Welding – widely used repair technology
Welding is a well-established technology in the repair of turbine stator and turbine rotor parts. Our carefully-designed repair procedures offer “good as new” performance of repaired components. Procedures are designed to suit your requirements, and are carried out by our highly skilled and well-trained welders.
A wide variety of alloys are used in gas turbines and each type presents specific challenges for welding. These challenges are addressed in different aspects of our welding procedures, such as:
- Welding atmosphere
- Interpass temperature
- Filler material composition
- Welding sequence
- Cooling rates
- Heat input rate during welding
- Post-weld heat treatment
Designing a weld repair procedure presents many challenges:
- Cracking sensitivity of the base material
- Degenerated structure of the base material in components exposed to service
- Cracking sensitivity of the weld filler
- Oxidation of the base material and weld filler material during welding
- Distortion due to thermal contraction during and after welding
- Hardness and brittleness of the weld and the Heat Affected Zone (HAZ) after welding
- High internal stress level of welded components
- Differences between the structure of the weld material and the base material
The following groups of gas turbine alloys require extra attention and special welding procedures:
- Low-alloy steels
- Cast-alloy steel and cast iron
- Ferritic/martensitic stainless steels
- Austenitic stainless steels, duplex steels
- Nickel- and cobalt-based sheet metal alloys
- High gamma prime forged and cast nickel based alloys
- DS and SX nickel base alloys
- Titanium alloys
Welding can fill in missing material with 100% bonding, thereby restoring components to 100% of the original dimensions and properties. Weld repairs are carried out by melting and depositing a filler metal on the material to repair.
Welding is based on two key principles:
- the fusion metal has to be molten
- the solidified material must have acceptable properties after the process
Cast steel and cast iron parts in gas turbines can usually be repaired by welding, provided the limited strength of the weld heat-affected zone is taken into account. Many cases of severe impact damages and corrosion damages thus can be repaired using this process.
Ferritic/martensitic stainless steels are widely used for compressor blades and steam turbine blades. Damages can be repaired by welding when cleaning and heat treatment procedures are well-designed and strictly adhered to. Since blades and vanes are not always easy to remove, repair welding sometimes has to be carried out in an assembled condition, using local heat treatment on the repair area.
Our service professionals know that a well-designed procedure and diligent execution are an absolute must for repairs with local heat treatment.
Nickel and cobalt-based sheet metal alloys are ductile alloys used in combustion components. These sheet metal alloys are not sensitive to stress corrosion and can usually be welded without any problems. Post-welding heat treatments are only required for components where residual stresses may affect resistance to high-cycle fatigue.
High gamma prime forged superalloys are commonly used in gas turbine low-pressure blades and in turbine disks. High gamma prime nickel-based alloys can only be repaired with lower alloyed weld filler materials that are less susceptible to cracking. Well-designed repair procedures take all of these factors into account and limit repairs to areas where the weld filler strength is adequate. Micro cracking is minimized by controlling heat input during welding.
DS and SX nickel-based alloys can be welded conventionally, like conventionally-cast components. The weld repair must be designed to take into account the lower strength of the available filler materials. Laser welding offers the potential to produce welds that have a continuation of the DS or SX structure.
Titanium welding requires special attention because of the sensitivity of titanium alloy for the pickup of oxygen. When this happens, the alloy becomes extremely brittle. Titanium welding should only be considered in facilities that allow welding with very low oxygen concentration. Sulzer has facilities equipped for titanium welding.
Repair welding is typically used to restore bearing surfaces, rabbet fits, coupling surfaces, and seal areas.