Stainless Steel Plate 304 vs 316: Chemical Composition and Weldability for Pressure Vessel Fabrication

Both grades are austenitic chromium-nickel stainless steels, and both are listed in ASTM A240 for pressure vessel plate. The entire difference comes down to one element: molybdenum. 316 carries 2.00 to 3.00 percent of it and 304 carries none. That single line on the mill test certificate changes chloride pitting behaviour, changes which intergranular corrosion test applies, changes the price, and - if you are designing to ASME Section VIII - changes almost nothing about the welding procedure. This guide separates the chemistry that matters from the chemistry that is marketing.

What is the difference between 304 and 316 stainless steel plate?

One element: molybdenum. 316 and 316L contain 2.00 to 3.00 percent molybdenum and 304 and 304L contain none. Chromium and nickel ranges move slightly in the opposite direction - 304 has the higher chromium range, 316 the higher nickel range - and the low carbon variants of both cap carbon at 0.030 percent. Everything you care about in service and in the weld follows from those four lines.

The quickest way to read a specification is to stop thinking of 304 and 316 as two different materials and start thinking of them as one material with an alloying option. Both are austenitic, which means both have a face-centred cubic crystal structure at room temperature, both are non-magnetic in the annealed condition, both cannot be hardened by heat treatment, and both are tough all the way down to cryogenic temperatures. The 300 series stainless steel basics are identical for the two grades.

ASTM A240 / ASME SA-240 chemical requirements, weight percent

Two details in that table surprise people. First, 304 has the higher chromium range - 17.5 to 19.5 percent against 16.0 to 18.0 percent for 316. The extra corrosion resistance of 316 is not more chromium; it is the molybdenum. Second, the nickel ranges are wide enough that a given heat of 316 can be nearly as nickel-rich as many nickel-bearing specialty grades, which is part of why the molybdenum surcharge alone does not explain the price gap.

Specify from the heat analysis on the mill test certificate, never from the typical values printed in a mill brochure.

A 316 heat at 2.02 percent molybdenum and one at 2.95 percent are both in specification and they do not behave the same in a hot chloride crevice. If the vessel is in a service where pitting is the governing mechanism, specify a minimum molybdenum of 2.5 percent as a supplementary requirement on the purchase order.

How does molybdenum change the corrosion behaviour of 316 plate?

Molybdenum slows down the initiation and early growth of pits and crevice attack in chloride-bearing environments. It does this by enriching in the passive film and in the solution inside a growing pit, which makes the local chemistry less aggressive and gives the surface more time to repassivate. It does not make 316 immune, and it does not buy resistance to chloride stress corrosion cracking.

Stainless steel protects itself with a chromium oxide film a few nanometres thick. In water containing chlorides that film is attacked at weak points - an inclusion, a scratch, a sulphide stringer - and a pit starts. Once a pit is open, the chemistry inside it becomes acidic and concentrated in chloride, and the pit digs faster than it did at the start. Molybdenum intervenes at two points: molybdate species in the film make it harder to break down in the first place, and molybdenum dissolved from the metal inside an active pit buffers the local acidity and helps the pit wall repassivate before it becomes self-sustaining.

Where the molybdenum in 316 helps, and where it does not

The single most expensive misunderstanding in this table is the chloride stress corrosion cracking row. Molybdenum does not help. A 316 vessel under insulation on a coastal site can crack from the outside in just as readily as a 304 one, because the mechanism needs chlorides, oxygen, tensile stress and a temperature above roughly 60 C - and molybdenum changes none of the four. The fix is a coating, low-chloride insulation, and keeping the metal dry. If you need the vessel to survive that duty in the bare metal, you need duplex 2205 or a higher alloyed material.

What are the PREN values of 304 and 316, and what do they actually predict?

Using PREN = percent Cr + 3.3 x percent Mo + 16 x percent N, typical 304 lands at roughly 18 to 20 and typical 316 at roughly 23 to 26. That is a real ranking difference of about 25 percent, and it is a useful way to compare heats - but PREN was developed for and validated on duplex and highly alloyed stainless steels, and on its own it is not a design number for a pressure vessel.

The molybdenum coefficient of 3.3 is the reason the gap exists: at 2.1 percent molybdenum, that term alone contributes about 6.9 points, more than wiping out the chromium deficit. Nitrogen contributes too, which is why 316LN - the nitrogen-alloyed variant - sits above plain 316L for the same molybdenum content. See stainless steel 316LN for the nitrogen-strengthened version.

