How to increase 304 SS hardness 3

[vosbaard]

I need some advice. I have a 304 SS ball (25-39 Rc according to manufacturer spec.) in an application where chemical compatibility is of great importance. My problem is that the 304 is just to soft, the 440C works well, however it is not compatible with the chemistry. Is there a way that i can increase the hardness of the 304?

 

[Metalguy]

Not with the ball you already have. BTW, that's a huge range of hardness. What sort of chem./temp. do you have?

 

[metalonis]

304 SS can only be hardened by work hardening.

 

[arunmrao]

I have often come across this statement that thehardness of 304 alloy is 35-40RC. How is it possible to achieve such hardnesses and also I would like it to be quallified alongwith the thickness of the piece. Please enlighten me on this aspect,that a predominantly austenitic material achieves such hardness levels.

 

[vosbaard]

The ball is 3/64 in dia. We are using it in the battery industry. It is subjected to room temp. and the chemistry is PCME:EC (Electrolyte) for MnO2/Lithium battery.

 

[Metalguy]

You can only increase the hardness of something like 304 SS via cold-working (and to a lesser extent, neutron radiation, but it's not too practical).

During cold-working some of the austenite (FCC), which isn't real stable in 304, changes to one of the forms of the much harder martensite (BCT, as in diamond, I think).

 

[TVP]

arunmrao,

As Metalguy stated, the FCC structure in Type 304 stainless steel is not stable during deformation, and martensite is formed. Depending on the degree of deformation, the subsequent strain hardening can produce strength levels of 1200 MPa, which would be in the high 30's HRC. The following document has a good graph of mechanical properties vs. degree of cold working for Type 304:

http://www.aksteel.com/pdf/markets_products/stainless/austenitic/304_304L_Data_Bulletin.pdf

 

[arunmrao]

Thanks . To what depth across the cross section is this phenomenon observed, please let me know.

 

[Metalguy]

All the way thru, *if* you can deform it that deep. It's simply deformation-related.

 

[TVP]

arunmrao,

As Metalguy states, the strain hardening occurs throughout the cross-section provided that the strain is developed throughout the cross-section. Wrought stainless steels are cold-rolled various amounts without subsequent annealing to produce increased strength throughout the cross-section. Shot-peening or roller burnishing would produce a similar effect, but only on the surface.

 

[arunmrao]

Thanks does that mean that we can use a component for corrosion application in the cold worked ondition? I have normally observed in the case of castings that it is a surface phenomenon,and that too in areas which are heavily worked.

Quite often customers request us to increase the hardness of 304, which I have normally declined hence my question in this thread.

To summarise in small cold worked components the hardness increases,

 

[Metalguy]

Cold-worked metals generally lose some corrosion resistance-sometimes a lot of it.

But there is no way *we* can tell if it would work for you because we know nothing about your environment.

 

[furula]

Your dilemma is shared by many industries. The good corrosion properties of austenitic stainless steels verses the hardness and strength of the lower corrosion resistant martensitic stainless alloys. The use of 17-4 precipitation hardening alloy would provide a hardness in the 35 to 38 HRC range and would equal the corrosion resistance of Type 304 stainless steel. It is easy to fabricate in the solution annealed condition, and hardening is easy with little distortion. Custom 455 (Cartech Cp) offers higher hardness and strength, at a higher cost, and has good corrosion resistance. A call to your local specialty steel supplier would be helpful as far as availability, cost and technical support.

 

[TEV]

There have been some recently developed case hardening processes for austenitic stainless steels that do not reduce the corrosion resistance, as the traditional carburizing or nitriding processes did. We use one that gives a case depth of about 25 to 30 microns with a hardness in the Rockwell 60's. The corrosion resistance is equivalent to the solution annealed product in most conditions, and superior in some. If this is of interest, I can give you a contact.

Furula is correct about choice of materials. Carpenter has some autenitics that can achieve very high hardnesses by strain hardening with good corrosion resistance, but at a price.

 

[KnifeKnut]

420 stainless perhaps?

 

[mmitpwdba]

What about Carpenter's Custom 465? 50 to 60 Rc.

See my predicament thread330-24316

 

[IFRs]

Take the 304 ball and shot-peen it. If the surface roughness is acceptable, the deformations will harden the surface.

 

[kenvlach]

Bodycote (

http://www.bodycote.com/)

has a relatively new Kolsterising® process for case hardening austenitic SS without loss of corrosion resistance. Similar or identical to the process TEV described.
“Wherever components are subjected to severe wear and corrosion attack.” Also useful for duplex SS & some nickel-based alloys (best corrosion resistance is for ferrite-free alloys containing some molybdenum).

"Kolsterising is a case-hardening process primarily for austenitic stainless steels. Wear resistance and resistance to galling is improved, while corrosion resistance remains unchanged.

The process involves diffusion of carbon into the work piece surface without the formation of chromium carbides. Post treatment surface hardness is equivalent to between 70 and 74 HRc. Standard case depths offered are 22 or 33 microns."

No change in dimension, so the treatment could easily be used for existing product lines. It also eliminates the galling problem of SS, so sounds like a winner.

 

[TEV]

Kenvlach - This is the process I was referring to. I am very familiar with the process and can assist with applications.

 

[brider]

TEV - Can you provide a rough cost comparison of Kolsterising vs Kolene QPQ? Is it much more expensive or comparable. Very interested in the possible use of Kolsterising, but cost is a critical factor in our product.