PAVIMENTO DE MACADAM CON RIEGO DE SEMIPENETRACION, IMPRIMACION Y ADHERENCIA

MASS HEATING

This book is devoted to a discussion of the use of induction heating principles for a large group of applications referred to as mass heating. This term applies to a variety of applications in which metal is heated for forging, forming, extru-sion, coating, etc. Typically, it is required to have a uniformly heated workpiece.

However, it is sometimes necessary to heat certain areas of the workpiece selec-tively and care must be taken to provide a required temperature distribution.

Temperature greatly affects the formability of metals. Heating of a component to temperatures that correspond to the plastic deformation range creates a favor-able condition for metal to be subsequently forced by various means into a desired shape. The most popular metal hot working processes for which induction heating is applied follow.¹

• Forging. Billets or bars are heated fully or partially, in cut length or continuously, and are forged in presses, hammers (repeated blows), or upsetters (which gather and form the metal). Steel components by far represent the majority of forged parts. At the same time, aluminum, copper, brass, bronze, cobalt, nickel, and titanium, as well as some other metal alloys, are also inductively heated and forged for a number of commercial applications.

• Forming. Hot forming includes a variety of metal working operations generally encompassing bending, expanding, and spinning. The versa-tility of induction heating is that it can selectively heat through specific areas of the workpiece or can heat areas to different temperatures providing required temperature gradients, making it a popular choice for hot forming operations.

• Extrusion. Extruding is the process of forcing or squeezing metal through a die. Ferrous and nonferrous metals are heated by induction prior to extrusion.

• Rolling. Bars, billets, rods, slabs, blooms, strips, and sheets are pro-cessed in rolling mills. These components are made from ingots or continuous cast metals and their alloys.

The goal of using induction heating in all of these applications is to provide the metal workpiece at the hot working stage with the desired (typically uniform) temperature across its diameter/thickness as well as along its length and across its width. The required final temperature typically depends on the material chem-ical composition and particularities of the postheating metal processing operation.

For example, for steel billets, the final temperature is in the range of 1000 to 1200°C and the required surface-to-core and noise-to-tail temperature uniformity in the billet is commonly specified as ±50°C for rolling and ±25°C for forming.

For heating of aluminum alloy billets before extrusion, the final temperature is

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20 Optimal Control of Induction Heating Processes in the range of 450 to 480°C and required uniformity is in the range of ±10 to

±15°C. Certain applications may require a nonuniform end-to-end temperature profile (for example, heating providing required temperature gradients).

In some cases, the initial temperature of the workpiece prior to induction heating is the ambient temperature. In other cases, the initial temperature is not uniform — for example, due to uneven cooling of the slab, transfer bar, strip, or bloom as it progresses from the caster. Surface layers, and particularly the edge areas, become much cooler than the internal regions.

The power ratings of induction heating machines range from less than 100 kW up to dozens of megawatts. The success of an induction mass heating system is based on an in-depth understanding of the process features. This is imperative for developing the sophisticated design concepts and precise engineering that lead to the achievement of a reasonable, commercially acceptable compromise among often contradictory process requirements and design criteria.

Cylindrical and rectangular solenoid multiturn induction coils are most often used in induction mass heating applications. The four basic heating modes in induction mass heating (Figure 1.4) follow.¹

• Static heating. In the static heating mode, a workpiece such as a billet or slab is placed into an induction heating coil for a given period of time while a set amount of power is applied until the component reaches the desired heating conditions. Upon reaching the required

FIGURE 1.4 Four basic heating modes used in induction mass heating.

Multi-turn solenoid induction coil

Heated workpiece

(a) Static heating

Coil #1

Heated workpiece

Heated workpiece

(c) Oscillating heating

(b) Progressive multi-stage heating and continous heating

Coil #2 Coil #3

Coil #1 Coil #2 Coil #3

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Theory and Industrial Application of Induction Heating Processes 21 thermal conditions, the heated component is extracted from the induc-tion heater and delivered to the metal forming stainduc-tion. The next cold workpiece is loaded into the coil and the process repeats.

• Progressive multistage heating. This heating mode occurs when two or more heated workpieces (e.g., billets) are moved (via pusher, index-ing mechanism, walkindex-ing beam, etc.) through a sindex-ingle-coil or multicoil induction heater. Therefore, components are sequentially heated (in a progressive manner) at certain predetermined heating stages inside the heater.

• Continuous heating. With the continuous heating mode, the workpiece is moved in a continuous motion through one or more induction heating coils. This heating mode is commonly used when it is required to heat long components such as bars, slabs, strips, tubes, wires, blooms, and rods.

• Oscillating heating. In this heating mode, a component moves back and forth (oscillates) during the process of heating inside a single-coil or multicoil induction heater with an oscillating stroke featuring a space-saving design approach.

