Knife heat treatment

Heat treatment changes the allotropy of an iron alloy. Steel doesn't equal steel; first, there are chemical differences (different amounts of carbon, nickel, etc. added) and second, there is a difference in the microcrystalline structure of the metal.

The different allotropes (= same material in different structures) of steel have different mechanical properties. They have different levels of brittleness, flexibility, softness, etc. So, if you have two steels made from the same proportion of elements, it is possible that one of them will be harder. If you make a knife out of it, it will keep its edge for a longer time.

Molten metal has no crystalline structure, it is a liquid. The structure forms when you cool it down to a solid state. But depending on the conditions while cooling down, different structures will form. There are a few factors determining which structure you'll get in your finished blade, but the most important one is the rate (and direction) of temperature changes to which the blade is subjected. Specific tempering processes will create different proportions of different allotropes, determining the final mechanical properties of the knife.

I don't know which process Shun is using, but it seems that it has properties which your shop owner doesn't like. This is not necessarily a bad thing: there is no perfect combination of properties in a knife, everything is a trade-off. Knives hard enough to keep an edge longer are very hard to impossible to sharpen at home once they go dull. Knives which achieve perfect sharpness with enough care will be much worse than the average knife if you don't invest enough time in them. And so on. So, you should choose a knife based on your sharpening skills and willingness to invest time in it, not on what professionals consider the best.

I suspect that the knife owner shut up because he didn't know what he was talking about.

So long as you don't mistreat them, Shun hold a blade rather well. The problem is that they're cut at a sharper angle than most European knives (16° vs. ~20°), so the blade is more delicate, and so is more prone to go out of alignment, especially if you use too hard of a cutting board. Cutting on a glass, ceramic, metal, or marble surface will slowly destroy a European blade ... you'll horribly screw up a Shun if you do it even once.

Note that Wusthof and Henkels recently (in the last 3-5 years) went to a steeper angle as well, some as low as 11°, so they'll likely have some of the same problems. (mine from those companies are older, so I don't know for sure).

You can also damage them if you're too aggressive with the knife (hacking / chopping, instead of slicing) or if you try to use it on hard items (bones, frozen meat), and they're more prone to damage if you hit them against something; I took the tip off of my Shun because I laid it in the sink ... the tip was against the side of the sink, and the weight was enough to break off a mm or so at the end. I then got another chip in it (hit against a metal utensil in the dish rack) all in my first week.

Even with that chip, and regular use over about 5 years, it's still holding a sharper edge than my Henkels, Wusthof or Sabatier blades, and it's the knife I use the most these days.

The blade of knives is made of metals or alloys (mixture of metals). Metals have a tendency to expand at high temperatures. So, it is possible that the sharp edge of the blade is expanding when heat treated, making the knife dull/blunt. This is dependent on the metal type of the knife he was using because each metal has its specific degree of expansion. Shun knives has different models with different metal type.