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Why do props rust?
- Thread starter Tony
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Ratickle
Founding Member / Super Moderator
Usually not quite enough nickel in the stainless. There are different grades. All grades have ranges of acceptable standards. Depending on the type of stainless, some are more brittle, some more flexible, some more rust resistant.
303, 304, 316, monel, etc. there are lots of different ones. Props are an oddity. Mercury has probably done the most testing with material types and grades.
15-5 seems to be the most dependable compromise at the moment. Mercury was working with the new material, propietary at the last I knew, but I never got my hands on one to perform a test burn to see what it was. Thinner blades same strength and corrosion resistance according to their info.
303, 304, 316, monel, etc. there are lots of different ones. Props are an oddity. Mercury has probably done the most testing with material types and grades.
15-5 seems to be the most dependable compromise at the moment. Mercury was working with the new material, propietary at the last I knew, but I never got my hands on one to perform a test burn to see what it was. Thinner blades same strength and corrosion resistance according to their info.
ThrottleUp Props
Sponsor / Charter Member
Keep in mind, Stainless steel is just what it's called, stainLESS not stainproof.
The 300 series of stainless steels are the most corrision resistant, 316 being considered the "marine grade" stainless steel with the greatest corrsion ressistance. The problem with 316 is it's lack of strength.
Props are typically made with PH (precipitation hardening) grades of stainless steel; 15-5, 17-4, 13-8, etc. These stainless steels can be hardened easily and have a great deal of strength, higher than many steels. The PH series of stainless steels are used extensively in military and aerospace applications. Important to us is the fact they are also considered to be "free machining" which makes them a material that machines well. 316 is "gummy" when machined and even though it's not as hard it is not a material any machinist would go out of thier way to machine.
If the prop has gone through a hardening process, which most Mercury props have not, it's tendency to rust is even greater. A polished prop will resist rust more than a satin finished prop.
It's a matter of finding a balance between strength and corrosion resistance. There are some materials that have greater strength and corrsion resistance than 15-5 and 17-4 but not many would be willing to pay for it as the material is several times the price of 15-5 and 17-4.
The 300 series of stainless steels are the most corrision resistant, 316 being considered the "marine grade" stainless steel with the greatest corrsion ressistance. The problem with 316 is it's lack of strength.
Props are typically made with PH (precipitation hardening) grades of stainless steel; 15-5, 17-4, 13-8, etc. These stainless steels can be hardened easily and have a great deal of strength, higher than many steels. The PH series of stainless steels are used extensively in military and aerospace applications. Important to us is the fact they are also considered to be "free machining" which makes them a material that machines well. 316 is "gummy" when machined and even though it's not as hard it is not a material any machinist would go out of thier way to machine.
If the prop has gone through a hardening process, which most Mercury props have not, it's tendency to rust is even greater. A polished prop will resist rust more than a satin finished prop.
It's a matter of finding a balance between strength and corrosion resistance. There are some materials that have greater strength and corrsion resistance than 15-5 and 17-4 but not many would be willing to pay for it as the material is several times the price of 15-5 and 17-4.
Ratickle
Founding Member / Super Moderator
Really? Why would they not run them through the hardening process? I'm at a complete loss on this one.....Cost savings only???? Or something I'm missing?
The strength of the prop would be way off from the specs of the steel standards.
ThrottleUp Props
Sponsor / Charter Member
Just about all stainless props go through no heat treatment at all. They are cast and finished. That's one of the differences between the higher end CNC props and the cast props. Most of the CNC props have been through a heat treat process.
Cost would be my guess. When you consider the street price of a Bravo one prop is less than $500 it's just not possible to put it through the heat treat process. That would include annealing, cooling, and then hardening. The process, including cooling time, can take as long as 8-9 hours.
It's also not practical to put it through the heat treat process after it's finished since the high temperature of the annealing process will cause the pitch to increase due to distortion of the blades.
By doing our own heat treat in house we are able to tailor the heat treat process to the intended application.
Cost would be my guess. When you consider the street price of a Bravo one prop is less than $500 it's just not possible to put it through the heat treat process. That would include annealing, cooling, and then hardening. The process, including cooling time, can take as long as 8-9 hours.
It's also not practical to put it through the heat treat process after it's finished since the high temperature of the annealing process will cause the pitch to increase due to distortion of the blades.
By doing our own heat treat in house we are able to tailor the heat treat process to the intended application.
