Can a Dishwasher or Hot Water Ruin Your Kitchen Knife? We Tested the Myth

There’s a myth that keeps coming back:
“Never wash your knife in hot water — you’ll ruin the heat treatment.”
“Boiling water softens the edge.”
“It drops the HRC.”

Instead of debating it in comments and reading posts that speculate with no actual physical testing behind it… we tested it.

🎥 The Video Test (Sorry, it's in french)

We attached a short French video showing the full experiment — before measurement, boiling test, and after-results — all done with our calibrated in-shop digital HRC machine (yes, the nearly $4,000 tester we invested in so we can measure instead of guess).

You’ll see:

• The initial HRC reading

• The knife in rolling boiling water

• The post-boil hardness results

• The exact variance numbers

No theory. Just data.

🔬 The Setup

We used a 2-logo Henckels knife — a common stainless kitchen knife with a softer heat treatment, ideal for a real-world test.

Starting hardness: 53.3 HRC

We then placed it in rolling boiling water for 3 minutes. Not warm tap water. Not “hot sink” water. Actual 100°C boil.

After removing it, we let it cool naturally and measured again.

📊 The Results

After boiling:

➡️ A feather 0.8% variance in HRC
➡️ Slightly lower at the middle 52.5 HRC BUT a little higher at the heel 55.1 HRC

So what does that mean?

✅ Boiling water does NOT change heat treatment
✅ It does NOT reduce hardness
✅ It does NOT soften the steel
✅ It does NOT affect edge retention

If 100°C water could alter hardened steel, professional kitchens would be destroying knives daily.

Heat treatment happens at temperatures in the hundreds of degrees Celsius, followed by controlled tempering cycles. Boiling water simply isn’t hot enough to change the steel’s structure.

🚫 What Actually Damages Knives in the Dishwasher

The dishwasher can damage knives — just not because of heat.

🧪 Abrasive Detergents

❌ Aggressive chemicals strip finishes
❌ Accelerate corrosion (especially carbon steel)
❌ Contribute to edge dulling over time

🥄 Banging & Contact

❌ Knives hit plates, racks, and other cutlery
❌ Edge-to-metal contact causes micro-chipping
❌ Rolling and flattening of the edge

💧 Moisture + Time

❌ Spotting
❌ Rust on carbon steel
❌ Handle swelling (especially wood)

That’s mechanical and chemical damage — not heat-treatment failure.

🔪 What About the Edge?

“Okay, maybe HRC doesn’t change… but does hot water make the edge less sharp?”

Simple answer:

If the water isn’t hot enough to change the temper of the steel, it’s not hot enough to change the edge geometry.

Hardness defines edge stability.
No hardness change = no structural edge change.

🟢 Final Verdict

Hot water? ✅ Fine.
Boiling water? ✅ Still fine.
Dishwasher detergent + banging around? ❌ That’s the real problem.

So yes — we still recommend hand washing and drying your knives. But we can officially retire the myth:

🔥 Hot water does NOT ruin heat treatment.

Mike