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National Advisory Committee for Aeronautics, Technical Notes - Effect of Variables in Welding Technique on the Strength of Direct Current Metal Arc Welded Joints in Aircraft Steel - I - Static Tension and Bending in SAE 4130 Steel Sheet

Arc—Welded butt Joints in l/8~inch SEE #130 steel sheet of aircraft
quality, which were made under various conditions of welding and heat
treatment, were tested to evaluate the effects of specific welding—
technique factors on the strength of the joints. The results of the ten—
sile tests indicated that crater blowholes, or crater cracks, produced by
‘interrupting the weld, had the most pronounced influence cn the trans- - __
verse tensile strength of the buttnwelded specimens; that position of ,p >“u_
welding had no significant effect on the tensile strength of any group Of
specimens; that preheating produced no increase in_ either the soundness
or strength of the welds; and that welds made with alloy-steel electrodes
were stronger than those made with plainrcarbonrsteel electrodes_. It was
found that the factor having the greatest influence on the plate—bending
fatigue strength of the welded specimens was the external stress-raiser
at the toe of welds in the reinforced specimens.

The increasing use of metal-arc welding in the fabrication of steel
aircraft structures, brought about by the desire fer increased production
and other advantages inherent in the metal—arc~welding method, has also
brought into focus certain of its limitations. The level of weldability
of aircraft steels, for example, is a very important factor, and the suit«
ability of various types of electrodes for aircraft welding has been ez-
tensively investigated. The development of proper welding techniques has
- also been given much attention, since experience has shown that many of
the difficulties in the fabrication of Welded airframes, such as distor—
tion, cracking, and certain types .of weld defects, may have their origin
in faulty welding technique.

The influence of weld penetration, contour, craters, undercutting,
and similar factors on the structural integrit-y of aiIn aft Joints is
recognized, but the degrees 12d which these factors affect the static and
dynamic strength are not yet fully understood. This investigation was
originally setup to study the effect of those factors and several others
such as hardness, grain size, and chemicaJ.—ccmpositi on gradients, on the
strength of Joints in SAE hl30 aircraft steel welded by the direct—current
metalraro method. These factors are influenced by the skill of the weld-
ing operator, type and size of electrode, welding current, speed and posi—
tion of welding preheat, heat treatment after welding, and the size, gecmr
etry, degree of restraint, and surface conditions at the weld Joint.