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16
AFRICAN FUSION
August 2016
Repair of graphitised pipe welds
P
lain carbon steels that are in service at elevated tem-
peratures (typically 400-450 °C) for prolonged periods
of time (typically in excess of 20 years) may experience
graphitisation, often on the border of the visible heat affected
zone (HAZ). This study aimed to determine the effects of this
HAZ graphite on the mechanical properties of plain carbon
steels. Additionally, it was required to evaluatewhether repair
welding of such graphitised material was viable.
A number of extended heat treatments were performed, in
associationwith various joint configurations. A combination of
gas-tungsten arc welding and shieldedmetal arc welding was
used. Transverse tensile tests were performed but tensile test
samples did not always fracture through the graphitised HAZ
region. Failure that did occur in the graphitised HAZ resulted
in a decrease in reduction in area; but no other mechanical
properties were affected by the presence of HAZ graphite.
It was demonstrated that it is possible to perform repair
welding on graphitised material using conventional welding
procedures.
Introduction
C-Mn steels are widely used in processes where the materials
are exposed to elevated temperatures and pressures for pro-
longed periods of time. This type of operating environment is
typical for pipelines that transport superheated steam. Pro-
longed exposure (in excess of 10 years) of a carbon-manganese
steel steam line subjected to moderately high temperatures
(in the range of 400-450°C) results in amicrostructural change
known as secondary graphitisation.
Secondary graphitisation has been the cause of several
catastrophic failures, the most notable of which was that of
the Springdale Generation Station in the USA in 1943 when a
high temperature steam pipe failed [1].
It is generally accepted that the presence of graphite
reduces the tensile strength, ductility, and hardness of con-
ventional C-Mn steels [2].
Graphite formation in carbon steel has classically been
defined as the decomposition of the metastable cementite
phase to form stable ferrite and graphite [3]. It has been dem-
onstrated that steelswith the same chemical composition, but
different microstructures (induced through heat treatment)
prior to graphitisation exhibit a rate of graphitisation thatmay
vary significantly [4]. This observation indicates that the rate
of graphitisation is likely to be determined by the stability of
the carbides present in the steel, with martensitic and higher
carbon containing carbides such as the chi phase decompos-
ing faster and providing free carbon for graphitisation atmuch
higher rates than cementite does. Much of the early literature
reports successful attempts to graphitise various steels using
prolonged heat treatments at temperatures ranging from
600‑760°C [4], [5], [6], [7], [8].
Secondary graphite manifests in steels in two ways:
1. Homogeneously nucleated throughout the material –
Figure 1.
2. Heterogeneously nucleated along the heat-affected zone
of welded joints (HAZ graphite) – Figure 2.
Homogeneously nucleated graphite nodules are generally
considered to have little to no effect onmechanical properties
and are rarely (if ever) a concern.
Heterogeneously nucleated graphite nodules, on the other
hand, tend to form along the heat-affected zone of welded
joints. This form of graphite tends to form near-continuous
planes that may affect mechanical properties.
The service temperature often determines whether
graphitisation or spheroidisation occurs. Figure 3 illustrates
that, for a particular steel, the transition temperature where
spheroidisation becomes more favoured than graphitisation
is at approximately 540°C. This transition temperature is dif-
ficult to determine and it is unclear which factors govern the
transition temperature [9].
Repair welding of carbon steel pipe
that has experienced partial graphitisation during
elevated temperature service
In this paper, presented at the 69
th
IIW Annual Assembly and Conference in Melbourne dur-
ing July, PGH Pistorius and KJ Kruger from the SAIW Centre for Welding Engineering at the
University of Pretoria, and CPM Orsmond from Sasol Synfuels report on an investigation
into the graphitisation effects of plain carbon steels that have been in high temperature
service for many years. The authors also investigate the viability of repair procedures to
rehabilitate the mechanical properties of pipe made from these steels.
Figure 1: Randomly distributed graphite in a 1%C, 0.23%Si,
0.34%Mn steel that has been austenitised at 1 000°C, quenched and
heat treated in air at 650°C for 100 hours [3].