Materials science

In the composition of malleable cast iron It is obtained by annealing a cast iron with 2.2...3.2% carbon at a temperature of 1050...800 ⁰ C for 24...60 hours. In this case, cementite (Fe ₃ S) in white cast iron is broken down into pearlite and piece by piece (a finer copy) into graphite (Fig. 10.3-v, g), as a result, the plasticity and impact resistance of cast iron increases, and its viscosity is sufficient. will be The metal base of malleable cast iron can be ferritic-pearlite or ferritic, depending on the type of annealing. Ferritic cast iron has high plasticity. Malleable cast iron of KCH 37-12, KCH 35-10, KCH 50-4 brands is used more in mechanical engineering .
Table 10.3
Description of malleable cast iron

Brand	Mechanical properties			Structure	Usage
	s V , kg/mm ^2	d,%	NV, kgk/mm ^2
KCH-35-10	35	10	163	Ferritic	Gear wheels, chain links, brake pads, wheel hubs, gears, reducers...
KCH 37-12	37	12	163	Ferritic
KCH 540-4	50	4	241	Perlite
KCH 60-3	60	3	241	Ferritic and pearlitic

Necessary equipment, materials and tools to perform the work: micrographs of various cast irons, metallographic microscope, circle and ruler, atlas of microstructures of metals and alloys
Laboratory procedure:

1. 
   Cast iron slices are observed under a microscope and their structures are determined;
   
2. 
   According to the determined structure, the type of cast iron is determined;
   
3. 
   The structure observed under the microscope is drawn.
   

Task: Explain the structural changes that occur in cast iron with x % (the value of x is taken according to the option) carbon content when heated to the melting temperature. Give information about the structures observed in this cast iron and their mechanical properties.

Options	1	2	3	4	5	6	7	8	9	10	11
Amount of C, in %	2.3	2.4	2.5	2.6	2.7	2.8	2.9	3.0	3.15	3.25	3.3
Options	12	13	14	15	16	17	18	10	20	21	22
Amount of C, in %	3.4	3.5	3.6	3.7	3.8	3.9	4.0	4.1	4.3	4.5	4.8

The following shall be included in the laboratory report:

1. 
   The part of the iron-cementite state diagram related to cast iron is drawn, and using it, the change in the structure of cast iron with an increase in carbon content is analyzed;
   
2. 
   The types, structures, properties and areas of use of cast iron are covered;
   
3. 
   Using the drawn microstructure, the following are determined: the components of the structures, the type of cast iron, the conditions and methods of its production, mechanical properties ( using NV , d _V , d -reference books);
   
4. 
   The influence of the form of graphite in cast iron on its properties is explained;
   
5. 
   General conclusions .
   

Review questions:

1. 
   What alloys are called cast iron?
   
2. 
   What types of cast iron do you know?
   
3. 
   How are cast irons marked?
   
4. 
   How to obtain gray, malleable and highly refined cast iron ?
   
5. 
   How does the shape of graphite in cast iron affect its properties?
   

- laboratory work
HEAT -TREATED CARBON
STUDY OF MICROANALYSIS OF STEELS.

Purpose of the work: Independent microanalysis of heat-treated carbon steels, chemical-heat-treated carbon and alloy steels.

Study of the structures of carbon steels; mastering the ability to determine the type of alloy, the amount of carbon in it, the approximate brand of steel and the field of application depending on the microstructure.

Heat-treated carbon steel during the work process -

studying the microstructure of The structure of steel at equilibrium can be determined from the state diagram of iron-cementite alloys. Iron-carbon alloys containing up to 0.025% carbon are called technically pure iron . Its structure consists of light-colored ferrite grains bounded by dark circles (Fig. 8.1, a).
Iron-carbon alloys containing 0.025...2.14% carbon are called steels. According to the structure in the equilibrium state, steels are classified as follows:
of pre-eutectoid steels (0.025... 0.8%S) consist of gray-colored ferrite grains and dark-colored pearlite grains (Fig. 8.1, b, v, g).
of eutectoid steel (0.8%S) consists of pearlite with plate-like structure (Fig. 8.1, d).
After the eutectoid, the structure of steels (0.8...2.14% С) consists of pearlite and secondary cementite (Fig. 8.1, e). When the steel is cooled slowly, the cementite structure is arranged in the form of flow nets at the boundaries of the pearlite grains.
Thus, with an increase in the carbon content of carbon steels in the equilibrium state, their structures and phase composition also change - the amount of soft pearlite structure decreases and the amount of hard cementite increases, and accordingly the hardness (NV) and strength limit of the steel ( s _V ) increases, and plasticity ( d ) and impact viscosity ( a _n ) decreases (Fig. 8.2). As the carbon content exceeds 1%, the fineness of the steel decreases because continuous cementite species are formed on the surface of the pearlite grains.
Depending on the structure of carbon steels up to the eutectoid in equilibrium, it is possible to determine the amount of carbon in the steel, as well as to determine the grade of the steel. For this purpose, the surfaces occupied by ferrite and pearlite grains in the steel microstructure observed under a microscope are roughly determined. The carbon content of ferrite is assumed to be zero, and the carbon content of pearlite is assumed to be equal to 0.8%.
For example, suppose that 60% of the pre-eutectoid steel structure is pearlite grains, and the remaining 40% is ferrite grains. In this case, the amount of carbon in the steel is determined from the following ratio:
in 100% perlite - 0.8%S,
in 60% perlite – X%S,

This amount of carbon is found in grade 45 structural steel.

Figure 8.1. Changes in the microstructure of steel depending on the amount of carbon in it:

α-0.01% С; b-0.3% С; v-0.5%S; g-0.6%S; d-0.8%S; ye-1.2%S.
( To metals nitrogen of acid ethyl 4% in alcohol solution with processing given , x500)
8.2.-r asm . Steel _ _ contained carbon of the amount of steel _ mechanic properties effect : s _b – consistency _ limit , kg . k / mm ² ; a _n - percussion viscosity , a _N , kJ / m ² ; NV – Brinell according to _ hardness kg . k / mm ² ; d is relative elongation , %.
Microstructures of heat-treated carbon steels and steels