Machining 304 Hot Rolled vs 304 Cold Drawn
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  1. #1
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    Default Machining 304 Hot Rolled vs 304 Cold Drawn

    In the middle of a very complicated part run, with lots of fun, small features; e.g. drilling 0.0315" diameter holes in 304 Stainless Steel to a depth of over 0.18".

    These parts also were my first venture into 304 Stainless Steel after doing a fair amount of milling and turning of 303 Stainless Steel. This was a bit of a shock, to say the least.

    The first series of parts have been made out of hot rolled 304. The hardness from the mill is 178 Brinell and had a very rough mill scale. I broke a couple hundred dollars in drills and endmills using the manufacturer's recommended speeds and feeds. After dialing back the speeds and feeds and adjusting a lot of the machining toolpaths, I was able to get a consistent process.

    I am now running a different set of parts in the series and the material is cold drawn 304, with a mill hardness of 216 Brinell. I started with a lot of the speeds and feeds that I developed for the hot rolled 304, but based on the sounds of the machining, I have been able to push the speeds and feeds back up to the manufacturer's recommended speeds and feeds.

    The harder material seems to cut better, but also has a lower chromium content 18.13% versus 18.33%.

    I know this is a bit subjective, but has anyone had a similar experience with hot rolled and cold drawn stainless steel?

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    304 is known to be an inconsistent material. Compare the sulfur content on the 2 bars. I try to avoid milling cold rolled stainless and really anything cold rolled. It will turn into a banana after you face mill it or squeeze if you machine out an L or U shape. Unless we don’t have to 100% machine the part (which is almost never) we don’t use cold.

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    Per the lot certifications for both,


    Element | Hot Rolled | Cold Formed
    Carbon | 0.014% | 0.023%
    Silicon | 0.38% | 0.400%
    Manganese | 1.72% | 1.560%
    Chromium | 18.33% | 18.130%
    Molybdenum | 0.44% | 0.390%
    Copper | 0.49% | 0.500%
    Nickel | 8.06% | 8.090%
    Cobalt | 0.133% | 0.150%
    Phosphorus | 0.032% | 0.040%
    Sulfur | 0.025% | 0.0300%
    Nitrogen | 0.093% | 0.0850%


    I am surprised that composition, in aggregate, can change the machining so dramatically. Certainly, the drastic difference is machining is not something I have read about w.r.t. 304 before your comment. I have a rather exciting pocket coming up with the cold formed/cold rolled 304 - I can certainly take it easier on the slotting cuts, have you found a tempering process that might relieve the stresses in the material to have less deformation?

    Regarding choice of hot vs cold 304, the choice was made mostly to minimize the material needed to be removed instead of process issues. Next time the preparation process is absolutely something I will pay attention to.

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    Just coincidentally ran 304 today removing 30 lbs from the under side of a turn table to reduce weight. I've milled 304 before, but only on small parts. I don't know what flavor of 304, but guess I got lucky and got the "good kind". Sounded good, end mill still looks good. YG-1 6 flute V7 Inox, 3/8" .75"loc. 140IPM, .375 doc, HSM 5% step over. Flood cooling on a Brother S500. Turn table 24" diameter, 1/2" thick.

    20211203_103059.jpg


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