The solid-solution hardening of carbon has a major effect on the strength of martensite, but ductility can only be obtained at low carbon levels. Although alloying elements affect hardenability, they have a minor effect on hardness except to reduce it at high carbon levels by causing austenite to be retained.

Alternative ways of improving the strength of alloy steels are:

(1) Grain refinement, which increases strength and ductility. This can be developed by severely curtailing the time after the cessation of forging at some low temperature of austenite stability or by rapid heating, coupled with a short austenitising period. Fine grain is produced in 9% Ni steel by tempering fine lath martensite.

(2) Precipitation hardening by carbide, nitride or intermetallic compounds.
(a) By secondary hardening, e.g. 12% Cr steel with additions.
(b) Age hardening a low carbon Fe-Ni lath martensite supersaturated with substitutional elements, e.g. maraging.
(c) Age hardening of austenite, e.g. stainless steels. Phosphorus and titanium are common additions. Stacking faults are often associated with fine carbide precipitates, and strength can be raised by increasing the number of stacking faults (i.e. lower fault energy).
(d) Controlled transformation 18/8 austenite steels in which transformation to martensite is induced by refrigeration or by strain.

(3) Thermomechanical treatments which may be classified into three main groups:
(a) Deformation of austenite prior to the transformation.
Ausforming consists of steel deforming in a metastable austenitic condition between Ac1 and Ms (e.g. 500°C called LT) followed by transformation to martensite and light tempering (Fig. 1). This results in increased dislocation density in the martensite and a finer carbon precipitation on tempering. Strengths up to 1800 MPa can be obtained without impairing the ductility (~6 % deformation). Steels must possess a TTT-curve with a large bay of stable austenite, e.g. 826 M40. Typical application is for leaf springs.

Figure 1. Methods of thermomechanical treatment

Deformation of stable austenite just above Ac₃ before cooling (called HT). The properties are somewhat inferior to those produced by ausforming.
Deformation induced transformation originally used in Hadfield 13% Mn steel, but can be adapted to metastable austenitic stainless steels. The fully austenitic steel is severely warm-worked above the lowest temperature at which martensite is produced during the straining. The distinctive property is the high rate of straining hardening, which increases ductility.

(b) Deformation of austenite during the transformation
Isoforming is the deformation of a steel (e.g. 1% Cr) during the isothermal transformation to pearlite, which refines the structure and improves fracture toughness (Fig. 1). A somewhat similar thermomechanical process can be used in the bainitic region, producing bainite and martensite.
Zerolling consists in forming martensite by deformation at subzero temperatures to strengthen 18/8 austenitic steels. The amount of martensite is influenced by alloy composition and increased with deformation and lowering of the temperature.

(c) Deformation after the transformation of austenite
Marforming consists of deforming the maraging steel in the soft martensitic condition, generally cold. There is a pronounced increase in strength of the subsequent maraged product. With other steels, considerable increases in strength can be obtained by straining martensite (~3 %) either in the untempered or tempered condition. A strengthening effect also occurs on re-tempering, probably due to the resolution and reprecipitation of the carbides in a more finely dispersed form.
Strain tempering and dynamic strain ageing
Both processes involve about 5% deformation at the room temperature between two stages of tempering -- strain tempering -- while in dynamic strain ageing deformation is concurrent with tempering.

 

 

Steel	Cooling Rate	TS MPa	Elongation	RA	Izod ft lbf	Izod J
Ni-Cr	Oil	896	18	60	64	87
Ni-Cr	Furnance	880	18	60	19	25
Ni-Cr-Mo	Furnance	896	18	61	59	80

	C	Si	Mn
1. Silico-manganese	0,5	1,5	0,8
2. Silicon steel	0,07	4,3	0,09
3. Silichrome	0,4	3,5	8

All commercial steels contain 0,3-0,8% manganese, to reduce oxides and to counteract the harmful influence of iron sulfide. Any manganese in excess of these requirements partially dissolves in the iron and partly form Mn3C which occurs with the Fe3C. There is a tendency nowadays to increase the manganese content and reduce the carbon content in order to get a steel with an equal tensile strength but improved ductility

If the manganese is increased above 1,8% the steel tends to become airhardened, with resultant impairing of the ductility. Up to this quantity, manganese has a beneficial effect on the mechanical properties of oil hardened and tempered 0,4% carbon steel. The manganese content is also increased in certain alloy steels, with a reduction or elimination of expensive nickel, in order to reduce costs. Steels with 0,3-0,4% carbon, 1,3-1,6% manganese and 0,3% molybdenum have replaced 3% nickel steel for some purposes.

Non-shrinking tool steel contains up to 2% manganese, with 0,8-0,9% carbon. Steels with 5 to 12% manganese are martensitic after slow cooling and have little commercial importance.

Hadfield`s manganese steel contains 12 to 14% of manganese and 1,0% of carbon. It is characterized by a great resistance to wear and is therefore used for railway points, rock drills and stone crushers. Austenite is completely retained by quenching the steel from 1000°C, in which soft condition it is used, but abrasion raises the hardness of the surface layer from 200 to 600 VPN (with no magnetic change), while the underlying material remains rough. Annealing embrittles the steel by the formation of carbides at the grain boundaries. Nickel is added to electrodes for welding manganese steel and 2% Mo sometimes added, with a prior carbide dispersion treatment at 600°C, to minimize initial distortion and spreading.

Nickel and manganese are very similar in behavior and both lower the eutectoid temperature. This change point on heating is lowered progressively with increase of nickel (approximately 10°C for 1% of nickel), but the lowering of the change on cooling is greater and irregular. The temperature of this change (Ar1) is plotted for different nickel contents for 0,2% carbon steels in Fig. 1, and it will be seen that the curve takes a sudden plunge round about 8% nickel. A steel with 12% nickel begins to transform below 300°C on cooling, but on reheating the reverse change does not occur until about 650°C. Such steels are said to exhibit pronounced lag or hysteresis and are called irreversible steels. This characteristic is made use of in maraging steels and 9% Ni cryogenic steel.

The addition of nickel acts similarly to increasing the rate of cooling of a carbon steel. Thus with a constant rate of cooling the 5-8% nickel steels become troostitic; at 8-10% nickel, where the sudden drop appears, the structure is martensitic, while above 24% nickel the critical point is depressed below room temperature and austenite remains. The lines of demarcation are not so sharp as indicated by Fig. 1, but a gradual transition occurs from one constituent to another.

