Titanium-Steel Clad Plate Defects: Causes, Detection and Repair

Why Bond Defects Dominate Titanium-Steel Clad Plate Quality

Titanium-steel clad plate is a two-metal product: a thin titanium or titanium-alloy layer carrying the corrosion duty, explosively or roll bonded to a structural steel backing plate that carries the pressure and the load. Because the two metals cannot be joined by conventional fusion welding along a full face, the interface is created by solid-state bonding under heat and pressure. That interface, not the titanium surface, is where nearly every field failure begins. A clad plate can look perfect on the plate line and still be rejected at the fabricator's receiving inspection, because the controlling property is shear bond strength across a plane that no eye can see.

Four defect families account for most rejections and most shop-floor rework: unbonded areas at the interface, blisters or bulges that open during bending or heat treatment, edge and corner cracks created during shearing or drilling, and metallurgical degradation of the interface through excessive intermetallic growth. Each family has a different root cause, a different detection method and a different corrective action. Treating them as one problem is the single most common mistake in procurement and fabrication.

The Interface Metallurgy That Creates the Defects

Bonding titanium to carbon steel or low-alloy steel is a race against intermetallic formation. At elevated temperature the two metals diffuse into each other and form brittle TiFe and TiFe2 phases, plus TiC where carbon from the steel is available at the interface. A thin, discontinuous reaction layer of a few micrometres can be tolerable and may even assist bonding; a continuous layer of tens of micrometres destroys toughness and turns the interface into a crack path. This is why bonding windows are narrow: enough temperature and reduction to achieve closure of the surfaces, but not so much time-at-temperature that the brittle layer thickens.

Three practical consequences follow. First, the bonding process must be qualified by shear testing, not by appearance. Second, any subsequent thermal cycle, including stress relief, forming preheat and repair welding, adds to the diffusion budget and must be counted. Third, a plate that passed ultrasonic examination at the mill can still fail after a fabricator's unnecessary normalising pass, because the added soak time grew the intermetallic layer to a critical thickness.

Titanium also picks up interstitial elements aggressively above roughly 400 degrees C. Oxygen, nitrogen, hydrogen and carbon all raise hardness and reduce ductility, and hydrogen in particular can precipitate brittle hydrides in the alpha phase. Any preheat, flame straightening or repair operation therefore has to be judged not only by the steel backing but by the titanium side as well.

Incoming Re-Inspection: The Rules That Actually Catch Defects

The most effective defect control is a disciplined incoming inspection at the fabricator, applied plate by plate rather than by certificate review alone. A workable regime, consistent with the ultrasonic examination practice for clad plate in ASTM B898 and with the pressure-vessel clad requirements of NB/T 47002.3, is built on three tiers of coverage.

Transition joints, nozzle reinforcement pads, flange attachments and any location where a weld will later cross from the titanium layer into the steel layer receive continuous 100 percent ultrasonic scanning. These are the regions where a hidden unbond can be opened by welding heat and where a leak path can develop through the bond line.

A continuous band around the full perimeter, typically 50 mm wide, is scanned 100 percent. Edge unbonds are both the most common defect and the most damaging, because every plate is subsequently cut, sheared or machined at its edges, and an edge unbond propagates inward under handling and forming loads.

The remaining field is scanned on a grid, for example at 200 mm intervals, with any suspicious indication expanded to full coverage. The grid is a sampling strategy, not an acceptance criterion: a single relevant indication found on the grid justifies 100 percent scanning of that plate.

Acceptance should be written against a defined reference. For roll-bonded clad plate, a flat-bottom hole or notch reference block of the type used for straight-beam examination of steel plate under ASTM A578/A578M gives a reproducible sensitivity, with the acceptance level agreed between buyer and mill for the clad thickness involved. For explosion-bonded reactive-metal clad, ASTM B898 supplies the bond-integrity and shear-strength framework. Whatever reference is chosen, the report should state probe frequency, grid, reference block, rejection threshold and the map of every recorded indication, so that a second party can reproduce the result.

