Thermal And Metallurgical Considerations in Welding
A welding engineer needs the knowledge of welding metallurgy in order to control :
— the chemistry and soundness of weldmetal.
— the micro-structure of the weldmetal and heat-affected-zones (HAZs).
Metallurgy consists of two parts:
— Process metallurgy (e. g.) convertion of ore to metals, refining and alloying, shaping through casting, forging and rolling etc.).
— Physical metallurgy (deals with heat-treatment, testing, metallographic studies related to design and application).
Welding involves both:
— Process metallurgy-electrode covering and SAW fluxes formulation.
— Physical metallurgy—control of cooling rates and controlling the microstructure of weldmetal and HAZs (through welding heat input control and pre-and post-heating). The ultimate aim is to obtain the desired mechanical properties.
5.1 GENERAL METALLURGY
5.1.1 Structure of Metals
The pattern of solidification of metals is shown in Fig. 5.1. As the liquid metal cools and solidification temperature approaches initial crystals are formed. The crystals then grow into large solid grains. At the end of solidification the large solid grains meet each other at grain boundaries. Each grain has a crystalline structure with the atoms in the crystals arranged in a specific geometric pattern (F. C.C., B. C.C., HCP. Fig. 5.2). The orientation of grain lattice in each grain is different as each grain has developed independently. This orderly arrangement is disrupted at the grain boundaries, and has its repercussions on the metal properties. This applies to pure metals. Metals are commonly used in the industries as alloys (in combination with other metals or non metals).
Initial crystals Solid grains Solid grains with
Fig. 5.1 Pattern of solidification of metals
Fig. 5.2 The three most common crystal structures in metals and alloys. Left: face centred cubic (FCC) Centre: Body centred cubic (BCC) and right: hexagonal close packed (HCP).
Alloying elements dissolve in parent metal as follows:
(a) Substitutional solid solution in which alloying atom replaces the parent metal atom in the lattice (Fig. 5.3 (b)). This occurs when the solute and solvent atoms are similar in size and chemical behaviour.
(b) Interstitial solid solution in which alloying atom places itself in the space between the parant metal atoms without displacing any of them. See Fig. 5.3 (a). Example of this is carbon in iron (mild steel).
(c) Multiphase alloys. In many alloys, several alloying elements are used which do not completely dissolve either way. They produce multiphase alloys in which several phases having their own crystalline structure exist side-by-side.
A suitably polished and etched specimen of an alloy when observed under a microscope at high magnification shows grains, grain boundaries and phases in the microstructure. This microstructure depends upon the alloy chemistry and its thermal history.
(d) Grain boundaries. Since the atomic arrangement here is in disarray, the interatomic space may be larger than normal, movement of individual atoms of elements, through the solvent structure may occur resulting in a phenomenon called segregation.
(e) Grain size. The grain boundaries also resist deformation of individual grains, thus improving the strength of an alloy at normal temperatures. At elevated temperatures the atoms at the grain boundaries slide more easily. Thus, for better strength at lower temperatures coarse-grained structures are desireable. Metals could be coarse-grained or finegrained depending upon the solidification rate. Grain-size control is more important in the case of weld-metal.
5.1.2 Phase Tranformation
Multiphases can coexist in an alloy as discussed earlier. Phase change occurs on melting. In some metals phase change occurs in solid state due to heating or cooling—called allotropic transformation. Iron, titanium, zirconium and cobalt show allotropic transformation.