Ultimate Guide to Super Duplex S32750

Super Duplex S32750 is a 25 percent chromium duplex stainless steel with a mixed ferrite-austenite microstructure, roughly 41-43 PREN, and about twice the yield strength of 316L. It is sold under four names - UNS S32750, alloy 2507, ASTM A182 grade F53 and EN 1.4410 - because four different naming systems describe the same chemistry, and confusing them with the near-neighbour grade S32760 is the most common purchasing error in this family.

Duplex stainless steels are defined by a microstructure of roughly equal parts ferrite and austenite. The ferritic phase delivers high strength and resistance to chloride stress corrosion cracking; the austenitic phase delivers toughness and ductility. 'Super duplex' is the industry shorthand for the higher-alloyed branch of the family, generally with PREN above 40, where standard duplex such as duplex 31803 or 2205 (S32205) sits around 34-38.

The naming confusion is worth settling early, because a purchase order that says '2507' and a mill certificate that says 'S32760' are not the same material. S32760 (Zeron 100, F55) adds deliberate copper and tungsten; S32750 does not. Some distributors ship dual-certified stock that satisfies both, which is fine when you have accepted it in writing and dangerous when you have not.

Four elements do the work: 24.0-26.0 percent chromium, 3.0-5.0 percent molybdenum, 0.24-0.32 percent nitrogen and 6.0-8.0 percent nickel, with carbon capped at 0.030 percent.

Chromium and molybdenum build the passive film and resist localised attack; nitrogen raises pitting resistance sharply, restores austenite in the heat-affected zone after welding, and adds solid-solution strength; nickel balances the phase fractions and carries toughness. Carbon is deliberately held low, not for weldability as it is in the L grades of austenitic stainless steel, but because carbon stabilises carbides and competes with the phase balance the whole alloy depends on.

One detail separates S32750 from its closest relative: there is no deliberate copper or tungsten addition. S32760 carries roughly 0.5-1.0 percent of each, which helps in reducing acids and in some sour environments.

316L is fully austenitic, S32750 is roughly half ferrite and half austenite, and that single difference explains almost every advantage and every fabrication difficulty of the material. Ferrite brings strength and chloride stress corrosion cracking resistance; austenite brings toughness; and the material only performs when both are present in the right proportion.

 

In an austenitic grade such as 316L stainless steel, a chloride stress corrosion crack can run continuously through the grain structure. In a duplex structure the crack has to cross a phase boundary every few micrometres, and the ferrite and austenite grains have different crystallographic orientations. The crack is repeatedly deflected and blunted. That is the metallurgical reason duplex survives chloride environments where 316L does not, and it is why the phase balance is a specification requirement rather than a metallurgical curiosity.

 

The same two-phase structure is also the reason super duplex has a narrow processing window. Between roughly 600 C and 950 C the ferrite phase is thermodynamically unstable and begins to decompose into sigma phase, chi phase and chromium nitrides. Every welding pass, every hot forming cycle and every slow cool through that band costs you toughness and corrosion resistance.

 

Feature	316L (austenitic)	2205 (standard duplex)	S32750 (super duplex)
Phases present	Austenite only	Roughly 50:50 ferrite and austenite	Roughly 50:50 ferrite and austenite
Specified ferrite range	None (0 percent)	35 - 55 percent	35 - 55 percent
Test method for phase balance	Not applicable	ASTM E562 point count	ASTM E562 point count or ASTM E1245 image analysis
Magnetic response	Non-magnetic when annealed	Magnetically attractive	Magnetically attractive
Chloride SCC resistance	Poor above roughly 60 C	Good	Very good
Risk from thermal exposure	Sensitisation 425 - 815 C	Sigma and chi phase from about 600 C	Sigma and chi phase from about 600 C
Toughness retained at low temperature	Excellent	Good; verify with Charpy at -46 C	Good; verify with Charpy at -46 C

 

• Ferrite is the stronger phase and the one that resists chloride stress corrosion cracking, so a weld that cools too quickly and re-forms too little austenite will be strong but brittle and less corrosion resistant
• Austenite is the tough phase and the one that dissolves nitrogen, so a weld that cools too slowly grows excessive ferrite and becomes a site for nitride precipitation
• The practical target is 35 - 55 percent ferrite in the parent metal and a weld metal and heat-affected zone inside the range the project specification allows, verified on the procedure qualification coupon rather than assumed from the filler datasheet

ASTM A240 guarantees 795 MPa (115 ksi) minimum tensile strength, 550 MPa (80 ksi) minimum 0.2 percent proof strength, 15 percent minimum elongation and 310 HBW or 32 HRC maximum hardness. Those are minimums, not typical values - real mill certificates commonly show 600-650 MPa yield and 25-30 percent elongation.

 

The 550 MPa minimum proof strength is the number that changes a design. It is roughly double the 205 MPa minimum of 316L and about 22 percent above the 450 MPa minimum of 2205. In a pressure-containing shell or a long pipe run, that translates directly into reduced wall thickness, less weld metal, lighter supports and smaller lifting equipment.

 

What the ASTM minimums do not tell you is what a project specification will add. NORSOK M-630 material data sheet D51 raises elongation to 25 percent and adds a Charpy V-notch requirement at -46 C, because elongation and impact energy are the properties that collapse first when sigma phase appears. A plate can pass every ASTM requirement and still be unacceptable for a Norwegian offshore project.

 

Requirement	ASTM A240 plate	ASTM A790 pipe	NORSOK M-630 MDS D51
0.2 percent proof strength, min	550 MPa (80 ksi)	550 MPa (80 ksi)	550 MPa
Tensile strength, min	795 MPa (115 ksi)	800 MPa (116 ksi)	800 MPa
Elongation, min	15 percent	15 percent	25 percent (A5)
Hardness, max	310 HBW / 32 HRC	310 HBW / 32 HRC	32 HRC (301 HB / 330 HV10)
Charpy V-notch	Not required by the base spec	Not required by the base spec	45 J average / 35 J single at -46 C for thickness at or above 6 mm
Corrosion test	Not required by the base spec	Not required by the base spec	ASTM G48 Method A, 50 C, 24 h, no pitting at 20x, weight loss below 4.0 g/m2
Ferrite content	Not required by the base spec	Not required by the base spec	35 - 55 percent by ASTM E562, free of intermetallic phase at 400x

