Why Equivalent Stainless Steel Grades Are Not Always Identical

There are some stainless steel grades that are considered equivalent since they have similar composition and properties according to various standards. These standards include ASTM, EN, JIS, and GB standards. These grades are commonly used all around the world. The equivalence helps to facilitate comparison of different materials according to the standards of various regions. However, such materials cannot be absolutely identical since there can be small differences in their composition, mechanical properties, manufacturing process, and testing requirements.

One should keep this in mind during the selection of material for his/her own needs. The grades can be identical at first look and have great differences in behavior. There can be differences in welding, machining, corrosion resistance, mechanical properties, and even heat treatment. These differences may affect the final quality of products, their performance, and compliance with the customer's requirements. In this article, we explain the reasons of non-identicalness of equivalent stainless steel grades and provide key points to compare them.

What "Equivalent Grade" Really Means

The equivalent grade of stainless steel is defined as the stainless steel grade from various standards having similar chemical properties and uses. This usually happens in ASTM, EN, JIS, and GB standards. It should be noted that equivalent does not mean totally identical.

Similar chemistry does not mean identical performance

Two types of stainless steel might share the same chemical composition but fail to exhibit the same behavior when used. There are allowed variations in composition of such elements as chromium, nickel, carbon, molybdenum, and manganese in different standards. Even small changes in these components can influence properties of the steel, which include corrosion resistance, strength, hardness, weldability, and formability, causing different performances of apparently equal grades in specific uses.

Higher levels of carbon will result in higher strength and lower corrosion resistance after welding, while an increased amount of nickel improves the properties of toughness and formability, especially in cold conditions. Therefore, two equivalent grades of steel might differ in their behavior during the manufacturing process due to the mentioned variations. Thus, material selection must be made depending on the properties needed, rather than just grade names.

Product standards may be different

The equivalent grades are applied to materials produced to different standards for various products. While one standard can apply to plates and sheets, the other can be developed for bars, forgings, tubes, or pipes. Different standards can have varying regulations for production and heat treatments. Therefore, the materials that have equivalent grades will not necessarily possess identical properties and requirements.

For instance, stainless steel bars may have a requirement for different strength characteristics compared to the corresponding plates. Also, tube standards may require additional testing of welding, flattening, and pressure resistance. Therefore, there can be different requirements for products made from equivalent grades. When choosing material, it is necessary to take into account not only its grade but also the applicable product standard.

Testing and tolerance may be different

Tests requirements may vary from one standard to another. The first standard may demand tests for tensile strength, hardness, resistance to corrosion, or impact resistance, whereas the other standard may not demand such tests. The number of samples to test, as well as acceptance criteria, may differ from one standard to another. Such disparities will affect the extent of quality control of the material.

The maximum size of a material is also subject to variations. Size limitations control aspects such as thickness, diameter, width, length, straightness, and flatness. A certain grade of stainless steel may conform to sizes according to one standard but not others. Even minor size differences may affect the functionality of the material. For this reason, one should always look at the testing requirements and size requirements before considering an equivalent stainless steel grade.

Common Problems in Grade Mapping

Grade mapping refers to the process of comparing the grades of different standards to identify equivalent grades. It is useful for engineers and buyers to select materials that come from various countries. Nonetheless, this approach can be misleading at times since many believe equivalent grades to be exact matches. Table 01 below depicts some of the commonly mapped grades of stainless steel in international standards. There may be minor variations in the chemical composition, mechanical properties, and test requirements of equivalent grades.

Table 01: Common Equivalent Stainless Steel Grade Comparisons Across Standards

304 vs SUS304 vs 1.4301

ASTM 304, JIS SUS304, and EN 1.4301 are often treated as equivalent grades of austenitic stainless steels. These types of steel are extensively used for a wide range of applications. Due to the similarity of their chemical composition, these types of steels are often used as substitutes for each other.

However, these types of steel differ in terms of their standards. It means that the chemical composition and mechanical properties of these steels and test methods might vary. Sometimes this fact can influence the project and customer specifications. That is why engineers need to refer to the complete standard and not only to the type of steel.

316L vs SUS316L vs 1.4404

These grades include molybdenum, which enhances their corrosion resistance. They are superior to stainless steel 304 in severe conditions due to their best corrosion resistance.

