Welding of Creep-Resistant Steels: Importance of Preheating, PWHT and Proper welding consumables
Creep-resistant steels, also called heat-resistant steels, are designed for applications under high temperature and long-term mechanical stress. In installations such as steam generators, heat exchangers and pressure vessels, they are indispensable. The weldability of these steels places high demands on knowledge of metallurgy, stress control and heat input. Welding, if performed improperly, can lead to internal stresses, brittle microstructures, hot cracks, hydrogen cracks and reduced creep strength. Therefore, preheating and PWHT are not optional steps, but fundamental to a reliable joint.
When steel is subjected to a constant mechanical stress and a high temperature for a long period of time, it will slowly elongate even if that stress is lower than the yield strength. Creep becomes relevant at temperatures higher than about 0.4 times the melting temperature of the material in Kelvin. For steel, this is typically above 400 °C. For example, a steel pipe in a power plant with constant pressure and an operating temperature of 600 °C will deform slowly even if the stress remains constant.
Creep-resistant steels are also called heat-resistant materials. Creep-resistant steels are alloyed steels that maintain their strength at high temperature over extended periods of time. These steels are characterized by their high tensile strength, combined with high creep strength and high toughness, even at elevated temperature. To make a comparison, unalloyed steels are “only” applicable up to about 350°C, while the high vanadium alloyed CrMo(Ni) steels are applied up to about 650°C (depending on the alloy).
Elements that increase resistance to creep are carbon, chromium, molybdenum, vanadium and titanium. The more chromium and molybdenum, the higher the temperature at which the steels can be applied.
Examples:
- 16Mo3: Simple 0,5% Mo-steel up to 530°C
- 13CrMo4-5 / 10CrMo9-10: for installations up to 560 - 600°C
- X10CrMoVNb9-1 (P91): up to 620 - 650°C
| Steel grade | C (%) | Cr (%) | Mo (%) | V (%) | Nb (%) | Rm (MPa) | Rp0,2 (MPa) | Max. operatingtemp. (°C) |
|---|---|---|---|---|---|---|---|---|
|
16Mo3 |
0.12 - 0.20 | - | 0.25 - 0.35 | - | - | 450 - 600 | ≥ 280 | 530°C |
|
13CrMo4-5 (P11) |
0.08 - .018 | 0.70 - 1.15 | 0.40 - 0.60 | - | - | 440 - 590 | ≥ 290 | 570°C |
|
10CrMo9-10 (P22) |
0.08 - 0.14 | 2.00 - 2.50 | 0.90 - 1.10 | - | - | 510 - 670 | 300 - 450 | 600°C |
|
X10CrMoVnB9-1 (P91) |
0.08 - 0.12 | 8.00 - 9.50 | 0.85 - 1.05 | 0.18 - 0.25 | 0.06 - 0.10 | 620 - 850 | ≥ 450 | 620 - 650°C |
| Group | Description | Types |
|---|---|---|
|
1.1 |
C-Mo (0.5Mo) | 16Mo3 |
|
5.1 |
CrMo-steel: 0.75% ≤ Cr ≤ 1.5%, Mo ≤ 0.7% (1.25Cr - 0.5Mo) |
13CrMo4-5 |
|
5.2 |
CrMo-steel: 1.5% < Cr ≤ 3.5%, 0.7 % < Mo ≤ 1.2% (2.25Cr - 1.0Mo) |
10CrMo9-10 |
|
6.4 |
Martensitic: 7.0% < Cr ≤ 12.5%, 0.7% < Mo ≤ 1.2%, V ≤ 0.35% (9 - 12% Cr-Staal) |
X10CrMoVNb9-1 |
| P-No. | Description | Types |
|---|---|---|
|
4 |
1.25Cr – 0.5Mo | SA-182 F11 CL1 SA 213 T11 SA-335 P11 |
|
5A |
2.25Cr – 1Mo | SA-182 F22 CL1 SA-213 T22 SA-335 P22 |
|
5B |
5-9Cr – 0.5Mo | SA-182 F5 & F9 SA-213 T5 & T9 SA-335 P5 & P9 |
|
15E |
9Cr-1Mo-V | SA-335 P91 |
Pre-heating is essential when welding creep-resistant steels for several reasons:
By heating the base material before welding, you prevent the weld pool and base material from cooling too quickly. Slow cooling gives less chance of:
• Martensite formation (hard and brittle)
• Residual stresses
• Hydrogen cracks (Cold cracks)
Hydrogen, coming from moisture in welding consumables, shielding gas or base material can cause major problems. At low temperatures it can trap in the HAZ (heat-affected zone) which will change the microstructure and possibly cause cracks. Pre-heating accelerates the diffusion of hydrogen from the weld metal before it forms harmful microstructures (such as martensite).