 

Figure 1. Effect of nickel on change points and mechanical properties of 0,2% carbon steels cooled at a constant rate

The mechanical properties change accordingly as shown in the lower part of Fig. 1. Steels with 0,5% nickel are similar to carbon steel, but are stronger, on account of the finer pearlite formed and the presence of nickel in solution in the ferrite. When 10% nickel is exceeded the steels have a high tensile strength, great hardness, but are brittle, as shown by the Izod and elongation curves. When the nickel is sufficient to produce austenite the steels become non-magnetic, ductile, tough and workable, with a drop in strength and elastic limit.

Carbon intensives the action of nickel and the change points shown in Fig. 1 will vary according to the carbon content. The influence of carbon and nickel on the structure are shown in the small inset (Guillet) diagram in Fig. 1, for one rate of cooling. Steels containing 2 to 5% nickel and about 0,1% carbon are used for case hardening; those containing 0,25 to 0,40% carbon are used for crankshafts, axles and connecting rods.

The superior properties of low nickel steels are best brought out by quenching and tempering (550-650°C). Since the Ac3 point is lowered, a lower hardening temperature than for carbon steels is permissible and also a wider range of hardening temperatures above Ac3 without excessive grain growth, which is hindered by the slow rate of diffusion of the nickel. Martensitic nickel steels are not utilized and the austenitic alloys cannot compete with similar manganese steels owing to the higher cost. Maraging steels have fulfilled a high tensile requirement in aero and space fields. High nickel alloys are used for special purposes, owing to the marked influence of nickel on the coefficient of expansion of the metal. With 36% nickel, 0,2% carbon, 0,5% manganese, the coefficient is practically zero between 0° and 100°C. This alloy ages with time, but this can be minimized by heating at 100°C for several days. The alloy is called Inver and it is used extensively in clocks, tapes and wire measures, differential expansion regulators, and in aluminium pistons with a split skirt in order to give an expansion approximating to that of cast iron.

A carbon-free alloy containing 78,5% nickel and 21,5% iron has a high permeability in small magnetic fields.

Chromium can dissolve in either alpha- or gama-iron, but, in the presence of carbon, the carbides formed are cementite (FeCr)3C in which chromium may rise to more than 15%; chromium carbides (CrFe)3C2 (CrFe)7C3 (CrFe)4C, in which chromium may be replaced by a few per cent, by a maximum of 55% and by 25% respectively. Stainless steels contain Cr4C. The pearlitic chromium steels with, say, 2% chromium are extremely sensitive to rate of cooling and temperature of heating before quenching; for example:

 

The reason is that the chromium carbides are not readily dissolved in the austenite, but the amount increases with increase of temperature. The effect of the dissolved chromium is to raise the critical points on heating (Ac) and also on cooling (Ar) when the rate is slow. Faster rates of cooling quickly depress the Ar points with consequent hardening of the steel. Chromium imparts a characteristic form of the upper portion of the isothermal transformation curve.

The percentage of carbon in the pearlite is lowered. Hence the proportion of free cementite (hardest constituent) is increased in high carbon steel and, when the steel is properly heat-treated, it occurs in the spheroidised form which is more suitable when the steel is used for ball bearings. The pearlite is rendered fine.

When the chromium exceeds 1,1% in low-carbon steels an inert passive film is formed on the surface which resists attack by oxidizing reagents. Still higher chromium contents are found in heat-resisting steel.

Chromium steels are easier to machine than nickel steels of similar tensile strength. The steels of higher chromium contents are susceptible to temper brittleness if slowly cooled from the tempering temperature through the range 550/450°C. These steels are also liable to form surface markings, generally referred to as "chrome lines".

The chrome steels are used wherever extreme hardness is required, such as in dies, ball bearings, plates for safes, rolls, files and tools. High chromium content is also found in certain permanent magnets.

 

 

 

The structural steels can be grouped conveniently on the basis of tensile strength. However, the dividing lines between the classes are ill defined owing to the wide variation in properties obtained from one steel by varying the heat-treatment, and the ruling section in which the properties are required.

The basis of design of machine components generally fall into either static or dynamic loading. For static loading the proof stress should be used with a safety factor of 2 or less. For dynamic loading the working stresses must be related to the fatigue limit, which is about 55% of tensile strength for mild steel, but only 50% at 770 MPa and 40% at 1540 MPa tensile strength. Notch sensitivity is also very important.

The main objective of steels in this class is to enable lighter structures to be built by the use of relatively high tensile steels, while retaining as far as possible the highly desirable properties. Properties include easy workability, adaptability and insensitivity to faulty manipulation possessed by mild steel. These steels may be used in the hot rolled condition.

Four characteristics are important:

(1) yield strength for design
(2) notch ductility to avoid brittle fracture
(3) weldability
(4) cost.

From the point of view of cost, therefore, quench and tempering processes on thick plates and sections are usually ruled out. However, the installation of special equipment, such as roller quench presses, may make the technique an alternative to expensive alloy additions.

Further, although killed steels are established in European technology, balanced steels remain in favor in UK because hot top practice is not widely used and a greater yield is obtained in balanced steels. Increased use of continuous casting may affect this in the future.

The most popular types are those with manganese raised to 1.3-1.7% with carbon 0,2-0,4%, but for welding the carbon is kept low. The American "Corten A" Steel has a composition of C, 0.12; Si, 0.5; Cu, 0.5; Cr, 0.8; P, 0.1 and Mn, 0.5%. Although the tensile strength is less than 494 MPa the yield is in the region of 371 MPa.

The combination of copper and phosphorus also increases the resistance to atmospheric corrosion which is important when thinner plates are used. The original steel "A" suffers a decrease in yield strength and notch ductility in thickness over 25 mm, to overcome which "Corten B" was developed-C 0.14; P 0.04; Mn 1.1; Cr 0.5; Cu 0.4; V 0.1; Bol Al 0.02.

The addition of 0.5 nickel and 0,25 % molybdenum to a manganese steel gives a good general purpose steel (785 M 19). Fortiweld steel containing Mo, 0.5; B, 0.003; C, 0.11%. has a TS of 618 MPa and is readily welded since it transforms in the bainitic region.

