Ear-infrared emitter of high emissivity and corrosion resistance and method for the preparation thereof

A far-infrared emitter of high corrosion resistance is prepared by an oxidizing heat treatment of a body made from a stainless steel of 20-35% by weight of chromium, 0.5-5.0% by weight of molybdenum, up to 3.0% by weight of manganese and up to 3.0% by weight of silicon at 900.degree.-1200 .degree. C. to form an oxidized surface film having a thickness of at least 0.2 mg/cm.sup.2. Further, a far-infrared emitter of a high emissivity approximating a black body is prepared by subjecting a body made from a stainless steel of 10-35% by weight of chromium, 1.0-4.0% by weight of silicon and up to 3.0% by weight of manganese to a blasting treatment to roughen the surface followed by an oxidizing heat treatment at 900.degree.-1200 .degree. C. to form an oxide film on the surface in the form of protrusions having a length of at least 5 .mu.m.

BACKGROUND OF THE INVENTION 
The present invention relates to a far-infrared emitter of high emissivity 
and corrosion resistance and a method for the preparation thereof. More 
particularly, the invention relates to a stainless steel-made far-infrared 
emitter having a high emissivity approximating that of a black body and 
excellent corrosion resistance suitable as a heater element in room 
heaters and drying or heating apparatuses utilizing far-infrared rays as 
well as a method for the preparation thereof. 
As is well known, far-infrared rays have a characteristic of easily 
penetrating human bodies and various kinds of organic materials so that 
room heaters utilizing far-infrared rays are advantagesous in respect of 
the high efficiency of heat absorption in the depth of the human body and 
far-infrared drying or heating ovens can be advantageously used for drying 
of paint-coated surfaces or heating of various kinds of food by virtue of 
the rapidness of heating. 
Several metal oxides such as zirconium oxide, aluminum oxide, silicon 
dioxide and titanium dioxide are known to emit far-infrared rays with a 
high efficiency at high temperatures so that many of the far-infrared 
emitters currently in use are manufactured from a ceramic material mainly 
composed of one or more of these metal oxides or by providing a metal-made 
substrate with a ceramic coating layer composed of these metal oxides. 
Such a ceramic-based far-infrared emitter, however, is practically 
defective in respect of the fragility to be readily broken by shocks and 
lack of versatility to the manufacture of large-sized emitters. 
Metal-based ceramic-coated far-infrared emitters are also not without 
problems because the ceramic coating layer is liable to fall during use 
off the substrate surface in addition to the expensiveness of such an 
emitter. 
In view of the above mentioned problems in the ceramic-based far-infrared 
emitters, many proposals have been made for metal-made heat radiators of 
infrared emitters. For example, Japanese Patent Publication 59-7789 
discloses a heat radiator made of an alloy of nickel and chromium, iron 
and chromium or iron, chromium and nickel provided with a black oxide film 
on the surface mainly composed of an oxide of chromium formed by the 
oxidation at a high temperature. Japanese Patent Publication 59-28959 
discloses a stainless steel-made infrared heater element provided with an 
oxide surface film having a thickness of 1 to 10 .mu.m formed by an 
oxidation treatment at a high temperature of 700.degree. C. or higher. 
Japanese Patent Publication 60-1914 discloses an infrared-radiating heater 
element made of a highly heat resistant alloy such as incoloy and 
subjected to an oxidation treatment at a high temperature of 800.degree. 
C. or higher. Further, Japanese Patent Kokai 55-6433 discloses a stainless 
steel-made radiator provided with an oxide surface film formed by a wet 
process after roughening of the surface to have a surface roughness of 1 
to 10 .mu.m. 