Temperature	Section II Part D Table 1A (Section VIII-1)	ASME B31.3 Table A-1
38 C / 100 F	33.1 ksi (228 MPa)	38.7 ksi (267 MPa)
93 C / 200 F	33.0 ksi (228 MPa)	38.5 ksi (265 MPa)
149 C / 300 F	31.2 ksi (215 MPa)	36.4 ksi (251 MPa)
204 C / 400 F	30.0 ksi (207 MPa)	35.1 ksi (242 MPa)
260 C / 500 F	29.6 ksi (204 MPa)	34.5 ksi (238 MPa)
315 C / 600 F	29.4 ksi (203 MPa)	34.3 ksi (236 MPa)

The two allowable stress columns differ by about 17 percent because the two codes apply different safety factors to the same tensile strength, not because the material changes. Section VIII Division 1 caps the tensile term at 3.5 while ASME B31.3 caps it at 3.0. If you size a Division 1 vessel with the B31.3 number, you under-design the shell. Always take S from the table that matches your code of construction, at the design temperature rather than at ambient.

Three matter in practice: the material is magnetic, it has a lower thermal expansion coefficient than austenitic stainless steel, and its high strength means springback during forming is much larger than a fabricator used to 316L will expect.

 

Thermal expansion of roughly 13.0 x 10-6 per K between 20 and 100 C sits between carbon steel and austenitic stainless steel. That is genuinely helpful in a duplex-to-carbon-steel transition joint, because the differential expansion across the weld is smaller than it would be with 316L. Thermal conductivity is modest at about 14-17 W/m.K, which is why heat stays in the weld zone and why interpass control matters so much.

 

Property	Typical value	Why it matters to you
Density	7.8 g/cm3	Slightly below austenitic stainless steel, so a like-for-like volume weighs marginally less
Modulus of elasticity	200 GPa	Similar to carbon steel; elastic deflection calculations carry over directly
Thermal expansion, 20-100 C	13.0 x 10-6 /K	Lower than 316L, which reduces thermal stress in mixed-material systems
Thermal conductivity	14 - 17 W/m.K	Low, so heat concentrates in the weld and interpass temperature climbs quickly
Specific heat capacity	About 460 - 500 J/kg.K	Used for heat input and cooling rate calculations on thick sections
Electrical resistivity	About 0.8 micro-ohm.m	Higher than carbon steel; affects resistance welding set-ups
Magnetic response	Attractive to a magnet	Duplex is ferromagnetic because of the ferrite phase. Plan for it near compasses, magnetic sensors and mine-countermeasures equipment

The magnetic property catches people out. A magnet sticks to super duplex plate. It is not a sign that the material has been mis-supplied, and it is not a defect. But it does matter on a vessel or a subsea structure where magnetic signature is controlled, and it is the easiest way to tell duplex from austenitic in a stores yard before you reach for a spectrometer.

PREN = percent Cr + 3.3 x percent Mo + 16 x percent N ranks stainless steels by resistance to chloride pitting. S32750 typically lands at 41-43, which places it above duplex 2205 at 34-38 and far above 316L at about 25. It does not predict a service temperature, a crevice limit or an allowable chloride level, and treating it as a design threshold is the most expensive mistake buyers make with this alloy.

Chromium, molybdenum and nitrogen raise the pitting and crevice thresholds, and the dual phase microstructure blocks chloride stress corrosion cracking. Typical critical pitting temperature is 55-70 C by ASTM G48 Method E and typical critical crevice temperature is 40-50 C by Method F, with project minimums commonly set at 50 C and 35 C respectively.

 

Pitting and crevice corrosion are the same mechanism under different geometry. Both start when the passive film breaks down locally in the presence of chlorides and the pit chemistry becomes self-accelerating. A crevice - under a gasket, a lap joint, a tube-to-tubesheet expansion or a deposit - is simply a pre-made pit, which is why the critical crevice temperature of any stainless steel is always well below its critical pitting temperature and why design, not alloy choice, eliminates most crevice problems.

 

Chloride stress corrosion cracking is where duplex earns its price. The crack must repeatedly cross ferrite-austenite phase boundaries, and the phase with the lower electrochemical potential can act as a sacrificial barrier. A 316L system that cracks in months above roughly 60 C in a chloride environment can often run for decades in S32750 at the same conditions. There is no code-approved upper limit for duplex, because the threshold depends on chloride concentration, pH, oxygen, stress level and surface condition simultaneously.

Test	Conditions	What it tells you
ASTM G48 Method A	6 percent ferric chloride, 50 C, 24 hours, as-delivered surface	Whether the delivered product is free of the gross defects that cause early pitting. NORSOK M-630 acceptance: no pitting at 20x and weight loss below 4.0 g/m2
ASTM G48 Method E	Ferric chloride, temperature stepped until pitting starts	Critical pitting temperature. Typically 55-70 C for S32750
ASTM G48 Method F	Ferric chloride crevice assembly, temperature stepped	Critical crevice temperature. Typically 40-50 C for S32750
ASTM A923 Method A	Sodium hydroxide electrolytic etch, metallographic review	Rapid screening for intermetallic phases in the microstructure
ASTM A923 Method B	Charpy impact at a specified temperature	Whether toughness has been lost to sigma or chi phase
ASTM A923 Method C	Ferric chloride exposure of a specific specimen	Whether corrosion resistance has been lost to intermetallic phases
ASTM G36	Boiling magnesium chloride under controlled stress	Comparative chloride stress corrosion cracking screening
ASTM A380 / A967	Pickling and passivation practice	Removal of heat tint and free iron, and restoration of the passive film after welding

Two practical notes. First, the as-delivered surface condition is part of the test - NORSOK requires the specimen to be tested with its production surface including pickling, because a bright annealed surface and a badly heat-tinted surface are not the same product. Second, the corrosion test is run on the parent metal unless you order it on the weldment as well. If your risk is in the weld, specify G48 on a production weld coupon, not just on the plate certificate.

 

For a full treatment of S32750 in seawater service - including chlorination dosing, velocity and stagnation, hydrotest traps and 20-year lifecycle cost - read Super Duplex S32750 for Seawater Desalination: A Technical Guide.

The codes permit 315 C. The metallurgy does not. Continuous service is normally capped around 250-300 C, the material must never sit in the 400-550 C band because of 475 C embrittlement, and it must never dwell between 600 and 950 C because that is where sigma and chi phase form.

