Alloy Desulfurization Nozzle Hardness Testing: Methods, Standards, and Quality Control

Alloy desulfurization nozzle hardness testing is an essential quality control procedure for verifying the mechanical integrity, heat treatment condition, and corrosion resistance of spray nozzles used in flue gas desulfurization systems. This article explains the testing samples, hardness testing items, applicable methods, and international standards such as ASTM, ISO, and GB used by accredited third-party laboratories to evaluate duplex stainless steel, super austenitic, and nickel-based alloy nozzles.

Technical Overview

Alloy desulfurization nozzles are key atomization components installed in spray absorption towers of wet flue gas desulfurization (FGD) systems. They spray limestone-gypsum slurry or process liquor into hot flue gas to remove sulfur dioxide, operating continuously in a combined environment of chloride-bearing corrosive media, abrasive solid particles, and thermal cycling. Because of these severe service conditions, the nozzles are typically manufactured from corrosion-resistant alloys such as duplex stainless steel (UNS S31803, S32205), super duplex stainless steel (UNS S32750, S32760), super austenitic stainless steel (904L), cast duplex alloy CD4MCu, or nickel-based alloys including Alloy 625 and Alloy C-276.

Hardness testing is one of the most informative and cost-effective inspection techniques for these components. It verifies the effectiveness of solution annealing and quenching, detects harmful phases such as sigma and chromium carbides that increase hardness and embrittle the alloy, confirms adequate strength after casting or forming, and screens for excessive hardness values that correlate with reduced resistance to chloride stress corrosion cracking. Hardness measurement is therefore specified in most purchase specifications for FGD nozzle assemblies, weld joints, and repair-welded areas, and is routinely performed by accredited third-party laboratories during incoming inspection, manufacturing surveillance, and failure analysis.

Testing Samples

Cast Alloy Nozzle Bodies

Investment-cast or sand-cast desulfurization nozzles, including spiral spray nozzles, tangential hollow-cone nozzles, and axial-flow nozzles, are tested in both the as-cast and solution-annealed conditions. Testing confirms that the heat treatment has dissolved secondary phases and restored the toughness and corrosion resistance of the cast structure.

Wrought Alloy Components

Nozzle bodies, connecting flanges, adapter sockets, and swirl inserts fabricated from wrought plate, bar, or forged blanks are tested to verify compliance with the hardness limits of the applicable material standards, typically after final solution annealing or stabilization treatment.

Weld Joints and Heat-Affected Zones

Nozzle-to-pipe welds, nozzle-to-flange joints, and internal overlay welds are tested across the weld metal, the heat-affected zone (HAZ), and the base metal. Hardness traverses reveal whether the welding procedure has caused excessive hardening, sensitization, or formation of detrimental intermetallic phases.

Repair-Welded Areas and Hardfacings

Areas that have undergone weld repair, as well as erosion-resistant hardfacing or weld overlay layers applied to extend nozzle service life, are tested to characterize the deposited hardness, the dilution zone, and the integrity of the bond line.

Testing Items

Typical hardness testing items for alloy desulfurization nozzles include:

• Bulk Rockwell hardness (HRB, HRC) of nozzle bodies and machined surfaces
• Brinell hardness (HBW) of castings and thick sections
• Vickers hardness (HV10, HV30) for precision verification and referee testing
• Microhardness profiling (HV0.5, HV1) across weld metal, HAZ, and base metal
• Hardness uniformity surveys at multiple locations and orientations around the nozzle
• Hardfacing or overlay layer hardness and effective depth verification
• Maximum hardness screening for chloride stress corrosion cracking resistance, commonly limited to approximately 22 HRC or 237 HV10 for susceptible alloys in aggressive chloride environments
• Estimated tensile strength conversion from hardness values for quality surveillance purposes

Testing Methods

Rockwell Hardness Testing

Rockwell testing is performed in accordance with ASTM E18, ISO 6508, or GB/T 230.1. Austenitic and duplex stainless steel nozzles are commonly tested using the HRBW scale with a 1.588 mm tungsten carbide ball; harder materials and hardfacings are tested on the HRC scale. The method is fast and well suited to production inspection on ground flat areas of the nozzle.

