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Tool steel
Technical Report
Tool Steel General Until the mid-19th century, only tools made from carbon steel were used for cutting. During the latter half of the 19th century, however, manganese, chromium and tungsten began to be used as alloys in steel and thus it was possible to achieve good wear resistance. During the Paris Exhibition in 1900, the American, Taylor, presented a high-speed steel containing 1.85 percent C (carbon), 3.8 percent Cr (chromium), and 8 percent W (tungsten). This had considerably better cutting properties than carbon steel. High-speed steel has been gradually improved since then by means of various alloying additives. The steel was named high-speed steel since it permitted much more rapid cutting than was possible with carbon steel tools. Around 1920, cemented carbides were introduced, although the first cemented carbide tools were only suitable for machining cast iron. Tools for machining steel were introduced in about 1930, and it was not until the beginning of the 1950's that cutting using ceramics was introduced, to be followed by boron nitrides. The latest addition to tool materials came in 1973, when polycrystaUine diamond was presented for the first time. Tool material can be basically divided into carbon steel, high-speed steel, cast hard-alloys, cemented carbide, boron nitrides and diamond. In this section we will deal only with high-speed steel and cemented carbide. High-speed steel High-speed steel retains its hardness and thereby cutting ability at considerably higher temperatures than carbon steel, permitting significantly higher cutting speeds. As mentioned earlier, Taylor presented a high-speed steel in 1900. This was developed gradually to the classic 18-4-1 grade - a high-speed steel with 18 percent W, 4 percent Cr and 1 percent V. In Sweden this steel has been standardised as SS 2750.
MATERIALS & DESIGN Vol. 5 FEBRUARY/MARCH 1984
The well-known M2 grade (SS 2722) originated in the 1930's~ The letter M here denotes molybdenum-alloyed high-speed steel, according to the American AISI standard. Ever since the beginning of the 1940's, tungsten-alloyed high-speed steel has been replaced increasingly by molybdenum-alloyed steel. Today, only a few countries still use tungsten-alloyed steel. During the 1960's, composition development advanced towards an increase of the carbon content in established high-speed steel, sometimes reinforced by a cobalt additive. During the 1970's, powder metallurgical high-speed steel was introduced and this was characterised by an extremely good structure which, among other things, has improved the grindability. The use of powder steel is restricted, however, but its high production costs. Cemented carbide Cemented carbide consists of hard materials and a binder metal. The hard materials (carbides) provide hardness and wear resistance. The binder metal, whose primary task is to act as a binder and bearer of the carbide grain, provides toughness. The basic material is a powder mixture. The hard materials, such as tungsten carbide, titanium carbide, tantalum and niobium carbides comprise 85 to 90 percent of this mixture. The remaining 10 to 15 percent consists of binder metal: cobalt, nickel or iron. By varying the content of the composite materials, it is possible to obtain cemented carbides with different properties, such as varying degrees of wear resistance and toughness. Cemented carbide normally contains two or three socalled phases, the ct phase,/3 phase and 7 phase. The binder metal is referred to as the /3 phase. The a phase is pure tungsten carbide and provides the cemented carbide with its basic strength. The ~/ phase comprises some titanium, tantalium, niobium and tungsten as well as carbon. The 3, phase generally provides increased wear resistance at higher machining temperatures.
27
The properties of cemented carbide The high wear resistance and hot hardness permit high speeds over the cut and a high temperature on the cutting edge. Hence it is possible to use a high cutting speed and thus obtain a high cutting capacity. The wear resistance is closely related to toughness: the hardest and most wearresistant cemented carbide grades are also the most brittle. This entails certain limits as regards the shaping of the tool, such as the cutting angle.
~oo ~.
Percentage volume of 7-phase (provides wear resistance) POt PIO
P20
50 Classification of cemented carbides During machining, the demands made on the roughness and wear resistance of the cemented carbide vary from one operation to another. In order to assist users in selecting a suitable grade of cemented carbide, ISO has classified the demands made on cemented carbides. This has been done by classifying the application areas which, in turn, have been divided up into application groups. There are three application areas which are referred to as P, K and M. P refers to the machining of long-turning material (steel), K refers to short-turning material (cast iron) and terms of machining methods, lies between the P and K materials. M includes steel castings, annealed castings and certain types of spheroidal cast iron. The application groups are marked with number combinations ranging from 1 to 50, where the demands on tough-
P30
30
P40 MI0 M20
M30
10
M40
0
5
10
15
20 25 30 35
Percentage volume of/3-1 Application groups according to ISO in a/3- v-phase diagram. Classification of application areas according to ISO. Machined material Designation Operationsand workingconditions
Steel steel castings long turning annealed castings
Steel, steel castings, manganese steel, alloyed cast iron, austenite steel, annealed castings, automatic steel.
Cast iron and chilled, short turning annealed castings hardened steal non-ferrous metals, synthetic resJ,~s, wood.
28
P01
Fine turning and fine boring, high cutting speed, small turning area, high dimensional precision and surface equality, freedom from vibrations.
