At a Shandong seamless steel tube factory, the quality inspection process for seamless steel pipes is comprehensive and strictly followed to ensure the highest standards of performance and reliability. The following are the key methods used in the inspection:
1. Chemical Composition Analysis: This involves both chemical and instrumental analysis. Instruments such as infrared C-S analyzers, direct reading spectrometers, and N-O analyzers are used to determine the presence of elements like carbon, sulfur, nitrogen, oxygen, and others. For example, the infrared C-S instrument is used to analyze ferroalloys and steel materials, while the direct reading spectrometer measures elements such as carbon, silicon, manganese, phosphorus, and more. The N-O analyzer is used to detect gas content like nitrogen and oxygen.
2. Geometry and Shape Inspection: This step ensures that the dimensions of the steel pipe meet required specifications. Wall thickness is measured using micrometers or ultrasonic gauges, with at least 8 points checked at each end. Outer diameter and ellipticity are assessed using snap gauges, vernier calipers, and ring gauges. The length of the pipe is verified using a steel tape measure, either manually or automatically. Bending is tested with rulers, horizontal rulers, feeler gauges, and thin lines to measure curvature per meter and overall shape. The end face angle and blunt edge are also inspected using squares and cardboard templates.
3. Surface Quality Inspection: A 100% visual inspection is conducted under proper lighting conditions, using experienced personnel to check for markings, surface defects, and overall appearance. Non-destructive testing (NDT) methods include ultrasonic testing (UT), eddy current testing (ET), magnetic particle testing (MT), electromagnetic ultrasonic testing, and penetration testing. Each method has specific standards and levels, such as GB/T 5777-1996 for UT, GB/T 7735-2004 for ET, and others for MT and penetration testing.
4. Mechanical Performance Testing: This includes tensile tests to evaluate strength and plasticity, impact tests to measure toughness, hardness tests to assess material resistance, and hydraulic tests to check pressure resistance. Tensile samples come in various forms, including longitudinal, transverse, arc-shaped, and round specimens. Impact tests use CVN values and different notch types, while hardness is measured using Brinell, Rockwell, or Vickers scales.
5. Process Performance Testing: These tests evaluate the pipe's ability to withstand deformation during manufacturing. Flattening tests, ring pull tests, flaring, curling, and bending tests are performed to ensure the material remains intact under stress. For large-diameter tubes, bending tests can replace flattening tests.
6. Metallographic Analysis: This involves high-magnification microscopic examination of non-metallic inclusions, grain size, and microstructure, following standards like GB/T 10561. Low-magnification tests, such as acid etching and sulfur testing, are used to identify macroscopic defects like segregation, bubbles, and inclusions. Additionally, the tower hairline test checks for surface cracks and their distribution.
The above inspection methods ensure that every seamless steel pipe produced meets strict quality and safety standards before it is released for use.
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