While it is desirable that a far-infrared emitter has an emissivity as high 
as possible, the above described ceramic-based or stainless steel-based 
emitters have an emissivity rarely exceeding 0.9 or, in most cases, 0.8 or 
smaller. Far-infrared emitters usually utilize the far-infrared rays 
emitted from the emitter body at a temperature in the range from 
100.degree. to 500.degree. C. As is understood from the Planck's law of 
radiation distribution, an emitter of low emissivity can emit a 
far-infrared radiation identical with that from an emitter of higher 
emissivity only when it is heated at a higher temperature. Needless to 
say, a larger energy cost is required in order to heat an emitter at a 
higher temperature. Moreover, certain materials are susceptible to 
degradation when exposed to a radiation of shorter wavelength such as 
near-infrared and visible rays so that heat radiators used for such a 
material are required to emit far-infrared rays alone and the far-infrared 
emitter should be kept at a relatively low working temperature not to emit 
radiations of shorter wavelengths. Accordingly, it is eagerly desired to 
develop a far-infrared emitter having a high emissivity even at a 
relatively low temperature. 
Apart from the above described problem in the emissivity, stainless 
steel-made far-infrared emitters in general have another problem of 
relatively poor corrosion resistance. Namely, the working atmosphere of a 
far-infrared emitter is sometimes very corrosive. For example, a large 
volume of water vapor is produced when a water-base paint is dried or food 
is heat-treated with a far-infrared emitter to form an atmosphere of high 
temperature and very high humidity. When the working hours of such a 
heating furnace come to the end of a working day, the furnace is switched 
off and allowed to cool to room temperature so that the water vapor in the 
atmosphere is condensed to cause bedewing of the surface of the stainless 
steel-made far-infrared emitter. Thus, it is usually unavoidable that 
rusting of the stainless steel-made far-infrared emitter starts within a 
relatively short time as a consequence of the repeated cycles of heating 
and bedewing. Once rusting has started, it would be before long that scale 
of the rust comes off the surface to enter the food under the heat 
treatment or to adhere to the fabric material under drying so that the 
heating furnace can no longer be used without entirely replacing the 
far-infrared emitter elements in order to obtain acceptable products. 
SUMMARY OF THE INVENTION 
The present invention accordingly has an object to provide a novel 
far-infrared emitter free from the above described problems and 
disadvantages in the conventional stainless steel-made far-infrared 
emitters in respect of the emissivity and corrosion resistance as well as 
an efficient method for the preparation of such a far-infrared emitter. 
Thus, the far-infrared emitter having, in an aspect of the invention, 
excellent corrosion resistance is a body made from a stainless steel, 
which is essentially consisting of: from 20 to 35% by weight of chromium; 
from 0.5 to 5.0% by weight of molybdenum, up to 3.0% by weight of 
manganese and up to 3.0% by weight of silicon, the balance being iron and 
unavoidable impurities, and having an oxidized surface film of a thickness 
corresponding to at least 0.2 mg/cm.sup.2. 
The above defined far-infrared emitter of the invention can be prepared by 
heating a body made from the above specified stainless steel in an 
oxidizing atmosphere at a temperature in the range from 900.degree. C. to 
1200.degree. C. for a length of time which is at least 5 minutes when the 
heating temperature is 1100.degree. C. or higher and at least (142.5-0.125 
T) minutes when the heating temperature is lower than 1100.degree. C., T 
being the heating temperature given in .degree.C. 
The far-infrared emitter of the invention having, in another aspect of the 
invention, an outstandingly high emissivity is a body made from a 
stainless steel, which is essentially consisting of: from 10 to 35% by 
weight of chromium; from 1.0 to 4.0% by weight of silicon and up to 3.0% 
by weight of molybdenum, the balance being iron and unavoidable 
impurities, and having an oxidized surface film with protrusions each 
having a length of at least 5 .mu.m. 
The above defined high-emissivity far-infrared emitter of the invention can 
be prepared by a method comprising the steps of (a) subjecting the surface 
of a body made from the above specified stainless steel to a blasting 
treatment and then (b) heating the body after the blasting treatment in an 
oxidizing atmosphere at a temperature in the range from 900.degree. C. to 
1200.degree. C. for a length of time of at least 15 minutes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The corrosion-resistant far-infrared emitter according to the first aspect 
of the present invention is made from a stainless steel based on iron, 
chromium and molybdenum as the essential alloying elements together with 
silicon and manganese as the optional additive elements each in a 
specified proportion. Such a composition of stainless steels is not novel. 
The amount of and the role played by each of the alloying elements in the 
stainless steel are as follows. 
