Work Hardening 304 SS above 50 HRC? 4

[metalman8357]

Hi all,

I recently came across a sharp point self tapping fastener that claims to be 304 stainless steel, and can achieve a tip/thread surface hardness between 52-58 HRC solely from cold working. I know it's not uncommon to get core hardness values in the mid 30's to low 40's when wire drawing and thread rolling 304, but I'm having a hard time figuring out how they can get the surface hardness this high just from cold working. Any thoughts as to how this might be possible? The fastener in the picture (link below) likely has a core hardness of 32-50HRC, a surface hardness of 52-58HRC, and a case hardness of 50HRC minimum up to 0.010". Any ideas on how this might be possible?

http://www.sealtite.com/wb-sshss-group.htm

 

[TVP]

Austenitic stainless steels based on the nominal 304 composition strain harden like a mother. Nice technical description, eh? For example, wire that is cold drawn to 0.127" (3.2 mm), which is the minor diameter of the fastener from the link, will have a UTS of ~ 1500 to 1700 MPa, which is approximately 46 to 50 HRC. Thread rolling over this will further increase this by several HRC points. So, this could easily be heavily cold drawn and thread rolled 304.

 

[metalman8357]

Interesting. I knew that mid 40's was certainly possible but 46-50HRC seems pretty impressive. If I wanted to achieve a core hardness of 46-50 HRC, roughly what size diameter wire would I need need to start with in order to cold draw to a final diameter of 0.130"?

Also, does all this strain hardening have a drastic decrease in the corrosion resistance of the material?

Thanks,
M

 

[Screwman1]

Well......you can get that kind of hardness with huge amounts of cold working, but if you draw the wire to that extent it gets so hard that you blow up the forming tools in the header. My experience is that the practical upper limit on the blank diameter wire going into the header is about 130 ksi ( roughly Rc 29).
I think that what they are advertising is that when they roll out the gimlet point on those parts they are getting a small area at the point that is work hardened up to a pretty high amount, but once you approach the full body diameter it will drop off. Looking at that site, most all those parts are made in China, so I would take the performance claims with a grain of salt.

 

[EdStainless]

It has no impact on the general corrosion resistance, but the stress corrosion cracking resistance gets even worse.
It sort of begs the question of why to use 304 in the first place.
You have formed so much martensite to reach this hardness that you have no toughness left.
It had might as well been an 18% Cr martensitic to start with.

We used to draw fine wire (0.004") and easily reach 240ksi UTS.

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[Screwman1]

I agree 100% about the ability to achieve 240ksi with drawing, it just makes a material that has such a high yield and low ductility that you can't form the head of a screw on it. that is why I am a little bit of a doubter about the claims in the link.
In the construction industry there seems to be a hunt for a magic material for fastening wall sheets on metal buildings that has high hardness (Rc 50 or more), extreme corrosion resistance (similar to 302 SS), and low cost (similar to 18-8) and a resistance to SCC. You can get 3 out of 4, but trying to get all of them is pretty difficult.

 

[kadirgodil]

EdStainless
do u form martensite in 304 ss?? i mean if its an austenitic SS then?
so much hardness comes from strain hardening if i am not wrong..

 

[EdStainless]

And the strain hardening causes a change in structure. 304 is meta-stable so it does not stay austenitic when strained. Look at a calculated Ferrite Number, for most 304 it is 5-12%. There are austenitic alloys that stay non-magnetic when heavily cold worked. We draw 21-6-9 to 150-200ksi all of the time. It still has great toughness and is fully austenitic (aircraft hydraulic lines).

For high strength fasteners the trick is to cold draw the rod as high as you can while still being able to form the heads. And then rely on the thread rolling to drive the point hardness higher.

They should try some lean duplex SS grades (2101, 2003, 2201 and so on). They will heave better corrosion resistance than 304 (or any 16-18%Cr grade) and they have some SCC resistance. Not fantastic but at least some.

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