High speed steel (HSS or HS) is a subset of tool steel, commonly used in tool bits and cutting tools. It is often used in power saw blades and drill bits. It can withstand higher temperatures without losing its temper (hardness) which allows it to cut faster than high carbon steel, hence the name high speed steel. Main use of high speed steels is in the manufacture of various cutting tools: drills, gear cutters, saw blades, etc.
Physical Composition : High Speed Steel is a multi-component Fe-C-X system (Iron-Carbon-X). "X" can represent one or more other elements, most commonly Tungsten plus chromium, molybdenum, vanadium, or cobalt.
Strength: High Speed Steel exhibits a density of 8.67x1000 kg/m3. This high density affords it incredible durability and hardness (even at high temperatures) and shock and vibration resistance while still allowing for its machinability into tools and drill bits. Low carbon percentage gives it a very high melting point. Its strength also leads to greater durability in tools even when used in conditions of mechanical and thermal stress.
Physical Properties of HSS Modulus Of elasticity: 31psi*10(6) or 221GPa Density : 0.288lb/in(3) or 7292kg/m(3) Thermal Conductivity: 21W/m/K
Thermal Properties: High Speed Steel has very low thermal expansion rate of 9.7 microns per meter per degree Celsius. It also conducts heat at a very low rate. This enables HSS tools to cut faster (hence the name, "high speed") and to be used for longer periods.
Tungsten and Molybdenum Tungsten and Molybdenum form M6C type carbides. These increase the wear resistance. HSS containing high amounts of Molybdenum are more sensitive with regard to grain growth during strain hardening, and thus require more precise temperature control.
Cobalt Cobalt does not form carbides, but dissolves in the alloy. Increasing cobalt content increases the hot hardness, which permits higher cutting speeds. It also increases the thermal conductivity, and the melting point of the alloy.
Chromium Chromium forms M23C6 carbide, and increases the hardenability
Vanadium Vanadium forms an MC carbide of high hardness. This increases the alloy’s wear resistance. Also, when the Vanadium content is high, the grindability is poor.
• HSS produced by Powder Metallurgy(PM) offers a higher content of alloy elements and a combination of unique properties: - higher toughness - higher wear resistance - higher hardness - higher hot hardness • Using HSS-PM prolongs tool life, makes tool life more predictable, improves performance (feed and speed) and offers a solution to chipping problems. HSS-PM is an excellent substrate for making the best possible use of coatings. • HSS-PM has many advantages in high performance applications such as rough milling, gear cutting tools, and broaching, and also in cases of difficult tapping, drilling and reaming operations. HSS-PM is used too in bandsaws, knives, cold work tooling, rolls, etc.
High speed steels belong to the Fe-C-X multi-component alloy system where X represents chromium, tungsten, molybdenum, vanadium or cobalt.
Alloying compositions of common high speed steel grades (by %wt) Grade
C
Cr
Mo
W
V
Co
Mn
Si
T1
0.65– 0.80
3.75– 4.00
-
17.25– 18.75
0.9–1.3 -
0.1–0.4 0.2–0.4
M2
0.95
4.2
5.0
6.0
2.0
-
-
-
M7
1.00
3.8
8.7
1.6
2.0
-
-
-
M35
0.94
4.1
5.0
6.0
2.0
5.0
-
-
M42
1.10
3.8
9.5
1.5
1.2
8.0
-
-
Comparative properties of high speed steel TYPE HARDNESS RANGE
WEAR RESISTANCE (RELATIVE)
TOUGHNESS (RELATIVE)
HARDNESS (RELATIVE)
M-2
64-66
G
G
F
M-32
65-67
G
F
G
M-7
63-65
G
G
P
M-35
64-66
G
F
E
M-42
66-68
E
G
E
M-43
67-69
E
F
E
E:Excellent; G:Good; F:Fair; P:Poor
Steel type C 18% tungsten
30% tungsten
6% cobalt
12% cobalt (super HSS)
Hardnes s (VPN)
--- Composition % ----
0.68
0.75
0.8
0.8
Cr 4.0
4.7
5.0
5.0
W 19.0
22.0
19.0
21.0
V 1.5
1.4
1.5
1.5
Mo ---
---
0.5
0.5
Typical uses
Co ---
Low quality alloy, 800-850 seldom used.
---
General purpose 850-950 workshop cutting tools.
6.0
Heavy duty 800-900 cutting tools.
11.5
Cutting tools for machining 850-950 of high tensile materials.
The tool that you produce is only as good as the heat treatment that it receives. There are four steps that should be followed in any heat treating process. They include in order: preheating, austenitizing, quenching, and tempering.
PREHEATING Since most tool and high speed steels are sensitive to thermal shock, a sudden increase from room temperature to the austenitizing temperature of 1500F/2250F may cause these tools to crack. The material should be preheated to just below this critical transformation temperature, and then held long enough for the entire cross-section of the part to equalize. Once the part is equalized, then further heating to the austenitizing temperature will allow the material to transform while undergoing a minimum amount of distortion.
Austenitizing In the annealed microstructure, the alloy content of the steel is primarily contained in the carbide particles that are uniformly distributed as tiny spheres. This condition is typically referred to as a spheroidized annealed microstructure. The idea behind austenitizing is to re-distribute this alloy content throughout the matrix by heating the steel to a suitably high temperature so that diffusion can take place. Higher temperatures allow more alloy to diffuse, which usually permits a higher hardness.
Quenching Once the alloy content has been redistributed throughout the matrix, the steel must be cooled fast enough to fully harden it. This process is called quenching. By quenching the microstructure changes from austenite to martensite. How rapidly this process must take place depends upon the chemical composition of the alloy. For some alloys, cooling in still air is sufficient. Other mediums used for quenching include water, brine, and salt bath. Whatever quenching process is used, the resulting microstructure is extremely brittle and under great stress. This necessitates the tempering step that follows.
Tempering Tempering is performed to soften the martensite that was produced during quenching. Most steels have a wide range of temperatures that can be used for tempering, and the one that is chosen depends upon the aim hardness. Most tool and high speed steels require several tempers before the part can be put into service. This is because these alloys will retain a certain percentage of austenite when they are quenched, and during the first temper some of this retained austenite will transform to untempered martensite. By performing a second temper, this new martensite is softened, thus reducing the chance of cracking. But by tempering a second time, some of the remaining austenite is transformed to untempered martensite, and so the process may need to be repeated several times.
Used for cutting tools in which strength and hardness must be retained at temperatures greater than equal to 760 degree C (1400 degree F).
M2 - usually used in the manufacturing of various tools like drill bits, taps and reamers. M42 - widely used in metal manufacturing because of its superior red-hardness.
Coatings tools are coated to increase the life of high speed steel It increases a tool's hardness, lubricity. The cutting edge of a tool can cleanly pass through the material without having the material gall to it. It also decreases the temperature of the cutting process and increases the life of the tool.