Firstly, silicon in the stainless steel has an effect to increase the 
oxidation resistance of the stainless steel so as to facilitate the 
oxidation treatment thereof at a high temperature. However, an excessive 
amount of silicon in the stainless steel is detrimental in respect of the 
decreased ductility of the material not only in the base metal but also in 
the welded portion. This is the reason that the amount of silicon in the 
stainless steel should not exceed 3.0% by weight. 
Secondly, addition of manganese to the stainless steel has an effect to 
decrease the tenacity of the material not only in the base metal but also 
in the welded portion along with an adverse effect on the oxidation 
resistance of the stainless steel at high temperatures. Accordingly, the 
amount of manganese in the stainless steel should not exceed 3.0% by 
weight. 
Thirdly, chromium is one of the essential elements in stainless steels in 
order that the stainless steel may have corrosion resistance. When the 
amount of chromium is smaller than 20% by weight, no satisfactory 
corrosion resistance can be imparted to the stainless steel. When the 
amount of chromium exceeds 35% by weight, on the other hand, the steel may 
have brittleness to cause difficulty in fabrication into an emitter body. 
This is the reason for the limitation in the amount of chromium in the 
range from 20 to 35% by weight. 
Fourthly, molybdenum is another essential element in the stainless steel 
for shaping the far-infrared emitter of the invention and has an effect to 
improve the corrosion resistance of the stainless steel after an oxidation 
treatment at high temperatures. When the amount of molybdenum is smaller 
than 0.5% by weight, the above mentioned advantageous effect cannot be 
fully obtained. When the amount of molybdenum exceeds 5.0% by weight, on 
the other hand, the steel may have brittleness so that the steel cannot be 
worked into a thin plate or sheet. This is the reason for the limitation 
in the amount of molybdenum in the range from 0.5% to 5.0% by weight. 
In addition to the above mentioned elements including chromium, molybdenum, 
silicon and manganese, various kinds of additive elements can be added to 
the stainless steel according to the established formulation of stainless 
steels. For example, addition of titanium, niobium or zirconium in an 
amount up to 0.5% by weight is effective in improving the tenacity and 
oxidation resistance of the stainless steel in the base metal as well as 
in the welded portions. Further, addition of a rare earth element such as 
yttrium, cerium, lanthanum, neodymium and the like in an amount up to 0.3% 
by weight is effective in preventing falling of the oxidized surface film 
off the surface of the emitter body. Addition of these auxiliary elements 
is of course optional in the chromium-molybdenum-based stainless steel 
used for shaping the far-infrared emitter of the invention. 
The above defined stainless steel is fabricated into a thin plate which is 
subjected to a heat treatment in an oxidizing atmosphere to be provided 
with an oxidized surface film. The temperature of the heat treatment is in 
the range from 900.degree. C. to 1200.degree. C. When the temperature is 
lower than 900.degree. C., the diffusion velocity of chromium in the steel 
is low from the core portion to the surface layer not to fully compensate 
the amount of chromium lost in the form of an oxide out of the surface so 
that a chromium depletion layer having a thickness of up to several tens 
of micrometers is formed on the surface with consequently decreased 
corrosion resistance of the emitter. Such a chromium depletion layer is 
not formed on the surface when the heat treatment is performed at a 
temperature of 900.degree. C. or higher as a result of the increased 
diffusion velocity of chromium to impart the plate with high corrosion 
resistance. When the temperature of the heat treatment exceeds 
1200.degree. C., however, high-temperature distortion takes place in the 
stainless steel plate so remarkably that the plate can no longer be used 
as a material of the far-infrared emitter of the invention. 
It is essential that the oxidized surface film formed by the heat treatment 
of the stainless steel plate in an oxidizing atmosphere has a thickness 
corresponding to a weight of at least 0.20 mg/cm.sup.2 in order that the 
emitter may have a satisfactory emissivity of far-infrared rays. Such a 
thickness of the oxidized surface film can be obtained by conducting the 
oxidizing heat treatment for a sufficient length of time. When the 
temperature of the heat treatment is in the range from 900.degree. C. to 
1100.degree. C., the length of time for the treatment must be at least 
(142.5-0.125 T) minutes, T being the temperature in .degree.C., and, when 
the temperature is in the range from 1100.degree. C. to 1200.degree. C., 
the heat treatment must be continued for at least 5 minutes. The oxidizing 
atmosphere used in the oxidizing heat treatment is not limited to the 
atmospheric air as such but can be an oxygen-enriched gaseous mixture of 
oxygen and a non-oxidizng gas such as nitrogen, argon, helium and the like 
together with or without water vapor. Various kinds of combustion gases 
are also used satisfactorily for the oxidizing atmospheric gas in the 
inventive method. 
