Diamond plates are not recommended on all steels with lower than 58 HRC because of one simple reason

The only diamond stones I see an issue with are the venev diamond bonded. They do fill with swarf on sifter steels. But my dmts and atomas do not. All I use are dmts for everything. Even though I have a load of other sharpening stones.
 
This absolutely can be a factor on soft steel, but all the reasons have already been mostly covered. Excess pressure is bad no matter the hardness of the steel, but is much worse with softer steel because the diamond can dig in deeper and thus with heavy pressure, more sideways force is applied to each diamond particle because more surface area is in contact. I don't think it's a huge issue unless the steel gets pretty far down the hardness range though. And with very light pressure you should not have any issues regardless.
 
I don't know about "58 HRC" being a significant threshold, that's still pretty hard steel.

But it is definitely true that with much softer metals, diamonds will become embedded in the workpiece (as they do with a cast iron lap) and you will be rubbing diamonds on diamonds, potentially damaging the hone.
 
I know people love to talk about pressure with diamond plates, but the fact is that you are applying perhaps 100 times as much pressure on the outer curve of the blade (where the contact area is tiny) compared to a straight edge. You cannot moderate pressure simply by limiting your downward force when the contact area varies so much.
 
It’s going to depend on how much pressure or force is used in contacting the diamond plate. If you press hard it will pop or sheat the diamonds off the plate. I’ve seen this happen when a person was trying to sharpen like he was using a stone. It only takes a light pressure for the diamonds to remove metal even with very hard steel. Diamonds cut easily and fast and remove metal much easier than conventional stone or sic .

With softer steel 58 hrc or less it is important to keep the plate clear of the cuttings that can accumulate at a faster rate than harder steel. That’s when I prefer a plate with the holes or inlets to help it clear out. Of course this is my personal technic that works very well for me .
 
Agree.. the statement on diamond stones for steel below Rc58 is Eye Wash.
Excessive pressure and swarf clogging due to lack of lubrication is main problem.
I have two DMT stones from the late 1980's (approx.) that work fine after hours of soft stainless usage.

Regards,
FK
 
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I'd really expect it to be more likely of an issue with steels below 55 RC and/or with power equipment, in which case you'd preferably use CBN instead of diamond to prevent carbon migration from the diamond anyhow, though the issue would apply equally to coated CBN abrasives. It's generally a best management practice to use CBN with any powered grinding of ferrous metals, even if you're unlikely to run into the issue under the temps usually experienced with knife grinding. In a coarse range of grits up to ANSI 400 (~JIS 700) conventional silicon carbide and aluminum oxide stones work fine on high-vanadium steels anyhow, and most stuff in the hardness range where diamonds are more likely to get yanked off their plates is easy enough to sharpen on conventional stones anyhow.
 
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