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★ Check point of assembly

- Horizontal accuracy of an assembly stand.

- Height of a bottom board of an assembly stand.

- Consider the way to control the horizontal accuracy.

- Easiness of attaching the jig for assembly.

Because in welding if the temperature is low or base metal is thick, the speed that a welding part is refrigerated fast and shrinkage crack is easily occurred in a welding part of one pass, preheating is necessary. Therefore, to retrain occurrence of cold crack is the purpose of preheating with making cooling speed slow by preheating a welding part.

Also, in case that hazardous articles (water, oil, rust, paint and so on) are interposed, to preheat for a moment a bevel zone regardless of thickness and quality of materials burns these. So preheating is important before everything. Cooling speed of a welding part is changed by welding heat input, thickness of a board, temperature of the open air but the relation of cooling speed, change of a part affected by heat and weakening of materials is generally affected by carbon equivalent of a steel plate.

* welding heat input

E : voltage (V) , I : ammeter (A) , V : welding speed (cm/min) carbon equivalent

Generally

In steel plate, SS41, SB41P, SWS41, carbon equivalent is low, so preheating is not necessary except a case that temperature of the open air is lower than 0℃ but preheating is necessary in case that a board is thick and the pace is fast.

It is better to get preheating temperature by y slit cracking test. If the air temperature is low, the cooling speed is fast although preheating. Thus, it is better to repeat welding and preheating with measure temperature in proper time. in the cold place, it is better to preheat regardless of a quality of materials and thickness of board due to the air temperature is low.

*carbon quantity and preheating temperature