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Grey Iron? Alum Bronze? 4140? 1144? What Metal/Steel Alloy Grade to select/use???

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KJPecho

KJPecho

Structural
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Apr 26, 2019
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Take a look at the image. I have 3 components making up an "EYEBALL" assembly. The 2 most important are depicted. We are just about ready to cut/machine parts but we need to make a call on what alloy/grade of metal we will be selecting to machine these components from. These components are machined SPHERICAL and rotate and clamp within one another. Their movement can be likened to the eyeball in your head. We have sliding/mating surfaces in a semi corrosive environment exposed to water/coolant. The "eyeball" depicted in GOLD/BROWN turns in all directions within the "Slew Ring", like your eyeball does. The "Slew Ring" is split so that it can contain/house the spherically machined "Eyeball" Component. The "Slew Ring" is also machined Spherically at the ID of the ring, but cylindrically at the OD of the ring. As these parts are sliding/rotating inside of one another machining tolerances are critical...so built in stresses from machining and any spring to the components would be detrimental. All mating/sliding surfaces will be greased sufficiently. I am told these components will experience pressures while in operation of around 200psi...so strength is not critical but is preferred.

So now down to the question at hand....when considering the "Slew Ring", what is the best material to choose? I am familiar with A36, A572-50 (all structural), A516-70 (PVQ) and 4140 (We make all of our case hardened tooling out of this)....but seeing as this component is much different in nature and in function I really have no clue what the best way to go about selecting a steel/metal alloy for this.

Initially Grey Cast Iron was recommended/suggested for the "Slew Ring" and that is due to the graphite in the iron and its lubricity. But the Slew ring is clamped down onto the Eyeball to stop its movement and freeze it in position and i am afraid that due to cast irons brittleness we could be in danger of cracking the slew ring during clamping and unclamping.

Another material that was suggested to look into was Aluminum Bronze. I am wondering if anyone has any insight on this and or any recommendations?
 
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CMRP-86-DE_Top_Bottom_Half_Slew_Ring_Details_v3_msvnsz.png
 
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Semi-corrosive needs better definition.

You'll want to use dissimilar metals to prevent galling. It doesn't really matter which piece is which metal but if one of the metals is very high cost or difficult to machine you may want to use it for the smaller or simpler part.

For some parings, bronze and stainless steel go well together. A 400 series stainless is going to give more wear resistance and a sacrifice to corrosion resistance compared to a 300 series. The precipitation hardening grades like 17-4 sit right in the middle. If you want to go stainless on stainless, Nitronic alloys pair well with conventional stainless steels. Nitronic 60 makes a very good bearing for corrosion resistant 316 stainless steel.

The Nickel Institute has a handy publication that compares wear rates of various metal pairings.


You can also save yourself the hassle of lubrication and use graphite for one component.


Plastics can also work but you'll have to take special care in selection and dimensioning if the environment is wet due to moisture absorption. Acetal, for example, may perform well here.
 
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@TugboatEng THANKS! For the reply!

Semi corrosive environment.... As these components will be metal and possibly out a carbon steel alloy, rust/corrosion was a concern. The machine works steel and is done so at elevated temperatures. Water is used to quench the steel being worked and as a consequence these components depicted/described above will be sprayed flooded with water.

Glad you mentioned galling...that was another worry we had. I had previously mentioned there were 3 main components to this "eyeball" assembly. The 2 most important mentioned above....The eyeball goes inside the slew ring and that assembly is housed in a frame which we call the main housing. Due to the spherical geometry of the eyeball and the ID of the slew ring, the eyeball is free to move in all directions but it cannot twist rotate about its central axis. That is the reason for the split and clamping action of the slew ring. The slew ring will rotate +/- 20deg once clamped around the eyeball all within the main housing body/frame. The OD of the slew ring is machined cylindrically as is the ID of the main houseing body/frame. So the slew ring is critical in that it has sliding surfaces at both its spherical ID and the cylindrical OD. I guess you can think of the slew ring as a large bushing/bearing....so some type of bronze (if were able to get a large enough billet cast) would make sense there.

