L6 Tool Steel Tensile Properties After Tempering

L6 is the low-alloy tool steel that shops reach for when a die has to absorb shock rather than resist abrasion, and the numbers that describe that behaviour are tensile strength, elongation and reduction of area measured on tempered specimens. The tables below collect one L6 heat carried through five tempering temperatures and one comparison of the same L2 grade with and without vanadium, both taken from published practice. Every figure belongs to the exact hardening and tempering condition printed beside it, so read each row as measured behaviour at that one condition rather than as a design value. This is a reference summary of published practice and not a specification of ours.

What the tables measure

The first table follows one L6 tool steel from a hard, brittle condition after a low temper to a softer, far more ductile one after a high temper. The steel was austenitized at 815 °C (1500 °F), oil quenched and tempered for 2 h, and nothing changes across the five rows except the tempering temperature, so the columns show exactly what a shop pays in hardness for every point of ductility it buys back. The figures come from Allegheny Ludlum Steel Corp.

The second table answers the neighbouring question of what vanadium does to this family of steels. Two heats of L2 with the same carbon level and nearly the same chromium are compared after the same treatment, one with 0.20 per cent vanadium and one without, so the difference between the two columns is close to the effect of that single addition.

Tensile properties of L6 tool steel after tempering

Austenitized at 815 °C (1500 °F), oil quenched and tempered 2 h

Source, Tool Steels (G. A. Roberts and G. Krauss), Table 8-10, Tensile properties of L6 tool steel, printed page 158 of the copy consulted. Composition 0.68% C, 0.60% Mn, 0.65% Cr, 1.40% Ni, 0.20% Mo; austenitizing temperature 815 °C (1500 °F), oil quenched, tempered 2 h. Figures from Allegheny Ludlum Steel Corp. The source prints the two temperature columns with their headings interchanged, and the pairing used here follows the equivalent temperatures on the axis of Fig. 8-24 on the same page. This is a reference summary of published practice and not an Aobo Steel specification.

The lowest tempering temperature in the table relieves quench stress rather than softening the steel, and it leaves 57 HRC with 1.3 per cent elongation and 2.0 per cent reduction of area, which is a part with almost no reserve before it cracks. From there the hardness falls in four further steps to 32 HRC at 650 °C while the reduction of area climbs from 2.0 to 47.2 per cent and elongation from 1.3 to 18.8 per cent. Strength follows hardness down, from 2055 MPa at the lowest temper to 966.2 MPa at the highest.

Two of the steps are worth marking. Between 205 and 315 °C the steel gives up about 50 MPa of tensile strength and 3.5 HRC while reduction of area rises by 7.3 points, so the first part of the tempering range buys ductility cheaply. The last step, from 540 to 650 °C, costs 10 HRC and 376 MPa and returns 13.2 points of reduction of area, which is the point at which the steel stops behaving like a cutting edge and starts behaving like a shock-resisting die material. The two working bands the source names for the grade, 45 to 50 HRC for wood-cutting saws and 58 to 62 HRC for most other applications, sit at the two ends of this table, so the table prices the choice between a hard edge and a die that survives impact.

L2 tool steel with and without vanadium

Source, Tool Steels (G. A. Roberts and G. Krauss), Table 8-7, Mechanical properties of medium-carbon L2 steels with and without vanadium, printed page 148 of the copy consulted. Both steels oil quenched from 840 °C (1545 °F) and tempered at 425 °C (800 °F). Steel A composition 0.49% C, 0.76% Mn, 0.21% Si, 1.07% Cr. Steel B composition 0.50% C, 0.79% Mn, 0.31% Si, 0.98% Cr, 0.20% V. The impact figure is an Izod value in joules with the foot pound figure in brackets. This is a reference summary of published practice and not an Aobo Steel specification.

The addition of 0.20 per cent vanadium raises tensile strength by 82 MPa and yield point by 162 MPa on the same treatment, which is why the yield to tensile ratio moves from 0.911 to 0.963, and it improves ductility slightly rather than costing it, with elongation rising from 9.6 to 10.4 per cent and reduction of area from 35.9 to 43.1 per cent. The hardness rises from 457 to 481 HV and the measured Izod impact is the same 16.3 J (12.0 ft·lbf) for both heats, so the gain sits in strength rather than in toughness.

Read the two tables together and the family reads consistently. L6 and L2 are both low-alloy steels chosen where a tool has to survive impact, and in both cases hardness, strength and ductility move in step with the tempering temperature. Keep the impact figure in its own context, because it belongs to an unnotched test on one of the L2 heats and is not comparable with notched readings published for other grades.

Related reference data

The two grades in these tables are described with their forms, section ranges and delivery conditions on L6 tool steel, and the hardening, quenching and tempering sequence for the same grade is set out on L6 tool steel heat treatment. The choice between L6 and a hot work grade for a forging die is worked through on H13 and 1.2714 (L6) tool steel, the hardness each grade keeps after a given temper is charted on tool steel tempering chart, impact values for the whole range are collected on tool steel impact toughness, and the group these steels belong to is introduced on oil-hardening tool steel.

Compiled from Tool Steels (G. A. Roberts and G. Krauss), Table 8-10 and Table 8-7, on printed pages 158 and 148, the chapter on low-alloy special purpose tool steels. The source prints the temperature columns of Table 8-10 with their headings interchanged and the values here are paired as the equivalent temperatures printed in the figure on the same page. The tables are reproduced as published and are a reference summary rather than an Aobo Steel specification, so treat them as the measured behaviour of one heat under one heat treatment rather than as a design allowable for a particular die.