Mech 473 Lectures

Professor Rodney Herring

Group 4 Steels: Tools Steels and Their Uses

High speed (HS) tools steels are used for high cutting speeds such as drills, mill cutters, taps and others.

Hot-worked tool steels are intended to withstand combinations of heat, pressure, and abrasion associated with shearing, punching or forming of metals during manufacturing at high temperatures.

Cold-work tool steels are those intended for applications that do not require prolonged or repeated heating in the range above 205-260 oC.

Group 4 Steels: Tools Steels and Their Uses

Shock resisting tool steels are those intended for applications requiring toughness and resistance to shock-loading such as hammers, chisels, punches, driver bits and others.

Water hardening tool steels are shallow hardened and have relatively low resistance to softening. They are suitable for woodworking tools, hand-metal cutting tools such as taps and reamers and cutlery.

Tool Steels for Room Temperature UseThe primary requirement of these tool steels is a room temperature (RT) hardness

of at least RC60* for a plain carbon steel. These steels are classified according to their quenching media.

W – water hardened grades are plain carbon steels with 0.6-1.0 %C. These have a low hardenability, ie., martensite only to a depth of 0.5 in. V can be added to improve the hardness of these steels.Vanadium forms a very hard carbide V4C3, although relatively expensive, a small amount goes a long way for improving wear resistance.

S – shock resistant grades contain small amounts of Cr or Mo and are quenched in oil. They have lower C contents (0.5) to improve impact strength.

* - Recall - A value of Rockwell C-54 = the hardness of 50/50 P/M in a eutectoid steel, which was used to determine the Ideal Quench Diameter, DI.

Tool Steels for Room Temperature UseO – oil hardened grades have small percentages of Cr and W with 0.9 %C. The have

medium hardness and are used to short run cold forming dies.A- air hardening and the addition of greater amounts of Cr and Mo and 1 %C are used

for complicated shapes and thread rolling.Mo and W are relatively expensive so they are only added in small amounts to give much improved hardenability.

D – high carbon, high Cr grades with 12 %Cr and 1.5-2.25 %C are extremely wear resistant and used for long run dies and for gauges.Chromium is a relatively low cost addition for increasing hardenability with the excess Cr, Cr23C6 is also formed, which improves wear resistance.

Isothermal Transformation Diagram for Water Quenched Carbon Tool Steel

These steels must be cooled very rapidly past 500 C to avoid pearlite. Additional Mn – 0.5% - is added for hardening saw blades by oil quenching.

Note the %Martensite as a function of temperature

Isothermal Transformation Diagram for “Water Quenched” Carbon Tool Steel

Higher austenitizing temperatures (790 – 870 C) promotes grain growth (rippening), which results in fewer grains per cm2 and reduced % of martensite upon cooling.

256 grains 128 grains 32 grains

Isothermal Transformation Diagram for Oil Quenched Carbon-Chromium Tool Steels

The slower cooling rate of oil results in small thermal gradients and more uniform transformations through the cross sections – eg., a ball bearing steel.

Proeutectoid cementite curve

pearlite

Isothermal Transformation Diagram for “Oil Quenched” Carbon-Chromium Tool Steel

Again, higher austenitizing temperatures result in larger grain size and reduced amounts of martensite upon cooling.

32 grains 16 grains 2 grains256 grains

Tool Steels for High Temperature UseThese steels are intended to withstand combinations of heat, pressure, and

abrasion associated with shearing, punching or forming of metals during manufacturing at high temperatures.

H – Chromium hot working steels 5-7 Cr, 0.4-1.0 V, 1.5-7.0 W, 1.5 Mo, 0.35 CMedium hot working for Mg and Al extrusion die-casting dies.

H – Also for Tungsten hot working steels 9.5-12 W, 3.5-12.0 Cr, 0.35 C

Hot working extrusion and forging dies for brass, nickel and steel.T – Tungsten high speed steel 12-18 W, 4.0 Cr, 1-5 V, 0.7-1.5 C.

