English Reader for Technical Students. Учебное пособие

powder metallurgy (P/M) parts. The furnace had a belt width of 18 in. (46 сm) and was configured as follows:

•Five heating zones: delube zone, 72 in. (1,8 m) long; oxide reduction zone, 72 in. (1,8 m) long; three sintering zones, each 60 in. (1,5 m) long.

•Three cooling zones: fast cooling zone, 48 in. (1,2 m) long; two slow cooling zones, each 120 in. (3 m) long.

For a long time engineers have used manual calculations to design and analyze furnaces. The trend today is to use spreadsheet computer software. Time is saved but the underlying calculations are the same.

Both manual and spreadsheet calculations start with certain assumptions or inputs.

FurnXpert simplifies the job of sizing, designing, and simulating furnaces used for heat treating metal parts. Running the software involves three main steps.

1. Specify a furnace, including physical dimensions, refractory type, thermocouple locations, and type of heating (electric or gas).

2. Select parts to be processed in the furnace. Specify part shape, size, material, and configuration.

3. Specify furnace settings. These might include zone temperatures or the temperature profile along the length of the furnace, atmospheric gas flow, production speed, and furnace pressure.

Heat transfer calculations are соmplex and tedious. Performing them in detail either manually or even with the help of a spreadsheet program is impractical. However, these details can make a significant difference in overall furnace design and cannot be ignored.

Accuracy in manual calculations depends on rules of thumb and empirical values. Manual calculations can provide accurate results if the empirical values used are on target. This is only possible if these values are calibrated properly by comparing the results of manual calculations with experimental results. Even so, the empirical values can only be used with the furnace for which they were developed.

Advantages: The primary benefits of using a software tool like FurnXpert is that it can perform a furnace simulation 10 times faster and with much greater accuracy than manual methods. In addition, the software also presents an overall picture of the process. The factors that contribute to achieving accurate results are:

•Calculations are based on furnace temperature and not on a predicted part temperature.

•Property changes (thermal conductivity, density, and specific heat) with temperature are accounted for at each calculation step.

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•All heat transfer modes are considered and solved at each calculation step.

•Use of the finite element method analyzes every detail of the part. Finally, the process of design and analysis of a furnace or furnace op-

eration involves running a series of cases. Each case would involve a different combination of furnace design, part parameters, and furnace settings. With a program like FurnXpert, these different cases or "what if" analyses can be run in a fraction of the time it takes to perform them manually. This not only makes the design process more efficient, but also gives more accurate results and ultimately a better product or process.

Exercise 1. Find English equivalents to the following Russian words

Exercise 2. Match the definitions with the following words:

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Exercise 3. Group the following words into sentences and translate them:

1.process, involves, iterations, calculations, this, usually, lengthy, several, of, engineering.

2.applies, the furnace, same, to, users, argument.

3.engineers, manual, to, and, furnaces, for, have, a, design, long, calculations, analyze, time, used.

4.details, a, overall, and, be, these, can, difference, design, ignored, make, significant, in, furnace, not.

Exercise 4. Are these statements true or false?

1.It’s well-known that heat does not flow from higher temperatures to lower temperatures.

2.Thermophysical properties change with temperature.

3.Process engineers and heat treaters are not always faced with the challenge of operating their furnaces at the highest efficiency without sacrificing product quality.

4.The trend today is not to use spreadsheet computer software.

5.Manual calculations can not provide accurate results if the impirical values used are on target.

Exercise 5. Answer the questions:

1.What is the task of designing industrial furnaces?

2.Can you name the key aspects of furnace sizing and design?

3.Why are more and more engineers know using software tools for accurate analysis of equipment and processes?

4.Can you illustrate the benefits of design software?

5.What three main steps does running the software involve?

6.What are the advantages of using a software tool in the process of design of a furnace?

4.2. Troubleshooting generated atmospheres

The question is often asked, "What is an atmosphere?" The simple answer A gas that can be introduced into a thermal processing furnace with the objective of providing:

−a surface protection environment for the workload;

−a controlled oxidation environment for the workload;

−the introduction of elements for surface modification.