The oxidized surface film should have a thickness corresponding to a weight 
of at least 0.2 mg/cm.sup.2 or, preferably, in the range from 0.2 
mg/cm.sup.2 to 10 mg/cm.sup.2 or, more preferably, in the range from 0.5 
mg/cm.sup.2 to 2.0 mg/cm.sup.2. When the thickness is too large, the 
oxidized surface film may readily fall off the surface of the substrate as 
a trend. 
It is sometimes effective to increase the surface roughness of the 
stainless steel plate in order to have an increased effective surface area 
for emission of far-infrared rays. For example, satisfactory results may 
be obtained with a stainless steel plate after a blasting treatment or 
dull rolling. 
In another aspect of the invention, as is mentioned before, the present 
invention provides a far-infrared emitter having an outstandingly high 
emissivity. The far-infrared emitter of high emissivity is a body made of 
a specific stainless steel and having an oxidized surface film with 
protrusions each having a length of at least 5 .mu.m. Such a unique 
oxidized surface film can be formed by subjecting the surface of a 
stainless steel-made base body to a blasting treatment followed by an 
oxidizing heat treatment at a high temperature under specific conditions. 
The essential alloying elements in the stainless steel are silicon and 
chromium in amounts in the range from 1.0 to 4.0% by weight and in the 
range from 10 to 35% by weight, respectively. Silicon is an essential 
element in the stainless steel in order that protrusions are formed in the 
oxidized surface film on the surface of the base body. Namely, no 
protrusions can be formed in the oxidized surface film when the content of 
silicon in the stainless steel is lower than 1.0% by weight. When the 
content of silicon in the stainless steel exceeds 4.0% by weight, on the 
other hand, the stainless steel is somewhat brittle to cause difficulties 
in fabrication of plates thereof. Chromium is also an essential element in 
the stainless steel to impart oxidation resistance thereto. When the 
content of chromium is lower than 10% by weight, the steel may have 
insufficient oxidation resistance. When the content of chromium exceeds 
35% by weight, on the other hand, the steel is somewhat brittle to cause a 
difficulty in fabrication into an emitter. 
The stainless steel may contain manganese in addition to the above 
mentioned essential elements of silicon and chromium but the content of 
manganese should not exceed 3.0% by weight because of the adverse effects 
of manganese on the tenacity of the steel in the base metal and in the 
welded portion and on the oxidation resistance of the stainless steel at 
high temperatures. In addition, the stainless steel may contain up to 0.5% 
by weight of titanium, niobium and zirconium with an object of increasing 
the tenacity to facilitate fabrication and improving the oxidation 
resistance and up to 0.3% by weight of a rare earth element such as 
yttrium, cerium, lanthanum, neodymium and the like with an object of 
preventing falling of the oxidized surface film off the surface of the 
base body. 
A base body of the inventive far-infrared emitter of the invention prepared 
by fabricating the above described stainless steel is first subjected to a 
blasting treatment prior to the high-temperature oxidizing treatment to 
impart the surface of the steel plate with a strong work strain which is 
essential in order that protrusions of a length of at least 5 .mu.m are 
formed on the surface by the oxidation treatment. The blasting treatment 
is performed by projecting an abrasive powder of alumina or silicon 
carbide having a roughness of #100 to #400 or steel balls or steel grits 
having a diameter of 0.05 mm to 1.0 mm to the surface until the surface is 
imparted with a surface roughness of at least 0.5 .mu.m in Ra. 
