2. Experimental procedure
The material used for the study is API X52 that is the most usable in existed gas pipelines. The specimens were
cut from a buried in-service pipeline. This pipeline with the outer diameter D=610mm and wall thickness t=11mm,
was in service approximately 50 years till it was cut for testing. The chemical composition of steel and its
mechanical properties in air are given in Table 1 and 2 respectively. The structural specificity of the components of
welded pipe can be seen in Fig 2. The base metal consists of grains of ferrite-pearlite, typical in all carbon steels
(Fig. 1a). The weld metal showed the acicular ferrite columnar grains (Fig. 1b). The HAZ is composed of refined
grains of ferrite-pearlite (Fig. 1c), product of the transformation of base metal with the heat provided by the welding
process.