Protective furnace atmospheres for heat treating fall into five main categories: endothermic, blended gases, exothermic, elemental gases, vacuum, and air. Atmospheres can be generated or synthetic, or simply gases that are premixed/prestored.

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Because of the growing demand for repeatable metallurgical results, great emphasis is being placed on obtaining consistent gas analyses in various metallurgical processes. To meet this objective, it is necessary to understand the basic reactions among the industrial gases used for metallurgical processing.

Gases used in heat treating.

Depending upon the application, either individual gases or combinations of gases are chosen. The principal gases used in the heat treatment of steel are oxygen, nitrogen, carbon dioxide, hydrogen, water vapor, various hydrocarbons, ammonia, and argon. More information about each gas follows.

Oxygen: Oxygen will react readily with iron in steel to produce iron oxide, or scale. In addition, it will react with the carbon in the surface of the steel and cause surface decarburization. Note, however, that some processes take advantage of the presence of oxygen to create a controlled surface oxidation. This is done to provide a corrosion-resistant barrier on the surface of the steel.

Nitrogen: Nitrogen is usually present in an atmosphere as molecular nitrogen, which is passive to ferrite and used most successfully in annealing of low – carbon steels. Clean, dry nitrogen is required for successful annealing. Nitrogen is often considered a neutral atmosphere, but these a misconception that neglects to consider how the gas behaves when heated. Nitrogen will prevent surface oxidation, but will not stop decarburization. To prevent decarburization, the carbon potential of the furnace atmosphere needs to be in equilibrium with the surface carbon potential of the steel.

Carbon dioxide: At austenitizing temperatures, carbon dioxide will react with the carbon in the steel surface to produce carbon monoxide CO2 is an oxidizing gas.

Hydrogen: Hydrogen is considered a reducing gas that will reduce iron oxide to iron, or copper oxide to copper.

Water vapor If an endothermic gas generator is being operated, then the air that is being used to cause the endothermic reaction contains water vapor. Water vapor will oxidize the iron at the surface of the steel and also will combine with carbon in the surface to form carbon monoxide and hydrogen gases.

Hydrocarbons: Hydrocarbon gases are rich in carbon and can be easily cracked. Those of particular importance to heat treaters include methane (СH4 propane (C3H8), ethane (С2Н6), and acetylene (C2H2).

These gases will produce carbon-rich atmospheres inside the furnace chamber.

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Ammonia: Often used for one or both of its elemental gases, ammonia (NH3) can be a source of nitrogen for nitriding, or a source of hydrogen for a reducing process. Ammonia can be generated, or supplied as a bottled or bulk storage gas.

Argon: Argon is a true inert gas and will not react with a metal surface. It's used most often in aerospace applications, where it provides a completely nonreactive atmosphere. Argon's major drawback is cost.

Sooting: This problem is a direct result of too much carbon in the furnace atmosphere, which actually can be seen precipitating out of the atmosphere. This condition usually occurs at carbon potentials of about 1,6 % С or greater. Sooting will cause furnace refractory brick to become overloaded with diffused carbon, which will lead to difficulties in atmosphere control. In addition, the surface of the steel being carburized will have a serious propensity for the formation of retained austenite.

Obvious remedies are to cut back on the enrichment gas flow or dilute the furnace atmosphere with air. Air dilution, however, increases the risk of forming grain boundary oxides and surface oxides.

If the furnace has been operated at high carbon potentials for long periods of time, it will be necessary to bum out the carbon from the refractories. Some modern furnaces feature built-in burnout systems. All the operator has to do is enter the program mode for burnout and the operation will be completed automatically.

With other furnaces, the standard procedure is to reduce the temperature to ~870 °C (1600 °F) and remove any atmosphere that might already be in the furnace chamber. The furnace's entry and exit doors are then opened and air is simply blown into the chamber. An external blower or a compressed air line can be used. Any carbon present in the refractory brick will subsequently ignite and bum. The temperature in the furnace will increase until all the of carbon is burned out. The process can be tracked using the furnace's temperature control instrument. Burnout generally takes 2 to 3 hours.

