Solar Energy Materials - Science Direct

4 downloads 4500 Views 396KB Size Report
A,J. Vdzquez et aL / Surface treatment of steels by solar energy ... In the case of solar concentrated beams we can also work at atmospheric pressure. There are ...
Solar Energy Materials 24 (1991) 751-759 North-Holland

Solar Energy Materials

Sarface treatment of steels by solar energy A.J. Vfizquez, G.P. Rodriguez and J. de D a m b o r e n e a Centro Nacional de Incestigaciones Metal~rgicas (CENIM), At,enida de Gregorio del Ainu, 8, E-28040 Madrid, Spain In the recent years, solar energy has started to be considered as a useful energy resource for industry. Outside of its traditional applications, domestic heating and electricity productions, there are scarce references to topics like Surface Treatment of Metals. When solar energy is adequately concentrated, a sufficient energy density is reached to affect a metal thermically, from a mere surface heating to a total fusion depending on the power and duration of the treatment. "lhe present paper sets out the initial results of work on treatment of carbon steel carried out at the Almeria Solar Platform. The results indicate that the maximun temperature (close to 1200 o C) is reached only a few seconds or minutes after the begining of the test as a function of thickness and flux density. This suppose an enormous advantage over conve~tionat thermal treatment in terms of time. Another aspect of interest is that the energy is free of charge but, on the other hand, we cannot control it as we like because we depend on weather characteristics. Maximum hardness was found on the metal surface (950 HV) with a treatment depth of between 1.5 and 2 mm. The conclusions reached are therefore that solar energy treatment is ideal for production of structural modifications and even the formation of surface alloys.

I. Introduction Solar e~ergy has been applied to electricity production, traditional salt recovery processes from marine water and to domestic water heating since many years and only very recently some new applications to metal surface treatment have been developed. The main advantages of th:~sprocedure is the possible application on the surface of the material to be treated without heating the whole piece, so he can compete with procedures as induction heating, electron beam, laser, etc. The possibilities of these procedures are different not only in technical terms but also because of the cost of equipment, cost of the applied energy, working conditions, etc. Another aspect of interest is the lack of influence of environment because there is no production of residues and, moreover, it is the most abundant source of energy. Recently, laser is obtaining a great attention as a versatile tool to be applied in heat treating of metallic materials because the high speed of heating and the possibility of working at ambient pressure. There are many equipments that work in various wavelengths with different power outputs. Actually the most popular in industrial applications in metallurgy (heat treating, cutting, welding, etc.) is the 0165-1633/91/$03,50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

A,J. Vdzquez et aL / Surface treatment of steels by solar energy

752

CO 2 laser because of its higher power output. Recently cxcimer laser, lower in power, can be used in some applications because the best absorbance of its wavelength on metallic ~urfaces.

2. C h a r a c t e r i s t i c s o f c o n c e n t r a t e d s o l a r b e a m s

In the case of solar concentrated beams we can also work at atmospheric pressure. There are many types of utilities to obtain a power density not as high as with laser equipment in small surfaces (100 MW/cm2), as in laser treatment, but we can achieve enough energy density for most of the low energy density applications in the field of surface engineering. Nevcrtherless there is a running project to obtain, with a higly refractive index secondary concentrator 50000 suns. A clear advantage of concentrated solar beam treating is the possibility to treat very much larger surfaces as in laser treatments. The main advantage of concentrated solar beams with reference to laser treatment is the broad wavelength of the solar spectrum with reference to the monocromatic wavelength of the laser. Moreover, the normal laser equipment used in this kind of applications is the more industrial CO 2 working with a wavelength 10.6 /tm. As fig. 1 shows with this wavelength the absorbance on metals is very low and the piece needs to bc coated with some kind of black paints to enhance the absorptivity. That means that with the same power a solar beam applied on surface the power absorption will be higher or that we nccd to apply a lower power on surface to gc: the same. This is of particular interest in the case of metals.

U

0.8

o

0.7

. fd-Nd-Yog

L o s e r e n e r g y sys'cems Nd-Ya9

J CO

2

¢-

.Q



Ruby

L..

0

m 0.6

".. L 0"51 ~u 0.41~ . _ ~ ~ m

AM1 solar spect rum -----Iron .......C o p p e r

12001~ 1000 'E

800 ~ 600 u

0.3

0

E 0.2

.%

e-

400 .o

0

"U

0.0 0.3

--~

_'....: ,

,

0.6 0.8 1.0 2.0 4.0 Wavelength ()Jm)

r= ' " 8

200 o 0 L I0.0

Fig. 1. Normal spectral absorbance of iron and copper. Operating laser wavelengths as well as the solar spectrum are added for comparison.

A.J. Vdzquez et al. / Surface treatment of steels by solar ene ,r~,y

753

Another characteristic of the solar beam is that the power consumption is to be absolutely gratis. The difference with laser is, with reference to this point, tremendous because the laser production is one of the most ineffective energy devices. The counterbalance of this fact is that we cannot use the energy source as we like but oniy when the insolation is high enough to work. With reference to this point one observation must be made: the site for application of solar energy to energy production needs a high solar flux density and a high number of insolation hours per year. In our case, and specially in research, taking in account the short time needed to do each experiment, we only need to fulfil the first of both requisites. That means that there are many places, if they have enough insolation, e.g. at noon, where we can do an interesting research work; nevertheless, if we fulfill both requisites it will be better.

