Development of Cr-Mo-V ferritic steels for high temperature hydroprocessing reactors with enhanced behaviour

The operating temperatures of the hydroprocessing reactors are at present in the range 400-480°C with a corresponding hydrogen partial pressure from 10 to 35 MPa. Currently the thick walled reactors are made by 2.25CrlMo or 3CrlMo steels. The last generation (the 5 t h ) with the addition of V was developed in 1990s. The reduction of the reactor wall thickness, with consequent reduction of production costs, was the continuous driving force for the development of alternative higher strength materials The aim of the project was the development of a new steel chemistry of Cr-Mo steels for petrochemical plants, with tailored additions of elements, like V, Nb, Ti, B to: increase the tensile properties to obtain, particularly, yield strength > 450 MPa, maintain the hardness in the welded zone below 250 HV, reduce the disbonding due to hydrogen absorption at the interface of the internal protective coating, reduce the wall thickness of the reactor. The main objectives are: Development of a new steel chemistry of Cr-Mo steels for petrochemical plants with higher yield strength and better behaviour in both hot and cold hydrogen; An increase of yield strength of the base material at 485°C compared to the commercial available materials. Target minimum yield strength at least 15% higher than that for the 5 t h generation of Cr-Mo steels; Development of SAW filler metal and welding procedures to obtain a good weldability and a welded joint with hardness values below 250 HV; Availability of a steel with more resistance to hydrogen attack under high stress and at the upper limit of the yield strength: improvement of the resistance to Nelson attack under severe conditions; An evaluation of the benefits achievable through the reduction of thickness and weight: workability, transport and erection costs; The project was co-ordinated by CSM and the partners involved were European producers of steels and forged components for the hydroprocessing plants such as ARCELOR and Societa delle Fucine (SdF), centres for research and development in materials as CSM, TNO and ISQ, as well as filler material producer (Metrode) and University (Lille). The approach to the aim of the project was semi-empirical and based on laboratory scale production route. The main tasks of the project, distributed to the different partners with CSM as coordinator, were: Design of the new steel chemistry version, also with Thermocalc analysis, and production of alloys on laboratory scale. Set up of the heat treatment. Set up of filler metal and welding procedure. Mechanical and microstructural characterisation of base material and welded joint. Evaluation of Nelson hydrogen attack and shut down behaviour. Production of an industrial proptotype heat for plates manufacturing. A matrix of compositions was defined, on the base of the following main lines of development: - addition of W to replace Mo, in order to increase the mechanical properties; reduction of C and impurities (P, S, As, Sb), to reduce temper embrittlement; - addition of Ti, B, Nb, N and balancing of them and other elements like Si, Mn and Ni, to improve the mechanical behaviour as well as the hydrogen resistance and disbonding, without reduction in weldability. For the alloy design Thermocalc analysis for the prediction of the carbides formation and stability and the transformation temperature was performed by CSM. Laboratory scale materials ingots by Vacuum Induction Melting (50-200kg) using low impurities raw materials up to 10-15 heats were produced by CSM and these were transformed in thick plate by hot rolling (CSM). The set up of the heat treatment conditions required determining typical CCT (ISQ and CSM). Also on the basis of laboratory tests, temperature, time and cooling rate for normalising and tempering were defined (TNO, CSM).