Steel may contain some of the following elements and effects of each are given below.
EFFECTS OF ALLOYING ELEMENTS
1. NICKEL: -
- Toughens steel by refining grains
- Strengthens ferrite
- Improves tensile strength and hardness
- It is an austenite stabilizer
- It increases and oxidation resistance
- It increases impact resistance
- It reduces coefficient of thermal expansion
2. CHROMIUM: -
- Increases resistance to corrosion and oxidation
- Stabilizes carbides and forms hard chromium carbides
- Resists abrasion and wear
- It increases hardenability of steel
- It improves service life
3. MANGANESE: -
- Acts as a de-oxidizer and de-sulphuriser
- Increases hardenability
- It increases strength, toughness, hardness and machinability.
4. MOLYBDENUM: -
- Raises grain coarsening temperature of austenite
- Deepens hardening
- Reduces temper brittleness in nickel chromium steel
5. VANADIUM: -
- Raises softening temperature of hardened steel
- Increases hardenability
- Promotes fine grain
- Increases hardness and vanadium carbides promote secondary hardening.
- Improves wear resistance.
- Improves fatigue and creep resistance.
- t is a strong de-oxidiser.
6. TUNGSTEN: -
- Reduces grain growth
- Rises the limiting creep stress at temperatures
- Promotes hardness and strength at elevated temperature
7. COBALT: -
- Contributes to red hardness by hardening
- Tends to cause de-carbonization
- It increases resistance to tempering
8. SILICON: -
- Alloying element for electrical and magnetic sheet
- Strengthens low alloy steel
- It acts as graphitizer.
9. ALUMINIUM: -
- It is a strong de-oxidizer and grain refiner
10. SULPHUR: -
- It increases brittleness and produces hot shortness.
11. PHOSPHORUS: -
- Increases strength and hardness
- It is solid solution strengthener
- Excess addition makes steel brittle and cold shortness.
COMPOSITION, PROPERTIES & USES OF ALLOY STEEL
1. NICKEL STEEL
Composition: Amount of nickel varies from 2% - 45%
Properties:
- Very high tensile strength
- Stainless and non-magnetic
- Reduces coefficient of thermal expansion to 0.0000002
- It is known as invar and super invar. It posses extension equal to that of glass
Uses:
- Boiler plates, rivets, cams and bolts, etc,
- High-pressure boilerplates, war ships, connecting rods, etc.
- Engine valves, turbine blades, electric resistance wires, etc.
- For measuring instruments, survey tapes, clock pendulum, etc.
2. NICKEL CHROME STEEL
Composition:
(a) Mild nickel chrome steel C-0.2-0.3% Ni – 3.75%
- Having High strength
- Used for Axle crank shafts
(b) Medium nickel chromium steel C–0.2-0.35% Ni-3-3.75%
- Stronger than mild nickel chromium
- Used for Cranks, connecting rods, etc. for parts subjected to high stress and shocks.
(c) High Tensile nickel chromium steel Cr-0.3%
- Very hard, tough, high tensile strength
- Used for Parts for aircraft and shafts.
3. MOLYBDENUM STEELS
(a) Chromium molybdenum steel (Mo-0.15-0.6%)
- Relatively cheep, good deep hardening characteristics, ductility, and weld ability.
(b) Nickel Molybdenum steel (Cr-1% Mo-0.15-0.25%)
- High strength ductility, deep hardening, improved machinability.
Uses:
- Transmission gears, roller bearings, chain pins, etc.
4. CHROMIUM STEEL
Composition: Cr-8-15%
Properties:
Permanent magnetism
Uses:
Electrical equipments, springs, ball and roller bearings.
5. MANGANESE STEEL
Composition: Mn –1-1.5% up to 2%, Properties:
Strong and tough reduce formation of sulphide and deoxidization of molten metal. hard and brittle, very hard, tough, non-magnetic and high tensile strength.
