Residual stress less than tensile yield stress of the material is obtained for usual groove welds of high-strength steels (See Fig. 1). In the present research attention was focused on the difference in thermal stress cycle between mild steel and high-strength steel welds. Experiments were made in such manner that a round bar specimen (Fig. 3) set in a rigid frame (Fig.2) was subjected to a thermal cycle simulating weld heat-affected zone. The thermal cycle was given by high frequency induction heating and air-cooling or controled-cooling. Thermal stress developed in the specimen was measured by a load-cell (capacity 2 tons) connected to the specimen. A mild steel and HY-80 steel were used for the experiments (See Table 1.)
Thermal cycles measured at several points on the specimen are shown in Figs. 4 and 5. Free expan-sions as shown in Figs. 6 and 7 were obtained between end chucks of the specimen during thermal cycles. Uniform temperature distribution along parallel part of the specimen was not expected because of high frequency induction heating. Peculiar shape of the free expansion curves is obtained as the result of nonuniform temperature distribution (See Fig. 8.).
Figs. 9 thru 11 show thermal stress cycles plotted for surface temperature at the center of the specimen. It was found that factors by which residual stress is lowered are the decrease in A_r transformation temperature range over which yield stress of the material is rather small and the increase in expansion during the transformation. Therefore, rapid cooling from higher temperature than A_c1 point lowers residual stress level in high yield-strength steel welds (See Fig. 12).
Degree of restraint for the rigid frame used was investigated by an analogical system shown in Fig. 13. The restraint was about one-third as large as a bar with uniform temperature fixed at the ends.

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A study has been made of aging characteristics of Al-Zn-Mg alloy welds.
The parent metal contained Al-4.1% Zn-1.6% Mg-0.3% Mn and the filler metals were 5056, 5356 alloys and parent metal fillers.
All of the weldments could make almost the maximum recovery of strength in 90 days of nutural aging after welding. The weld strength, however, difinitely depended on welding processes, filler metals and previous heat treatments of parent metal. All of the undressed TIG and MIG-welds of 2 mm thick plate fractured at the heat-affected-zone when they were tensile-tested after natural aging of 90 days. The undressed TIG-welds of 5 mm thick plate fractured at the heat-affected-zone when they were tensiletested after natural aging of 90 days, but the dressed TIG-welds and all of the MIG-welds fractured at the weld metal.
The strength of welds depends on the structure of heat-affected-zone and the aging characteristics of weld metal. The former is affected by the reversion of G-P zone and the growth of M' particles during welding, and the latter by the chemical composition of weld metal.

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The characteristic of electric resistance welding process for making pipe from steel strip was considered and the effect of chemical composition on the weldability especially by low frequency current was studied.
There are many factors affecting the weldability of electric resistance welding steel pipe. The following four factors are most important from metallurgical view-point:
a) Deformability from steel strip to round pipe,
b) Upset-butt weldability by heavy current and short transmission time,
c) Hardenability of welded part by rapid heating and cooling,
d) Morphology of non-metallic inclusion in steel strip,
The deformability of steel strip seems to be mainly dependent upon the level of yield strength of steel strip. The formula between the carbon equivalent and the yield strength of welding structural steel plate will be useful.
On the upset-butt weldability the carbon content has the greatest influence. The carbon in steel strip most decreases the weldability. Silicon is the greatest element after carbon. Manganese and chromi-um have smaller effect on the weldability than carbon and silicon. Molybdenum and nickel have no effect up to about 1.0 weight percent respectively in steel. Phosphorus, sulphur, aluminum, nitrogen, titanium, vanadium, zirconium, niobium and cerium in steel have also no effect within the range of content in practical use respectively.
As for the hardenability of welded part the formula between the carbon equivalent and the hardness in arc-welding heat-affected zone of heavy steel plate will be useful.
The non-metallic inclusion has also great influence on the soundness of welded part. The sort of deoxidizing and desulphurizing agent, the usefulness of sulphide-fouming element, the size of steel ingot and the rolling ratio of steel strip on both directions longitudinal and transverse must be considered. The decrease of amount of non-metallic inclusion and the change of its morphology from ribbon to oval type will be effective.
In selecting the sort of steel strip for electric resistance welding pipe from metallurgical view-point, these four factors must be considered synthetically.

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It is generally admitted that the weld of zinc plated mild steel has poor mechanical properties due to inclusion of zinc compounds in the deposit metal.
At first, mechanical properties of zinc plated steel welds were investigated. Mild steel 3.2 mm in thickness and zinc plated steel with 0.018 mm thick zinc layer were used for base metal.
Various types of coated electrode for mild steel welding were used.
There was little difference between mild steel and zinc plated steel in tensile test and bending test on single bead or butt welded specimens.
Next, relation between types of electrodes and zinc contents in weld metal was investigated (shown in Fig. 1 to Fig. 7).
The electrode coatings were composed of several different elements such as silica, lime, iron oxide, rutile, ferro-silicon, and others.
Using those electrodes, the zinc plated steel plates were welded, and chemical analysis of zinc in deposit metals was made.
Zn-content in the deposit metal was higher in the coating with rich iron oxide than in one with rich lime or rich silica. And when ferro-silicon was added to the coating, Zn content decreased.
Analysis shows that Zn in deposit metal exists as zinc oxide. In this case, to decrease the Zn in deposit metal, a coating of which the dissociation pressure is lower than that of zinc oxide must be used.

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Dynamic strength of plastics has been studied by engineers for many years, but no satisfactory solutions arc available as yet.
What strength the weldments possess in comparison with the base materials is considered in this report.
The disturbance characteristics and the states of destruction are studied by varying the temperature.
The number of repeated cycles to failure of P.V.C. base plates reduces with the rise of temperature, and this reduction is remarkable above 50°C. The endurance limit at 20°C is 2.5 Kg/mm². This is about 37% of its tensile stress.
The maximum joint efficiency of P.V.C. plates processed with hot jet welding is over 74% at 30°C.
The endurance limit is 2.37 Kg/mm², and the endurance limit ratio of the weld to the base materials is nearly equal.
The endurance limits of P.V.C. pipe base material and welds by hot plate welding are 1 Kg/mm² and 0.5 Kg/mm² respectively. The S-N curve of the heated plate welded P.V.C. plate at 30°C augrees approximately with the base materials at 30°-40°C.
The endurance limit of low pressure polyethylene treated with frictional welding is 0.44 Kg/mm² at 20°C, and that of its base materials is 0.58 Kg/mm² when the number of repeated bending cycles is 5×10⁶.

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