Welding Titanium-Steel Clad Plate: Strip Backing and Argon Purity

Why Titanium-Steel Clad Plate Is Difficult to Weld

Titanium-steel clad plate joins two metals that cannot form a sound fusion weld with each other. At welding temperature titanium absorbs iron and forms intermetallic compounds such as TiFe and TiFe2, which are hard and brittle, and the joint cracks either during cooling or under the first service load. The dilution zone also picks up interstitial elements from the steel side, and titanium reacts readily with oxygen, nitrogen, hydrogen and carbon above roughly 400 degrees Celsius. A successful weld therefore does not attempt to fuse titanium to steel at all: the steel is welded to steel, the titanium is welded to titanium, and the two weld systems are separated by a transition detail that never brings the dissimilar metals into a molten pool.

The base material itself is specified separately. Titanium-steel clad plate is covered by ASTM B898 for reactive and refractory metal clad plate, and in Chinese practice by GB/T 8546 for titanium-steel clad plate, with the titanium cladding layer normally conforming to ASTM B265 grade 1, grade 2 or grade 7 depending on corrosion duty. The clad plate is supplied in the explosion-bonded, roll-bonded or explosion-plus-roll-bonded condition, and the bond shear strength and bond integrity from the mill are the starting point for any welding procedure; a weld cannot compensate for a poorly bonded plate.

The Strip-Backing Method: Geometry and Sequence

The standard industrial solution is strip backing, sometimes called the strip or batten method. The titanium layer is removed from a band along the joint edge, typically 15 mm wide on each side of the joint centreline, so that the steel substrate is exposed. The steel seam is then welded and inspected as a conventional steel weld, with the titanium cladding kept well clear of the arc. Finally a titanium strip, commonly 50 mm wide and matching the cladding grade, is placed over the completed and dressed steel weld and welded to the remaining titanium cladding on both sides, giving a continuous corrosion-resistant cover.

The dimensions are not arbitrary. The 15 mm removal width must be wide enough that the steel welding arc, spatter and heat-affected zone do not reach the titanium edge, and the 50 mm strip must overlap the original cladding by enough width to place a sound titanium fillet without running into the step left by the removal. For thicker cladding or higher heat input procedures, both dimensions are usually increased. The strip is bent or pre-formed to follow curvature on vessels and heads, and the overlap edges are fitted with a gap small enough to be bridged by the titanium weld without burn-through.

Sequence control matters on large equipment. Steel seams are completed and non-destructively tested first, and only then is the titanium cover welded, because any later steel welding near the strip would oxidise the titanium and destroy the cover. On nozzle and manway connections the same principle applies: the steel nozzle is welded to the steel shell before the titanium lining strip is closed out.

Shielding Gas and Atmospheric Protection

Titanium welds are protected by argon of not less than 99.99 percent purity, with a dew point low enough that moisture does not introduce hydrogen. Three separate gas paths are normally required: torch shielding, a trailing shield that extends coverage until the weld has cooled below about 400 degrees Celsius, and a purge on the back side of the joint. On strip backing, the back purge is delivered through a grooved backing bar or a sealed channel under the strip, and the oxygen content of the purge gas is checked at the outlet before the arc is struck, with a target well below 100 parts per million.

Colour is the field indicator. A silver or light straw weld surface is acceptable; a blue or purple tint indicates excessive oxygen and a grey or white powdery scale indicates contamination severe enough that the affected metal must be removed. Because the covering strip is thin and the weld is short, heat input is kept low with small electrodes, stringer beads and controlled travel speed. Contact of the hot titanium with the steel substrate must be avoided, since iron pickup produces a hard, brittle layer that shows up as cracking along the fusion boundary in later service.

Cleaning, Fit-Up and Environmental Conditions

Before welding, the joint area is cleaned of oxide, oil, grease, fingerprints and embedded iron. Oxide is removed mechanically with a titanium or carbide burr, or by pickling followed by thorough rinsing, in line with the descaling and cleaning guidance of ASTM B600. Stainless steel wire brushes, carbon steel tools and grinding wheels previously used on steel must not touch the titanium, because embedded iron becomes a contamination site. Only dedicated titanium tools, clean aluminium oxide or silicon carbide abrasives, and lint-free wipes with an approved solvent are used.

When the ambient temperature is below 5 degrees Celsius, the steel surface is preheated by flame before welding, primarily to remove moisture and to avoid a cold, fast-cooling joint; the titanium side is not flame heated directly, since a flame deposits carbon and hydrogen. Wind, draughts and humidity defeat gas shielding more often than any other single factor, so welding of titanium is carried out under screens, and in workshops with high humidity the shielding gas line is purged for a longer period.

Filler Metal and Procedure Qualification

Titanium filler is selected to match or slightly under-match the cladding grade, typically ERTi-1 or ERTi-2 to AWS A5.16, and the wire must be free of cracks, laps and interlayer defects and handled with gloves so that chlorides from skin contact do not enter the weld. Filler and base metal are stored in sealed packaging and, once opened, kept in a dry cabinet. Hydrogen and moisture are the two contaminants that most often cause delayed cracking in titanium welds.