Uses:-
Manganese steel castings are used for special purpose such as the parts of stone crushing grinding and dredging plant, forging of component can also be done
6.SILICON STEEL
Composition: 3.5% silicon
Properties:
- Improved electrical properties, low magnetic hysterises.
Uses:
-Chemical industries IC engines etc.
7.SILICON MANGANESE STEEL
Composition: Si-2%, Mn-0.7%
Properties:
-High resilience, strength and toughness.
Uses: -
-Used for leaf spring, coil spring, chemical industry and I C engine parts.
SPECIAL ALLOY STEEL
1. MAGNET STEEL
Composition 15-40% cobalt, 0.4-1% carbon1.5-9% chromium, 10%tungsten
Properties:
Possess improved magnetic properties
Uses:
Permanent magnets for loud speakers and other electrical machines and instruments.
2. HEAT RESISTING STEEL:
Composition 23-30% chromium, less then 0.35% carbon
Properties
Suitable for working at very high temperatures.
Resistance to acid corrosion.
Uses:
Furnace parts, annealing boxes and flame nozzles
3. SHOCK LOADING STEEL:
Composition:-0. 5% carbon, 2.25% tungsten, 1.5% chromium, 0.25% vanadium
Properties
It resists high shock loads and impact loads
Uses:
Axles, leaf and coil springs, wheel drums, etc,
4. STAINLESS STEEL:
a) Martensitic stainless steel
Composition 16-20% Cr, 0.1-0.3% C, 1-3% Ni
High strength, excellent toughness, resistance to oxidation, high corrosion and abrasion resistance.
Uses: -
Dairy and chemical Industries
All types of surgical dental instruments
Utensils
b) Ferritic stainless steel
Composition:-14-27%Cr
Properties:-
Magnetic, good resistance to corrosion, very soft, and low impact values.
Uses: -
Nuclear power plant parts.
c) Austenitic stainless steel:
Composition: - 15-20% Cr, 7-10% Ni (18/8) 18% Cr & 8% is widely used.Properties:
High red hardness, good corrosion resistance, better resistant to high temperature, high ductility
Uses:
Used in consumer products, architectural constructions, chemical equipments, etc,
5. HIGH SPEED STEEL:
(a) Tungsten HSS (18-4-1)
Composition: 18% W, 4% Cr. 1% V, 0.75%C
Properties:-
High red hardness, excellent toughness and cutting ability.
Uses:-
Used for cutting tools used on lathe, shaper, planer and drills, taps, dies etc,
(b) Molybdenum HSS (6-6-4-2 HSS)
Composition: 6% Mo, 4%Cr, 6%W, 2%V
Properties: -
It possesses same properties as that of tungsten HSS but it is cheaper.
Uses: -
Used for cutting tools
(c) Cobalt HSS
Composition: 10% Co, 18%W, 4%Cr, 2%V
Properties:-
High hardness, increased cutting ability at elevated temperature, more resistant to crack in quenching, subjected to decarborization if held for too long at the heating temperature.
Uses:
Suitable for cutting tools, which are used, in high production, in increased speed and more depth of cuts.
(d) Vanadium HSS
Composition: more than 1% V, 0.7% C
Properties:-
It has excellent abrasive resistance and good cutting efficiency.
Uses:-
Used for cutting tools.
TOOL STEELS
A steel containing alloying elements that enable it to be heat treated to obtain desirable characteristics such as strength, hardness and wear resistance are referred as tool steel.
The term tool steel refers to a group of high quality carefully manufactured steels that are characterized by high hardness and resistance to abrasion. Certain groups of steels also have a high resistance to softening at high temperatures.
A vacuum process produces tool steels for critical applications. Rigorous inspection procedures are employed throughout the manufacture of tool steel. Entire bars may be subjected to magnetic particle and ultrasonic inspection for surface and internal defects.