PREN worked from typical heat analyses

What PREN does not tell you matters just as much. It says nothing about the critical crevice temperature, which is always lower than the critical pitting temperature and is the number that governs a gasketed flange or a lap joint. It says nothing about weld metal, where segregation during solidification can leave the last metal to freeze with a different composition from the bulk. And it is a ranking index, not a threshold: a PREN of 24 does not tell you the chloride concentration or temperature at which a specific vessel will pit. For that you need measured critical pitting temperature data for the actual environment, or a service history.

Why does carbon content decide whether you buy 304L or 316L plate?

Because carbon is what makes a weld heat-affected zone lose its corrosion resistance. Above about 0.03 percent carbon there is enough of it to combine with chromium and precipitate chromium carbide at the grain boundaries when the metal passes through 425 to 815 C. The carbide pulls chromium out of the metal immediately around it, that metal drops below the roughly 12 percent chromium needed to stay stainless, and intergranular corrosion has a path to follow.

A weld does to the heat-affected zone what a heat treatment would. Metal a few millimetres from the fusion line is heated into the carbide precipitation range and held there long enough to matter, then cooled. On a multipass weld in thick plate the zone is reheated several times. In a grade with 0.06 percent carbon, chromium carbide precipitation can begin in minutes near 675 C. In a grade with 0.02 percent carbon, the same thermal cycle does not produce a continuous carbide network, because there simply is not enough carbon in solution to form one.

Carbon, sensitisation and the practical consequence

The L grade is not a free lunch. Its yield strength is 170 MPa against 205 MPa for the straight grade, and Section II Part D lists its allowable stress at 16.7 ksi against 20.0 ksi. You pay roughly 17 percent of design strength for the weldability. And the exemption is not permanent: hold an L grade between roughly 500 and 800 C for years and it will still sensitise, a phenomenon called low-temperature sensitisation. That matters for vessels that run warm, and it is one of the reasons 316H exists - see stainless steel 316H for the high-carbon variant.

What is dual-certified 304/304L or 316/316L plate and does it help?

Dual-certified plate is a single heat that meets every requirement of both the straight grade and the L grade - carbon at 0.030 percent or below, and tensile and yield at the straight grade minimums. It is the standard way mills supply stainless plate today, and it lets one stock item serve both an L-grade welding specification and a straight-grade design allowable.

There is no metallurgical trick here. The heat is simply a low-carbon heat that also happened to test above the straight grade mechanical minimums, which most modern melts do once nitrogen is controlled. The mill certifies it to both specifications and the certificate lists both.

What dual certification does and does not give you

For a vessel that will be welded and is not in high-temperature creep service, specify dual-certified 304/304L or 316/316L and design to the straight grade allowable where your Authorised Inspector agrees. It is the cheapest way to get L-grade weldability without giving up 17 percent of allowable stress. Confirm the certificate shows carbon at or below 0.030 percent before you accept it, and make sure the stainless steel plate buying guide requirements for marking and traceability are written into the order.

What are the mechanical properties of 304 and 316 plate per ASTM A240?

304 and 316 are identical: 515 MPa (75 ksi) minimum tensile, 205 MPa (30 ksi) minimum 0.2 percent yield, 40 percent minimum elongation. 304L and 316L are also identical to each other and lower: 485 MPa (70 ksi) tensile and 170 MPa (25 ksi) yield. Molybdenum changes corrosion behaviour; it does not change strength. The only mechanical difference in the whole comparison is the one caused by carbon.

ASTM A240 minimum mechanical properties, annealed condition

The last two rows are the ones that matter more in the shop than on the drawing. Austenitic stainless conducts roughly a third as much heat as carbon steel and expands roughly 40 to 50 percent more for the same temperature rise. Those two facts together are why a stainless vessel distorts in ways a carbon steel vessel of identical geometry does not, and they are the same for 304 and for 316 - choosing between the grades will not change your distortion problem.

Which grade gives a thinner and cheaper pressure vessel shell?

Neither. 304 and 316 carry the same ASME allowable stress at ambient temperature, so the calculated shell thickness is the same. What changes thickness is the carbon variant: designing to 304L or 316L at 16.7 ksi instead of 304 or 316 at 20.0 ksi makes the shell about 20 percent thicker. Molybdenum buys corrosion resistance; it buys no strength at all.

Work it through a representative shell. Take an internal pressure of 150 psi, an inside radius of 30 in, and a Type 1 double-welded butt joint with full radiography giving a joint efficiency of 1.0 per UW-12. The UG-27 equation for the longitudinal joint is t = PR / (SE - 0.6P).

