Why Carbide and Steel Expand Differently: What It Means for Brazed Joints

Scenario: a supplier says its carbide is brazed to steel, and a buyer wonders why the joint is a weak point. Direct answer: carbide expands and contracts less than steel when temperature changes, so when a brazed joint cools, the steel pulls on the carbide and can put it in tension. Carbide is sensitive to tension. Good joint design and cooling control aim to limit this. The figures below are from two technical sources read on 24 October 2026; they are typical values, not your supplier’s.

What the sources say

The sources describe the mechanism, and the mechanism is what matters for the joint.

• A brazing-materials maker’s paper lists the coefficient of thermal expansion as 5 to 7 (in 10^-6 per K) for cemented carbide and 11 to 14 for low-alloy steel.
• The same paper explains that once the filler metal sets, the two materials are rigidly joined, so steel, prevented from contracting freely, produces tensile stress along the joint. It adds that cemented carbides are sensitive to tensile stress, and the stress built up may cause damage and cracking.
• A materials-testing firm’s page gives about 5.3 for a 10% cobalt carbide grade and 13.2 for a DIN 1.2343 tool steel (20 to 400 °C), says failure can result from such stresses during cooling, and notes that slow cooling gives some reduction in brazing stress.

Plain-language picture

Imagine two bars of different materials glued together while hot. As they cool, one wants to shrink more than the other, but they cannot. The result is a built-in stress that stays in the part. Carbide is hard but brittle, so it is the part more likely to crack if that stress is in tension.

What a buyer can ask

The joint is where the difference shows, so the questions are about process rather than about materials.

1. How is the carbide attached, and how is the joint cooled and inspected? Brazing and cooling decide the residual stress.
2. What measures do you use to control joint stress? Ask for the method, not the claim.
3. Do you test joints, and how often? A destructive or shear test on a sample is the evidence.
4. What do you see as the common failure mode, and what returns do you get? The return pattern is the most honest quality signal.

What this does not tell you

The mechanism explains the risk; it does not size it.

• It gives no failure rate for any product.
• Values vary by grade, steel and temperature range.
• Design, braze alloy and process control strongly affect the result.

Boundary: Values are typical figures from two technical pages read on 24 October 2026, not your supplier’s data; they predict no failure rate.