The high price of alloying elements and precise production requirements and quality control are the reason for the high cost of tool steels. When it is considered that the tool steels are made into a complicated tool or die which requires hundreds of man-hours in its manufacture and will produce thousands or even millions it is well worth the extra cost.
The importance is given to different steels used for tool because of feasible properties of such steels: -
Slight change of form during hardening.
Little risk of cracking during hardening
Good toughness
Very good machinability
Good wear resistance.
A definite cooling rate during hardening
A definite hardening temp
Resistance to decarburization.
Resistance to softening on heating, i.e., red hardness
The term tool steel is used in present day industry, refers to a group of high quality, carefully manufactured steels that are characterized by high hardness and resistance to abrasion.
Classification of tool steels
Classification as per American Iron and Steel Institute (AISI) divides tool steels into the following main groups for majority of application.
- Cold work tool steels
- Hot work tool steels and
- High-speed tool steel.
Cold work tool steel
Typical uses are engraving tools, files, and razors, shaping tools, woodworking tools heading tools, etc.
Hot Work Tool steel
The term hot work tool steels are intended for all tooling applications except cutting tools where temperature attained by the tool, die and mould is above 200°C. Typical uses are for hot extrusion; hot stamping dies, die-casting dies, forging dies, etc.
High-speed tool steel
The term High Speed Tool Steel is defined as a special variety of tool steel, which by virtue of its composition retains its cutting hardness at low red heat. Typical uses are drills, slitting saws and other high speed and heavy cut tools, etc.
CLASSIFICATION AND COMPOSITION OF PRINCIPAL TYPES OF TOOL STEELS
1. Cold Work Tool Steels
It is seen that each group of tool steel processes a distinguishing property or properties, which identify and establish its superiority over other groups and no group has all the requisites viz, best in wear resistance, best in toughness. The increase in wear resistance and decrease in toughness is due to the increase of carbon and alloying elements.
Similarly it is seen that the shock resistance tool steel due to its medium carbon and alloying content is the best in toughness but less wear resistant.Wear resistance-toughness combination is the primary criterion for satisfactory tool operation.
S.NO. GROUP AISI CODE
1. Water hardening tool steel (High Carbon Steel) W
2. Shock resisting Tool Steel S
3. Oil Hardening Cold work Tool steel O
4. Air hardening Cold work Tool Steel A
5. High Carbon High Chromium Cold work Tool Steel D
Present trends in the cold work tool steels
High carbon water hardening tool steels are used for tools, which are simple in shape for short runs and where there are no sharp changes in section etc. The present trend is towards greater use of High Carbon High Chromium for complicated and interacted tooling and for long runs. This trend is principally to obtain longer tool life to avoid distortion or cracking during heat treatments and also the dimensional changes resulting from hardening are exceptionally small. AISI has therefore assigned symbol ″D″indicating dimensional stability. During tools fabrication if the changes in the size control and position of the tooling are to be kept to the obsolete minimum particularly when the precision operations like jig boring, die sinking etc. are involved, then high carbon high chromium becomes the obvious choice.
The special purpose tool steels coming under cold work tool steels are discussed like, Low alloy special purpose tool steel (L) are similar to group W. Greater wear resistance and hardenability is there due to the addition of Chromium and other elements. L6 has additional toughness and hardenability due to the presence of Nickel.
Carbon Tungsten tool steels (F) is similar to group W and have high wear resistance due to high carbon and tungsten content.
Used for Wire drawing dies, forming tools, burnishing tools etc. where high wear, low temperature and low shock applications.
Low Carbon mould steels (P) are alloy carburising steels produced to tool steel quality
Used for Moulds for injection or compression molding of plastics.
WATER HARDENING TOOL STEELS
The water hardening or carbon steels are one of the oldest types of tool steels. They depend primarily upon carbon content of their treatable properties with addition of chromium or vanadium. Chromium is added to increase harden ability and wear resistance. Vanadium is added to refine grain for added toughness. Carbon content varies from 0.6 to 1.4%. . They will harden with hard case and tough core. They posses low resistance to heat softening. They are easy to machine compared to other steels and require relative simple heat-treating methods. They are suitable for light or medium cold impact operations such as coining, cold heading, punching, knurling, embossing and for wood and metal hand cutting tools.