3. Characteristics of the in~talation There are three main different types of equipments or facilities~to be used in this application:

Type 1: Central tower receiver system. This installation consists of a field of mirrors directed against a central tower receiver furnace where the beams are focused. There are different facilities in each installation to select the number of mirrors to be employed in each test. The main advantage of this Type I system is the broad surface where the reflected beams can be directed. The control of the mirrors is made with a computerized systems that takes i~tc~ account the positioa of the sun, i.e., the hour of the day and the day of the year, the position of the mirror and the position of the furnace, etc. The total power of the plant is very high cd. 10 MW~. With this type of installation the samples are placed in a vertical position or slightly facing down and we h'ke to have a beam normal to the sample'g surface.

Type II: Flat and parabolic mirror system. This instalation has also one flat mirror but here the reflected beam is directed against a big multi-faceted parabolic mirror that concentrates the beam in a small surface. The parabolic mirror is static and the flat mirror follows the sun movement also with a computerized system. This kind of facility let to work almost in horizontal position. The maximun power of this system is very. much lower so the maximun surface to be treated is correspondingly lower. The position of the samples is vertical or slightly facing up.

Type 111: One parabolic mirror. This installation is the simplest one and consists merely in a parabolic mirror that follows directly the sun movement. In its focus is placed an appropriate device

A,J. Vdzquez et al. / Surface treatment of steels by solar energy

754

to put the sample to be treated. Some additional devices let the samples have a one- or two-axis displacement to obtain a track on the surface. As a function of the quality of the parabolic mirror the concentration on the focus can be the highest but the surface to be treated will then be the lowest. The control of the position of this mirror can be made by computer or, taken in account the short length in time of the experiments with a hand control. Because the mirror axis must be in the direction of sun beams the surface of the samples is always facing down.

4. Experimental work The equipment utilized in this work corresponds to one of the Type I facilities existing in Plataforma Solar de Almerla (PSA), in SE of Spain, named SSPS; its main characteristics are: Flat mirrors number 93 Max. surface 3655 m 2 Max. power 3600 kW(t) The main objective of this preliminary work was to identify the possibilities of this facil"ty in PSA, in particmar the heating rate and also the possibility of solid difussion reactions. The test c~_~nsists in the heating of cylindrical samples of various steels and of different thicknesses to determine the obtained heating rates. Moreover, some of the samples were cooled in water and the structure was identified by optical microscopy and the hardness obtained tested. The composition of employed steels is presented in table 1. The samples were slides of a round bar of steel 35 ran, in dianeter and 30 mm height. The samples were put inside holes of the same size made on alumina pane!s and then placed on the solar furnace. Some push rods in the back of the panel sewe to put out of the holes some of the samples for fast cooling in water (fig. 3). Behind the panel, close to the floor, a big pool of water was placed to receive the samples to be cooled. The remainder samples were cooled slowly inside the holes. Some thermocouples were placed in different samples throughout its generatr~ near the front surface, near the bottom and at half depth and the temperature was recorded dv~ring the experiment.

Table 1 Composition % Steel

C

Mn

Si

P

S

Cr

Ni

Mo

A B

0.38 1.00

0.83 0.29

0.91 0.22

0.014 0.015

0.009 0.018

0.90 0.10

0.33 0.12

0.20 < 0.05

755

A.J. Vdzquez et al. / Surface treatmem of steels by solar energy I 200

10o0 u

~- 800 a: 600

~00

n

2oo

60

120

I

I

180

240

300

T I M E , .~ Fig. 2. Heating rate,with solar energy.

This experiment was made with a relatively low heating rate, so we begin putting in place only a group of mirrors of the whole field and, when the rate begins to decrease we add another group and so on until we reach a temperature between 900 and 1000 ° C on the thermocouple at the surface of the sample. At this m o m e n t we push some of the samples in water for quenching and let the remaining in holes to cool slowly. We did al~.~ some experiments rising the temperature up to 1200 ° C.

5. Discussion In fig. 2 the maximun and minimun heating rate obtained on different samples is presented. The diferent parabolics branches of the curve correspond to the addition of a new group of mirrors with an in.crease in heating rate. The whole curve has also a parabolic shape. At the end of the experiment we can observe some differences in the temperature between samples, almost 150 o C; this difference corresponds to differences in the position of samples, probably not perfectly well aligned in the same plane, or beca~Jse the differences in the insolation on each sample is not balanced with a heating flow between samples to make a more uniform heating because we work with individual samples inside holes of non conductive material. We realize small differences in the angle of the surface of samples ha'~,~, ~ a very bi~ influence on the absorption of heat and therefore in the rising of the temperature.

A.J. Vdzqt,ez et al. / Surface treatment of steels by solar energy

756 C

io-

Si

Hn

zz

29

P

$

1.5 1.8 ,_

1200 Surface. Center,

¢.) 1000