P91 PWHT diagramm

SI was called into assess whether heat had had any detrimental impact on P91 after a fire at a power plant, taking metalurgical replicas for analysis to assess any softening in HAZ regions.

Gressler stressed the fact that hardness alone does not determine material health. Instead, an effective screening measure would be monitoring creep strain accumulation using high-temperature strain gauges for creep strain accumulation monitoring as well as engineering analysis to predict its lifetime.

Hardness

P91 steel family is a 9-12% Cr ferritic/martensitic stainless steel used in nuclear and power plant environments to achieve superior mechanical properties at elevated temperatures. Due to its low carbon, niobium, and nitrogen contents it reduces welding-induced cracking; however due to its higher hardenability it may form an excessively hard heat affected zone (HAZ), leading to hydrogen induced cracking – leading to failure of both welds and components.

SI was recently called upon by Siemens plant to assess a large weld repair of a pressure vessel valve following an incident involving fire, in order to assess whether any off-spec material may have resulted from it and, if that were indeed the case, whether this could be corrected before its warranty expired.

SI performed an analysis on the material and determined that its softness was not due to fire but was likely caused by ineffective PWHT.

PWHT temperature and duration are of vital importance in creating dissimilar welds with optimal microstructure and hardness, such as this study conducted with multi-pass shielded metal arc welds of Inconel 625 as filler metal. Results demonstrated that using PWHT at 750 degC for 2, 4, or 6 hours as the preferred PWHT time and temperature achieved both, effectively decreasing hardness in HAZ regions while simultaneously maintaining an appropriate microstructure to enable high performance in mechanical properties at elevated temperatures.

Toughness

P91 steel was developed specifically for use in Gen IV nuclear reactors. Due to its high chromium content and addition of vanadium and niobium as alloying elements, P91 offers significantly increased creep resistance compared to its lesser alloyed counterparts.

Material properties: this alloy is highly weldable and features excellent thermal fatigue and corrosion resistance, yet is susceptible to hydrogen-assisted cracking (HAC). Both gas and solid arc welding methods may be used; however, post weld heat treatment must take place for optimal creep properties.

HAC in P91 weldments may be caused by incorrect welding conditions and incorrect filler material selection, and may also be compounded by non-uniform weld geometry and use of slag. Additionally, it may manifest as dissimilar welds between grade 91 weldments and other weldable materials such as P22 or Inconel 625 welding processes.

Note it is currently impossible to repair grade 91 components without subsequent PWHT due to current weld repair methodologies that produce welds with unrefined microstructure in the HAZ and only limited tempering taking place. Therefore, additional extensive weld repair trials and testing must be conducted with regards to HAZ toughness and cross-weld creep strength, along with an analysis of any causes for failure such as grain coarsening and microstructure issues.

Elastic Modulus

The elastic modulus is a measure of how easily materials can be stretched or bent, and is determined by dividing their stress by their strain. A stress-strain curve test is an invaluable way to identify this parameter; small incremental strains are applied, then plotted onto a graph revealing their elastic modulus values – essential when calculating mechanical properties of materials!

Stress-strain curves typically exhibit linear behavior, signifying that materials are elastic. This behavior is explained by Hooke’s law which states that stress applied to materials should be proportional to its strain; Young’s modulus measures this correlation and measures stress divided by strain.

Stress-strain curves typically feature nonlinearity at a certain point when material has entered its plastic deformation region, signaling plastic deformation. At this stage, stress and strain no longer vary inversely; instead they respond predictably to stresses. Elasticity of material is determined by slope of non-linear portion of curve; this equation gives its values: EDL L0 L L 0/D L 0/D

Korrosioonikindlus

Grade 91 steel (EN designation X10CrMoVNb9-1) is an alloyed 9Cr-1Mo steel with high levels of chromium, vanadium and niobium that boasts improved creep resistance. This grade of 9Cr-1Mo is often utilized for nuclear power plant headers as well as other cladding and ducting applications at nuclear power plants and has outstanding radiation damage resistance as well as oxidation and corrosion resistance at elevated temperatures.

Dissimilar Shielded Metal Arc Welded (SMAW) joints formed of P91 and P22 steels can produce weak impact toughness due to the formation of untempered martensite in the weld fusion zone, necessitating selection of an optimal preheating and postweld heat treatment (PWHT) temperature range for successful welds.

PWHT temperatures vary based on the composition and percentage of manganese and nickel in steel, specifically when considering Mn + Ni content; higher percentages lower the critical transformation temperature.

Studies were performed to investigate the impact of diffusible hydrogen levels and heat treatment conditions on the microstructure and mechanical properties of multi-pass Shielded Metal Arc Welded (SMAW) grade 91 butt weldments using field emission scanning electron microscopy, mercury diffusible hydrogen measurement, room-temperature tensile testing, Charpy testing and energy dispersive X-ray spectroscopy. Results demonstrated that using subcritical PWHT with normalizing/tempering treatments produced an optimal microstructure with good tensile properties as well as Charpy toughness.