 

There are two independent limits and engineers routinely confuse them. The code limit is an administrative ceiling derived from allowable stress and creep data. The metallurgical limit is set by phase stability and is almost always the binding constraint. A vessel can be fully code-compliant at 300 C and still embrittle in service if the operating profile keeps it there for years.

 

Limit	Value	Source and consequence
ASME Section VIII-1 maximum	315 C (600 F)	Section II Part D listing for SA-240; above this there is no allowable stress
ASME B31.3 maximum	315 C (600 F)	Table A-1 listing; minimum design temperature without impact testing is about -51 C
ASME B31.1 maximum	250 C (480 F)	Power piping listing is more conservative
Impact test exemption floor	About -29 C for VIII-1, -51 C for B31.3	Below these temperatures you must demonstrate toughness, usually with Charpy testing
Practical continuous service ceiling	250 - 300 C	Risk of long-term embrittlement rises sharply at the top of this band
475 C embrittlement band	400 - 550 C, worst near 475 C	Chromium-rich alpha prime precipitates in ferrite. Hardness rises, toughness collapses
Sigma phase formation band	600 - 950 C, fastest near 800 - 850 C	Sigma and chi phase consume chromium and molybdenum from the surrounding matrix
Chi phase formation band	700 - 900 C	Forms faster than sigma and is equally damaging to toughness
Chromium nitride precipitation	600 - 900 C in ferrite	Cr2N in the weld heat-affected zone; controlled by heat input and cooling rate
Solution annealing range	1025 - 1125 C	The only thermal cycle the material is designed to receive, followed by water quench

 

• Sigma phase is the serious one: it can cut Charpy impact energy by 80 percent or more and it removes chromium and molybdenum from the matrix around it, so pitting resistance drops at the same time as toughness
• 475 C embrittlement is insidious because it happens at temperatures that feel harmless compared with welding. A flare line, a hot gas bypass or a badly designed trace heating loop can sit in the band for years
• Both conditions are reversible only by a full solution anneal at 1025-1125 C with a water quench, which is usually impossible on an installed system - so the control has to be applied at the design stage
• Cryogenic service down to about -50 C is generally acceptable with impact-tested material; below that, verify with Charpy testing at the design temperature.

Do not use it above about 300 C, in strong reducing acids without copper-bearing S32760 or a nickel alloy, in a design you cannot weld within the heat input and interpass window, or where the corrosion mechanism is one that PREN does not address. S32750 solves chloride and sour service problems; it does not solve every corrosion problem.

 

Being explicit about exclusion is what separates a materials guide from a sales page. S32750 is a superb alloy inside its envelope and an expensive mistake outside it. The table below lists the common cases where a different material is the correct answer.

Situation	Why S32750 fails or is wasted	Better choice
Continuous service above about 300 C	Sigma phase and 475 C embrittlement are time-temperature driven; no Code allowable stress exists above 315 C	Inconel 625 or another nickel alloy for strength and stability
Hydrochloric or hot sulfuric acid at meaningful concentration	Duplex stainless steels rely on a passive film that reducing acids destroy regardless of PREN	Hastelloy C276 or another nickel-chromium-molybdenum alloy
Strong reducing acid where copper helps	S32750 has no deliberate copper addition	S32760 / F55 with copper and tungsten
Ambient fresh water with negligible chlorides	Paying for PREN that will never be used, and accepting harder fabrication for nothing	316L or duplex 2205
Complex cold-formed parts with tight radii	High proof strength means high springback and roughly double the forming load of 316L	An austenitic grade, or redesign the part around a larger bend radius
Thick section welded in the field with limited process control	Holding 0.5-1.5 kJ/mm and 100 C interpass on site is hard; failures concentrate in the weld HAZ	Shop-fabricated spools - see prefabrication versus field welding
Cathodically protected structure with high strength and hardness	Hydrogen induced stress cracking risk under cathodic protection rises with strength, hardness and applied stress	Control hardness, limit cathodic protection potential, and follow a HISC assessment such as DNV guidance
Service requiring non-magnetic material	Duplex is ferromagnetic because of the ferrite phase	An austenitic stainless steel or a nickel alloy

 

If none of these exclusions applies and your driver is chlorides, seawater, brine or sour service, S32750 is very likely the right shortlist candidate. The next question is whether you need the copper-bearing variant or whether standard duplex will do - which is what the comparison section below is for.

Use ER2594 or E2594 filler metal, hold heat input between roughly 0.5 and 1.5 kJ/mm, keep interpass temperature at 100 C maximum, shield with argon containing 2-3 percent nitrogen, purge the root, and never apply post-weld heat treatment other than a full solution anneal. Every one of those controls exists to keep the ferrite-austenite balance in the weld and heat-affected zone inside the band the alloy was designed for.

 

Welding is where super duplex projects succeed or fail. The parent plate arrives with a correctly balanced microstructure produced under mill control. The weld thermal cycle destroys that balance locally: the heat-affected zone is heated into a temperature range where ferrite is stable and austenite dissolves, and what happens on cooling determines whether the joint performs. Cool too quickly and the HAZ stays too ferritic, with poor toughness. Cool too slowly and nitrides and intermetallic phases precipitate.

 

Filler metal choice follows a single rule: the weld metal must be at least as corrosion resistant as the parent metal after dilution. For S32750 that means ER2594, a 25 percent chromium, 9 percent nickel, 4 percent molybdenum, nitrogen-bearing filler. ER2209 is a 2205 filler and is under-matched for 2507; a 316L filler is far worse. Dilution with the parent metal pulls the weld composition down, which is exactly why the filler is over-alloyed in the first place.

