Our process uses 304L exclusively and a PMI is absolutely necessary. This requires at least a laboratory carbon analysis as most portable elemental analysers will not analyze for carbon.
The SS 'L' Grade has lower carbon percentage compared to ' normal' grade. The SS 304L has approximately 0.03% carbon while in normal grade the carbon percentage is around 0.08%. The necessity of having low percentage of carbon is to avoid sensitization which is when stainless steel goes through the termperature range of 425 C to 815C during welding or any process annealing or fabrication process. A sensitized structure leads to Cr depletion in narrow band and render the material vurnerable to inter-granular attack.
It is thus imperative to perform PMI (Positive Material Identification) to segregate low carbon grade SS from relatively higher grade of carbon containing SS.
But, your standard field PMI methods cannot detect carbon. You must rely on lot/heat control and lab testing to back up your carbon numbers.
Unless you have need for "H" grade,(high carbon) material you should make of point of only using "L" grade materials to minimize the risk of a mixup. In many parts of the world the price premium for "L" over standrad material is $0.02/lb or less. Further, almost all of the "L" grade material manufactured has sufficent mechanical properties to be dual certified as 304/304L and used at the stanrd 304 strength levels.
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
Ed,there is a mix up perhaps in your comment regarding pricing of L grades. In our country L grade carries a premium price tag.The margins are definitely higher. Ofcourse now who talks of L grade it is just sufficient if you have a standard grade and get along.
For castings the price differential becomes very huge as we need to use virgin grade raw materials to get the C right. We have no converter facilities.
Yes, for castings there can be large premiums.
If a steel mill is charging more than a few cents/pound premium then either you are being gouged, or the material is not being made via and oxygen decarb process (I would say AOD, but that is trademarked). Blowing the carbon down is trivial in a refined steel. In the US (and from sources in Europe) we often get 304L/316L with carbon in the range of 0.012-0.016.
I have seen stainless foundries that used AOD to refine heats before casting. This let them use lower cost, higher carbon feedstock.
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
It is absolutely necessary to ask PMI for a S.S L-Grade (low carbon). PMI to detect carbon is more difficult to perform in the field because carbon analyzers are heavier and less available than other hand portable elements analyzers, normally carbon content is checked in laboratories through RX chromatography analysis.
To sum up, L-Grade SS is good for intergranular corrosion resistance but not good for strength application like pressure containing parts owing to the low carbon.
The main purpose of PMI is for alloy composition identification but not for carbon. So mill certificate is necessary to make sure the carbon content is less than .03%.
ZJL, The strength of "L" grade material really only comes into play at high temp (creep limited). The specified minimum strengths for the traditional grades (304, 316, 317, 309, 310, and so on) are so low that even well annealed "L" material easily is strong enough.
For newer alloys (lean duplex, 6% Mo, super ferritics) the spec minimum strengths are very close to reality.
The time that you NEED "L" grade material is when you will be using material with welds that are not annealed. If there are no field welds and all fabrication welds are annealed then there are no sensitized welds to worry about.
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
The price premium can vary a lot by foundry. The one I worked for had an AOD, and refined all stainless in it to remove carbon as well as sulfur. If a customer want high carbon we had to add it back in.
Also a general note on "L" grade stainless- if it is weld metal, up to .04%C is allowed in most cases.
zjliang, I have occasionally seen PMI specs for heat exchangers that did not require PMI for 304 stainless steels, while requiring it for other alloy materials. I don't know why they would not require it for 304, but it's not unheard of.
stanweld: that's news to us, and to all the firms we hire to do PMI. Everybody around here will only do carbon contents by chemical analysis. The handheld units they use are XRFs, and incapable of quantifying carbon to the required levels. PMI for carbon around here means removing metal from fittings: fine if it's big stuff, and a little risky if it's small pipe.
We find that there is zero premium regionally for using dual grade material (304/304L or 316/316L dual certified). That's all we use, unless we need H grade material in which case we specify and segregate it.
And there's a lot more to working with L grade material than merely buying L grade material in the first place. Keeping carbon out of the HAZ of piping during welding is critical, which means good cleanliness and materials control during welding and fitting, no grease pencils, being careful with tape used to cover openings for purging etc.
moltenmetal: You are absolutely correct with regard to XRF PMI instruments. Light emitting spectrometers using high purity W, Mo, Cr, Ta metal electrodes can readily determine Carbon content. We used one of these units to perform 100% PMI on a Project in the UK in 1993/1994 and compared the Carbon contents obtained therefrom on samples with those having conventioanal laboratory analysis of the same samples with repeatable high accuracy demonstrated.
Recent PMI requirements by Engineer/Owners have effectively prohibited the use of such PMI equipment on our current projects because of the arc burn produced.
the L grade also demonstrate better forgebility, I used to run a cold heading process which I see lower defect rate(craking at the head) with L grade than the normal grade.
