The effect of repeated post weld heat treatment on the mechanical properties of ASTM A302 grade B
Date
2016-03-16
Authors
Filmalter, Rudi
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Abstract
Erosion-corrosion on ash lock internal surfaces means that these equipment are
continuously refurbished through weld build-up, followed by post weld heat treatment
(PWHT). The deterioration of mechanical properties of the ash locks after numerous PWHT
cycles has been a concern. A graphical prediction model based on experimental work on
this material grade has been derived previously to predict the mechanical properties after a
number of PWHT cycles. The validity of the model was, however, questioned. Ash lock,
210AL-3401, was scrapped for the purpose of testing the effect of several PWHT cycles on
the mechanical properties and microstructure to determine whether the current model is
valid or needs to be optimised.
Test samples of the ASTM A 302 Grade B manganese-molybdenum material were cut from
the shell and top dome parent metal and shell-to-dome weld of the test ash lock and
subjected to a number of additional simulated PWHT cycles with holding times ranging from
2 to 100 hours. This was followed by mechanical testing, i.e. tension -, hardness - and
Charpy V-notch impact testing, in accordance with SA-370, and metallographic examination
after each PWHT cycle.
Test results showed that actual measured tensile properties for the shell and dome samples
were generally higher than what was predicted by the current model and, similar to
hardness, followed an overall downward trend with increasing PWHT cycles for the shell,
dome and weld. Impact toughness for the shell and dome was mostly above the minimum
required values, but after 100 hours of additional PWHT impact toughness was below the
minimum required values when tested at 0°C and above minimum required values when
tested at higher temperatures. Impact toughness for the shell-to-dome weld showed an
overall increase with increasing PWHT. Metallographic examination showed an overall
increase in quantity and size of molybdenum carbides and spheroidisation of the cementite
phase for the shell and dome samples with increasing PWHT. Significant coarsening of the
carbides together with an increase in the level of spheroidisation resulted in a rapid
decrease in tensile properties and impact toughness with increasing PWHT. Similar effects
were observed on the microstructure of the weld samples, but no significant effect on
impact toughness was observed.
Description
A dissertation submitted to the Faculty of Engineering and the Built
Environment, University of the Witwatersrand, in fulfilment of the
requirements for the degree of Master of Science in Engineering.
Johannesburg 2015