Thanks for the links i will scope those out. And for the suggestion on the use and pairing of stainless with bronze/alum bronze. I do see some information out there on functional machined components out of stainless steels.

Not sure if anyone has any experience with Grey Cast Iron for functional machine components? I know they make gears, pistons, sleeves cylinders out of this material. With the graphite it would make for a nice bearing surface. Were looking at a product called DuraBar....
 
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It seems Dura-Bar is preferred for machinability vs. other properties.

One suggestion, as I didn't notice that this is 34 inches in diameter at first, would be to select a weldable material so that it can be ring rolled instead of cast.

Making the slew ring of steel or cast iron and coating with hard chrome or electroless nickel may also provide you with improved life. Hard chrome sliding on bronze play well together.
 
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Is there any temperature change to worry about?
Normally using Ni-Resist cast iron (D-2 would be my choice) and Al Bronze together would a great way to so, but they have very different CTE.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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Worm gearing generally uses an aluminum bronze gear with a hardened steel worm. Or a cast iron gear with a steel worm. But both are usually lubricated in an oil bath. Plain old cast iron rusts easily.
 
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I agree with Edstainless. IF CTE is an issue you could consider Invar36.

 
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Wow you all are great! Thank you for the responses on this.

Heat input to these components will be minimal. The machine is an open frame design. The elevated temps are introduced into the steel by way of induction coil...worked then quenched with water to cool/temper. This assembly is after the quenching and thus i do not believe much of that heat existing in the steel will remain when it gets to the tooling installed in the eyeball assembly.

So it does sound like Cast Iron is a viable material from your responses. So long as it is the right kind/type and it has the correct finish. I'm thinking the slew ring is specified to be made from some Alum Bronze Alloy and that we go to the MFG for recommendations on specific alloy. The Eyeball itself and the Main Housing to be made from this Ni-Resist D2 cast iron or Invar36.

Do we think rust could be an issue with the cast iron? And if so does chroming make sense like TugBoatEng had suggested?

 
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KJPecho
this is good size assembly
lubrication system?
cast or forged, bar may not be an option.
so no slew bearings? so limits material choice. aluminum bronze with graphite
with these diameters heat treating material will be expensive difficult to maintain roundness
 
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Yes these parts are of decent size. And if this machine works out there are talks of building an even larger machine! These are all very important considerations to keep in mind. There is an automated high pressure lubrication system pumping grease into grease ways between the eyeball and the slew ring and the slew ring and the main body/housing. I should mention that the movements of the machine are very slow and very minute/miniscule. Bar is definitely not an option on this as its not produced large enough. This will either have to be a forging or a casting. Initial talks with the MFG of the alum bronze mentioned sand casting. No bearings external of the slew ring....the idea is that the material for the slew ring itself would act as a bearing surface and be sufficiently greased. Again any turning/rotating of the slew ring will be slow and miniscule...the ring at max will only rotate off center +/- 20deg. Once the eyeball assembly reaches its position in roll, pitch and yaw the hydraulic clamps are engaged and the eyeball is locked within the slew ring and the slew ring is locked within the main body/housing.

If we are using Ni-Resist D2 for the eyeball and the main body/housing and aluminum bronze for the slew ring...would there be a need to heat treat? At initial conception of this project we had talked about the use of a steel alloy and heat treating/hardening of closely toleranced, mating/rubbing/sliding surfaces was brought up as a concern.
 
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Don't mean to highjack your thread but I wanted to ask Ed a question: Does ductile iron (D-2) machine well (at least compared to cast iron)? Cast iron machined easily but created a lot of dust when we used to use it for our gear blanks.
 
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My preferable tool steel is A2, it has the least amount of heat treat distortion, my second choice D2, but in my experience D2 has material imperfections.
we made gears from D2 and had a few failures from our customers. but mainly pump gears. but D2 has very low distortion.
now 34 Inches that may be an other issue.
 
