A range of HSLA steels reflecting the two major
processing routes, quench and tempering and controlled
rolling, have been tested in fatigue to assess their
potential wider application in the offshore Industry. The
six steels chosen have a wide range of yield strenghts (470
to 690Nmm-2), fracture toughness (31 to 260J at -40°C) and
carbon equivalent values (0.19 to 0.33). Fatigue testing
has in general been carried out at low frequency (0.5Hz) and
high load ratio (0.6) however some tests have been conducted
at very low frequency (0.1Hz) and low load ratio (0.1). An
in-air study was first used to assess the fatigue
performance of all six parent plates. Five steels were
welded by the Submerged Arc Welding process at high heat
input (1.5kJmm -1) to evaluate the Heat Affected Zone (HAZ)
fatigue performance. A new test was devised to grow a
fatigue crack through a single pass, bead on plate-, HAZ
whilst maintaining a constant stress intensity range. The
surface crack length was continuously monitored and recorded
to an accuracy of 0.01mm.
A corrosion fatigue study evaluated the
performance of one controlled rolled and one quenched and
tempered steel at three levels of impressed current cathodic
protection.
Extensive metallographic examination was made to
study the influence of microstructural features and types on
fatigue crack propagation. Techniques used include optical
microscopy, scanning electron microscopy, fatigue crack and
surface replication and crack profile digitising. These
techniques give an assessment of crack path deviation and
branching, the influence of precipitates and inclusions, and
an indication of the mode of fatigue crack propagation.
The wide range of microalloyed HSLA steels tested have shown
a significant improvement in fatigue crack propagation
resistance over structural steels conforming to BS 4360
grade 50D. Typically an improvement by a factor of two has
been observed. Whilst the observed increase in fatigue life
was slightly reduced by high heat input welding the slope of
the Paris curves remained unaffected thus indicating a
similar stress intensity range sensitivity in the HAZ to
that shown by the parent plate.
The newly developed crack monitoring system
coupled to a computer controlled fatigue testing machine has
shown a wide variation in fatigue crack propagation rates
through a heat affected zone microstructural gradient.
Growth rates have increased by a factor of ten in localised
coarse grained microstructural regions compared to the
adjacent weld metal and outer heat affected zone.
The corrosion fatigue study has also indicated
that in general HSLA steels retain their superiour fatigue
resistance compared to structural steels and in particular
respond more favourably to cathodic protection. Both in-air
and corrosion fatigue studies have indicated that the
controlled rolled steel microstructures developed mainly for
line pipe application has the greatest potential for
increased use offshore
Is data on this page outdated, violates copyrights or anything else? Report the problem now and we will take corresponding actions after reviewing your request.