Pearlite Reduced Steel. Pearlite increases the tensile strength but not the yield stress, and since it raises the brittle-ductility transition temperature, there is a good case for reducing the carbon content. Low carbon steels (< 0.15%) strengthened with Mn + Nb and control rolled have good weldability and toughness and are called Pearlite Reduced Steel (PRS).

Grain refinement. Decreasing the ferrite grain size significantly increases both yield strength and notch ductility without increasing the carbon equivalent, which affects weldability. The relation of yield strength s_y to structure is given by the Petch equation:

s_y = s_i + K_y d^-1/2

Fine grained steels using Al N have to be produced as killed steel with low (production) yield (BS 50D). Niobium and vanadium have a lower affinity for oxygen than aluminium and can be used in semi-killed steels, an economic advantage. Since 1960 about 0.30-0.1% Nb forming Nb3C4 has been increasingly used as a grain refiner and precipitation hardening element and is the basis of several weldable steels in BS 4360, replacing 968 7762 3706, and includes 4 tensile ranged (40, 43, 50, 55 h bar) with several sub-grades which are distinguished by increased stringency of yield stress and notch ductility requirements. Both ladle and product analyses and carbon equivalents are included.

Precipitation Hardening. In Niobium steels-Nb4C3 dissolves above 1250°C, large grains and subsequent precipitation hardening is pronounced, but brittle transition temperature is high. Normalising at 900-950°C forms a precipitate, resulting in a grain refined steel, slight precipitation hardening, but low impact transition temperature.

By controlled hot rolling from 1250°C to a low finishing temperature (900°C) with a substantial amount of deformation in range 950-850°C, a fine ferrite grain size is obtained in sections up to 25 mm, with a minimum yield of 463 MPa due to dispersion hardening occurring in the ferrite during cooling.

Thick plates present difficulties in getting the required drop in rolling temperature. Holding at an intermediate temperature produced a partially recrystallised structure of large grains surrounded by small ones. The final structure is of very mixed size with poorer mechanical properties.

It is possible to quench similar steels from 1050°C to form a low carbon martensite or with lower carbon content, acicular ferrite followed by tempering to give higher properties. Nicuage steel, C, 0.06, Ni, 1, Cu, 1.1, Nb, 0.02, Mn, 0.5 rolled and aged at 500-570°C has a yield of 600 MPa and a tensile strength of 700 MPa, elongation 23% and 34-82 Joules charpy V notch impact at -20°C coupled with good weldability and corrosion resistance.

Relative to the steels just discussed, those in this group are designed solely for their mechanical properties, which depend on accurately controlled heat-treatment.

In 1941 BS 970 covering bars, billets and forging in this strength range rationalized steel specifications to conserve essential alloys. The basic principle was the specifying of mechanical properties related to size of bar when heat-treated rather than chemical composition. The cheapest steel will develop the requisite properties in the limited ruling section of the component used although other factors may modify the choice such as forging characteristics and die wear, machinability and ease of heat treatment.

Suitable compositions within the range have to be chosen in relation to mass. Thus nickel alone has a low efficiency in spite of its pre-war popularity. A conception new to many engineers is that the actual steel used for a given tensile strength depends on the size of the article at the time it is heat-treated.

In 1970, BS 970 was revised and the En designation was replaced by a six digit system. The first three digits refer to alloy type, the fifth and sixth digits represent 100 times the mean carbon content. At the fourth digit letters, A, M and H indicate if the steel is supplied to analysis, mechanical property or hardenability requirements, which are the new alternative methods.

Interest in (1544-2160 MPa) tensile steels for use in the aircraft industry is leading to the development of modified steels which can be used up to 400°C in supersonic aircraft and which possess adequate ductility and notch impact strength. Vacuum arc or Electro-slag melting of some of these steels reduces inclusions and impurities and gives a more suitable cast structure. As a result, the transverse properties of the resultant forging tend to be superior to those from conventionally air cast ingots. A few typical compositions are given in Table 1. These are based on Ni-Cr-Mo or use silicon with nickel or copper. Silicon reduces the expansion in g-martensite change and hence reduces risk of quench cracking; while copper is useful for producing secondary hardening at 450-550°C.

TABLE 1. TYPICAL 0,37% C, 1850 MPa STEEL

With such low alloy contents protection against rust by cadmium plating or aluminium spraying is necessary. Unfortunately, cleaning and plating processes can introduce hydrogen into the metal and embattlement at the high strength level can be serious. Such embattlement becomes evident under sustained loading, for example a 2,5 Ni Cr Mo steel hardened to 2220 MPa tensile broke within 100 hours at MPa and in 12 hours at 1544 MPa sustained load in air and much shorter times in corrosive media. With such high tensile steels it is highly desirable to (a) avoid notches, (b) to reduce internal stresses by tempering at as high a temperature as possible, (c) minimize the introduction of hydrogen (abrasive blasting preferred). Cadmium coats should be avoided for service above 250°C. For other high strength steels refer to precipitation hardening transformation and controlled transformation stainless steels.

These use the martensitic reaction which end high hysteresis occurs in Fe-Ni alloys. They are iron based alloys containing 18 Ni 8 Co 5 Mo with small amounts of Al and Ti and less than 0,03% C, which makes such a difference to fracture toughness and ease of welding. The strength is maintained with increase in section thickness and also up to 350°C. Alloy cost is balanced by lower production cost, virtually no risk of decarburisation, distortion or cracking. These steels are used for air frame and engine components, injection moulds and dies.

On cooling from the austenitic condition the alloy transforms to a fine lath type martensite, and precipitation hardening is induced by "maraging" at 480°C. Many types of precipitates have been reported (e.g. Ni3TiAl) but the main hardener is probably orthorhombic Ni, Mo, the solubility of which is probably reduced by Cobalt. The steels have high fracture toughness, KIc due to a combination of fine grain size of the martensite and the high dislocation density, leading to fine precipitation. The steels can be nitrided. The corrosion resistance is only slightly improved but the 12% Cr variety has been developed for corrosion resistance.

