfurization scrubbers and other pollution-control installations
- Pressure vessels, storage tanks, and structural members in marine and bridge engineering
- Welding procedure qualification, welder performance qualification, failure analysis, and independent verification by accredited third-party laboratories during project surveillance
FAQ
What is the heat-affected zone in duplex stainless steel welding?
The HAZ is the band of base metal adjacent to the weld that was heated high enough to transform microstructurally but not melted. In duplex steel it consists of a high-temperature region that ferritizes completely and reforms austenite during cooling, and a lower-temperature region where partial transformation and intermetallic precipitation risks occur.
What ferrite content is acceptable in the duplex HAZ?
Acceptance depends on the governing project specification, but a balanced target near 50% ferrite / 50% austenite is standard, and many specifications accept approximately 35–65% ferrite in the HAZ. Excessive ferrite indicates overly rapid cooling and is commonly accompanied by chromium nitride precipitation, reduced toughness, and lower pitting resistance.
Why do chromium nitrides form, and how are they controlled?
Rapid cooling through roughly 800–500 °C leaves nitrogen supersaturated in ferrite, which precipitates as Cr2N at ferrite boundaries and within grains, locally depleting chromium. Control measures include adequate (but not excessive) heat input—commonly about 0.5–2.5 kJ/mm for grade 2205 and lower ranges for super duplex—controlled interpass temperatures (typically maximum 150 °C for standard duplex and 100 °C for super duplex), and nitrogen additions to shielding and root backing gases to stabilize reformed austenite.
What is secondary austenite (γ2) and why does it matter?
Secondary austenite forms during cooling or reheating, either coupled with Cr2N or sigma precipitation or by transformation of ferrite. Because γ2 is depleted in chromium, molybdenum, and nitrogen relative to primary austenite, its pitting resistance equivalent is lower, making it a preferred initiation site for pitting attack.
Which standards govern HAZ evaluation and acceptance?
Core references include ASTM A923 (detection of detrimental intermetallic phases, Methods A/B/C), ASTM E3 and E407 (preparation and etching), ASTM E562 and E1245 (phase quantification), ASTM E384 and ISO 6507-1 (microhardness), ISO 17639 (weld macro- and micro-examination), ISO 8249 (ferrite measurement of welds), ASTM G48 and G150 (corrosion), and relevant GB/T duplex product standards for Chinese-market fabrications.
Can the HAZ be evaluated non-destructively?
Partially. Magnetic-induction ferrite measurement per ISO 8249 provides a rapid surface screening of ferrite content, and field metallographic replication allows on-site optical examination of polished and etched surfaces. Definitive characterization of nitrides, intermetallics, and sub-surface phase balance, however, requires sectioned laboratory specimens examined by optical microscopy, SEM/EDS, and EBSD.