Procedure qualification follows the applicable pressure vessel code, in most cases ASME BPVC Section IX, or ISO 15614-5 for titanium and zirconium alloys where the European route applies, with procedure qualification and welder performance qualification tested separately. Essential variables include gas purity and flow, trailing shield arrangement, interpass temperature, cleaning method and the strip geometry. Mechanical testing of the qualification assembly normally includes bend tests on the titanium cover weld and, where the design requires it, a shear or peel assessment of the strip-to-cladding joint. Visual and penetrant examination of the titanium cover, plus a helium leak test on the finished closure weld for vessels, complete the acceptance chain.

Weld Defects, Repair Rules and Acceptance Criteria

Defect	Typical cause	Acceptance or repair route
Blue or grey oxidation tint	Shielding breakdown, excessive heat input, draught	Remove contaminated metal and reweld; discolouration is not acceptable on the process side
Porosity in the titanium cover	Moisture or hydrocarbon on filler or joint faces	Excavate to sound metal and repair weld with fresh shielding
Cracking at the titanium-steel interface	Iron pickup, dilution, direct arc on steel adjacent to titanium	Cut out the affected region, reinstate the strip detail rather than local patch welding
Burn-through or undercut on thin strip	Excess current, poor fit-up gap, wrong strip thickness	Replace the strip section; do not bridge with weld metal
Unbonded area in the clad plate	Mill bonding defect, not a welding fault	Small areas: plug or rivet repair; large areas: replace the plate

Repair of micro-cracks on the clad surface follows a strict order: the defect is fully removed by grinding or machining to sound metal, the cavity is cleaned, and only then is repair welding performed. Small, shallow cracks may be repair welded directly after preparation, while deeper or branched cracks are excavated through their full depth, because welding over an uncleaned crack simply traps contamination and the crack reopens under thermal cycling. Each repair is recorded by location, and the number of repairs per unit area is limited by the project specification.

Unbonded areas found during fabrication or final inspection are handled differently from weld defects. Where the unbonded area is large, the plate is replaced; where it is small and outside critical zones, the area can be remedied by mechanical means, for example titanium rivets or plug welds that clamp the cladding to the substrate, using the minimum number of fasteners consistent with restoring the bond. Each rivet is itself a potential crevice, so the pattern is agreed with the design authority and documented on the as-built drawing.

Inspection, Testing and Documentation

Bond integrity of the clad plate is normally examined by straight-beam ultrasonic testing to ASTM A578 or the equivalent method of GB/T 2970, with the acceptance level agreed for the service. After welding, the titanium cover weld is examined visually over its full length, by dye penetrant where geometry allows, and by helium mass spectrometer leak testing for pressure-retaining equipment. Ferrite or iron contamination checks, using a chemical spot test or an instrument, confirm that no free iron remains on the titanium surface, since free iron causes pitting corrosion in service even when the weld itself is sound.

Documentation for the finished item should include the mill certificate for the clad plate with bond test results, the welding procedure specification and qualification record, welder qualification certificates, gas purity records including purge oxygen readings, the repair log with locations, and the final examination reports. For equipment in corrosive service, the process-side condition of the titanium surface is the record that matters most: a weld that passes penetrant testing but carries a blue oxide may still fail by corrosion or hydrogen embrittlement within a few years.

Safe Practice and Field Considerations

Titanium dust and fine grinding swarf are a fire hazard, and the metal burns in air once ignited, so grinding is kept wet or under extraction and swarf is stored in covered metal containers away from ignition sources. Chlorinated solvents must never be used on titanium surfaces, because stress corrosion cracking follows chloride contamination at elevated temperature. Welding is done in a cleared area away from galvanising, painting and steel grinding, since airborne iron particles settle on the titanium and contaminate it before the arc is struck.

In field erection the two most common failures are shielding loss in wind and premature welding of the titanium cover before the steel side is complete. Both are procedural rather than metallurgical problems, and both are prevented by a written sequence that holds the titanium closure until all steel welding, stress relief and testing in the area is finished.

FAQ

Q: Why can titanium not be welded directly to steel in clad plate?
Titanium and iron form brittle intermetallic compounds such as TiFe and TiFe2 in the molten pool, and the joint cracks during cooling, so the steel is welded to steel and the titanium to titanium, with a strip covering the transition.

Q: How wide should the titanium be removed at the joint edge?
About 15 mm of titanium cladding is removed from each side of the joint to expose the steel for welding, and a titanium strip of roughly 50 mm width then covers the completed steel weld and overlaps the remaining cladding.

Q: What argon purity is required for titanium welding?
Argon of not less than 99.99 percent purity is used for torch, trailing and backing shielding, with purge oxygen monitored before welding and coverage maintained until the weld cools below about 400 degrees Celsius.

Q: Should the plate be preheated when ambient temperature is low?
When the ambient temperature is below 5 degrees Celsius the steel side is preheated with a flame to remove moisture and slow cooling, while the titanium surface is not flame heated because the flame introduces carbon and hydrogen.

Q: How are unbonded areas in clad plate repaired?
Large unbonded areas require replacement of the plate, while small areas outside critical zones may be remedied by titanium rivets or plug welds, using the fewest fasteners that restore the bond and recording the repair on the drawing.

Q: What tests confirm the finished weld is acceptable?
Visual examination and dye penetrant testing of the titanium cover weld, helium leak testing on pressure equipment, ultrasonic bond examination of the plate, and an iron contamination check on the titanium surface, all supported by the qualified welding procedure and gas purity records.