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INTRODUCTION• High strength low alloy (HSLA) steels are designed to provide conventional carbon steels better mechanical properties and/or greater resistance to atmospheric corrosion than a hardening mechanism.• The material for a pressure retaining component should have a sufficient strength and fracture toughness for the assurance of the structural integrity. For a nuclear reactor vessel, a resistance to an irradiation embrittlement is also an important property. In general, the fracture toughness of a material is decreased when its strength is increased by a hardening mechanism.
INTRODUCTIONLow-alloy steels (LAS) are widely used for pressure vessel andpiping in light water reactors. The reactor pressure vessel (RPV) isthe most critical pressure-boundary component as far as safetyand plant life are concerned. The possible effect ofenvironmentally- assisted cracking (EAC) on RPV structuralintegrity, therefore, continues to be a key concern within thecontext of both reactor safety and evaluation/extension of plantservice life.
INTRODUCTIONThe HSLA steels have low carbon contents (0.05-0.25% C) in orderto produce adequate formability and weldability, and they havemanganese contents up to 2.0%. Small quantities of chromium,nickel, molybdenum, copper, nitrogen, vanadium, niobium,titanium and zirconium are used in various combinations.
The various types of Microalloyed• Vanadium microalloyed steels• Niobium microalloyed steels• Niobium-molybdenum steels• Vanadium-niobium microalloyed steels• Vanadium-nitrogen microalloyed steels• Titanium microalloyed steels• Niobium titanium microalloyed steels• Vanadium-titanium microalloyed steels
HEAT TREATMENTS OF LOW ALLOY STEELS• Most of the engineering properties of metals and alloys are related to their structure. Equilibrium structure can be predicted for an alloy with the help of an equilibrium diagram.• Mechanical properties can be changed by varying the relative proportions of micro constituents. In practice, change in mechanical properties is achieved by a process known as heat treatment.• This process consists of heating a metal or alloy to a specific predetermined temperature, holding at this temperature for required time, and finally cooling from this temperature. All these operations are carried out in solid state.
Heat treatment may be undertaken for the following purposes:• Improvement in ductility• Relieving internal stresses• Refinement of grain size• Increasing hardness or tensile strength
RADIATION EMBRITTLEMENT OF LOW-ALLOY STEELS• Neutron irradiation of reactor pressure vessel (RPV) steels increases density of point defects, enhances diffusivity of all atoms in solid solution and produces phase transformations, precipitation, micro voids, etc. that results in considerable change in mechanical properties of low-alloy steels.• The most dangerous of them are the loss of plasticity and increase of brittle fracture. The prediction of radiation embrittlement of RPV materials during their operation is of great applied importance.
CHARACTERIZATION of Ni–Mo–Cr LOW ALLOY STEELS FOR NUCLEAR APPLICATION• The Mn–Mo–Ni low alloy steels such as SA508 Grade 3 and SA533 Grade B, have been used widely for nuclear reactor pressure vessels for more than 30 years due to a combination of their good strength, toughness and weldability in addition to economy.• Several types of advanced PWR’s (pressurized water reactors) are under development, which are from a smaller modularized reactor to a much larger capacity reactor than the currently operating reactors.• ferritic low alloy steels still have a priority under the operating conditions of PWR type reactors.• , Ni and Cr are known to be effective elements for an increase of the hardenability of ferritic steels.
EFFECT OF CYCLIC STRAIN RATE ON LOW ALLOY STEEL• Low alloy steels (LASs) used as the structural materials of nuclear power plants (NPPs) are subject to cyclic stress during plant operation. Consequently, fatigue damage is one of the most significant degradation mechanisms of them.• Moreover, fatigue crack growth rate is accelerated in the high temperature water environment of NPPs, thereby reducing the fatigue life Therefore, the environmental fatigue behaviors of LASs should be considered to assess the integrity and the safety of NPPs.