What are the key points for quality control of seamless stainless steel pipes

Seamless stainless steel pipes are widely used in piping systems for high-pressure, low-temperature, and corrosion-resistant environments. Mechanical properties, technological properties, and corrosion resistance are three crucial indicators for evaluating the quality of steel pipes and are also essential guarantees for ensuring safe navigation of ships. In actual ship applications, a stainless steel chemical tanker discovered leaks in its deck cargo piping system after its maiden voyage; pressure testing revealed more than 10 pinhole leaks. During the manufacturing process of cargo tanks for an ethylene ship, multiple surface quality problems were found in the tank penetration pipes, including pitting and surface cracks. So how can these problems be fundamentally solved? Seamless steel pipes are made from round steel billets through main processes such as piercing, cold drawing/cold rolling, solution treatment, straightening, and pickling. Each stage of pipe manufacturing is complementary. In particular, quality control of raw materials and intermediate products is crucial to ensuring that the finished pipes meet specifications and standards in terms of chemical composition, surface quality, mechanical properties, technological properties, and corrosion resistance.

1. Controlling the Quality of Raw Materials for Seamless Stainless Steel Pipes. 

The raw materials used in the production of seamless steel pipes are mainly continuously cast round billets and rolled (forged) round steel. Ensuring the quality of the billets is the first step in ensuring the quality of the steel pipes. This mainly includes the steelmaking level, casting and cooling processes, and forming quality.

(1) Improving the Steelmaking Level. It is crucial to reduce harmful elements and gases (nitrogen, hydrogen, oxygen), improve the uniformity and purity of the composition, reduce non-metallic inclusions, and change their distribution morphology. When the composition of the billet is uneven and severe segregation occurs, the rolled steel pipe will exhibit a severe banded structure, thereby reducing the mechanical and corrosion properties of the steel pipe, and even rendering it unqualified. Non-metallic inclusions (such as sulfides, oxides, and silicates) are pressed into thin sheets, which not only affects the performance of the steel pipe but may also cause cracks during the production process.

(2) Improving the Casting and Cooling Processes. Reducing subcutaneous bubbles, subcutaneous cracks, porosity, and shrinkage cavities is also crucial, as these defects, regardless of type, can cause problems during piercing and rolling. Some defects amplify after use, shortening product lifespan; in severe cases, they lead to direct scrapping of intermediate products, such as internal folds.

(3) Deviations in the billet's shape, such as straightness, diameter, and ovality, directly affect piercing quality, causing defects in the rough tube. Therefore, during approval, it is necessary not only to focus on steelmaking equipment and processes but also to conduct tests extending to seamless steel pipes. This approval process determines the inspection items, sampling quantities, and acceptance standards for billet factory inspection, especially quality items not explicitly stated in specifications and guidelines but requiring special attention, such as low-magnification and microstructure testing.

2. Ensuring the accuracy and uniformity of furnace temperature control for seamless stainless steel pipes. 

The furnaces used in the manufacture of seamless stainless steel pipes include billet heating furnaces and steel pipe heat treatment furnaces. Temperature control accuracy and uniformity are two important indicators for evaluating the quality of heating equipment and are crucial for ensuring the heating process. Therefore, manufacturers should strictly adhere to the regulations regarding the service life and calibration cycle of thermocouples, as well as the testing of furnace temperature uniformity. The formulation of the heating process should comprehensively consider the inherent properties of the hot furnace equipment, the type and quantity of billets or steel pipes, and strictly control the heating rate, holding time, and cooling rate to avoid cracking, overheating, or burning. Taking the billet heating during the piercing stage as an example, considering the low thermal conductivity (i.e., slow heat transfer) and high expansion coefficient of stainless steel at room temperature, a relatively long preheating time is necessary in the furnace. The initial heating rate should be slow to prevent hot cracking. Once the billet temperature exceeds a certain level (generally around 850℃), the thermal conductivity and plasticity of stainless steel increase rapidly. Simultaneously, prolonged residence in the high-temperature zone will produce the α phase, i.e., ferrite. If the α phase exceeds a certain proportion, the metal's hot plasticity decreases sharply, potentially preventing piercing. Furthermore, high temperature and prolonged holding can lead to coarse internal grains. Therefore, rapid heating is crucial during the homogenization stage, completing homogenization in a short time. In addition, attention should be paid to controlling the furnace atmosphere, maintaining a weakly oxidizing atmosphere to reduce surface oxidation and prevent surface carbonization.

3. Optimizing the Inspection Process and Equipment for Seamless Stainless Steel Pipes.

The inspection of intermediate products during the seamless stainless steel pipe manufacturing process is key to controlling the quality of the finished pipes, especially the inner wall surface quality, which must be addressed from the source.

(1) Establish reasonable and effective inspection and handling procedures. Ensure that existing problems can be identified and effectively addressed. Taking the raw pipe as an example, shot blasting or pickling/passivation is generally used to remove surface oxides to achieve the inspection state. To determine whether defects have been completely removed, it is necessary to combine dye penetrant testing, remelting, and pickling as inspection methods. This effectively ensures that defective intermediate products do not flow into the next process.

(2) Sufficient and effective inspection equipment is also required. This includes endoscopes, thickness gauges, and high-intensity flashlights. Older equipment has a low identification rate, and even experienced inspectors may miss defects. Newer equipment can quickly identify the type and location of defects.

(3) Establish an effective defect feedback system. Analyze and statistically process defects promptly, including defect type, stage of occurrence, frequency of occurrence, and handling method. This allows for timely analysis of the causes of defects and guidance on removing defects according to a unified process.