Duplex stainless steel microhardness testing is a precision laboratory technique used to evaluate the localized hardness of ferrite and austenite phases, weld zones, and heat-affected regions in duplex and super duplex alloys such as UNS S31803, S32205, and S32750. Performed in accordance with ASTM E384, ISO 6507-1, and GB/T 4340.1, the method detects detrimental precipitates such as sigma phase and alpha-prime embrittlement that conventional bulk hardness testing cannot reveal. This article explains the applicable standards, specimen preparation, test parameters, measurement procedures, and typical applications of microhardness testing for duplex stainless steels.
Technical Overview
Duplex stainless steels are chromium-nickel-molybdenum alloys engineered with a two-phase microstructure consisting of approximately equal proportions of ferrite and austenite. This dual-phase structure combines the high strength and chloride stress-corrosion cracking resistance of ferritic grades with the toughness and weldability of austenitic grades. Common commercial grades include lean duplex UNS S32304 (EN 1.4362), standard duplex UNS S31803 and S32205 (EN 1.4462), and super duplex UNS S32750 and S32760.
Because the two phases respond differently to thermal processing and mechanical work, bulk hardness measurements alone cannot reveal localized changes in the material. Microhardness testing uses a diamond indenter applied under very low loads, typically between 10 gram-force and 1 kilogram-force, to produce indentations small enough to be placed within individual grains or specific microstructural features. This allows independent evaluation of the ferrite and austenite phases, weld metal, heat-affected zones, and intermetallic precipitates.
- ASTM E384 covers Vickers and Knoop microindentation hardness testing of materials.
- ISO 6507-1 and GB/T 4340.1 specify the Vickers hardness test method, including the micro-load range.
- ISO 6507-3 and GB/T 4340.3 cover the calibration of reference blocks used for indirect verification of testing machines.
- ASTM E92 covers Vickers and Knoop hardness testing more broadly, complementing E384.
In properly solution-annealed duplex stainless steel, typical hardness values range from approximately 200 HV for lean grades to 310 HV for super duplex grades. Local hardness values substantially above the base-metal level usually indicate the presence of sigma phase, chromium nitrides, alpha-prime precipitation, or cold work, all of which can degrade toughness and corrosion resistance.
Testing Samples
Plate, Sheet, and Strip Samples
Flat-rolled products are sectioned to a convenient size, typically 20 to 30 millimeters square, and mounted with the test surface perpendicular to the through-thickness direction or parallel to the rolling plane depending on the evaluation objective. Full-thickness sections allow hardness traverses from surface to mid-thickness to detect decarburization, surface hardening from grinding, or uneven annealing.
Pipe and Tube Samples
Ring sections or longitudinal strips are removed from seamless and welded pipe. For longitudinally welded pipe, the specimen must retain the complete weld cross-section, including weld metal, heat-affected zones on both sides, and adjacent base metal, so that a continuous hardness traverse can be performed across all zones.
Welded Joint Samples
Weldment specimens are the most frequently tested duplex samples because duplex welds are sensitive to heat input, cooling rate, and nitrogen loss. Cross-sectional specimens are prepared for multipass welds, dissimilar joints between duplex and austenitic steels, cladding overlays, and repair welds. Both root, fill, and cap regions are evaluated, as secondary austenite formation and chromium nitride precipitation are often concentrated near the root and interpass regions.
Forging, Casting, and Component Samples
Coupons cut from forgings, investment castings, flanges, valve bodies, pump components, and service-aged parts are tested to verify heat treatment condition, assess long-term embrittlement, or support failure analysis. Additively manufactured duplex components may also be tested in as-built and heat-treated conditions to map anisotropy and process effects.
Testing Items
Typical microhardness test items for duplex stainless steel include the following:
- Vickers microhardness at loads of HV0.05, HV0.1, HV0.2, HV0.5, and HV1 for general mapping and zone evaluation.
- Knoop microhardness for elongated indents suitable for very thin layers, coatings, and near-edge measurements.
- Phase-specific hardness measurements placed individually within ferrite and austenite grains to quantify phase-by-phase strengthening or embrittlement.
- Hardness traverses across welded joints covering base metal, heat-affected zone, fusion line, and weld metal at defined intervals.
- Hardness verification of intermetallic phases such as sigma, chi, and chromium nitride precipitates at grain boundaries and ferrite-austenite interfaces.
- Through-thickness hardness profiles to detect surface effects such as grinding damage, cold-work hardening, or decarburized layers.
- Statistical hardness distribution reporting including minimum, maximum, mean value, and standard deviation for each defined zone.
Testing Methods
Specimen Preparation
Accurate microhardness results depend heavily on metallographic preparation quality. The specimen is sectioned with abrasive cutting under abundant coolant to avoid heat alteration, mounted in epoxy or thermosetting resin, ground sequentially with silicon carbide papers from approximately 120 grit to 1200 grit or finer, and then polished with diamond suspensions down to 1 micrometer, finishing with oxide polishing suspension of approximately 0.05 micrometer where phase contrast is needed. Any smeared metal, embedded abrasive, or surface relief will distort the indentation diagonals and bias the results.
Etching and Phase Identification
Light etching is applied to reveal the two-phase structure so that indentations can be positioned within a specific phase. Common etchants and methods include:
- Kalling reagent number 2, which leaves austenite bright and colors ferrite, providing clear phase differentiation.
