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Choosing between 303 and 304

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misterwilling

Plastic
Joined
Jun 21, 2017
Hi guys, I have a part to build that is called out "300 SST as required" and requires "stress relief before final machining". I have attached a rough image of what the part looks like, but I can get in to too much detail about the part. As you can see in the image, the part has lots of material removal and some undercutting. The part also has close tolerance features, +.0009 -0 bores, .001 concentricity, +/-.001 depth of O-ring groove all the way around, .01 True position, ect.

The blank size is either 10" diameter x 2.5" long or 2.5"thk 8x9" plate. I believe the round bar would be our best best if we can find it, due to typically lower cost and less internal stresses than plate. Please let me know if I am thinking correctly on this.

I've already read through this post, http://www.practicalmachinist.com/vb/cnc-machining/premium-grades-304-ss-easiest-machine-245172/ which covers selecting easier to machine variants of 304, and I have also looked around for easier to machine variants of 303.

I've would like to choose 303 as my primary choice, due to it being easier to machine. In my experience though, 304 is more stable than 303. I'm wondering if building the part out of 304 might make it easier to hold all of our close tolerances. Do you think its worth also ordering some 304 material and test them side by side? It would be some expensive testing, but we are expected to build quite of few of these if the first run goes well. I'm also not familiar with the process for stress relieving SS. I'm currently reaching out to our heat treating company for guidance, but if anyone has insight into that process or how much material to leave for a final machining that would be great.
 

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I would agree with 303 for ease of machining. However that part doesn't look so bad to do from 304. Would think the heat treater should have final say though. I don't know if that stress relieving will distort anything, and to what degree. I have never had that done that on 303. This configuration looks like it would be fairly stable for heat treat, i would think you would maybe get a couple tenths of change.


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Is there a spec called out in addition, like ASTM ####? I just had a part where the print said, Material, any 300 series stainless, per astm 3$%^ ( I don't recall the number offhand)

At first I thought, well I'll just make it from 303. But then it turns out, the only stainless that meets that spec is 316 vacuum melt.

Be careful
 
misterwilling said:
Hi guys, I have a part to build that is called out "300 SST as required" and requires "stress relief before final machining". I have attached a rough image of what the part looks like, but I can get in to too much detail about the part. As you can see in the image, the part has lots of material removal and some undercutting. The part also has close tolerance features, +.0009 -0 bores, .001 concentricity, +/-.001 depth of O-ring groove all the way around, .01 True position, ect.

The blank size is either 10" diameter x 2.5" long or 2.5"thk 8x9" plate. I believe the round bar would be our best best if we can find it, due to typically lower cost and less internal stresses than plate. Please let me know if I am thinking correctly on this.

I've already read through this post, http://www.practicalmachinist.com/vb/cnc-machining/premium-grades-304-ss-easiest-machine-245172/ which covers selecting easier to machine variants of 304, and I have also looked around for easier to machine variants of 303.

I've would like to choose 303 as my primary choice, due to it being easier to machine. In my experience though, 304 is more stable than 303. I'm wondering if building the part out of 304 might make it easier to hold all of our close tolerances. Do you think its worth also ordering some 304 material and test them side by side? It would be some expensive testing, but we are expected to build quite of few of these if the first run goes well. I'm also not familiar with the process for stress relieving SS. I'm currently reaching out to our heat treating company for guidance, but if anyone has insight into that process or how much material to leave for a final machining that would be great.
Click to expand...

I'd be worried about deburing that part. Looks like a knife edge is created where the holes intersect each other. You may have got this job because everyone else no bid it.
 
Can you get 10'' dia 303 bar? .....and I'm not sure 303 plate is made, let along in that thickness.

And Re Mtndews post, I'd choose 316 over 304 any day of the week.
 
I'd just like to win the Loto and only worry about whether to cut my drink with an cube of ice or not...


Regarding that part, get more info from customer. One simply does not put tenths tolerances and then say " but make out of whatever you want ". But yeah, 303 would be easier to machine, not so great in other ways though. Here its all 316.
 
Limy Sami said:
Can you get 10'' dia 303 bar? .....and I'm not sure 303 plate is made, let along in that thickness.
Click to expand...

Fry Steel shows up to 22" diameter 303. Although I've never seen any with my own two eyes, 303 plate does appear to be made. I think its apparent availability is a relatively recent development.
 
Me too would worry about the geometry. THe rectangular opening between holes is a nightmare and dimensions will be totally undefined after machining. Also, totally "undeburrable".
Looks like somebody had an "idea" and you got the draft. 110% redesign is needed !
Especially if you need SS. Perhaps ceramics would leave acceptable edges ;)
 
If you have a whole lot of them and permissible...303 Investment Casting. Near net size saves on material, tooling, manpower and machine time. Keep pattern simple for a quick ROI.

