Metallurgy/Stainless Steel Weldability-316L
I am a fresh mechanical engineering graduate and have been struggling to find literature to support a claim.
Basically we are looking to use SS 316L which will be welded. It will contain more than 10% ferrite, which according to some authors “degrades the mechanical properties at low or high temperature service. At low temperatures excess ferrite (more than 10%) can promote crack paths when the temperature is below ductile-brittle transition temperature. At high temperatures continuous brittle Ϭ Phase may form at the interface between austenite and the ferrite"(ASM International 2006, Corrosion of weldments [#05182G])
According to this article, if the temperatures are above the ductile to brittle transition temperature, the SS 316L will be alright to use at ferrite levels in excess of 10%. They would also need to be below the high temperatures mentioned above to make sure the mechanical properties remain undegraded.
Hence I wish to find out the DBTT (Ductile to brittle transition) and use it as elementary proof that we will be operating at temperatures above that level, and below the high temperatures listed and will need literature to support my claim. Also, if there is no reason the ferrite levels should not be used, i would like another source of literature perhaps that would clearly say that fe levels in excess of 10% should never be used?
Any help or light shed in this matter would be greatly appreciated .
I would greatly appreciate any more light you can shed on this
I cannot provide you with a good answer to your question, just a few remarks about your situation:
You would need to find the impact test data of a ferritic stainless steel with a composition as close as possible to the delta ferrite in your steel and remember that it is not the average composition of your grade because partitioning of alloying elements causes a composition deviation from the average. Ferrite stabilizing elements such as Cr, Mo, S, and P tend to segregate in ferrite whereas austenite formers such as Ni, C, and N are present in austenite at concentrations above the average of the steel. So the question now is how pronounced this segregation is and what the temperature-dependence of ferrite with a composition close to that of the delta ferrite is.
Now, the most convincing answer to the users of your product would probably be to measure or somehow estimate the chemical composition of the delta ferrite, make a fully ferritic alloy with that composition and experimentally determine the impact energy as a function of temperature. Or in a very lucky case, you might bump into such data for a steel composition approximating yours so that you can use it as a reference.
The other possibility is to perform impact testing of 316L steel that you are dealing with and see how its impact energy changes with temperature. This allows you to determine whether or not the low-temperature embrittlement mechanism you referred to is operative at the service temperature of the parts.
There are literature data which discuss the composition dependence of DBTT in steels. Chapter 5 of "Handbook of Stainless Steels" (by Peckner and Bernstein) is a good source to refer to in order to find some impact data on ferritic stainless steels. It has graphs showing the detrimental effect of interstitial elements C and N on the impact properties. It also demonstrates the effect of Cr and Mo. Please let me know your email address if you would like to have a look at some of the impact energy plots therein.