Worked UG-27 shell calculation: P = 150 psi, R = 30 in, E = 1.00

The allowable stress itself is derived the same way for every line in that table. Section II Part D sets S as the lower of two fractions of the specified minimum properties at temperature: two thirds of the yield strength, and one 3.5th of the tensile strength. At ambient that gives 2/3 x 30 = 20.0 ksi against 75 / 3.5 = 21.4 ksi for the straight grades, so 20.0 governs; and 2/3 x 25 = 16.7 ksi against 70 / 3.5 = 20.0 ksi for the L grades, so 16.7 governs. Above roughly 540 C the creep and stress rupture properties take over and the two families diverge, so read Table 1A for the exact grade and design temperature rather than interpolating.

The practical takeaway is that the carbon variant is a design variable and the molybdenum is not. If your vessel is designed to 304L and you then swap to 316L for corrosion reasons, the shell thickness is unchanged and the only bill that moves is the plate price. If you swap from 316 to 316L, the shell grows by about a fifth. Confirm the joint efficiency you are entitled to under UW-12 before you finalise the wall, because full radiography at E = 1.00 is worth more material than the grade choice usually is.

At what temperature should you stop using 304L or 316L?

Three separate limits apply and the lowest one governs. Low-temperature sensitisation starts to matter above roughly 500 C in long service, the allowable stress falls away sharply above roughly 540 C as creep takes over, and molybdenum-bearing 316L is the more sigma-prone of the two in the 550 to 900 C band. For sustained metal temperature above about 540 C, move to 304H or 316H, or to a stabilised grade.

Temperature limits and what governs at each band

Sigma phase deserves a note because it is specific to the molybdenum side of this comparison. Sigma is a hard, brittle intermetallic that forms from ferrite in roughly the 550 to 900 C range, and molybdenum accelerates it. In 316L base metal there is normally no ferrite to convert, so the risk sits in the weld metal, where 3 to 10 ferrite number was deliberately put there to prevent hot cracking. If a 316L vessel is going to run in that temperature band, hold the weld metal ferrite at the bottom of the recommended range and keep the filler heat's ferrite number low.

Is 316 better than 304 for chloride stress corrosion cracking?

No. This is the most common and most expensive mistake in the whole 304 versus 316 decision. Molybdenum improves pitting and crevice resistance; it does not improve resistance to chloride stress corrosion cracking. Both grades crack, and a 316 vessel under insulation on a warm, chloride-bearing site will fail the same way a 304 one does.

Chloride stress corrosion cracking needs four things at once: chlorides, dissolved oxygen, tensile stress, and a temperature above roughly 50 to 60 C. Remove any one and you do not get cracking. Molybdenum has no effect on any of the four. What it does change is the pitting that often precedes the crack, and in narrow laboratory window that can shift the threshold slightly - but a shift in a laboratory threshold is not a licensing basis for a pressure vessel.

What actually prevents chloride stress corrosion cracking

If you are writing the material line for a vessel that will be insulated and located outdoors or on a coastal site, do not write 316 as the mitigation for external chloride stress corrosion cracking. Write the coating specification, the low-chloride insulation requirement and the weatherproofing detail, and budget for maintaining them. The material upgrade that works in that duty is a change of alloy family, not a change within the 304 and 316 pair.

How much more does 316 plate cost than 304, and does it matter on a vessel?

316 plate typically carries a 25 to 45 percent premium over 304, driven mainly by the molybdenum and nickel alloy surcharges. On a finished pressure vessel the penalty is smaller than that gap suggests, because plate is only part of the delivered cost and welding, examination and heat treatment are identical for the two grades.

Where the cost difference shows up, and where it does not

The last row is not a joke. When a 304 vessel pits because the process turned out to contain chlorides, the root cause is easy to find and the fix - upgrade the material - is straightforward. When a 316 vessel cracks under insulation, the fabricator is usually the first suspect even though the material was never capable of resisting that mechanism. Paying the premium for 316 and then using it to solve a problem molybdenum does not address is the worst of both outcomes.

For the alloy options beyond this pair, 904L versus 254 SMO versus 316L covers where the super austenitic grades take over, and stainless steel 904L gives the composition and properties of the most widely used of them.

Which filler metal do you use for 304 versus 316 plate?

Match the grade and keep the carbon low. Weld 304 and 304L with ER308L for GTAW and GMAW or E308L for SMAW; weld 316 and 316L with ER316L or E316L. Use the silicon-bearing variants for GMAW to improve wetting. Both base metals sit in P-No. 8 Group 1 and all of these fillers sit in F-No. 6, so one procedure qualification structure covers the whole family.

