Stainless steel seamless pipes are widely used in high pressure, low temperature, and corrosion resistant pipe systems. The mechanical properties, technological properties, and corrosion resistance are the three important indicators for measuring the quality of steel pipes, and they are also important guarantees for ensuring safe navigation of ships. According to the production process diagram of stainless steel seamless pipes, seamless steel pipes are made from round steel billets through main processes such as perforation, cold drawing, cold rolling, solution treatment, straightening, pickling, and other supporting manufacturing processes. Each manufacturing process of the pipe complements each other, especially the quality control of raw materials and intermediate products, which is an important guarantee for ensuring that the chemical composition, surface quality, mechanical properties, technological properties, and corrosion resistance of finished pipes meet the standards and specifications.
The main raw materials used for producing seamless steel pipes are continuous casting round billets and rolled (forged) round steel. To ensure the quality of the steel billets is the first line of defense for ensuring the quality of steel pipes, which mainly includes the steelmaking level, casting and cooling process, as well as forming quality. Firstly, improving the steelmaking level requires reducing harmful elements and gases (nitrogen, hydrogen, oxygen), improving the uniformity and purity of the composition, reducing non-metallic inclusions, and changing their distribution morphology, which are key factors. When the composition of the billet is uneven and serious segregation occurs, it will cause severe banded structure of the steel pipe after rolling, which will reduce the mechanical and corrosion resistance of the pipe and even make it unqualified. Therefore, when approving, not only should the steelmaking equipment and process be concerned, but also the testing of stainless steel seamless pipes is required to determine the inspection items, sampling quantity, and acceptance criteria for the inspection of billets, especially the quality items that need to be focused on but are not explicitly designated in the standards and guidelines, such as low magnification and microstructure testing.
The furnaces used in the production of seamless steel pipes include billet heating furnace and steel pipe heat treatment furnace. Temperature control accuracy and uniformity are two important indicators for evaluating the quality of the heating equipment and are important guarantees for the heating process. Therefore, the manufacturer should strictly abide by the provisions of the period of use and calibration period of the thermocouple, as well as the uniformity test of furnace temperature.
The development of the heating process should comprehensively consider the inherent properties of the furnace equipment, the type and quantity of billets or steel pipes, and strictly control the heating speed, holding time, and cooling speed to avoid cracks, overheating or burning. For example, taking the billet heating in the perforation stage as an example, considering that the thermal conductivity of stainless steel is small (slow heat transfer) at normal temperature and the coefficient of thermal expansion is large, there should be a long preheating time in the furnace. The heating rate in the early stage should be slow to prevent thermal cracking. When the billet temperature exceeds a certain temperature (generally around 850°C), the thermal conductivity and plasticity of stainless steel increase rapidly. At the same time, if stainless steel stays at high temperature for too long, α phase will be generated, that is, ferrite will be formed. If α phase exceeds a certain proportion, the metal thermal plasticity will drop sharply. When serious, perforation cannot be performed, and high temperature and long holding time will also make the internal grain size coarse. Therefore, in the homogenization stage, the rapid heating and short-time homogenization should be carried out. In addition, attention should also be paid to the regulation of furnace atmosphere and maintaining a weak oxidizing atmosphere. This can not only reduce the degree of surface oxidation but also prevent carbon enrichment.
The intermediate product inspection in the process of stainless steel seamless pipes control is the key to control the quality of the finished pipe, especially the quality of the inner wall surface, which must be paid attention to from the source.Firstly, a reasonable and effective inspection and processing procedure should be established to ensure that any problems can be detected and addressed effectively. Secondly, sufficient and effective inspection equipment should be provided. Additionally, an effective defect feedback system should be established to analyze and statistics the defects in a timely manner, such as the type of defect, the stage of occurrence, the occurrence frequency, and the processing method, etc., so as to analyze the causes of defects in a timely manner and guide the removal of defects according to unified processes.