Low alloy or carbon steel roll with a built-up weld layer of an iron alloy containing carbon, chromium, molybdenum and cobalt

A roll of which surface cracks are hardly caused under the condition of repetitive heating and cooling. A roll comprises that the alloy consisting of no more than 0.10 percent by weight of carbon, 10.about.14 percent by weight of chromium, 0.1.about.1.0 percent by weight of molybdenum, 0.5.about.2.0 percent by weight of cobalt and essentially balanced iron is coated on the surface of a roll body consisting of low alloy or carbon steel by build-up welding. Therefore, build-up welding layer of the above-mentioned compositions excel in view of resistance to corrosion, and that fact is related with a long-life of a roll.

A roll having a built-up weld layer thereon. 
BACKGROUND OF THE INVENTION 
The present invention relates to a high order of durable roll. In the cases 
of a roll which is used at hightemperature ranges such as a hot rolling 
process or a continuous casting process, its strength, toughness and 
resistance to corrosion at high temperature ranges are required for the 
roll. As the material of a roll or the material which is formed as a 
built-up weld layer on the surface of a roll used at high temperature 
ranges, an alloy steel containing 10.about.14 percent by weight of 
chromium is generally used. However, the aforesaid alloy steel containing 
10.about.14 percent by weight of chromium does not have enough resistance 
to repetitive thermal stress, and there are defects which appear when the 
surface of a roll such as a continuous casting roll sustains continuously 
repetitive heating and cooling treatment such as cracks which lead to the 
breakdown of the roll as a consequence of the thermal stress. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a high order of durable 
roll. Another object of the invention is to provide a high order of 
durable roll especially at high temperature ranges. These objects of the 
present invention are achieved with an alloy having excellent toughness, 
resistance to heat, to corrosion, and to repetitive thermal stress. The 
aforementioned alloy consisting of no more than 0.10 percent by weight of 
carbon, 10.about.14 percent by weight of chromium, 0.1.about.1.0 percent 
by weight of molybdenum, 0.5.about.2.0 percent by weight of cobalt and the 
balance essentially iron, is deposited on the surface of a steel roll body 
by build-up welding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The roll of the present invention is comprised of a roll body (1) and a 
built-up weld layer (2) which is welded on the surface of the roll body as 
shown in FIG. 1. The roll body (1) consists of a low alloy carbon steel 
and the built-up weld layer (2) consists of an alloy having a composition 
of no more than 0.10 percent by weight of carbon, 10.about.14 percent by 
weight of chromium, 0.1.about.1.0 percent by weight of molybdenium, 
0.5.about.2.0 percent by weight of cobalt and the balance essentially 
iron. The experiments for achieving the present invention and their 
results are shown in the following. 
Test Pieces 
Firstly, a built-up weld layer having various component compositions as 
shown in Table 1, is formed on a steel plate substrate (JIS G3101 SS41) by 
the multi-layer welding method, and then test pieces after the post weld 
heat treatment at 625.degree. C. for 2 hours, are prepared for the tension 
test, the impact test, the hardness test, the high-temperature tension 
test and the repetitive thermal impact test. Therefore, as regards the 
shape and size of the above-mentioned test pieces, JIS Z2202, No. 14 test 
piece (6 mm in diameter) was used for the tension test, and JIS Z2202, No. 
4 test piece (5 mm in width) for the impact test. Also, a test piece of 
the shape and size of FIG. 2 was used for the hightemperature tension 
test. A test piece as shown in FIG. 3 was used for the repetitive thermal 
impact test; the built-up layer (2') of 10 mm thickness was coated on a 
steel plate substrate of SS41 (1'). 
Repetitive Thermal Impact Test 
As to the repetitive thermal impact test for testing resistance to cracks, 
one set of two induction coils was placed slightly separated from the 
surface of a test piece as shown in FIG. 4. Then the test piece was 
subjected to repeated cycles of heating and cooling 1,000 times as shown 
in FIG. 5, and after that, the surface condition of the test piece was 
observed. 
TABLE 1 
______________________________________ 
(% by weight) 
No. C Si Mn P S Ni Cr Mo Co 
______________________________________ 
1 0.06 0.70 1.55 0.011 
0.009 
0.02 10.59 
0.03 -- 
2 0.06 0.63 0.73 0.018 
0.010 
0.02 10.22 
0.67 0.74 
3 0.07 0.67 0.71 0.021 
0.012 
0.02 10.37 
0.76 1.56 
4 0.05 0.69 1.83 0.012 
0.009 
0.03 12.54 
0.02 0.01 
5 0.05 0.59 0.71 0.017 
0.012 
0.03 13.01 
0.59 0.75 
6 0.05 0.62 0.73 0.020 
0.014 
0.06 12.32 
0.61 1.63 
7 0.08 0.61 0.71 0.020 
0.013 
0.02 10.77 
0.04 0.014 
8 0.08 0.69 0.45 -- -- 0.16 9.01 -- -- 
______________________________________ 
The Results Of Each Test 
The mechanical properties of each test piece are shown in the following 
Table 2. 