The next step is a heat treatment of the thus blasting-treated base body of 
the emitter in an oxidizing atmosphere at a temperature in the range from 
900.degree. C. to 1200.degree. C. for at least 15 minutes so as to form an 
oxidized surface film in the form of protrusions having a length of at 
least 5 .mu.m whereby the surface of the emitter body is imparted with a 
greatly enhanced emissivity of far-infrared rays. The oxidizing atmosphere 
used here can be the same as in the oxidizing heat treatment of the 
emitter body made from the chromium-molybdenum-based stainless steel to 
impart enhanced corrosion resistance. The temperature in the oxidizing 
heat treatment should be in the range from 900.degree. C. to 1200.degree. 
C. because an oxidized surface film in the form of protrusions cannot be 
formed at a temperature lower than 900.degree. C. while the base body of 
the emitter is subject to a high-temperature distortion at a temperature 
higher than 1200.degree. C. to such an extent that it can no longer be 
used as a far-infrared emitter of the invention. The length of time for 
the heat treatment is usually at least 15 minutes at the above mentioned 
temperature in order that the oxidized surface film may have a form of 
protrusions of a sufficient length. 
In the following, examples are given to illustrate the inventive 
far-infrared emitters in more detail. 
EXAMPLE 1 
Eight kinds of steels A to H were used in the tests each in the form of a 
plate having a thickness of 1.0 mm after annealing and pickling including 
six commercially available steels A, B, D, E, F and G and two 
laboratory-made steels C and H prepared by melting, casting and rolling. 
Table 1 below shows the grade names and chemical compositions of these 
steels. 
Each of these stainless steel plates was cut by shearing into 10 cm by 10 
cm square plates, referred to as the samples No. 1 to No. 12 hereinbelow, 
which were subjected to a surface treatment I, II or III specified below 
excepting for the samples No. 2, No. 5 and No. 12 followed by a 
high-temperature oxidizing treatment in air under the conditions shown in 
Table 2. 
Surface Treatment 
I : sand blasting with #180 SiC abrasive powder 
II : shot blasting with steel balls of 0.1 mm diameter 
III : dull rolling, i.e. rolling with a surface-roughened roller 
TABLE 1 
______________________________________ 
Steel No. 
C Si Mn Cr Mo Ni Others 
______________________________________ 
A 30Cr2Mo 0.003 0.2 0.1 30.1 1.9 &lt;0.1 Nb 0.14 
B 26Cr4Mo 0.003 0.2 0.1 26.2 3.7 &lt;0.1 Nb 0.16 
C 30Cr1Mo 0.005 0.4 0.2 29.2 0.9 &lt;0.1 Ti 0.1 
REM 0.1 
D 18Cr2Mo 0.004 0.1 0.3 17.8 1.8 0.3 Nb 0.3 
E SUS 430 0.04 0.4 0.4 17.4 &lt;0.1 0.2 Ti 0.2 
F SUS 304 0.06 0.5 1.5 18.5 &lt;0.1 8.2 
G Incoloy 0.024 0.4 0.4 20.4 &lt;0.1 31.1 
Ti 0.3 
Al 0.3 
H 25Cr 0.011 0.4 0.2 24.8 &lt;0.1 &lt;0.1 
______________________________________ 
The stainless steel test plates after the high-temperature oxidation 
treatment were subjected to the measurement of the center-line average 
height of surface roughness R.sub.a defined in JIS B 0601 by using a 
tracer-method surface roughness tester specified in JIS B 0651. The test 
plates were accurately weighed before and after the high-temperature 
oxidation treatment to determine the increment in the weight by the 
oxidation treatment per unit surface area. The amount of oxidation in 
mg/cm.sup.2 shown in Table 2 is the thus obtained value after 
multiplication by a factor of 3.3. 
TABLE 2 
__________________________________________________________________________ 
Amount 
Surface 
Conditions of high-tempera- 
Rough- 
of oxi- 
Sample Steel 
treat- 
ture oxidation treatment 
ness, 
dation, 
Emmis- 
Corrosion re- 
No. No. 
ment (142.5-0.125 T, minutes) 
.mu.m 
mg/cm.sup.2 
sivity 
sistance 
__________________________________________________________________________ 
Inven- 
1 A I 16 
hours at 900.degree. C. (30) 
0.9 0.3 0.8 no rusting 
tive 2 A -- 4 hours at 1000.degree. C. (17.5) 
0.1 0.6 0.7 no rusting 
exam- 
3 A III 4 hours at 1000.degree. C. (17.5) 
1.8 1.0 0.9 no rusting 
ple 4 B II 1 hour at 1100.degree. C. 
