Thank-you in advance for your comments!
My client manufacturers fuel & water tanks. These are mounted on a truck chassis and sold as a complete water truck and/or fuel truck system.
In the fuel truck arena, I have been helping them put the procedures and processes in place to meet DOT 406 (CFR 178). One of the issues that is continually debated is whether fuel tanks (gas, diesel & methanol) should be made from 304 or 304L? (Less than 5 psig MAWP). My client and I are not on the same page with 304L vs 304.
My understanding is that 304L (or 316L) with the lower carbon content helps to minimize intergranular corrosion. These carbides that precipitate out in 304 (or 316) during welding also lead to other cracking at the tank seams, joints and early tank leakage failure. We have seen this on several truck tanks made from 304 SS.
I take my responsibility very serious...since I am the design certifying engineer (DCE) under DOT 406, to select the correct material based on my engineering judgement, is my selection of 304L over 304 correct?
Regards,
John Dowdy, PE
Hey John,
You will hear from better heads than mine, but your thinking sounds good to me.
However, I would still be willing to bet that the cracks and failures you have observed were due to process control issues rather than 304 vs 304L
Without heat in service the difference between these are pretty slim.
John;
I'm with Lawrence in that you'll likely hear from far better minds than mine (perhaps Chuck Meadows may respond or js55) but that said, I also agree with Lawrence' evaluation that any failures are likely more due to process control than intergranular corrosion cracking... just my 2 cents. But I do like your thinking.
I am not an engineer, nor a graduate metallurgist. I think I understand what you are up against there. Good Luck!
I used to do weld repairs on SS truck tanks, both 304, 316 and 304/316L and even ELC. I once took a sample of a cracked area to a metallurgical engineer/ failure analyst, and he told me that the material had work hardenned and that was the cause of failure. (This is what I suspected because of the nature of the cracks) I then remembered that the comopany had some product tanks that had many more suports internally and longitudinal suppports along the rails, but which were heavier (and could not carry as much product), and those tanks never failed. These were MIlk tanks, Fuel tanks, mixed product tanks.
I would look to fatigue loading in the original design, as well as the difference between the low carbon and non-low carbon base metals..
The choice between 304 and 304L is usually associated with a concern about sensitization. Or as some people mentioned carbide precipitation.
Get ready for a long sentence! Specifically, it is chromium carbide formed along the grain boundaries that result in a depletion of corrosion resistant chromium along the grain boundaries that leads to intergranular corrosion and stress corrosion cracking when the heat affected zones are subjected to water containing halide (chlorides are often the concern) ions.
I don't believe sensitization is going to be a major concern in a fuel oil tank unless there is a concern for water contamination. In the case of sea water, you may definately have a problem if there is any seawater contamination. The chlorides in saltwater will attack any HAZ that has been sensitized in very short order.
Single pass welds on light gage sheet are not usually a problem, however, intersecting welds welds can lead to knifeline corrosion and multiple pass welds are more prone to sensitization due to the accumulated time at temperature that leads to sensitization.
Sensitization is a defusion process. It is strongly influenced, i.e., promoted, by time at temperature. That is why the interpass temperature is closely controlled and interpass cooling is used to reduce the time at temperature.
Best regards - Al
John,
Don't let these guys fool ya. There ain't no better heads.
I am assuming the debate is generated due to the lower cost of 304 as opposed to 304L or dual cert stuff.
Therefore, they are dead on, be sure you are not dealing with a process issue that would cause failure as opposed to a material issue.
Understanding the theory of material responses is indespensable. But make sure that your actual failures are unavoidable sensitization related.
Changes in process parameters may be considerably cheaper than a material change.
So......are both grades acceptable for DOT 406 service?
John,
I for one do not have a copy of DOT 406 to answer your question, but for codes and standards in general, if it does not specifically prohibit something then it is assumed acceptable based upon sound engineering judgment.
John,
As these guys have said, and rightfully so, the main difference between a 304 and a 304L is the carbon content. Lower content will definitely delay sensitization (chromium carbide precipitation) that leads to intergranular corrosion. Using a 308L filler metal with a FN of approximately 6-10, and the 304 (or 304L) with a FN of around 4-6, should eliminate the probability of weld cracking. One thing that has not been mentioned is the fact that the heat discoloration in the HAZ definitely needs to be removed. The discoloration in the HAZ is a chromium-depleted-oxide layer that definitely compromises the corrosion protection. If you definitely feel the problem is with the base metal (304), you might want to try a 316L that has moly that aids in pitting corrosion resistance. Your decision of using 304L over straight 304 will only aid you in the delay of sensitization, but I think that is a remote possibility. What welding processes are you using to fabricate these tanks? I sounds to me like it is not a base metal/filler metal problem, but more of a process problem.
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