Joint	Recommended filler	Why
S32750 to S32750	ER2594 / E2594 (GTAW, GMAW, SMAW)	Over-alloyed 25Cr-9Ni-4Mo-N composition keeps weld metal PREN at or above the parent after dilution
S32750 to S32760	ER2594	Satisfies both parents; confirm the project specification allows a single filler
S32750 to duplex 2205	ER2594	Match the higher-alloyed parent. ER2209 would under-match the 2507 side
S32750 to 316L	ER2594 or a nickel-based filler such as ERNiCrMo-3	Avoids a diluted weld with less molybdenum than either parent; choose the nickel filler where thermal expansion mismatch or service temperature is a concern
S32750 to carbon steel	ER2594 for the buttering layer, then a suitable transition filler	Butter the carbon steel side first, then weld the duplex joint, so the fusion zone never forms a martensitic band
Root runs	ER2594 with argon back purge	Unpurged roots form heavy chromium oxide - 'sugaring' - that is a corrosion initiation site no amount of downstream pickling fully removes

Parameter	Target	Consequence of getting it wrong
Heat input	0.5 - 1.5 kJ/mm typical; 2.5 kJ/mm is the usual absolute ceiling	Too low: excessive ferrite in the HAZ, low toughness. Too high: sigma, chi and nitride precipitation
Interpass temperature	100 C maximum for 25Cr super duplex	Above the limit the joint spends too long in the precipitation band during multipass welding
Preheat	None, beyond removing moisture	Duplex does not need hydrogen crack prevention. Heating simply pushes the joint toward the precipitation band
Shielding gas	Argon with 2 - 3 percent nitrogen for GTAW and GMAW	Nitrogen helps reform austenite in the weld and HAZ. Too much nitrogen risks porosity
Root purge	Argon purge verified with an oxygen meter before striking the arc	Oxygen above a low ppm level produces heavy oxide scale and an immediate corrosion initiation site
Post-weld heat treatment	None, or full solution anneal at 1025 - 1125 C with water quench	A 600 - 700 C stress relief precipitates sigma phase and ruins the joint
Ferrite in weld metal	Within the range the specification allows, commonly 30 - 65 percent	Outside the band, verify with a point count on the qualification coupon
Consumable condition	Dry, sealed, handled like any stainless consumable	Moisture and hydrocarbon contamination from handling are a common source of porosity

 

• Qualify the procedure on the actual thickness range you will weld - heat flow, and therefore cooling rate, changes completely between 6 mm and 40 mm
• Include a hardness traverse across weld metal, heat-affected zone and parent metal in the procedure qualification. Hardness above about 330 HV10 is a warning sign for both susceptibility to hydrogen induced stress cracking and loss of ductility
• Run ASTM G48 Method A on a weldment coupon, not only on the plate certificate, when the weld is the corrosion-critical location
• Remove all heat tint mechanically or by pickling, then passivate. Heat tint is a chromium-depleted oxide layer and it pits preferentially
• Use dedicated stainless wire brushes and grinding wheels. Free iron from carbon steel tools becomes a rust initiation site on the finished surface

Our Inconel 625 welding guide and Hastelloy C276 welding guide show the same qualification discipline applied to nickel alloys, which is useful if your project uses both families.

Exactly one thermal cycle: solution annealing at 1025-1125 C followed by rapid water quenching. There is no ageing, no tempering, no normalising and no subcritical stress relief. Anything else you do to this alloy in a furnace is either a full solution anneal or damage.

 

The purpose of the solution anneal is twofold: dissolve any intermetallic phases and precipitates that formed during hot working, and re-establish the correct ferrite-austenite balance. The quench is not an optional finishing step. Its only job is to move the material through the 600-1000 C band fast enough that sigma and chi phase do not have time to nucleate. A slow cool through that band undoes the entire heat treatment.

 

Thermal cycle	Temperature range	Result
Solution annealing (the specified condition)	1025 - 1125 C, water quench	Correct condition. Dissolves precipitates and sets the phase balance
Hot forming	Roughly 1000 - 1150 C	Acceptable, but the component must be re-solution annealed and quenched afterwards
Post-weld solution anneal	1025 - 1125 C, water quench	Used for heavy sections, hot-formed parts and safety-critical weldments
Stress relief - never do this	550 - 700 C	Precipitates sigma and chi phase. Destroys toughness and corrosion resistance
475 C embrittlement band	400 - 550 C	Alpha prime precipitation in ferrite. Progressive hardening and loss of toughness
Sigma phase band	600 - 950 C, fastest near 800 - 850 C	Brittle intermetallic; removes chromium and molybdenum from the surrounding matrix
Chi phase band	700 - 900 C	Forms faster than sigma; equally damaging to impact energy
Sensitisation band (austenitic grades, for contrast)	425 - 815 C	An austenitic problem, not a duplex one. Do not carry the 316L rule across

 

NORSOK M-650 is the qualification standard that matters for mills and heat treaters. It requires the manufacturer to demonstrate that the furnace can hold temperature uniformity, that the quench is fast enough, and that production material meets the full test set. A certificate that states 'solution annealed' without stating temperature, soak time and cooling medium does not tell you whether the heat treatment was performed correctly - and NORSOK M-630 requires all three to be stated.

 

The most common heat treatment failure is not a furnace that ran too cold. It is a furnace that cooled too slowly, or a heavy section whose core did not quench as fast as the surface. Thick plate and heavy forgings are the highest risk, which is why specifications call for testing at mid-thickness and at quarter-thickness rather than only at the surface.

Expect roughly twice the forming force of 316L, larger minimum bend radii, and noticeably more springback. Hot form above about 1000 C and re-solution anneal afterwards. Machine it with rigid set-ups, sharp positive-rake carbide tools and generous coolant, and finish with pickling and passivation using dedicated stainless tooling.

 

The high proof strength that makes super duplex attractive in service is exactly what makes it harder to form. Press brake tonnage scales with strength, springback scales with the ratio of yield strength to modulus, and the material work hardens quickly. A fabricator who sets up the job from 316L experience will get parts that are out of tolerance and a die that is overloaded.

 

• Cold bending: use a larger inside bend radius than you would for austenitic stainless steel - a common starting point is a minimum of about 3 times the plate thickness for super duplex, then verify on a trial piece
• Hot forming: work above roughly 1000 C and follow with a full solution anneal and water quench. The component is not in specification until it has been re-annealed
• Machining: rigid machine, short tool overhang, sharp positive-rake carbide, constant feed rather than a light cut that rubs and work hardens the surface, and plenty of coolant
• Drilling and tapping: use sharp tools and avoid dwelling. Power requirements run significantly above 316L
• Cutting: plasma and laser both work; the heat-affected zone from thermal cutting must be removed before welding
• Grinding: aluminium oxide or zirconia abrasives dedicated to stainless steel. Never use a wheel that has touched carbon steel
• Pickling and passivation: remove heat tint mechanically or chemically to ASTM A380, then passivate to ASTM A967. This is not cosmetic - heat tint is a chromium-depleted layer that pits first
• Handling: use stainless or polymer slings, separated storage from carbon steel, and clean lifting equipment. Free iron contamination is the cheapest failure to prevent and one of the most expensive to explain afterwards

For product-form specific guidance, see S32750 plate to ASTM A240 and duplex and super duplex pipe.

