Mechanical properties and microstructure of large steel forgings for applications in the energy sector

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 water environments 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 of final 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 largescale forged segments with two different cross-sections (100 and 250 mm) were separately subjected to water, aqueous polymer solution and vegetable oil quenching and then tempered at 590°C, 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 0°C 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 800°C and 500°C (λ, t 8/5) were obtained by water, polymer and vegetable oil respectively. Kinematic viscosity may be the main variable controlling the cooling performance of the different cooling media evaluated due to changes in the 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 - 1˚C/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 forged component. 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 associated with the different fractions of tempered martensite and tempered bainite generated by changes in cooling rate. Accordingly, the higher strength and impact toughness values were observed at mixtures of tempered martensite (TM) and tempered bainite (TB) with proportions of 85% (TM) -15% (TB). The above due to partitioning effect of the acicular lower bainite on the austenitic grains in association with tempered martensite. Regarding the fracture toughness assessment, it can be argued that the yield strength variation between the selected specimens, along with the carbide size variation observed between tempered martensite and tempered bainite, were not large enough to induce significant changes leading to negligible variation in the final CTOD properties. Construction of CCT diagrams by means of quenching dilatometry has proven to be an effective technique to predict the microstructures industrially produced at the central part of large forgings, although the unavoidable effect of macro segregation must be considered for comprehensive analyses. Finally, in spite of the fact that vegetable oil provided slow quench rate compared with those of water or aqueous polymer quenchants, it is true that the mechanical properties produced by this bio-quenchant were similar to those produced by water and polymer quenching. As such, this finding indicates the possibility of implementing vegetable oil as an alternative quenchant in material specifications used in the production of large scale forgings made of low alloy steels, in particular when a balance between mechanical properties, dimensional stability (distortion) and reduced crack susceptibility is desired after quenching and tempering.