Characteristic of alloying elements

Extrait:

Very important elements for alloy steels are manganese, nickel, chromium, molybdenum, vanadium, tungsten, silicon, copper, cobalt and boron. All commercial steels contain 0,3-0,8% manganese, to reduce oxides and to counteract the harmful influence of iron sulphide. 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. Nickel and manganese are very similar in behaviour and both lower the eutectoid temperature. Nickel steels are noted for their strength, ductility and toughness, while chromium steels are characterized by their hardness and resistance to wear. 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. The chrome steels are used wherever extreme hardness is required, such as in dies, ball bearings, plates for safes, rolls, files and tools. The combination of nickel and chromium produces steels having all these properties, some intensified, without the disadvantages associated with the simple alloys. Molybdenum dissolves in both alpha- or gama-iron and in the presence of carbon forms complex carbides (FeMo)6C, Fe21Mo2C6, Mo2C. Molybdenum is also a constituent in some high-speed steels, magnet alloys, heat-resisting and corrosion-resisting steels.

Manganese

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

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

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:

Temp. of Initial Heating, °C

Critical Hardening Rate
(Mins to cool from 836° to 546°C)

836

3,5

1010

6,5

1200

13

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.

Nickel and chromium

Nickel steels are noted for their strength, ductility and toughness, while chromium steels are characterized by their hardness and resistance to wear. The combination of nickel and chromium produces steels having all these properties, some intensified, without the disadvantages associated with the simple alloys. The depth of hardening is increased, and with 4,5% nickel, 1,25% chromium and 0,35% carbon the steel can be hardened simply by cooling in air.

Low nickel-chromium steels with small carbon content are used for casehardening, while for most constructional purposes the carbon content is 0,25-0,35%, and the steels are heat-treated to give the desired properties. Considerable amounts of nickel and chromium are used in steel for resisting corrosion and oxidation at elevated temperatures.

Embattlement. The effects of tempering a nickel-chromium steel are shown in Fig. 2, from which it will be noticed that the Izod impact curve No. 1 reaches a dangerous minimum in the range 250-450°C in common with many other steels. This is known as 350°C embattlement. Phosphorus and nitrogen have a significant effect while other impurities (As, Sb, Sn) and manganese in larger quantity may also contribute to the embattlement.

Figure 2. Effect of tempering on the mechanical properties of nickel-chromium steel, C 0,26, Ni 3, Cr 1,2, 29 mm diam, bars hardened in oil from 830°C. Izod (2) for steel with 0,25% molybdenum added

Temper brittleness is usually used to describe the notch impact intergranular brittleness (Grain boundaries are revealed in temper brittle samples by etching in 1 gm cetyl trimethyl ammonium bromide; 20 gm picric acid; 100 cc distilled water, 100 cc ether. Shake mixture, allow to stand for 24 hrs; use portion of top layer and return to tube afterwards) induced in some steels by slow cooling after tempering above about 600°C and also from prolonged soaking of tough material between about 400° and 550°C.

Temper brittleness seems to be due to grain boundary enrichment with alloying elements-Mn, Cr, Mo-during austenitising which leads to enhanced segregation of embattling elements P, Sn, Sb, As-by chemical interaction on slow cooling from 600°C. The return to the tough condition, obtained by rehearing embattled steel to temperatures above 600°C and rapidly cooling, is due to the redistribution and retention in solution of the embattling segregation. Antimony (0-001 %), phosphorus (0-008 %), arsenic, tin, manganese increase, while molybdenum decreases the susceptibility of a steel to embattlement. 0-25 % molybdenum reduces the brittleness as shown by Izod curve No. 2. Table 1 illustrates the effect rate of cooling after tempering and the influence of an addition of 0-45 % molybdenum:

Table 1. Steel 0,3% C, 3,5 % Ni, 0,7%, Cr, tempered at 630°C

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

Molybdenum

Molybdenum dissolves in both alpha- and gama-iron and in the presence of carbon forms complex carbides (FeMo)6C, Fe21Mo2C6, Mo2C.

Molybdenum is similar to chromium in its effect on the shape of the TTT-curve but up to 0,5% appears to be more effective in retarding pearlite and increasing bainite formation. Additions of 0,5% molybdenum have been made to plain carbon steels to give increased strength at boiler temperatures of 400°C, but the element is mainly used in combination with other alloying elements.

Ni-Cr-Mo steels are widely used for ordnance, turbine rotors and other large articles, since molybdenum tends to minimize temper brittleness and reduces mass effect. Molybdenum is also a constituent in some high-speed steels, magnet alloys, heat-resisting and corrosion-resisting steels.

Vanadium

Vanadium acts as a scavenger for oxides, forms a carbide V,C, and has a beneficial effect on the mechanical properties of heat-treated steels, especially in the presence of other elements. It slows up tempering in the range of 500-600°C and can induce secondary hardening. Chromium-vanadium (0,15%) steels are used for locomotive forging, automobile axles, coil springs, torsion bars and creep resistance.

Tungsten

Tungsten dissolves in gama-iron and in alpha-iron. With carbon it forms WC and W2C, but in the presence of iron it forms Fe3W3C or Fe4W2C. A compound with iron-Fe3W2-provides an age-hardening system. Tungsten raises the critical points in steel and the carbides dissolve slowly over a range of temperature. When completely dissolved, the tungsten renders transformation sluggish, especially to tempering, and use is made of this in most hot-working tool ("high speed") and die steels. Tungsten refines the grain size and produces less tendency to decarburisation during working. Tungsten is also used in magnet, corrosion- and heat-resisting steels.

Silicon

Silicon dissolves in the ferrite, of which it is a fairly effective hardener, and raises the Ac change points and the Ar points when slowly cooled and also reduces the gama-alpha volume change.

Only three types of silicon steel are in common use-one in conjunction with manganese for springs; the second for electrical purposes, used in sheet form for the construction of transformer cores, and poles of dynamos and motors, that demand high magnetic permeability and electrical resistance; and the third is used for automobile valves.

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

It contributes oxidation resistance in heat-resisting steels and is a general purpose deoxidizes.

Other elements

Copper dissolves in the ferrite to a limited extent; not more than 3,5% is soluble in steels at normalizing temperatures, while at room temperature the ferrite is saturated at 0,35%. It lowers the critical points, but insufficiently to produce martensite by air cooling. The resistance to atmospheric corrosion is improved and copper steels can be temper hardened.

Cobalt has a high solubility in alpha- and gama-iron but a weak carbide-forming tendency. It decreases hardenability but sustains hardness during tempering. It is used in "Stellite" type alloys, gas turbine steel, magnets and as a bond in hard metal.

Boron. In recent years, especially in USA, 0,003-0,005% boron has been added to previously fully killed, fine-grain steel to increase the hardenability of the steel. The yield ratio and impact are definitely improved, provided advantage is taken of the increased hardenability obtained and the steel is fully hardened before tempering. In conjunction with molybdenum boron forms a useful group of high tensile bainitic steels. Boron is used in some hard facing alloys and for nuclear control rods.

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