Metal strength changes with temperature, but a single alloy name is not enough to choose a value. Product form, heat-treatment condition, test basis and the property being compared all matter. This reference brings together selected minimum 0.2% proof-strength values for Alleima Sanmac 304/304L bar and thermally treated Sanicro 69 steam-generator tube. Use each row only for the named product condition; the values are not design allowables.

Temperature effects on metal strength

What the values describe

Proof strength is the stress associated with a specified permanent strain. The table uses the 0.2% proof-strength column, Rp0.2, in megapascals. One MPa is one newton per square millimetre. It does not mix proof strength with ultimate tensile strength, hardness or creep strength.

The Sanmac 304/304L source reports minimum high-temperature values for bar steel tested in its delivery condition. The Sanicro 69 rows are the supplier’s higher guaranteed proof-strength values for steam-generator tubes after thermal treatment. Those are distinct products and conditions. Do not compare them as though they were interchangeable grades or identical test populations.

How to use the table

First identify the exact grade, product form and condition on the material certificate or specification. Then select a listed test temperature and read the minimum proof strength attached to that source row. If the source does not list the needed temperature, do not interpolate or extrapolate from a neighboring row. Obtain the applicable product data or controlling specification.

For example, Alleima lists 155 MPa minimum Rp0.2 for Sanmac 304/304L bar at 100 °C and 92 MPa at 500 °C. That is a 63 MPa difference within this specific published table. It demonstrates why the room-temperature value should not be carried into a hot-service calculation. It does not provide a creep allowable or a safe stress for a component.

Conditions and limits

Temperature-dependent tensile tests are not the same as long-term exposure tests. At elevated temperature, time under load, oxidation, thermal cycling, environment, section size, welds and prior heat treatment can change performance. A short-duration tensile value cannot establish resistance to creep or stress rupture. The values also do not replace pressure-vessel, piping, structural or material-selection codes.

The Sanicro 69 source specifically separates general material values from a higher guaranteed proof-strength table for thermally treated steam-generator tubing. This article uses the latter only with that condition named in every row. Do not apply those figures to solution-annealed tube, bar, plate or a different alloy designation.

Avoid common mistakes

Do not treat ultimate tensile strength as the yield or proof limit. The data sheet lists these as separate columns because they answer different questions. Also do not assume that a value at 500 °C establishes performance over years at that temperature. Long-duration service needs the controlling design basis and time-dependent material data.

Compare equivalent property definitions and source conditions before selecting an alloy. If you need a certified design value, check the applicable code edition, product specification, heat number, thickness range and qualification records with the material supplier or responsible engineer.

For a real hot component, also check thermal gradients and restraint. A component may experience a different temperature through its wall, and constrained thermal expansion can add stress even when the applied mechanical load stays constant. The table contains material test points, not a coupled thermal-stress analysis or a limit for a particular geometry.

The values should be treated as supplier-specific product data. A different melt, size range or processing route may have separate requirements. If the delivered certificate and published data sheet do not match, use the governing specification and resolve the discrepancy before using a value in a design review.

Sources

Values are transcribed selectively from Alleima’s Sanmac 304/304L bar data sheet and Sanicro 69 tube data sheet. Consult the current data sheet and governing material specification for a specific design.