| The application of quenched and tempered high strength steel is one of the important measures to lighten and largen the pressure equipments. Reheat embrittlement which might induce cracking is a considerable matter that must be faced while pressure equipment is designed and fabricated with those materials. Taking the07MnNiVDR steel as main study object, the sensitivity of reheat embrittlement and cracking was studied in detail. The mechanism of reheat embrittlement and cracking was discussed. The influence of welding and heat treatment to the occurrence of reheat cracking was analyzed.Results showed that the large heat input of welding was the technologic reason of the reheat embrittlement of07MnNiVDR steel. The content of carbon and alloy elements is unable to cause obvious precipitation harden in postweld heat treatment as it is comparatively low. So it is hardly to cause apparent reheat embrittlement that can induce cracking. However, when the welding heat input increase, the total grain boundary of coarse grain heat affected zone decreases remarkably as the grains become excessively large. Carbides of strong carbide forming elements, such as chromium, molybdenum and vanadium, will precipitate along the grain boundary distinctly. This results in the lack of alloy elements along the grain boundary and forms a certain district that we may call it poor alloy layer (PAL) in which the alloy elements are considerably scarcer than around area. The emergence of PAL will make the material exhibit reheat embrittlement as it weakens the strengthen mechanism of grain boundary and localizes the deformation. At lower temperature, there is no apparent PAL as the carbide precipitation of alloy elements is difficult. While at higher temperature, PAL will be mitigated by the diffusion of alloy elements. Thus, PAL exists in a certain temperature scope that corresponds to the sensitive temperature of reheat embrittlement. The sensitive temperature will be higher while the alloy content is high as it is more difficult to form the PAL. Whereas, the07MnNiVDR steel has lower alloy content, so it is easer to bring PAL at lower temperature and cause a lower sensitive temperature. Tests indicated that the sensitive temperature of07MnNiVDR was about600℃.Impurity elements such as phosphorus segregate at grain boundary worsen the situation of PAL. It causes more serious reheat embrittlement. Although the reheat embrittlement still takes place, it is beneficial to avoid reheat cracking by controlling the phosphorus content to relieve the embrittlement degree. As the phosphorus content of07MnNiVDR is about half of the earlier quenched and tempered steel CF-62and the toughness is enhanced by adding nickel, the reheat cracking risk of07MnNiVDR welding structure should be decreased.As a comparison, similar study of ASME material SA543was executed. This material is nickel-chromium-molybdenum alloy steel plate for use in the quenched and tempered condition, intended for the fabrication of welded pressure vessels and other pressure equipment. Results validated the reheat embrittlement mechanism deduction of07MnNiVDR. To some extent, this material also has reheat embrittlement tendency. It has higher sensitive temperature as its higher alloy elements content, but the reheat cracking risk is higher than07MnNiVDR.In order to eliminate some blindness of our pressure equipments fabrication and change the low reliability situation of some domestic pressure equipments, it is necessary to adopt the risk based design method to guide the production process rationally. This would control the risk of equipment and production in time and effectively. As an example of quenched and tempered high strength steels, this study analyzes the possible risk of pressure equipment which was made of07MnNiVDR in its whole life. Parameters of welding and postweld heat treatment were recommended too. The results of this study have been practiced in engineering successfully. |
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