Volume 60, Issue 8

Steels with different mechanical and chemical properties are used together in the oil and gas industry. In this case, it has brought about the necessity of joining steels with different properties by welding. Most of the time, there are problems in welding these steels with different chemical properties. Therefore, in this study, AISI 304 stainless steel, AISI 4140 tempered steel, and S235JR structural steel with different chemical and mechanical properties used in the oil and gas industry were joined by friction welding. Then, macro and microstructure studies as well as hardness measurements, tensile tests, and torsion tests were applied to the produced samples. As a result of the micro-structure studies, it was observed that the martensitic structure was formed in the full deformation region of the joint made of AISI 304 stainless and AISI 4140 tempered steel, while it was determined that the other joints were formed of recrystallized fine grains in the full deformation region with the effect of friction. When the hardness measurement results were examined, it was determined that the hardest region was the full deformation region in all welded joints produced. As a result of the tensile tests, the highest tensile strength obtained was 622.94 N/mm 2 in the joint made of AISI 304 stainless steel and AISI 4140 tempered steel. As a result of the torsion tests, the highest torsion moment was measured as 250 Nm in the sample produced from AISI 304 stainless steel and AISI 4140 tempered steel.

Super 304HCu is an advanced ultra-super critical (A-USC) boiler grade austenitic stainless steel with the distinct addition of 3 wt.-% of Copper. A-USC power plants intended to operate in chloride rich environments (sea shore, feed water residues, etc.) are susceptible to chloride assisted corrosion failures. In this study, the pitting corrosion behaviour of the Super 304HCu parent material and tungsten inert gas weld joints was studied using a potentiodynamic cyclic polarization test in 3.5 % NaCl solution at three different pH levels (pH = 3, pH = 7, and pH = 11). The E pit values of the parent material is found to be much nobler than that of the weld joints. The micrographs of the pitted weld joints and the oxalic acid etched structure of Super 304HCu joints are presented. From the micrographs it is revealed that the heat affected zone is the most susceptible region to pitting corrosion.

One of the objectives of this series of case studies presented in section Failure Analysis of Practical Metallography is to educate colleagues who are new in the field, to inform them about failure mechanisms, not least about the rarer, more exotic ones and about metallurgical causes of failure that were utterly unexpected to occur in the particular failure case studies presented here. The subject of this contribution is a failure mechanism called Strain-assisted Grain Boundary Oxidation (SAGBO). This failure mechanism is not rare at all in hot-going nickel-base components in the aerospace industries. However, SAGBO was certainly unexpected to occur in large pipes that remained well below 400 °C in service.