Group 4 Steels: Tools Steels and Their Uses

2. 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.

3. 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. Group 4 Steels: Tools Steels and Their Uses

4. Tool Steels for Room Temperature Use The primary requirement of these tool steels is a room temperature (RT) hardness of at least RC60* for a plain carbon steel. These steels are classifiedaccording 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.

5. Tool Steels for Room Temperature Use O – oil hardened grades have small percentages of Cr and W with 0.9 %C. The have medium hardness and are used to short runcold formingdies. 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, Cr23C6is also formed, which improves wear resistance.

6. Isothermal Transformation Diagram for Water Quenched Carbon Tool Steel These steels must be cooled very rapidlypast 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

7. Isothermal Transformation Diagram for “Water Quenched” Carbon Tool Steel 128 grains 32 grains 256 grains Higher austenitizing temperatures (790 – 870 C) promotes grain growth (rippening), which results in fewer grains per cm2 and reduced % of martensite upon cooling.

8. Isothermal Transformation Diagram for Oil Quenched Carbon-Chromium Tool Steels Proeutectoid cementite curve g pearlite The slower cooling rate of oil results in small thermal gradients and more uniform transformations through the cross sections – eg., a ball bearing steel.

9. Isothermal Transformation Diagram for “Oil Quenched” Carbon-Chromium Tool Steel 256 grains 32 grains 16 grains 2 grains Again, higher austenitizing temperatures result in larger grain size and reduced amounts of martensite upon cooling.

10. These 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 C Medium hot workingfor 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 C Used for 85% of US cutting steels before the advent of ceramic cutting tools. Tool Steels for High Temperature Use

11. Tool Steels for High Temperature Use Although Mo and W are expensive, when present in large amountsthey 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.

12. This diagram below indicates temperature of phase changes but not the phase compositions, except for the carbon content. The 18W 4Cr 1V raises theeutectoid temperature from 727 to 840 oC. Also, the eutectoid composition is reduced from 0.8 to 0.25 C. the solubility of C in austeniteis reduced from 2.1 to 0.7 C. Pseudo-Binary Phase Diagram for High Speed Steels The increase in eutectoid temperature means that the HS steels have to be heat treated athigher temperatures than plain carbon or low alloy steels. In addition, higher temperatures are needed to dissolve the carbides. Note

13. 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. 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. No decomposing region

14. 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. 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. No decomposing region

15. Types and Uses of Tools Steels

16. Types and Uses of Tools Steels C Mn Cr V W Mo Co * * * - See next slide

17. 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)

18. “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 weight C36-43 in annealed state or C69-72 in oil quenched and tempered state 2) 45% (WC + TiC) – 55% high speed steel by volume 34% W – 10% Ti – 5% C – 3% Cr – 1% V by weight C44-46 in annealed state or C71-72 oil quenched and tempered 3) Co-based Hard Alloys – referred to as Stellites 30% Cr – 19% W – 2% C – 3.5% Ni balance Co Hardness of C60 – 65 Also very resistant to corrosion. Used as blade edges in mining equipment. Tool Steels Based on Undissolved Carbides

20. The End(Any questions or comments?)