Nevertheless, the requirements for the grades of these alloys may differ. They may vary in terms of chemical composition limitations, mechanical properties, or testing. Inspection requirements may also be different. Such a difference will influence customers' demands, compliance, or legal requirements. That is why it is necessary to compare the whole standard.

Duplex stainless steel mapping

It is much more difficult to substitute duplex stainless steel grades compared to austenitic stainless steel. Duplex stainless steel has austenite and ferrite present in its microstructure, giving it increased strength and excellent resistance to stress corrosion cracking. Examples of duplex grades include ASTM S31803 and S32205, as well as EN equivalent grades.

The most frequent error when dealing with such steels is to consider that one can substitute all duplex grades of a certain name. The change of such elements as chromium, molybdenum, nitrogen, and nickel could have a huge influence on corrosion resistance and strength. Some duplex grades are produced with increased corrosion resistance, some with improved strength. That is why, duplex grade substitution should always be checked.

400 series mapping mistakes

Stainless steel 400 series is often challenging when comparing across various standards, as they consist of varying grades of materials. These may include ferritic grades such as grade 430, martensitic grades such as 410 and 420, and free machining grades such as 416. Despite being part of one series, these materials have entirely different properties and applications.

One major error that people make is assuming that any member of the 400 series of stainless steels is equivalent to another. Some of the differences between the materials include 410 having moderate corrosion resistance properties but can be heat-treated to increase strength. On the other hand, grade 430 has high corrosion resistance but low hardness. 416 is mostly focused on machinability.

What Buyers Should Check Before Accepting Equivalent Grades

Buyers have to look into more than just the names of the grades before deciding on equivalent grades for stainless steel. This is because although the names of these equivalent grades may seem the same, the grades are not always identical due to differences in chemical composition, mechanical properties, product standards, and certification requirements, among others. Buyers need to go through the list provided in Table 02.

Table 02: Checklist for Verifying Equivalent Stainless Steel Grades

Chemical composition range

This will be the first thing to do because the standards for stainless steel stipulate the minimum and maximum amounts of such components as chromium, nickel, carbon, molybdenum, manganese, and nitrogen. Even when the grades are regarded as equal, the compositions of each might differ. The variation in amounts of alloying components may have an impact on such factors as corrosion resistance, weldability, strength, and toughness. The lower amount of carbon makes it possible to avoid post-welding corrosion. It is crucial to verify the composition according to the certificate.

Mechanical property requirements

The mechanical properties of the material explain its reaction under applied loads. Such properties include tensile strength, yield strength, elongation, hardness, and toughness. There could be various requirements with different minimum requirements. This is the case even among grades regarded to be equivalent.

The material could comply with certain mechanical properties based on one standard while failing in compliance with others. This is important in pressure vessels, constructions, and other demanding applications. The buyer needs to look at the mechanical properties of the material and compare them to the tests conducted by the manufacturer.

Product form standard

Equal grades need to be checked along with the product standard for stainless steel. Stainless steel products are available in various forms including plates, sheets, bars, pipes, tubes, wires, forgings, and castings. Different standards apply to different product forms which also specify the manufacturing and inspection procedures.

For instance, an equivalent grade could require different things depending on the product form. The plate standard could require something else from the bar or tube standard. The surface finish size limitations and heat treatment regulations could also differ. Even the testing procedure could vary. It is best that the buyer checks the grade as well as the product standard.

MTC and inspection report

Both the Material Test Certificate and the inspection report indicate that the material is meeting the necessary standards. This usually involves information on chemical and mechanical tests performed, as well as heat treatment information. In addition, this includes information regarding the dimensions, heat numbers, and inspection reports.

When considering accepting any equivalent grade, it is necessary to go through all relevant documentation. The Material Test Certificate must indicate the grade, the standard to which it was tested, and the results of the test. The inspection reports must include information regarding the dimensions and appearance of the test.

Conclusion

The equivalent grades of stainless steel may be useful in the process of material selection. They simplify the process of material selection. However, they do not necessarily mean the materials are exactly alike and must always be checked before being used. The chemical and mechanical properties of the grades may vary. In addition to that, standards and test conditions may also vary. Before adopting an equivalent grade, all standards and certificates need to be checked.