Ratickle
Founding Member / Super Moderator
I've managed a few foundries and owned a machine shop. Fairly familiar with all the different materials and processes. It just seems like you would do the pre-harden after cast to the 15-5 or 17-4, then all the work, then the final harden holding in a form with a vacuum cool to minimize distortion within acceptable parameters for final tweaking. That's the way we cast and finish impellor pump vanes or gun pieces.
If not, what is the purpose of using the PrecipHardening materials?
Ratickle
Founding Member / Super Moderator
Just don't spray it with bottom or toilet bowl cleaner. Learned that the hard way.....:ack2:
ThrottleUp Props
Sponsor / Charter Member
The PH stainless steels, even when not heat treated, are stronger than any other material when you consider cost. I'm not familiar with high volume heat treating so I can't comment on that. We heat treat all of our props but we don't do the volume that other prop manufactures of cast props do. I know some manufacturers do a quick H900 heat treat to thier props but that is much too hard for a prop on a high horsepower application.
ThrottleUp Props
Sponsor / Charter Member
bbladesprops
Sponsor
Yes, #6 Mercury CNC and prior version #6 propellers are heat treaded.
Heat treating, as most of the contributors to this thread know, is a touchy business. Too much too brittle. The quality of the heat treat can also only be as good as the quality of your (or however is doing your castings) process. Casting flaws cause cracks and failures more than the fact that the prop hasn't been heat treated. (sorry to say)
I have seen non heat treated AND heat treated castings from all manufactures fracture and crack whether heat treated or not. The quality of the casting tends to make a better future for the propeller.
If we look at the failure of Mercury castings compared to Powertech, Precision, Solas, Mich. Wheel and a ton of little guys (many local) who have pored castings for the big name propeller guys around the world, Mercury has as low of a failure rate or better than others. Therefore, why heat treat to add strength if the failures are few per the volume or intended HP? That way the props can stay at a lower price for a probable same expected life time. That being said, going into the high end HP levels with ultra high "X" dimentions.....why not get heat treated props for that little extra help.
For the record, all Mercury Racing propellers are heat treaded including #6 props, OB cleavers, Lightning ET's for drag racing etc.
Yes heat treating the prop pre the customizing process is key to maintaining the specification we decide to bring the propeller to. When I was was at Merc Racing and to this day the prop would be cast, inspected........trade secrected...heat treated, re-inspected and Labbed (now CNC'd). Point being the heat treat may adjust the propeller geometry.
Back to the original question. I guess Julie answered it. Propellers are not pure stainless. Satin finished props tend to hold and release water better making them that .1-1mph faster. This testing was done many moons ago by my mentor. However, the satin finish process opens the porosity in the prop. Areas of stock propellers which have not been high polished well during production will also have this porosity and will begin to rust first. If you want bling, go with high polish. If you want performance, go with satin, just maintain them with a coat of CRC or WD40 after you use them.
Brett
Heat treating, as most of the contributors to this thread know, is a touchy business. Too much too brittle. The quality of the heat treat can also only be as good as the quality of your (or however is doing your castings) process. Casting flaws cause cracks and failures more than the fact that the prop hasn't been heat treated. (sorry to say)
I have seen non heat treated AND heat treated castings from all manufactures fracture and crack whether heat treated or not. The quality of the casting tends to make a better future for the propeller.
If we look at the failure of Mercury castings compared to Powertech, Precision, Solas, Mich. Wheel and a ton of little guys (many local) who have pored castings for the big name propeller guys around the world, Mercury has as low of a failure rate or better than others. Therefore, why heat treat to add strength if the failures are few per the volume or intended HP? That way the props can stay at a lower price for a probable same expected life time. That being said, going into the high end HP levels with ultra high "X" dimentions.....why not get heat treated props for that little extra help.
For the record, all Mercury Racing propellers are heat treaded including #6 props, OB cleavers, Lightning ET's for drag racing etc.
Yes heat treating the prop pre the customizing process is key to maintaining the specification we decide to bring the propeller to. When I was was at Merc Racing and to this day the prop would be cast, inspected........trade secrected...heat treated, re-inspected and Labbed (now CNC'd). Point being the heat treat may adjust the propeller geometry.
Back to the original question. I guess Julie answered it. Propellers are not pure stainless. Satin finished props tend to hold and release water better making them that .1-1mph faster. This testing was done many moons ago by my mentor. However, the satin finish process opens the porosity in the prop. Areas of stock propellers which have not been high polished well during production will also have this porosity and will begin to rust first. If you want bling, go with high polish. If you want performance, go with satin, just maintain them with a coat of CRC or WD40 after you use them.
Brett
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