/
~
P10
Turning, reproduction turning, thread cutting, milling, high cutting speed,small to medium sized turning area
P20
Turning, reproduction turning, milling, medium cutting speed, medium-sized turning area, planing with a small chip area.
,~ J-
P30
Turning, milling, planing, medium to low cutting speed, medium to large turning area, even under unfavorable working conditions
~ ~
P40
Turning, planing, milling, shaping, low cutting speed, large turning area, large turning area possible, unfavorable working conditions, even automatic work.
P50
In cases of very high demands on the hardness of the cemented carbide, turning, milling, shaping, low cutting speed, large turning area, large turning angle possible, unfavorable working conditions, automatic work.
--
Z
~""
M10
Turning, medium to high cutting speed, small- to medium-sized turning area.
M20
Turning, milling, medium cutting speeds, medium-sized turning area.
t~
M30
Turning, milling, planing, medium steel cutting speed, medium-sized turning area.
,~
M40
Turning, profile turning, cutting, especially in automated machines.
K01
Turning, fine turning and fine boring, fine milling, peeling.
KI0
Turning, milling, boring, countersinking, peeling, notching.
K20
Turning, milling, planing, countersinking, notching high toughness demands on cemented carbides,
K30
Turning, milling, planing, shaping, unfavorable working conditions, large turning angle possible.
K40
Turning, milling~ planing, shaping, very unfavorable working conditions, vary large turning angle possible.
7
Z
l
- - Z f I" ~ ' '
~
"z
z ,~.,g, ~ ~ "~' ""
"~
MATERIALS & DESIGN Vol. 5 FEBRUARY/MARCH 1984
ness grow as the numbers rise. Since the wear resistance of the cutting material increases as toughness decreases, the application grouping also expresses the decrease in wear resistance when increased toughness is required. ISO's system of application areas and application groups is designed to be combined with the product lines of cemented carbide manufacturers.
Cemented carbides are classified according to composition and internal structure There is a general systematic relationship between the group M refers to material which, in classification and ISO's classification by application, even if it is not uncommon
MATERIALS & DESIGN VoL 5 FEBRUARY/MARCH 1984
that the same type suits several application groups or that the same application group - in order to be effectively covered - requires more than one type. In the case of treatment by cutting, apart from the ct phase content, the 3, phase content is also vitally important for the wear resistance of the material. In combination with the ~ phase content, which primarily determines the toughness of cemented carbide, the 3' phase content provides a guide in evaluating the properties of a cemented carbide grade. A diagram with the 3' phase (primarily wear resistance) plotted on one axis and the ]3 phase (primarily toughness) on the other, offers a practical aid for studying the importance of the composition for the various application areas
29
Tool Steel General Until the mid-19th century, only tools made from carbon steel were used for cutting. During the latter half of the 19th century, however, manganese, chromium and tungsten began to be used as alloys in steel and thus it was possible to achieve good wear resistance. During the Paris Exhibition in 1900, the American, Taylor, presented a high-speed steel containing 1.85 percent C (carbon), 3.8 percent Cr (chromium), and 8 percent W (tungsten). This had considerably better cutting properties than carbon steel. High-speed steel has been gradually improved since then by means of various alloying additives. The steel was named high-speed steel since it permitted much more rapid cutting than was possible with carbon steel tools. Around 1920, cemented carbides were introduced, although the first cemented carbide tools were only suitable for machining cast iron. Tools for machining steel were introduced in about 1930, and it was not until the beginning of the 1950's that cutting using ceramics was introduced, to be followed by boron nitrides. The latest addition to tool materials came in 1973, when polycrystaUine diamond was presented for the first time. Tool material can be basically divided into carbon steel, high-speed steel, cast hard-alloys, cemented carbide, boron nitrides and diamond. In this section we will deal only with high-speed steel and cemented carbide. High-speed steel High-speed steel retains its hardness and thereby cutting ability at considerably higher temperatures than carbon steel, permitting significantly higher cutting speeds. As mentioned earlier, Taylor presented a high-speed steel in 1900. This was developed gradually to the classic 18-4-1 grade - a high-speed steel with 18 percent W, 4 percent Cr and 1 percent V. In Sweden this steel has been standardised as SS 2750.
MATERIALS & DESIGN Vol. 5 FEBRUARY/MARCH 1984
The well-known M2 grade (SS 2722) originated in the 1930's~ The letter M here denotes molybdenum-alloyed high-speed steel, according to the American AISI standard. Ever since the beginning of the 1940's, tungsten-alloyed high-speed steel has been replaced increasingly by molybdenum-alloyed steel. Today, only a few countries still use tungsten-alloyed steel. During the 1960's, composition development advanced towards an increase of the carbon content in established high-speed steel, sometimes reinforced by a cobalt additive. During the 1970's, powder metallurgical high-speed steel was introduced and this was characterised by an extremely good structure which, among other things, has improved the grindability. The use of powder steel is restricted, however, but its high production costs. Cemented carbide Cemented carbide consists of hard materials and a binder metal. The hard materials (carbides) provide hardness and wear resistance. The binder metal, whose primary task is to act as a binder and bearer of the carbide grain, provides toughness. The basic material is a powder mixture. The hard materials, such as tungsten carbide, titanium carbide, tantalum and niobium carbides comprise 85 to 90 percent of this mixture. The remaining 10 to 15 percent consists of binder metal: cobalt, nickel or iron. By varying the content of the composite materials, it is possible to obtain cemented carbides with different properties, such as varying degrees of wear resistance and toughness. Cemented carbide normally contains two or three socalled phases, the ct phase,/3 phase and 7 phase. The binder metal is referred to as the /3 phase. The a phase is pure tungsten carbide and provides the cemented carbide with its basic strength. The ~/ phase comprises some titanium, tantalium, niobium and tungsten as well as carbon. The 3, phase generally provides increased wear resistance at higher machining temperatures.