The steel should not be used when the tool has drastic changes, sharp corners or holes near the edge of the tool. Tools of this nature have a tendency to crack during heat treatment because of quenching stresses caused by uneven cooling stress concentration points. They should not be used where distortion during heat treatment will present a problem; the steels have a tendency to wrap during quenching.
THEY SHOULD NOT BE USED WHEN WORKING TEMPERETURE WILL BE EN COUNTERED, AS THE TOOL WILL SOFTEN.
When the tool is relatively complicated and there is a possibility to crack during Heat Treatment water-hardening steel should not be used. Oil or air hardening steel should be used.
SHOCK RESISTING STEELS:
SHOCK RESISTING STEELS:
These steels are used for shock operations at normal temperatures and where maximum abrasion resistance is not required. They have less carbon and have higher toughness. The hardness will be below 60HRC.They are oil and water hardening steels.
The chief alloying elements are silicon, chromium, and tungsten and molybdenum. . These alloys increase Hardenability and provide heat and wear resistance. The tungsten types have higher heat resistance.
Pneumatic chisels are heavy duty; shear blades and similar tools are manufactured from the steel
OIL HARDENING TOOL STEELS:
They are safer to harden and have less dimensional change during heat treatment. They have good machinability, good resistance to decarborisation and have a high carbon content to provide good wear resistance. The depth of hardening is greater than that of water hardening steels and as a result they are usually less tough. They do not have high red hardness and there fore must be used for tools that will operate near room temperature. Blanking, hardening, trimming, and cooling tools thread rolling dies, knurling tools, gauges etc., are manufactured.
AIR HARDENING TOOL STEEL:
This possesses better properties than oil hardening steels. Manganese, chromium, molybdenum and vanadium are the chief alloying elements. These alloying elements promote air-hardening characteristics, which result in excellent dimensional stability. The high carbon content provides good wear resistance. The high manganese grades may be hardened at lower temperatures thus reducing scaling and further reducing dimensional change .The machinability is lower than water and oil hardened grade steel. This steel has poor machinability. The air hardening grades are used to manufacture intricate dies, punches, and thread rolling dies, long slender broaches and other applications where resistance to distortion and abrasion are important.
HIGH CORBON HIGH CHROMIUM COLD WORK STEELS:
They combine the high wear resistance with deep hardening properties. The have low dimensional change during hardening and have a medium resistance to heat softening. They are susceptible to edge brittleness which makes them unsuitable for edge cutting tools.
Intricate dies and punches, master gauges and other applications where dimensional stability and long wearing properties are important are manufactured from this steel.
HOT WORK TOOL STEELS:
Hot work tool steels (AISI code H) form a special group of steels intended mainly for industrial applications used in high temperature metal forming operations. H steels are intended to cover all applications except cutting tools, where the operating temperature is above 200 C.
The desirable properties for this type of tool steels are red hardness; hot wear resistance and toughness at elevated temperature besides having a good thermal conductivity. The three main hard working activities are forging, hot extrusion and die- casting.
These steels have been alloyed to withstand high working temperatures for such application as die casting dies etc, hot forging dies, plastic molds, hot extrusion dies etc., the main alloying elements are chromium, molybdenum and tungsten. The hot work steels are divided into three groups depending upon their alloying elements.
CHROMIUM HOT WORK TOOL STEELS:
They contain 5to7% chromium and smaller amounts of vanadium, tungsten, and molybdenum. In addition to good red hardening properties, they are extremely deep hardening and good dimensional stability during hardening.
TUNGSTEN HOT WORK STEELS:
They contain 9to18%tungsten and 2to12%chromium. These alloys are good resistance to high temperature softening. They are however more brittle at working hardness.
