Metallurgical Examination

TM 750-245-4

d. Insufficient pressure is sometimes evidenced by

metal expulsion and sometimes by : he appearance of a

heat affected zone (IIAZ) at the point of contact of the

electrode. Metal expulsion is usually prevalent when

welding heat is high and the foregoing pressure is not

great enough to retain the molten metal. This condition

may also be a result of poor heat inertia. The heat

affected zone results because the low pressure of the

electrodes creates a high -contact resistance at the

electrode and material interface.

e. Excessive pressure is evidenced by metal

expulsion which results in blow holes. In this case, the

pressure is so great that it expels the molten material

Figure 8-11. Methods of pull testing.

from the weld zone.

f. Insufficient heat results in the lack of fusion and,

c. Method A is referred to as the torsion-shear

consequently, a poor weld joint (fig. 8-12).

method of pull-testing. This method places a torsional

force in addition to a tension and shear force on the

weld.

In general, this method results in pull-test

strengths which are much lower than the results

obtained by either of the other methods. Mechanically

locked interconnections are detected by the torsion-

shear method, whereas they may not be detected by

other methods.

d. The significance of this test is that a quantitative

analysis of the weld strength may be obtained and used

as a basis for determining optimum welding machine

settings. Once the machine has been set, periodic pull-

tests performed on samples taken from the production

line can be evaluated and the results used as an in-

process check.

8-26. Metallurgical Examination

a. Metallography is the only feasible analytical

Figure 8-12. Lack of fusion caused by

technique available for evaluating the quality of a weld.

insufficient heat.

Metallographic examination reveals the interior of the

weld, enabling the observer to determine what type of

weld has been made, the amount of fusion present, and

g. Excessive heat can cause excessively large

any defects present within the weld. However, it takes a

nuggets (over penetration) which, in extreme cases,

trained and experienced metallographer to interpret the

extend the full width of the material. Over penetration is

photomicrographs.

undesirable since the recrystallized nugget lacks

b. Before the interior of the weld can be observed,

ductility. Excessive heat also contributes to metal

the sample must be encapsulated in a rigid substance

expulsion, gas pockets, and shrinkage cavities. Figure

and then subjected to a series of grinding and polishing

8-13 is a photomicrograph of a weld containing

operations. These operations leave a free surface and a

shrinkage cavities.

disturbed crystalline layer above the basic metal for

h. The nugget of a fusion type weld should exhibit

examination. The free surface and disturbed crystalline

equal penetration and be free of porosity, inclusions,

layers are removed by an etching process which, if done

blow holes, and shrinkage cavities. Penetration is

properly, will reveal the true structural characteristics of

defined as the depth to which the fusion extends into the

the weld.

material and should be at least 20 percent of the

c. Improper control or selection of welding

material thickness is. Figure 8-14 illustrates proper

variables can be readily determined by metallographic

penetration.

analysis. Such conditions as excessive or insufficient

i. Improper heat balance can, in many cases, be

pressure or energy, heat unbalance, or combinations of

detected by the presence of a nugget existing within one

these have a direct effect on the metallurgical

of the materials. Figure 8-15 (top) shows a nugget

appearance of the weld.

8-8