In structures with large wall thicknesses, preheating ensures that there are no large temperature differences between the cold base material and the hot weld material. Without preheating, the base material would absorb the heat of the weld metal too quickly, which can lead to internal stresses and deformations in the material
Pre-heating of a grade P91 welding joint
Post Weld Heat Treatment (PWHT) of creep-resistant steels is necessary for several reasons, all related to the mechanical and metallurgical properties of the material after welding. PWHT is essential for creep-resistant steels for the following reasons:
During welding, thermal expansion and contraction occurs, resulting in residual stresses. When liquid weld metal solidifies, it will shrink, resulting in residual stresses. PWHT reduces these residual stresses.
Creep-resistant steels have a specially designed microstructure (often ferritic-perlitic, bainitic or martensitic) that provides creep resistance at high temperature. Welding disrupts this microstructure, especially in the heat-affected zone (HAZ). Martensitic steels, such as P91, contain a hard and brittle weld metal after welding and in the HAZ. PWHT causes conversion of martensite to tempered martensite, with fine carbides along the grain boundaries. This increases creep resistance and ductility.
Without PWHT, there is a high risk that the welded part will fail prematurely during prolonged exposure to high temperatures and stresses. PWHT stimulates the formation of stable carbides that stabilize the steel structure in the long term and increase creep resistance.
Example of PWHT diagram from Grade P91
GTAW:
Root welding, low hydrogen diffusion
GMAW:
Root welding, fill and Cap
FCAW:
Production welding in position
SMAW:
Robust, fieldwork
The choice of a welding consumable for welding creep-resistant steels is crucial to ensure the mechanical properties and creep resistance of the welded joint. This choice is based on several important criteria:
• The welding filler material must be chemically compatible with the base material.
• Often, a filler material with slightly higher alloying elements is chosen in order to maintain the desired strength and creep resistance after welding.
• At high temperatures, the welding filler material must be resistant to creep deformation.
• Materials must be resistant to temperature aging and oxidation.
• Typical operating temperatures are often >450°C, and sometimes up to 600–650°C.
• Yield strength, tensile strength, and creep strength must be equal to or slightly higher than those of the base material.
• Fatigue strength and toughness must be sufficient, especially in transition zones.
• Most filler materials require Post Weld Heat Treatment (PWHT) to reduce stresses and normalize structure.
• The additive material must be resistant to this treatment without becoming brittle.
| Base materiaal | GTAW | GMAW | FCAW | SMAW |
| 16Mo3 / P1 | CEWELD SG Mo Tig CEWELD ER80S-D2 Tig |
CEWELD SG Mo | CEWELD AA R Mo | CEWELD E 7018-A1 |
| 13CrMo4-5 / P11 | CEWELD SG CrMo1 Tig CEWELD ER 80S-B2 Tig |
CEWELD SG CrMo1 CEWELD ER 80S-B2 |
CEWELD AA R CrMo1 | CEWELD E 8018-B2 |
| 10CrMo9-10 / P22 | CEWELD SG CrMo2 Tig CEWELD ER 90S-B3 Tig |
CEWELD SG CrMo2 CEWELD ER 90S-B3 |
CEWELD AA B CrMo2 | CEWELD E 9018-B3 |
| X10CrMoVNb9-1 / P91 | CEWELD ER90S-B9 (P91) Tig | CEWELD ER90S-B9 (P91) | CEWELD AA 90S-B9 | CEWELD E 9018-B9 |
Important: Always use matching welding consumables. Wrong choice of materials can lead to differences in creep resistance, stress concentrations and will lead to cracks in the HAZ.
Example of cracks in the heat-affected zone in a CrMoV weld made with 2.25Cr-1Mo filler material.
(A) Macro of the fracture
(B) Micro of the fracture location (not in the same weld)
(C) Example of the microstructure in the heat affected zone
Gjerde, M. (2018). Designing with urban daylight: A social agenda. Lighting Research & Technology, 50(3), 366–380. https://doi.org/10.1080/09506608.2017.1410943
| Material | Preheat (°C) | Interpass (°C) | PWHT (°C) | Holdtime (min/mm Minimum 30 minutes |
| 16Mo3 / P1 | 100 - 150 | <250 | 580 - 620 | 2 min/mm |
| 13CrMo4-5 / P11 | 150 - 200 | <300 | 630 - 700 | 2 min/mm |
| 10CrMo9-10 / P22 | 200 - 250 | <300 | 660 - 700 | 4 min/mm |
| X10CrMoVNb9-1 / P91 | 200 - 250 | <300 | 740 - 780 | 4 min/mm |
Notice:
Cooling too quickly after welding Grade P91 gives untempered martensite and causes brittleness. Without timely PWHT, non-optimal precipitates occur which seriously deteriorates long-term mechanical properties, such as creep resistance.
| Causes | Consequence |
| No preheating | Hydrogen cracks, hard HAZ |
| No PWHT or too short PWHT | Embrittlement, creep cracking |
| Excessive interpass temperature | Coarse-grained structure, loss of creep strength |
| Incorrect welding consumable selection | Cracks in the HAZ |
| Cooling too quickly | Material remains martensite resulting in brittleness |
- Pre-heating prevents cracking and improves hydrogen diffusion
- PWHT is essential for stress relieving and metallurgical stabilization
- Use correct welding consumables, equivalent to base material
- Control interpass temperatures to prevent coarse-grained areas