P91 Tabla PWHT

P91 steel is an attractive material for power boilers and other high-temperature applications, offering excellent combinations of mechanical properties at higher temperatures such as strength, creep resistance and Charpy impact toughness.

Recent studies have demonstrated that different conditions for normalizing and tempering (NT) of P91 steel welds can produce optimal mechanical properties in the HAZ without needing post weld heat treatment.

Hardness

SA 335 grade 91 is a modified high-chromium-moly martensitic steel designed for elevated temperatures in power plants and other high performance applications, such as welding. It boasts good creep strength and toughness as well as corrosion resistance and weldability; however, during welding or postweld heat treatment (PWHT), its hardness may decrease leading to cracking or early failure.

Studies on the effects of various welding processes, types of filler wires and preheat/interpass temperatures on P91 were once rare; however, recently it has become more prevalent to repair P91 pipelines with dissimilar welds using similar welding processes and filler metal as original welds; this requires HAZ hardness levels being lower than their parent metal; how this can be accomplished within reasonable amounts of time in PWHT awaits further investigation.

Some power plant projects reported type IV cracks in their weld HAZ during operation. These cracks were likely the result of insufficient PWHT. Our investigation concluded that PWHT at 750 degC for two hours is ideal to reduce hardness in weld HAZ while providing sufficient microstructure and an acceptable hardness difference with parent metal.

Creep Strength

Creep strength of p91 measures how much stress a material can withstand before deforming, with creep resistance depending on factors like grain size and operating temperature as well as chemical composition that impacts transformation temperatures and other properties of its composition.

To establish the creep limit of a metal, designers use a ratio between rupture stress and its stress-deformation curve, extrapolated up to 100,000 hours, and calculated minimum creep rates over this timeframe. Rupture stress measures both end point rupture stress as well as necessary stress to reach this point – it is reported as both stress and time values.

Martensitic heat resistant p91 steel (9Cr-1Mo-V-Nb) is commonly employed in fossil fuel power plants for boiler headers and other components due to its excellent steam corrosion resistance, low thermal expansion rate, high thermal conductivity and weldability; however in-service failures of components constructed of this metal have been reported after relatively short service duration.

Failures occur because of reduced creep strength properties due to fine-grained areas in the heat affected zone (HAZ).

Charpy Toughness

P91 steel is ideal for use in long-term power plant components that operate at high temperatures, including those exposed to creep and thermal load resistance, high ductility and minimal thermal expansion. As its oxidation resistance falls below 610 degC, its applications for power generation become limited. Furthermore, poor impact toughness limits its use under dynamic loads; to enhance this property further it is recommended to post-weld heat treat (PWHT). DIN EN 288-3 provides mechanical tests on real weld HAZ specimens, in which crack propagation through each subzone differs depending on its microstructure and results in variations in impact energy between tests.

Multi-pass shielded metal arc welds of P91 and P22 steel produce poor impact toughness in their weld fusion zones (FGHAZs). This is due to an accumulation of untempered martensite in this area due to improper PWHT.

Studies have demonstrated the effects of diffusible hydrogen level and PWHT duration on weldments’ mechanical properties and hardness gradient. Pandey et al. investigated this factor’s effect on microstructure and tensile properties for multi-pass welded P91 plate weldments using SMAW with four different hydrogen levels, showing higher diffusible hydrogen and PWHT duration resulted in improved tensile properties but decreased impact toughness.

Resistencia a la corrosión

Grade 91 steel is known for its excellent corrosion resistance; however, stress-induced martensite and creep damage may cause corrosion to occur. Postweld heat treatment (PWHT) may help mitigate these effects; however, this method may not always be feasible or cost effective, particularly during repairs in service.

As part of its objective to develop weld repair procedures for grade 91 without PWHT, the research conducted in this project focused on investigating the metallurgical behavior of materials during welding and heat-treating processes, by evaluating microstructures of weld and HAZ specimens produced after welding; mechanical tests were then conducted on real welds to measure HAZ toughness.

Comparisons were also performed on different welding techniques, focusing on their effect of preheat/interpass temperatures and welding currents. Electrodes with diameters matching those found in grade 91 proved most suitable for developing welding procedures without PWHT; using combinations of SMAW electrodes of diameters 3.2mm (1/8in, first layer) and 4.0mm (5/32in, second layer) produced significant refinement while tempering was limited within HAZs.

Postweld heat treatments of TIG weld joints between P22 and grade 91 steels using Inconel 625 filler metal are best performed at 750 degC for 2 hours to obtain optimal compromise between hardness of interface and weld metal properties, while maintaining high mechanical properties across a broad temperature spectrum.