CF8M hardness/toughness 2

[andysf]

Good day!
My company have been producing castings in CF8M and WCB material for a few years now.
The jobs in particular require a drilling and tapping operation, and this has been causing one or two issues for the machinists (in both materials).
Compared to our competitors castings, our own have regularly appeared much harder to drill and tap.
More recently, a separate machinist has complained of our 25/12 material breaking several taps.
Whenever the hardness has been checked, there has never been any difference between our own and our competitor. Is this more of a toughness problem? I have heard sulphur levels can alter machinability, but cannot see any difference between chemistry samples from our own and our competitors castings... Can anyone think of any other avenues for me to look down? Is there something inherently wrong in the way we are casting?

 

[sjlivesey]

have you looked at your drill bits and taps? the facility I work at has been "saving" money by buying cheap drill bits and taps made Red Ch*na. even though the label says "M42" they are not the same quality as ones made in USA. do an Ishikawa(fishbone) of your process and see if any of your processes or tools has changed. could be something as simple as the type and quantity of tapping oil.

 

[bcd]

You compared hardness and chemistry, but have you compared microstructres to see if there is a difference in heat treatment (for WCB) or solution annealing (for CF8M)? I suspect the difference is this.

 

[andysf]

bcd, I haven't actually been down this route yet, but think it may be the next step. I wanted to be sure there was nothing untoward in our melting process.
I can't imagine the tooling is an issue, however, due to the fact the same tooling is used for both our castings and our competitors castings.

 

[arunmrao]

Blaming the foundry is easiest way of passing the buck. I have travelled the same road for years. WCB will not pose a problem unless you encounter inclusions, but CF8M and HH grade or any of the other austenitic grades always meets such comments.

Austentic Stainless steels tend to work harden as you progress, requiring more pressure .

Look for the right drill bit (point angle 120-135Degrees), use low speeds, a rigid drill chuck and clamp the job well. I have used coconut oil,while drilling or tapping. A Cobalt based drill bit will be helpful.

I recently drilled Hardox 550, like butter with Co drill bit and low speed.

Hope it helps.

 

[redpicker]

What arunmrao said.
If this is a customer saying your castings are more difficult than your competitors, I would suspect that the reason they are having trouble with yours is becasue that is what they are working with now.

IME, machining problems get blamed on:

1) Material
2) Tooling
3) Drawing

In that order. And, in the case of material, the other lot, which they don't have any more of, ran without any problems.

rp

 

[jwhit]

As previously stated, keep the tip of the tool ahead of the work hardening, that is, use a fairly high feed rate and a reduced tool speed for CF8M. Another thought on this material, based more on observation than book knowledge is that lower ferrite contents tend to be tougher and more sensitive to work hardening. A higher ferrite content seems to break the chip a little better as it is less gummy. I suspect the ferrite pools, having higher hardness influence that. Ferrite in these alloys is generally controlled by the foundry for specific application, with Cryo and biopharm use and low permeability applications having low ferrites between 0 and 5%. Other applications at 5 to 20%, with 8-15% being pretty much optimum. Weldability with 5-20% ferrite is also improved.

 

[kumkumvijay]

My opinion is to eliminate the root cause from casting ( Material) just compare the microstructure of your supplier difficult to machine casting and competitor casting in thin and thick sections.
If you find same microstructure then its something related to your machining process.

the following microstructure will affect your machining tool life:
1. carbide precipitate.
2. Ferrite pahse transfer to sigma phase during machining due to temperature increase to the level of 650C.