Why Square Metal Substrates Deform and Crack – What We've Seen

We've made a lot of square metal substrate. And we've seen a lot of them come back from the field cracked, bowed, or twisted like a potato chip.

Sometimes it's our fault. Bad brazing, wrong foil thickness. But most of the time, the deformation and cracking come from predictable causes. The same ones show up again and again.

Here's my list – what breaks square catalytic carriers, and how to stop it from happening.

Thermal Expansion – Corners vs. Edges

Squaremetal substrates expand when they get hot. But not evenly.

The corners have more material – more foil layers meeting. They get hotter and expand more. The middle of the flat sides has less material. It expands less.

That difference creates stress. The corners want to pull away from the center. If the substrate can't flex, something cracks. Usually at the corner joints.

We saw this on a generator square carrier. After 500 heat cycles, the corners had hairline cracks. The middle was fine. The customer had used thin foil – 0.05 mm. No margin for the stress.

Fix: Thicker foil (0.08 mm minimum). Ductile brazing filler. Extra brazing material at the corners.

Thin Foil – Too Flexible

Foil thickness is the biggest factor in deformation resistance.

0.05 mm works for round substrates. For square, it's often too thin. The flat sides bow out when hot. The corners lift.

We had a customer insist on 0.05 mm for a square catalytic carrier in a paint booth oxidizer. 550°C. After 1,000 hours, the middle of each side had bulged outward by 2 mm. The substrate no longer sealed against the can. Exhaust bypassed. Conversion dropped.

Fix: 0.08 mm for most square applications. 0.1 mm for high temp or large sizes.

Brazing Voids – Weak Corners

The brazed joints are what hold a square metal honeycomb together. If the braze doesn't fill the corner completely, that's a weak spot.

When the substrate heats up, stress concentrates at the void. A crack starts. Then it spreads along the braze line.

We cut open a failed square carrier once. The corner looked fine from outside, but under magnification, the braze had only filled half the joint. The rest was empty space. The crack followed the void.

Fix: Extra brazing filler at the corners. Slow furnace ramp to let filler flow. Peel test samples from corners, not just the flat faces.

Cell Density – Too Many Walls, Too Thin

Higher cell density means more walls. More walls means more brazed joints. That can make the substrate stiffer – up to a point.

But high cell density also means thinner walls. Thin walls are weaker. They can buckle under thermal stress.

We tested a 400 cpsi square metal substrate against a 200 cpsi one. Same foil thickness. The 400 cpsi was stiffer to twist, but it cracked sooner in thermal cycling. The 200 cpsi lasted longer because the thicker walls could absorb stress without breaking.

Fix: 300 cpsi is a good balance. 200 cpsi for high‑stress applications. 400 cpsi only for clean, low‑temp, well‑supported applications.

Material – Aluminum Doesn't Belong

Aluminum expands more than stainless. It's also weaker at high temperature. Put an aluminum square catalytic carrier in a 500°C exhaust stream, and it will warp. Not maybe. It will.

We had a customer buy square aluminum substrates from a cheap supplier. They put them in a diesel generator aftertreatment system. Three months later, every single one had bowed and cracked. The customer blamed us – we weren't the supplier. But it reinforced the point: stainless only for square.

Fix: 304 stainless for most. 316 for corrosive gas. 347 for >650°C.

Mounting – Squeeze Too Hard, It Bows

Even a perfect square metal substrate can fail during installation.

If the can is too tight, pressing the carrier in can bow the flat sides. You might not see the bow until after heat cycling. Then the stress from expansion and contraction amplifies the damage, turning a small bow into a permanent warp or a full crack along the edge.

We had a case where a customer used a press-fit can with no tolerance for thermal expansion. The square substrate fit snugly at room temperature – too snug. After the first heat cycle to 600°C, the flat sides bowed inward by 1.5 mm. Over the next 200 cycles, the bow worsened, and small cracks formed at the corners where the can pressed hardest. The customer thought the substrate was defective, but when we measured the can, we found the internal dimensions were 0.8 mm smaller than the substrate’s outer dimensions – a classic case of over-tightening.

The opposite is just as bad: a can that’s too loose. If the substrate rattles inside the can during operation, the constant vibration wears down the brazed joints, especially at the corners. We saw this in a mobile generator application, where the substrate shifted back and forth in the can every time the generator started or stopped. After six months, the corners had chipped, and the honeycomb structure began to separate.

Another mounting mistake: uneven pressure. If the can is bent, or the mounting brackets pull harder on one side than the other, the substrate twists. This twist doesn’t always show up immediately, but thermal cycling will exaggerate it, leading to diagonal cracks across the flat faces.

Fix: Leave a 0.3–0.5 mm gap between the substrate and the can for thermal expansion. Use a high-temperature ceramic mat or wire mesh to secure the substrate – it absorbs vibration and allows for slight movement. Ensure the can is perfectly square and the mounting brackets apply even pressure. For large substrates, add support ribs inside the can to distribute stress evenly across the flat sides.

Contaminants – Hidden Stressors

We often overlook contaminants, but they’re a silent cause of deformation and cracking. Oil, grease, or dirt left on the foil before brazing creates weak spots in the braze joint. When the substrate heats up, these contaminants burn off, leaving tiny voids or porous areas that can’t handle thermal stress.

A customer once sent us failed square substrates where the cracks traced along braze lines that looked discolored. Under testing, we found traces of machine oil on the foil surfaces – the cleaning process before brazing had been rushed. The oil burned during brazing, creating porous braze joints that failed under repeated heat cycles.

Another culprit: corrosive gases. If the exhaust stream contains sulfur, chlorine, or other corrosive elements, they eat away at the stainless steel over time, making the foil thinner and more prone to cracking. We saw this in a waste incineration application, where the substrate’s corners corroded first (due to higher heat and more exposure) and eventually cracked.

Fix: Thoroughly clean foil and components before brazing – use a degreaser and dry completely. For corrosive environments, use 316 or 317 stainless steel (more corrosion-resistant than 304). Add a protective coating (like alumina) to the substrate surface to shield against harsh gases.

Final Thoughts: Prevention Beats Repair

Square metal substrates aren’t inherently fragile – their failure almost always comes down to predictable, avoidable mistakes. Thermal stress, thin foil, poor brazing, wrong materials, bad mounting, and contaminants are the main culprits, and each has a clear fix.

The key is to match the substrate’s design to the application: use the right foil thickness, cell density, and material for the temperature and environment. Don’t cut corners on brazing or cleaning, and ensure the mounting allows for thermal expansion and vibration absorption.

We’ve learned these lessons the hard way – through hundreds of failed substrates and field tests. By following these guidelines, you can avoid the same mistakes and keep your square catalytic carriers working reliably, even in the harshest conditions.