I did this on a specialty item I had planned to make from round stock. Product took off for my customer, I could not get close to keeping up machining from solid...drums of castings casting arrived and my 10 minute op went to 45 seconds and done. ROI by the 2nd order...think we ran 20-30 orders over the next 2 years.
 
mikespahn said:
I would agree with 303 for ease of machining. However that part doesn't look so bad to do from 304. Would think the heat treater should have final say though. I don't know if that stress relieving will distort anything, and to what degree. I have never had that done that on 303. This configuration looks like it would be fairly stable for heat treat, i would think you would maybe get a couple tenths of change.
Click to expand...

Never heard back from our heat treater, but I'm hoping to do the stress reliving in house. I found this article Stress relieving heat treatments for SS and it suggests 210-475C with a slow cool. We have the ability to do this in house and would be great to do it one at a time so I can measure the movement. Not having to wait on sending parts to sublet service in also much better.

Larry Dickman said:
Is there a spec called out in addition, like ASTM ####? I just had a part where the print said, Material, any 300 series stainless, per astm 3$%^
Click to expand...

No ASTM# specified. Says exactly and only: 300 SST as required

John Welden said:
I'd be worried about deburing that part. Looks like a knife edge is created where the holes intersect each other. You may have got this job because everyone else no bid it.
Click to expand...

We got this job because the current supplier can't keep up. I know why! The deburing is going to suck. Inside the part there are actually flats to get rid of the knife edges, but its still going to be a nightmare to reach and cut those flats. There is going to be a lot of shopping for undercutting tools.

Milland said:
If using a "premium" material is in the budget, Carpenter makes a variety of specialty stainless steels, including more free-machining variants than just 303. You can check here for information, or contact Carpenter's tech service department.
https://www.cartech.com/globalassets/literature-files/carpenterguidetomachining.pdf
Click to expand...

Thanks! I was planning on looking into Project 70+ Materials

Mtndew said:
Any time you can choose 303 over 304... do it.
Click to expand...

That's what I'm thinking now after these comments. Screw a "303 vs 304" test; just order some premium 303 and start cutting!

SND said:
Regarding that part, get more info from customer. One simply does not put tenths tolerances and then say " but make out of whatever you want ". But yeah, 303 would be easier to machine, not so great in other ways though. Here its all 316.
Click to expand...

It doesn't make much sense, especially because most of the parts spec'd in the assembly are 304, but I don't want to kick a sleeping dog.

Madis Reivik said:
Me too would worry about the geometry. THe rectangular opening between holes is a nightmare and dimensions will be totally undefined after machining. Also, totally "undeburrable".
Looks like somebody had an "idea" and you got the draft. 110% redesign is needed !
Especially if you need SS. Perhaps ceramics would leave acceptable edges ;)
Click to expand...

A version of this part is being built by someone else in the 100's per year

SIM said:
If you have a whole lot of them and permissible...303 Investment Casting. Near net size saves on material, tooling, manpower and machine time. Keep pattern simple for a quick ROI.

I did this on a specialty item I had planned to make from round stock. Product took off for my customer, I could not get close to keeping up machining from solid...drums of castings casting arrived and my 10 minute op went to 45 seconds and done. ROI by the 2nd order...think we ran 20-30 orders over the next 2 years.
Click to expand...

Thanks for the tip. Sounds like a good idea, but I'll have to revisit it after we get the prototypes built. We don't have enough time at the moment, but I'll set aside some time to research it. Unfortunately this customer doesn't let you change your methods from the FAI part without informing them. So there would be lots of paperwork to make the change.
 
Misterwilling,

What are the finish requirements for the inside and outside of this part?

Any chance that you could use small rounded triangular stone media in a vibratory deburrer? That would knock off all burrs and break all sharp edges. It will also provide an even satin finish, inside and out.

Don't leave parts in a vibratory deburrer too long though, or they will be undersize on the outside and oversize inside; or even missing :).

Seriously, the cutting action of these stones focuses on sharp points and edges; and does very little to large radii or flats. Just use the right size and shape stones to reach inside the part.

I would choose 303 over 304, as the latter work hardens in spectacular fashion. If you must use 304, anneal it first...
 
303 is really stable if you stress relieve the material. If you have access to a heat treatment furnace you just need to heat to around 1850 degrees F and air quench to room temp, it recommended to water quenched but I have never done it. But the material will hold tenths after annealing.
 
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