dmalicky
Mechanical
- Aug 20, 2003
- 34
We are seeking a stainless steel casting alloy for a pump housing. The pump moves ~dry granular solids (it is a rotary airlock valve for pneumatic conveyance). Here are our requirements, and my understandings. I would appreciate any suggestions/corrections on this!
1) Low thermal expansion coefficient (5.0 to 6.5 in/in/F, between 70F and 250F).
---> Austenitic alloys are out. Ferritic (CB-30, CC-50...), duplex alloys with high ferrite (CD-4MCu), and PH/martensitic (CB7Cu...) alloys are good.
2) Corrosion resistance equal to or better than CF-8 (304) for these materials: salt, organic acids, food products, fish, sodium hydroxide (optional). Surface corrosion is the main concern, not SCC as our stresses are very low.
---> CC-50 is good. CD-4MCu excellent. Don't know about CB7Cu.
3) Machinability equal to or better than CF-8. These housings are 16"x16"--a lot of metal to bore out.
---> I'm finding machinability data scarce/contradictory. Many sources say ferritics are good since they don't work-harden much; but another specifically lists CC-50 as worse than CF-8. One source lists CD-4MCu as better than CF-8 with HSS tools but lists the opposite using carbide tools. Does that make sense? If someone has practical experience with these or other appropriate alloys, I would welcome input.
4) Hardness better than CF-8. The harder the better, as erosion and erosion-corrosion are concerns.
---> PH alloys are best, followed by duplexes, then ferritics, then austenitics.
5) Cost equal to or less than CF-8.
6) A "stainless steel" alloy for marketing considerations. (We would otherwise use Ni-Resist type 3 or 4 as they seem to meet our requirements quite well.)
7) Preferable: a relatively non-exotic alloy, for sourcing considerations.
Note: we *don't* need high strength -- 20-30kpsi yield is plenty. As many of these alloys are high strength, are there established variants that trade strength for machinability, without sacrificing (much) corrosion?
Some specific questions:
a) Which alloy meets our requirements best?
b) CD-4MCu and ferrite content: using the alloy's constituents and the Schaeffler diagram, I find it should be about 95% ferrite. But it is called a duplex and one source listed it at 65% ferrite. Its CTE is listed at 6.1 in/in/F. I would think that CTE is linearly predicted by the ferrite %, but 65% ferrite should give a much higher CTE than 6.1 (CTE for 100% ferrite =~5.5, CTE for 100% austenite =~9.5). Is CTE not linear to ferrite %, then, or is CD-4MCu's CTE not 6.1? In duplexes, is CTE governed by the fact that the ferrite is the matrix?
c) What are the relative machinabilities of these alloys, compared to CF-8:
CC-50, CD-4MCu, CB7Cu#?
d) Are any of these alloys worth pursuing (I know little about them):
Sea-Cure 29-4..., 2204, 2205, 20Cb-3?
Many thanks for any advice you may have,
David Malicky
Horizon Systems, Inc.
Lawrence, KS
1) Low thermal expansion coefficient (5.0 to 6.5 in/in/F, between 70F and 250F).
---> Austenitic alloys are out. Ferritic (CB-30, CC-50...), duplex alloys with high ferrite (CD-4MCu), and PH/martensitic (CB7Cu...) alloys are good.
2) Corrosion resistance equal to or better than CF-8 (304) for these materials: salt, organic acids, food products, fish, sodium hydroxide (optional). Surface corrosion is the main concern, not SCC as our stresses are very low.
---> CC-50 is good. CD-4MCu excellent. Don't know about CB7Cu.
3) Machinability equal to or better than CF-8. These housings are 16"x16"--a lot of metal to bore out.
---> I'm finding machinability data scarce/contradictory. Many sources say ferritics are good since they don't work-harden much; but another specifically lists CC-50 as worse than CF-8. One source lists CD-4MCu as better than CF-8 with HSS tools but lists the opposite using carbide tools. Does that make sense? If someone has practical experience with these or other appropriate alloys, I would welcome input.
4) Hardness better than CF-8. The harder the better, as erosion and erosion-corrosion are concerns.
---> PH alloys are best, followed by duplexes, then ferritics, then austenitics.
5) Cost equal to or less than CF-8.
6) A "stainless steel" alloy for marketing considerations. (We would otherwise use Ni-Resist type 3 or 4 as they seem to meet our requirements quite well.)
7) Preferable: a relatively non-exotic alloy, for sourcing considerations.
Note: we *don't* need high strength -- 20-30kpsi yield is plenty. As many of these alloys are high strength, are there established variants that trade strength for machinability, without sacrificing (much) corrosion?
Some specific questions:
a) Which alloy meets our requirements best?
b) CD-4MCu and ferrite content: using the alloy's constituents and the Schaeffler diagram, I find it should be about 95% ferrite. But it is called a duplex and one source listed it at 65% ferrite. Its CTE is listed at 6.1 in/in/F. I would think that CTE is linearly predicted by the ferrite %, but 65% ferrite should give a much higher CTE than 6.1 (CTE for 100% ferrite =~5.5, CTE for 100% austenite =~9.5). Is CTE not linear to ferrite %, then, or is CD-4MCu's CTE not 6.1? In duplexes, is CTE governed by the fact that the ferrite is the matrix?
c) What are the relative machinabilities of these alloys, compared to CF-8:
CC-50, CD-4MCu, CB7Cu#?
d) Are any of these alloys worth pursuing (I know little about them):
Sea-Cure 29-4..., 2204, 2205, 20Cb-3?
Many thanks for any advice you may have,
David Malicky
Horizon Systems, Inc.
Lawrence, KS
