Heat-Resisting Stainless Steel: 304H, 309S, 310S, 321 and 347

A heat-resisting stainless steel is a stainless steel selected for service in hot air, combustion gases or process streams at temperatures above about 500 degrees Celsius. At these temperatures a steel must meet three requirements that do not arise at room temperature: it must form an oxide scale that protects the metal beneath, it must carry its load without slow continuous stretching, known as creep, and its structure must stay stable over years of exposure. The austenitic grades used for this service fall into three groups. The H grades, such as 304H, keep a minimum carbon content for creep strength. The stabilised grades 321 and 347 bind their carbon with titanium or niobium so that long exposure does not destroy their corrosion resistance. The high-chromium grades 309S and 310S resist scaling to 1,095 and 1,150 degrees Celsius in continuous service. The service temperatures in this article are maximum temperatures in dry air from published handbook tables. A temperature limit always belongs to a stated atmosphere and to continuous or intermittent heating.

Oxidation resistance and scaling temperature

Oxidation resistance is the ability of a steel to form a thin, adherent oxide layer that slows further attack of the metal. In stainless steel the protective oxide is rich in chromium, and the higher the chromium content, the higher the temperature at which the layer stays protective. Silicon and rare earth elements such as cerium improve the adhesion of the layer. Above a certain temperature the oxide grows fast, cracks and flakes away, and the metal loses section steadily. That temperature is the scaling temperature, and handbooks state it as a maximum service temperature in dry air.

Handbooks give two figures for each grade. Continuous service means the part stays hot. Intermittent service means the part is heated and cooled repeatedly. Austenitic steel expands more on heating than its chromium oxide does, so each cooling cycle cracks the scale and exposes fresh metal. For the austenitic grades the intermittent limit is therefore lower than the continuous one.

These figures are oxidation limits in clean, dry air. They are not strength limits, and they fall in atmospheres that contain water vapour, sulphur, chlorides or fuel ash. A part that carries a load reaches its useful limit at a lower temperature, set by creep. How these limits compare with the melting range of the same grades is set out in the melting point of steel article.

Creep strength and the H grades

Creep is the slow, continuous deformation of a metal under a constant load at high temperature. Austenitic stainless steels begin to creep at about 500 to 550 degrees Celsius. Above that range the design stress of a part comes from creep and stress-rupture data for the grade and not from its room-temperature yield strength. The ASME Boiler and Pressure Vessel Code, Section II Part D, gives allowable stresses for each grade up to a stated temperature; for 304 the listing stops at 816 degrees Celsius.

Carbon raises creep strength. Carbon in solution and in fine carbides hinders the sliding of grain boundaries and the movement of dislocations at temperature. The H grades exist for this reason. AISI 304H specifies 0.04 to 0.10 percent carbon, where 304 sets only a maximum of 0.08 percent and 304L a maximum of 0.03 percent, and it requires a grain size of ASTM No. 7 or coarser, because coarse grains have less boundary area to slide. The same pattern gives 309H, 310H, 321H and 347H. The H grades have the same room-temperature minimum strengths as their standard grades; they differ in the allowable stresses at 550 to 750 degrees Celsius. Pressure codes specify 304H for structural and pressure-containing parts between about 500 and 800 degrees Celsius, in superheater and reheater tubing, steam headers and refinery piping.

The low-carbon L grades are the wrong choice for this service. Their low carbon protects a weld from sensitisation, but it also lowers creep strength, and a 304L part loaded at 650 degrees Celsius creeps sooner than a 304H part.

309S and 310S

AISI 309S and AISI 310S are the high-chromium austenitic grades for scaling resistance. 309S contains 22.0 to 24.0 percent chromium and 12.0 to 15.0 percent nickel; 310S contains 24.0 to 26.0 percent chromium and 19.0 to 22.0 percent nickel, with silicon up to 1.5 percent. The extra chromium raises the scaling temperature, and the extra nickel keeps the structure fully austenitic despite the chromium, which on its own would form ferrite. 310S has the higher limits of the two, 1,035 degrees Celsius intermittent and 1,150 degrees Celsius continuous, and it is the usual grade for furnace parts, radiant tubes, muffles, retorts, heat treatment baskets and fixtures, and kiln supports.

