Materials science

Review questions:

1. 
   What alloys are called steels?
   
2. 
   What types of steel do you know?
   
3. 
   What is the structure of pre-eutectoid equilibrium steel? What about eutectoid steel? What about post-eutectoid steel?
   
4. 
   How does carbon affect the mechanical properties of steel?
   
5. 
   How is the amount of carbon in steel determined ?
   

Mechanical properties of carbon steels and areas of use Table 8.1

Steel grade	Mechanical properties				Usage
	V kGk/mm ^2	eg kgk/mm ^2	d,%	NV, kGk/mm ^2
10	36-45	20	31	-		Axle, roller, stud, nut, bushing, groove, etc.
20	44-54	25	25	-
25	48-58	28	23	-		Light weight parts: shaft, washer, shaft, coupling, bolt, nut...
35	56-66	32	20	-
40	60-72	34	19	187		High-precision details: connecting rod, lever, lever, axle, shaft,...
50	68-80	33	14	207
60	73-85	42	12	229		Joints of rolling mills, stock, cable, spring, spring,...
U7	78-90	44	20-22	229
U8, U8A	60	-	19-23	187 (62)*		Kerner, punch, matrix, metal cutting scissors, ...
U9, U9A	95-98	-	15-16	192 (62)*		Carpentry tools, stone chisels, kerner, metal chisels, ...
U10, U10A	60	-	23	197 (62)*		Metal grating cutter, tap, plate, rasveriot, ...
U11, U11A	90-95	-	23-24	207 (62)*		Milling cutter, shaver, drill, tap, plate, ego, razviyortka, ...
U12, U12A	65	-	23-24	207 (62)*
U13, U13A	70-75	-	28-36			Finally, tools that are required to be hard: drill, sickle, stamp

62- steel hardness after quenching in water,
Rockwell along a ( N R S ).
CONTROL QUESTIONS

1. 
   What is called ferrite, pearlite, cementite? What kind of steels are formed during thermal processing?
   
2. 
   What is martensite and what is its structure?
   
3. 
   What is troostite and what is its structure?
   
4. 
   What is sorbitol and what is its structure?
   
5. 
   Martensite is formed during thermal treatment of what kind of steels?
   
6. 
   Martensite is formed during thermal treatment together with what structures?
   

6 - laboratory work
COLLECTION OF STEEL SAMPLES BY TORES
THROUGH THE DEPTH OF MOUNTING
DETERMINE .


The purpose of the work: to learn to find the real critical diameter of steel 45, 40X, 40XH when cooling in water and oil.


Theoretical part
Hardening depth of steel is the ability to harden to a certain depth. Tempering depth refers to the distance from the surface to the semi-martensitic zone (50% martensite, 50% troostite), because it is in this zone that the hardness of tempered steels decreases sharply. The hardness of the semi-martensite zone in steels depends on the amount of carbon and alloying elements in its composition ( Table 12.1)
Table 12.1
Hardness of semi-martensitic zone in steels
dependence on carbon content

t/r	of carbon amount, %	The hardness of the semi-martensitic zone is H 50
		For carbon steels	For alloy steels
1	0.08-017	-	25
2	0.18-0.22	25	30
3	0.23-0.27	30	35
4	0.28-0.32	35	40
5	0.33-0.42	40	45
6	0.43-0.52	45	50
7	0.53-0.62	50	55

It is possible to determine the depth of penetration of the detail and the hardness of the cross section.

Due to the fact that it is very difficult to determine the depth of penetration of real details, the depth of penetration is determined using the " end-to-end " method.
In this method, a specially prepared sample is heated to the required temperature and then cooled by spraying water from one end. The cooling rate of the sample is maximum at the tip of the sample and decreases further away from it. (Figure 12.1)
The sample is heated in a furnace in a steel container (the heating temperature is shown in Table 12.2), coal or graphite is placed inside the container and the tip of the sample touches it.

Figure 12.1. Sample cooling scheme

After standing in the oven for a certain time, the sample was cooled in a special device. The time of placing the sample in the device should not exceed 5 seconds. The edge of my sample is then cooled by spraying water (the cooling conditions are always the same). In such conditions, the cooling rate on the surface of the end of the cooled sample is equal to 1000 ⁰ C/c.

After the specimen has completely cooled, 0,0787 ina flat surface of width li is ground along the cylindrical surface, and then the hardness of this surface is measured along the length of the specimen in Pockwell equipment (measurement procedure is shown in Table 12.3).
Table 12.2
Annealing temperature for tempering of carbon and alloy steels

Steel classes	Carbon content, %	Heat to find temperature,^0C
Carbon, manganese, chromium, molybdenum, chromium-nickel, micelle-molybdenum	up to 0.25 O.36-0.35 O.37-0.55	900 870 840
Silicon, silicon-manganese, chromium-nickel-manganese, chromium-vanadium, chromium-molydene-vanadium	O.15-0.24 0.25-0.44 0.45-0.65	900 880 870

After completing Table 12.3, the curve of hardness of the sample as a function of the distance starting from the cooled end is seen. On this curve, the distance to the semi-martensitic zone is found (see Figure 12.2 and Table 12.1).

Table 12.3
Variation of steel hardness with distance from the cooled tip

Method of measurement	1	2	3	4	5	6	7	8	9	10	11	12
Distance starting from the cooled end, mm	1.5	3	4.5	6	9	12	15	21	27	33	42	51
Hardness value, HRC

The distance found is proportional to the minimum cooling rate such that a semi-matrenitic structure forms in the center of the actual detail. In a given cooling medium of the parts, the real critical diameter of the center is found using a special nomogram (Fig. 12.3).

Figure 12.2. The method of determining the distance to the semi-martensitic zone

Figure 12.3. Nomogram for determining the critical diameter of the penetration depth

Equipment, tools, samples and materials needed to complete the work: different brands of steel alloys, special containers, laboratory furnaces, grinding wheel, digital seals, special equipment for one end of the sample, Rockwell equipment, manual and questionnaires.

Order of work:

1. 
   A steel sample of a given brand is placed in containers;
   
2. 
   The container in which the sample is placed is heated in the oven (heating temperature is given in table 12.2) and kept for 40 minutes;
   
3. 
   Take the sample out of the container and cool it in a special device for 10 minutes;
   
4. 
   The side of the sample is smoothed by sharpening;
   
5. 
   The burnished surface is marked as shown in Table 12.3;
   
6. 
   The hardness is measured and a graph of the change of hardness is drawn starting from the end of the specimen;
   
7. 
   Depending on the amount of carbon in steels, the hardness of the semi-mertinsite zone is determined;
   
8. 
   The distance to the semi-martensitic zone is determined;
   
9. 
   From the nomogram, the real critical distribution in water and oil is determined.
   

The following shall be included in the laboratory report:
The topic and purpose of the laboratory work, the brand and chemical treatment of the tested steels, the procedure for heating the sample, the cooling scheme of the sample, the hardness value of the semi-martensitic zone, the hardness table measured along the length of the sample, the graph showing the hardness value of the semi-martensitic zone , the distance from the cooled end of the sample to the semi-martensitic zone, the critical yield diameter of the investigated steels, summary and list of references.