MOLYBDENUM HOT WORK STEELS:
These contain 5to 9.5% molybdenum, 4% chromium, 1.5to6.5% tungsten and smaller amounts of vanadium.
SPECIAL PURPOSE TOOL STEELS:
LOW ALLOY SPECIAL PURPOSE TOOL STEELS:
These steels are similar to the water hardening steels. Chromium and other elements are added for grater wear resistance and hardenability. They are used where high wear resistance and toughness are required.
CARBON TUNGSTEN TOOL STEELS:
These water hardening Steels have high wear resistance because of the high carbon and tungsten content. They are used where high wear resistance and low shock applications.
LOW CORBON MOULD STEELS:
They are carbonizing steels produced to tool steel quality. They have a low hardness in the annealed state and are generally carbureted for greater wear resistance after being machined their major application is moulds for injection or compression molding of plastics.
HIGH SPEED STEEL:-
High-speed steel (HSS) is highly alloyed tool steel. These steels owe their name to the fact that they were originally developed for high-speed metal cutting. HSS have a high degree of red hardness and high abrasion resistance along with a comparable degree of shock resistance. Their primary use is as a material for cutting tools, although they have other applications, such as extrusion dies and blanking punches and dies. Their major alloying elements are Tungsten, Molybdenum, Chromium and Vanadium and in special grades cobalt is added to give superiority in red hardness and abrasion resistance. Also HSS is more difficult to machine and grind HSS can be divided into two groups:
- Tungsten HSS (Group T) and
- Molybdenum HSS (Group M).
Even though the characteristics of both are almost same, cost is different. Group M is about 30% less than group T in cost. The general purpose grades of HSS are T1, M1, and M2 & M10. When the highest possible red hardness is required in a cutting tool material, Cobalt high speed steel such as M6 may be used. But this is higher in cost, difficult to machine, heat treat and grind.
INFLUENCE OF ALLOYING ELEMENTS ON THE PROPERTIES OF HSS-
The desired high hardness, hot strength, red hardness, toughness and wear resistance of the tools depends mainly on the presence of alloying elements present in the HSS.
Tungsten and Molybdenum-
They form carbides (M6) in HSS. These carbides increase the wear resistance and red hardness of the tools.
Cobalt
This will increase the hot hardness. Which can permit higher cutting speeds. Cobalt does not form carbides. Cobalt will increase the thermal conductivity and the coercive force while the red hardness remains unchanged. The durability of cobalt (content) HSS is 2 to 3 times more due to higher secondary hardness, wear resistance and thermal conductivity. Also the melting point of HSS is raised by the presence of Cobalt.
Chromium
4% chromium is used in HSS to promote the depth of hardenability.
Vanadium
Due to Vanadium wear resistance increases. But when the Vanadium content is at higher level the grindability will be poor.
Selection of HSS
It is clear that no group has all the requisites. The increase in wear resistance and decrease in toughness of high carbon high Vanadium super HSS is due to the increase in carbon and Vanadium content. This is due to the formation for Vanadium carbide.
The following list can be used for selecting HSS for different applications.
- Boring tool and broaches: - T1, T15, and M2.
- Drills of all kind: -T1
- Hand Taps – T1
Some applications of typical HSS are given below:-
T1----- Hot work drawing, shearing, piercing, press tools for M.S, twist drills and reamers etc.
M1----- Twist drills, thread cutting tools, reamers and cutters.
Cast Iron
Cast iron is sometimes used as the main body of jigs and fixture. By casting metal may be placed to better advantage and as results the weight of the fixture or jig may be reduced. The stability and compressive strength of grey cast iron, as well as its ease of casting make it suitable for this purpose. Cast iron is also used in construction of large forming and drawing dies and as a material for die set shoes.
