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Work Hardening Stainless - The Key

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Robert LaLonde

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Yesterday I did a little bit of work with 304 stainless. I recently picked up several 12' bars in different sizes from my local metal vendor to just have some onhand. Just about everybody seems to be a little bit afraid of 304-316 stainless. They say to use 303 or 416 for its machinability. Well I have some 303 and some 416 also, but not in sizes suitable for yesterday's project. The 304 is stocked locally primarily because its food grade and its stronger than 303, and more common and cheaper than 308 or 316. As an agricultural community most makers and builders build machines with food grade alloys. Coolers, precoolers, processing lines, etc. My vendor doesn't stock 416, but he let me know he can get the 416 for me if I don't mind waiting for their regular supply trucks. I like 416 because its both stronger and cheaper than 304.

Anyway, I had some bars of 304 on hand because its available. Yesterday I needed to make some parts requireing turning, tapping, drilling, and threading. I was a little bit afraid of it because of other peoples comments about it. My solution or key... Use the horsepower of the machine and cut very aggressively. I drilled a modestly deep hole in one bar on the lathe and then POWER tapped it. Not a large thread. Just 3/8-16. I only had a hand tap so I was a bit nervous, but by over drilling the hole, and backing out about every 4 threads it went right in and came right out. I did use LOTS of tap magic cutting oil. I didn't even use a cheater bar on the chuck key. Just two handed it. I had the lathe on low gear turning about 70IPM for power tapping, but the drilling was with a LOT of pressure at 300 RPM. Lots of oil for both.

I cross drilled the bar for a handle the same way. Well. not on the lathe. On my drill press. Mid range RPM and aggressive heavy pressure on the handle. Two hands on the handle after I locked the drill vise down so I could reverse quickly without rubbing.

Then I turned a stud to go with it. One end was 7/16-14 and the the other was 3/8-16. I thought about single pointing it, but then I said screw this. Its seems to really like hard heavy cutting. I power turned it with a die holder. Both ends.

In conclusion. Hard heavy cutting seems to work better on work hardening stainless. I can see why so many folks with small, light weight, not so rigid machines might sometimes have issues with stainless. They just can't do heavy aggressive machining. I know I struggled with it years ago on my old HF 7x10. This 3HP 14x40 made short work of it.

Maybe I'm nuts, and maybe I just got lucky, but on those hoity toity machining sites I have read a comment or two were they said they just went hog wild on stainless and did ok. I never really believed them before.

(P.S. The cross handle was a piece of 1144 stress proof with some lock nuts on the end. As usual it was a dream to thread.)

P.P.S. I did cheat a bit on diameters going go a little over on inside threads and a little under on outside threads before threading. Just a few thousandths. I'm sure that helped.
 
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RJSakowski

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#2
A great write-up.

Can anyone explain from a metalurgical standpoint what causes stainless to work harden?
 

Robert LaLonde

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#3
Correction. I had a spiral point machine tap. Not a hand tap. I misrecalled.
 

mikey

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#4
A great write-up.

Can anyone explain from a metalurgical standpoint what causes stainless to work harden?
It's the addition of nickel to the alloy that is most responsible for the tendency to work harden. Nickel is added to chromium-containing steel to form the Austenitic class of SS. The most common formulation in this class is 304, which has 18% chromium and 8% nickel; 304 is also known as 18-8. The nickel increases ductility so chip breaking is more diffiucult; hence more of the heat from a cutting operation tends to stay in the cutting zone and hardening results.

When they add sulfur to the 304 mixture, we get 303. The sulfur reduces ductility and the chip breaks more readily. 303 will still work harden but not as readily.

Martensitic SS, like 416, contain no nickel and are the easiest of the SS to machine. They can work harden, too, but less so than the Austenitic class.

As Bob suggests above, 304 likes rigid, powerful machines but a smaller hobby machine will also readily machine it provided the user knows what he is doing.
 
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RJSakowski

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Is it localized heating, then, that causes the hardening? If so, will aggresive cooling such as flood cooling reduce the tendency to work harden?
 
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Many years ago before insert tooling became the norm. I had a government inspectors raising doubts about my using insert mill cutters on 316 clamp rings for missiles . But we were burning up high speed and cobalt end mills . The problem came with changing the tooling out and waste of time . So I'm on the second shift and the boss gives me the mill job that night. I burned up a few endmills and said this isn't going to do . It hardened the steel and it wasn't working . I had an 2" triangle insert tool, set it up and could cut the entire surface with one pass and with the mister they never got hot or hard . And yes it was a faster cut and that made it more aggressive. Next afternoon they questioned why and where I got the mill cutter. Later it meant a promotion and raise . Just for using what I knew. It would eat four endmills and the inserts would eat the 316 one set of three inserts finished the run .
 

mikey

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Is it localized heating, then, that causes the hardening? If so, will aggresive cooling such as flood cooling reduce the tendency to work harden?
Yes, high heat in the cutting zone promotes work hardening. Flood cooling will help but not that many hobby guys have that option. So, what can we do?
  • Mount the work piece as rigidly as the lathe allows. Keep work extension as short as possible and use a live center when needed.
  • Mount the tool as rigidly as possible, keeping extension of the tool to a minimum.
  • Use tools with smaller nose radii to reduce deflection. If the tool deflects, it does not cut and if it does not cut it will build heat.
  • Use tools with high positive rake to enhance chip clearance so that the heat goes out with the chip. 303 has high ductility so chips do not break easily; they string. Even so, getting the chips out fast removes heat and high positive rake tooling does this.
  • Use sharp tools. For me, HSS with larger side and back rake angles works well. I don't use carbide for this material, although that is certainly not an industrial practice. I opt for cobalt tools that handle higher heat better and I keep the tool sharp.
  • Use lubricants. I prefer sulfur-bearing cutting oils but have found AnchorLube to work well. This is not so much to cool the cut; it is more to lube the cut without boiling away.
  • Attend to cutting conditions. The smaller the lathe, the more important this becomes. The speed, depth of cut and feed have to be adjusted to allow the tool to cut continuously; if you dwell in the cut then heat builds and the work hardens. Stainless, especially Austenitic steels like the 300 series, is not particularly hard so cutting speeds in the 100-120 SFM range is recommended but I have found that on smaller lathes it is better to go slower so that we can keep up with the feeds. I normally use 60 SFM. Depths of cut need to be realistic and applicable to the lathe in use; smaller lathes require smaller depths of cut in order to keep feed consistent. Feeds in industry may be measured in IPM but that is meaningless to a hobby guy who is cranking manually; here, our speeds and depths of cut have to be adjusted to enable us to turn the wheel at a consistent and achievable rate and that comes with experience.
  • The final thing I can think of is to use a steel that is easier to cut. 303 is the sulferized version of 304 and cuts better. 416 does not contain nickel and is much easier to machine than the 300 series.
I know that work hardening of SS is an issue for us but if we do it smart, we can cut it. I have turned a fair amount of 303, 304 and 416 on my Sherline and Emco lathes and have had few issues. However, it took awhile to figure out what I needed to do to cut it. If I had to pick one thing to remember about stainless, it would be to not dwell in the cut.
 
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