Filler selection for 304 and 316 plate welds

The filler is deliberately not a perfect match to the base metal. ER308L runs about 19.5 to 22.0 percent chromium against 17.5 to 19.5 percent in the plate, and ER316L runs about 18.0 to 20.0 percent chromium against 16.0 to 18.0 percent in the plate. That chromium enrichment is what puts the weld metal on the ferritic-austenitic solidification path instead of the fully austenitic one, which is the next question - and it is also why a weld deposited with the wrong filler can look perfect on radiography and still be a cracking problem.

The qualification structure is common to both grades. ASME Section IX assigns 304, 304L, 316 and 316L to P-No. 8 Group 1, assigns all of these austenitic stainless fillers to F-No. 6, and covers the weld metal composition under the A-Number system. Practical consequence: qualifying a procedure on 316L generally qualifies the essential variables for the rest of the family, which is why most shops qualify on 316L and work down rather than qualifying every grade separately.

Why does ferrite number matter in austenitic stainless weld metal?

Because a small amount of delta ferrite is the only thing standing between an austenitic weld and solidification cracking. Target 3 to 10 ferrite number. Below about 3 FN the weld metal solidifies as coarse columnar austenite grains with impurity-rich liquid films between them and can crack while it cools; above about 10 FN the ferrite can convert to brittle sigma phase in warm service.

Ferrite is magnetic and austenite is not, which is why the whole measurement can be done with a calibrated magnetic instrument on the finished weld. Ferrite number is defined by AWS A4.2 and is not exactly volume percent - below about 8 to 10 the two are close, above that they diverge - which is one more reason the recommended target band sits where it does.

Ferrite number bands and what each one means

Predicting it before you weld is done with the WRC-1992 constitution diagram, which uses chromium and nickel equivalents calculated from the diluted weld metal composition: Cr equivalent equals percent Cr plus percent Mo plus 0.7 times percent Nb, and Ni equivalent equals percent Ni plus 35 times percent C plus 20 times percent N plus 0.25 times percent Cu. The nitrogen term is the reason WRC-1992 displaced the older Schaeffler diagram for modern low-carbon, nitrogen-bearing heats.

Predicted ferrite number is a calculation; measured ferrite number is the acceptance record. Measure it on the procedure qualification coupon with a calibrated instrument and again on production test coupons if the filler heat changes. Note also that the cryogenic impact test exemptions in UHA-51 carry their own ferrite limits that sit lower than the 3 to 10 band. If your vessel needs both crack-resistant welds and a very low design metal temperature, resolve that conflict on the PQR with your Authorised Inspector rather than discovering it after the welds are in.

Do you need preheat or post weld heat treatment for 304 and 316 vessel welds?

No to both, in the usual sense. There is no hydrogen cracking mechanism to preheat against, because austenite does not transform to martensite on cooling and has high hydrogen solubility. And ASME Section VIII-1 Table UHA-32 states that post weld heat treatment for P-No. 8 austenitic stainless steel is neither required nor prohibited - there is no thickness trigger at all.

Thermal treatment of austenitic stainless vessel welds

The UHA-32 position is genuinely different from the carbon steel world and it catches fabricators out. For a P-No. 1 carbon steel vessel there is a hard thickness threshold - 38 mm (1.5 in) for P-No. 1 Group 1 - above which PWHT is mandatory. For austenitic stainless there is no threshold, no hold time and no minimum temperature. If the user specifies heat treatment anyway, the right cycle is a solution anneal followed by rapid cooling, not a subcritical stress relief. A 650 C stress relief on a vessel welded from non-L plate is an excellent way to sensitise the entire heat-affected zone.

Formed heads are the exception worth knowing about. Cold forming strain and any hot forming cycle both change the material, and heavily cold-formed austenitic heads are frequently solution annealed after forming to remove the strain and restore corrosion resistance. That is a forming operation heat treatment, not a weld PWHT, and it happens before the head is welded into the shell.

How do you stop sensitisation in the heat-affected zone of a vessel weld?

Control carbon first, then time and temperature. Order L grade or dual-certified material so there is not enough carbon to form a continuous carbide network, then keep interpass temperature low and heat input moderate so the HAZ spends as little time as possible between 425 and 815 C. Where the vessel will run warm for years, consider a stabilised grade, and where the consequence is high, verify with an ASTM A262 test.

