Grade Identity and Standard History
13CrMo44 is a chromium-molybdenum heat resistant steel originally listed in DIN 17175, the German standard for seamless tubes of heat resistant steels, with material number 1.7335. The designation indicates approximately 0.13 percent carbon, about 1 percent chromium and about 0.5 percent molybdenum, which is why the steel is generally described as a 1.25Cr-0.5Mo alloy. DIN 17175 has been withdrawn and the grade now appears in EN 10216-2 as 13CrMo4-5, retaining material number 1.7335. Old drawings, spare part listings and Chinese documentation often still carry the DIN 17175 designation, and the material route is the same grade.
The steel sits in the middle of the low alloy heat resistant family. It offers more creep strength than carbon steel but is less highly alloyed, and therefore cheaper and more easily welded, than the 2.25Cr-1Mo grade and the 9 percent chromium grades used at higher steam temperatures.
Chemical Composition
| Element | Typical specification range | Function |
|---|---|---|
| Carbon | 0.10 - 0.17 percent | Strength, carbide formation |
| Silicon | 0.35 percent maximum | Deoxidation |
| Manganese | 0.40 - 0.70 percent | Strength, deoxidation |
| Phosphorus | 0.025 percent maximum | Restricted for toughness and creep ductility |
| Sulphur | 0.020 percent maximum | Restricted for hot workability |
| Chromium | 0.70 - 1.10 percent | Oxidation resistance, carbide stability |
| Molybdenum | 0.45 - 0.65 percent | Creep strength at temperature |
The combination of chromium and molybdenum is what gives the steel its creep resistance. Chromium stabilises the carbides that carry load at temperature and provides resistance to steam side oxidation, while molybdenum strengthens the ferrite matrix and slows the coarsening of carbides during long term exposure.
Heat Treatment and Delivery Condition
Tubes are normally supplied in the normalised and tempered condition. The normalising treatment produces a uniform ferritic-bainitic structure, and the subsequent tempering step is carried out at a temperature above the intended service temperature, which stabilises the carbide distribution before the tube enters service. The heat treatment temperatures are recorded on the material certificate, because creep performance is directly related to the condition in which the tube is supplied.
Cold finished tubes receive a final heat treatment after the last cold forming pass, since the cold work would otherwise reduce creep ductility. For superheater and reheater applications the tube is commonly supplied with a controlled surface condition on the inside diameter, free of heavy scale, to limit steam side oxidation in service.
Service Limits and Applications
The grade is used for superheater and reheater tubes, steam piping, headers and economiser sections where the metal temperature exceeds the practical range of carbon steel but remains below the range where higher chromium grades are required. The permissible metal temperature is fixed by the creep rupture data adopted in the design code, and in German practice the material data sheets have traditionally provided those creep values. Above the practical temperature ceiling the design must move to the 2.25Cr-1Mo grade or to a 9 percent chromium grade, which is precisely how boiler superheater and reheater stages are graded from lower to higher temperature along the steam path.
For orientation, the American counterpart A335 grade P11, UNS K11597, specifies a minimum room-temperature tensile strength of 415 MPa and a minimum yield strength of 205 MPa, and the EN grade is in the same band. As with any cross-standard substitution, the certificate must be checked against the design data rather than relying on the grade name.
Frequently Asked Questions
Q: What does the name 13CrMo44 mean?
A: It is a legacy German composition designation indicating roughly 0.13 percent carbon, about 1 percent chromium and about 0.5 percent molybdenum, the steel commonly described as 1.25Cr-0.5Mo.
Q: What is the current standard for this grade?
A: DIN 17175 is withdrawn; the grade is now covered by EN 10216-2 as 13CrMo4-5 with material number 1.7335.
Q: Is 13CrMo44 equivalent to A335 P11?
A: They are counterparts in the same composition family, but each standard has its own property tables, so an equivalence statement should be based on the actual certificate.
Q: Why is the tube normalised and tempered?
A: Normalising gives a uniform structure and tempering above the service temperature stabilises the carbides, which is what preserves creep strength over the design life.
Q: What service temperature range is typical?
A: It is used above the practical range of carbon steel and below the range where the higher chromium grades take over, with the exact ceiling set by the creep data adopted in the applicable design code.
Q: Where is the grade most often used in a boiler?
A: In superheater and reheater tube banks, steam headers and steam piping stages where the metal temperature exceeds the limit for carbon steel.