Classification and Identification of Iron-Carbon Alloys

 

1.8.1. Steels

 

All the elements, with the exception of carbon, nitrogen, hydrogen and, to some extent, boron form substitutional solid solution with iron. Dissolving in the iron, they change the temperature intervals in which a- and g-iron exits. With respect to their effects on the temperature intervals in which the allotropic forms of iron exit, alloying elements can be classified into two groups.

Elements of the first group include nickel and manganese. They lower point A₃ and raise point A₄. As a result the range of the a-phase is narrowed. As shown in Fig. 1.44a, alloys, having an alloying element (Ni or Mn) exceeding certain limit, undergo no phase transformations (a«g) when cooled down to room temperature. Their structure at room temperature consists of g-phase and they are called austenitic alloys.

Alloys which partly undergo an a«g transformation are called semiaustenitic alloys.

Elements of the second group (Cr, W, Mo, V, Si, Al, etc.) narrow and completely enclose in a loop the g-phase region (Fig. 1.44b). By this at certain alloying element content the alloys consist at all temperatures of the solid solution of the alloying element in the a-iron. These are called ferritic alloys, and alloys with only a partial a®g transformation are said to be semiferritic.

With respect to their relation to carbon, all alloying elements can be classified into three groups:

- graphitizing elements: silicon, nickel, copper and aluminum (these elements are in the solid solution);

- neutral elements: e.g. cobalt, which neither forms carbides nor causes graphitization;

- carbide-forming elements, which can be arranged in the following order of their increasing affinity for carbon and the stability of their carbide phases:

Fig. 1.44. Schematic binary equilibrium diagrams of iron-alloying element (Ni, Mn) (a)

and iron-alloying element (Cr, W, Mo, V) (b) systems

 

If the Mn, Cr, W and V content is small in steel, they dissolve in cementite, in which they substitute iron atoms. The composition of the cementite can be represented in this case by the formula (Fe, M)₃ C, where M denotes the alloying element.

Special carbides (Fe, Cr)₇ C₃, (Cr, Fe)₂₃C₆, Fe₂ Mo₂C, Fe₂ W₂C are formed in the steel when alloying element content is sufficient.

All steels can be classified according to:

- their structure;

- their purpose;

- their quality;

- their deoxidization and etc.

According to their structure under equilibrium conditions, steel can be classified as
(Fig. 1.45):

- hypoeutectoid (F+P);

- eutectoid (P);

- hypereutectoid (P + Carbides);

- ferritic (F);

- semiferritic (F + P);

- austenitic (A);

- semiaustenitic (A+F);

- ledeburitic (P+C).

According to their purpose steels can be classified as:

- machine (constructional) steels;

- boiler steels;

- die steels;

- high speed (rapid-tool, red-hard) steels;

- electrical steels;

- heat-resistance steels;

- stainless (rustless, corrosion-resistant) steels;

- welding steels, etc.

According to their deoxidation steels can be classified as:

- rimming steel s (deoxidized by Mn only);

- semikilled steels (deoxidized by Mn and Si);

- killed steel (deoxidized by Mn, Si and Al).

According to their quality, or method of production steels can be classified (in the former USSR) as:

- ordinary quality steels (S£0.05 %, P£0.06 %);

- quality steels (S£0.035 % P£0.035 %);

- high-quality steels (S£0.025 %; P£0.025 %);

- super-grade steels (S£ 0.015 %; P£0.015 %).

Fig. 1.45. Structural class diagrams of steel

 

Identification of Steels in Ukraine and Community of Independent States. Ordinary quality steels (common steels), according of their purpose and guaranteed properties, are classified into three groups: A, Б and B.

Group A: Cт0, Cт1кп, Cт1пс, Cт1сп, Cт2...Cт6сп,

where Cт is means steel;

 0...6 is conventional steel grade number;

кп is riming, пс is semikilled, cп is killed steel.

Group A consists of steels that are supplied with their mechanical properties guaranteed, but not their chemical composition. The higher the number, the more the carbon content, the higher the strength and the lower the ductility.

The group Б comprises steels which are supplied with their chemical composition guarantied: Б Cт0, Б Cт 1кп...Б Cт6.

The group В consists of steels of improved quality which are supplied with their mechanical properties and chemical composition guarantied. The available grades are B Cт2,
B Cт3, B Cт4, B Cт 5.

Quality engineering carbon steels are identified by the numbers 08, 10, 15, 20... 85, which indicate the average content of the carbon in hundredths of one per cent. But У7, У8...У13 denote tool carbon steels, where the numbers stand for the average carbon content in tenths of
1 %.