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KJPecho
Quote: If we are using Ni-Resist D2 for the eyeball and the main body/housing and aluminum bronze for the slew ring...would there be a need to heat treat? At initial conception of this project we had talked about the use of a steel alloy and heat treating/hardening of closely tolerance, mating/rubbing/sliding surfaces was brought up as a concern.

you will have to run the numbers for for scuffing, contact stress, tensile stress safety factors (S/N curve)

depending on the tolerance, surface finish, tensile strength of the parent material.
 
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Hey guys! Hope everyone is doing well today! So in reaching out to a handful of Foundries I am coming short of someone who is constantly producing Ni-Resist Ductile Cast Iron or someone that can sand cast in the size we need. Dura-Bar produces a Ni-Resist but it is a flake graphite. Any suggestions on foundries/contacts? i have reached out to a handful already, the closest i got and still waiting for a reply (at least they didnt immediately say no) was Stainless Foundry out of Milwaukee, WI.
 
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KJPecho
these are to be machined after casting?
 
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Yes certain surfaces will require machining after casting. We have also begun to consider an austenitic stainless in place of the Ni-Resist Cast Iron if we are unable to find someone to produce for us.

Question with respect to casting vs forging. In talking to these foundries, making a casting of a part that is 1 off and is not production may be prohibitive as most foundries requires certain size pours. Also we have been asking for a billet style casting which is poured to a rough square shape or round shape....the weight of such a casting as large as these parts are have been an issue and it was suggested to us that we get a die/tool made for the casting so the casting will be near net shape with maybe 1/4" added to girth for machining purposes...that way it reduces the weight of the casting and are not machining nearly 70-80-% of the casting. I guess my request of large rectangular or round billets is more so geared to a forging as what i have been told....I was under the assumption that cast iron is a cast product....would Ni-Resist Cast Iron be available in a large solid billet style forging?
 
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KJPecho

mileage will very with ones experience, I believe it depends on which casting process is used. I lightly worked with the lost wax process.
were a master mold is produced, and the sand cast molds for production are for net near castings.
pro's saves material
less machining saves on labor
once in production saves time

con's
parts have a tendency to change dimensionally, and a very well receiving inspection required to monitor change.
and the shop has to be on there toes or it can cause a lot of scrap.
difficult to to tool for first turning and milling, since cast surfaces are a challenge.
up front cost for tooling for the casting and machining.

My preferences are a rolled ring forging (easier to tool less material waste)
pancake forging ( allows for easier tooling but has more waste of material)
castings
last is castings, to me castings are a PITA to manufacture from and are hard to clamp in the initial manufacturing
cycles, and I had bad experience with cast dimensionally changing, however knowing is gold, and as
the shop is made aware and proper risk management is followed, then it is doable.
for a very small lot a forging may be be less expensive. get quotes for all of the above.
 
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Ni-resist is a highly alloyed austenitic cast iron. We used to sand cast both grades. It isn't truly 'stainless' but it does have great corrosion resistance. As far as machining goes we did it all with water based coolant. The only dry operations where in the foundry (cut off and snag grind).
The type 2 can ge cast either as gray or ductile, for a large structural part I would prefer ductile. There is no HT to strengthen this alloy, and if the casting does not have large variations in thickness I don't think that a stress relief would be needed.
I know that there are still large pump bowls and impellers cast from Ni-Resist, you may need to widen you search some. The nice thing about a casting is that non-mating surfaces could just be blasted and left as-cast.

If you wanted a forged or formed part and wanted some corrosion resistance then a 400 stainless may be the only real option. In the low Cr grades (410, 420) you will get staining but fair corrosion resistance but you would need to make sure that all surfaces are clean. By adjusting the C levels you can adjust the strength (higher C = higher strength = lower toughness) when it is heat treated.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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Niresist is a friendly alloy and does not pose many challenges in processing. As Ed states, we are in the process of being awarded large pump bowls for lifting seawater.



 
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