This chapter covers a broad range of low-carbon steels optimized for structural applications. Low-carbon structural steels are generally considered the highest-strength steels that can be welded without undue difficulty, even in the field. They include mild steels, carbon-manganese and niobium- and vanadium-containing steels, and high-strength low-alloy steels. Chapter 5 discusses the composition, microstructure, and properties of these workhorse materials and explains how to identify the cause of production-related issues such as lamellar tearing and ferrite-pearlite banding. It also describes some of the alloying variations that have been developed to improve machinability and the mechanisms by which they work.

The formation of defects in materials that have been fusion welded is a major concern in the design of welded assemblies. This article describes four types of defects that, in particular, have been the focus of much attention because of the magnitude of their impact on product quality. Colloquially, these four defect types are known as hot cracks, heat-affected zone microfissures, cold cracks, and lamellar tearing.