3.6 1.4 0.9 no rusting 
5 C -- 0.5 
hour at 1200.degree. C. 
0.2 0.8 0.7 no rusting 
Compar- 
6 A I 12 
hours at 850.degree. C. 
2.4 0.1 0.5 rusting in part 
ative 
7 A I 10 
minutes at 1000.degree. C. (17.5) 
0.7 0.1 0.5 no rusting 
exam- 
8 D II 4 hours at 1000.degree. C. (17.5) 
3.6 1.0 0.8 rusting in part 
ple 9 E II 4 hours at 1000.degree. C. (17.5) 
1.8 2.2 0.9 rusting all over 
10 F II 4 hours at 1000.degree. C. (17.5) 
2.4 0.8* 
0.8 rusting all over 
11 G II 4 hours at 1000.degree. C. (17.5) 
1.6 0.3 0.7 rusting in part 
12 H -- 4 hours at 1000.degree. C. (17.5) 
0.2 0.8 0.7 rusting all 
__________________________________________________________________________ 
over 
*falling of a part of oxide film 
This is because an X-ray analysis of the oxide film on each of the test 
plates indicated that the oxide film had a chemical composition 
approximately corresponding to Cr.sub.2 O.sub.3 to give a weight ratio of 
Cr.sub.2 O.sub.3 to oxygen equal to 3.3. 
In the next place, the infrared emissivity of each of the test plates was 
obtained as an average ratio of the intensity of infrared emission at 
400.degree. C. in the wavelength region of 5 to 15 .mu.m to the black body 
emission at the same temperature in the same wavelength region. The 
results are shown in Table 2. 
The results in Table 2 indicate the criticality of the oxidation 
temperature and the length of the oxidation treatment. Thus, the sample 
No. 6, oxidized for 12 hours at a low temperature of 850.degree. C., and 
sample No. 7, oxidized at 1000.degree. C. for a short time of 10 minutes, 
each had an amount of oxidation of only 0.1 mg/cm.sup.2 to give an 
emissivity of 0.5 which should be compared with the emissivity of 0.8 and 
0.7 obtained in the samples No. 1 and No. 2 prepared from the same kind of 
the stainless steel A. A practically acceptable emissivity of 0.7 or 
higher could be obtained in all of the test plates excepting No. 6 and No. 
7. In this regard, dull rolling for the surface treatment was effective to 
give an emissivity of 0.8 or higher on the test plates having the thus 
roughened surface. In particular, an improvement in the productivity of 
the oxidation treatment was obtained by using the steel C as is shown by 
the sample No. 5 which could be fully oxidized at a high temperature of 
1200.degree. C. within a short time of 0.5 hour by virtue of the addition 
of 0.1% by weight of rare earth elements, i.e. mixture of cerium, 
lanthanum and neodymium, to the 30Cr1Mo steel with an object to prevent 
falling of the oxide film from the surface. 
Finally, the salt spray test specified in JIS Z 2371 was undertaken for 4 
hours to determine the corrosion resistance of the test plates to give the 
results shown in Table 2. As is shown there, no rusting at all was found 
on each of the test plates No. 1 to No. 5 according to the invention while 
rusting was found in part on the sample No. 6, prepared from the 30Cr2Mo 
steel but oxidized at a low temperature of 850.degree. C., sample No. 8, 
prepared from the 18Cr2Mo steel of low chromium content of 18% by weight, 
and sample No. 11, prepared from incoloy, and rusting was found allover 
the surface on the samples No. 9, No. 10 and No. 12 prepared from SUS 430, 
SUS 304 and 25Cr steel, respectively. 