Mild Steel
M.S. or low carbon steels are used as component parts of jigs and fixtures where wear resistance and maximum strength are not necessary. Standard structural shapes are used in the construction of frame works for large jigs and fixtures.
Non-metallic tooling materials include Masonite, densified wood, plastics, ceramics, diamonds etc.
SPECIFICATION OF STEELS:
Steels are specified on the basis of certain criteria like, the method of manufacturing, chemical composition, heat treatment, mechanical properties etc.
Every country has a different way of specifications. Majorities are based on chemical composition.
INDIAN STANDARD DESIGNATION SYSTEM (ISI 1961):
Indian standard code for designation of steel was adopted by ISI in 1961 and it was revises in to two parts in 1974. Part 1 covers the designation of steel based on Letter symbols and part 2 covers the designation of steel based on Numerals.
Code designation on the basis of Mechanical properties is based on tensile strength or yield strength. Symbol Fe is used to designate minimum tensile strength. Symbol FeE is used to designate minimum yield strength in N/mm2, symbol St is used when tensile strength is in Kg/ mm2 and StE is used for Yield strength in Kg/mm2. It is followed by special characteristics covering method of de-oxidation, steel quality, degree of purity, surface condition, weld ability and heat treatment.
Designation of steel on the basis of chemical composition consists of a numerical figure indicating 100 times the average percentage of carbon content. Letter C is used for plain carbon steels and Letter T are used for tool steel and these are followed by a figure indicating ten times the average percentage of manganese.
Symbols S, Se, Te, Pb, are P are used to indicate free cutting steels followed by a figure indicating 100 times the percent content of the respective element.
Alloy steel are designated in the symbolic form on the basis of their alloy content by first specifying the average content of carbon in 100ths of a percent followed by the chemical symbols of the significant element in the descending order of percentage content, the nominal or average percent of each alloying element is indicated by an index No just after its chemical symbol. If it is tool steel, letter T should be prefixed before carbon content.
If the average alloy content is up to 1%, the index No is expressed up to two decimal places underlined by a bar except for B, N. etc. where they will be indicated by an alloy symbol only.
For e.g. chromium content is between 0.5% and 0.8%, it is represented as Cr 65.
If alloy content is between 1 % and 10% the index no is rounded to the nearest whole no. If it is necessary, the alloy contents may be rounded of one place of decimal, the decimal digit being underlined by a bar e.g. If the chromium content varies between 1 .20% and 1.60%, it may be represented either as Cr 1 or Cr 1 4.
If the alloy content is more than 10%, the index no is rounded to the nearest whole no e.g. if chromium content is between 12% and 18%, it is represented as Cr 15.
If two or more significant alloying elements have the same alloy index, their chemical symbols may be grouped together followed by their alloy index, e.g. if a steel contains nickel between 0.9% and 1.3% and chromium between 1.2% and 1.6% both may be represented as NiCr 1.
Some typical e.g. are shown below.
1.Fe 410 K - Killed steel with minimum T.S of 410 N/mm2
2.St 42- Steel with minimum T.S of 42 Kg/mm2
3.Fe E 270 - Steel with minimum Y S of 270 N/mm2
4.C 20- Steel with average carbon of 0.2%.
5.25 C5 - Steel with average carbon of 0.25% and Mangesese of 0.5%
6.80 T 11- Tool steel with average carbon of 0.8% and 1.1% mangesese.
7.15 Ni 1 3 Cr 1 Mo 12 - Steel with carbon 1.5%, Nickel 1.3%, Cr 1%, Mo 0.12%.
8.35 Mn 1 S 18 - Steel with carbon 0.35%, Mn 1%, sulphur 0.18%.
9. T 75 W 18 Cr 4 V 1 - Tool steel with carbon 0.75%, Tungsten 18%, Chromium 4%, Vanadium 1%.
10.T 105 Cr 1 Mn 60 - Tool steel with carbon 1.05%, chromium 1% and manganese 0.6%.
COMMENTS