Tool Steel Color Code Chart for Grade Identification
The colour painted on the bar end to keep one tool steel grade out of another grade’s rack, with the AISI and SAE numbering system that sits behind it, from a tool and die shop handbook. A reference summary of published practice and not an Aobo Steel specification.
Why the ends of the bars are painted
Two grades that look alike in the rack harden differently, and the mistake is usually found after the heat treatment, when the part has already been machined. A grade mixed into the wrong job comes out undersized, cracked or soft, and what is lost is the tool rather than the bar. Colour marking exists to make that mix-up visible before the cut.
That is why the same grade can appear twice in the list below in two different colours. The mills marked the standard condition and the special quality condition of a grade separately, so a storeman could tell the two apart without reading a tag.
Colour code used to mark tool steel grades
AISI grade and the colour painted on the bar
| AISI grade | Colour used on the bar end |
|---|---|
| A2 | Blue |
| D2 | White |
| H13 | Black |
| H13 SMQ | Black and white |
| M2 | Purple |
| O1 | Brown |
| S7 | Red |
| S7 SMQ | Yellow |
| S5 | Pink |
| W1 | Green |
| 4140 PH | Orange |
| 1018 CRS | Gold |
| 410 SS | Grey |
| 420 SS | Blue and red |
Source, Table 5-1, colour coding scheme for various tool steels, printed page 70 of Tool and Die Making Troubleshooter (Richard M. Leed), PDF page 87 of the copy consulted.
The colour is painted on the sawn end of the bar or on the surface near the end. SMQ in the table is the mill’s special quality marking; the same grade in that condition carries a different colour from the standard condition. 4140 PH, 1018 CRS, 410 SS and 420 SS appear in the scheme because they are stocked beside tool steel in the same rack. One mill’s colour scheme is not another mill’s, so the code is only usable together with the supplier’s own list and the certificate that came with the bar.
Reading the AISI and SAE numbers
The colour tells a storeman which bar to pick. The four digit number tells a buyer what is in it. The first digit gives the steel category, and for tool steels that is the letter of the group. The second digit gives the approximate percentage of the main alloying element, and the last two digits give the carbon content in hundredths of a per cent. The same logic runs through the alloy steels, so 4150 is a chromium molybdenum steel with 0.50 per cent carbon.
The tool steel groups are separated by how they harden rather than by what they contain. The least alloyed are water hardening, the next group oil hardening, and the most heavily alloyed air hardening. Shock resisting, hot work, high speed and plastic mold groups sit alongside them, and each group takes its own letter. A purchase that goes out on the letter and the number still leaves the condition open, and that is the part the colour code was invented to close.
What the colour cannot tell you
The paint is on the bar end, so it disappears with the first saw cut, and a bar that has been cut for stock is the hardest one to identify afterwards. A shop that buys cut lengths receives the colour only if the supplier marks them. Colour is therefore a rack check at goods inwards and not an identity test, and it is no substitute for the certificate that comes with the bar.
Two further limits are worth stating, because the list above is one mill’s scheme. Mills and distributors set their own colours, and the same colour is used for a different grade at a different supplier, so the chart has to be read with the supplier’s own list beside it. The colour also says nothing about the condition of the steel beyond the grade marking, so hardness, decarburisation and the annealing state are still checked on the certificate or with a hardness test rather than by eye.
Where the grades in this list are used
The cold work grades in the chart are covered on their own pages, with the composition, working hardness and heat treatment for each of them, for D2 tool steel, O1 tool steel and A2 tool steel. The hot work grades that appear in the same racks are on H13 tool steel and S7 tool steel, and the stainless grades are on 420 stainless steel. Where a grade number has to be traced across standards, the tool steel equivalent grades chart carries the AISI, DIN, JIS and GB equivalents side by side.
Compiled from Tool and Die Making Troubleshooter (Richard M. Leed), the colour coding scheme on printed page 70 and the discussion of AISI and SAE numbering on printed pages 69 to 71. The codes are reproduced from the published table and are a reference summary rather than an Aobo Steel specification; colour schemes are set by each mill and distributor, so use the scheme that came with your material.