A240 for plate, A790 for seamless and welded pipe, A789 for tube, A182 grade F53 for forgings and flanges, A815 for wrought fittings, A276 and A479 for bar. ASME adopts most of these with an SA prefix. Then, on top of the ASTM base specification, project standards such as NORSOK M-630, NORSOK M-650 and NACE MR0175 / ISO 15156 add the requirements that actually determine whether the material performs.

 

An ASTM specification is a floor, not a specification. It defines chemistry, mechanical properties, permissible testing and dimensional tolerance. It does not, by default, require a corrosion test, an impact test, a ferrite measurement or a freedom-from-intermetallic-phase check. Those come from supplementary requirements you invoke, or from a project standard such as NORSOK M-630 that bundles them.

 

Product form	ASTM specification	ASME adoption	Notes
Plate, sheet and strip	ASTM A240 / A240M	SA-240	Solution annealed 1025-1125 C and water quenched; PREN above 41 is a supplementary requirement
Seamless and welded pipe	ASTM A790 / A790M	SA-790	The standard specification for pressure pipe in this grade
Seamless and welded tube	ASTM A789 / A789M	SA-789	Commonly used for heat exchanger and instrumentation tubing
Electric fusion welded pipe	ASTM A928	SA-928	Classes 1 to 5 define the extent of radiography and whether the weld is cold worked
Forgings, flanges, forged fittings	ASTM A182 grade F53	SA-182	F53 is the grade designation for S32750; F55 is S32760
Wrought buttwelding fittings	ASTM A815	SA-815	Elbows, tees, reducers and caps; the S32750 designation appears with the WP prefix
Bar and shapes	ASTM A276, ASTM A479	SA-479	A276 for general shapes, A479 for bar intended for pressure-containing use
Castings	ASTM A995 grade 5A	SA-995	Cast equivalent; mechanical properties differ from wrought material and must be verified
Pipe fittings, forged	ASTM A182 F53 with ASME B16.11	SA-182	Dimensional standard is separate from the material standard

Standard	What it governs	Why it matters for S32750
NORSOK M-630	Material data sheets that select the options and add requirements to the base standard	MDS D51 for 25Cr duplex sets PREN at or above 40, elongation at 25 percent, Charpy at -46 C, G48 Method A at 50 C and ferrite at 35-55 percent
NORSOK M-650	Qualification of manufacturers of special materials	Requires the mill or heat treater to demonstrate furnace uniformity, quench capability and reproducible properties before supplying
NORSOK M-601	Welding and inspection of piping	Sets welding consumable qualification, heat input, interpass and examination extent
NACE MR0175 / ISO 15156-3	Materials for H2S containing environments	S32750 is listed in the solution annealed condition; maximum hardness is commonly 32 HRC and projects frequently tighten it to 28 HRC
ASME Section VIII Division 1	Pressure vessel construction	Provides the allowable stress and the design rules; maximum temperature 315 C
ASME B31.3	Process piping	Provides a different allowable stress basis; maximum temperature 315 C
ASME Section IX	Welding procedure and welder qualification	Defines how the WPS and PQR must be qualified for the thickness and process you use
ASTM A923	Detection of detrimental intermetallic phase in duplex stainless steel	The test that catches sigma and chi phase before it reaches your fabrication shop
EN 10204	Types of inspection documents	3.1 is a mill declaration with test results; 3.2 adds third-party witness

 

Two documentation points save the most arguments later. First, always name the standard edition on the purchase order - a 2019 edition and a 2024 edition can differ in testing requirements. Second, decide early whether you need EN 10204 3.1 or 3.2. Type 3.1 is a mill declaration with actual test results signed by an authorised representative; Type 3.2 adds independent third-party witness of the testing. Critical offshore and sour projects almost always ask for 3.2.

UNS S32750 equals EN 1.4410 (X2CrNiMoN25-7-4), equals ASTM A182 grade F53, equals the trade name 2507, and is close to GB 022Cr25Ni7Mo4N. It is not the same as S32760, S32550, S39274 or 1.4507, and it is not the same as the 2205 family despite the similar numbering.

 

Equivalence here means 'the same chemistry within tolerance', not 'automatically substitutable'. Standards bodies set slightly different limits on minor elements and on mechanical properties, so an equivalent designation is a starting point for a technical review, not a permission slip. The check that matters is whether the substitute meets every supplementary requirement on your purchase order - PREN, ferrite, impact energy and corrosion test included.

System or country	Equivalent designation	Status
UNS	S32750	Primary designation
EN / Werkstoff	1.4410, X2CrNiMoN25-7-4	Full equivalent
ASTM forging grade	F53	Full equivalent for A182 forgings, flanges and fittings
Trade names	2507, SAF 2507, UR 52N+, Avesta 2507	Mill trade names for the same UNS chemistry
GB / China	022Cr25Ni7Mo4N	Near equivalent; verify minor element limits
ASTM cast grade	A995 grade 5A (J93404)	Cast form; different mechanical properties, verify before substituting for wrought
ISO	X2CrNiMoN25-7-4	Aligned with the EN designation
Not equivalent	S32760 / F55 / Zeron 100, 1.4501	Different UNS number with deliberate copper and tungsten
Not equivalent	S32550 / Ferralium 255, 1.4507	Different alloy with copper; not a drop-in substitute
Not equivalent	S32205 / S31803 / 2205 / F51, 1.4462	Standard duplex with lower PREN and lower strength

When a supplier proposes an equivalent, ask for the actual heat analysis and calculate PREN with the formula on your purchase order. Then ask for the ferrite, hardness, impact and corrosion results. If all five match, the substitution is defensible. If the supplier can only offer the designation and not the data, it is not an equivalence argument, it is an assumption. Our Inconel 625 equivalent grades guide applies the same method to a nickel alloy.

Start with ASTM A790 for seamless or welded pipe, size it with the allowable stress from the code you are building to, choose seamless versus welded on the basis of thickness, diameter and service, and add supplementary requirements for corrosion testing and impact toughness. Then specify the examination extent, because a pipe is only as good as the inspection written into the order.

 

The seamless versus welded decision is usually driven by availability and wall thickness. Seamless A790 is the default for high-pressure, small-to-medium diameter and sour service. Welded A790 or A928 electric fusion welded pipe becomes practical at larger diameters and heavier walls, provided the weld is fully radiographed and the class requirement is understood. A928 class 1 through class 5 differ precisely in how much of the weld is examined and whether the weld seam is cold worked and re-annealed.

