WELDING METALLURGY

Metallurgy is the science, which deals with the internal structure of metals and the relationship between those structure and the properties exhibited by metals.  When referring to welding metallurgy, the concerns are about the various changes that occur in metals when joined by welding especially those affecting the mechanical properties.

It is certainly appropriate for the welding inspector to knowledgeable in the basics of welding   metallurgy granted, it is unlikely that   the inspector will be responsible for the specification of base or weld metal alloys or their treatment However, an understanding of the basics of welding metallurgy is not only helpful to the welding inspector, but often a requirement for many inspection function.  One reason for this is that the mechanical properties of metals, such as strength, hardness, ductility, toughness, fatigue strength, and abrasion resistance, are all affected by the metallurgical transformations as result of welding.

These properties are affected by various metallurgical factors, including alloy additions, thermal treatment. The welding inspector who has an understanding of these properties will have a better feel for why certain fabrication operations are necessary. Certain fabrication requirements, such as preheat, post heat, interpass temperature control, heat input control, peening, thermal stress relief, and other heat treatments can all result in some type of metallurgical change which. In turn will, affect the metal ‘s mechanical properties. Therefore, this section will primarily describe certain aspects of ferrous   [Iron based] welding metallurgy with emphasis on the need for fabrication methods to control the changes, which can occur.

Since the topic of welding metallurgy includes numerous facets, it would be unreasonable to think that this discussion could cover them al l. so, we will limit the coverage to those more important changes, which may occur during the welding operation. These changes can be summarized and divided into two categories.                        

        

The first category includes those changes, which occur in a metal as it is heated from room temperature to a higher temperature and those changes occurring when a metal is cooled from a higher temperature to lower temperature. The second category is the effect on the metal’s properties versus the rate at which these temperature changes occur. More specifically, we are concerned with how quickly a hot metal cools to room temperature, that is, metal’s cooling rate.                     

Our discussion will begin with specific references to the changes, which occur in a metal as they are heated and cooled uniformly. However, it must be noted that welding presents some very different problems since the welding operation tends to heat very localized areas of the metal. Consequently, this non-uniform heating/cooling creates the need for some additional considerations.