Applications of alloy steels

Structural steels

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.

Structural Steels Below 680 MPa tensile

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.

Steels Above 680 MPa Tensile

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.

Ultra-high Tensile Structural Steels

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

	Si	Mn	Ni	Cr	Mo	V	Temper	El
3% Cr-Mo-V	0,2	0,5	0,2	3,0	0,9	0,2	300	8
Rex 539 Si-Mn-Ni	1,6	1,6	1,8	0,1	0,4	0,2	350	7
H50	1,1	0,5	-	5,0	1,3	1,1	560	7
SAE 4340	0,3	0,6	1,8	0,8	0,25	-	220	9
Si-Cu steel	2,0	Cu=2	-	-	0,7	0,2	400	9

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.

Maraging 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.

Hair-line Cracks

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:

Dia bar (metre	0.025	0.25	0.5	1
Hours	1	100	400	1600

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.

High and Low Thermal Expansion 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

Creep-resisting Steels for Use at Steam Temperatures

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.