 

Decision	Options	How to decide
Specification	A790 seamless, A790 welded, A928 EFW classes 1-5	A790 seamless for critical and sour duty; A928 only with the class and radiography extent stated on the order
Wall thickness	ASME B36.10 or B36.19 schedules	Calculate from the design pressure with the correct allowable stress, then add corrosion allowance and any erosion allowance - see ASME B36.19 dimensions and weights
Corrosion test	ASTM G48 Method A at 50 C for 24 hours	NORSOK acceptance: no pitting at 20x, weight loss below 4.0 g/m2, tested on the as-delivered internal and external surface
Impact testing	Charpy V-notch at -46 C	45 J average and 35 J single for wall thickness at or above 6 mm under NORSOK M-630
Ferrite content	35 - 55 percent by ASTM E562	Verified near surface and at mid-thickness; the test must report both
Nondestructive examination	UT, RT, PT to the applicable ASME or project requirement	State the method, the extent and the acceptance standard on the order, not just 'NDE'
Length and end finish	Random or fixed length, plain, bevelled or threaded	Bevel to the WPS joint design; confirm bevelling does not introduce carbon steel contamination
Certification	EN 10204 3.1 or 3.2	Choose 3.2 when a third party must witness the mechanical and corrosion testing

 

• Do not design pipe wall with the B31.3 allowable stress if the system is being built to ASME Section VIII-1 - the Div 1 value is about 17 percent lower
• Specify maximum hardness if the line is sour or cathodically protected; hardness above about 330 HV10 raises hydrogen induced stress cracking risk
• Order with a documented heat number chain from melt to finished pipe, and require marking that survives handling
• For subsea and deepwater service, check subsea flange and component requirements alongside the pipe specification

Order to ASTM A240 or ASME SA-240, specify the solution annealed and water quenched condition, add PREN, ferrite, hardness, impact and corrosion testing as supplementary requirements, and specify ultrasonic examination for plate that will be welded into a pressure boundary. Thickness drives almost every additional requirement.

 

Plate is where thickness effects bite hardest. A 10 mm plate quenches evenly. A 60 mm plate has a core that cools far more slowly than its surface, and the core is where sigma phase and an unbalanced phase fraction hide. This is why heavy plate specifications call for testing at quarter-thickness and mid-thickness, and why ultrasonic examination to a standard such as ASTM A578 or equivalent matters before you cut expensive shapes out of it.

 

Decision	Options	How to decide
Specification	ASTM A240 / ASME SA-240	The only flat-rolled specification for this grade; confirm the edition on your order
Condition	Solution annealed at 1025 - 1125 C and water quenched	Require the certificate to state temperature, soak time and cooling medium
Thickness range	Typically 3 mm to about 80 mm in production	Above roughly 40 mm, require through-thickness testing and discuss quench capability with the mill before ordering
PREN	Minimum 40 (NORSOK) or 41 (ASTM supplementary requirement)	Write the formula on the order: percent Cr + 3.3 x percent Mo + 16 x percent N
Corrosion test	ASTM G48 Method A at 50 C for 24 hours	No pitting at 20x and weight loss below 4.0 g/m2 on the as-delivered surface
Intermetallic phase check	ASTM A923 Method A, B or C	Method B Charpy and Method C corrosion give quantitative evidence; Method A is a screen
Impact testing	Charpy V-notch at -46 C for thickness at or above 6 mm	45 J average, 35 J single, tested at the location the specification names
Ultrasonic examination	ASTM A578 or equivalent, with the acceptance level stated	Detects mid-thickness laminations before they become a weld repair
Surface finish	Hot rolled annealed and pickled; No.1 or 2B depending on route	Match the finish to the service; a rougher surface pits more readily
Cutting	Plasma, laser or waterjet with the heat-affected zone removed	Thermal cut edges must be ground back to sound metal before welding

 

For general guidance on specifying stainless plate dimensions and finishes, see the stainless steel plate buying guide and how to choose plate thickness. For the product itself, see alloy plate supply.

Against 316L, S32750 roughly doubles proof strength and lifts PREN from about 25 to about 42. Against 2205 it adds roughly 5 PREN points and about 100 MPa of proof strength. Against S32760 it is nearly identical in strength and PREN, and the difference is the deliberate copper and tungsten in S32760 that helps in reducing acids.

 

This is the decision most project teams actually face, and it is worth stating plainly: the choice between these four grades is a choice about which corrosion mechanism governs. If the mechanism is chloride pitting or crevice corrosion, PREN is your ranking tool. If it is chloride stress corrosion cracking, the duplex microstructure matters more than PREN. If it is a reducing acid, neither PREN nor the microstructure saves you and you need copper or a nickel alloy.

 

Factor	S32750 (super duplex)	S32760 / F55	Duplex 2205 (S32205)	316L
UNS / grade	S32750, F53, 1.4410	S32760, F55, 1.4501	S32205, F51, 1.4462	S31603
Chromium / molybdenum / nitrogen	25 / 4.0 / 0.28	25 / 3.6 / 0.25	22 / 3.1 / 0.17	17 / 2.1 / 0.06
Typical PREN	41 - 43	40 - 42	34 - 38	About 25
Proof strength, min	550 MPa (80 ksi)	550 MPa (80 ksi)	450 MPa (65 ksi)	170 MPa (25 ksi)
Tensile strength, min	795 MPa (115 ksi)	750 MPa (108 ksi)	655 MPa (95 ksi)	485 MPa (70 ksi)
Chloride SCC resistance	Very good	Very good	Good	Poor above about 60 C
Reducing acid resistance	Fair	Better, copper-bearing	Fair	Poor
Welding difficulty	Highest of the four	Highest of the four	Moderate	Easiest
Relative material cost	High	High	Moderate	Lowest
Best used for	Seawater, brine, sour service, high-chloride process duty	Reducing acid plus chloride duty	Mild chloride duty, cost-sensitive projects	Non-chloride process and general service

 

• Need the copper and tungsten? See the full F53 versus F55 comparison
• Deciding between standard and super duplex? Read duplex 2205 versus super duplex 2507
• Weighing a nickel alloy instead? Compare with Inconel 625 versus 316L and Inconel 625
• Not sure whether any stainless grade survives your water chemistry? Start with what alloy to use for seawater piping

Almost every field failure traces back to one of five causes: thermal exposure that precipitated sigma phase, a weld made outside the heat input and interpass window, the wrong filler metal, a crevice that the design created and no alloy can survive, or the wrong material delivered into the order. Four of the five are preventable at the design and purchasing stage.