The mechanism is worth understanding once so the controls make sense. Chromium carbide precipitates at grain boundaries because carbon diffuses to the boundary faster than chromium can diffuse out of the surrounding metal. The carbide that forms is rich in chromium, so the narrow band of metal immediately beside the boundary is depleted. Below roughly 12 percent chromium that band is no longer stainless, and in a corrosive fluid it dissolves preferentially, leaving a continuous intergranular path. The fastest precipitation happens near 650 to 750 C, which is squarely inside the range a weld HAZ passes through.

Sensitisation controls, in the order they should be applied

Which A262 practice to call out depends on the grade, and this is one of the few places where the 304 versus 316 choice genuinely changes a test method. Practice B, the ferric sulfate-sulfuric acid test, is the appropriate one for molybdenum-bearing austenitic grades, so specify it for 316 and 316L. Practice C, the nitric acid or Huey test, is the conventional choice for 304 and 304L. Practice A is the rapid oxalic acid etch used for screening, and Practice E is the copper-copper sulfate-sulfuric acid test used when a general austenitic assessment is wanted.

Why do austenitic stainless vessel welds distort more than carbon steel welds?

Two physical properties, both the same for 304 and 316. Austenitic stainless conducts roughly a third as much heat as carbon steel, so the weld heat stays in the joint instead of spreading out; and it expands roughly 40 to 50 percent more per degree, so the same temperature field produces more thermal strain. More strain concentrated in a narrower zone means more distortion, more shrinkage and more residual stress.

Physical properties that drive fabrication behaviour

The controls follow directly from the physics. Use moderate heat input rather than the high deposition rates you would use on carbon steel. Use copper or aluminium backing bars to pull heat out of the root. Use a balanced welding sequence - backstep or symmetrical - so shrinkage is distributed rather than accumulated. Clamp to a strongback where the geometry allows, and remember that restraint raises residual stress even while it lowers visible distortion.

One more consequence is specific to thin-walled vessels. A shell under about 6 mm is stiffness-limited rather than strength-limited, so distortion and handling damage, not the UG-27 calculation, set the practical minimum. That is why thin austenitic vessels are usually built with stiffening rings or an increased nominal wall rather than to the calculated minimum, and why the thickness guidance in how to choose stainless steel plate thickness is worth reading before you finalise the shell.

What examination does ASME Section VIII require on austenitic stainless vessel welds?

Radiography per UW-11 at the level needed to earn the joint efficiency you designed to, plus liquid penetrant examination on all austenitic chromium-nickel welds with a nominal thickness over 19 mm (3/4 in) as required by UHA-34. If any heat treatment is performed, the examination is done after it. The penetrant requirement is an austenitic-specific rule with no carbon steel equivalent.

Examination requirements that apply to 304 and 316 vessel welds

The UHA-34 rule is the one most often missed. A fabricator whose habits come from carbon steel will radiograph, will perhaps ultrasonic test, and will not think about penetrant examination on a 25 mm shell seam. The Code requires it, and it matters: austenitic weld metal solidifies with coarse columnar grains, and a radiograph taken through coarse columnar grains is poor at finding the planar, tightly closed lack-of-fusion defects those grains tend to hide.

What is the hydrostatic test trap for stainless steel pressure vessels?

The water. A vessel that passes its hydrostatic test can be ruined by the test water left standing in it, because stagnant chloride-bearing water pits austenitic stainless faster than most of the process fluids the vessel will ever see. Limit chloride, drain immediately, dry completely, and do not leave a partially filled vessel sitting through a weekend.

Hydrostatic test controls for austenitic stainless vessels

There is a second, quieter trap in the same area. Vessels are often hydrotested, drained, and then left with the nozzle blanks on and the manway closed for weeks before commissioning. The remaining film of water evaporates and recondenses, concentrating whatever chloride was there onto the warmest and coolest surfaces. If a vessel has to sit, leave it open and dry, or preserve it with dry nitrogen. Fabricators who have handled stainless steel for years will recognise this pattern: the damage is never where the water was deepest, it is at the water line.

How do you verify you actually received 316 and not 304?

Positive material identification with a handheld X-ray fluorescence analyser, checking molybdenum. It takes seconds, it works on the plate and on the finished weld, and it is the only practical way to catch a mix-up in the shop, because 304 and 316 are visually, magnetically and dimensionally identical.

Verification steps from purchase order to nameplate

Molybdenum is the discriminator because it is the only element in the specification that one grade has and the other does not. Chromium and nickel ranges overlap enough that they cannot settle the question, and no visual or magnetic test can. A five-second PMI shot on every plate before it goes into the roll, and on every seam after welding, removes an error mode that otherwise surfaces years later as unexplained corrosion. The same check is worth doing on the filler wire spool, because the wire is the other place a mix-up can hide.

