Grade Identity and Where It Sits in the Standard System
13CrMo44 is a low-alloy chromium-molybdenum steel for elevated-temperature service. It appears in DIN 17175 for seamless tubes, carries the material number 1.7335, and is applied to low and medium pressure boiler heating surfaces, superheater and economiser tubing and to petrochemical piping. The equivalent modern European designation is grade 13CrMo4-5 in EN 10216-2, which superseded the older DIN 17175 delivery conditions; the closest American counterpart is ASME SA335 grade P11 for high-temperature piping.
Because the grade is a creep-resisting alloy rather than a plain carbon steel, both chemistry and heat treatment have to be controlled together with the cold forming route.
Chemical Composition and Mechanical Properties
Typical specification limits for 13CrMo44 are given below. They follow the classic DIN 17175 table for the grade and match the 13CrMo4-5 composition in EN 10216-2.
| Element | Requirement (%) |
|---|---|
| Carbon | 0.10-0.18 |
| Silicon | 0.10-0.35 |
| Manganese | 0.40-0.70 |
| Phosphorus (max) | 0.035 |
| Sulphur (max) | 0.035 |
| Chromium | 0.70-1.10 |
| Molybdenum | 0.45-0.65 |
| Room Temperature Property | Requirement |
|---|---|
| Tensile strength Rm | 440-590 MPa |
| Minimum yield strength ReH | 290 MPa |
| Delivery condition | normalised or normalised and tempered |
| Test temperature for impact | room temperature unless specified otherwise |
Cold Drawing Route for 13CrMo44 Tubes
For cold rolled or cold drawn 13CrMo44 tube, a pierced and hot rolled mother shell is first pickled and lubricated, then drawn over a mandrel or plug in successive passes with intermediate annealing. Cold working is used to reach the tight wall tolerance and concentricity demanded by heat exchanger service, and each drawing pass requires a lubricant and phosphate or oxalate coating that will not introduce surface defects.
After the final draw, the tube is heat treated to restore the ferritic-pearlitic structure and to meet the Rm and ReH figures above, then straightened, cut and given the required surface finish. Because cold reduction strongly affects residual stress, cold drawn tubes are often supplied in a normalised or normalised-and-tempered condition rather than as cold worked.
Tolerance, Testing and Inspection
Seamless tubes for this service are generally ordered with wall thickness tolerance tighter than for plain carbon grades, because the wall controls both pressure rating and heat transfer. Routine inspection includes dimensional and visual examination, 100% non-destructive testing of the body (ultrasonic or eddy current) and a hydrostatic test where specified. Impact testing is normally requested when the wall thickness is sufficient to extract a standard specimen, and the delivery condition is fixed in the purchase order because it decides the acceptance limits.
Ordering information should state the standard and edition, grade designation, outside diameter and wall thickness, length, delivery condition, test category and any third-party inspection requirement. Where the specification must match an existing installation, both the DIN grade name and the EN designation should be quoted so that the mill can confirm equivalence.
Frequently Asked Questions
Q: What is the material number for 13CrMo44?
A: The material number is 1.7335. The grade is listed in DIN 17175 for seamless tubes and appears as 13CrMo4-5 in EN 10216-2.
Q: What are the typical chromium and molybdenum contents?
A: Chromium 0.70-1.10% and molybdenum 0.45-0.65%, with carbon 0.10-0.18%. The combination provides the elevated-temperature strength for boiler and petrochemical service.
Q: Is 13CrMo44 equivalent to ASME SA335 P11?
A: The two are very close lower-chromium-molybdenum grades and are frequently treated as counterparts, but they are not interchangeable under every code. The applicable design code decides which is acceptable for a given pressure and temperature.
Q: Why use a cold drawn tube instead of a hot rolled one?
A: Cold drawing gives tighter wall tolerance, better concentricity and a smoother surface, which matters for heat exchanger tubing. It also requires controlled heat treatment after drawing to restore the specified properties.
Q: What non-destructive testing is normal for these tubes?
A: 100% ultrasonic or eddy current testing of the body plus a hydrostatic test where specified, together with dimensional and visual checks. Additional impact or hardness tests can be added as an order option.