A study of fatigue crack propagation in quenched and tempered and controlled rolled HSLA steels.

摘要

A range of HSLA steels reflecting the two majorprocessing routes, quench and tempering and controlledrolling, have been tested in fatigue to assess theirpotential wider application in the offshore Industry. Thesix steels chosen have a wide range of yield strenghts (470to 690Nmm-2), fracture toughness (31 to 260J at -40°C) andcarbon equivalent values (0.19 to 0.33). Fatigue testinghas in general been carried out at low frequency (0.5Hz) andhigh load ratio (0.6) however some tests have been conductedat very low frequency (0.1Hz) and low load ratio (0.1). Anin-air study was first used to assess the fatigueperformance of all six parent plates. Five steels werewelded by the Submerged Arc Welding process at high heatinput (1.5kJmm -1) to evaluate the Heat Affected Zone (HAZ)fatigue performance. A new test was devised to grow afatigue crack through a single pass, bead on plate-, HAZwhilst maintaining a constant stress intensity range. Thesurface crack length was continuously monitored and recordedto an accuracy of 0.01mm.A corrosion fatigue study evaluated theperformance of one controlled rolled and one quenched andtempered steel at three levels of impressed current cathodicprotection.Extensive metallographic examination was made tostudy the influence of microstructural features and types onfatigue crack propagation. Techniques used include opticalmicroscopy, scanning electron microscopy, fatigue crack andsurface replication and crack profile digitising. Thesetechniques give an assessment of crack path deviation andbranching, the influence of precipitates and inclusions, andan indication of the mode of fatigue crack propagation.The wide range of microalloyed HSLA steels tested have showna significant improvement in fatigue crack propagationresistance over structural steels conforming to BS 4360grade 50D. Typically an improvement by a factor of two hasbeen observed. Whilst the observed increase in fatigue lifewas slightly reduced by high heat input welding the slope ofthe Paris curves remained unaffected thus indicating asimilar stress intensity range sensitivity in the HAZ tothat shown by the parent plate.The newly developed crack monitoring systemcoupled to a computer controlled fatigue testing machine hasshown a wide variation in fatigue crack propagation ratesthrough a heat affected zone microstructural gradient.Growth rates have increased by a factor of ten in localisedcoarse grained microstructural regions compared to theadjacent weld metal and outer heat affected zone.The corrosion fatigue study has also indicatedthat in general HSLA steels retain their superiour fatigueresistance compared to structural steels and in particularrespond more favourably to cathodic protection. Both in-airand corrosion fatigue studies have indicated that thecontrolled rolled steel microstructures developed mainly forline pipe application has the greatest potential forincreased use offshore.