Many alloy-steel ingots and large forging are susceptible to the formation of small silvery cracks or flakes in their interior. These cracks often form at room temperature after an incubation period and the cause of them is not completely known but is related to the cracking of welded hightensile steels in that hydrogen has a large influence in promoting embattlement which increases with the tensile strength of the steel. Less trouble is experienced with acid open-hearth steel (usually containing 4 cc per 100 gm) than with basic electric steel containing about 6-8 cc per 100 gm of hydrogen in the ladle. Slow cooling and also isothermal transformation at about 600°C tends to reduce the incidence of hair-line cracks and this is materially assisted by the vacuum melting. To reduce the above hydrogen concentrations to about 1 cc per 1 00 gm requires beat-treatments of the following magnitude at 650°C:

The problem is therefore more acute with large ingots.

Alloy spring steels of the chromium-vanadium (Cr, 1; V, 0,2; C, 0,6) (Ni, 0,5; Cr, 0,5; Mo, 0,2; C, 0,6) and silico-manganese (Si, 2; Mn, 1) types are also oil hardened from 80°C and tempered at 480°C to give a Vicker`s hardness of 400, tensile strength of 1390 MPa, with appreciable ductility.

Springs are made from steel treated as follows:

a) Cold drawn patented steel wire.
b) Cold worked annealed steel.
c) Quenched and tempered (0,5/1,0%C) steel (VPN 340-430). After having been formed, the springs, (a, b, c) are only given a low temperature temper (170-300°C) to relieve forming stresses.
d) Annealed steel. After having been formed, the spring is then quenched and tempered. All heavy springs are formed hot and frequently hardened immediately after forming. Aero-engine valve-spring steel must be free from any kind of surface defect. Springs for watches and aircraft instruments, Bourden tubes, diaphragms etc. are often made from Ni-span containing 42 Ni, 5 Cr, 2,3 Ti, 0,02 C, 0,55 Al which has low mechanical hysteresis and can be heated to reduce the effect of changes in service temperatures.

Faults in springs are due to:

1) Decarburisation due to annealing, etc., affecting the fatigue properties.
2) Segregations forming lines of weakness in the material which may open up into splits in service.
3) Internal cup and cone fractures due to overdrawing.
4) Mechanical damage, such as rolling laps, deep grooves, scratches due to wire drawing, vice marks and scoring due to winding.
5) Incorrect tempering, especially in chromium-vanadium steels.

There are cases in engine construction where steel has to work in conjunction with light alloys, such as cylinder-head bolts, valve seating, or cylinder liners in aero engines. The comparatively high thermal expansivity of aluminium leads to looseness unless the steel has a similar coefficient of expansion. The austenitic steel of the following composition

C, 0,59; Ni, 12; Mn, 5,1; Cr, 3,4

has a thermal expansion of 0,000021 per degree C up to 400°C, which is only slightly lower than that of aluminium, and it combines good mechanical properties with resistance to abrasion.
Cold rolled austenitic stainless steel is another alternative. Where an abnormally low coefficient of expansion is required, Inver, containing 36% Ni, is used.

Ball-race steel. A typical composition is C, 1,0; Mn, 0,5; Cr, 1,36%. After quenching in oil from 810°C the steel is usually tempered at 100-200°C to
a) reduce hardening stresses,
b) reduce cracks in grinding.

Tempering at 100°C also increases the hardness slightly, e.g.:

Tempering temperature nil 100 200 250 VPN 800 876 750 736

The use of higher temperatures and pressures in modern power stations has necessitated the use of special steels for the pipe-lines and other parts. The essential characteristic of these steels is higher resistance to creep at temperatures varying from 400° to 565°C. A common steel used for this purpose is one containing approximately 0,55% molybdenum with a carbon content of approximately 0,15-0,2%.

Failures of pipes have occurred in America, which have been traced to the formation of a network of graphite in the heat-affected zone of the pipe adjacent to the weld. Heating the steel to approximately 750°C appears to accelerate the formation of this graphite, which is also largely affected by the process of making a steel, particularly as regards the use of aluminium as a deoxidizes, which is more commonly used in America than Great Britain. It has been found that a small addition of a carbide stabilizer, such as 1,0% chromium, is beneficial in minimizing this trouble.

The addition of 25% V raises the creep resistance still further. The 1% Cr Mo V steels currently in use are chiefly of two types, those for steamchest castings where due to welding considerations the carbon level is limited to 0,5%; and those used in HP/IP rotors in which the need for improved hardenability with large rotors has necessitated carbon levels of 0,25-0,30%.

Creep seems to be related to the uniformity of the V4C3 and its interparticle spacing which should be less that 1000A (10-5 cm). This distribution is affected by cementite which promotes regions denuded of V4C3 and to minimize this problem the cooling rate from the austenitising temperature must be such as to give an entirely upper bainitic structure.

Low-carbon 3 and 6% chromium steels containing 0,5% molybdenum have carbides which resist hydrogen attack and embattlement at elevated temperatures and pressures and are useful in synthetic ammonia plants and oil refineries. Columbium or titanium is sometimes added to minimize the air-hardening tendencies in the steel by forming carbides less soluble at heat-treating temperatures. Air-hardening still occurs after welding but air-cooling from 800°C is sufficient to soften the steel. Resistance to corrosion and oxidation increases with the chromium content; the elongation at rupture is also increased.

 

 

Standard BS 4659:1971 groups tool steels into six types:

1. high speed,
2. hot work,
3. cold work,
4. shock resisting,
5. special purpose and
6. water hardening.

The designations follow the AISI with the addition of B. Thus BTI and BMI designates high speed steel of tungsten and molybdenum grades respectively.

This term refers to steels which show little change in volume from the annealed state when hardened and tempered at low temperatures. Usually the following volume changes occur.

In non-shrinking steels the volume changes counterbalance each other, and such steels are required for master tools, gauges and dies which must not change size when hardened after machining in the annealed condition. The cheapest non-shrinkage steel contains 0,9% carbon and about 1,7% manganese. A better steel is,

C, 1.0; Mn, 0.95; W, 0.5; Cr, 0.75; V, 0.2

Both steels are oil quenched from 780° to 800°C and tempered 224-245°C. High carbon 5% and 12% chromium steels are also used for non-distortion.

