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Discontinuities may be classified as defects depending on acceptance criteria in a particular specification or code. Discontinuities are rejectable only if they exceed
specification requirements in terms of type, size, distribution, or location. Discontinuities may be found in the weld metal WM, heat-affected zones HAZ,
and base metal BM of weldments.
These may eventually lead to final failure by one of the instant failure modes described above. Welded joints are particularly susceptible to fatigue, typically
initiating from discontinuities at the weld toe. The region affected is called the heat affected zone that lies outside the fusion zone in pure metals and outside the partially
melted zone in alloys similar to the area in the undisturbed tank metal next to the actual weld material. Messler, R. W. 1999a. This area is less ductile than either the
weld or the steel plate due to the effect of the heat of the welding process. Literature show that HAZ is frequent where damage start to occur. This zone is most vulnerable
to damage as cracks are likely to start here. Thus, the zone is uncovered for exposure to influence the serious damage.
1.3 Objectives of project
This research project is to i.
Investigate on the fracture toughness of welded pressure vessel steel. In this project, study will be carried on investigating of fracture toughness properties
for each zone of welded pressure vessel steel. ii.
To differentiate the fracture behavior on area which are Base Metal BM and Heat Affected Zone HAZ
1.4 Scope of project
Pressure vessel steels with good ductility and weldability have been widely used in oil and gas refinery, power generating stations and chemical industries. The weld of
pressure vessel steel plates has mechanical and metallurgical inhomogeneity due to
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the weld thermal cycle in the base metal BM, the heat affected zone HAZ and the weld metal WM. The extent is to differentiate microstructure of base metal zone
and heat affected zone on welded structures. Welded structures are subjected to the dynamic loading usually, for example the construction structure during the life cycle
on-off. It is necessary to guarantee the base steel and its welded joint for own enough fracture toughness at the loading rate which the structure subjected.
Recently, a progressive methodology called as local approach is proposed to address the specimen geometry effect on the fracture resistance. The constituent relation for
the structure steel at dynamic loading is decided by the experiment result. The local approach is employed to correlate the fracture toughness at the dynamic loading for
HAZ zone. In the project, the fracture crack propagation in the HAZ of commercial pressure vessel steels is studied with regard to the influence of microstructure,
inclusion size and distribution, and the hardness distribution.
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CHAPTER 2
LITERATURE REVIEW
2.1 Pressure Vessel Steel
Pressure vessel steel is defined as a container with a pressure differential between inside and outside. The inside pressure is usually higher than the outside, except for
some isolated situations. The fluid inside the vessel may undergo a change in state as in the case of steams boilers, or may combine with other reagents in the case of a
chemical reactor. Pressure vessels often have a combination of high pressures together with high temperatures, and in some cases flammable fluids or highly
radioactive materials.
In pressure vessel steels, carbon is of prime importance because of it strengthening effect. It also raises the transition temperature, lowers the maximum energy values
and widens the temperature range between completely tough and completely brittle behavior. Manganese on the other hand up to 1.5 improves low temperature
properties.
Of all the different kinds of steel, those produced in greatest quantity fall within the low carbon classification. These steels generally contain less than about 0.25 wt C
and are unresponsive to heat treatment intended to form martensite; strengthening is accomplished by cold work. Microstructures consist of ferrite and pearlite
constituents. As a consequence, these alloys are relatively soft and weak, but have outstanding ductility and toughness; in addition they are machinable, and of all steels
are the least expensive to produce.
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They typically have yield strength of 275 MPa, tensile strengths between 415 and 550 MPa and ductility of 25 EL. A516-Grade 70 is one such kind of steel and has
applications in low-temperature pressure vessels. Samit, S. 1998a.
The important mechanical properties for pressure vessel are: i.
Yield Strength ii.
Ultimate Strength iii.
Reduction of Area a measure of ductility iv.
Fracture Toughness v.
Resistance to Corrosion
2.2 Fracture Mechanics
