Structure fusion welds

Joining is another problem. Fusion welding in inert atmospheres often develops recrystallized structures in the heat-affected zones, so that welded pans lose strength and are embrittled. Special joining techniques being used In help overcome these deficiencies include electron-beam and solid-phase welding, and the development of special brazing materials. 

VI.16] BRITISH STANDARDS INSTITUTION, Guidance on Methods for Assessing the Acceptability of Flaws in Fusion Welded Structures, PD 6493, BSI, London (1991). 

Properties. Thermal expansion near zero from 5 to 35°C, thus it can be fusion welded at room temperature, enabling fabrication of complex structures having extreme dimensional stability. (See Tables 6.1 and 6.2.)... 

The methods available for the control of cast structures see Section 6.fi) can be applied to fusion welds, but there are practical difficulties. Magnetic stirring, ultrasonic vibration - and... 

The integrity of welded structures depends on the integrity of the welds, and much attention is given to testing methods, such as destructive tests, nondestructive tests, and general weld inspection. An objective of many tests is to determine whether welds contain specific defects, such as porosity, slag inclusions, cracks, or lack of fusion (14,15). 

Welded microstructures can be extremely complex and often change drastically over a very short distance. The fusion zone or weld metal is a dendritic structure that has solidified from a molten state. Bordering the fusion zone are transition, unmixed and partially melted zones, and the heat-affected zone (HAZ). These zones can be reheated and altered by subsequent weld passes, in multipass welding. For alloys with structures that depend strongly on thermal history, such as steels, the final microstructure can be extremely complex. Since welded structures are often quite susceptible to corrosion, overalloyed filler metals are often used to enhance the weld corrosion resistance. For stainless steels with sufficiently high carbon content, sensitization in the HAZ is another major problem. (Frankel)5... 

Usually, welded joints of 5 and 9% Ni steels possess adequate workability within the range 300 to 77 K for static, dynamic, and alternating loads. In arc and electroslag welding, there is no need for the welded joints to be heat treated. In spite of the growth of primary austenite grains in the base metal at the fusion line, the subsequent y a transformation results in the formation of a disoriented cellular structure, which accounts for the satisfactory ductility at temperatures down to 77 K. 

---