Tool steels are steels that are primarily used to make tools used in manufacturing processes as well as for machining metals, woods and plastics. These steels are fabricated into many types of tools or into the cutting and shaping parts of power-driven machinery for various manufacturing operations. They contain tungsten, molybdenum, and other alloying elements that give them extra strength, hardness, and resistance to wear. They can also be termed as special steels which have been developed to form, cut or otherwise change the shape of a material into a finished or semi-finished product. Although forming a small percentage of total steel production, its nature stands a major factor affecting any finished component.
Tool steels are generally ingot-cast wrought products, and must be able to withstand high specific loads as well as be stable at elevated temperatures. Other properties possessed are
(i) Slight change to form during hardening
(ii) Little risk of cracking during hardening
(iii) Good wear resistance, good toughness, very good machinability
(iv) A good resistance to decarbonization during heat treatment
Tool steels are fabricated in to many types of tools or into cutting and shaping parts of power-driven machinery for various manufacturing operations. They contain Tungsten, Molybdenum and other alloying elements which give them extra strength, hardness and resistance to wear. Because cutting processes involve high local stresses, fractions and considerable heat generation, cutting tool materials as carbon steels (Steel containing 1 to 1.2% carbon), high speed steels (Iron alloys containing tungsten, chromium, vanadium and carbon) Tungsten carbide and diamonds and by such recently developed materials as ceramic, carbide ceramic and aluminum oxide.
In many cutting operations fluids are used to cool and lubricate. Cooling increases tool life and helps to stabilize the size of the finished part. Lubrication reduces friction, thus decreasing the heat generated and the power required for a given cut. Cutting fluids include water based solutions, chemically inactive oils, and synthetic fluids.
Based on Standard BS4659:1971, tool steels are grouped into six types; high speed, hot-work, cold-work, shock resisting, special purpose and water hardened tool steels.
Classification of tool steels is done based on uses and application, composition or heat treatment process. Putting into consideration the constraints of this discussion, classification based on composition will be looked into. We have the classes of Carbon tool steels, low-alloy tool steel and the High Speed Tool Steel (HSS).
1.1 CARBON TOOL STEEL
They are perhaps the oldest known tool material. In general, carbon tool steels usually contain 1.2% - 1.5% carbon, which are present in the Martensite phase. This gives it the ability to harden. More than 90% of all steels are carbon steels. They contain varying amounts of carbon but not more than 1.65% Mn, 0.60% Si and 0.60% Cu. Increase in carbon increases wear resistance. When heated to recommended temperature, say 1800 -1950 ⁰F, the steel is free of excess carbides that might decrease forge ability. Other alloying elements are frequently used and supplement the carbon i.e. aiding the hardness. They are Silicon, Manganese, Chromium, Vanadium, Cobalt and Molybdenum.
Silicon acts as a deoxidizer, though influencing the properties of steel only slightly unless the amount exceeds 0.5% if carbon content is high, Silicon reacts to form Silicon Carbide, which increases hardness and wear resistance but toughness decreases. Sulphur and phosphorus, of 0.3% have a negligible effect on properties. Manganese leads to a greater penetration of hardness.
1.2 LOW ALLOY TOOL STEELS
These are tool steels with total percentage of alloying element less than 2.5%. Moderate amounts of Cr, Ni, Mn, Si or Mo is added to increase hardenability, as compared to carbon tool steels i.e. their percentage alloying element is higher than that of carbon tool steels. These principal alloying elements are Si, Ni with low or high carbon. Cr a ferrite stabilizer and relatively strong carbide former which behaves differently form Ni, which is an austenite stabilizer and non carbide former.
Due to this fact, both possess different effects on low alloy steel. Ni lowers the critical temperature range while Cr raises it. It implies tat the presence of Ni brings about low hardening temperature, slightly lowers annealing temperature and longer annealing times. The reverse is the case for Cr. also Ni-Steel is less machinable due to strong solid solution hardening effect it exerts has it enters the matrix rather than carbide. The reverse is also the case for Cr-Steel.
Low alloy tool steels have a specified composition, containing percentages of vanadium, Molybdenum or other elements as well as large amounts of manganese, silicon and copper than regular carbon steels. The low-alloy tool steels cost less than the regular alloy steels because they contain only small amount of the expensive alloying elements. They have however been specially processed to have much more strength then carbon steels of the same weight.