27
The properties of cemented carbide The high wear resistance and hot hardness permit high speeds over the cut and a high temperature on the cutting edge. Hence it is possible to use a high cutting speed and thus obtain a high cutting capacity. The wear resistance is closely related to toughness: the hardest and most wearresistant cemented carbide grades are also the most brittle. This entails certain limits as regards the shaping of the tool, such as the cutting angle.
~oo ~.
Percentage volume of 7-phase (provides wear resistance) POt PIO
P20
50 Classification of cemented carbides During machining, the demands made on the roughness and wear resistance of the cemented carbide vary from one operation to another. In order to assist users in selecting a suitable grade of cemented carbide, ISO has classified the demands made on cemented carbides. This has been done by classifying the application areas which, in turn, have been divided up into application groups. There are three application areas which are referred to as P, K and M. P refers to the machining of long-turning material (steel), K refers to short-turning material (cast iron) and terms of machining methods, lies between the P and K materials. M includes steel castings, annealed castings and certain types of spheroidal cast iron. The application groups are marked with number combinations ranging from 1 to 50, where the demands on tough-
P30
30
P40 MI0 M20
M30
10
M40
0
5
10
15
20 25 30 35
Percentage volume of/3-1 Application groups according to ISO in a/3- v-phase diagram. Classification of application areas according to ISO. Machined material Designation Operationsand workingconditions
Steel steel castings long turning annealed castings
Steel, steel castings, manganese steel, alloyed cast iron, austenite steel, annealed castings, automatic steel.
Cast iron and chilled, short turning annealed castings hardened steal non-ferrous metals, synthetic resJ,~s, wood.
28
P01
Fine turning and fine boring, high cutting speed, small turning area, high dimensional precision and surface equality, freedom from vibrations.
/
~
P10
Turning, reproduction turning, thread cutting, milling, high cutting speed,small to medium sized turning area
P20
Turning, reproduction turning, milling, medium cutting speed, medium-sized turning area, planing with a small chip area.
,~ J-
P30
Turning, milling, planing, medium to low cutting speed, medium to large turning area, even under unfavorable working conditions
~ ~
P40
Turning, planing, milling, shaping, low cutting speed, large turning area, large turning area possible, unfavorable working conditions, even automatic work.
P50
In cases of very high demands on the hardness of the cemented carbide, turning, milling, shaping, low cutting speed, large turning area, large turning angle possible, unfavorable working conditions, automatic work.
--
Z
~""
M10
Turning, medium to high cutting speed, small- to medium-sized turning area.
M20
Turning, milling, medium cutting speeds, medium-sized turning area.
t~
M30
Turning, milling, planing, medium steel cutting speed, medium-sized turning area.
,~
M40
Turning, profile turning, cutting, especially in automated machines.
K01
Turning, fine turning and fine boring, fine milling, peeling.
KI0
Turning, milling, boring, countersinking, peeling, notching.
K20
Turning, milling, planing, countersinking, notching high toughness demands on cemented carbides,
K30
Turning, milling, planing, shaping, unfavorable working conditions, large turning angle possible.
K40
Turning, milling~ planing, shaping, very unfavorable working conditions, vary large turning angle possible.
7
Z
l
- - Z f I" ~ ' '
~
"z
z ,~.,g, ~ ~ "~' ""
"~
MATERIALS & DESIGN Vol. 5 FEBRUARY/MARCH 1984
ness grow as the numbers rise. Since the wear resistance of the cutting material increases as toughness decreases, the application grouping also expresses the decrease in wear resistance when increased toughness is required. ISO's system of application areas and application groups is designed to be combined with the product lines of cemented carbide manufacturers.
Cemented carbides are classified according to composition and internal structure There is a general systematic relationship between the group M refers to material which, in classification and ISO's classification by application, even if it is not uncommon
MATERIALS & DESIGN VoL 5 FEBRUARY/MARCH 1984
that the same type suits several application groups or that the same application group - in order to be effectively covered - requires more than one type. In the case of treatment by cutting, apart from the ct phase content, the 3, phase content is also vitally important for the wear resistance of the material. In combination with the ~ phase content, which primarily determines the toughness of cemented carbide, the 3' phase content provides a guide in evaluating the properties of a cemented carbide grade. A diagram with the 3' phase (primarily wear resistance) plotted on one axis and the ]3 phase (primarily toughness) on the other, offers a practical aid for studying the importance of the composition for the various application areas
29