Prepare standard procedures.

The need to troubleshoot furnace atmosphere problems can be dramatically reduced if standard operating procedures are written for both incoming material inspection and heat treating equipment operation.

Incoming material can cause considerable atmosphere problems if it has not been properly prepared and cleaned. Residual surface contaminants can cause, for example, corrosion or pitting, or an appearance problem. Oils, greases, lapping compounds, marker ink, paint, and cutting fluids should be completely removed by washing or degreasing.

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Standard operating procedures for generators and furnaces also should include burnout procedures. Both the process and equipment operating procedures should be well understood by furnace operators/technicians.

Exercise 1. Give Russian equivalents to the following words and word combinations:

A thermal processing furnace, a surface protection environment, surface modification, the growing demand, the basic reactions, the industrial gases, heat treating, oxygen, nitrogen, carbon dioxide, hydrogen, ammonia, a corrosion resistant barrier, methane, propane, ethane, acetylene, air dilution, oils, greases, lapping compounds.

Exercise 2. Find English equivalents to the following Russian words and word combinations:

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Exercise 3. Fill in the blanks with prepositions (of, with, in, of, to, upon, of, in, with, of, in):

1.Depending … the application, either individual gases or combinations … gases are chosen.

2.… addition, it will react … the carbon … the surface … the steel and cause surface decarburization.

3.… prevent decarburization the carbon potential … the furnace atmosphere needs to be … equilibrium … the surface carbon potential … the steel.

Exercise 4. Group the following words into sentences and translate them:

1.To understand, the industrial gases, for, used, it, among, is metallurgical, necessary, the basic reactions, processing for.

2.To provide on, this, the surface, is, a barrier, of, done, the steel, corro- sion-resistant.

3.For, clean, is, dry, annealing, for, nitrogen. required, successful.

4.And, argon, gas, with, will, a metal surface, not, is, true, react, a gas, inert.

Exercise 5. Answer the following questions:

1.What is an atmosphere of a furnace?

2.What main categories of protective furnace atmospheres can you name?

3.What are the principal gases used for?

4.What is sooting?

5.What is the standard procedure with other furnaces?

6.How can the need of troubleshoot furnace atmosphere problems be reduced?

4.3. Secondary metallurgy for steelmaking

Decarburizing and degassing of high-grade and special steels today is widely used in ladle metallurgy plants. The main purposes of ladle metallurgy, i.e. the combination of a ladle furnace and vacuum degassing, are to increase production capacity of the melting furnace to make the entire production process more economical and improve the quality of the liquid steel.

In modern ladle metallurgy all refining and fine-tuning operations, apart from desulfurization, degassing and, partly decarburization, are carried out in the ladle. After tapping from the melting furnace a new reactive basic slag is added. The temperature loss during tapping and all subsequent treatment steps such as degassing, alloying and casting is compensated in the ladle furnace (LF-process).

For temperature and composition homogenization the melt has to be continuously mixed by means of inert gas stirring through a porous bottom plug.

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In order to adjust close tolerances of analysis and final temperature, most of the alloying additions are made during tapping and finally in the ladle furnace. The aim is to adjust the lower analysis limit for all elements in the ladle furnace and make only the fine-tuning and addition of oxygen-reactive elements in the vacuum station. If a reduction of gaseous elements like hydrogen and nitrogen and extremely low-contents-like of oxygen, sulphur and aluminium are required, a subsequent vacuum treatment is necessary.

During vacuum degassing (VD process) the entire melt is intensively mixed with the reactive slag. The degassing reaction takes place at the interface-between the melt and surrounding atmosphere at-considerably- lower-partial-pressure. Therefore, intensive inert gas stirring is very important in order to bring all particles of the melt in contact with the reactive surface. This stirring-action is further enhanced by the nucleation of gas bubbles caused by the reaction of carbon with dissolved oxygen from the melt to form CO gas. This so-called Vacuum Carbon-Deoxidation (VCD) process helps to achieve a low final oxygen content.

