PSI - Issue 23

Ivo Černý et al. / Procedia Structural Integrity 23 (2019) 493 – 498 Ivo Černý / Structural Integrity Procedia 00 (2019) 000 – 000

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3.3. Hardness profile

As already mentioned, hardness was evaluated as HV 10 profiles from the surface of additive layers to the depth, to base material and horizontal profiles – in lines parallel to the surface at the depth of approximately 0.5 mm. The results are in the diagrams in Fig. 5 a,b.

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a) b) Fig. 5. a) Vertical and b) horizontal profiles of HV10 hardness in additive welded layers.

It follows from the diagrams in Fig. 5 that the maximum hardness HV10 is between 613 and 635. If recalculated to strength according to the EN ISO 18265 Standard (2014), the strength of the additive layer is R m = 1910 – 1980 MPa, which is a very high value. In Fig. 5a, the drop of hardness approximately 10% corresponds to the end of the additive layers – to the fusion zone and further decrease of hardness follows in the HAZ towards the base metal, which is also partially quenched close to the HAZ. The depth of the maximum hardness in the double layer is approximately twice as much as in the single layer, which corresponds to the metallographic evaluation of the depth of the layers. The hardness of the not affected base metal is only HV 138 -140, which corresponds to R m = 440 – 450 MPa, this values is slightly lower than typical strength of the S355 steel, e.g. EN 10025-2 Standard (2004). The horizontal hardness profile is similar for both single and double layers and is characteristic by local minima and maxima, the minima resulting from partial local material tempering, when the next overlapping layer was welded. It should be pointed out, however, that the scale of the vertical axis in Fig. 5b is much finer in comparison with Fig. 5a and the hardness fluctuations are only within 14% range, which is a very good result. Survey of high-cycle fatigue test results is in Fig. 6. The stress range was calculated considering actual height of the specimen at the position of crack initiation, not nominal height outside the additive layers. It follows from Fig. 6 that unlike the region up to 10 5 cycles, where fatigue strength of specimens with additive layers is comparable to those of base material (BM), endurance limit was quite significantly reduced by the additive welding process, by more than 40%. The reduction of the endurance limit was caused by microscopic lack of fusion at edges of each single layer track, visible in Fig. 7b. All fatigue cracks initiated at such defects, either inside the additive layers at defects between neighbouring tracks or at defects of marginal tracks, where lack of fusion was more distinct – Fig. 7a. Unlike the defects inside the layers, which could be removed by grinding, the suppression of marginal defects (left and right sides in Fig. 7b) would need to optimize the laser welding parameters. 3.4. High cycle fatigue