Alloy steels are to be identified by numbers, letters (conventional symbols of the chemical elements) and certain letters at the end, for example, 15X, 45XA, 12XH3A, 20X2H14A, etc. The two-digit number at the beginning of the designation indicates average carbon content in hundredths of one per cent; the chemical symbols indicate the alloying elements. The number following each alloying element symbol indicates the approximate content of the element in whole percent. The absence of the number following the symbol indicates that the content of the particular element is about one percent. The letter at the end of designation indicates the quality of steel:

- the absence of a letter -quality steel;

- letter A - high-quality steel;

- letter Ш - super-quality steel;

For example: 12XH3A is high quality steel, containing 0.12 %C; 1 %Cr, 3 %Ni,
06X18H9-Ш is super-quality steel, containing 0.06 %, 18 %Cr, 9 %Ni.

In Ukrainian and Russian standards the alloying elements are indicated by single capital Russian letters, representing the following elements:

A-nitrogen (in the middle of the designation), E-niobium, B-tungsten, Г-manganese,
D-copper, E-selenium, K-cobalt, H-nickel, M-molybdenum, П-phosphorus, P- boron, T-titanium, Ф-vanadium, X-chromium, Ц-zirconium, Ч- rare earths, Ю-aluminum.

 

1.8.2. Cast Irons

 

Alloys of iron and carbon in which the carbon content exceeds 2.14 % are called cast irons. Carbon in cast iron may be in the form of either cementite (according to the metastable diagram
Fe-Fe₃C), or graphite (according to the stable diagram Fe-C), or in both forms.

Cast iron in which all carbon is in the form of cementite (Fe₃C) is called white cast iron.

Cast iron in which part of the carbon (more than 0,8 %) is in the form of cementite is called mottled cast iron.

Cast iron in which not more than 0.8 %C is combined in Fe₃C is called graphited cast iron.

Graphited cast iron may be pearlitic, pearlitic-ferritic and ferritic.

The degree of graphitization depends on cast iron composition and the rate of crystallization and cooling (Fig. 1.46). Carbon and silicon promote the graphitization; Mn, Cr, Ti, V, Nb and other combine with carbon in carbides and prevent the graphitization.

With respect to the graphite form cast irons can be classified into the following groups:

- grey cast iron which has lamellar graphite inclusions (see Fig.1.32 g);

- high-strength cast iron with graphite as a spheroidal inclusions (see Fig. 1.32 h);

- malleable cast iron, which has flaky nodules of graphite (temper carbon). Grey cast iron normally has composition: 2.2…3.8 %C, 1.0…3.0%Si, 0.5…0.8 %Mn, up to 0.2 %P, up to
0.15 %S.

Fig. 1.46. Structural diagram of cast iron with a wall thickness of about 50 mm and various carbon

and silicon content (a) and at different wall thickness (b): I-white cast iron; II- mottled cast iron;

III-pearlitic grey cast iron; IV-ferritic-pearlitic grey cast iron; V-ferritic grey cast iron

 

Average mechanical properties of grey cast irons:

pearlitic cast iron                          ferritic cast iron

                               

              

                                  

The less amount of C and Si and the more Mn content the higher strength and hardness of cast iron.

Grey cast irons are graded as:

The grade means: CЧ grey-cast iron: (10…45) - s_u=100…450 MPa.

Grey cast iron is produced by melting and pouring the metals of specified composition into moulds. During solidification the lamellar graphite precipitates.

Inoculated cast iron (CЧ30...CЧ45) is obtained by special additions, called inoculants (ferro-silicon with 75 % Si, calcium-silicon, etc), in amount from 0.3 to 0.8 percent to the liquid cast iron just before pouring the moulds. Inoculation is resorted to obtain iron castings of various wall thicknesses with pearlite metallic matrix and the graphite lamellar of small sizes.

White and chilled cast iron, owing to the presence of cementite, are extremely hard
(HB 4000…5000 MPa), brittle and practically unmachinable. The high hardness of the casting surface provides good resistance against wear, especially abrasive wear. Thus, chilled cast iron is used to make rolls of sheet mills, wheels, balls of ball mills, etc. For such components, cast iron with low silicon content, lending itself well to chilling, is used. Its approximate composition is from 2.8 to 3.6 %C, 0.5…0.8 %Si, 0.4…0.6 %Mn.

The alloyed with Cr, Mn, Ti, etc white cast irons are also used. Their identification is similar to alloy steel: 300X, 250X2, 300X28H2, etc.

High-strength cast iron is obtained by making small ladle additions of certain alkali or alkali-earth metals (Mg, Ce, Y, Ca) to the liquid metal. In the most cases, the residual magnesium content amounts to 0.03…0.07 %. With respect to other constituents, high-strength cast iron does not differ from ordinary grey iron. Magnesium and other elements cause the graphite to precipitate in the process of solidification of the cast iron as spheroidal inclusions instead of lamellar. Spheroidal graphite, having minimum surface for a given volume, weakens the metallic matrix to a lesser extent than lamellar graphite. These cast irons have higher mechanical properties than ordinary grades of grey cast iron: s_u=(400…1000) MPa, d»(1.5…10) %, HB=(1800…2200)MPa.