TABLE 2 
__________________________________________________________________________ 
room temperature high temperature .sigma.B 
high temperature .phi. 
No. 
.sigma.B 
.delta. 
.phi. 
Hv IV 
500.degree. C. 
600.degree. C. 
700.degree. C. 
800.degree. C. 
500.degree. C. 
600.degree. C. 
700.degree. C. 
800.degree. C. 
__________________________________________________________________________ 
1 68.2 
5.0 
55.7 
221 
2.1 
45.1 
36.6 
23.6 51.5 
59.9 
81.2 
2 68.4 
11.8 
51.0 
238 
4.2 
46.9 
39.7 
25.9 
15.9 
44.7 
50.4 
69.0 
84.7 
3 73.0 
15.3 
54.4 
239 
2.8 
50.5 
42.5 
28.0 
16.1 
51.9 
58.3 
72.8 
74.6 
4 55.3 
21.0 
43.2 
192 
1.9 
35.7 
28.9 
17.6 
11.0 
44.8 
50.8 
75.5 
83.7 
5 59.7 
17.5 
38.8 
206 
2.9 
41.3 
33.7 
22.5 
14.1 
41.0 
48.7 
68.0 
81.9 
6 59.3 
19.3 
45.4 
210 
2.2 
40.5 
33.5 
22.3 
13.2 
40.4 
44.8 
64.0 
79.0 
7 66.0 
16.7 
49.8 2.7 
44.0 
36.2 
24.0 
13.7 
45.7 
49.9 
75.0 
78.8 
8 81.2 
15.4 
49.0 
252 
2.1 
50.1 
41.7 
26.8 43.1 
45.1 
64.2 
__________________________________________________________________________ 
In the above identified Table 2, .sigma.B presents the tensile strength 
(Kgf/mm.sup.2), .delta. presents the elongation (%), .psi. presents the 
reduction of area (%), Hv presents Vickers hardness, and IV presents 
Charpy impact value (Kfgm/cm.sup.2), (hereinafter referred to in the same 
manner). 
The results of the above-mentioned Table 2, diagrammatically shown in FIG. 
6 to FIG. 10, clearly show the effects of the addition of Cr and Co in 
particular at room temperature. Referring now to FIG. 6 to FIG. 10, " " 
indicates the case of 10 percent by weight of Cr, " " indicates the case 
of more than 12 percent by weight of chromium, ".DELTA." indicates the 
case of 9 percent by weight of chromium and small sized numbers in the 
above identified Figures indicate the numbers of the test pieces which are 
shown in Table 1. 
Secondly, photomicrostructures of the surface of test pieces (No. 
1.about.No. 8) after the repetitive thermal impact test are shown in FIG. 
11 to FIG. 18. In the case of the 9 percent by weight of chromium test 
piece, as shown in FIG. 18, surface cracks and corroded grooves along the 
boundary of weld beads were caused on the test piece by the repetitive 
thermal impact test. Therefore, at least 10 percent by weight of chromium 
is necessary for resistance to thermal impact cracks and to corrosion. 
However, with large quantities of chromium, the strength and the toughness 
seen to become law as indicated in FIG. 6 to FIG. 10. Also, the effects on 
the improvement of the mechanical properties by adding cobalt are not 
observed. Hereby it is proved that no more than 14 percent by weight of 
chromium is a desirable chromium quantity. 
Next, cobalt is used for the improvement of the mechanical properties, 
especially resistance to the occurrence of surface cracks by repetitive 
heating and cooling, and resistance to roughness on the surface as shown 
in Table 2 and FIG. 11 to FIG. 18. That is to say, the roughness on the 
surface of a roll is observed to be pronounced in test pieces (FIG. 11, 
14, 17 and 18) which essentially do not contain cobalt, and in particular 
in a test piece (FIG. 18) which contains a small quantity of chromium, 
cracks were caused along weld beads. On the other hand, the occurrence of 
surface cracks is not observed in the materials containing cobalt which 
were used for the built-up layer of the present invention (FIGS. 12, 13, 
15 and 16), and also it is proved that roughness on the surface is hardly 
observed. However, as regards more than 2.0 percent by weight for the 
addition of cobalt, it can hardly be expected that the effects of adding a 
cobalt quantity as clarified in Table 2 and FIG. 6 to FIG. 10, so that 
0.5.about.2.0 percent by weight of cobalt is a sufficient cobalt quantity. 
Also 0.1.about.1.0 percent by weight of molybdenum is included for the 
purpose of improving resistance to temper brittleness. Carbon is the 
component for improving the strength of steel, but in the case of more 
than 0.10 percent by weight of carbon, its toughness becomes low and 
weldability is impaired. Therefore, the quantity of carbon is limited to 
0.1 percent by weight as the maximum.