Methods and Standards for Rating Steel Cleanliness via SEM Analysis

From towering skyscrapers to bridges with massive spans, steel is one of the most widely used and important building materials of the modern age. The strength that steel structures can provide depends largely on the quality of the steel used. Steel cleanliness ratings were created in response to the need to ensure uniformity in product quality. Ratings began with simple inclusion counting under an optical microscope, evolved with the development of sulfur printing, and became more sophisticated in the 1970s when minicomputer technology was introduced.

Today, automated scanning electron microscopy (ASEM) represents the state-of-the-art in steel cleanliness evaluation. The future lies in higher magnification analysis of ever finer precipitates and their relationship to complex microstructures. Advancements will come from emerging technologies like Field Emission Microscopes (FEM) with in-lens detectors and A.I.-enabled software to analyze ever-increasing amounts of raw data.

In this article, we’ll look at the fascinating history of steel cleanliness ratings and how this pioneering work resulted in a set of standards that keep our steel infrastructure as safe and reliable as possible.

Why Standard Methods?

The introduction of standards for steel cleanliness and other materials dates to the early 20th century. Standards were driven by the need for high-performance, reliable materials in military applications, though they quickly influenced the broader development of material quality standards across military and commercial sectors. Cleanliness standards were an essential component for increased trade, allowing contracts to specify specific quality requirements.

World War I and II: Laying the Foundations

World War I and World War II both drove a demand for high-quality materials for military equipment, leading to the establishment of more rigorous standards. The need for consistent and reliable materials, especially steel, became crucial to producing military vehicles, ships, aircraft, and weapons.

The first significant standardization in the evaluation of steel cleanliness was the development of sulfur printing (using the Baumann Method) in the early part of the 20^th century.   Combined with chemical analysis, this procedure provided a quantitative evaluation of the quantity and size of sulfide inclusions in steel. As the sulfur content of steels has gone down, the method has less applicability but is still in use for quality reasons in some applications. As optical and electron microscopic procedures developed, they have replaced most of the sulfur print rating of steel

1940s and 1950s: The Birth of Standards

The American Society for Testing and Materials (ASTM) published the first standard test method for determining inclusion content in steel with ASTM E 45 in 1942. This seminal standard continues to play a vital role in the steel industry, providing a standardized approach for evaluating and ensuring the cleanliness of steel products.

The formalization of military standards, often referred to as MIL-STD (Military Standard), also began in earnest during this period. These standards were established by the U.S. Department of Defense and other military organizations to ensure that materials and products met the stringent requirements necessary for military applications.

• MIL-STD-105 (1950): One of the earliest and most influential military standards was MIL-STD-105, which was introduced in 1950. This standard provided sampling procedures and tables for inspection by attributes, which were crucial for ensuring the quality and reliability of materials, including steel, used in military equipment.
  
  
• MIL-S-18729 (1955): Introduced in 1955, this standard specified the requirements for the quality and cleanliness of steel for military use. It included methods for detecting and rating non-metallic inclusions, ensuring the steel used in critical applications met high purity standards.

1970’s: Continued Evolution

Military standards continued to evolve, incorporating the rapid advancements in materials science and testing methods. During this period, standards became more detailed and specific, addressing various aspects of material quality, including cleanliness, mechanical properties, and performance under different conditions.

Over time, military standards began to integrate more closely with industry standards, such as those developed by ASTM, ISO, and other organizations. This integration helped ensure that materials used in both military and commercial applications met consistent quality criteria.

This period also saw the introduction and commercial availability of technologies that allowed for detailed imaging of microstructures, including Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and X-ray Spectroscopy. Large steel producers began to adopt SEM as part of their research and development and quality assurance programs.

1980’s – Today: The Computer Era

Beginning in the 1980s, computers and software began to significantly shape the way the industry analyzed steel in much the same way they shape the rest of society. For the first time, computerized image analysis allowed for greater automation increasing efficiency and accuracy while providing more consistent measurements.

Imaging technology also continued to evolve with the introduction of Electron Backscatter Diffraction (EBSD), Energy Dispersive X-ray Spectroscopy (EDS), and higher-resolution optical microscopes and SEMs. This era also saw the development of new techniques for sample preparation that greatly improved the quality of metallographic samples.

In recent years advanced software technology combined with ever more powerful imaging technology has resulted in the ability to image in 3D, monitor quality in real-time during production, and find inclusions down to the nanoscale.