 

That fifth cause deserves emphasis because it is the one nobody budgets for. S32750, S32760 and 2205 look identical, machine identically and weigh the same. A plate marked 2507 that is actually 2205 will pass dimensional inspection, weld normally and then pit years early in seawater. Positive material identification is the only routine control that catches it, and it costs a few minutes per heat.

Problem	How you see it	Root cause	Corrective action
Sigma phase embrittlement	Charpy energy collapses; hardness rises above 32 HRC; pitting in service	Slow cooling through 600 - 950 C, or a subcritical stress relief applied by mistake	Full solution anneal at 1025 - 1125 C with water quench. On installed equipment, cut out and replace. Prevent by specifying ASTM A923 and quench documentation
475 C embrittlement	Progressive hardening and loss of toughness after years of service	Long-term exposure between 400 and 550 C	Limit service temperature. The condition is only reversible by full solution annealing, so control it in design, not in maintenance
Chromium nitride in the weld HAZ	Low HAZ toughness and reduced pitting resistance near the fusion line	Excessive heat input, or too slow a cooling rate through 600 - 900 C	Reduce heat input, enforce 100 C maximum interpass, add nitrogen to the shielding gas, and qualify the procedure on the production thickness
Excess ferrite in the weld	High hardness, low impact energy, susceptibility to hydrogen cracking	Heat input too low, so austenite did not reform on cooling	Raise heat input into the 0.5 - 1.5 kJ/mm band, use nitrogen-bearing shielding gas, and verify ferrite on the coupon
Under-matched weld metal	Preferential corrosion of the weld bead or the fusion line	ER2209 or a 316L filler used on S32750, or excessive dilution on a thin joint	Weld with ER2594 or E2594; on dissimilar joints match the higher-alloyed parent
Crevice corrosion at flanges and gaskets	Localised attack under gasket faces, lap joints and tube-to-tubesheet joints	Geometry, not alloy. A crevice defeats any stainless grade at the right temperature	Redesign to eliminate the crevice, use welded instead of flanged joints where possible, control chlorination and temperature, and select gasket material deliberately
Heat tint and iron contamination	Rust staining and shallow pitting after commissioning	Welding without purge, or carbon steel tools, slings and grinding wheels	Purge the root, pickle to ASTM A380, passivate to ASTM A967, and segregate stainless handling tooling
Hydrogen induced stress cracking under cathodic protection	Cracking in highly stressed, cathodically protected components	Combination of high hardness, high applied stress and hydrogen from cathodic protection	Limit hardness, control cathodic protection potential, reduce stress concentrations, and complete a dedicated HISC assessment
Wrong grade delivered	Material performs below expectation with no visible defect at receipt	Supply chain substitution of 2205 for 2507, or S32760 for S32750, without agreement	Positive material identification on every heat or piece; certificate review against the purchase order line by line
Pitting after hydrostatic testing	Corrosion found at or near the water line rather than at the lowest point	Chloride-bearing test water left standing, then drained slowly and not dried	Limit test water to about 50 ppm chloride, drain immediately, and dry all internal surfaces

 

Notice how many of those rows end with 'specify it on the purchase order'. That is not coincidence. Super duplex is a specification-driven material: the alloy is capable, but the capability only arrives if you ask for the tests that prove it.

What is Super Duplex S32750?

Super Duplex S32750 is a 25 percent chromium duplex stainless steel with a mixed ferrite-austenite microstructure, roughly 41 to 43 PREN and a minimum 0.2 percent proof strength of 550 MPa. It is supplied solution annealed and water quenched, and it is used for seawater, brine, high-chloride process duty and sour oil and gas service where 316L and standard duplex 2205 do not last.

 

Is 2507 the same as S32750?

Yes. 2507 is the mill trade name for UNS S32750. The same chemistry also appears as ASTM A182 grade F53 for forgings and as EN 1.4410 or X2CrNiMoN25-7-4 in European standards. Always put the UNS number on the purchase order so there is no ambiguity.

 

What is the difference between S32750 and S32760?

S32760 contains deliberate copper at 0.5 to 1.0 percent and tungsten at 0.5 to 1.0 percent, which improves resistance in reducing acids. S32750 has neither, with copper capped at 0.50 percent as a residual. Strength and PREN are very similar, so the choice rests on whether the process fluid is a reducing acid.

 

Is S32750 stronger than 316L?

Yes, substantially. ASTM A240 lists a minimum 0.2 percent proof strength of 550 MPa for S32750 against 170 MPa for 316L, and 795 MPa minimum tensile against 485 MPa. In practice that lets a designer reduce pressure-containing wall thickness appreciably for the same design conditions.

 

What is the PREN of S32750?

Typically 41 to 43, calculated as percent Cr + 3.3 x percent Mo + 16 x percent N. ASTM A240 carries a supplementary requirement for PREN above 41 and NORSOK M-630 sets a floor of 40.0. Always write the formula on the purchase order, because more than one PREN convention is in circulation.

 

What is the maximum service temperature of S32750?

ASME Section VIII-1 and B31.3 list SA-240 S32750 to 315 C, and B31.1 stops at 250 C. Metallurgically, continuous service is normally capped around 250 to 300 C because sigma phase forms from about 600 C and 475 C embrittlement develops between 400 and 550 C.

 

Can you stress relieve S32750?

No. A subcritical stress relief at 550 to 700 C precipitates sigma and chi phase and destroys both toughness and corrosion resistance. The only heat treatment this alloy receives is a solution anneal at 1025 to 1125 C followed by water quenching.

 

What heat treatment does S32750 require?

Solution annealing at 1025 to 1125 C followed by rapid water quenching. There is no ageing, no tempering and no normalising step. The certificate should state the annealing temperature, the soak time and the cooling medium.

 

What filler metal do you use to weld S32750?

ER2594 for GTAW and GMAW, E2594 for SMAW, and ER2594 with an argon back purge for root runs. Do not use ER2209 or a 316L filler, because dilution with the parent metal would leave the weld metal with less chromium and molybdenum than the plate it joins.

 

What heat input should be used when welding super duplex?

