ABSTRACT:

Between the weld metal which reaches its melting point and above, and the cold parent metal, there are zones, within which all intermediate temperatures are reached promoting localized changes in the microstructure due to which pitting and severe corrosion were located. These are due to the difference in thermal expansion coefficients of the involved materials. In the present research, the effect of some of the heat treatment operation that are commonly used have been investigated. Also, an attempt was made to calculate residual thermal stresses at weldments due to the difference in thermal expansion coefficients at the involved materials.

SELECTION OF THE STEEL UNDER INVESTIGATION

Steels are selected on a performance basis rather than purely on strength. Where strength is a criterion however, the ferritic steels will be less strong and ductile than austenitic, and martensitic and precipitation hardening steels will be the strongest. In the present work, however, the environmental factors (corrosivity, operational temperature, resistance to pick-up of elements, e.g., sulphur, carbon from service atmospheres) were of considerable importance in selection of the material used in the present investigation, i.e., 316L authentic stainless steel. The chemical composition of which is given in Table I. The use of 316L austenitic stainless steel was encouraged by the knowledge that it offers much better welding characteristics than the other stainless steels (Baker, 1972). Metallographic examination of the 316L stainless steel showed that the structure consisted of homogeneous stable austenite with an assembly of more or less polygonal grains, see Fig. 1. The present steel belongs to a group of steels which constitutes the more highly alloyed austenitic steels, which contain chromium (12-30%) plus nickel (7-25%) and in most cases, smaller proportions of other elements. These steels are structurally unaffected by heating and cannot be hardened by quenching.

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