We got a set of very nice knives as a wedding present and the instructions specifically say not to put them in the dishwasher. Why? What goes on inside the dishwasher that could ruin a knife? It’s just hot water and detergent, right?
Several reasons come to mind:
Water is hotter coupled with a more agressive detergent can damage some metals.
Edges banging against one another and other silverware.
All my knives get hand washed and dried.
Also, some knives have exotic wood handles that will dry and crack and delaminate if exposed to repeated dishwasher cycles.
FYI dishwashing detergent is not just surfacants- it includes silica (essentially sand) to sand-blast the crud off of the dirty dishes. This is why cut crystal is never put in the diswasher- it would get scratched all to heck and turn cloudy. I collect fiestaware, and the pre dishwasher era plates cannot be put in the dishwasher at all- they were fired at lower temperatures and not “fully vitrified” as the new plates are- making them much harder and not susceptible to scratching from the silica in the detergent. Older plates with transfer decorations, gilding, will lose their decoration over time because you are sand-blasting them.
Don’t ever put any good knife in the washer- you are sand-blasting off the very fine (thin) edge. Mo mother did this once with a good set of knives I bought her and they can literally barely cut an onion now. I always hand-wash my Henckles knives as soon as I am done with them- they are still razor sharp after fifteen years (frequent honing on the steel and professional re-sharpening once.) You want dull, dangerous, ruined knives? put 'em in the diswasher. Although if you do, they can be re-sharpened (Contact your local knife retailer, they usually do it)
Hope this helps!
It’s the same reason you do not put quality non-stick cookware in the dishwasher, either.
I… did not know this. Thanks!
It’s not really likely to seriously damage the stainless steel of the knives via heat; the temps are just too low.
The concern is primarily for the edge and handles- the banging, and possibly chemical action against the edge may dull and nick it overly much, and mess up any pretty handles.
What do I do? I get relatively cheap, but good quality knives (Mercer Genesis) and just dishwasher them, and sharpen them when they’re a little dull. I figure I may be wearing them down or something, but an 8" chef’s knife only costs $34, so if it takes me 5 years to destroy it, I’ll just toss it and buy another. And for the record, I’m going on 7 years like this, and it’s as sharp and unworn as ever. (and FTR, it’s as sharp, and keeps an edge as well as my wife’s Wusthof Classic)
Thanks for the info. I grew up without a dishwasher, so when I got one as an adult I was like, “Wow! Now I don’t have to wash anything by hand ever agian!” 
Um, no. And thanks to the Dope, now I know why.
Word to the wise – take your time hand-washing the sharp knives. Haste is…bad.
I didn’t know that about the silica.
Ignorance fought.
The sad thing is, I did not know this when I bought my parents a set of Henckels to replace the crap knives they’ve been using for twenty years. The Henckels have probably been through the dishwasher twice a week for three years now.
They still cut better than the other knives.
Also, if you drop the knife, do not try to catch it. Just get out of the way and pray it survives. My mom, the RN, has told a manager to take his employee to the ER for stitches because the poor kid was so terrified of screwing up the knife that he tried to catch it. Lacerated his palm, and they had to clear out the prep area to re-sterilize it.
Also, I can’t recall the name for this effect now, but when two pieces of metal of different composition touch, one of them will rust more easily than if it wasn’t touching the other one. And the one rusting is likely to be the “good one”, as my mother has already found out several times 
My brother (the mechanical engineer, who damn it should know better) is now officially forbidden from filling her dishwasher.
ETA: found it, galvanic corrosion. Wiki
Heh. I was once cutting box straps with an X-Acto razor knife, and set the knife atop the pallet of boxes at one point. When it rolled off, I reached without thinking and caught it – blade-first in the center of my palm. Owie.
I still wouldn’t do it. The steel on Mercer knives is going to be about the same as many lines of more expensive German and American knives. I have mostly Mundial knives, with a few German and Japanese knifes thrown in, and I keep them out of the dishwasher.
The carbon content of higher-end kitchen knives is higher than for other types of “stainless” steel. A knife made of 440 A/B/C steel isn’t going to rust if it’s exposed to water, like a French-made carbon steel knife would, but they’re not entirely rustproof, either.
Forchner or Vitornox
Cheap, well balanced, take and hold an edge, can be tossed about, no tears if lost or stolen, cuts like a champ, ergonomic, just fine in the dish washer.
I shall have to pass this along to my homebrewing friends who use the dishwasher set to Volcano Heat Dry to sterilize their bottles. Microscratches r bad.
Handles would be the primary reason to avoid dishwashers. No matter what the material it’s an opportunity for water and detergent to get between the handle and the blade material, and the repeated cycles of heating can loosen rivets holding the handle on. The detergent and heating cycles aren’t good for the edge, but that can be resharpened. For some knives there is the potential for warping from uneven heat as well.
They could use regular liquid detergent for hand washing instead of the abrasive dishwasher stuff.
Danger! Danger! Don’t use any non-dishwasher soap in the dishwasher. It foams like crazy and then overflows. Ask me how I know. 
But for homebrew, you don’t really need the soap anyway. They’re probably hand washing the bottles, since there’s no way a DW would get enough spray inside the bottle to clean it, and just using the heat dry setting to sanitize. No detergent, no scratches.
I’ve heard it said that everyone tries to catch a dropped knife once. 