The suffix fixes the carbon. AISI 310 permits up to 0.25 percent carbon, 310S limits it to 0.08 percent, and 310H requires 0.04 to 0.10 percent for creep strength. 309 and 309S follow the same pattern at 0.20 and 0.08 percent. The lower carbon of the S grades leaves less chromium carbide at the grain boundaries after welding.

The high nickel of 310S is a weakness in reducing atmospheres that contain sulphur. Nickel and sulphur form a low-melting compound that attacks the grain boundaries, so 310S can fail faster in reducing sulphur-bearing gas than a lower-nickel grade. Grades for that duty rely on silicon and rare earth additions rather than on nickel. 310S resists oxidising and carburising atmospheres well. Its properties and limits in detail are on the AISI 310S grade page.

The stabilised grades 321 and 347

Austenitic stainless steel held between about 425 and 860 degrees Celsius forms chromium carbides at its grain boundaries. The metal next to each boundary loses chromium and, once the part cools and meets water, corrodes along the boundaries. This change is sensitisation. A weld passes through the range in seconds, and a low carbon content is enough to protect it; a part that operates continuously in the range stays there for years, and low carbon does not protect it.

The stabilised grades solve the problem by adding an element that binds carbon more strongly than chromium does. AISI 321 adds titanium, at least five times the carbon plus nitrogen and at most 0.70 percent. AISI 347 adds niobium, at least ten times the carbon and at most 1.00 percent. The carbon forms titanium or niobium carbide, and the chromium stays in solution. Both grades keep the ordinary carbon level, and with it a higher strength at temperature than an L grade. Both have the scaling limits of 304, 870 degrees Celsius intermittent and 925 degrees Celsius continuous, and their typical uses are expansion joints and bellows, exhaust systems, superheater parts and hot process piping.

The two grades differ in welding and in stability. Titanium does not transfer across a welding arc, so 321 is welded with a 347 consumable, and 347 is far more common as filler metal than as parent metal. Niobium carbide stays stable to a higher temperature than titanium carbide, which gives 347 more reliable protection near the top of the range. 321H and 347H add a minimum carbon content for creep strength. Both stabilised grades contain no molybdenum and resist chlorides no better than 304. The general mechanism of sensitisation is described in the corrosion of stainless steel article.

EN 10095 heat-resisting grades and S30815

EN 10095, Heat resisting steels and nickel alloys, is the European standard for steels made for service in hot gases and combustion products. It is separate from EN 10088, which covers the corrosion-resisting stainless steels. Its austenitic grades include 1.4828 (X15CrNiSi20-12), 1.4833 (X12CrNi23-13), 1.4835 (X9CrNiSiNCe21-11-2), 1.4845 (X8CrNi25-21) and 1.4878 (X8CrNiTi18-10). The nearest counterpart of 310S is 1.4845. The match between systems is less exact for heat-resisting grades than for corrosion-resisting grades, and the standard stated on an order governs.

Two of these grades raise oxidation resistance with silicon rather than with more chromium and nickel. 1.4828 contains 19 to 21 percent chromium, 11 to 13 percent nickel and 1.5 to 2.5 percent silicon, and resists scaling in air to about 1,000 degrees Celsius. UNS S30815, which corresponds to 1.4835, contains 20 to 22 percent chromium and 10 to 12 percent nickel, with 1.4 to 2.0 percent silicon, 0.14 to 0.20 percent nitrogen and 0.03 to 0.08 percent cerium. Its producers give a maximum service temperature in air of about 1,150 degrees Celsius, the same as 310S, at about half the nickel. The cerium improves the adhesion of the scale under thermal cycling, and the nitrogen and controlled carbon raise creep strength.

Sigma phase and embrittlement in service

Sigma phase is a hard, brittle intermetallic compound of iron and chromium. In austenitic stainless steel it forms slowly during long exposure at about 600 to 900 degrees Celsius, faster in high-chromium grades and in any ferrite present. In 310S it precipitates at roughly 650 to 900 degrees Celsius. Sigma raises strength slightly but lowers toughness and ductility at room temperature, so a part that worked safely when hot can crack when it is cooled, handled or repaired. It also removes chromium from the surrounding metal.

