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From the above statement, it is obvious that a wide range of steels, having different compositions and heat treatments, are available. Afrox welding consumables are available for welding three types of low alloy steels, with widely varying uses, i.e. creep resisting steels, high tensile steels and steels for use at low temperatures. The welding consumables, steels and procedures for welding these steel types are discussed separately.
Steels for Elevated Temperature Service
Creep is a property of great importance in materials used for elevated temperature applications. Creep is defined as the plastic deformation of steel occurring at an elevated temperature under constant load. Creep is a time dependent failure and occurs at stresses below the yield strength for the particular temperature to which the material is subjected.
Creep occurs in three stages:
• Primary creep (transient stage) – In this stage the creep rate is initially high and gradually decreases due to the effect of work hardening.
• Secondary creep (steady state creep) – The stage in which deformation continues at a constant rate, which results from a balance being maintained between the competing processes of work hardening and recovery.
• Tertiary creep – If the stress is sufficiently high in this stage, the creep rate accelerates until fracture occurs. For practical purposes the resistance to creep is expressed by:
• Creep strength - The stress which, at a given temperature and after a given time, causes failure.
• Creep limit - The stress which, at a given temperature and after a given time, causes a certain amount of deformation, e.g. 1%.
Creep Resisting Steels
When materials are subject to elevated temperatures, the following properties are of major importance – the resistance of the materials to oxidation and the maintenance of an adequate level of tensile strength and creep resistance. Furthermore, the steels must be capable of operating at these elevated temperatures for an indefinite period. It is a well-known fact that chromium increases the strength and oxidation resistance of steel while molybdenum increases the red hardness of steel and its elevated temperature tensile properties. It is not surprising, therefore, that these two elements are the major alloying additions to these steel types. A wide range of creep resisting steels containing between 0,5 and 1% molybdenum and up to 12% chromium have been developed for use in the power generation and petroleum refining industries.
While the addition of chromium and molybdenum improves the elevated temperature properties of the steel, they also significantly increase the hardenability of the steel. It is therefore of the utmost importance that these steels be preheated prior to welding and maintained at the preheat temperature for the duration of welding. Immediately after welding, the fabrications should be stress relieved before cooling below the preheat temperatures.