Page 391 - Failure Analysis Case Studies II

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Most alloy steels suffer some degree of embrittlement in the coarse-grained region of the HAZ when
heated at 600°C. Elements that promote such embrittlement are Cr, Cu, Mo, B, V, Nb and Ti, while
S, and possible P and Sn, influence the brittle intergranular mode of reheat cracking. Molybdenum-
vanadium and molybdenum-boron steels are particularly susceptible, especially if the vanadium is
over 0.1 %. The relative effect of the various elements has been expressed quantitatively in formulae,
due to Nakamura (1) and It0 (2):

P = Cr+3.3Mo+8.1V-2 (1)
P = Cr+Cu+2Mo+IOV+7Nb+5Ti-2 (2)
When the value of the parameter P is equal to or greater than zero, the steel may be susceptible to
reheat cracking. The cracks are intergranular relative to prior austenitic grains and occur pref-
erentially in the coarse-grained HAZ of the weld, usually in the parent metal but also sometimes in
the weld metal. There are two distinct fracture morphologies: low-ductility intergranular fracture
(as shown in Figs 2 and 3) and intergranular microvoid coalescence. The former is characterised by
relatively smooth intergranular facets with some associated particles, and occurs during heating
between 450 and 600°C. The brittle intergranular mode is initiated by stress concentrators such as
pre-existing cracks or unfavourable surface geometry.
Compositions that have suffered reheat cracking in practice are Mo or Cr-Mo steels with more
than 0.18% V, all of which have parameter values greater than zero. ASTM steels that are known
to be subject to reheat cracking in thick sections are A508 Class 2, A517 Grades E and F, A533B,
A542 and A387 Grade B. Time constraints precluded a detailed chemical analysis of the nozzle
forging, and the material certificate does not quote residual values of V, Cu, Nb or Ti. According
to eqn (1) above, however, the value of parameter P is 0.571 which would indicate susceptibility to
reheat cracking.
The cracks generally occur during the PWHT heating cycle before reaching soaking temperature,
probably in the 450-700°C range. The heating and cooling rates do not appear to have any significant
effect on the result. Reheat cracks may also form or extend in service if the welded component is
operating at elevated temperature and if joints are exposed to tensile stress, due to either inadequate
PWHT or service loads.
The literature indicates that reheat cracking may be avoided by the following means:
(a) Material selection: for heavy sections, alloy content should be limited as indicated by the
Japanese formulae and vanadium should be limited to 0.10% maximum.
(b) Design to minimise restraint: where restraint is unavoidable, consideration should be given to
intermediate PWHT after the vessel is part welded.
(c) Use of a higher preheat temperature: dressing the toes of fillet and nozzle attachment welds; use
of a lower-strength weld metal.

2.4.1. Fabrication considerations.. Consideration of the foregoing discussion in relation to the
cracking experienced in the nozzle C1 of the catalyst reduction reactor indicates that many of the
factors contributing to reheat cracking were present during fabrication. The following factors are
believed to be particularly pertinent:
(a) Very heavy section thickness (133 mm) of forged nozzle in susceptible material.
(b) High stress concentration at toe of shell-to-nozzle attachment weld.
(c) Over-matched high yield strength filler metal E801 5-B2 (UTS 567 MPa/+83,000 p.s.i.) com-
pared with parent metal specification requirements of A182 gr F12 (min UTS 485 MPa/70,000
p.s.i.) and A387 gr 12 cl 2 (min UTS 450 MPa/65,000 p.s.i.).
(d) Significant delay between completion of welding and subsequent PWHT. The vessel code data
book indicates that the completed weld was subjected to NDE examination by MT and UT on
15 November 1990 but the vessel was eventually only post weld heat treated some 2 months
later on 11 January 1991.

2.5. Conclusions and recommendations
Metallographic evidence, supported by a review of the circumstantial evidence leads to the
conclusion that the crack in the hydrogen inlet nozzle C1 was a reheat crack which probably initiated