Tool steels: AISI classification

American Iron and Steel Institute (AISI) tool steels are classified into the following main series:
1) steel "W" – carbonaceous, water quenched;
2) steel "L" – low alloyed;
3) steel «S» – shockproof;
4) steel "O" – oil quenched;
5) steel "A" – air quenched;
6) steel "D" – high carbon, high chromium;
7) steel "H" – with high hardness, heat resistant;
8) steel "M" – high speed alloyed with molybdenum;
9) steel "T" – high speed tungsten alloyed.

American tool steels (AISI)

Each of these types of tool steels is further subdivided into specific steels.. In the designation of steel, a number is added to the letter, eg, W2. Each such series of steels includes up to ten, sometimes more, various steels. for instance, the W series includes steels from W1 to W7. In the table 1 for clarity the classifications are presented by 1-2 representative of all series of tool steels with medium chemical composition.

klassifikaciya-instrumentalnyx-staleyTable 1 - AISI classification of tool steels (average chemical composition)

Rating of tool steels for wear resistance, viscosity and heat resistance

The most important properties of tool steels are considered to be wear resistance., viscosity and heat resistance. In the table 2 the ratings of these three main properties are given for each of the steels, indicated in the table 1. This rating is a number from 1 to 10: number 10 denotes the highest rating.

reyting-instrumentalnyx-staleyTable 2 - Wear rating, toughness and heat resistance of tool steels

In general, the main "levers" for achieving a high level of the three main properties of tool steels are as follows:
1) higher wear resistance - more carbides;
2) higher viscosity - lower carbon content;
3) higher heat resistance - more alloyed carbides.

Tool steels series "W"

The letter "W" stands for water quenching (water). These steels are similar to ordinary carbon steels and have very low hardenability.. As shown in the table 2, the toughness of this steel increases, if the steel is surface hardened. It means, that steel is hardened at a rate, which ensures the formation of martensite only near the surface, allowing the core to remain unhardened and therefore tough. Low heat resistance in comparison with other steels is explained by the fact, that other steels contain alloyed carbides, such as M3C, not ordinary Fe3C. Alloy carbides resist coarsening and dissolve at high temperatures, which gives steels an increased heat resistance.

In the last column of the table 1 the total content of alloying elements in steel is indicated. This total content of alloying elements increases when moving along the table from top to bottom.. Which means, that the volume fraction of carbides also increases.

L series tool steels

The letter "L" went to this series from the word "low" – all of these steels are low alloy and similar to conventional low alloy steels. for instance, essentially L6 steel is very close to steel 4340 with carbon content from 0,4 to 0,7 %.

"S" series tool steels

The letter "S" of these steels is from the word "shock" – hit, since they are all shockproof. High viscosity, which is necessary for shock resistance, achieved by reducing the carbon content of these steels to a very low level. However, this is also the reason for the low wear resistance and heat resistance., typical for these steels.

Tool steels series "O"

The letter "O" in the designation of these steels from the word "oil" – oil, as they all receive oil quenching. Key alloying elements in the table 1 highlighted in green. In steels of the "O" series – it's manganese and vanadium. These elements give the steels "O" increased hardenability compared to steels of the "W" series, which allows them to be quenched when cooled in oil.

Tool steels series "A"

The letter "A" stands for air quenching (air). The hardenability of these steels is improved to such an extent, that they are capable of being quenched when cooled in air. Key alloying elements, which provide high hardenability, are chromium and molybdenum.

Tool steels series "D"

These steels are called high carbon and high chromium.. Steel are capable of being hardened in air. As seen in the table 2, the combination of a high carbon content with a high alloying element provides high wear resistance, quite high heat resistance, but very low viscosity.

Tool steels of the "N" series

The letter "H" here stands for "hot hardness" – retention of hardness at elevated temperatures, heat resistance. These steels are usually used for making dies and other pressing tools for hot pressing aluminum and its alloys. The combination of a low carbon content and a fairly high alloying content gives good toughness and heat resistance., but very moderate wear resistance.

Tool steels of the "M" and "T" series

The letters "M" and "T" stand for molybdenum and tungsten (tungsten) in these high speed steels. Both carbides of these elements are stable up to very high temperatures.. Therefore, the high level of these alloying elements gives a large volume fraction of carbides in these steels.. This provides them with good wear resistance and heat resistance., but low viscosity. T15 high carbon steel exemplifies the development of tool steels for maximum wear and heat resistance at the expense of toughness.

Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007