Stainless steel Hull Yacht

[gonzo]

Welder/fitter: do you know at what temperature range is Monel magnetic?

[welder/fitter]

Originally Posted by gonzo

Welder/fitter: do you know at what temperature range is Monel magnetic?

The most honest answer I could give to that is that I have no idea, but it is easy enough to look up. I had to learn a bit about it when I was a weld inspector for my father's consulting firm, back in the mid/late 1980s &, again, in the mid/late 1990s. My father was a metallurgical engineer whose clients, among others, included the shipyards that I worked for in later years. I recall that it depends on curie temperature resulting from the alloying, some of the series having such a low curie point that they would be insignificant to the discussion at hand, 400 series is slightly magnetic through room temperature, not, if heated. I was taught to, among other things, differentiate between Monel & other alloyed steels by the results of polishing, due to the lack of chromium.

Is your question based on an interest in the magnetic range of various Monel alloys, or just dick measuring? I wonder, as there are members far more qualified to answer your question than I. Like yourself, I'm just trying to help the OP as best I can.

Mike

[davptt]

This morning, a welder passed by and examined the hull. He believed the hull is not stainless steel, it is in fact mild steel. The gray silver color is due to stray current, that what cause the surface turn gray silver.. Is that true?... still it does not explain how the below waterline does not have any rust..

David

[Wynand N]

David, its easy to find out if it is indeed a mild steel hull.

There are few pieces of spare stainless steel metal about 5mm left in the yacht...

and herein lies your answer. Take a piece of those off cuts and do the following;

1. If it is magnetic - 3CR12 or mild steel
2. If non magnetic, its stainless steel - note that the lower grade 302L might be very lightly magnetic. Above 308L it should be completely non magnetic.
3. Arc weld a piece with an E6013 (or E6012) and if it is problematic, burns, spatter etc and basically does not weld as smooth such as on mild steel - chances are it is stainless or 3CR12. Magnetic test will tell.
The latter can only be welded with AWS ER 309L, 308L, or 316L should be used.

IOW, if the plate is magnetic, but cannot be welded with a mild steel electrode, you most certainly have 3CR12.

Edit: Here is a spec sheet for 3CR12 - I use it all the time due to its relative low cost over stainless where marine conditions are not applicable. http://www.vrnsteel.co.za/3cr12.html

[welder/fitter]

David,
Do you notice any dis-similarities between the two surfaces on the scrap that you have? Regardless, if you have the chance to do as Wynand suggests, run a pass on each surface of a piece of the scrap. Note whether there is any discrepancy in weldability.

[pdwiley]

Originally Posted by Wynand N

David, its easy to find out if it is indeed a mild steel hull.



and herein lies your answer. Take a piece of those off cuts and do the following;

1. If it is magnetic - 3CR12 or mild steel
2. If non magnetic, its stainless steel - note that the lower grade 302L might be very lightly magnetic. Above 308L it should be completely non magnetic.
3. Arc weld a piece with an E6013 (or E6012) and if it is problematic, burns, spatter etc and basically does not weld as smooth such as on mild steel - chances are it is stainless or 3CR12. Magnetic test will tell.
The latter can only be welded with AWS ER 309L, 308L, or 316L should be used.

IOW, if the plate is magnetic, but cannot be welded with a mild steel electrode, you most certainly have 3CR12.

Edit: Here is a spec sheet for 3CR12 - I use it all the time due to its relative low cost over stainless where marine conditions are not applicable. http://www.vrnsteel.co.za/3cr12.html

A simple spark test with an angle grinder should give useful information as to material. The sparks are radically different between carbon (mild) steel and stainless steel. A couple of scraps will show this straight away.

Wynand, interesting information about 3CR12. I've got a sheet of it in the shop, am planning on making doubler plates, wear plates etc from it for my boat - all above the WL. I thought seriously of buying enough to do the entire deck & cabin as it was going to be cheaper (marginally) than the A36 plate after blasting & priming. I would have painted it as well. Chickened out as I couldn't find anyone that had done it and didn't want to be the first. Maybe next time, if there is one.

PDW

[welder/fitter]

Originally Posted by pdwiley

A simple spark test with an angle grinder should give useful information as to material. The sparks are radically different between carbon (mild) steel and stainless steel. A couple of scraps will show this straight away.
PDW

LOL, I completely forgot to suggest a spark test, most welders primarily use this method - part of basic welding program. Good thing someone had their thinking cap on!

[pdwiley]

Originally Posted by welder/fitter

LOL, I completely forgot to suggest a spark test, most welders primarily use this method - part of basic welding program. Good thing someone had their thinking cap on!

I don't know (because I haven't done it) if there's a visible difference between say 3CR12 and 316, monel or other high nickel alloys. I'm assuming this hull isn't cupronickel because then it should look bronze-ish and it definitely wouldn't rust.

My first thought re inside rust, bright under paint outside was a carbon/stainless explosive bonded/clad steel but it's horribly expensive so why would anyone do that for a pleasure boat? Wynand's comments about 3CR12 made more sense.

PDW

[welder/fitter]

Originally Posted by pdwiley

I don't know (because I haven't done it) if there's a visible difference between say 3CR12 and 316, monel or other high nickel alloys. I'm assuming this hull isn't cupronickel because then it should look bronze-ish and it definitely wouldn't rust.

My first thought re inside rust, bright under paint outside was a carbon/stainless explosive bonded/clad steel but it's horribly expensive so why would anyone do that for a pleasure boat? Wynand's comments about 3CR12 made more sense.

PDW

I agree. I have seen/heard of a few boats with Monel cladding on mild steel. That's why I suggested running a pass on each side, just in case. But, 3CR12 would fit the bill. If the OP could post a couple of photos, int/ext, it'd be interesting to see.

[davptt]

According the spark test in wikipedia, it is very hard to distinguish between ferric stainless steel such as 3cr12 and mild steel...

I just began learn how to weld.. so it would be difficult for the to differentiate between the two..

I am planning cut small piece 2cm of rudder (rudder are same material as hull) and cut similar piece of known mild steel... I left them in damp condition for few weeks... see of they will be any differences in corrosion rate....

[murdomack]

Originally Posted by gonzo

Welder/fitter: do you know at what temperature range is Monel magnetic?

I seem to remember monel being slightly magnetic at normal ambient temperatures.

Regarding the rust on the inside surface, could this area have been ground or pollished with a carborundum disc? This can cause rust to appear on alloys.