Exploration • Drilling • Field Services
Working with specialised steels
Hans-Willi Bonn, Ingo Detemple & Jörg Maffert reveal their experience with welding and stress-relieving heat treatment of CrMoV steels.
C
hromium-molybdenum steels are used for the construction of apparatus and equipment in which complex processes take place. Tese generally take the form of petrochemicals plants
and refineries, also referred to as ‘downstream’, where chemical products and/or synthetic fuels are produced from natural gas or crude oil. Two of these process operations are
‘hydrotreating’ and ‘hydrocracking’. Hydrocracking is a catalytic cracking process conducted in the presence of hydrogen, in order to convert higher molecular-weight hydrocarbon fractions to intermediate products for the production of motor gasoline, kerosene and diesel fuel. Te process is operated at hydrogen pressures of up to 200 bar and temperatures of up to 480° C. Te apparatus in which these processes take place are referred to as ‘reactors’ and are generally designed for 40 years of productive operation. What makes these steels so special? In addition
to their resistance to high-pressure hydrogen and their mechanical and technological properties, they are also required to withstand a further damage mechanism: temper embrittlement. Te steel therefore needs low levels of tramp elements such as tin (Sn), antimony (Sb) and arsenic (As), targets that can be achieved using the oxygen-blowing steelmaking process. Tese steels are alloyed using Cr, Mo and, in some cases, with V, which fix the carbon contained in the steel in special carbides, i.e., chemical compounds between the alloying elements and carbon. Any hydrogen that enters the steel is fixed, at non-critical concentrations, to the finely dispersed vanadium carbides, and is thus not available for any reaction of carbon with hydrogen to form methane, a reaction that can seriously damage the steel. Welding also requires special properties in the
parent steel material. Stress relief of the structure is necessary as well as the precipitation of carbon- fixing special carbides in the weld metal. Te completed vessels are annealed under extreme conditions for this purpose. Te parent material is, in fact, required to withstand these heat-treating conditions multiple times and nonetheless remain able to bear its operating loads safely after completion of
fabrication. Tese production and welding requirements, and also subsequent heat-treating requirements, usually originate from the API (American Petroleum Institute) 934 series of documents. Tese documents are extremely high-ranking recommendations to industry, and are used around the world as the basis for the definition of the technical requirements.
In the following paper, the authors examine the necessity and influence of post-weld stress-relieving annealing (PWHT = Post Weld Heat Treatment) in accordance with API RP 934 A, and also of heat input, on the welded joint. Requirements concerning PWHT originate from weld filler material manufacturers, material users and equipment operators. Te presence of the necessary microstructure is
the precondition for attainment of the necessary toughness in the weld metal. PWHT thus serves not only to reduce welding residual stresses but also, and in particular, for the achievement of the desired mechanical properties in all the possible operating states that the reactor can undergo during its service-life. Tis requirement must be harmonised with the
parent material, the mechanical and technological properties of which are significantly impaired by excessive heat treatment. Te end customer, equipment fabricator, weld filler material manufacturer and steel producer cooperate closely in agreeing PWHT conditions.
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Fig. 1. Temperature-time diagram: rise in HP value via various heat treatments.
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