Solution treatment dissolves sigma: 1,040 to 1,150 degrees Celsius for 310S followed by rapid cooling. Furnace parts in 310S that run above 650 degrees Celsius are re-annealed periodically, about every 1,000 hours, to restore ductility. Installed pipework cannot be annealed in this way, so its design uses aged properties rather than the properties as supplied. Long service also coarsens carbides and, in unstabilised grades, sensitises the steel. A header that stays hot is unaffected, because intergranular corrosion needs water, but the same header corrodes along its grain boundaries if wet chloride reaches it during a shutdown. The heat treatments involved are described in the heat treatment of steel article.

Grade selection by temperature band

The table ranks the grades by the property that governs each temperature band. The bands overlap, and the atmosphere, the load and the presence of water at shutdown move a grade up or down.

Above about 1,150 degrees Celsius, and in strongly carburising or sulphidising gas, the austenitic stainless steels give way to nickel-chromium alloys. Duplex stainless steel has no place in heat-resisting service, because its ferrite embrittles above about 300 degrees Celsius.

Heat-resisting bar from Laxcon Steels

Laxcon Steels rolls hot rolled round bar from 16 to 125 mm in lengths up to 8 metres, with solution annealing among the heat treatments supplied. AISI 310S round bar and AISI 321 round bar are rolled to order, and so are 310S equal angles, annealed and pickled. The composition limits, designations and limits of 304H, 321, 347 and 310S are set out on their grade pages in the grade reference.

Frequently asked questions

Which stainless steel resists the highest temperature?

Among the standard austenitic grades, 310S resists scaling to the highest temperature: about 1,150 degrees Celsius in continuous service and 1,035 degrees Celsius in intermittent service in dry air. S30815 reaches a similar limit with less nickel. These are oxidation limits; a part under load is limited by creep at a much lower temperature.

What is the difference between 321 and 304 stainless steel?

AISI 321 is 304 with a titanium addition that binds carbon, so it does not sensitise during long service between about 425 and 860 degrees Celsius. The two grades have the same room-temperature minimum strengths and the same scaling limits. 321 is chosen for welded parts that work hot; 304 or 304L is chosen below that range.

What is the difference between 309 and 310 stainless steel?

AISI 310 contains more chromium and nickel than 309: 24 to 26 percent chromium and 19 to 22 percent nickel, against 22 to 24 and 12 to 15. Its scaling limits are higher, 1,150 degrees Celsius continuous against 1,095 for 309. 309 is used where the lower temperature allows it.

What is the difference between 304 and 304H?

AISI 304H requires 0.04 to 0.10 percent carbon and a grain size of ASTM No. 7 or coarser, where 304 sets only a carbon maximum of 0.08 percent. The higher carbon and coarser grain give 304H greater creep strength above about 500 degrees Celsius. Room-temperature minimum strengths are the same.

Why is the intermittent service temperature lower than the continuous one?

Austenitic stainless steel expands more on heating than its oxide scale does. Each cooling cycle cracks the scale and exposes fresh metal to oxidation, so a part that is heated and cooled repeatedly loses metal at a lower temperature than a part that stays hot.

Sources

• Stainless Steel World, March 2021, "Austenitic stainless steel grades for high-temperature applications", Table 1: maximum service temperatures in dry air for 304, 309, 310, 316 and 321, after the Atlas Steels technical handbook.
• Atlas Steels grade data sheets for 304/304H, 310/310S/310H, 321/321H and 347: scaling limits, sigma phase, solution and stabilising treatments.
• Outokumpu Therma 253 MA and Alleima 253 MA data sheets: UNS S30815, EN 1.4835, maximum service temperature in air.
• Swiss Steel Group and Deutsche Edelstahlwerke data sheets for 1.4828: scaling resistance in air.
• EN 10095, Heat resisting steels and nickel alloys.
• ASTM A240/A240M, Chromium and chromium-nickel stainless steel plate, sheet and strip: compositions of the H, S and stabilised grades and of S30815.
• ASTM A276/A276M and ASTM A479/A479M, stainless steel bars and shapes.
• ASME Boiler and Pressure Vessel Code, Section II Part D, allowable stresses.