301 FH post tempering to raise RC

[tmalinski]

I am stamping and forming .030 thick 301ss FH RC45 and then grinding a cutting edge on it. My problem is I need to increase the hardness to achieve a better cutting edge but I can't use Extra Full Hard because my tooling won't hold up to the abuse prior to the grinding especially when piercing small holes. So I was told I could stamp, pierce, and form the FH up to the grind operation, then temper the parts to raise the RC up to around RC50 and then do the finish grind. Does anybody have any information on post tempering or any other method of increasing the hardness?
Thanks,
tom

Tom Malinski
Sr Design Engineer
OKay Industries
New Britain CT

 

[TVP]

Sandvik has quite a bit of information available on their website, and I highly recommend them as a source of spring temper stainless steels (

http://www.smt.sandvik.com/sandvik/0140/Internet/se01596.nsf/).

The following is an excerpt for their grade 12R11 which is equivalent to Type 301:

The strength of cold rolled Sandvik 12R11 can be increased by a tempering operation at 350°C (662°F) for 3 hours. An increase in tensile strength up to approx. 150 MPa (22 ksi) can be expected, depending on the initial cold rolled tensile strength. Further information on the nominal tempering effect can be seen under the "Mechanical properties" section. This heat treatment is also beneficial for relaxation and fatigue resistance.

Tempering is normally carried out by the customer after forming. To avoid discolouration, parts should be carefully cleaned before heat treatment. Tempering in open air furnaces gives a harmless brownish oxide on the surface.

 

[tmalinski]

Thanks, I'll check it out

Tom Malinski
Sr Design Engineer
OKay Industries
New Britain CT

 

[EdStainless]

You can stress relieve and get an increasein strength, but the hardness shouldn't change measurably, unless you are causing the formation of precipitates in the structure.

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Rust never sleeps
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[TVP]

Ed,

For high-C stainless steel grades like Type 301 precipitates do form upon heating to the range of 350-500 C. According to Sandvik ("Stainless Spring Steels", April 2003 issue of SPRINGS), "During tempering, a fine dispersion of carbides (M₂₃C₆) is precipitated in the microstructure, giving the material an increase in tensile strength of up to about 200 MPa maximum. Mike McGuire previously remarked upon this being a creative use of "sensitization", except that the carbides are uniformly distributed instead of being confined to the grain boundaries.

 

[EdStainless]

As I recall from my spring wire days, the carbide dispersion is very sensitive to temperature. The specific chemistry and degree of cold work will need to be matched with time and temperature.
I have seen this go wrong many times. You can form the carbides, but stress relive so much that you don't gain strength. Or you can get the carbides too coarse and then you have no fatigue life. And in all of the cases you do increase sensitivity to CSCC.

The other approach is to use full hard strip, and anneal the areas that need to be pierced. I have seen this done by Induction heating and in thin strip I have seen it done with high intensity lamps.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[tmalinski]

EdStainless
Your suggestion of using Full Hard Strip and annealing the holes area is interesting. Although I am already using Full Hard so we are talking about going to Extra Full Hard in the RC45-50 range. I can pierce 40-45 but If I get 45-50 we have trouble.
Thanks for the Tip

Tom Malinski
Sr Design Engineer
OKay Industries
New Britain CT

 

[TVP]

Tom,

Just out of curiosity, what is the failure mode when the strip material has hardness is 45-50 HRC? Tool fracture? Chipping?

 

[tmalinski]

TVP
I'm working with a sample of Extra Full Hard .030 x 3/4 x RC49 material processing a blank, several piercings, and forming operation, All secondary operations. No prog die because the quantities are too low. Generally it's the blanking and piercing tools that break down. I have to pierce several .035 dia holes and a few larger ones. That are very close to each other. I tried close space punches with a quill holder, and heavy bodied punches with just enough point length to pierce, basically throw away's can't sharpen them. Tried punches in Carbide, CPM10V & 3V but they wont hold up. Sometimes the punch edges chip, but usually they shatter near the tip. None of them break near the heads. These punches are .035 with a .125 body. I am guiding the punches by the punch body in a spring stripper because the point dia is too short to guide. I have opened up the die clr to 12% per side to reduce the blanking pressure, but I have to be careful to control the amount of break due to assembly requirements of the components. The dies are CPM3v they hold up fairly well.

Tom

Tom Malinski
Sr Design Engineer
OKay Industries
New Britain CT