​13CrMo4-4 (now commonly designated 13CrMo4-5 or 1.7335)​​ is a specific grade of ​chromium-molybdenum (Cr-Mo) alloy steel​​, standardized primarily under ​European norms (EN 10028-2:2017)​​ as ​13CrMo4-5​​ and under its historical ​DIN designation 1.7335​​.

It's widely used for ​high-temperature and high-pressure applications​​ due to its combination of good ​creep strength​​, ​oxidation resistance​​, and ​weldability​​.

Here's a breakdown of its key aspects:

​Chemical Composition (Typical Range - EN 10028-2):​​

​Carbon (C):​​ 0.08 - 0.18%

​Silicon (Si):​​ ≤ 0.50%

​Manganese (Mn):​​ 0.40 - 1.00%

​Phosphorus (P):​​ ≤ 0.025%

​Sulfur (S):​​ ≤ 0.010%

​Chromium (Cr):​​ 0.70 - 1.15%

​Molybdenum (Mo):​​ 0.40 - 0.60%

The "13CrMo" indicates approx. ​1.3% Cr​​ (historically from mean Cr content range), and "44" (or "4-5") indicates the Mo content range.

​Material Number:​​

​1.7335​​ (Unified Material Number according to EN 10027-2).

​Key Properties & Characteristics:​​

​Good Creep Strength:​​ Can withstand high loads/stress at elevated temperatures over long periods without significant deformation (creep). Suitable for temperatures up to approximately ​550-580°C (1022-1076°F)​​ for long-term service.

​Good Oxidation Resistance:​​ The chromium content forms a protective oxide layer, resisting scaling in steam, flue gas, and other oxidizing atmospheres at high temperatures.

​Good Weldability:​​ Generally weldable using suitable procedures and filler metals (like G 24CrMo4-5 or AWS/ASME equivalents).

​Good Ductility & Toughness:​​ Exhibits good mechanical properties at ambient and elevated temperatures.

​Good Thermal Stability:​​ Maintains strength well at high temperatures.

​Requires Heat Treatment:​​ Typically supplied in the ​Normalized (+N)​​ or ​Normalized and Tempered (+NT)​​ condition to achieve the necessary microstructure and mechanical properties.

​Mechanical Properties (EN 10028-2, for thickness t ≤ 16mm, +NT condition):​​

​Yield Strength (Rp0.2):​​ ≥ 290 MPa (min)

​Tensile Strength (Rm):​​ 450 - 600 MPa

​Elongation (A):​​ ≥ 20% (min)

​Impact Toughness (KV):​​ ≥ 40 J (tested at -10°C to +20°C, depends on thickness and test temperature specification).

​Delivery Conditions:​​

​+N:​​ Normalized

​+NT:​​ Normalized and Tempered (most common for pressure vessel/piping applications).

​Common Applications:​​

​Power Generation:​​ Boiler tubes, superheater tubes and headers, steam pipes & fittings (within its temp range), pressure vessels, heat exchangers for fossil fuel plants.

​Petrochemical & Refining:​​ Piping, tubing, pressure vessels, heat exchangers operating at elevated temperatures and pressures in hydrogen and hydrocarbon service.

​Industrial Furnaces:​​ Components subjected to heat.

​High-Temperature Pipework:​​ Transporting steam or hot fluids.

​Important Considerations:​​

​Mandatory Post-Weld Heat Treatment (PWHT):​​ Welded components must undergo PWHT after welding. This is critical to:

Relieve harmful welding stresses.

Restore the required toughness in the Heat-Affected Zone (HAZ).

Maintain creep strength.

Temper any hard, brittle microstructures formed during welding. The temperature range for PWHT is typically ​650°C - 700°C (1202°F - 1292°F)​​.

​High-Temperature Design Values:​​ Material allowable stresses for design at different temperatures are defined in pressure vessel codes like EN 13445, ASME Section II Part D, AD 2000 Merkblätter, etc. Always use the latest applicable design code.

​Similar Grades:​​

​ASTM/ASME:​​ A213 T11, A335 P11, A199 T11, A387 Gr.11 Cl.1 (Closest equivalents in chemical composition, though properties and exact ranges vary).

​ISO 9327-1:​​ 14CrMo4-5 (Former designation, largely superseded by EN).

​JIS G4109:​​ STPA24, STBA24.

​In summary:​​ 13CrMo4-5 (1.7335) is a well-established Cr-Mo alloy steel chosen for its reliable combination of strength, oxidation resistance, and weldability at elevated temperatures up to ~550°C. Its primary use is in pressure parts for boilers, power plants, and refineries. The requirement for strict welding procedures and mandatory PWHT cannot be overstated for its safe application.