While high-speed steels are very efficient with heavy cuts and high speeds they are incapable, at slow speeds and lighter cuts, of holding the keen edge necessary for obtaining a very smooth finish on certain articles. Special steels have been produced for this purpose, known as finishing steels, which are capable of retaining a keen cutting edge for much longer periods than carbon steel used under similar conditions. The usual type has the approximate composition:

C, 1.1 to 1.4; W, 4; Cr, 0.7 to 1.5; V, 0.3

After preheating to 650°C it is water hardened at 820-840°C and immediately tempered at 150-180°C. Anneal at 750°C. Tungsten steels containing 1 to 5,5% and 1 to 1,3% carbon are used for twist drills, taps, milling cutters, drawing dies and also tools for rifling gun barrels, boring cylinders and expanding tubes, which require long continuous cutting without interruption for regrinding. They are tempered at 200-230°C.

The standard oil hardening die steels contain 1 C, 1 Mn, 0,3-1,6 W, 0,5 Cr, hardened from 800°C and immediately tempered at 170-250°C. For cold obtrusion punches high-speed steels are satisfactory, e.g. 6W6 Mo.

High carbon-chromium (A)

This steel has good resistance to oxidation at elevated temperatures, high hardness and good wearing properties. lt is suitable for intricate sections, dies for blanking, coining, toller threading and drop forging hard materials. The structure is martensitic on cooling in air but the carbides can be precipitated and the steel softened by very slow cooling from 840°C.

This is the best type of steel for hot work except where resistance to scaling or oxidation is important. lt is used for hot-drawing, hot-forging, extrusion dies and dies for die casting aluminium, brass and zinc alloys. Die-casting die steels often fall through surface cracking caused by cyclic expansion and contraction, aggravated by the erosive action of the molten metal. Increased die life necessitates regular maintenance and careful preheating before use.

Sensitivity of die steels to distortion during heat-treatment is largely affected by directionality and particle size of the carbides in the microstructure. Expansion is greatest in the direction of carbide stringers. Fine random distribution of carbides are therefore desirable. For die casting and extrusion dies molybdenum containing 0,5 Ti + 0,08 Zr is useful in critical applications. Thermal conductivity, resistance to thermal shock and attack by molten metal is high and no heat treatment is required. Nimonic 80(a) and 90 have also been used satisfactorily for dies and inserts. Die block steels for drop forging have been standardised into four type. These are:
1) 0,6 carbon steel,
2) 1% nickel, 0,6 C,
3) 1,5 Ni, 0,7 Cr, 0,6 C,
4) 1,5 Ni, 0,7 Cr, 0,6 C, 0,25 Mo.

Hardness ranges from 425/455 for dies with shallow impressions to 298/355 for very large forgings.

Some examples of alloy steels used for shearing are given in Table 3.

The evolution of high-speed cutting tools commenced with the production of Mushet`s self-hardening tungsten-manganese steel in 1860. The possibilities of such steels for increased rates of machining were not fully appreciated until 1900, when Taylor and White developed the forerunner of modern high-speed 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°C. This allowed the tool to cut at speeds of 80-50 meters per minute with its nose at a dull red temperature and it was one of the astonishing exhibits at the Paris Exhibition of 1900.

Table 3. Shear blade Steel

The main constituents in high-speed steel are 14 or 18% tungsten, 3 to 5% chromium and 0,6% carbon. 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 in 6,5×2 M₂S.

Vanadium improves the cutting qualities of the tools and increases the tendency to air hardening. 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 decarburisation. The high vanadium steel is somewhat brittle, but is excellent for cutting very abrasive materials.

The study of the structures of such highly alloyed steels is complex, but it can be simplified by converting the amounts of the various elements to an equivalent percentage of tungsten as regards the effect on the closed g-loop:

Hence 18 W, 4 Cr, 1 V is equivalent to 25% tungsten and the section of the FE-W-C equilibrium diagram is shown in Fig. 1.

 

Figure 1. Section of the Fe-W-C equilibrium diagram at 25% tungsten

In the ingot the structure is similar to cast iron, but the cementite consists of mixed carbides (Fe, W Cr, V),C with the balance of the elements in solution in the ferrite. In this condition the steel is extremely brittle and the eutectic net-work has to be broken up into small globules, evenly distributed by careful annealing, followed by forging. "Strings" or laminations of carbides should be avoided, otherwise cracks are liable to form during hardening.

High-speed steel is softened by annealing at 850°C for about four hours, followed by slow cooling. The steel must be protected against oxidation. After forging, tools should be heated to 680°C for -If hour and air cooled before hardening in order to reduce risk of fracture. The annealed structure consists of carbide globules in a matrix of fine pearlite.

From Fig. 1 it will be seen that on heating, austenite forms at about 800°C, but contains only 0-2% carbon (eutectoid E). Quenching produces martensite, which tempers readily and has no advantage over carbon tools. More carbide dissolves on heating, as indicated by line EB, and quenching produces structures of increasing red-hardness, due to the effect of the larger amounts of alloying elements in solution, which render the steel sluggish to tempering. Even at 1300°C, when melting occurs, only 0,4% carbon (B) is dissolved and the remainder exists as complex carbides. It will be seen, therefore, that to attain maximum cutting efficiency sufficient carbon and alloying elements must be dissolved in the austenite and this necessitates temperatures little short of fusion, usually 1150-1350°C.

Grain growth and oxidation occur rapidly at such temperatures. Hence the tools are carefully preheated up to 850°C, then heated rapidly to the hardening temperature and quenched in oil or cooled in an air blast without soaking. To reduce the severe stresses set up by quenching, the following modifications can be used to reduce the temperature gradient from outside to center prior to the austenite-martensite transformation:
a) cool in salt bath at 600°C until temperature is uniform; then quench in oil, or
b) oil quench to 425°C, then air cool to room temperature.

When quenched from high temperatures high-speed steels contain an appreciable amount of retained austenite which is softer than martensite. This is decomposed by tempering, or by sub-zero cooling to -80°C. Multi-tempering is often more effective than a single temper of the same duration.
Tempering at 350-400°C slightly reduces the hardness but increases toughness. Tempering at 400-600°C increases the hardness, frequently to a value higher than that produced by quenching. This phenomenon is known as secondar hardening. The structure of the hardened high-speed steel consists of isolated spherical carbides embedded in an austenite-martensite matrix.
Dark etching grain boundaries are frequently evident. Tempering produces a general darkening of the matrix. "Stellite" type alloys consist of a cobalt base with about Cr, 30; W, 15 with other additions, including carbon. The structure consists of a cobalt matrix with complex tungsten-chromium carbides. lt has a high resistance to corrosion and to tempering and is used for tools, gauges, valve seatings and hard facing.