High-strength cast irons can be identified by the letters BЧ followed by a number. The number indicates the average tensile strength in MPa·10^-1: BЧ 60, BЧ 70, BЧ 100.

Malleable cast iron is obtained by prolonged heating of white-iron castings at high temperatures (annealing).This leads to the formation of rounded graphite nodules. Compared with the lamellars, such nodules, called temper carbon, reduce the strength and ductility of the metallic matrix in the cast iron structure considerably less. The metallic matrix of malleable iron is commonly ferrite (ferritic malleable cast iron), or less frequently pearlite (pearlitic malleable iron). Ferritic malleable iron has the higher ductility and employed in the engineering industries.

The thickness of the cross sections of the casting should not exceed 50 mm to obtain white iron and to prevent the precipitation of lamellar graphite during crystallization.

The malleablizing procedure to obtain pearlitic and ferritic malleable irons is illustrated in Fig. 1.47.

Malleable iron can be identified by letters KЧ followed by two numbers. The first number indicates the tensile strength in MPa-10^-1 and the second is the percent elongation: KЧ 35-10,
KЧ 60-3, KЧ 35-10, KЧ 60-3, etc.

Fig. 1.47. Temperature vs time diagram of the malleablization procedure to obtain malleable cast iron

 

Non-ferrous Metals

 

1.9.1. Aluminum and Its Alloys

 

Typical features of aluminum are its low density (2.7 g/cm³), low melting point (660°C) and high electrical and thermal conductivity, high corrosion resistance due to the film Al₂O₃ on its surface.

On the basis of its purity, distinctions are made between aluminum of extra-high purity grade A999 (99.999 % Al), high purity: grades A995, A99, A97, A95 (99.95 %A1) and commercial purity: grades A85, A8, A7, A6, A5, AO (99.0 % Al).

The mechanical properties of high-purity annealed aluminum are: s_t=50 MPa, s_0.2 =15MPa, d»50%.

Commercial Al is used for the elements of structures and for components not subjected to loads, under conditions when high ductility, good weldability, corrosion resistance and high thermal and electrical conductivity are required. Aluminum is used for wires in electrical lines and equipment (its electrical conductivity is 65 % of the electrical conductivity of copper), for cables, electrical conductors, for various pipelines, milk tanks, doors, panels, etc.

Al-Cu, Al-Si, Al-Mg, Al-Cu-Mg, Al-Cu-Mg-Si, Al-Mg-Si and Al-Zn-Mg-Cu are most extensively used alloys.

All alloys of aluminum can be divided into 3 groups:

- wrought alloys, intended for the manufacture of sheets, plates, pipes, bars, rolled shapes, etc.;

- casting alloys, intended for foundry castings;

- alloys, obtained by powder metallurgy techniques: SAP-sintered aluminum powders (Al+Al₂O₃) and SAA- sintered aluminum alloys.

Typical wrought alloys are duralumins Д1 and Д16 (4…5 % Cu, 1…1.5 % Mg, ~ 0.5% Mn) which have s_t=400…500 MPa, s_0.2=250…380 MPa, d=10…15% and are widely used for manufacture of sheets for airplanes, rockets, etc.

Besides duralumins, high-strength (s_t=500…520 MPa), forging and heat-resistance (for components operating at 250…350°C) aluminum alloys are used.

Aluminum casting alloys are intended for foundry castings. The best known are the
Al-Si alloys, called silumins, eutectic alloys containing from 10 to 13 % Si. Due to eutectic these alloys have good foundry properties: low melting point (~600°C), high fluidity, small shrinkage, etc.

Cast alloys are identified: AЛ1, AЛ2...AЛ21, where, A-aluminum Л-cast, 1...21-number of grades.

Alloys based on Al-Al₂О₃ composition have the designation SAP (sintered aluminum powder) and consist of aluminum and disperse flakes of A1₂О₃ (6…22%). Compared with other
Al alloys the SAP materials have high corrosion resistance and heat resistance when heated for a long time up to 500°C, or when subjected to a short-term load at 1000°C.

Sintered aluminum alloys (SAA) contain a great number of alloying elements (in powder) and have special properties (low coefficient of linear expansion, etc.)

SAP and SAA are obtained by cold briquetting of powder mixtures, vacuum degassing and sintering under pressure.

