Originally Posted by Trim50
Since you're going to be welding...wouldn't 316L be a better material selection?
Ken, great question. Here is my opinion... (I'm not a materials "expert" but just an old engineer with some practical application knowledge)
Bear with me because I'll delve a little deeper than my original post and it will get a little technical.
Let's look at the chemical composition of the two.
Type---C Max---Mn Max---P Max---S Max---Si Max------Cr-------------Ni------------Mo--------Cu Max
The "L" version only differs by the lower Carbon content. Since the corrosion resistance comes mainly from the Chromium and Nickel content, both types are basically equal in their corrosion resistance.
The lower Carbon content comes into play when the steel is heated within the Temperature Range of 800 - 1500 degrees.
At these temperatures, the steel matrix loosens and certain materials can propagate and some elements, such as carbon, tend to bind together to make little microscopic "pockets" of one element within the matrix. This "pocket" tends to be more brittle then the surrounding matrix which is more ductile. Under stress, this discontinuity between the two areas can cause the Carbon to "break away" from the surrounding matrix. This microscopic inclusion can cause high local stresses in the matrix and become the origin of a stress crack. Once a crack has started, even microscopically, the tip of the crack is always under much higher stress than the surrounding matrix, causing the tip to crack further and move through the material.
Three major factors come into play when it comes to Carbon precipitation in the surrounding matrix, the first two being temperature and time. The third factor is "work hardening" which is when the material is put under extreme localized stress which opens the matrix and allows precipitation at lower temperatures. Many stainless steels are considered non "work hardening" because their low Carbon content does not allow a change in material properties after "work hardening" such as occurs in mild steel. None the less, it could allow for carbon precipitation in the matrix.
Now lets look at the application. There are only two times after the steel leaves the furnace that it is subjected to high temperatures and stress, when it is formed into it's final state for sale as a product, and when it is welded. The forming process generally does not alter the matrix appreciably because it is done in a fashion (usually with annealing, i.e. heating and slowly cooling the material to return the material to it's original matrix) so that the material properties are maintained. When welded, localized areas of the base material can have elevated temperatures, and in a perfect world, the weldment would then be annealed by heating it and slowly lowering the temperature to preserve the original matrix structure. In a welding application, the material is under high temperature for only a short period of time, and this steel has a fairly low carbon content to start out with, so the benefits of annealing are not large, but would be done in a perfect world.
An additional treatment after welding is passivation. To passivate the steel means to apply a chemical to the surface of the material which lifts off any pure iron from the surface, therefore removing microscopic areas of the surface matrix that is not uniform stainless steel. This is actually what you are doing to some degree when you apply stainless steel cleaners to remove rust spots. In all but a very few specific examples, the highest stress seen by a material is on the surface.
But all of this is a little digression, because in most applications, the weld bead itself is what is undergoing the maximum stress, not the base material. Since the material in the weld bead comes from weld rod/wire used in the welding process, that is the material that we must examine.
There are in general use three types of TIG wire used when welding stainless steel. Lets take a quick look at those.
18/8-308L-------.01----1.8-----0.4-----20-----10.00----- - ------0.10
23/12-309-------.02----1.6-----0.4-----24-----13.00----- - -------- -
As you can see, all three of these have very low Carbon content specifically to avoid Carbon precipitation as high temperatures.
Since the weld material should match the base material as closely as possible for uniform material properties, the 316 weldment should be welded with the 316L wire. Though the 308L is not a bad choice, it has slightly less corrosive protection to salt.
So for a practical application where your davit is manufactured by a professional fabricator, your weld bead is in actuality probably 316L and you didn't realize it.
On another note, 316L is usually not made into pipes and tubes and to find tubular 316L would probably require a special order. It is generally only available in plates and billets. 316 is readily available in the market in most forms and dimensions.
So when is 316L used in the real world? It is generally used for weldments that will see extended periods of elevated heat and stress such as occurs in manufacturing processes for chemicals.
So in a nutshell,
1.) Your weld bead is most likely 316L to begin with.
2.) In a perfect world, your weldment would be annealed and passivated, but probably isn't. (and in this application would be a topic of debate whether it would be worth the extra effort for a marginal gain)
3.) Have another sundowner and take this one off your worry list.
Hope this explains the material selection a little better and didn't make your head hurt too much.