When should a pressure vessel be built from 304 instead of 316?

Whenever there is no localised corrosion mechanism for the molybdenum to address. If the fluid contains no meaningful chloride, the temperature stays below roughly 60 C in the presence of any chloride at all, the service is oxidising, or the vessel is dry, clean and indoor, 304L is the correct and cheaper material and the molybdenum is money spent on nothing.

Selection matrix for 304L versus 316L pressure vessel plate

Where the answer is neither, the upgrade paths are well established. Duplex 2205 roughly doubles the yield strength and handles chloride stress corrosion cracking far better than either grade. 254 SMO and 904L take over where pitting resistance is the governing requirement, and for the most aggressive duties the comparison becomes Inconel 625 versus 316L. Welding those materials is a different discipline again - see the Hastelloy C276 welding guide and the Inconel 625 welding guide for how the procedures change.

How do you specify 304 versus 316 plate for a pressure vessel?

Work in order: define the environment, choose the carbon variant, confirm Code listing and certification, design with the allowable stress for the grade you actually bought, qualify the procedure with a measured ferrite number, control interpass and shielding, specify examination and any intergranular corrosion test, then control the hydrotest water and clean the vessel afterwards.

Step 1. Write the process environment down before you write the grade down: Record chloride content, temperature, pH, whether the fluid is oxidising or reducing, what happens during shutdown, and what is under the insulation. 304 and 316 are both austenitic chromium-nickel grades; the molybdenum in 316 only buys you anything if a localised corrosion mechanism is actually present. See 304 vs 316 stainless steel properties and applications for the general property picture.

Step 2. Choose the carbon variant from the welding and service temperature together: For a welded vessel that will not see sustained service above about 425 C, order the L grade. If the design temperature sits in the creep range, move to 316H or 304H instead of trying to stretch an L grade. If you want one stock item to serve both, buy dual-certified material and make sure the mill certificate lists both specifications.

Step 3. Confirm the plate is Code material and get the certificate before it is cut: The plate must be SA-240 (the ASME adoption of ASTM A240) and listed for Section VIII-1 use, with a heat number traceable to the plate marking and an EN 10204 3.1 or 3.2 document. Read the heat analysis, not the typical value: check carbon against 0.030 percent for an L grade and molybdenum against 2.00 percent for 316. ASTM A240 lists the full chemistry and mechanical requirements.

Step 4. Design the shell with the allowable stress for the exact grade you bought: Use t = PR / (SE - 0.6P) from UG-27 for the longitudinal joint and t = PR / (2SE + 0.4P) for the circumferential joint, with S read from Section II Part D Table 1A at the design temperature and E from UW-12 for your joint type and examination. Do not design with 304 allowables and then buy 304L plate.

Step 5. Qualify the welding procedure with the right filler and a measured ferrite number: Qualify per ASME Section IX using P-No. 8 Group 1 base material with ER308L or ER316L under F-No. 6. Predict the ferrite number with the WRC-1992 diagram on the actual filler heat, then confirm it on the qualification coupon with a calibrated magnetic instrument and hold 3 to 10 FN.

Step 6. Control heat input, interpass temperature and root shielding: Keep interpass at 175 C (350 F) maximum for general work and 150 C (300 F) maximum where corrosion resistance is critical. Back purge the root with argon so the oxygen content stays low enough to avoid sugaring, and use a balanced weld sequence with copper or aluminium backing bars to pull heat out of the joint.

Step 7. Write the examination and the intergranular corrosion test into the order: Specify radiography per UW-11 for the joint efficiency you designed to, liquid penetrant per UHA-34 on welds over 19 mm nominal thickness, and an ASTM A262 test on a production weld coupon when the vessel will see sensitising temperatures or a corrosive fluid. Use Practice B for 316L and Practice C for 304L.

Step 8. Control the hydrostatic test water and then dry the vessel: Limit chloride to roughly 50 ppm, drain the vessel as soon as the test is complete, and dry all internal surfaces. Remove heat tint by pickling to ASTM A380 or by mechanical means with dedicated stainless tools, passivate to ASTM A967, and finish with positive material identification on the completed welds to confirm molybdenum is present where 316 was specified.

Two habits make the difference between a specification that survives contact with a shop and one that does not. First, write the supplementary requirements on the purchase order, not in the body of the drawing notes - minimum molybdenum, minimum ferrite number, intergranular corrosion test, and certificate type are all purchase order items. Second, make the material identification check a hold point in the inspection and test plan rather than a request. For the full route from plate through to spool and vessel assembly, pipe spool fabrication covers the fabrication sequence and prefabrication versus field welding covers where the work is best done.

