African Fusion August 2016

Repair of graphitised pipe welds

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.

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].

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].

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August 2016

AFRICAN FUSION