 

1.9.2. Copper and Copper-base Alloys

 

Copper is a red metal with a rose-colored fracture. The melting point is 1083°C, the density is 8.94 g/cm³. It has the highest electrical and thermal conductivity among all metals, except silver and gold.

With respect to purity copper is available in the following grades: MOO (99.99% Cu), MO (99.95% Cu), Ml (99.9% Cu), M2 (99.7% Cu), M3 (99.5% Cu), M4 (99.0% Cu). Impurities found in copper have a strong effect on its properties.

Copper has good resistance to corrosion under ordinary atmospheric conditions in fresh and sea water and aggressive media, but can't withstand sulphurous gases and ammonia. Mechanical properties of copper are given in table 1.2.

 

Table 1.2 - Mechanical Properties of Copper.

Condition	st, MPa	s0.2 MPa	d,%
as-cast	160	35	25
hot-worked	240	95	45
cold-worked	450	250	3

 

Copper is used in electrical, electronics and electrovacuum engineering (mainly for conductors).

Distinction is made between two main grades of copper alloys: (1) brasses, alloys of copper with zinc and (2) bronzes, alloys of copper with other elements, among which there may be zinc, but only in a combination with other elements. Copper alloys have high mechanical and processing properties and good resistance to wear and corrosion.

The alloys are identified by the letters: Л for brass and Бp for bronze. This symbols, Л or Бp, are followed by the symbols of other components. The numbers following the symbols are separated by hyphens, in the same order as the components are given. In the grade symbols for brasses the first number is copper content and, the remainder points to zinc content. In those for bronzes, copper content is not given, but it is remainder.

Thus, for example, grade Л Ж Mц 59-1-1 is the brass, containing 59 % Cu, 1 % Fe, 1 % Mn, and the remainder is zinc. Grade Бp OC 6.5-0.15 is the bzonze containing 6.5 % Sn, 0.15 % Pb and the remainder is copper.

In the Ukrainian and Russian Standards the alloying elements are denoted by the letters which represent: O-tin, Ц-zinc, Mц-manganese, Ж-iron, Ф-phosphorus, Б-beryllium, X-chromium, H-nickel, C-lead, Al-aluminum, K-silicon.

Distinction is made between wrought and casting brasses and bronzes.

 

1.9.3. Magnesium and Its Alloys

 

Magnesium is light-grey metal; its characteristic features are low density (1,74 g/cm³) and melting point (651°C). Magnesium has adequate corrosion resistance in the atmosphere, but only poor resistance in fresh and sea water. Magnesium is combustible in air. Pure magnesium is used in pyrotechnics and the chemical industry.

The most widely used alloys are those with Al (up to 10 %), Zn (up to 6 %), Mn (up to
2.5 %) and with Zr (up to 1.5 %).

Magnesium casting alloys MЛ1...MЛ12 and wrought alloys (MA1...MA14) owing to their high strength to density ratio (s_t/g) have found wide application in aircraft construction, in rocket engineering, in electrical and radio engineering, in automobile, textile industry, etc.

 

1.9.4. Titanium and Titanium-base Alloys

 

Titanium is a silvery-white metal. Its melting point is 1665±5°C, density is 4.5 g/cm³. Three grades of commercial titanium are available (in Ukraine): BT1-00 (99.53 % Ti), BT 1-0 (99.48) and BT 1 (99.44 % Ti). Pure Ti has s_t=250 MPa, d=70 %.

A stable oxide film readily forms on the surface of Ti. As a result, it has high corrosion resistance in fresh and sea water and in certain acids. It is also stable against cavitation corrosion and corrosion under voltage.

Ti is alloyed with Al, Mo, V, Mn, Cr, Sn, Fe, Zr, Nb, Si. Titanium alloys have a high strength-to-density ratio (s_t/g), higher than that of steel. They are widely used in aviation and rocket engineering, in equipment engineering, in shipbuilding, etc. Ti-alloys have high ductility at low temperatures. This makes them suitable for cryogenics engineering.

 

1.9.5. Babbits

 

Babbits are antifriction alloys based on either tin or lead (table 1.3). These alloys are used for lining (babbiting) sleeve bearings. Distinguishing features of babbits are their low melting point (350…450°C), capacity for running-in and the absence of a tendency to seize with steel. An alloy with high antifriction properties has heterogeneous structure consisting of a soft and ductile matrix (Sn or Pb) with hard inclusions.

 

Table 1.3 - Chemical Compositions of Babbits

Grades	Composition, %
	Sn	Cu	Sb	Pb
B89	89	3.5	Rest	-
B83	83	6.5	Rest	-
B16	16	2.5	16	Rest

 

The soft matrix of babbits B83 and B89 is the solid solution of antimony and copper in tin. The hard particles in the structure are crystals SnSb, Cu₆Sn₅, Cu₃Sn.

METALLURGY