Metallurgical analysis of seamless steel pipe

Metallographic analysis of seamless steel tubes is crucial for assessing their internal structure and identifying any potential defects. This analysis is typically divided into high magnification and low magnification tests, each serving different purposes and providing various insights into the material's properties.

High Magnification Test

A high magnification test involves using a microscope to examine the internal structure and defects of the material at a high level of detail. For seamless steel tubes, common high-power test items include:

Microscopic Evaluation of Non-Metallic Inclusions: This test is generally performed at a magnification of 100 times the field of view. The most severe inclusions in the sample are compared with a standard level map to assess the inclusion levels in the steel.

Determination of Average Grain Size: Evaluating the grain size helps in understanding the material's mechanical properties.

Determination of Decarburized Layer Depth: This measures the depth of the layer where carbon content has been reduced, which can affect the tube's strength and hardness.

Microstructure Inspection: This involves examining the microstructure to identify phases, grain boundaries, and any defects.

Non-metallic inclusions are graded according to the A method in GB/T 10561 and can be classified as:

Type A: Monosulfide type

Type B: Aluminum oxide type

Type C: Monosilicate type

Type D: Spherical oxide type

DS Inclusions

Each of these characteristics is compared with corresponding standard rating charts to ensure consistency and accuracy in evaluation.

Low Magnification Test

Low magnification tests, also known as macroscopic analysis, involve assessing low-degree tissue defects by visual observation or under a magnification of 10 times or less. Common low magnification test methods include:

Acid Etching Test: Used primarily for tube blank inspection to reveal the low magnification structure and defects such as looseness, segregation, subcutaneous bubbles, peeling, white spots, and inclusions.

Sulfur Test: Mainly used when the sulfur content in the steel is high; it has gradually been replaced by acid etching when the sulfur content is low.

Tower Hairline Test: This method involves using the naked eye or a magnifying glass (up to 10 times magnification) to observe and verify the number, length, and distribution of hairlines on each turning surface of the sample.

These tests are essential for ensuring the quality and reliability of seamless steel tubes. By conducting both high and low magnification tests, manufacturers can identify and address any potential issues, ensuring that the final product meets the required standards and specifications.