Page 81 - Handbook of Materials Failure Analysis

1 Introduction 75

an HK-40 and two HP-40 grades of two different manufactures are showed for
comparison.
Entering in the diagram with a given σ level, the HP alloys show greater LMP
values than the HK-40 grade.
The superior creep properties of the HP-40 alloys, compared to the HK-40 grade,
provide longer rupture life at the same service temperature and allow to produce
tubes with thinner wall or, alternatively, fewer tubes with larger diameter and save
in tubing cost and furnace size. Moreover, it is worth to notice that the slightly dif-
ferent creep behavior of the two HP-40 alloys is probably due to the little differences
of carbon and niobium contents that have percentages of 0.40 and 0.85, respectively,
in the Cronite alloy and 0.45 and 1.5 in the G4852 Micro alloy.

1.3 A PROCEDURE FOR NONDESTRUCTIVE TESTING
ON REFORMER TUBES
Creep properties depend on material microstructure and it is well known that tensile
and creep strength decrease with the increase of tube working time, when changes in
metallurgical features become clearly observable [22].
Investigations on these phenomena require to extract some tubes during planned
maintenance shutdown of plant and then cut samples from them. Unfortunately, it
must be taken into account that this procedure is really expensive, being the cost
of each tube very high (some tens of thousands of Euros); moreover, the condition
of a sample tube may or may not be representative of the total number of tubes in the
furnace. Therefore, it seems worth to develop methods that allow to perform mea-
surements on a large number of tubes, preferably without extracting them from plant.
Assessment on creep damage and residual life of reformer tubes has been
addressed to monitor defects accumulation: as quoted in Ref. [23], conventional non-
destructive testing techniques, such as eddy current and ultrasonic measurements,
allow to detect internal cracks and are useful during the last service stage of tubes;
material degradation phenomena, as carburization and oxidation, can be inspected
observing ferromagnetic layers on tubes surfaces; measurements of magnetic perme-
ability and low magnetic fields can give satisfactory information on effective tube
wall thickness, but cannot indicate creep failures unless micro cracks appear.
In particular, a comprehensive approach should include reformer tube inspection,
remaining life evaluation, and assessment of operational problems [19], as internal
and external inspection methods to check periodically tube conditions at various
creep stages and identify problems with burners or local hot spots, due to gas flow
conditions, evaluating also the effect of high-temperature corrosion. Unfortunately,
assessments of tube residual life are affected by several uncertainty factors.
As quoted previously, investigations on relationship between mechanical prop-
erties and microstructure degradation are current; however, reliable criterion
describing the amount of degradation in creep-resistant tubes during service has
not been established so far [24]. Experiments to predict residual life should be based
on investigations performed during service, but up today they require the tube