Jul 9 2012
Topics Covered
IntroductionOverviewPropertiesComposition Applications
Introduction
Hot work tool steels are steels capable of withstanding high abrasion, heat
and pressure conditions that prevail in manufacturing units that perform
processes such as forming, shearing and punching of metals at high temperatures
of 480 to 760°C (900 to 1400°F). These steels have wear resistance up to 540°C
(1000°F).
Hot work tool steels are designated as group H steels and they have 0.35% to
0.45% carbon, 6% to 25% chromium, with vanadium, molybdenum, and
tungsten as the other alloying elements. Tungsten is primarily used in hot forming tool steels due to its high
temperature strength, toughness and resistance to grain growth.
Overview
Tungsten hot-work steels constitute the H21 to H26 types of hot work steels.
These steels have similar characteristics as those of other of high-speed steels. The hot
work steel type H26 has low carbon content when compared to that of T1 high
speed steel. The primary alloying elements of tungsten hot-work steels include
tungsten, carbon, chromium and vanadium.
Properties
Tungsten hot work steels have high alloy content, which enhances their heat
resistance. The high alloy content also makes the tungsten steels brittle and
unsuitable for the water-cooling process. Breakage of tungsten hot work steels
can be reduced if they are preheated to operating temperatures prior to use.
Tungsten steels have normal working hardness of 45 to 55 HRC. Thermal shock
resistance and toughness of these steels can be improved by reducing the carbon
content. In such cases, it is necessary to adjust the tungsten and vanadium
content also as these two reduce the hardenability of steel by trapping large
amount of carbon in the form of carbides.
Scaling can be reduced by quenching tungsten hot-work steels in oil or hot
salt. Tungsten hot-work steels are resistant to distortion when they are air-
hardened, and have higher hardening temperature when compared to chromium
hot-work steels.
Composition
The following table shows the composition limits of tungsten hot-work
steels.
| Designation |
Compositiona, % |
| AISI |
UNS |
C |
Mn |
Si |
Cr |
Ni |
Mo |
W |
V |
Co |
| H21 |
T20821 |
0.28-0.36 |
0.15-0.40 |
0.15-0.50 |
3.00-3.75 |
0.30 max |
- |
8.50-10.00 |
0.30-0.60 |
- |
| H22 |
T20822 |
0.30-0.40 |
0.15-0.40 |
0.15-0.40 |
1.75-3.75 |
0.30 max |
- |
10.00-11.75 |
0.25-0.50 |
- |
| H23 |
T20823 |
0.25-0.35 |
0.15-0.40 |
0.15-0.60 |
11.00-12.75 |
0.30 max |
- |
11.00-12.75 |
0.75-1.25 |
- |
| H24 |
T20824 |
0.42-0.53 |
0.15-0.40 |
0.15-0.40 |
2.50-3.50 |
0.30 max |
- |
14.00-16.00 |
0.40-0.60 |
- |
| H25 |
T20825 |
0.22-0.32 |
0.15-0.40 |
0.15-0.40 |
3.75-4.50 |
0.30 max |
- |
14.00-16.00 |
0.40-0.60 |
- |
| H26 |
T20826 |
0.45-0.55b |
0.15-0.40 |
0.15-0.40 |
3.75-4.50 |
0.30 max |
- |
17.25-19.00 |
0.75-1.25 |
- |
aExcept group W, all steels contain 0.03 max S, 0.03
max P and 0.25 max Cu; group W contain 0.025 max S, 0.025 max P, and 0.20 max
Cu. As mentioned, amount of sulfur may be increased from 0.06% to 0.15% to
enhance the machinability of group A, M, T, H and D
steels..
Applications
Tungsten hot work steels find applications in the following areas:
- Manufacturing mandrels and extrusion dies for high temperature applications,
such as extrusion of brass, nickel alloys, and steel
- Hot-forging dies of rugged design.