Frequently Asked Questions

Molybdenum. 316 and 316L contain 2.00 to 3.00 percent molybdenum and 304 and 304L contain none. The chromium ranges differ slightly in the other direction - 304 runs 17.5 to 19.5 percent chromium against 16.0 to 18.0 percent for 316 - and 316 carries the higher nickel range at 10.0 to 14.0 percent. Everything else, including all mechanical properties, is the same.

How much molybdenum is in 316 stainless steel plate?

ASTM A240 requires 2.00 to 3.00 percent for both 316 and 316L. A heat at 2.02 percent and one at 2.95 percent are both compliant but do not perform identically in a hot chloride crevice, so specify a minimum molybdenum as a supplementary requirement when pitting is the governing mechanism.

Is 316 stronger than 304?

No. ASTM A240 lists identical minimum mechanical properties: 515 MPa (75 ksi) tensile and 205 MPa (30 ksi) yield for both, and 485 MPa (70 ksi) tensile and 170 MPa (25 ksi) yield for both L grades. Molybdenum changes corrosion behaviour, not strength.

What is the PREN of 304 versus 316?

Using PREN = percent Cr + 3.3 x percent Mo + 16 x percent N, typical 304 is about 18 to 20 and typical 316 is about 23 to 26. The gap comes almost entirely from the molybdenum term. PREN is a ranking index, not a design threshold - it does not give you a critical pitting temperature or a critical crevice temperature.

Which is better for a pressure vessel, 304 or 316?

It depends entirely on whether a localised corrosion mechanism exists. If there are no chlorides, or trace chlorides below about 50 ppm at ambient temperature, 304L is correct and cheaper. If chlorides are present or the fluid is a dilute reducing acid, choose 316L. If chlorides are present above about 60 C, neither is adequate because chloride stress corrosion cracking is then the governing risk.

Why is 316L preferred over 316 for welded pressure vessels?

Carbon. 316 permits up to 0.08 percent carbon, which is enough to form chromium carbide at grain boundaries in the heat-affected zone during welding, depleting the adjacent metal of chromium and creating a path for intergranular corrosion. 316L caps carbon at 0.030 percent, which is too little to form a continuous carbide network in a normal weld cycle.

Can you weld 304 to 316?

Yes. Use ER316L or E316L filler, which matches the higher alloyed of the two base metals. Never downgrade to a 308L filler on a 304 to 316 joint, because the diluted weld metal would then carry less molybdenum than either parent. Both grades are P-No. 8 Group 1, so the joint is within one procedure qualification structure.

What filler metal do you use to weld 316 plate?

ER316L for GTAW and GMAW, E316L for SMAW, and ER316LSi for GMAW when you want better wetting and bead shape from the silicon. For 316L to carbon steel use ER309LMo so the transition zone keeps molybdenum. Target 3 to 10 ferrite number in the as-deposited weld metal.

What is the correct ferrite number for 308L and 316L weld metal?

Target 3 to 10 FN, with 3 to 8 the usual optimum. Below about 3 FN the risk of solidification cracking rises sharply because the weld solidifies as coarse columnar austenite with impurity films between the grains. Above about 10 FN, and especially above 12 FN, delta ferrite can convert to brittle sigma phase in the 550 to 900 C band, and molybdenum accelerates that transformation.

Do you need post weld heat treatment for 304 or 316 vessel welds?

No. ASME Section VIII-1 Table UHA-32 states that PWHT for P-No. 8 austenitic stainless steel is neither required nor prohibited, so there is no thickness trigger as there is for carbon steel. If the user specifies heat treatment anyway, the right cycle is a solution anneal at 1040 to 1095 C followed by rapid cooling - not a subcritical stress relief, which would sensitise any non-L material.

Do you preheat stainless steel before welding?

Not to prevent cracking, and not to the temperatures used on carbon steel. Austenite does not transform to martensite on cooling and holds hydrogen readily, so there is no hydrogen cracking mechanism to preheat against. Preheat is used only to remove moisture or thaw a cold joint, and excessive preheat makes distortion worse.

What is the maximum interpass temperature for austenitic stainless steel?

175 C (350 F) is the common maximum for general fabrication, and 150 C (300 F) is used where corrosion resistance is critical. Interpass is controlled as a maximum rather than a minimum precisely because holding the heat-affected zone in the 425 to 815 C band is what causes sensitisation.

What is sensitisation and how do you prevent it?