When modern induction mass heating systems are designed, temperature uniformity of the heated component is only one of the goals. Additional design criteria include maximum production rate, minimum metal losses, and the ability to provide flexible and compact systems that have high electrical efficiency. Other important factors include quality assurance, process repeatability, automation capability, environmental friendless, reliability, and maintainability of the equip-ment. The last criterion, but not the least, is the competitive cost of an induction heating system.

One of the challenges in induction heating arises from the necessity to provide the required surface-to-core temperature uniformity. Due to the physics of the process, the workpiece core tends to be heated more slowly than its surface. The main reason for the heat deficit of the heated component is the skin effect discussed earlier.

It has been pointed out that, in induction heating, the heat transfer by con-vection and radiation reflects the value of surface heat losses. This heat loss varies with temperature. The analysis shows that the convection losses are the major part of the heat loss in low-temperature applications such as induction heating of tin, lead, and aluminum alloys. In hot working applications (including induction heating of steel, titanium, cobalt, and nickel), radiation losses are much greater than convection losses, representing the major portion of total heat loss from the workpiece surface.

It is typically much easier to provide surface-to-core temperature uniformity for metals with high thermal conductivity, such as aluminum, silver, or copper.

Metals with poor thermal conductivity, including stainless steel, titanium, and carbon steel, require extra care to obtain the desired temperature uniformity. This

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22 Optimal Control of Induction Heating Processes

“extra care” includes proper selection of heat mode, frequency choice, process time, and other parameters.

Figure 1.5 shows the typical time–temperature curve for static induction heating of a nonmagnetic solid cylinder. As one can see, immediately after heating begins, the surface temperature and average temperature begin to rise. In contrast, there is a delay before the core temperature starts to grow. If the materials’

properties vary according to a linear function with temperature and surface heat losses are absent, there will be a linear region where all three temperatures are represented by three straight lines (Figure 1.5, solid lines). The surface-to-core temperature difference in this area is proportional to the power density during the heating cycle, the frequency, geometry, and material properties of the heated components.

As soon as power is cut off, the surface temperature decays rapidly, due to heat transfer toward a cooler core, and there is a corresponding rise in the core temperature. During the soaking stage, the surface-to-core temperature differen-tial decreases and the heated component approaches the temperature uniformity required for hot working.

In reality, the surface-to-core temperature differential starts to decrease before the soaking stage begins. This temperature differential starts to decline during the heating stage (Figure 1.5, dotted curve representing surface temperature). This takes place due to the increasing surface heat loss with temperature and the depth FIGURE 1.5 Time–temperature profile during static heating of a nonmagnetic solid cylinder.

Heating

Surface Average

(mean)

Core

Time

Time

t_f t₁

TemperaturePower

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Theory and Industrial Application of Induction Heating Processes 23 of induction heating during the heating cycle. Because the electrical resistivity of most metals increases with temperature, current penetration depth increases.

It is important to note that the soaking stage can be performed when the heated workpiece is inside the induction coil and/or during the workpiece transfer stage to the hot forming machinery. The latter approach allows minimizing the total process time.

It should be mentioned that, in reality, the time–temperature profiles are more cumbersome than those shown in Figure 1.5. To improve the performance of an induction heating machine and reduce the heating time while providing surface-to-core temperature uniformity, power pulsing can be applied. Power pulsing refers to a technique that applies short bursts of power to maintain a desired surface temperature or a maximum allowable surface-to-core temperature differ-ence. Pulse heating consists of a series of “Heat ON” and “Heat OFF” cycles until the desired uniformity is obtained (Figure 1.6). Depending upon the partic-ular application, the process time reduction with pulse heating can exceed 40%.

The heating time can be reduced even more when applying accelerated heating (Figure 1.7). Obviously, to provide this type of heating, it is necessary to have a power supply that allows a gradual reduction of output power. The accelerated heating approach may have several modifications. Figure 1.8 shows

FIGURE 1.6 Power pulsing.

Surface

Average (mean) Core

Time Time

t_f t₁ t₂ t₃

Temperature

T_max

Maximum permissible temperature

Required temperature

T_final

Power

Time reduction compared to

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24 Optimal Control of Induction Heating Processes

FIGURE 1.7 Accelerated heating.

FIGURE 1.8 Modification of accelerated heating.

Surface Average

(mean) Core

Time Time

t_f

t₁ t₂

Temperature

T_max

Maximum permissible temperature

Required temperature

T_final

Power

Surface

Avoidance of exceeding the critical values of thermal gradients

Core

Time Time

Temperature

T_max T_final

Power

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Theory and Industrial Application of Induction Heating Processes 25 one of the modifications of accelerated heating that is particularly useful when heated bars, slabs, or billets have a tendency to crack. These cracks appear due to excessive thermal stresses (thermal shocks) that appear when thermal gradients exceed the maximum permissible level. These levels vary depending upon the metal’s chemical composition, microstructure, and temperature.

Although it has been mentioned that the curves shown in Figure 1.5 through Figure 1.8 illustrate typical time–temperature profiles that take place during static