©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A 1.4.1.3
Initial crystal formation begins at
the coolest spot in the weld. That spot is at the point
where the molten metal and the unmelted base metal meet. As the metal
continues to solidify,
you will note that the grains in the center are smaller and finer in texture than
the grains at
the outer boundaries of the weld deposit. This is explained by the fact
that as the weld
metal cools, the heat from the center of the weld deposit will dissipate into
the base metal
through the outer grains that solidified first. Consequently, the grains
that solidified first
were at high temperatures for a longer time while in the solid state and this
is a situation
that encourages grain growth. Grain size can have an effect on the soundness
of the weld. The
smaller grains are stronger and more ductile than the larger grains. If
a crack occurs,
the tendency is for it to start in the area where the grains are largest.
1.4.1.4 To
summarize this section, it should be understood that all metals are composed
of crystals of grains. The shape
and characteristics of crystals are determined by the arrangement
of their atoms. The atomic pattern of a single element can change its
arrangement at different temperatures,
and that this atomic pattern or microstructure determines
the properties of the metals. 1.5
HEAT TREATMENT
The temperature that metal is heated,
the length of time it is held at that temperature, and the
rate that it is cooled, all have an effect on a metal's crystalline structure.
This crystalline structure,
commonly referred to as "microstructure," determines
the specific properties of metals.
There are various ways of manipulating the microstructure, either at the
steel mill or
in the welding procedure. Some of the more common ways are as follows:
1.5.1
Preheat
- Most metals are rather good conductors of heat. As
a result, the heat in
the weld area is rapidly dispersed through the whole weldment to all surfaces
where it is radiated
to the atmosphere causing comparatively rapid cooling. In some metals, this
rapid cooling
may contribute to the formation of microstructures in the weld zone that are detri-
mental. Preheating the weldment
before it is welded is a method of slowing the cooling FIGURE
3 GRAIN
BOUNDARIES DENDRITE
INITIAL
COMPLETE FORMATION
CRYSTAL FORMATION
SOLIDIFICATION
BASE METAL
