Nitrogen is not always detrimental to weld metal. It may have an effect of increasing of hardness of wear resisting steel and strength of steel at elevated temperature. Moreover it may stabilize austenitic phase. It was studied in this report to use nitrogen from welding atmosphere as alloying element of weld steels. three kinds of electrode wire were used : 1 Mn-0.5 Si, 2.5 Cr-1 Mo and 5 Cr-0.5 Mo. Bead-on-plate specimens were made by each of these electode wires with CO2-N2 or Ary-N2 shielded arc welding. Nitrogen content of supplied gas was varied from 0 to 10%. Weld steels were given various heat treatment : as welded, water quench and anneal at temperature from 100 to 600°C after water quench. Vickers hardness was measured about those specimens. And then mechanical properties of all weld metals by these electrode wires with CO2, CO2-A, CO2-N2 shielded arc welding were compared at room temperature and 400°C. The experiment has revealed the following ; 1) There is a certain relation between a nitrogen content of weld steel and one of supplied gas, and it is variable according to a content of alloying element of weld metal. Chromium decreases a nitrogen content of weld metal and increases a critical nitrogen content of atmosphere in which a weld metal becomes porous. 2) Increasing the nitrogen content of weld metal, it becomes harder. 3) The more chromium content of weld metal increases, the harder nitrogen makes weld metal. 4) A weld metal which contains nitrogen and some alloying elements became harder in the case it was annealed at 100 to 300°C after water quench than in the case it was only water quenched. 5) A weld metal by 5 Cr-0.5 Mo electode wire with CO2-N2 shielding gas revealed a higher value of ultimate tensile strength than CO2, or CO2-A shielding.
The phenomena of horizontal fillet and vertical welding have been studied in the light of inte rfacial tension theory, and it was concluded as follows, (1) The sectional curve of harizontal fillet weld is determined by both gravity and the interfacial tension γ of the molten metal drop, the mother drop, covered with molten slag, and there seems to be a limit value of leg length, for a system of metal and slag. (2) The possibility of vertical welding depends upon that a mother drop can adhere on the vertical plate surface, and it can be realized by the most essential fact that the mother drop, confined within the concave penetration surface, behaves as if it were the more wettable liquid drop. (3) The contact angle of mother drop determines the sectional shape of the weld bead. The great contact angle of the mother drop with high iron oxide type electrode yields the rather convex bead in flat welding, and bead of equal leg length in horizontal fillet welding, on the other hand, the small contact angle with ilumenite type electrode gives the flatter bead in flat welding and concave one in horizontal fillet welding. These facts has been proved by means of many measurement of solidification angle in the penetration surfaces. (4) There are twe types of the state of the mother drop within the penetration surface. One of them is the advancing type and the other, receding one, each having the following characteristics. advancing type receding type blowing force of arc weak strong contact angle small great penetration shallow deep bead appearance good undercut bead surface smooth wavy sectional shape of bead flat convexe
Authors investigated the influence of the various welding condition on the shape of penetration surface caused by TIG arc torch fixed perpendiculer to the mild steel plate. After discussing the results obtained we have come to the following conclusions. (1) The surface of penetration obtained was roughly one of revolution and its sectional profile was clearly divided into three groups, that is, peripheral, central and simple type. Penetration in p type consists of deep peripheral groove and shallow and flat bottom at the center part. Much deeper center hollow is added to the former in case of c type. Sectional profile curve in s type is of simply concaved arc. (2) Type of penetration changes abruptly from p to c as welding current is increased or arc length is decreased. Stype seems to be formed when the arc and mother drop is unstable. (3) Authors proposed as a probable hypothesis that the deep peripheral groove in p type is originated from the strong outerward surface flow of over-heated molten metal caused by the difference of surface tension under the arc. (4) If the contral depression of molten metal surface, caused by plasma jet stream or abovementioned surface flow, become deep enough to reach the bottom of penetration, then the direct action of the arc begins to act on the bare surface of base plate forming the central deep penetration (5) If the contact angle of molten metal on the solid metal has a definit value being not zero, occurence of the bare surface is not continuous to the welding condition, jumping up to a certain wide range from zero, which may satisfactorily explain the discontinuity of the change of penetration type.
The precipitation characteristics of boron constituent in the weld heat affected zone had been already reported in the Report No. 1. As a continuation of the Report No. 1, the effect of born constituent upon the propertis of weld heat affected zone has been investigated and the following results were obtained. 1. The precipitation of boron constituent has the tendency to decrease the hardenability of heat affected zone. 2. The predominant precipitation of boron constituent decreases the ductility of heat affected zone including elongation, reduction in area in tention, test and also crack initiation angle as well as fracture angle in bend test. The precipitation also impairs impact value. With regard to the influence of the precipitate upon the ressistance for fatigue, the precipitate has no effect on the fatigue limit but it decreases the number of repeated cycles required for fracture under the stress more than fatigue limit. From these results, it is advisable to avoid the precipitation of boron constituent in the weld heat affected zone by selecting the suitable steel composition and correct welding condition. Much efforts has been made in our Steel Works for this purpose and the object has been almost achieved by selecting the most suitable steel composition.