ASME SA213 TP321H Stainless Steel Tubing for High Temperature and Creep Service

Overview of ASME SA213 TP321H Tubing

ASME SA213 TP321H stainless steel tubing is the high carbon variant of the titanium stabilised 18-8 austenitic grade, supplied as seamless tube for boiler, superheater and heat exchanger service under ASME SA213 and its ASTM A213 equivalent. The additional carbon compared with plain TP321 is deliberate: it raises elevated temperature strength and creep resistance, which is what allows the grade to be used in tubes that operate hot for decades.

The material is produced over a wide range of sizes and is normally delivered after hydrostatic testing and intergranular corrosion testing, so that both pressure integrity and resistance to grain boundary attack are demonstrated before the tube is installed.

TP321H Compared with TP321

The two grades look similar in a specification table but they are intended for different duties, and the differences are worth stating precisely.

Carbon content: TP321H requires 0.04 % to 0.10 % carbon, while TP321 allows up to 0.08 % with no minimum, which lifts the high temperature strength of the H grade.

Titanium requirement: TP321H needs titanium of at least four times the combined carbon and nitrogen content, whereas TP321 requires at least five times, in both cases with 0.70 % as the maximum.

Grain size: the H grade is controlled to a coarser grain size than the standard grade, because coarse grains improve creep behaviour at elevated temperature.

Typical duty: TP321 suits general corrosive service at moderate temperature, while TP321H is selected for tubes that must hold pressure in the creep range.

Substituting one grade for the other is therefore a design decision rather than a purchasing convenience, and the allowable stress tables for the two grades differ at elevated temperature.

Chemical Composition

The table gives the specified heat analysis ranges for TP321H.

Element	Requirement, %	Element	Requirement, %
Carbon	0.04 - 0.10	Chromium	17.0 - 19.0
Manganese	2.00 max	Nickel	9.0 - 12.0
Silicon	0.75 max	Titanium	4 x (C + N) min to 0.70 max
Phosphorus	0.045 max	Nitrogen	Residual, as specified
Sulphur	0.030 max	Molybdenum	Not specified for this grade

Molybdenum is not a specified element in this grade, so a composition table that lists a molybdenum range for 321H is mislabelled. The only deliberate strengthening additions are carbon and titanium, and the nitrogen content is limited so that the titanium requirement remains achievable within the 0.70 % ceiling.

Mechanical and Physical Properties

Property	Requirement
Tensile strength	515 MPa minimum
Yield strength, 0.2 % proof	205 MPa minimum
Elongation in 50 mm gauge length	40 % minimum
Hardness, Rockwell B	95 HRB maximum
Hardness, Brinell	217 HB maximum

For thermal and mechanical design the following typical physical values apply: density 8027 kg/m3, elastic modulus 193 GPa, mean coefficient of thermal expansion about 16.6 µm/m per degree Celsius from 0 to 100 Celsius, 17.2 from 0 to 315 Celsius and 18.6 from 0 to 538 Celsius. Thermal conductivity is about 16.1 W/m·K at 100 Celsius and about 22.2 W/m·K at 500 Celsius, specific heat at room temperature is about 500 J/kg·K and electrical resistivity is about 720 nΩ·m.

Standard Equivalents

Standard system	Designation
United States, UNS	S32109
European, EN number and name	1.4878, X10CrNiTi18-10
Japan, JIS	SUS 321H
United Kingdom, old BS designation	321S51

The higher carbon content is reflected in the UNS number and in the EN name, so a certificate quoting S32100 or 1.4541 describes TP321 rather than TP321H and should be queried before the material is accepted.

Size Range, Testing and Delivery Condition

TP321H seamless tube is produced with outside diameters from about 6 mm to 762 mm and wall thicknesses from 0.5 mm to 50 mm, with lengths reaching 18000 mm; special combinations can be negotiated with the mill before rolling rather than after.

Hydrostatic testing to demonstrate pressure integrity, or an agreed alternative non destructive test.

Intergranular corrosion testing to confirm that the titanium stabilisation is effective in the delivered condition.

Chemical analysis and tensile testing per heat, with hardness results reported against the maximum limits.

Dimensional inspection of outside diameter, wall thickness, ovality and straightness.

Tube is normally supplied in the solution annealed condition, and where the finished component will be hot formed or welded the heat treatment condition should be confirmed before fabrication begins. Pickling or bright finishing is agreed with the purchaser according to the cleanliness requirement of the process fluid.

High Temperature Applications

Superheater and reheater tubes in boilers, where creep strength governs the wall thickness.

Heat exchanger tubes in refinery and petrochemical units handling hot hydrocarbon and hydrogen bearing streams.

Feed and discharge lines around hydrogenation reactors.

Auxiliary steam generator and economiser tube bundles.

Molten salt heat exchangers in concentrated solar thermal plants, where the tube must tolerate thermal cycling as well as high temperature.

Where the tube is welded into a header or tube sheet, matching filler metal of the stabilised type is used and the welded joint is normally given a solution anneal, because the coarse grain structure that gives TP321H its creep strength also makes the heat affected zone sensitive to premature cracking if it is left in a stressed condition.

FAQ

Q: What distinguishes ASME SA213 TP321H from TP321?
TP321H has a higher minimum carbon content, a lower titanium multiple requirement and a controlled coarser grain size, all of which raise elevated temperature strength and creep resistance compared with standard TP321.

Q: What is the UNS number for TP321H?
The grade is designated UNS S32109, with EN 1.4878 and the name X10CrNiTi18-10, and the Japanese designation SUS 321H is also widely used.

Q: Does TP321H contain molybdenum?
No. Molybdenum is not a specified element in this grade, and composition tables that quote a molybdenum range for 321H are referring to a different steel.

Q: What size range can be supplied?
Outside diameters from about 6 mm to 762 mm and wall thicknesses from 0.5 mm to 50 mm are produced, with lengths up to 18000 mm and special sizes available by agreement.

Q: Which tests are normally performed before delivery?
Chemical analysis and tensile testing per heat, hardness testing, dimensional inspection, hydrostatic or an agreed non destructive test, and intergranular corrosion testing to verify the stabilising treatment.

Q: Why is intergranular corrosion testing added for this grade?
Because the tube operates hot and is usually welded, the test confirms that titanium has stabilised the carbon and that the grain boundaries will not be attacked during service or after fabrication.