Hardening & Tempering

Austenitising and quenching

Steel is heated to austenitic region, held there until carbon is dissolved and then cooled rapidlycooled rapidly

Quenching is performed to cool the hot metal rapidly by immersing in brine, water or oil

Carbon does not get time to escape and martensite phase is the result of quenching

Martensite is very hard and brittle.

Tempering is applied to hardened steel to reduce brittleness, increase ductility, and toughness and relieve stresses in martensite structure.

Tempering

In this process, the steel is heated to a relatively low temperature, held there for prescribed period of time and then cooled to room temperature.

This process increases ductility and toughness but also reduces hardness, strength and wear resistance marginally. Increase in tempering temperature lowers the hardness.

Main battle tank with the T-90 Gun Barrel fitted to it

Tempered at 350-400C

Tempered at 500C

Tempering IzodTemperature impact°C UTS YS El RA Hardness energy

MPa MPa % % HB J

205 1765 1520 10.0 33.0 475 18 260 1670 1430 11.5 37.0 455 14 315 1570 1340 13.0 41.0 425 14 370 1475 1250 15.0 45.0 400 20 425 1380 1170 16.5 49.0 375 34 425 1380 1170 16.5 49.0 375 34 540 1170 1000 20.0 56.0 325 81 650 965 830 22.0 63.0 270 135

Typical mechanical properties of heat treated 4130 steel as a function of tempering temperature

Radial hardness profiles of cylindrical steel bars

•Hardenability is the ability of theSteel to be hardened by formingmartensite.•Hardenability is not “hardness”.

Hardenability

•Hardenability is not “hardness”.•High hardenability means theability of the alloy to produce a highmartensite content over a largecross sectional area.

Hardenability is a qualitative measure of the rate at which hardness decreases with distance from the surface because of decreased martensite content.

Hardenability is measured by the Jominy end-

Hardenability

Hardenability is measured by the Jominy end-quench test, performed for standard cylindrical specimen, standard austenitization conditions, and standard quenching conditions (jet of water at specific flow rate and temperature).

Hardenability Curve

Quenched end cools most rapidly and contains most martensite

Cooling rate decreases with distance from quenched end:greater C diffusion, more pearlite/bainite, lower hardness

High hardenability means that the hardness curve is relatively flat.•

Hardenability generally increases with C content

Hardenability

Alloying elements delay formation of pearlite, bainite : more martensite

Alloys in figure above all have 0.40 wt% C, but have different additional alloying elements

Radial hardness profiles of cylindrical steel bars

25 1040 880 18 55

Effect of mass on typical properties of heat treated 4130 steel

Bar size, mm UTS (MPa) YS (MPa) El (%) RA (%)

25 1040 880 18 55

50 740 570 20 58

75 710 540 22 60

Compositions of the ultrahigh-strength medium carbon low alloy steels used for automotive applicationsTrade Composition, wt % (a)Name C Mn Si Cr Ni Mo V 4130 0.28−0.33 0.40−0.60 0.20−0.35 0.80−1.10 ... 0.15−0.25 ... 6150 0.48−0.53 0.70−0.90 0.20−0.35 0.80−1.10 ... ... 0.15−0.25 6150 0.48−0.53 0.70−0.90 0.20−0.35 0.80−1.10 ... ... 0.15−0.25 8640 0.38−0.43 0.75−1.00 0.20−0.35 0.40−0.60 0.40−0.70 0.15−0.25 ...

(a) P and S contents may vary with steelmaking practice. Usually, these steels contain no more than 0.035 P and 0.040 S.

Piston (top) and connecting rod from typical automotive engine (scale is in centimetres)

Ultra high strength Steel Grade

Automotive applications

4130 Automotive connecting rods, engine mounting lugs, shafts, fittings, bushings, gears, bolts, axles

6150 Gears, pinions, springs 6150 Gears, pinions, springs (both coiled and flat), shafts, axles, studs, fasteners

8640 Gears, pinions, shafts, axles, studs, fasteners

Grade Hardening treatment Tempering treatment

4130 Heat to 845 to 870 °C and hold, then water quench; or heat to 860 to 885 °C; hold and then oil quench. Holding time depends on section thickness

Hold at least 1/2 h at 200 to 700 °C; air cool or water quench. Tempering temperature and time at temperature depend mainly on desired hardness or strength level

6150 Austenitize at 845 to 900 °C; oil Hold at least 1/2 h at 200 to 650 6150 Austenitize at 845 to 900 °C; oil quench

Hold at least 1/2 h at 200 to 650 °C. Tempering temperature and time at temperature primarily

depend on desired final hardness

8640 Austenitize at 815 to 845 °C; quench in oil or water

Hold at least 1/2 h at 200 to 650 °C.

4340 steel (0.4%C,1.8%Ni, 0.8%Cr, 0.25%Mo)has a higher hardenability which enables slower quenching rates with less

0.4% C Ni-Cr-Mo low alloy steel issuperior to 0.45 carbon steel

enables slower quenching rates with less residual stresses.

The ideal critical diameter for the 4340 steel 65 mm, as compared to 15 mm for a 1040 steel.

0.4% C Ni-Cr-Mo low alloy steel issuperior to 0.4% carbon steel 0.4% C steel, when quenched and tempered at 450C has the

following properties:Y.S. 650 MPaT.S. 800 MPa%El 15Izod Impact 30 J

4340 steel, when quenched and tempered for the same elongation and impact value, has the following strength properties:Y.S. 1200 MPaT.S. 1350 MPa%El 15Izod Impact 32 J

It is clear that, for the same ductility and toughness, the low alloy steel possesses superior strength. Conversely, for the same strength, the low alloy steel would have larger ductility and toughness.