Original high speed (HS) cutting steel with excellent HT wear resistance.M – Molybdenum HS steel 3.5-8.0 Mo, 1.5-6.0 W, 4.0 Cr, 1-5 V, 5 Co, 0.8-

1.5 CUsed for 85% of US cutting steels before the advent of ceramic cutting tools.

Tool Steels for High Temperature UseAlthough Mo and W are expensive, when present in large amounts

they form very hard carbides, (Mo-W)6C , which precipitate as fine particles during secondary tempering of martensite.

(Mo-W)6C resists growth at dull red temperatures, which is the basis of High Speed (HS) steels.

Vanadium forms V4C3, which is the hardest of all the carbides. It resists dissolving into solution in austenite and thus remains unchanged in the microstructure after successive heat treatment cycles producing the hardest alloys.

For high speed machining operations, the tool tip may be heated to >600 oC (red hot) by the friction heat for long periods.

Present developments are to coat these tool steels used as bits with TiC, or use ceramic tool tips.

Pseudo-Binary Phase Diagram for High Speed SteelsThis diagram below indicates temperature of phase changes but not the phase compositions, except for the carbon content.The 18W 4Cr 1V raises the eutectoid temperature from 727 to 840 oC.

Also, the eutectoid composition is reduced from 0.8 to 0.25 C.the solubility of C in austenite is reduced from 2.1 to 0.7 C.

The increase in eutectoid temperature means that the HS steels have to be heat treated at higher temperatures than plain carbon or low alloy steels.In addition, higher temperatures are needed to dissolve the carbides.

Note

Isothermal Transformation Diagram for “Air Hardened” High Speed Tool Steels

High speed tool steels are used for high cutting speeds such as drills, mill cutters, taps and others.

No decomposing region

The pearlite knee is displaced upward and well to the right for these steels.

In the temperature interval of ~360-600 oC, the metastable austenite shows no sign of decomposing even after a period of weeks.

Isothermal Transformation Diagram for Air Hardened High Speed Tool Steels

High speed tool steels are used for high cutting speeds such as drills, mill cutters, taps and others.

No decomposing region

One can use hot-quenching (like martempering) in which the steel is quenched in a salt bath at 500-550 oC, held until a uniform temperature and then quenched in oil.

The advantage is economic as there is less time to process the steel compared to slow air cooling the steel.

Types and Uses of Tools Steels

C Mn Cr V W Mo Co Types and Uses of Tools Steels

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* - See next slide

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Tool Steels Based on Undissolved Carbides

“Super-Speed Steels”Compositions are given under T15 and M15• These steels contain higher carbon (1.5%) compared to the original High

Speed Steels, and in addition to the 4%Cr, they contain 5%Co.• The Tungsten steel contains 12%W. The Mo steel contains 6%W + 3.5%Mo.These steels have undissolved V4C3 present in the quenched state and this is not

affected by the double tempering treatment to develop (W,Mo)6C.Their overall hardness is thus in the region of C70 - C72 and they have a much

longer tool life than the normal high speed steels.(cont’d)

Tool Steels Based on Undissolved Carbides“Cemented Carbide Tool Tips”Made of 90% WC (or WC + TiC) embedded in a ductile matrix of Co metal.There are 3 Steel Bonded “Carbide Tool Tips” (Compositions listed)1) 45% TiC – 55% Cr-Mo alloy steel by volume

26% Ti – 7% C – 2% Mo by weightC36-43 in annealed state or C69-72 in oil quenched and tempered state

2) 45% (WC + TiC) – 55% high speed steel by volume34% W – 10% Ti – 5% C – 3% Cr – 1% V by weightC44-46 in annealed state or C71-72 oil quenched and tempered

3) Co-based Hard Alloys – referred to as Stellites30% Cr – 19% W – 2% C – 3.5% Ni balance CoHardness of C60 – 65Also very resistant to corrosion. Used as blade edges in mining equipment.

The End

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