Stainless Steel Grades

[KenMcCulloch]

I am having a new fitting fabricated as part of an improved arrangement to support the inner forestay where the load passes through the deck. Stainless steel is the obvious material, but what are the advantages / disadvantages of 304 (A2) Vs 316 (A4)? I know the latter is more corrosion resistant, however this is a pretty dry location and strength is more of a concern than the possibility of the odd speckle of rust.

 

[saxonpirate]

I am having a new fitting fabricated as part of an improved arrangement to support the inner forestay where the load passes through the deck. Stainless steel is the obvious material, but what are the advantages / disadvantages of 304 (A2) Vs 316 (A4)? I know the latter is more corrosion resistant, however this is a pretty dry location and strength is more of a concern than the possibility of the odd speckle of rust.

You're right regarding the A4 being more corrosion resistant but in this case it's well worth bearing in mind that whilst some properties are improved over A2, others deteriorate. The extra Chromium makes the metal more succeptable to fatigue and cracking. I'd go for A2.

 

[rob2]

For most applications on deck strength is not much of an issue between 304 and 316. However, 304 does rust quite a bit in a salty environment, so 316 is the universal choice. Upmarket self steering manufacturers use 316L which has even better corrosion resistance, but it isn't so commonly available from most fabricators.

Other grades are available with inferior corrosion resistance or duplex grades sometimes chosen for prop shafts as they are stronger and particularly resistant to crevice corrosion, like you may experience at seals and bearings.

Rob.

 

[saxonpirate]

For most applications on deck strength is not much of an issue between 304 and 316. However, 304 does rust quite a bit in a salty environment, so 316 is the universal choice. Upmarket self steering manufacturers use 316L which has even better corrosion resistance, but it isn't so commonly available from most fabricators.

Other grades are available with inferior corrosion resistance or duplex grades sometimes chosen for prop shafts as they are stronger and particularly resistant to crevice corrosion, like you may experience at seals and bearings.

Rob.

Rob, to a point I'd agree with you there, but if A2 is well polished after fabrication it can well stand the test of time in a marine environment. I've used it on countless occasions and have never been let down by it and it's not as succeptable to fatigue and cracking as A4.

 

[KellysEye]

316 is regarded as the marine standard.

 

[saxonpirate]

316 is regarded as the marine standard.

The OP asked about the advantages/disadvantages of the two grades of stainless A2 and A4. The main difference being the chromium content. I described above the advantages and disadvantages.

316 I'm afraid has become the yachties buzz word for anything stainless, as are JCB for all earth moving equipment and Hoover for all vacuum cleaners amongst the general public.

I think you will find that A2-AISI 304 is the material of choice for all s/s standing rigging including bottle-screws, although these are also manufactured in 316.

 

[vyv_cox]

Lots of misinformation in this thread. AISI 304 contains 18% of chromium, 8% nickel. AISI 316 contains exactly the same chromium and nickel with the addition of 2% molybdenum. This improves its pitting resistance and also its crevice corrosion resistance, both being due to pretty much the same condition. There is no significant difference between the two so far as fatigue, strength, ductility are concerned.

The L suffix refers to low carbon content, essential for a fabricated component. There are also versions with small titanium and niobium additions to mop up excess carbon in the weld pool but these are probably over the top for this job. The welds in a standard alloy can corrode rapidly, although this is not very likely when not immersed.

For this job 316L would be the preferred choice as its appearance will remain better for longer. 304 will suffer more frequent pitting that will require polishing fairly regularly. 316 would probably be good enough for a stem fitting but not for a fuel tank, for example.

 

[KenMcCulloch]

Thanks Vyv, I was hoping you would be along! Part of the reason for having this part made is that the existing fitting is poorly made in some sort of S/Steel and the welds are badly corroded. 316L it will be if the metalworker can provide that..

 

[Bosun Higgs]

Lots of misinformation in this thread. AISI 304 contains 18% of chromium, 8% nickel. AISI 316 contains exactly the same chromium and nickel with the addition of 2% molybdenum. .

Sorry Vyv but you're not right. A typical 316 will have a couple of extra % of chrome compared to 304 as well as 2.5% of moly. However the fatigue resistance of both grades is for all practical purposes the same and unlikely to be an issue if the forestay tang is well made of a decent size. As for polish and shinyness, I would not hesitate to use 304 below deck on a dry modern boat but not above deck where it will require cleaning - which is work.

I would not go near either grade if they were to go through ( say) a wet wooden deck - I've seen to much worm eaten austenitics to risk that.

 

[VicS]

Sorry Vyv but you're not right. A typical 316 will have a couple of extra % of chrome compared to 304 as well as 2.5% of moly

Figures I am looking at indicate more Cr in 304 than 316.

16 to 18.5% in 316, 18 to 20% in 304.

 

[vyv_cox]

Sorry Vyv but you're not right. A typical 316 will have a couple of extra % of chrome compared to 304 as well as 2.5% of moly.
.

As with all alloys there is a huge range of compositions made for a variety of purposes by hundreds of manufacturers. I have some cutlery at home that is 20% chromium, 10% nickel, described as 316. Nevertheless the standard composition is 18/8/2.