Testing hardness / toughness of annealed 304ss

D

dbb-the-bruce

Dave
H-M Lifetime Diamond Member
Joined
Feb 28, 2019
Messages
499
TLDR: looking for a less subjective test for checking 304ss 16g sheet before and after annealing. I don't have any actual equipment (RC hardness tester for example) and I'm stuck with scratching it with a carbide scribe or attempting to file. There have to be other tricks, what are they?

Application: I'm micro machining (largest end mill .125, smallest .010 inch) 18g 304ss and finding that it's challenging with the equipment I have (small desktop CNC, no cooling). I have been successful - really not a lot of issues other than my carbide bits don't last very long.

I've read that annealing SS can / will make it more machinable.

Annealing process I'm using is to heat to 1900F in a heat treat oven for a while (soak) and then quench in water

Comparing pieces annealed/not annealed with a file test - feels like - the annealed is a littler softer than not annealed. Scratch test with a scribe is much more vague - hard to gauge how hard I'm scratching.

I've considered using a spring-auto punch tool, the assumption being that the strike will be more repeatable than anything I do by hand.

Other methods or comments greatly apprecieated.
 
Last edited:
300 series stainless steel isn't heat treatable. Thinking that if it gets annealed will make it softer and easier to machine isn't the right way to approach your problem. The problem is the material is getting cold worked when machining and this wearing out your tooling. So speeds and feeds are probably going to be the best way to deal with your problem. If you have use stainless, maybe look for 302 as it may machine a little better
 
vinnito1 said:
300 series stainless steel isn't heat treatable. Thinking that if it gets annealed will make it softer and easier to machine isn't the right way to approach your problem. The problem is the material is getting cold worked when machining and this wearing out your tooling. So speeds and feeds are probably going to be the best way to deal with your problem. If you have use stainless, maybe look for 302 as it may machine a little better
Click to expand...
You are confounding hardening with annealing, both of which are heat treatments.
AZOM Stainless Steel - heat treatment

It does appear that 300 series can not be heat treated to increase hardness.

Regarding feeds and speed, yes very important. I've reached workable feeds and speed based on the limitations of my machine - it is small and has a high spindle speed range. I will say that because of the small / tiny size of the end mills I'm using, the available speed and feed calculators are useless. They do not provide workable answers for tiny mills (in my experience).

I'd love some help with this.
What you say is a correct range / combination of speed and feed for using a 2 flute carbide .015D x .045L end mill to clean up pocket corners at a depth of .01?

I do think that cooling would help me but I've been reluctant to invest and I'm not thrilled about what I perceive will be messy.
 
Let me back up. What are you trying to accomplish through the annealing process? From your statements i though you were trying to make it softer and therefor more machinable. I was simply stating the change is softness isn't going to be all that much on HB scale, so probably not going to make a noticeable difference.

I can't help with feeds and speed.....there are going to be more knowable people than me on this. Have you looked at NYC CNC stuff? A few years ago ago they were promoting cutting speeds/feeds for the hobbyist application, so their website might have info that could help.

What kind of desktop CNC do you have? Have you checked tool runout when installed. Since your using pretty small end mills, this might be affecting tool life.

What are you trying to make and good luck
 
Yes, annealing to soften. And yes it may not make a big difference (that's why I'm trying to compare pre / post - my original question). You can anneal - it may not be enough of a difference to matter. So I'm attempting to answer (for myself): Am doing it right? (annealing) and how can I check the results?
On the CNC machine - it's a Nomad 3 from Carbide 3d. It has an 8x8x3 envelope, A decently powered spindle that runs from 10K - 22K (yes fast). And it is quite rigid for its size and also very accurate / repeatable in positioning. I've been able to do rather precise, fine detail work in brass and aluminum. I doubt there is an issue spindle run out but will check.

The NYC CNC site does have a lot of good information, but nobody seems to work with stuff this small (I've looked around ALOT).
Thanks for the suggestions.

Asm109 said:
Why would you not buy what you need?
Click to expand...
I kind of did. Can poke around more now that I know more. The most available and inexpensive 16g SS that I could find was 304 sheet. I don't think it's annealed (I wasn't aware / thinking much about machinability at the time, just jumping in).

I think that I did check McMaster - the 18g .060 thick stuff that's annealed only comes 4" wide. I'm guessing that the process of making big thin sheets is tailored for stuff / processes that typically don't include machining (stamp, plasma cut, fold, spot wield etc, that kind of work instead) and it's not worth annealing large sheets at production.

I'll look harder before I buy more.
 
So back to the original question of other hardness tests one could easily do.....
I did try using a punch, not a significant difference in the sizes of the divots.
 
dbb-the-bruce said:
I've seen those, I'll add a set to my purchase list.
Click to expand...
They are not super cheap but they sure are better than nothing! I used them to test my first stab at annealing and then re hardening and tempering. I am a happy customer.
 
You must log in or register to reply here.
Back
Top