Methods of evaluation welded seams durability for cold cracks formation (Part 2).

Metallurgical processes, occurred during point fusing, in spite of short-term of their behavior, are reproduced, apparently, with the same completeness level, as during real production facing.

The symmetry of fused point allows to apply to it strains of two types: tearing-off, which causes normal stresses σ in fusion zone and torsion, which causes tangential stresses Ƭ in fusion zone.

It is known that during metal fusion, which endures structural changes of martensitic type, tangential stresses of significant magnitude occur in fusion zone, during which relaxation microcracks occur. That is why stressing the point by torsion gives additional possibility for more detailed studying both the phenomenon of detained destruction itself and strengtheningcharacteristics of fusion zone of tested fused metal.

Two-position experimental machine was constructed and made for experimental examination of the method. The kinetic scheme of the machine is shown on fig. 1.

The comparative evaluation of susceptibility to chipping-off origination was made for two steel grades. Fusion was executed using flux cored wire that provides fused metal chemical composition, indicated in table 1.

On fusion completion the sample was stressed by tearing-off or torsion. Time period between the end of fusion and stressing the sample was 2 min.

As the index of fused metal susceptibility to chipping-off origination the maximal (critical) stress is taken, during which sample destruction doesn’t occur for 20 h.

Experimental results are shown on fir. 2 for alloys of two grades. It is obvious that during torsion and under conditions of tearing-off the time before sample will destruct depends on magnitude of applied stress, at that with decreasing external stresses it increases.

The significant difference in experimental results of samples when applying tearing-off and torsion strains is worth noticing. Thus, when stressing samples by tearing-off (fig. 2, a) the time before destruction in all stress range right up to bend point on the curve is significantly low, that indicates high speed of cracks origination. At the same time when stressing by torsion it is noticeably bigger. The relation of critical stresses is also changes. When stressing by tearing-off the critical stress in У250Х5Т5 alloy is twice bigger, then in У250Х5 alloy. When stressing by torsion the stresses relation rate is equal approximately to 4.5.

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