L Series and S Series Tool Steel Properties, Annealed and Hardened

The L series and the S series are the two tool steel groups that are bought for strength and shock resistance rather than for wear resistance or hot hardness. L is the low alloy special purpose group, and S is the shock resisting group, and both are used for parts that take a blow rather than a rub: chisels, punches, rivet sets, shear blades, arbors, cams, chucks and collets. This page carries the published mechanical properties for the grades still in commercial use, in the annealed condition and at each tempering temperature, so the strength and the toughness at the hardness the job needs can be read off together.

Group L, the low alloy special purpose steels

Group L steels carry small amounts of chromium, vanadium, nickel and molybdenum, and the group once held seven grades. Only two remain, L2 and L6. L2 is available in several carbon contents from 0.50 to 1.10 per cent and its chromium and vanadium make it an oil hardening steel of fine grain size. L6 adds about 1.50 per cent nickel for toughness. Both are oil hardening in name, and in practice a large L2 section is often quenched in water instead. The hardenability range inside the group is wide and it is a carbon effect as much as an alloy one: an L2 at 0.50 per cent carbon reaches about 57 HRC as oil quenched but will not through harden in sections over about 12.7 mm, while L6 at 0.70 per cent carbon reaches about 64 HRC as quenched and holds over 60 HRC through 76 mm.

Table 7. L2, nominal room temperature mechanical properties

Table 7. L6, nominal room temperature mechanical properties

Source, ASM Handbook Vol.1, article Wrought Tool Steels, Table 7. The annealed rows are the as-supplied condition. Source footnotes, (a) elongation measured on a 50 mm (2 in.) gauge length, (b) Charpy V-notch, (c) Charpy unnotched. The L grades are quoted on the notched test and the S grades on the unnotched test, so the impact columns of the two groups are not comparable with each other. A dash means the source lists no value.

Group S, the shock resisting steels

Group S steels are the ones specified when the tool has to absorb impact. They vary from shallow hardening, S2, to deep hardening, S7, and hardenability here is controlled by composition rather than by the grain size and melting practice effects that dominate group W. The group needs relatively high austenitizing temperatures to reach its optimum hardness, and because of that undissolved carbide is not the hardenability control it is in other groups. S2 is normally water quenched, S1, S5 and S6 are oil quenched, and S7 is normally cooled in air except in large sections, which are oil quenched.

The mechanical properties of the three grades in this table are the reason group S is sometimes considered for non-tooling work. At 205 °C (400 °F) temper, S5 reaches 2345 MPa (340 ksi) tensile with 206 J of unnotched impact energy, and S7 reaches 358 J at the 650 °C (1200 °F) temper while still holding 1240 MPa (180 ksi). Strength and toughness move together across the tempering range in a way that is unusual in a tool steel, and the 650 °C rows are where that shows most clearly: hardness falls into the high thirties and the impact energy goes up by roughly an order of magnitude.

Table 7. S1, nominal room temperature mechanical properties

Table 7. S5, nominal room temperature mechanical properties

Table 7. S7, nominal room temperature mechanical properties

Source, ASM Handbook Vol.1, article Wrought Tool Steels, Table 7. The annealed rows are the as-supplied condition. Source footnotes, (a) elongation measured on a 50 mm (2 in.) gauge length, (b) Charpy V-notch, (c) Charpy unnotched. The L grades are quoted on the notched test and the S grades on the unnotched test, so the impact columns of the two groups are not comparable with each other. A dash means the source lists no value.

Reading the two tables together

Three cautions belong with these numbers. The first is the one already noted in the source footnote, that the L grades are quoted on a Charpy V-notch test and the S grades on an unnotched test, so an impact figure from one table cannot be compared with a figure from the other. The second is that every value is a nominal room temperature value from a single source, and strength and elongation vary with section size, with the actual austenitizing temperature and with the number of tempers, none of which the table variables. The third is that the hardness figures in the annealed rows are on the Rockwell B scale and the hardened rows on Rockwell C, so the annealed hardness is a machinability indicator rather than a working hardness.

The conversion between the scales, and the reason a 96 HRB annealed condition and a 54 HRC working condition cannot be compared directly, is on the Vickers to Rockwell C conversion page. The group W and group O properties are on the tool steel properties chart, the hardness each of these grades keeps at temperature is on the tool steel hot hardness chart, and the impact energy of the families as a whole is on the tool steel impact toughness chart. Aobo Steel supplies L6 and S7 as forged and hot rolled bar, and the annealed hardness on the mill certificate follows the same table.

Source: ASM Handbook, Vol 1, Properties and Selection: Irons, Steels and High-Performance Alloys.