Brinell Hardness Testing

Brinell testing per ASTM E10, ISO 6506, or GB/T 231.1 applies a 10 mm tungsten carbide ball, typically with a 3000 kgf load, and averages hardness over a large area. This makes it ideal for coarse cast structures where local microstructural variations must be smoothed out. Test surfaces are prepared by sawing or flat grinding to obtain a smooth, parallel face.

Vickers and Microhardness Testing

Vickers testing per ASTM E92, ISO 6507, or GB/T 4340.1 uses a diamond pyramid indenter and is employed for precise measurements, thin sections, and referee testing. Microhardness testing per ASTM E384 or ISO 14577 uses low loads such as HV0.5 or HV1 on polished and etched cross-sections to map hardness distributions across welds, HAZ boundaries, and overlay dilution zones with indentation spacing in accordance with the standards.

Portable and On-Site Hardness Testing

For installed nozzles or large assemblies that cannot be brought to a bench machine, portable methods are used. The Leeb rebound method per ASTM A956, ISO 16859, or GB/T 17394 enables rapid on-site surveys, while the ultrasonic contact impedance (UCI) method per ASTM A1038 is suitable for thin walls and weld zones. Results from portable instruments are verified against bench-method correlations before acceptance.

Sample Preparation and Result Evaluation

All indentation methods require removal of scale, decarburized layers, or sprayed coatings to expose representative base material, surfaces that are flat and free from vibration, and specimen thickness of at least ten times the indentation depth. Minimum spacing between indentations and from edges follows the requirements of the respective standards. At least three to five valid readings are taken and averaged for each location, and hardness conversions between scales are made only in accordance with ASTM E140, ISO 18265, or GB/T 1172.

Applications

Hardness testing of alloy desulfurization nozzles is applied throughout the energy, metallurgical, and environmental protection industries:

• Wet limestone-gypsum FGD spray absorption towers in coal-fired power plants
• Sintering machine and pelletizing plant flue gas desulfurization in the iron and steel industry
• Waste incineration flue gas scrubbing and acid gas removal systems
• Refinery and petrochemical gas scrubbers handling chloride-rich media
• Marine exhaust gas cleaning systems (scrubbers) on ships and offshore platforms
• Incoming inspection, weld procedure qualification, repair validation, and root-cause failure analysis of nozzles

FAQ

Why is hardness testing important for desulfurization nozzles?

Hardness reflects the combined effects of chemical composition, casting quality, and heat treatment. Abnormally high hardness can indicate sigma phase precipitation, carbide formation, or cold work, all of which reduce toughness and increase susceptibility to chloride stress corrosion cracking in FGD slurry environments. Hardness also provides a rapid estimate of tensile strength for batch conformity checks.

What hardness values are typical for alloy desulfurization nozzles?

Solution-annealed duplex stainless steel nozzles typically measure about 180 to 280 HBW, super austenitic grades about 150 to 220 HBW, and nickel-based alloys about 150 to 250 HBW. Hardfacing layers may range from 40 to 60 HRC depending on the deposited alloy. Specific limits are defined by the applicable material specification and purchase contract.

Can hardness testing be performed on installed nozzles without removal?

Yes. Portable Leeb and UCI instruments allow on-site hardness surveys of installed nozzles and headers, provided the surface is prepared correctly and the instrument is calibrated on a reference block of similar material and mass. Bench Rockwell, Brinell, or Vickers testing remains the preferred referee method for acceptance decisions.

How is weld hardness evaluated?

A hardness traverse is taken across the polished cross-section of a production test coupon, measuring the weld metal, fusion line, HAZ, and base metal at defined intervals. Peak HAZ hardness is compared against specification limits to confirm that the welding procedure avoids sensitization, excessive hardening, and detrimental phase formation.

Can hardness values be converted to tensile strength?

Yes, approximate tensile strength can be estimated from hardness using standardized conversion tables such as ASTM E140, ISO 18265, or GB/T 1172. These conversions are indicative only; contractual strength verification still requires tensile testing on representative specimens when specified.