Roughly 0.5 to 1.5 kJ per millimetre is the practical window, with 2.5 kJ/mm a common absolute ceiling. Too little heat input leaves the heat-affected zone over-ferritic and brittle; too much allows sigma, chi and nitride precipitation.

 

What is the maximum interpass temperature for S32750?

100 C for 25Cr super duplex. Standard 22Cr duplex such as 2205 is commonly allowed up to 150 C, and the tighter limit for super duplex is one of the details most often missed on site.

 

Does S32750 need post weld heat treatment?

No. PWHT for duplex stainless steel is not a stress relief in the carbon steel sense. It can only be a full solution anneal at 1025 to 1125 C with a water quench, which is normally impractical on a fabricated assembly and unnecessary if the welding procedure is controlled.

 

What is sigma phase and why does it matter?

Sigma phase is a brittle intermetallic compound rich in chromium and molybdenum that precipitates in the ferrite between roughly 600 and 950 C, fastest near 800 to 850 C. It can cut Charpy impact energy by 80 percent or more and removes alloying elements from the surrounding matrix, so pitting resistance falls at the same time.

 

What is 475 C embrittlement?

Between about 400 and 550 C, and worst near 475 C, the ferrite phase separates into chromium-rich alpha prime and iron-rich alpha. Hardness rises and toughness collapses progressively with time. It is the reason duplex service temperature is capped well below the code allowable limit.

 

How do you test for sigma phase in S32750?

ASTM A923 is the standard route. Method A is a sodium hydroxide electrolytic etch with metallographic review, Method B is Charpy impact testing, and Method C is a ferric chloride corrosion test. A falling Charpy energy and pitting in Method C together are conclusive.

 

Is S32750 magnetic?

Yes. The ferrite phase makes duplex stainless steel ferromagnetic, and a magnet will stick to it. This is normal and not a sign of incorrect supply, but it matters where magnetic signature is controlled or where a magnet is being used as a quick sorting tool.

 

Is S32750 suitable for seawater?

Yes, it is one of the standard alloys for seawater service. Typical critical pitting temperature is 55 to 70 C and typical critical crevice temperature is 40 to 50 C, which puts it well above 316L and above duplex 2205. Design still governs: crevices, stagnation and chlorination practice decide the outcome as much as the alloy does.

 

Is S32750 suitable for sour service?

Yes. It is listed in NACE MR0175 / ISO 15156-3 in the solution annealed condition. Maximum hardness is commonly 32 HRC, and many sour seawater projects tighten that to 28 HRC. The H2S partial pressure, chloride, temperature and pH envelope must be checked against the current edition for the specific application.

 

What is the hardness limit for S32750 in sour service?

32 HRC is the commonly cited maximum under ISO 15156-3 for 25Cr duplex in the solution annealed condition. Projects frequently specify 28 HRC where cathodic protection and hydrogen induced stress cracking are concerns. NORSOK M-630 also caps hardness at 32 HRC, 301 HB or 330 HV10.

 

Which ASTM specification covers S32750 pipe?

ASTM A790 covers seamless and welded ferritic-austenitic stainless steel pipe, and ASME adopts it as SA-790. ASTM A789 covers tube, ASTM A928 covers electric fusion welded pipe in classes 1 to 5, and ASTM A815 covers wrought buttwelding fittings.

 

Which ASTM specification covers S32750 plate?

ASTM A240, adopted by ASME as SA-240. It requires solution annealing between 1025 and 1125 C with a water quench, and it lists 795 MPa minimum tensile, 550 MPa minimum proof strength, 15 percent minimum elongation and 310 HBW or 32 HRC maximum hardness.

 

What is the ASME allowable stress for SA-240 S32750?

About 33.1 ksi (228 MPa) at 38 C from ASME Section II Part D Table 1A for Section VIII Division 1 construction, falling to roughly 29.4 ksi at 315 C. ASME B31.3 Table A-1 lists about 38.7 ksi at ambient. The difference is a safety factor, not a material difference, so use the table that matches your code of construction.

 

What is the ferrite content requirement for S32750?

35 to 55 percent by ASTM E562 point count under NORSOK M-630, measured near the surface and at mid-thickness, with the microstructure free of intermetallic phases at 400x magnification. Weld metal ranges are set by the project specification and are commonly wider than the parent metal range.

 

How do you tell S32750 apart from 2205 on receipt?

You cannot by eye. Both are magnetic, both look identical and both machine the same way. Use positive material identification to confirm chromium at 24 to 26 percent, molybdenum at 3 to 5 percent and nitrogen at 0.24 to 0.32 percent, and check the heat analysis on the EN 10204 certificate against the purchase order.

 

What should be on the mill certificate for S32750?

The UNS number, the specification and edition, the heat analysis, mechanical test results, the solution annealing temperature, soak time and cooling medium, PREN with the formula, ferrite content, hardness, impact energy and test temperature, ASTM G48 Method A results, and ASTM A923 results where specified, issued to EN 10204 Type 3.1 or 3.2.

 

Why is S32750 more expensive than 316L?

Chromium, molybdenum and nickel content drive the raw material cost, and the melting, hot working and quenching route is tighter. Against that, roughly double the proof strength lets you buy less tonnage, use less weld metal and install lighter supports, so the installed cost gap is considerably smaller than the mill price gap.

 

When should you choose S32750 instead of duplex 2205?

Choose S32750 when chloride pitting or crevice corrosion governs and the temperature or chloride concentration is beyond what PREN 34 to 38 can carry, when sour service requires the higher alloy, or when you need the extra proof strength to reduce wall thickness. Choose 2205 when the duty is milder and cost matters more.

 

Can you weld S32750 to carbon steel?

Yes. Butter the carbon steel face with ER2594 or a suitable transition filler first, then complete the joint so the final fusion zone never forms a hard martensitic band. Control heat input and interpass as you would for a duplex-to-duplex joint, and verify hardness across the transition.

 

What causes pitting in S32750 after a hydrostatic test?

Standing chloride-bearing test water. Limit test water to about 50 ppm chloride, drain the system immediately after the test and dry all internal surfaces. Damage from hydrotesting almost always appears at a water line rather than at the lowest point of the vessel.

 

How thick does S32750 plate need to be before extra testing is required?

The main trigger is 6 mm: NORSOK M-630 requires Charpy V-notch testing at -46 C for thickness at or above 6 mm. Above roughly 40 mm the concern shifts to quench capability, and specifications call for testing at quarter-thickness and mid-thickness rather than at the surface only.

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