Which Steel Grade for Steam Lines in a Power Plant?

Steam lines are among the most heavily stressed components in a power plant. They carry media at high temperature and high pressure over long service lives – and often for decades. The choice of steel grade is therefore not a matter of taste, but a design decision that depends directly on the operating parameters. This article provides technical orientation on the material spectrum and the key criteria. The specific design remains a matter for the responsible specialist planning.

What the operating parameters dictate

Three quantities largely determine the material choice: the operating temperature, the internal pressure and the required service life. At higher temperatures, steel loses strength, and a time-dependent failure under load occurs – creep. Decisive is therefore no longer the yield strength at room temperature, but the creep rupture strength: the stress that a material withstands at a given temperature over a defined period (typically calculated for 100,000 hours and more) without failure. Added to this is scaling resistance, i.e. the resistance to scaling on hot surfaces. Where saturated steam is carried under moderate conditions, different requirements apply than with superheated steam.

The material spectrum at a glance

In simplified terms, a graduation can be described that leads, with rising temperature and pressure loading, from non-alloy to alloyed creep-resistant steels:

• P235GH, P265GH – non-alloy or low-alloy creep-resistant steels for saturated steam and moderate operating conditions.
• 16Mo3 – molybdenum-alloyed; the Mo improves the creep strength and shifts the field of application to higher temperatures.
• 13CrMo4-5 – chromium-molybdenum-alloyed; the chromium raises creep strength and scaling resistance further.
• 10CrMo9-10 – more highly chromium-molybdenum-alloyed for superheated steam at higher temperatures and pressures.

The exact application limits of each grade follow from the material data sheets and the relevant standards – not from blanket temperature values. A sound assignment is always made on the basis of the actual design parameters.

The right standard for pipe and plate

For seamless tubes for pressure purposes made of creep-resistant steels, EN 10216-2 is the governing standard. It regulates materials, technical delivery conditions and testing for precisely this application. For plates and flat products made of pressure-vessel steels with elevated temperature properties, EN 10028-2 applies accordingly. Anyone who assigns material and standard cleanly avoids the most common source of error – namely a material that bears the right designation but was not supplied in the version required for pressure applications.

Welds and traceability

With creep-resistant steels, the processing determines the operational safety of the entire line. The weld is regularly the critical point. Practically relevant are, among others:

• a cleanly executed TIG root for the inner contour of the weld;
• preheating according to material and wall thickness, to avoid hardening and cracking;
• with the Cr-Mo grades, depending on the specification, a post-weld heat treatment to reduce residual stresses and to set the microstructure.

Equally important is seamless traceability. For pressure-bearing components, an inspection certificate 3.1 to EN 10204 is standard – it documents the tested properties of the specific batch and is the basis of all documentation vis-à-vis the operator and inspection body.

In short: The steel grade for steam lines follows the operating parameters – from P235GH/P265GH for saturated steam through 16Mo3 to the Cr-Mo grades 13CrMo4-5 and 10CrMo9-10 for superheated steam, designed according to creep rupture strength and scaling resistance. Tubes are governed by EN 10216-2, plates by EN 10028-2, and the weld as well as the inspection certificate 3.1 determine the operational safety. The binding design belongs in every case in the hands of the specialist planning.