Sensitisation is the precipitation of chromium carbide at grain boundaries when the metal is held between about 425 and 815 C, which depletes chromium from the metal immediately beside the boundary and allows intergranular corrosion. Prevent it by using L grade or dual-certified material with 0.030 percent carbon maximum, limiting interpass temperature and heat input, and using a stabilised grade such as 347 for warm service.

What temperature causes sensitisation in stainless steel?

Between about 425 and 815 C (800 and 1500 F), with the fastest precipitation near 650 to 750 C. A weld heat-affected zone passes through this band in minutes, which is why carbon content - not chromium content - determines whether a weld stays corrosion resistant.

Which ASTM A262 practice tests 316L for sensitisation?

Practice B, the ferric sulfate-sulfuric acid test, is the appropriate practice for molybdenum-bearing austenitic stainless such as 316 and 316L. Practice C, the nitric acid or Huey test, is the conventional choice for 304 and 304L. Practice A is the rapid oxalic acid screening etch, and Practice E is the copper-copper sulfate-sulfuric acid test.

What is the allowable stress of SA-240 304 versus 316?

Identical at ambient temperature: 20.0 ksi (138 MPa) for both 304 and 316, and 16.7 ksi (115 MPa) for both 304L and 316L, per ASME Section II Part D Table 1A. That means 316 does not let you build a thinner vessel than 304 - the molybdenum buys corrosion resistance, not strength.

Is 316 resistant to chloride stress corrosion cracking?

No. Molybdenum improves pitting and crevice resistance but has no useful effect on chloride stress corrosion cracking, which needs chlorides, oxygen, tensile stress and a temperature above roughly 60 C. Molybdenum addresses none of the four. The fix is to remove one of the four conditions or to change alloy family to duplex, 6Mo or a nickel alloy.

Can 316 be used for seawater service?

Not for continuous immersion or any creviced duty. 316L pits and crevice corrodes in seawater, particularly in stagnant or shielded areas. Use super duplex such as S32750, a 6Mo grade such as 254 SMO, or a nickel alloy depending on velocity, temperature and crevice geometry.

What is the hydrostatic test water chloride limit for stainless vessels?

Commonly specified at 50 ppm maximum, and 25 ppm or lower where the vessel cannot be reliably dried. Whatever the limit, drain the vessel immediately after the test and dry all internal surfaces. Stagnant chloride water pits austenitic stainless faster than most process fluids it will ever contain, and the damage usually appears at the water line.

How do you tell 304 from 316 in the shop?

Positive material identification with a handheld X-ray fluorescence analyser, checking molybdenum. The two grades are visually, magnetically and dimensionally identical, and the chromium and nickel ranges overlap, so molybdenum is the only reliable discriminator. Check it on every plate at receiving, on each piece after cutting, and on the completed welds.

Is 316 stainless steel plate magnetic?

No, not in the annealed condition - both 304 and 316 are austenitic and non-magnetic. Weld metal in either grade contains enough delta ferrite to give a weak magnetic response, which is exactly what a ferrite measurement instrument detects. Cold work can also induce slight magnetism in the formed areas.

At what temperature should you stop using 304L or 316L?

Three limits apply. Low-temperature sensitisation matters above about 500 C in long service, allowable stress falls away above roughly 540 C as creep governs, and molybdenum-bearing 316L is the more sigma-prone grade in the 550 to 900 C band. For sustained metal temperature above about 540 C, move to 304H or 316H, or to a stabilised grade such as 347.

What is dual certified 316/316L plate?

A single heat that meets every requirement of both specifications: carbon at 0.030 percent maximum together with the straight grade tensile and yield minimums. It lets one stock item satisfy an L-grade welding specification and a straight-grade design allowable, subject to the purchase order, plate marking and certificate all naming the grade you are designing to.

Does 316 weld metal need stress relieving?

No. There is no Code requirement to stress relieve austenitic stainless welds, and a subcritical stress relief in the 600 to 700 C range is actively harmful because it sensitises non-L material and can grow sigma phase in molybdenum-bearing weld metal. Residual stress is managed through joint design, weld sequence and, where necessary, mechanical methods.

How much more does 316 plate cost than 304?

Typically 25 to 45 percent on plate price, driven mainly by the molybdenum and nickel alloy surcharges and moving with the alloy markets. On a finished vessel the penalty is smaller, because welding, examination and heat treatment cost the same for both grades and plate is only part of the delivered cost.

Is 316 better than 304 for nitric acid service?

No, it is worse. Molybdenum dissolves preferentially in strongly oxidising acids, so 304L is the standard choice for nitric acid service and 316L is avoided. This is one of the few cases where the more expensive grade is the technically wrong one.