Destroying Drill Bits

[m1kemex]

Work hardening is the ONLY way a 200 or 300 series stainless increases strength (aka hardens)
You can buy cold rolled stainless with an ultimate tensile strength of 200 ksi.

Technically speaking you're correct. But stainless steel doesn't work harden to the point of challenging high speed steel's hardness:

Rockwell C hardness - Material
15-40 - 300 series stainless steel
63-65 - high speed steel
63-70 - chrome oxide

Chrome oxide can easily overcome HSS's hardness, however.
So, in the context of drilling, what of the above is more likely to be the source of fast abrasion wear?

 

[pavyan]

If we are talking about stainless steel machining, I do not think that chromium oxides formed by passivation on the surface of stainless or acid-resistant steel are a significant cause of tool dullness during machining. This is an extremely thin layer, especially in the first moments.

According to available information, the most important reasons for difficulties during machining of alloy chromium steels include:
1. The cause is chromium itself, its high content in this steel - theoretically minimum of 11%, and most often above 13% (popular stainless steels) or even 18% (acid-resistant steels, e.g. 304, 1H18N9, which is what I use most often). Chromium is a very hard metal, sources says that hardness ranges from 40HRC to even 68HRC (hard chrome plating).

2. Poor thermal conductivity of stainless/acid-resistant steel - it can be even several times worse than ordinary carbon steel. This in turn causes rapid overheating of the cutting edges with insufficient cooling/lubrication - a fact that is especially common in amateur machining. This property of this steel is also "unpleasant" when welding 304/316 type steel - it causes large deformations of welded details.

3. These steels relatively easily form build-up on the cutting edges, which creates difficulties in cutting and dulls (by overheating) the tools.

4. Additionally, in acid-resistant austenitic steels (with low carbon content, i.e. non-hardening 304, 316, etc.) small amounts of chromium carbides can form (precipitate) in the cutting area - and we all know what the hardness of carbides is.

To simplify as much as possible : from my experience - the most important thing when machining stainless/acid-resistant steel is low cutting speed and CONTINUOUS cooling/lubrication: i.e. pour the coolant, pour it and pour it again
Then even ordinary HSS steel tools (and even more so HSS Co) will work for in stainless steel a long time.

 

[JPMacG]

This is a wild thought, but could your chuck be distorting the ID of your work piece? And as you drill the wall thinner the distortion increases?

As an experiment try holding the work piece in a collet?

 

[pacifica]

Stainless steel doesn't really work-harden. It's called passivation. Chrome in the alloy gets exposed during cutting, which causes it to oxidize (air), and cover the material (again) with a tiny layer of chrome-oxide. It's a ceramic and it's really hard, so getting through it causes a softer metal (in this case, HSS) to give in, instead.

It doesn't minimize heating. It minimizes chrome exposure to oxygen, and thus re-oxidation (passivation restores the tiny ceramic layer that originally protects stainless steel from rust).

It's not necessary to drill stainless steel fast or use cobalt drills. You can use clay to make a sort of dam around the bore and fill it with some liquid (such as oil) that prevents the cutting area from replenishing of air (oxygen).

It's actually best to drill as slow as you can as the turbulence caused by the tool can cause foaming, and thus transfer air via microscopic bubbles to the bottom / cutting area.

thanks for explanation, always wondered why when drilling stainless it wasnt that hot but didnt cut well. I would slow speed and increase feed which usually worked. Now I will use clay and oil too.

 

[Batmanacw]

Always take a chip. Rubbing is bad.

 

[m1kemex]

Chromium is a very hard metal, sources says that hardness ranges from 40HRC to even 68HRC (hard chrome plating).

Hard chrome plating is a layer of chrome oxide (a ceramic). Metallic chrome doesn't get nearly that hard.

P. S. Regardless of the reasons mentioned earlier, I just discovered, looking at the specifications sheets, that 304 steel (the grade I checked) has, among other things, 1.0% silicon. No doubt on why it's is such a nightmare...