- Electrolytic etching in 10 to 20 percent sodium hydroxide or potassium hydroxide solution, which outlines intermetallic phases and differentiates ferrite for intergranular corrosion assessment.
- Glyceregia for revealing the general microstructure and weld zone boundaries.
Etching must be light and uniform, because aggressive etching creates surface relief that rounds the indentation edges. Where phase identification by etching is not compatible with measurement, an alternative sequence is to place indentations first on the polished surface and then etch lightly afterward to confirm which phase each indent occupied.
Test Load and Dwell Time Selection
Test load selection balances two requirements: the indentation must be small enough to remain within a single phase, yet large enough for the diagonals to be measured with acceptable precision. For duplex stainless steel with typical annealed grain sizes, HV0.1 and HV0.2 are the most commonly selected loads for phase-specific measurements, while HV0.5 and HV1 are suitable for zone averages across welds. The dwell time is normally 10 to 15 seconds in accordance with ISO 6507-1 and GB/T 4340.1.
Indentation Placement and Spacing
Indentations must be placed at least one-half of the expected diagonal length away from grain boundaries and phase interfaces so that the plastic zone does not interact with an adjacent phase. In accordance with ISO 6507-1 spacing rules for steels, the distance between the centers of two adjacent indentations should be at least three times the diagonal length, and the distance from the specimen edge should be at least two and one-half times the diagonal. For weld traverses, indentations are commonly spaced at 0.5 to 1.0 millimeter intervals or closer where steep hardness gradients are expected, such as near the fusion line.
Equipment Verification and Data Processing
The microhardness tester is verified before use with certified reference blocks covering the test load range, following the indirect verification procedures of ISO 6507-3 and ASTM E384. Diagonals are measured optically at magnifications that allow resolution consistent with the standard, and hardness values are calculated from the standard Vickers equation. Reports should record the test force, dwell time, number of indentations, etching condition, and the locations of each measurement relative to recognizable microstructural features.
Applications
Microhardness testing is a key quality-control and diagnostic tool throughout the life cycle of duplex stainless steel products:
- Welding procedure qualification and production weld surveillance, where hardness maps confirm acceptable heat input, cooling rate, and nitrogen management in the heat-affected zone.
- Detection of sigma phase and other intermetallic precipitates formed by slow cooling after solution annealing, extended exposure in the 600 to 950 degree Celsius range, or improper multipass welding, typically indicated by local hardness values well above the base-metal level.
- Assessment of 475 degree Celsius embrittlement in service-aged components, in which alpha-prime precipitation raises the hardness of the ferrite phase markedly while austenite remains comparatively unchanged.
- Verification of solution-annealing heat treatment by confirming that both phases have returned to expected baseline hardness values.
- Evaluation of cold-work hardening in formed heads, bent pipe, deep-drawn components, and fasteners, where through-thickness hardness profiles quantify the degree of deformation.
- Failure analysis of cracked or corroded components, where elevated local hardness helps identify embrittled zones, sensitized areas, or mechanically damaged surfaces.
- Research and development of lean and super duplex compositions, additive manufacturing parameters, and dissimilar metal joints.
- Compliance demonstration for sour service and other demanding specifications in which hardness limits are imposed on duplex weldments and base metal.
FAQ
What is the typical hardness range of annealed duplex stainless steel?
Solution-annealed lean duplex grades such as UNS S32304 typically measure approximately 200 to 230 HV, standard duplex grades such as UNS S32205 measure approximately 230 to 290 HV, and super duplex grades such as UNS S32750 and S32760 measure approximately 270 to 310 HV. In the annealed condition, the ferrite phase is usually slightly harder than the austenite phase because of its higher chromium and molybdenum content in solid solution.
Which test load should be used for phase-specific measurements?
Loads of HV0.1 to HV0.2 are generally recommended for placing indentations entirely within a single phase of standard duplex microstructures. Lower loads such as HV0.05 may be used for fine microstructures or individual precipitates, while HV0.5 and HV1 are preferred for average zone hardness across welds where individual phase resolution is not required.
How does microhardness detect sigma phase and 475 degree Celsius embrittlement?
Sigma phase and alpha-prime precipitates are far harder than the matrix phases. When microhardness measurements within the ferrite phase or at phase boundaries return values substantially above the annealed baseline, for example local values approaching or exceeding 400 HV, the presence of embrittling precipitates is strongly indicated. Microhardness data are then confirmed by metallography, and where necessary by corrosion testing and quantitative phase analysis.
Which standards govern duplex stainless steel microhardness testing?
The primary method standards are ASTM E384 for microindentation hardness of materials, and ISO 6507-1 or the equivalent GB/T 4340.1 for the Vickers test. Machine and reference block verification follow ISO 6507-3 and GB/T 4340.3. Product-specific acceptance limits are defined by the applicable material specifications, fabrication codes, or end-user requirements.
Can microhardness results be converted to Rockwell or Brinell values?
Standardized hardness conversion tables exist for steels, but conversions between microhardness and macro scales are approximate and should never substitute for direct measurement in acceptance testing. For duplex stainless steel with its two-phase structure, conversions carry additional uncertainty, and the specified test scale should be applied directly whenever a specification defines one.
How many indentations should be recorded for a valid result?
For zone-average measurements, a minimum of three to five valid indentations per zone is customary, and larger statistical populations are recommended where hardness scatter is expected, such as in weld metal. Phase-specific studies typically report separate mean values and standard deviations for ferrite and austenite so that embrittlement of one phase is not masked by averaging both phases together.