During the last fifty years engineers have demanded steels with higher and higher tensile strength, together with adequate ductility. This has been particularly so where lightness is desirable, as in the automobile and aircraft industries. An increase in carbon content met this demand in a limited way, but even in the heat-treated condition the maximum strength is about 700 MPa above which value a rapid fall in ductility and impact strength occurs and mass effects limit the permissible section.

Heattreated alloy steels provide high strength, high yield point, combined with appreciable ductility even in large sections. The use of plain carbon steels frequently necessitates water quenching accompanied by the danger of distortion and cracking, and even so only thin sections can be hardened throughout. For resisting corrosion and oxidation at elevated temperatures, alloy steels are essential.

The principal alloying elements added to steel in widely varying amounts either singly or in complex mixtures are nickel, chromium, manganese, molybdenum, vanadium, silicon and cobalt.

The effect of the alloying element in the steel may be one or more of the following:

(1) It may go into solid solution in the iron, enhancing the strength. The general effectiveness is shown in Fig. 1.
(2) Hard carbides associated with Fe,C may be formed.
(3) It may form intermediate compounds with iron, e.g. FeCr (sigma phase), Fe,W,.
(4) It may influence the critical range in one or more of the following ways:

(a) Alter the temperature. For example, 3% nickel lowers the Ac points some 30°C, while 12% chromium raises the Ac1, temperature to about 800°C and also forms a range of 150/200°C above this in which the pearlite changes to austenite. Fig. 2 shows the effect of alloys on the eutectoid temperature.

(b) Alter the carbon content of the eutectoid (Fig. 2). The carbon content of the pearlite in a 12% chromium steel is 0,33%, as compared with 0,87 in an ordinary steel. Nickel also reduces the amount of carbon in the pearlite and consequently increases the volume of this constituent at the expense of the weaker ferrite.

(c) Alter the “critical cooling velocity”, which is the minimum cooling speed which will produce bainite or martensite from austenite. Typical critical speeds obtained by quenching from 950°C are given in Table 1.

 

 

Figure 1. Hardening effects of alloying elements in solid solution in fully annealed ferrite (Austin)

Figure 2. Effects of alloying elements on the carbon content and temperature of the eutectoid point (Bain)

 

Table 1. Effect of alloying on the critical cooling speed of steel

 

 

The efficiency of the additions of the various alloy elements in reducing the effect of mass during quenching may be judged by the relative reduction of the critical velocity of the steel. Chromium and manganese respectively are far more effective than nickel.

(5) Combinations of elements can be chosen so that the volume change is reduced and also the risk of quench cracking. It may produce effeets characteristic of the alloying element.

(a) It may render the alloy sluggish to thermal changes, increasing the stability of the hardened condition and so producing tool steels which are capable of being used up to 550°C without softening and in certain cases may exhibit an increase in hardness.

(b) It may have a chemical effect on the impurities. Under suitable slag conditions vanadium, in quite small quantities, "cleans" the steel and renders it free from slag inclusions. Manganese and zirconium form sulphides.

(c) Certain elements such as chromium, Aluminium, silicon and copper tend to produce adherent oxide films on the surface of the steel which increase its resistance to corrosion and oxidation at elevated temperatures.

(d) Creep strength may be increased by the presence of a dispersion of fine carbides, e.g. molybdenum.

Classification of alloying metals according to their effect in the steel is difficult, because the influence varies so widely with each addition depending on the quantity used and other elements present. A useful grouping, however, is based upon the effect of the element on (a) the stability of the carbides and (b) the stability of the austenite.

(1) Elements which tend to form carbides. Chromium,tungsten,titanium, columbium, vanadium, molybdenum and manganese. The mixture of complex carbides is often referred to as cementite.

(2) Elements which tend to graphitise the carbide. Silicon, cobalt, aluminium and nickel. Only a small proportion of these elements can be added to the steel before graphite forms during processing, with attendant ruin of the properties of the steel, unless elements from group 1 are added to counteract the effect.

(3) Elements which tend to stabilise austenite. Manganese, nickel, cobalt and copper.

These elements alter the critical points of iron in a similar way to carbon by raising the A₄ point and lowering the A₃ point, thus increasing the range in which austenite is stable, and they also tend to retard the separation of carbides. They have a crystal lattice (f.c.c.) similar to that of g-iron in which they are more soluble than in a-iron.

(4) Elements which tend to stabilise ferrite. Chromium, tungsten, molybdenum, vanadium and silicon .

These elements are more soluble in a-iron than in g-iron. They diminish the amount of carbon soluble in the austenite and thus tend to increase the volume of free carbide in the steel for a given carbon content. On the binary equilibrium diagram of these elements with pure iron the A₄ point is lowered and A₃ raised (although it may be lowered initially), until the two points merge to form a “closed gamma loop”.

Thus, with above, a certain amount of each of these elements the austenite phase disappears and ferrite exists from the melting-point down to room temperature. No critical points exist and such steels (e.g. 18% chromium irons) are not amenable to normal heat treatment, except recrystallisation after cold work. This effect, however, can be counteracted by adding elements from group 3. For example, 2% of nickel is added to the 18% chromium stainless steel to enable it to be refined by normal heat-treatment; carbon has the same effect. Aluminium has the reverse effect in 12 % chromium steel.

Steels used for making tools, punches, and dies are perhaps the hardest, the strongest, and toughest steels used in industry. It is obvious that tools used for working steels and other metals must be stronger and harder than the steels or material they cut or form.

The metallurgical characteristics of various compositions of tool steels are extremely complex. There are hundreds of different makes and types of tool steels available and each may have a specific composition and end use.

In the United States, the Society of Automotive Engineers, in cooperation with the American Iron and Steel Institute, has established a classification system which relates to the use of the material and its composition or type of heat treatment. This classification system divides the tool and die steels into separate categories that are shown in Table 1.You may get more details on properties of these steels within the KEY to METALS·Steel Database, as well as on steels defined by other standards, such as European,Japanese,Chinese,Indian and others.