EXAMPLE 2 
Stainless steel plates having a thickness of 1.0 mm were prepared by 
rolling two different chromium-silicon steels I and J having a chemical 
composition shown in Table 3 followed by annealing and acid washing. Test 
plates of infrared emitters were prepared from these laboratory-made 
stainless steel plates I and J as well as from commercially available 
plates of stainless steels SUS 430 and SUS 304 (steels E and F, see Table 
1) having a thickness of 1.0 mm for comparative purpose. 
TABLE 3 
______________________________________ 
Steel No. 
C Si Mn Cr Ni Others 
______________________________________ 
I 11Cr1.5Si 
0.01 1.5 0.2 11.2 0.2 Ti 0.2 
J 25Cr3Si 0.005 2.9 2.1 25.1 &lt;0.1 Ti 0.2 REM 0.1 
______________________________________ 
Each of the stainless steel plates I, J, E and F was cut into 10 cm by 10 
cm squares which were subjected first to a blasting treatment and then to 
a high-temperature oxidation treatment in air under the conditions shown 
in Table 4 given below. The conditions of the blasting treatments I and II 
shown in the table were the same as in Example 1. 
Each of the test plates after the blasting treatment excepting the sample 
No. 16 was subjected to the measurement of the surface rougness in the 
same manner as in Example 1 to find a substantial increase in the surface 
roughness from about 0.3 .mu.m on the plates of the steels I and J and 
about 0.2 .mu.m on the plates of the steels E and F to about 1.8 to 2.9 
.mu.m on the plates after the shot blasting treatment with steel balls and 
about 0.8 to 1.4 .mu.m on the plates after the blasting treatment with the 
silicon carbide abrasive powder. 
The surface condition of these test plates after the oxidation treatment 
was examined using an electron microscope to give the photographs of FIGS. 
1 and 2 indicating the surface condition of the sample No. 13 according to 
the invention and the sample No. 16 for comparative purpose, respectively. 
Further, microphotographs of 800 magnifications were taken of the surface 
of the test plates inclined at an angle of 60.degree. to estimate the 
length of the oxide protrusions, of which an average of the actual values 
was calculated and shown in Table 4. As is shown in the table, no 
protrusions of the oxide film were found on the sample No. 16 prepared by 
omitting the blasting treatment and the samples No. 18 and No. 19 prepared 
from the stainless steels SUS 430 and SUS 304, respectively, containing no 
silicon. The length of the oxide protrusions was about 3 .mu.m on the 
sample No. 17 prepared by the high-temperature oxidation treatment for a 
relatively short time of 30 minutes. The samples No. 13 to No. 15 each had 
oxide protrusions of a length of at least 7 .mu.m. 
The test plates were subjected to the measurement of the emissivity in the 
wavelength region of 5 to 15 .mu.m in the same manner as in Example 1 to 
give the results shown in Table 4. The emissivity was 0.7 to 0.9 on the 
samples No. 17 to No. 19 having no protrusions of the oxide film and on 
the sample No. 16 of which the length of the oxide protrusions was only 
about 3 .mu.m while the samples No. 13 to No. 15 had a quite high 
emissivity of 1.0 to approximate a black body. 
TABLE 4 
__________________________________________________________________________ 
Surface 
Conditions of high- 
Rough- 
Sample Steel 
treat- 
temperature oxidation 
ness, Emis- 
No. No. 
ment treatment .mu.m 
Condition of oxide film 
sivity 
__________________________________________________________________________ 
Inven- 
13 I I 4 hours at 1000.degree. C. 
0.8 10 .mu.m long protrusions 
1.0 
tive 14 J I 16 
hours at 950.degree. C. 
1.4 7 .mu.m long protrusions 
1.0 
exam- 
15 J II 0.5 
hour at 1100.degree. C. 
2.9 10 .mu.m long protrusions 
1.0 
ple 
Compar- 
16 I -- 4 hours at 1000.degree. C. 
0.3 smooth 0.7 
ative 
17 I I 0.5 
hour at 1000.degree. C. 
1.1 3 .mu.m long protrusions 
0.8 
exam- 
18 E II 4 hours at 1000.degree. C. 
1.8 smooth 0.9 
ple 19 F II 4 hours at 1000.degree. C. 
2.4 smooth, falling in part of the 
0.8m 
__________________________________________________________________________