The effects of other alloying elements are summarized below:
Si
1. Deoxidizer
2. Hardness penetration and increases strength.
Mo
1. Increases tempering resistance
2. Increases hardenability and minimizes temper brittleness.
Manganese also acts like Ni i.e. lowering the critical temperature, improves rolling and penetration of hardness. The Ni bases are used for manufacturing tools such as shear bladed, stamping tools, thread rolling dies, chisels, brake dies e.t.c. Drills and broaches, knurls and files, tap dies, rock drills, swaging dies are Cr-based.
1.3 HIGH SPEED TOOL STEELS
The evolution of high-speed cutting tools commenced with the production of Mushet’s self-hardening tungsten-Manganese steel in 1960. The possibilities of such steels for increased rates of machining were not fully appreciated until 1990, when Taylor and White developed the forerunner of modern high-speed tool steels. In addition to tungsten, chromium was found to be essential and a high hardening temperature to be beneficial. The steel resisted tempering up to 600 ⁰C. This allows the tool to cut at speeds of 80-50m/min with its nose at a dull red temperature. The main constituents of high-speed tool steels are 14 or 18% W, 3 to 5% Cr and 0.6%C. Other elements are frequently added to modern steels which vary considerably in composition and cost. 0.09-0.15% Sulphur is sometimes added to give free machining for unground form tools, e.g. gear hobs. Vanadium improves the cutting qualities of the tools and increases the tendency to air harden. Cobalt, often added to the “Super high-speed steel”, raises the temperature of the solidus and enables a higher hardening temperature to be used, with consequent greater solution of carbon. Secondary hardness is marked in such steels, and this permits the use of deep cuts at fast speeds.
The molybdenum steel is susceptible to decarburization. The high vanadium steel is somewhat brittle, but it is excellent for cutting very abrasive materials. Vanadium is also added for grain refinement.
High speed tool steels are characterized by being heat treatable to very high hardness (Usually Rockwell C64 or over) and of retaining their hardness and cutting ability at temperature as high as 540⁰C, thus permitting truly high-speed machining. Possess good wear resistance, shock resistance, good non-deforming property.
High speed tool steels find application in drills, taps, reamers, milling cutters, broaches, power-saw blades, lathe, shaper and planer tool bits.
1.4 HEAT TREATMENT
HSS are annealed at 850⁰C for four hours, followed by slow cooling. During heating the steel should be protected against oxidation. The annealed structure consists of carbide globules in a matrix of fine Pearlite.
HARDENING: When heated at 900⁰C, Austenite forms, but contains about 1.2% C. Quenching produces martensite, which tempers readily and has no advantage over carbon tool steels. To attain maximum cutting efficiency, sufficient carbon and alloying elements must be dissolved in the austenite and for this reason; HSS is heated between 1150 and 1350⁰C. They are carefully preheated up to 850⁰C and then heated rapidly to the hardening temperature so as to minimize grain growth and deoxidation that occur rapidly at such temperature. Holding at the hardening temperature should be sufficient for dissolving that part of the Carbide which can pass into solution at this temperature. The holding time in a molten salt bath for tools10-15mm in thickness may be taken as 8-9secs/mm cross sectional area. Next the tool may be quenched to 425⁰C, kept there until temperature is uniform throughout the section, and then air cooled to room temperature. This reduces the sever stresses otherwise set up if quenched in oil or cooled in air blast without soaking.
Carbon and low-alloy steels may be quenched in water or brine.
TEMPERING: Since retained Austenite in the structure of the quenched HSS is softer than Martensite and thus lowers the cutting properties, sub zero treatments at a temperature 70-80⁰C below zero, is often applied after 30-60min of hardening to prevent stabilization of the Austenite. Tempering follows at 560⁰C to relieve internal stresses and transform retained Austenite to Martensite.
1.5 MICROSTRUCTURE
The final structure of HSS consists of carbide globules in a matrix of fine Pearlite.
1.6 MECHANICAL AND ENVIRONMENTAL CONSIDERATIONS
Carbon tool steels with high percentage carbon should be selected as the cover for the required wear resistance. Also to be noted is the fact that, increase in carbon increases wear resistance but reduces toughness. Tool steels that lack Cr should be kept away form corrosive atmosphere. In situations where high strength steels are to machined, tool steels with greater resistance to tempering at the temperature required for machining should be selected. HSS stands a better option for this condition as they posses longer life, retain sharp edges at high temperatures, provide good finish, they are also possess high resistance to corrosive environments. It should also be noted that in a situation where by a lubricant will be needed in the course of machining, the tool to be selected must be able to withstand corrosion.
