Recently, plasma process has been applied to such industrial uses as cutting and spraying. Arc plasma is constricted by the gas flow and/or metallic wall of nozzle in a plasma process. The temperature and the thermal energy intensity of the constricted arc plasma are much higher than those of ordinary open arc. In the cutting and welding applications, it will be expected that the plasma stream has a property of line heat source similar to the one as observed in Electrlon Beam Welding.
In this report the experimental results on the plasma welding of stainless steel plate are described briefly. The electric capacity of plasma welding torch used in these experiments is about 8 KW. The plasma welding is capable of producing a narrow and deep weld bead by the action of "Key Hole". And a sound weld is obtained by one pass of torch even for a 15 mm thick plate. With the above welding energy source, optimum welding conditions are established for various thickness of plate. Welding speed in plasma process is much higher than that in TIG welding and others at the same welding energy.
Various tests on the microstructure, mechanical property and the deformation of weld specimen are carried out with satisfactory results.
It was confirmed that the formation of key hole in crater of bead had much influence upon the welding results. When the key hole is not formed, porosity is contained in most weld beads. The formation of key hole is sensitive to the travelling speed.

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The maximum temperature rise in electrode tip is estimated from the measured heat input to the electrode during one spot welding. It reaches as high as about 700°C for ordinary spot welding conditions of steel plates of 0.8-3.2 mm thickness.
For calculation the electrode cross section is assumed constant along the electrode length, and the effect of wave form of heat flow from the tip is investigated. Q in Table 1 is the measured heat input to each side of the electrode per one welding under the conditions shown in the table. θ₂ in the table shows the calculated results using the equation (9) under the following assumption, i.e. the heat flow wave form is taken as Fig. 11(a), heat flow time T_H is assumed as equal to current flow time T₀ shown in the table, the cross section of the electrode is taken as S₂ corresponding to the root diameter of the electrode.
Fig.14 shows the temperature distribution curves along the length when a constant heat H=1000 cal/cm² sec flows in from the electrode tip for the duration T_H=0.25, 0.5, 1.0, 1.5sec. Curves A-D show the temperature rise when the distance x₀ of cooling end from the tip is infinity and A, B'-E' for x₀=1 cm. For the calculation of the effect of cooling water, it is assumed that equation (33) holds, according to the model shown in Fig. 13 (h is taken as 1 for Fig. 14 and 18).
Heat flow time T_H is divided into two parts as shown in Fig.17 and the temperature at the moment C is calculated as the sum of the parts. Temperature due to the part AB is calculated using equation (49), T₁ being time constant defined as in equation (43) and the period of BC is chosen far longer than time constant T₂.
It is easily shown that the magnitude of temperature fluctuation at a point I cm distant from the tip is negligible when intermittent heat input cycle is less than 1 sec/spot and we see that the cooling effect of cooling water is only effective to reduce the average temperature rise in the electrode under intermittent welding.
Table 3 shows the temperature rise at the tip for cyclic heat flow from the tip for various combinations of x₀, h, T_H and T_w (i.e. cyclic period). The value shows the temperature at the moment C in Fig. 19 for H=1000 cal/cm² sec.
Fig.2, 4 shows the oscillogram of the temperature rise at 2 mm distant from the tip (Fig.1). Electrode pressure is also shown in Fig.2.

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A study has been made of the effect of additional elements on the weld-crack susceptibility of Al-4.5% Zn-1.5% Mg and Al-5.0% Zn-2.0% Mg alloys.
Weld-crack susceptibility was evaluated by MIG-spot cracking and Fish-bone cracking tests. The former was carried out with 5356 filler and the latter was carried out without filler, with Al-Zn-Mg filler, 5356 filler or parent metal filler.
The results were as follows:
1). The addition of zirconium to parent metal has a remarkable effect for reducing weld-crack susceptibility.
2). Neither titanium nor boron addition has any appreciable effect, though these elements serve as well to refine the structure of the weld metal as zirconium does. Titanium however, has some effect for reducing weld-crack susceptibility, only when 5356 filler is used.
3). None of other additional elements such as copper, silver, chromium and manganese was found to be effective for reducing weld-crack susceptibility.
4). Weld-crack susceptibility depends not only on the grain size but also on the volume and form of the liquid film, which is formed with boundary segregation of eutectic composition far from equilibrium.

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磁気分析装置を用いて得られた本研究結果を総括すると次のようである.
1)最高加熱温度1350℃のC.C.T.図は含AIN低炭素鋼とその比較材であるアルミキルド低炭素鋼とではほとんど差はない.
2)最高加熱温度1200℃のC.C.T.図では,含AIN低炭素鋼の方がその比較材にくらべて少し短時間側に移行しており,ferrite,pearliteおよび中間段階組織の臨界冷却時間はそれぞれ0.86,3および0.42secである.
3)1350℃で10min加熱後730℃に保持した場合,Ar3変態速度は含AIN低炭素鋼とその比較材とでは差異はなかった.
4)1200℃で10min加熱後730℃に保持した場合,変態開始後約9secまでは含AIN低炭素鋼のAr₃変態速度はアルミキルド低炭素鋼にくらべて大きかった.とくに変態開始後数秒閥はAN-1はAN-1Cの2～2.5倍の変態速度を有している.

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Effect of hot forming process such as hot bending, line heating and spot heating on the mechanical properties of double normalized and tempered 9% Ni steel was studied by examining both tensile properties and impact behaviours.
The results are summarized as follows:
1. Most suitable heating temperature in hot bending was 820°C to 950°C, and normalizing at 900°C, normalizing at 790°C or normalizing at 790°C, and tempering at 570°C followed by air cooling were needed to obtain good notch toughness after forming.
2. The examinations of microstructures showed that good notch toughness was correlated with solution increase, spherical degree of carbides.
3. Line heating tended to harden especially the heating surface, and greatly reduce the notcc toughness. However the notch toughness reduction at the mid-wall and the bottom surface (plate thickness was 13.5 mm) was slight in the case when water spray cooling from maximum heaating temperature of 550°C to 900°C was applied.
4. Spot heating must be avoided, for local shrinkage makes the notch toughness scatter and reduce the toughness value itself.

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