Machine learning and artificial intelligence have led to even more powerful automation, allowing testing laboratories to process large volumes of samples and characterize hundreds of thousands of inclusions at the click of a button.

Current Steel Cleanliness Standards

By leveraging the latest technology and techniques, steel cleanliness standards have continually evolved to keep pace with the growing demand for steel that is lighter, stronger, and more cost-effective. Standards are used across the industry to maintain quality, giving manufacturers and contractors access to consistent and predictable steel. Today, steel quality is assured through a series of international standards

ASTM: US-based ASTM International is the oldest and perhaps the best-known standards organization, with standards that are used and accepted worldwide.

• ASTM E45: This standard covers the microscopic determination of the inclusion content of steel. It outlines methods to evaluate the type, size, and distribution of non-metallic inclusions in steel. It is the most general standard, allowing for either chart comparison, severity rating, or SEM analysis, known as Methods 1 – 3.

Inclusions are separated into 4 categories based on morphology and classified based on their width ranging from thin (T) to heavy (H) to oversized (O).

• Type A - deformable sulfides
• Type B - non-deformable oxides
• Type C - deformable oxides/silicates
• Type D - isolated inclusions/globular.

Tables categorizing the inclusions into groups based on parameters defined in the standards constitute the severity rating. The rating can be based on the worst field, average field, or severity.

• ASTM E2142: This practice provides a standard for evaluating the cleanliness of steel using automated scanning electron microscopy and energy-dispersive X-ray spectroscopy. It mimics E45 in its methods and rating procedures.
  
  
• Method 1 mimics E45 using morphology as the rating criteria.
• Method 2 adds the inclusion typing and severity rating for stringers but replaces the optical identification by color or shape with a chemical classification.
• Method 3 can be tailored (individualized) to the specific application's needs, ie, reports on abundance and composition can be customized.

ISO Standards: The International Organization for Standardization (ISO) is a European-based standards organization that develops standards across a range of industries.

• ISO 4967: This standard specifies the methods for determining the content of non-metallic inclusions in steel using microscopic examination of polished sections.
  
  
• ISO 13520: This standard specifies a method for determining the content of inclusions in steel using the step-by-step method of microscopic examination.

SAE Standards: The Society of Automotive Engineers (SAE) is a professional association that provides standards, in the fields of automotive, aerospace, and commercial vehicle engineering.

• SAE J422: This standard describes the methods for evaluating the cleanliness of steel by measuring the number of non-metallic inclusions.

European Standards: A consortium of European standards organizations are responsible for developing standards used across Europe,

• EN 10247: This standard specifies the microscopic method for determining the inclusion content of steel.

JIS Standards: The Japanese Industrial Standards Committee (JISC) is responsible for creating and updating industrial standards in Japan.

• JIS G 0555: This standard outlines methods for determining the cleanliness of carbon steel for machine structural use.

The Future of Steel Cleanliness Rating

As steel gets lighter and stronger, the demands on cleanliness go up exponentially. Accordingly, the need to characterize smaller and more complex inclusions and precipitates also increases. The result is that optical microscopes as a rating tool, and even tungsten filament-based SEMs, will soon become obsolete. More powerful technologies, such as Field Emission SEMs, will replace them as the standard.

New sampling schemes will be required to quantify abundance, and more complex algorithms for multiphase inclusion characterization will be required. Testing and analysis of steel samples will continue to evolve, and the standards that they inform will continue to be updated. The challenge for the steel industry will be to keep pace with these changes so they can continue to provide ongoing innovation to their customers.

RJ Lee Group

RJ Lee Group has deep roots in the steel industry. Dr. Richard J Lee was instrumental in developing the very first scanning electron microscope procedures for analyzing steel inclusions in the 1960s. We have continued to support steelmaking clients by not only adopting the latest in microscopy technology but also leading the charge in developing new methods and techniques to address the changing needs of the industry.

One example is our IntelliSEM™ software, developed in-house by RJ Lee Group to meet the needs of our customers. IntelliSEM represents the very latest in SEM-based automatic image analysis (AIA) because not only can it produce steel cleanliness ratings, but it also allows multiphase inclusions to be characterized. The current IntelliSEM, with its sophisticated workbench and multiphase mapping, is leading the way into the future of automated particle characterization and inclusion analysis.

From cleanliness to coatings to failure analysis, RJ Lee Group offers a wide range of services in support of the steel industry. To discuss your needs with a scientific expert with experience in the steel industry, contact RJ Lee Group.