Tool steels are generally ingot-cast wrought products, and must be able to withstand high specific loads as well as be stable at elevated temperatures. Other properties possessed are
(i) Slight change to form during hardening
(ii) Little risk of cracking during hardening
(iii) Good wear resistance, good toughness, very good machinability
(iv) A good resistance to decarbonization during heat treatment
Tool steels are fabricated in to many types of tools or into cutting and shaping parts of power-driven machinery for various manufacturing operations. They contain Tungsten, Molybdenum and other alloying elements which give them extra strength, hardness and resistance to wear. Because cutting processes involve high local stresses, fractions and considerable heat generation, cutting tool materials as carbon steels (Steel containing 1 to 1.2% carbon), high speed steels (Iron alloys containing tungsten, chromium, vanadium and carbon) Tungsten carbide and diamonds and by such recently developed materials as ceramic, carbide ceramic and aluminum oxide.
In many cutting operations fluids are used to cool and lubricate. Cooling increases tool life and helps to stabilize the size of the finished part. Lubrication reduces friction, thus decreasing the heat generated and the power required for a given cut. Cutting fluids include water based solutions, chemically inactive oils, and synthetic fluids.
Based on Standard BS4659:1971, tool steels are grouped into six types; high speed, hot-work, cold-work, shock resisting, special purpose and water hardened tool steels.
Classification of tool steels is done based on uses and application, composition or heat treatment process. Putting into consideration the constraints of this discussion, classification based on composition will be looked into. We have the classes of Carbon tool steels, low-alloy tool steel and the High Speed Tool Steel (HSS).
1.1 CARBON TOOL STEEL
They are perhaps the oldest known tool material. In general, carbon tool steels usually contain 1.2% - 1.5% carbon, which are present in the Martensite phase. This gives it the ability to harden. More than 90% of all steels are carbon steels. They contain varying amounts of carbon but not more than 1.65% Mn, 0.60% Si and 0.60% Cu. Increase in carbon increases wear resistance. When heated to recommended temperature, say 1800 -1950 ⁰F, the steel is free of excess carbides that might decrease forge ability. Other alloying elements are frequently used and supplement the carbon i.e. aiding the hardness. They are Silicon, Manganese, Chromium, Vanadium, Cobalt and Molybdenum.
Silicon acts as a deoxidizer, though influencing the properties of steel only slightly unless the amount exceeds 0.5% if carbon content is high, Silicon reacts to form Silicon Carbide, which increases hardness and wear resistance but toughness decreases. Sulphur and phosphorus, of 0.3% have a negligible effect on properties. Manganese leads to a greater penetration of hardness.
1.2 LOW ALLOY TOOL STEELS
These are tool steels with total percentage of alloying element less than 2.5%. Moderate amounts of Cr, Ni, Mn, Si or Mo is added to increase hardenability, as compared to carbon tool steels i.e. their percentage alloying element is higher than that of carbon tool steels. These principal alloying elements are Si, Ni with low or high carbon. Cr a ferrite stabilizer and relatively strong carbide former which behaves differently form Ni, which is an austenite stabilizer and non carbide former.
Due to this fact, both possess different effects on low alloy steel. Ni lowers the critical temperature range while Cr raises it. It implies tat the presence of Ni brings about low hardening temperature, slightly lowers annealing temperature and longer annealing times. The reverse is the case for Cr. also Ni-Steel is less machinable due to strong solid solution hardening effect it exerts has it enters the matrix rather than carbide. The reverse is also the case for Cr-Steel.
Low alloy tool steels have a specified composition, containing percentages of vanadium, Molybdenum or other elements as well as large amounts of manganese, silicon and copper than regular carbon steels. The low-alloy tool steels cost less than the regular alloy steels because they contain only small amount of the expensive alloying elements. They have however been specially processed to have much more strength then carbon steels of the same weight.
