CPM® REX® 121(HS)*
High hardness high vanadium cobalt high speed steel
CPM REX 121 is a new high vanadium cobalt bearing tool steel designed to offer a combination of the highest wear resistance, attainable hardness, and red hardness available in a high speed steel. Its superior red hardness permits higher cutting speeds compared to other high speed steels. Its superior wear resistance (due to the high vanadium content) helps maintain a sharp cutting edge at increased cutting speeds.
    CPM REX 121 also bridges the performance gap between high alloy tool steels and carbide materials. It may be used in cutting tools, where high cutting speeds demand higher heat resistance, but carbide is too brittle, or in high wear tooling applications (punches and dies) where carbide tools are too fragile.
    Because of the extremely fine and uniform CPM microstructure, sulfur may be added if desired, to improve the machinability. The higher sulfur content benefits the toolmaker by allowing ease of manufacture, and the tool user by allowing easier resharpening. The CPM process ensures that the additional sulfur will not detrimentally affect the tool’s toughness.
Typical Chemistry (Patent Pending)
Carbon 3.4%
Tungsten 10.0%
Chromium 4.0%
Molybdenum 5.0%
Vanadium 9.5%
Cobalt 9.0%
Sulfur 0.03% (0.12%)*
Typical Applications
Hobs Broaches
Milling Cutters End Mills
Form Tools Punches and Dies
Guide Rolls Wear Parts
* The addition of higher S in larger diameter rounds (e.g. 2-9/16" and over) provides a uniform dispersion of small sulfides throughout the structure, resulting in machinability and grindability benefits with no deleterious effect on toughness.
Thermal Treatments

Annealing: Heat to 1650F (870C), hold 2 hours, slow cool no faster than 25F (15C) per hour to 1000F (540C), then air or furnace cool.

Annealed hardness: about BHN 360/410 (HRC 38/44)

Stress Relieving: Annealed material - Heat to 1100-1300F (595-705C), hoId 2 hours and air or furnace cool. Hardened material - Heat to 25-50F (15-30C) below original tempering temperature, hold 2 hours and air or furnace cool.

Hardening: (Salt Baths or Vacuum Furnace preferred.)

Preheat: Preheat at 1500-1550F (815-845C), hold long enough to soak through. A second preheat at 1850-1950F (1010-1065C) is recommended, when hardening from over 2100F (1150C).

High Heat: 1875-2240F (1025-1225C)

Quench: Quench rapidly to below 1100F (595C), equalize, then air cool to hand warm, 125F (50C). Salt or interrupted oil quenching usually gives best heat treat response for high speed steels. Vacuum hardening may result in slightly lower hardness for larger tools. The quench rate from the hardening temperature to below 1100F (595C) is critical to achieve optimum results. Cooling rate below 1000F (540C) may be slower to minimize distortion if desired.
Temper: Temper 2 to 4 times at 1000F (540C) or higher. Hold at least 2 hours at temperature for each temper. 2 to 4 tempers are required, depending on hardening temperature. Air cool to room temperature between tempers.

Straightening: Best done warm - 500-1000F (260-540C). Straightening after salt quenching, before cooling to below 500F (260C) is preferred if possible. This grade has limited toughness; straightening may present a risk of cracking.
Hardening Data
(Salt or oil quenched, vacuum HT may result in ~1 point lower HRC)
Tempering
Hardening Temperature
Temperature
1875F
2050F
2150F
2200F
2240F
°F
°C
(1025C)
(1120C)
(1175C)
(1205C)
(1225C)
As Quenced
71
69
67
65
63
950
510
69
70.5
71
70.5
70
1000
540
68.5
70
70.5
70.5
70.5
OPTIMUM FOR MAXIMUM TOUGHNESS AND EFFECTIVE STRESS-RELIEVING.
1025
550
67.5
69.5
70.5
70.5
70
1050
565
66.5
68.5
70
69
69.5
1100
595
63
66
67.5
68
68.5
1200
650
55
56
58
59
60
NOTE A:
RESULTS MAY VARY WITH HARDENING METHOD AND SECTION SIZE. SALT OR OIL QUENCHING WILL GIVE MAXIMUM RESPONSE. VACUUM OR ATMOSPHERE COOLING MAY RESULT IN UP TO 1-2 HRC POINTS LOWER.
MINIMUM TIME AT
AUST TEMP (MINS)
30
20
15
10
5
MINIMUM NUMBER
OF TEMPERS (2 HRS)
2
3
3
4
4
Comparative Properties

Wear resistance (depends on application and hardness) approximately 50%-100% better than T15, CPM Rex 76, or CPM 10V.

Toughness (depending on heat treatment) approximately comparable to CPM 15V, CPM Rex 76 at maximum hardness, or conventional T15.

Red hardness higher than CPM Rex 76. Note that CPM Rex 121 can retain a room temperature hardness of HRC 60 even after tempering at 1200F (650 C)

    The high red hardness of CPM Rex 121 may prove particularly useful for high cutting speed operations, or as a substrate for TiAIN or similar coatings, designed for higher temperature exposure than other coatings.
    When CPM Rex 121 is used in place of conventional cobalt-bearing high speed steels in cutting tools, cutting speeds may be increased by approximately 25%-50%, depending on application experience.

Physical Properties
Modulus of Elasticity ...................................................................31 psi x 106 (214 GPa)
Specific gravity & density slightly less than CPM Rex 76
Coefficient of Thermal Expansion similar to CPM Rex 76
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