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Gas Turbine Rotor Life AssessmentpdfLanguages:
Heat treatment
Withstanding high temperatures using appropriate treatment
Turbine and compressor parts are often exposed to high temperatures during operation. This can change the steel structure and result in failure. Sulzer offers specialized heat treatment processes to improve strength, ductility, corrosion resistance, creep resistance, and hardness.
Alloyed steels
These are used for turbine shafts and turbine casings, as well as for the blades and vanes of both compressors and steam turbines.
- Steel hardening: Steel that contains carbon can be heated to a red hot condition in which it is very weak and ductile, and can be shaped easily. Quenching in water will make it very hard and strong, but also very brittle. This hardness can be reduced by tempering at an intermediate temperature.
- Well-designed heat treatments can make use of these effects in order to create high-strength yet ductile steels. We use these heat treatments to optimize mechanical properties of your components and increase the operational life of your equipment.
- Low-alloy steels contain a lower percentage of alloying elements. These alloys have different crystal structures below and above approx. 800°C. When they are heated or cooled, these materials recrystallize whenever this temperature is passed. Such a process can be used to refine grains in large forgings and castings. A fine-grained structure offers better strength and ductility.
- When cooled down rapidly, recrystallization will not be complete. The resulting structure is called “martensite”. Martensite contains high internal stress levels. It is both hard and strong, but also brittle, and therefore is too sensitive to cracking for most applications.
- An extra heat treatment at an intermediate temperature (300°C to 750 C) reduces the cracking sensitivity and the hardness to appropriate values.
- Any activity, such as welding, that locally heats an alloy over 800°C will inevitably create martensite in that area upon cooling. Welds will thus be unacceptably brittle unless a proper heat treatment is performed.
- Sulzer has these procedures in place and can customize them to your needs.
The properties of most materials are determined by the manufacturing heat treatment as well as by their composition. For most materials, heat treatments are an absolute necessity in order to obtain the required strength, ductility, creep resistance, corrosion resistance, or hardness.
The type of heat treatment required varies considerably for different groups of materials. For a given material, different heat treatments may be required for manufacturing, welding, hardening, or restoration.
Experienced welding personnel
Every day our experienced staff welds turbine shafts and turbine casings, as well as blades and vanes of both compressors and steam turbines. A key factor for success is a well-designed heat treatment procedure during and after the welding operation.
Austenitic steels and forged superalloys
These alloys are typically used in combustion components and similar low-stressed components.
The alloys contain a high percentage of alloying elements. Their austenitic structure is dense, which is beneficial for creep resistance, and it is stable at all temperatures. Since there is no phase change, austenitic alloys will not harden by thermal cycling in a heat treatment or during welding.
Solution heat treatments can be used in finished components to dissolve brittle phases and grain boundary carbides. Overheating can create irreparable grain growth. We select the process with care to avoid damage to your components.
High-alloyed and cast superalloys
These alloys are used in blades and vanes in the high-temperature section of gas turbines.
- Superalloys contain elements that create secondary phases. Secondary phases can be seen as grit particles embedded in the crystals and along the alloy’s grain boundaries. Their presence greatly increases creep resistance. For the best quality, their shape and distribution must be optimized.
- In cobalt-based alloys, carbides are these secondary phases. In nickel-based alloys, aluminum (plus titanium, niobium and/or tantalum) combines with nickel to form Ni3Al. This phase is usually called “gamma prime” (γ’) and the particles are usually referred to as “precipitates”.
- These secondary phases, which can make up to 50% or more of the alloy’s volume, are created and shaped by heat treatments. Although their hardening effects are low, these heat treatments are often called “precipitation-hardening heat treatments”. Their goal is not to harden the alloy but to create a fine distribution of the particles that is optimal for creep resistance at specific temperatures.
- Fine precipitates are best for high strength and intermediate temperature creep resistance; coarse precipitates are best for high-temperature creep resistance.
- High alloyed and cast superalloys operate at high temperatures. “Operation” can be considered as a long time heat treatment that can create substantial changes over time. Fine precipitates tend to coalesce or to dissolve, and coarse precipitates tend to become even coarser.
- Heat treatment of these alloys, both in production and in restoration, consists of a high-temperature solution heat treatment followed by one or more precipitation steps at intermediate temperatures. Both the temperature and duration of these steps must be selected with care and intermediate cool downs may also be required.
Restorative heat treatments
As described above, used components may have developed a modified (degenerated) structure. A degenerated structure exhibits coarsening of existing phases and formation of new but deleterious phases, like the sigma-phase which is extremely brittle.
These phases create local inhomogeneous areas but the overall composition of the alloy remains unaffected. A restorative heat treatment starts with a very high-temperature solution heat treatment that dissolves both precipitates and deleterious phases. After this step, a modified version of the standard heat treatment is used to recreate the intended original structure and distribution of the precipitates.
Hot Isostatic Pressing (HIP)
A HIP treatment is a heat treatment in a high-temperature solution carried out with 1,000 to 2,000 bar of argon pressure. This isostatic pressure closes internal voids that may be present in a casting when the alloy is weak and ductile at high temperatures. It is a proven technology for improving precision-cast products such as gas turbine blades and vanes.Heat treatments are crucial steps in the service and repair of turbine and compressor components. Our extensive experience with heat treatment can increase the lifetime of your components. Our experts tailor the procedure for every alloy, component, and component condition.