Defect Types, Mechanisms and Corrective Actions

Defect	Typical mechanism	Detection	Corrective action
Area unbond at interface	Insufficient local pressure or temperature during bonding; surface contamination by oxide, oil or grit	Straight-beam ultrasonic scanning; shear test on coupons	Reject or cut out; small isolated areas may be repaired only where the design code permits and with documented procedure
Blister or bulge opening during forming	Sub-critical unbond that grows when bending strain exceeds the ductility of the bond line	Ultrasonic before forming; visual after each forming step	Re-anneal to restore ductility, re-form with preheat, or cut out the affected zone
Edge crack or tear	Shearing or drilling driven from the steel side pushes the titanium layer into tension at a stress concentration	Visual and dye penetrant on cut edges; macro-section	Machine or drill from the titanium side toward the steel; dress and re-inspect edges
Interface delamination after heat treatment	Excess diffusion forming a continuous brittle intermetallic layer; thermal stress from mismatched expansion	Ultrasonic after heat treatment; shear test on witness coupon	Reject; revise the thermal cycle; limit soak time and cooling rate
Surface contamination and iron pickup	Carbon steel tooling, grinding dust or chloride-bearing fluids contacting the titanium layer	Visual; iron-contamination test with ferroxyl-type reagent	Dedicated stainless or titanium tooling; pickling and passivation per ASTM B600 practice
Waviness and flatness loss	Residual stress imbalance between the two layers after rolling or asymmetric machining	Straightedge and dial gauge on a flat bed	Stress relief within the qualified window; machine equal stock from both faces

Pre-Forming Controls: Annealing, Preheat and Tool Direction

Forming is where latent defects become visible. Two controls carry most of the risk reduction.

The first is condition of the plate. If the clad plate has been cold worked, or if its ductility has been reduced by an earlier thermal cycle, it must be re-annealed before bending, flanging or head pressing. Annealing serves two purposes: it removes residual stress that would otherwise add to forming strain, and it restores ductility in the titanium layer so that the bond line is not forced to accommodate deformation it cannot tolerate. The anneal must be written as a qualified cycle with temperature, hold time, atmosphere and cooling rate, and it must respect the diffusion budget discussed above. Furnace atmosphere matters: a nitrogen or hydrogen-bearing atmosphere at temperature will contaminate the titanium surface, so a clean, controlled atmosphere or an inert purge is required.

The second is temperature during forming. Titanium-clad plate bent cold at low ambient temperature, or bent at high strain rate, is prone to bubbling at the interface and to surface cracking on the tension side. Flame preheating of the bend zone is a standard shop remedy, provided it is done with a soft, neutral or slightly oxidising flame held in motion, with temperature checked by contact or infrared measurement and kept in the moderate range that improves ductility without driving interstitial pickup. Local overheating produces exactly the defect the preheat was meant to prevent.

Tool direction is equally decisive in cutting and drilling. Whenever the geometry allows, shear, punch, drill and machine from the titanium layer toward the steel layer. Driving the tool from the steel side forces the titanium layer to deform in tension against a constrained backing, which produces edge tearing, layer separation and crescent-shaped cracks at the exit of a drill. Drilling from the titanium side lets the titanium cut cleanly and the steel exit into free space. Where access forces work from the steel side, reduce feed rate, support the titanium face with a backing plate, and expect to dress and inspect the exit edge afterwards.

Repair, Re-Examination and the Discipline of Not Repairing

Clad plate repair is often technically possible and commercially the wrong answer. The governing rule is that repair must be permitted by the design code and by the purchase specification, and must be supported by a qualified procedure and a post-repair examination. A weld repair that penetrates the titanium layer introduces a new heat-affected zone, a new diffusion zone and a new source of residual stress; if the repair is in a pressure-retaining or corrosion-critical area, its long-term integrity is usually inferior to that of an intact plate.

Where repair is permitted, the sequence is: map the indication precisely, remove the defective region by machining to a profile that allows access, clean to bright metal, weld with matching titanium filler under inert shielding with a trailing purge, verify by penetrant and by ultrasonic scanning of the repaired zone plus a defined surrounding band, and record the location, extent and procedure on the plate map. The repaired area must be included in the final documentation, because it will be examined again by the end user's inspector.

The cheaper discipline is prevention at the two points that are fully under the fabricator's control: do not accept a plate whose bond integrity is unproven, and do not run an unqualified thermal cycle on a plate that has already passed.

