Dear, Marios Kazasidis

This is most likely due to phase transitions (see iron-carbon phase diagram) when the zone near the weld is heated. In hardened steels, typical hardening structures may be formed: ductility is reduced, welding stresses, cracks and other defects may occur. To reduce the resulting stresses, it is necessary to conduct steel temper.

In most cases, the composition of the weld metal is more alloyed than that of the base metal. It has higher hardness, tensile strength and yield strength.

When a weld is tested in that manner, you can expect that the material is not homogenous in the direction of loading. A number of issues can affect the result: strength mismatch, residual stress due to heat at the weld, fusion boundary properties etc.

Inspecting the fracture surface of the tensile specimen could reveal the 'weakest link' which contributed to the failure.

Considering the case in the asked question is of the transverse tensile loading type; and the tested sample had same material of similar thickness on either side of the weld, the probable reason might be as below:

1. Mostly the location of failure is not exactly at the base metal; however, it is at the heat affected zone (HAZ). It is well known, in HAZ, grain coarsening takes place which leads to significant softening at the local region; that usually prone to failure in a transverse loading. In this context, for a detailed explanation, readers are requested to go through the below article:

https://link.springer.com/article/10.1007/s11665-014-1361-5

2. Apart from the grain coarsening at the HAZs, the process of welding (mostly the fusion type) causes some significant mismatch in the material properties across the weld. Once applied an external loading, an internal crack can easily initiate and propagate in a local region where there is a drastic transition in the material property. Therefore, as suggested by Wee Khor above, inspection of the fractured surfaces of the tensile specimen is highly essential. Again in this regard, for the detailed explanation, readers are requested to refer the below article that deals with shear-induced fracture:

https://www.sciencedirect.com/science/article/pii/S0924013610001925

Failure away from weld indicates that the strength of the weld is higher than the strength of the failed area (not the base metal).

During welding the weld area and the area just beside the weld line/zone goes under different heating cycle. This changes the original characteristic of the material. Now the material near the weld area is not having same characteristics as base metal anymore.

This different heating cycle may affect the grain size of the area around the weld as well. This ultimately will lead to change its ductility after the welding.

This may be the probable reason that the failure is away from weld still the elongation values are less than parent metal.

I hope this might be helpful.

Good luck.

Dear Kazasidis,

You have tested cross weld specimen and that includes the weldment regions in gauge length. In this context, your specimen is not truly homogeneous in gauge length, it includes the combined properties of HAZs and weld metal. This is the main reason, and the fracture location is base metal which signifies the weld qualification. You cannot quantify it as propertie of base metal. If you perform the test using 2D DIC technique, then probably you can quantify it for elongation at acceptable extent.

Is diameter of the samples with (weld + Base metal) and only base metal samples are same?

Due to the differences in properties of the different areas of a joint, i.e. weld metal, HAZ and base metal, not all parts act physically the same in transverse tensile. Contribution of the different areas in the transverse tensile test will not be homogenous as in test where all material is the same. The orientation of the tensile specimen with respect to the mill rolling direction of the test plates might play a role as well.