The effects of other alloying elements are summarized below:
Si
1. Deoxidizer
2. Hardness penetration and increases strength.
Mo
1. Increases tempering resistance
2. Increases hardenability and minimizes temper brittleness.
Manganese also acts like Ni i.e. lowering the critical temperature, improves rolling and penetration of hardness. The Ni bases are used for manufacturing tools such as shear bladed, stamping tools, thread rolling dies, chisels, brake dies e.t.c. Drills and broaches, knurls and files, tap dies, rock drills, swaging dies are Cr-based.
1.3 HIGH SPEED TOOL STEELS
The evolution of high-speed cutting tools commenced with the production of Mushet’s self-hardening tungsten-Manganese steel in 1960. The possibilities of such steels for increased rates of machining were not fully appreciated until 1990, when Taylor and White developed the forerunner of modern high-speed tool steels. In addition to tungsten, chromium was found to be essential and a high hardening temperature to be beneficial. The steel resisted tempering up to 600 ⁰C. This allows the tool to cut at speeds of 80-50m/min with its nose at a dull red temperature. The main constituents of high-speed tool steels are 14 or 18% W, 3 to 5% Cr and 0.6%C. Other elements are frequently added to modern steels which vary considerably in composition and cost. 0.09-0.15% Sulphur is sometimes added to give free machining for unground form tools, e.g. gear hobs. Vanadium improves the cutting qualities of the tools and increases the tendency to air harden. Cobalt, often added to the “Super high-speed steel”, raises the temperature of the solidus and enables a higher hardening temperature to be used, with consequent greater solution of carbon. Secondary hardness is marked in such steels, and this permits the use of deep cuts at fast speeds.
The molybdenum steel is susceptible to decarburization. The high vanadium steel is somewhat brittle, but it is excellent for cutting very abrasive materials. Vanadium is also added for grain refinement.
High speed tool steels are characterized by being heat treatable to very high hardness (Usually Rockwell C64 or over) and of retaining their hardness and cutting ability at temperature as high as 540⁰C, thus permitting truly high-speed machining. Possess good wear resistance, shock resistance, good non-deforming property.
High speed tool steels find application in drills, taps, reamers, milling cutters, broaches, power-saw blades, lathe, shaper and planer tool bits.
1.4 HEAT TREATMENT
HSS are annealed at 850⁰C for four hours, followed by slow cooling. During heating the steel should be protected against oxidation. The annealed structure consists of carbide globules in a matrix of fine Pearlite.
HARDENING: When heated at 900⁰C, Austenite forms, but contains about 1.2% C. Quenching produces martensite, which tempers readily and has no advantage over carbon tool steels. To attain maximum cutting efficiency, sufficient carbon and alloying elements must be dissolved in the austenite and for this reason; HSS is heated between 1150 and 1350⁰C. They are carefully preheated up to 850⁰C and then heated rapidly to the hardening temperature so as to minimize grain growth and deoxidation that occur rapidly at such temperature. Holding at the hardening temperature should be sufficient for dissolving that part of the Carbide which can pass into solution at this temperature. The holding time in a molten salt bath for tools10-15mm in thickness may be taken as 8-9secs/mm cross sectional area. Next the tool may be quenched to 425⁰C, kept there until temperature is uniform throughout the section, and then air cooled to room temperature. This reduces the sever stresses otherwise set up if quenched in oil or cooled in air blast without soaking.
Carbon and low-alloy steels may be quenched in water or brine.
TEMPERING: Since retained Austenite in the structure of the quenched HSS is softer than Martensite and thus lowers the cutting properties, sub zero treatments at a temperature 70-80⁰C below zero, is often applied after 30-60min of hardening to prevent stabilization of the Austenite. Tempering follows at 560⁰C to relieve internal stresses and transform retained Austenite to Martensite.
1.5 MICROSTRUCTURE
The final structure of HSS consists of carbide globules in a matrix of fine Pearlite.
1.6 MECHANICAL AND ENVIRONMENTAL CONSIDERATIONS
Carbon tool steels with high percentage carbon should be selected as the cover for the required wear resistance. Also to be noted is the fact that, increase in carbon increases wear resistance but reduces toughness. Tool steels that lack Cr should be kept away form corrosive atmosphere. In situations where high strength steels are to machined, tool steels with greater resistance to tempering at the temperature required for machining should be selected. HSS stands a better option for this condition as they posses longer life, retain sharp edges at high temperatures, provide good finish, they are also possess high resistance to corrosive environments. It should also be noted that in a situation where by a lubricant will be needed in the course of machining, the tool to be selected must be able to withstand corrosion.
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