Carbon steels

Carbon steels find most extensive use for shaped castings. The composition includes carbon, manganese, silicon, phosphorus, and sulfur. It is carbon that exerts the main influence on the mechanical and casting properties of steels. Sulfur and phosphorus are harmful admixtures of steel. Sulfur reduces the fluidity of steel at high temperatures and makes it more susceptible to hot cracking. Phosphorus decreases the impact strength of steel, and does so more intensely as its content grows. The contents of phosphorus and sulfur in steel are specified within certain limits. The permissible total content of sulfur and phosphorus must be lower than 0.1%.

With an increase in the carbon content of steel, the ultimate strength, hardness, and yield strength grow, while the elongation, necking (reduction in area), and impact strength decrease. Manganese has a deoxidizing action and neutralizes the adverse effect of sulfur, forming in the reaction with sulfur the chemical composition MnS. The contents of manganese and sulfur in carbon steel must conform to the relation Mn > 1.71 S. The content of manganese usually varies from 0.3 to 0.8%. Silicon present in carbon steel acts as a deoxidizer and fixes gases. Carbon steels contain 0.2-0.5% Si.

Depending on the purpose and the requirements they have to fulfil, steel castings are divided into three groups: group I includes general-purpose castings; group II, high-purpose castings; and group III, special-purpose castings which have to comply with the most stringent requirements.

These groups differ by the contents of sulfur and phosphorus. For castings of group I, the allowable contents of these elements are 0.05-0.07% S, and 0.05-0.08% P; for group II castings, 0.045-0.06% S and 0.04-0.07% P; for group III castings. 0.045-0.05% S and 0.04-0.05% P. The content of other elements is the same in castings of all the three groups. Castings show the same mechanical properties, irrespective of the group they belong to.

Low-carbon steels contain 0.12-0.2 % C, 0.35-0.65% Mn, 0.15-0.25% Si, about 0.06% P, and near 0.05% S. Low-carbon steels have a lowered fluidity and enhanced tendency to form hot cracks in castings. The tensile strength of these steels after cooling in the annealing furnace or normalizing is 394-410 Mpa, the elongation being 24-23%. Low-carbon steel castings are used in electrical engineering and machine-building industry.

Medium-carbon steels contain 0.22-0.45 % C, 0.35-0.8 % Mn, 0.2-0.45% Si, 0.03-0.05% P, and 0.03-0.05% S. These steels have better casting properties than low-carbon steels, that is, they are more fluid and less susceptible to hot cracking. The tensile strength is 410-540 MPa (42-55 kgf mm-²), and the elongation 23-12%. These steels are used for casting small, medium-sized, and large parts employed in various branches of the machine industry.

High-carbon steels contain 0.45-0.6% C, 0.5-0.8% Mn, 0.04-0.05% P, and 0.04-0.05% S. These steels show a high fluidity and are not subject to hot cracking. But since the thermal conductivity of these steels is low, castings are more exposed to the danger of developing internal stresses. High-carbon steels are used for casting hot rolls, gears and drums of mobile hoist machines.

Task 1

Give English equivalents to the following words and word combinations.

широке використання, включати, шкідливі домішки, розтріскування, межа міцності, межа текучості, зменшення площини поперечного розрізу, ударна стійкість, містити, рідко текучість, піч для відпалу, видовження, різні галузі, чутливий до.

Task 2

Match the following words with their translation:

1. composition	a) подовження
2. influence	b) межа міцності на розтягнення
3. properties	c) склад
4. content	d) відновник, розкислювач
5. elongation	e) вміст
6. deoxidizer	f) вплив
7. requirements	g) вимоги
8. tensile strength	h) властивості

Task 3

Fill in the correct words from the list below. Use each word only once.

fluidity, hot, reaction, specified, casting, cracking, castings,

deoxidizing, extensive, strength, content, special-purpose

1. in the … with	7. mechanical and … properties
2. susceptible to hot …	8. … action
3. … castings	9. … cracking
4. the most …use	10. impact … of steel
5. high …	11. steel …
6. be … within certain limits	12. permissible total …

Task 4

Give Ukrainian equivalent to the following words and word combinations.

shaped castings, mechanical and casting properties, harmful admixtures, hot cracking, impact strength of steel, permissible total content, ultimate strength, yield strength, the most stringent requirements, irrespective of, lowered fluidity, annealing furnace, thermal conductivity, be exposed to, internal stresses.

Task 5

Fill in the gaps with the verbs given below used in the proper tense form.

to exert, to show, to use, to specify, to divide, to grow, to be, to vary

1. Carbon _____ the main influence on the mechanical and casting properties of steels.

2. The content of phosphorus in steel _____ within certain limits.

3. With an increase in the carbon content of steel, the ultimate strength, hardness, and yield strength _____, while the elongation, necking, and impact strength decrease.

4. The content of manganese usually _____ from 0.3 to 0.8%.

5. Depending on the purpose and the requirements they have to fulfil, steel castings _____ into three groups.

6. The content of other elements _____ the same in castings of all the three groups.

7. Castings _____ the same mechanical properties, irrespective of the group they belong to.

8. Medium-carbon steels _____ for casting small, medium-sized, and large parts employed in various branches of the machine industry.

Task 6

Make up sentences from the given words. Start with the underlined word.

1. are casting Medium-carbon for machine steels used the parts for industry different.

2. more The growing strength of phosphorus impact intensely the content of steel decreases.

3. by hot The least to is susceptibility high-carbon shown steels cracking.

4. reduced high The fluidity steel of the impact at is by sulfur of while decreases strength phosphorus temperatures steel.

Task 7

Answer the following questions.

1. What is the composition of carbon steels?

2. How many harmful admixtures of steel do you know? What are they?

3. What influences the impact strength of steel?

4. What types of carbon steels are there?

5. What is the highest carbon content in steel?

Task 8	Compare low-carbon, medium-carbon and high-carbon steels in such aspects as: 1) content; 2) fluidity; 3) susceptibility to hot cracking; 4) tensile strength; 5) elongation; 6) usage. Use such expressions as: The main difference lies in …, It is known that …, The best …(the highest …) is shown by …, etc. Have in mind the ways of linking ideas.
Task 9	Fill in the table, using the information from the text.

admixture	influence	content
Carbon
Manganese
Silicon
Phosphorus
Sulfur

Task 10

Write an abstract to the text “Carbon steels”.

Дата добавления: 2015-07-20