 

AISI-SAE Types	Classification of Tools Steels	COMPOSITION %
		C	Cr	V	W	Mo	Other
W1	Water hardening	0.6	-	-	-	-	-
W2		0.6	-	0.25	-	-	-
S1		0.5	1.5	-	2.5	-	-
S5	Shock resisting	0.55	-	-	-	0.4	0.80 Mn
							2.00 Si
S7		0.5	3.25	-	-	1.4	-
O1	Oil hardening	0.9	0.5	-	0.5	-	-
O6		1.45	-	-	-	0.25	1.00 Si
A2	Cold work	1	5	-	-	1	-
A4	Medium alloy	1	1	-	-	1	2.00 Mn
	air hardening
D2	Cold work	1.5	12	-	-	1	-
	High carbon
	High chromium
M1	Cold work	0.8	4	1	1.5	8	-
M2	Molybdenum	0.85	4	2	6	5	-
M10		0.9	4	2	-	8	-
H11	Hot work	0.35	5	0.4	-	1.5	-
H12	Chromium	0.35	5	0.4	1.5	1.5	-
H13		0.35	5	1	-	1.5	-
P20	Die casting mold	0.35	1.25	-	-	0.4	-

 

                    Table 1.

 

In general, tool steels are basically medium- to high-carbon steels with specific elements included in different amounts to provide special characteristics. The carbon in the tool steel is provided to help harden the steel to greater hardness for cutting and wear resistance. Other elements are added to provide greater toughness or strength.

In some cases elements are added to retain the size and shape of the tool during its heat treat hardening operation or to make the hardening operation safer and to provide red hardness so that the tool retains its hardness and strength when it becomes extremely hot. Various alloying elements in addition to carbon are chromium (Cr), cobalt (Co), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), and vanadium (V).

The addition of elements produces different effects on the resultant composition as follows:

·                     Chromium produces deeper hardness penetration in heat treatment and contributes wear resistance and toughness.

·                     Cobalt is used in high-speed steels and increases the red hardness so that they can be used at higher operating temperatures.

·                     Manganese in small amounts is used to aid in making steel sound and further additions help steel to harden deeper and more quickly in heat treatment. It also helps to lower the quenching temperature necessary to harden steels. Larger amounts of manganese in the 1.20-1.60% range allow steels to be oil quenched rather than water quenched.

·                     Molybdenum increases the hardness penetration in heat treatment and reduces quenching temperatures. It also helps increase red hardness and wear resistance.

·                     Nickel adds toughness and wear resistance to steel and is used in conjunction with hardening elements.

·                     Tungsten added to the steel increases its wear resistance and provides red hardness characteristics. Approximately 1.5% increases wear resistance and about 4% in combination with high carbon will greatly increase wear resistance. Tungsten in large quantities with chromium provides for red hardness.

·                     Vanadium in small quantities increases the toughening effect and reduces grain size. Vanadium in amounts over 1% provides extreme wear resistance especially to high-speed steels. Smaller amounts of vanadium in conjunction with chromium, and tungsten, aid in increasing red hardness properties.

The tool or die steels are designed for special purposes that are dependent upon composition. Certain tool steels are made for producing die blocks; some are made for producing molds, others are made for hot working, and still others for high-speed cutting applications.

The other way for classifying tool steels is according to the type of quench required to harden the steel. The most severe quench after heating is the water quench (water-hardening steels). A less severe quench is the oil quench obtained by cooling the tool steel in oil baths (oil-hardening steels). The least drastic quench is cooling in air (air-hardening steels).

Tool steels and dies can also be classified according to the work that is to be done by the tool. This is based on class numbers.

·                     Class I steels are used to make tools that work by a shearing or cutting action, such as cutoff dies, shearing dies, blanking dies, trimming dies, etc.

·                     Class II steels are used to make tools that produce the desired shape of the part by causing the material being worked, either hot or cold, to flow under tension. This includes drawing dies, forming dies, reducing dies, forging dies, etc. This class also includes plastic molds and die cast molding dies.

 

·                     The Class III steels are used to make tools that act upon the material being worked by partially or wholly reforming it without changing the actual dimensions. This includes bending dies, folding dies, twisting dies, etc.

·                     Class IV steels are used to make dies that work under heavy pressure and that produce a flow of metal or other material compressing it into the desired form. This includes crimping dies, embossing dies, heading dies, extrusion dies, staking dies, etc. It is important to understand and have sufficient information concerning the composition of the tool or die, the type of heat treatment that it has received, and the type of work that it performs.

Stainless steels are iron-based alloys containing at least 10.5% Cr. Few stainless steels contain more than 30% Cr or less than 50% Fe. They achieve their stainless characteristics through the formation of an invisible and adherent chromium-rich oxide surface film. This oxide forms itself in the presence of oxygen.

Other elements added to improve characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulfur, and selenium. Carbon is normally present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic grades.

The selection of stainless steels may be based on corrosion resistance, fabrication characteristics, availability, mechanical properties in specific temperature ranges and product cost. However, corrosion resistance and mechanical properties are usually the most important factors in selecting a grade for a given application.

Stainless steels are commonly divided into five groups: martensitic stainless steels, ferritic stainless steels, austenitic stainless steels, duplex (ferritic-austenitic) stainless steels, and precipitation-hardening stainless steels.

The development of precipitation-hardenable stainless steels was spearheaded by the successful production of Stainless W by U.S. Steel in 1945. The problem of obtaining raw materials has been a real one, particularly in regard to nickel during 1950s when civil wars raged in Africa and Asia, prime sources of nickel, and Cold War politics played a role because Eastern-bloc nations were also prime sources of the element. This led to the development of a series of alloys (AISI 200 type) in which manganese and nitrogen are partially substituted for nickel. These stainless steels are still produced today.

Over the years, stainless steels have become firmly established as materials for cooking utensils, fasteners, cutlery, flatware, decorative architectural hardware, and equipment for use in chemical plants, dairy and food-processing plants, health and sanitation applications, petroleum and petrochemical plants, textile plants, and the pharmaceutical and transportation industries. Some of these applications involve exposure to either elevated or cryogenic temperatures; austenitic stainless steels are well suited to either type of service.

Modifications in composition are sometimes made to facilitate production. For instance, basic compositions are altered to make it easier to produce stainless steel tubing and casting. Similar modifications are made for the manufacture of stainless steel welding electrodes; here combinations of electrode coating and wire composition are used to produce desired compositions deposited weld metal.

