What Is a Heat Number?

A heat number is a unique identifying code a technician stamps on a piece of metal to provide information about its origins. Metal plate and pipes are most likely to bear this number, and in some cases, the law requires its use on metal products. The heat number provides a method for tracing materials and is an important part of quality assurance and control. People can usually find it near the edge or end of a finished metal piece, and other information may be on the stamp as well.

There are three segments in a heat number. The first digit indicates which furnace a technician used to prepare a batch of molten metal. The next digits indicate the year of the casting, and the last two or three digits describe the number of the casting. A number like 222121 indicates that the metal came from furnace two in the year 2022, and it was the 121st melt for that furnace that year. This information corresponds to a record the manufacturer keeps for future reference.

In quality control, people subject a sample piece of metal from a given batch to some testing to make sure it meets the required tolerances. People will test for contaminants, weak spots, and other issues. If there is a problem with the metal, people can recall other pieces with that heat number to check them and see if they share the issue. The number also corresponds to records about the shift that handled the metal, what time of day it was cast, and so forth. This information can be useful when people want to determine why the same quality control problem keeps recurring.

With the heat number, people can trace all the materials from a particular factory. Along with the heat number, the stamp includes a disclosure of the manufacturer and the facility, if the manufacturer has more than one metalworking shop. The stamp may also discuss the mold number, referring to the mold the company used to make the product. This can be useful for matching replacement parts, as people can use the mold number to get an exact replacement.

Manufacturers may apply the heat number even if the law does not require it because it can be a useful tool for monitoring materials. As long as the company maintains current records, it should be able to look up any piece of metal by its heat number and provide insight into its origins. This can be useful for activities like accident forensics, where investigators may want to find out more about a particular metal product to see if a problem with the metal contributed to the accident. Heat numbers are also useful for recalls, allowing people to identify a bad batch of metal and issue a warning to the public.

Heat numbers allow every piece of steel to be traceable. Steel milling companies must conform to a variety of standards. For instance, the American Institute of Steel Construction's Code of Standard Practice requires steel producers to have an identification system for its materials. The AISC certification process also mandates a documented procedure for identifying materials.

Since around 98% of steel material is reclaimable, modern production processes mostly rely on recycled scrap for melting and alloying into new forms. Meanwhile, pig iron production hasn't gone the way of the dinosaur — it's still vital to the steel industry's supply chain. Still, heat numbers as a materials identification and tracking process help establish audit trails for both the mill and the customers purchasing steel from it.

Heat numbers also prove that quality control was performed for every production run. As previously mentioned, the heat number is a unique identifier stamped onto a metal plate indicating the furnace, year and casting number. Heat numbers are typically applied after each piece has been removed from the ladle and rolled.

Two kinds of quality tests are usually performed on steel: mechanical properties and chemical composition. Mechanical properties testing checks for yield stress, elongation, hardness and toughness. Tensile tests measure yield stress and elongation by placing tension loads on a steel sample until it experiences plastic strain — in other words, deformed past the point of recovery. Yield stress indicates the point at which steel permanently bends out of shape. Hardness ad toughness tests measure how well the steel holds up to deformation, bending, scratching and indentation.

Chemical composition testing checks for the percentage of various metallic elements by weight. Several types of chemical analysis tests are in use today:

Chemical analysis determines whether or not the actual contents of the steel sample match the correct composition and grade that should be produced in the production run from which it came. After all, there can be massive differences between two alloys of similar (but not exactly the same) composition.

Certified mill testing reports detail the results from quality control tests conducted on specific steel samples. As mentioned previously, mechanical properties and chemical composition tests confirm that steel production processes are operating within specified parameters. They also verify that the steel being produced meets quality standards and will perform as customers expect. Similarly, customers rely on mill certificates to know that they're getting the exact materials they need for their own production processes.

Besides appearing on steel material plates, heat numbers also appear on certified mill testing reports. Heat and lot numbers are both included on these mill test certificates. Because each furnace can have more than one heat treatment process, separate lot numbers indicate different heat treatments. Some materials certificates may also indicate the number of pieces in a particular lot or batch. Together, the heat and lot numbers identify the exact production run that the tested sample is from.

As mentioned earlier, a heat number usually consists of three components: the furnace's identifying number, the year in which the steel was produced and the melt number that the piece came from. While this format isn't universal, it's followed by most major steel producers around the world.

Keep in mind that there may be minor formatting differences between steel companies. Some heat numbers are alphanumeric, mixing letters and numbers together. Unless you're told otherwise, the first digit in the heat number identifies the furnace and the second two numbers are the production year. The following digits are t the melt number. If you're not sure, you can confirm details with the mill from which the steel came.

Basically, the terms "heat number" and "heat code" refer to the same set of digits stamped on the ingot or printed on a certified mill test report. Both identify productions run of steel that came from a specific mill's furnace. While industry professionals may use either one of these terms, they are interchangeable.