How to Specify Duplex Stainless-Steel Tower Internals and Their Welds

Duplex stainless steel can provide higher strength and improved resistance to chloride stress corrosion cracking compared with common austenitic grades in suitable services. Those benefits depend on a controlled mixture of ferrite and austenite and on avoiding damaging intermetallic phases. Incorrect heat treatment, welding heat input, filler selection, or cooling can leave a component with the correct chemical composition but unacceptable toughness and corrosion resistance.

This distinction is critical for thin tower internals. PMI may confirm chromium, nickel, molybdenum, and nitrogen within the expected alloy family, but it cannot prove phase balance, weld quality, or freedom from sigma phase. Procurement must control the complete metallurgical condition.

Why Duplex Performance Depends on Phase Balance

Duplex stainless steels derive their properties from both ferritic and austenitic phases. Excessive ferrite can reduce toughness and corrosion performance; excessive austenite may reduce strength or indicate an unsuitable thermal cycle. Harmful intermetallic phases can precipitate when material spends too long in critical temperature ranges.

Base plate or sheet is normally supplied in a solution-annealed condition with rapid cooling. Subsequent welding changes the microstructure in the weld metal and heat-affected zone. Very low heat input can cool too quickly and leave excessive ferrite. Excessive heat input or high interpass temperature can slow cooling and promote intermetallic precipitation or excessive grain growth.

The acceptable range depends on alloy grade, product form, thickness, welding process, service, and project specification. A single ferrite number copied from another project is not a complete welding requirement.

Select Grade from the Actual Environment

“Duplex” includes lean duplex, standard duplex, super duplex, and other compositions with different pitting resistance, strength, weldability, temperature limits, and availability. Selection should consider chlorides, temperature, pH, oxidizing species, sulfides, solids, crevices, cleaning chemicals, and shutdown conditions.

High alloy content does not eliminate every mechanism. Duplex grades can suffer hydrogen-related damage in certain sour environments, intergranular attack after improper heating, crevice corrosion under deposits, and reduced toughness outside suitable temperature ranges. The materials engineer should define the approved grade and condition for each component.

Thin perforated sheets, wire, fasteners, castings, and weld filler may not share identical properties or availability. Every product form should be specified, including small clips, nuts, washers, mesh, and attachment welds. Substituting austenitic hardware may introduce galvanic, strength, or thermal-expansion differences.

Control Forming and Heat Treatment

Cold forming changes strength, residual stress, and local microstructure. Tight bends in tray panels, distributor troughs, or clips may require larger bend radii or procedure qualification. Springback differs from austenitic stainless steel and can lead to forced fit-up if fabrication drawings use unsuitable allowances.

Hot forming or heat straightening must stay within an approved thermal procedure. Local torch correction can expose material to damaging temperatures without controlled cooling. Flame straightening after welding should not be accepted as ordinary dimensional repair.

Surface contamination from carbon-steel tools can initiate rusting and localized corrosion. Cutting, grinding, and forming equipment should be controlled. Thermal-cut edges may require removal of oxide and affected metal according to the fabrication specification.

Qualify Welding for Thin Internals

The welding procedure should define base and filler metals, joint design, shielding and backing gases, heat input range, interpass temperature, travel speed, cleaning, and repair limits. Nitrogen-containing shielding or backing gas may be used in qualified procedures to support phase balance, but gas composition should not be improvised in production.

Root shielding is important where the process contacts the reverse side. Heavy oxidation reduces corrosion resistance and can conceal defects. Weld surfaces should be accessible for cleaning and examination without grinding thin sheet below minimum thickness.

Autogenous welding may produce excessive ferrite in some applications because no nickel-enriched filler is added. It should be used only when qualified for the alloy, thickness, joint, and service. Tack welds and small intermittent attachments receive rapid thermal cycles and deserve the same metallurgical control as major seams.

Repair welding adds repeated heat exposure. Set limits on repair cycles and require engineering review when defects recur. Large weld buildup used to correct poor fit can distort the internal and create unfavorable microstructure.

Inspection Beyond PMI

Material certificates and PMI establish identity but not final condition. Depending on criticality, inspection may include ferrite measurement, corrosion testing, impact testing, hardness, macro examination, metallography, surface examination, or testing of production coupons. The specification should state method, location, sampling, surface preparation, and acceptance criteria.

Ferrite measurement on thin, curved, or rough welds can be influenced by probe size, calibration, geometry, and nearby base metal. Results should be interpreted using a qualified method rather than collected as isolated numbers. A compliant average should not hide one untested repair or heat-affected zone.

Visual inspection should check oxidation, undercut, lack of fusion, excessive grinding, arc strikes, distortion, and incomplete cleaning. Dye penetrant examination can detect surface-breaking defects on suitable prepared surfaces. Finished dimensions, levelness, and opening sizes remain essential because metallurgical compliance does not guarantee hydraulic performance.

Surface Restoration and Installation

Heat tint and embedded contamination should be removed by approved mechanical or chemical methods. Pickling and passivation procedures must be compatible with the duplex grade, thin geometry, and environmental requirements. Thorough rinsing and drying prevent chemical residue or chloride contamination.

Protect cleaned components during packaging and site storage. Field welding, grinding, bending, or mixing hardware can invalidate shop controls. Any necessary field modification should use the qualified procedure, controlled consumables, shielding, examination, and surface restoration.

Installation should avoid forced alignment and over-tightening. Duplex strength may be high, but thin panels and bolt holes can still deform. Dissimilar-metal contact and electrical bonding details should be reviewed for the specific service.

The duplex specification succeeds when chemistry, microstructure, fabrication, welding, inspection, and surface condition remain linked through traceable records.