Handling, Storage and Contamination Control

Titanium's corrosion resistance depends on a passive oxide film that is a few nanometres thick and easily destroyed by embedded iron, chloride residue or oxygen starvation in a crevice. Clad plates should be stored on clean timber or polymer bearers, never directly on carbon steel skids, and should be lifted with slings that do not embed iron into the surface. Grinding and brushing of the titanium face must use dedicated abrasives that have never touched carbon steel. Cutting fluids and marking pens should be chloride-free, and any temporary paint or adhesive must be removable without leaving a carbonaceous residue that will later be baked into the surface during welding.

Edges deserve particular attention. A rolled or sheared edge exposes the steel core to the environment and to any subsequent weld; most clad-plate specifications require the steel edge to be trimmed back or the titanium layer to be formed over the edge so that the steel is not left as an exposed strip. Where the design keeps a bare steel edge, it must be coated and the coating recorded in the maintenance file.

What the Buyer Should Put in the Purchase Specification

Defects are cheapest to control in the purchase order. A specification that reduces rework and dispute should state: the clad grade and thickness combination (for example 3 mm titanium Gr.2 on 20 mm Q345R backing), the bonding method, the applicable clad-plate standard, the shear-strength acceptance value and test frequency, the ultrasonic examination reference and acceptance level with the required coverage map, the heat-treatment condition on delivery, the flatness tolerance, the permitted repair scope, and the documentation package.

On the documentation side, insist on a plate-by-plate map rather than a single certificate for the lot. The map should link each plate to its steel heat number and its titanium heat number, state the ultrasonic results with the scan grid, list any repair with its procedure reference, and include the mechanical test results from the lot's witness coupons. That package is what allows a fabricator to prove, years later, that a failure originated in service rather than in the delivered plate.

Common Misconceptions Worth Correcting Early

A polished titanium surface is not evidence of a sound bond; the bond line is invisible and must be examined by ultrasonic means. A thicker titanium layer is not automatically safer; it increases residual stress and cost without improving bond integrity. A plate that passed examination at the mill does not stay qualified through an extra normalising pass. And a small unbond is not automatically harmless: its significance depends entirely on whether it sits in a corrosion path, a weld path or a high-strain forming zone.

FAQ

Q: What shear strength should a titanium-steel clad plate achieve?
For roll-bonded and explosion-bonded titanium-steel clad plate, tensile shear strength across the interface is commonly specified at 140 MPa minimum in the as-delivered condition, tested per the shear method of GB/T 6396 or the equivalent clause of ASTM B898; the exact value must be fixed in the purchase specification for the specific thickness combination.

Q: Can an unbonded area be repaired by welding?
Only when the design code and purchase specification permit it. Repair welding of the titanium layer is done with matching filler under full inert shielding, followed by penetrant and ultrasonic re-examination of the repaired zone plus a surrounding band, and the repair must be recorded on the plate map.

Q: Why does a clad plate blister during bending when it passed ultrasonic inspection?
Ultrasonic examination detects a mechanical gap, not a weak but closed bond. A sub-critical bond can survive handling and still open when bending strain exceeds the local bond ductility, especially at low ambient temperature or high forming rate, which is why preheat and prior annealing matter.

Q: Should the plate be annealed before forming?
Yes, when the plate has been cold worked or its ductility has been reduced by an earlier thermal cycle. Annealing removes residual stress and restores ductility in the titanium layer, but it consumes part of the diffusion budget, so temperature, hold time and cooling rate must follow a qualified cycle.

Q: Why must drilling start from the titanium side?
Drilling or shearing from the steel side pushes the constrained titanium layer into tension at the tool exit, producing tears and crescent cracks. Working from the titanium side lets the titanium cut cleanly and the steel exit unsupported, which greatly reduces edge damage.

Q: How is iron contamination on the titanium surface detected?
Ferroxyl-type reagent testing reveals free iron on the surface as coloured spots. The test is used after fabrication, after pickling and passivation per ASTM B600 practice, and before any final corrosion-resistant acceptance, because embedded iron becomes a pitting initiation site in chloride service.