Martensitic stainless steels are essentially alloys of chromium and carbon that possess a distorted body-centered cubic (bcc) crystal structure (martensitic) in the hardened condition. They are ferromagnetic, hardenable by heat treatments, and are generally resistant to corrosion only to relatively mild environments. Chromium content is generally in the range of 10.5 to 18%, and carbon content may exceed 1.2%. The chromium and carbon contents are balanced to ensure a martensitic structure after hardening.

General corrosion is often much less serious than localized forms such as stress corrosion cracking, crevice corrosion in tight spaces or under deposits, pitting attack, and intergranular attack in sensitized material such as weld heat-affected zones (HAZ). Such localized corrosion can cause unexpected and sometimes catastrophic failure while most of the structure remains unaffected, and therefore must be considered carefully in the design and selection of the proper grade of stainless steel.

Corrosive attack can also be increased dramatically by seemingly minor impurities in the medium that may be difficult to anticipate but that can have major effects, even when present in only part-per-million concentrations; by heat transfer through the steel to or from the corrosive medium; by contact trimmed only on the ends.

Stainless steels are available in the form of plate, sheet, strip, foil, bar, wire, semi-finished products, pipes, tubes, and tubing.

Sheet is a flat-rolled product in coils or cut lengths at least 610 mm wide and less than 4.76 mm thick. Stainless steel sheet is produced in nearly all types except the free machining and certain martensitic grades. Sheet from the conventional grades is almost exclusively produced on continuous mills. Hand mill production is usually confined to alloys that cannot be produced economically on continuous mills, such as certain high-temperature alloys.

The steel is cast in ingots, and the ingots are rolled on a slabbing mill or a blooming mill into slabs or sheet bars. The slabs or sheet bars are then conditioned prior to being hot rolled on a finishing mill. Alternatively, the steel may be continuous cast directly into slabs that are ready for hot rolling on a finishing mill. The current trend worldwide is toward greater production from continuous cast slabs.

Sheet produced from slabs on continuous rolling mills is coiled directly off the mill. After they are descaled, these hot bands are cold rolled to the required thickness and coils off the cold mill are either annealed and descaled or bright annealed. Belt grinding to remove surface defects is frequently required at hot bands or at an intermediate stage of processing. Full coils or lengths cut from coils may then be lightly cold rolled on either dull or bright rolls to produce the required finish. Sheet may be shipped in coils, or cut sheets may be produced by shearing lengths from a coil and flattening them by roller leveling or stretcher leveling.

Strip is a flat-rolled product, in coils or cut lengths, less than 610 mm wide and 0.13 to 4.76 mm thick. Cold finished material 0.13 mm thick and less than 610 mm wide fits the definitions of both strip and foil and may be referred to by either term.

Cold-rolled stainless steel strip is manufactured from hot-rolled, annealed, and pickled strip (or from slit sheet) by rolling between polished rolls. Depending on the desired thickness, various numbers of cold rolling passes through the mill are required for effecting the necessary reduction and securing the desired surface characteristics and mechanical properties.

Hot-rolled stainless steel strip is a semi-finished product obtained by hot-rolling slabs or billets and is produced for conversion to finished strip by cold rolling.

Heat Treatment. Strip of all types of stainless steel is usually either annealed or annealed and skin passed, depending on requirements. When severe forming, bending, and drawing operations are involved, it is recommended that such requirements be indicated so that the producer will have all the information necessary to ensure that he supplies the proper type and condition. When stretcher strains are objectionable in ferritic stainless steels such as type 430, they can be minimized by specifying a No 2 finish. Cold-rolled strip in types 410, 414, 416, 420, 431, 440A, 440B, and 440C can be produced in the hardened and tempered condition.

Experience in the use of stainless steels indicates that many factors can affect their corrosion resistance. Some of the more prominent factors are:

·                     Chemical composition of the corrosive medium including impurities

·                     Physical state of the medium-liquid, gaseous, solid, or combinations thereof

·                     Temperature

·                     Temperature variations

·                     Aeration of the medium

·                     Oxygen content of the medium

·                     Bacteria content of the medium

·                     Ionization of the medium

·                     Repeated formation and collapse of bubbles in the medium

·                     Relative motion of the medium with respect to the steel

·                     Chemical composition of the metal

·                     Nature and distribution of microstruc-tural constituents etc.

Surface Finish. Other characteristics in the stainless steel selection checklist are vital for some specialized applications but of little concern for many applications. Among these characteristics, surface finish is important more often than any other except corrosion resistance. Stainless steels are sometimes selected because they are available in a variety of attractive finishes. Surface finish selection may be made on the basis of appearance, frictional characteristics, or sanitation.

Plate is a flat-rolled or forged product more than 250 mm (10 in.) in width and at least 4.76 mm (0.1875 in.) in thickness. Exceptions include highly alloyed ferritic stainless steels, some of the martensitic stainless steels, and a few of the free-machining grades. Plate is usually produced by hot rolling from slabs that have been directly cast or rolled from ingots and that usually have been conditioned to improve plat surface. Some plate may be produced by direct rolling from ingot.

For strip, edge condition is often more important than it usually is for sheet. Strip can be furnished with various edge specifications:

·                     Mill edge (as produced, condition unspecified)

·                     No.1 edge (edge rolled, rounded, or square)

·                     No.3 edge (as slit)

·                     No.5 edge (square edge produced by rolling or filing after slitting)

Foil is a flat-rolled product, in coil form, up to 0.13 mm thick and less than 610 mm wide. Foil is produced in slit widths with edge conditions corresponding to No.3 and No.5 edge conditions for strip. Foil is made from types 201, 202, 301, 302, 304, 304L, 305, 316, 316L, 321, 347, 430, and 442, as well as from certain proprietary alloys.

The finishes, tolerances, and mechanical properties of foil differ from those of strip because of limitations associated with the way in which foil is manufactured. Nomenclature for finishes, and for width and thickness tolerances, vary among producers.

Mechanical Properties. In general, mechanical properties of foil vary with thickness. Tensile strength is increased somewhat, and ductility is lowered, by a decrease in thickness.

Bar is a product supplied in straight lengths; it is either hot or cold finished and is available in various shapes, sizes, and surface finishes. This category includes small shapes whose dimensions do not exceed 75 mm and, second, hot-rolled flat stock at least 3.2 mm thick and up to 250 mm wide.

Hot-finished bar is commonly produced by hot rolling, forging, or pressing ingots to blooms or billets of intermediate size, which are subsequently hot rolled, forged, or extruded to final dimensions.