First things First and Disclaimers
Steel is a complex engineered material that comes in almost an infinite variety of chemistries and designations, each tailored to a specific application. The material science is complicated enough by itself, to say nothing of the steel selection process, even for a seasoned engineer. The situation certainly isn’t helped by the fact that there are multiple international standards with their own unique designators for the steel chemistries and mechanical properties. Steel is essentially the foundation of modern industrial economies, and it is employed in many different modern manufacturing capacities. As such, it’s important to understand how two of the biggest international specifications, JIS and the AISI standards match up against each other.
Before this can be undertaken, it must be recognized that two exactly equivalent steel specifications do not exist. Even if two comparable steel specifications across different standards possess the same mechanical properties and even the same chemical composition, the steels manufactured to these two standards may not be exactly equivalent. The simple realization that the two different standards are maintained by two separate organizations means that they may diverge in the future, or have diverged at some minor inflection in the past. The fact is that finely tuned nuances may exist in the chemical compositions and processing techniques that make two identical steels across standards impossible.
AISI/SAE and UNS Designation Definitions
On one side of the Pacific Ocean stands the American Iron and Steel Institute (AISI) which is simply a trade association consisting mainly of North American Steel manufacturers. This organization, originally chartered in the US, traces its roots back to the middle of the 19th Century, staking claim to the title of one of the first trade associations. One of the ways that it accomplishes its primary mission of advancing the steel industry is through the creation and maintenance of a common set of standards, which are today maintained by the Society of Automotive Engineers (SAE).
The Unified Numbering System (UNS) steel standards are very similar to the AISI standards because they are simply a renumbering of the underlying AISI standards. The only minor differences between the two standards are the fact that the UNS standard typically possesses an extra letter in the front of the specification (in this case “S” for “Steel”) and a few extra digits of identifying information at the end of the specification. This is apparent by examining the expansive table shown at the end of this discussion.
AISI Carbon and Alloy Steels
For carbon and other common alloy steels, the naming convention is actually intuitive to understand, and it’s based on a simple four-digit numbering system, where the first digit tells you the classification of the steel. For example, 1XXX is reserved for carbon steels, while 9XXX is reserved for Silicon-Manganese steels. Next, the final two digits in the specification denote the alloy weight % of carbon in “hundredths” of a percent. In this way, it’s possible to know a bit about the composition of the steel, simply from its name. For example, 1040 AISI steel is carbon alloy steel that contains 0.40 weight % Carbon alloy. The full breakdown of this table is shown below.
Stainless Steels and High Strength/Low Alloy Steels
For Stainless steels, the designation is shorted slightly to a 3 digit number, but the same general rule applies: there are 100,200,300 etc., up to 900 stainless steel series levels depending on the main alloying ingredient and iron phase. For example, 300 series stainless steels are austenitic phase, Chromium alloy steels while 400 series stainless steels are ferrous and martensitic phase steel with chromium with chromium alloying ingredients. However there is a slight difference here in how stainless steels are numbered in the UNS/AISI standard, as they are simply numbered in a somewhat sequential fashion such that the chemistry of 301 Stainless Steel is entirely different from 304 Stainless Steel.
On the opposite side of the Pacific the “Japanese Industrial Standards” or JIS. These standards are maintained by the Japanese Industrial Standard Committee and set the standard for all industrial activities in Japan.
JIS standards are denoted as follows: “JIS X 0208:1997″, where X is a division letter (A-Z) represents the specific area division), followed by a four-digit identification number. Sometimes there are actually 5 digits cited when the JIS standard directly references a corresponding International Standards Organization (ISO) standard. Finally the last four digits are the revision release year.
The industrial disciplines covered by these standards are incredibly expansive- JIS standards exist for an amazing breadth of topics from ceramics to aviation, but for the purposes of this discussion, the steel specifications are found in the G standard.
However, it’s not too difficult to link the AISI standards to JIS specifications Since the JIS specs post-date the AISI standards, the JIS specs pretty much follow along in the path that the AISI standards blazed with the exception being that the numbers is JIS specs are simply sequentially numbered and do typically contain any information about the steel chemistry,
The JIS number roughly follows the AISI format wherever possible. For example, SUS 304 is equivalent to AISI 304. Note also that the JIS standards for steel also contain a three letter identifier in front of the specification; for example, stainless steels are denoted as SUS XXX while tool steels are designated as either SKH/SKD/SKS XXX, etc. Additionally the special purpose steels such as chromium or silicon steels, which would be the AISI 5000/6000/9000 series steels, are generally designated as SUP/SUJ/SUM in the JIS specification.
After all this discussion, it’s important to remember the disclaimer at the front of this section- although steel specifications might be comparable, they should not ever be assumed to be equivalent, especially for new steel chemistries such as high speed tool steels like AISI T4/T5. It is only after a careful study of the chemical composition of the steel and processing parameters across both standards, that an assessment of equivalence can really be made. These nuances exist for every steel grade across all international standards, and the differences are waiting to be discovered!