The economic production of high chromium-steel grades can only be accomplished by using low-cost raw materials in the melting furnace. This requires carbon removal by means of oxygen blown into the melt. Consequently, heavy chromium losses due to oxidation during oxygen blowing cannot be avoided at the usual melt temperature of 1700 °C. With the application of the VOD process (Vacuum Oxidation Decarburization) in the ladle or in a vacuum converter (VODC) it is possible to limit chromium oxidation. The VOD treatment takes advantage of the fact that under vacuum conditions decarburization with oxygen takes place before chromium oxidation. Thus very low carbon values with very little loss of chromium can be obtained.

Exercise 1. Find English equivalents to the following Russian words

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Exercise 2. Fill in the blanks with prepositions (in, of, by, at, of, from, in, at, with, of, to, between, with, by):

1.The degassing reaction takes place … the interface… the melt and surrounding atmosphere … considerably lower partial pressure.

2.This stirring action is further enhanced … the nucleation … gas bubbles caused … the reaction … carbon …. dissolved oxygen … the melt .. form CO gas.

3.… the application … the VOD process … the ladle or … a vacuum converter it is possible to limit chromium oxidation.

Exercise 3. Are these statements true or false?

1.Decarburizing of special steels today is not widely used in ladle metallurgy plants.

2.The mail purposes of ladle metallurgy are to increase production capacity of the melting furnace.

3.In modern ladle metallurgy all refining operations are not carried out in the ladle.

4.After tapping from the melting furnace a new reactive basic slag is not added.

5.During vacuum degassing the entire melt is intensively removed with the reactive slag.

6.The economic production of high-chromium steel grades can only be accomplished by using high-cost raw materials in the melting furnace.

7.Thus very low carbon values with very high loss of chromium can be obtained.

Exercise 4. Answer the following questions:

1.What is widely used in ladle metallurgy plants?

2.What are the main purposes of ladle metallurgy?

3.What operations are carried out in the ladle?

4.When are most of the alloying additions made? What is the aim?

5.Why is the entire melt mixed with the reactive slag?

6.What is called VCD? What does VCD help to achieve?

7.When is it possible to limit chromium oxidation?

8.What is VOD?

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4.4. Vacuum induction melting

In contrast to electric arc furnace/ladle metallurgy, there are many different factors to be considered in vacuum induction melting which significantly affect the metallurgical process. In a VIM or VDP furnace the slag is transported to the crucible wall by the bath movement, resulting in the solidification of the slag at the wall. For this reason, metallurgical reactions, such as dephosphorization and desulphurization. are limited.

VIM metallurgy is primarily-aimed at pressure-dependent reactions, such as reactions of carbon, oxygen, nitrogen and hydrogen. The removal of undesired volatile trace elements, such as antimony, tellurium, selenium and bismuth, in vacuum induction furnaces is of considerable practical importance. These elements must be kept at very low concentrations in order to avoid the risk of premature part failure - particularly in the production of special alloys for critical applications, such as jet engine parts. Because of the high vapour pressures of most of the undesirable trace elements, these can be kept at very low levels by distillation during melting under vacuum. This is very important, particularly for alloys with extremely high strengths at higher operating temperatures.

For special applications (i. e. rotating parts) the quality of the material produced by VIM is, however, not sufficient to satisfy the extremely exacting demands with respect to cleanliness and primary structure. In this case, the VIM produced material must undergo a remelting and/or resolidification process.

The VIDP concept is based on a modular design that can be extended to melting and casting under vacuum or protective gas atmosphere. The casting process is realized by using a ceramic launder which transfers the liquid metal through a pouring tunnel to the casting (mould) chamber.

The VIDP furnace has the following advantages in comparison with a conventional VIM furnace:

−vacuum chamber size is reduced to a minimum, the result being lower pressure, shorter pumping time or smaller pump system capacity;

−better control of the process atmosphere;

−faster replacement of different furnace bodies with shorter downtime for crucible exchange;

−high flexibility in the choice of the pouring technique;

−reduced risk of contamination by eliminating all flexible power cables, water hoses and hydraulic lines inside the vacuum chamber;

−lower desorption and leakage rates compared to a conventional VIM furnace.

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