Abstracts Category : Other

Mechanical Properties and Microstructure ofLarge Steel Forgings for Applications in theEnergy Sector

by Edgar Ivan Saldana-Garza

Due to its optimal balance of strength and toughness, AISI 8630M low alloy steel forgings are extensively used as structural components for subsea applications in the oil and gas energy sector. However, considering the high pressures and low temperatures conditions in deep waterenvironments the oil and gas industry has continuously been demanding higher reliability levels on the structural integrity of large steel forgings in order to avoid in-service premature failures. In particular, special attention has been given to the CTOD fracture toughness as a critical parameter for structural design. Heat treatment is the last stage in the manufacturing sequence of large forgings and to certain extent, defines the metallurgical characteristics offinal component. The cooling rate during industrial quenching treatment represents one of the most important processing parameters controlling the microstructure before tempering treatment.The research programme involved industrial-scale experimental heat treatments in which largescaleforged segments with two different cross-sections (100 and 250 mm) were separately subjected to water, aqueous polymer solution and vegetable oil quenching and then temperedat 590C, to evaluate the influence of cooling rate on the microstructure and mechanical properties produced under industrial conditions. Tensile, CVN and CTOD fracture toughness properties were measured at RT, -30 C and 0C respectively as per specification requirements.Microstructural evolution and fracture surfaces were evaluated by high resolution scanning electron microscopy. A CCT diagram was constructed by means of quenching dilatometry in order to validate the microstructural changes produced during industrial quenching.In summary, the present investigation, showed that irrespective of the cross section, the faster,intermediate, and slower cooling times between 800C and 500C (, t 8/5) were obtained bywater, polymer and vegetable oil respectively. Kinematic viscosity may be the main variablecontrolling the cooling performance of the different cooling media evaluated due to changes inthe thermophysical properties of the quenchants.In addition, the predominant microstructures for the different thickness-quenchant conditions were found to be associated with mixtures of tempered bainite and tempered martensite. This was evidenced by the fact that the majority ofthe industrial cooling curves fell within a similar microstructural region of the CCT (0.03 -1C/s) dilatometric diagram which consisted of mixtures of martensite and bainite.Accordingly, the mechanical properties evaluated were similar among the different thickness quenchant conditions investigated. In this sense, all conditions evaluated showed strength and impact toughness properties well above the material specification limit for the selected forgedcomponent. It can be argued that the strength and impact toughness are controlled by changes in the distribution and size of carbide precipitates and packet substructure