Surface-hardened chain link

A surface-hardened chain comprises a plurality of connected chain links, each of which links is made from a killed steel having a specified chemical composition and comprises a surface-hardened layer of a high carbon tempered martensite structure and a core layer of a low carbon tempered martensite structure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a surface-hardened chain used as a load chain of 
an electric chain block or a pneumatic chain hoist, a chain of a chain 
conveyor or the like. 
2. Description of Related Art 
In this type of the chain, it is required to have higher wear resistance 
and fatigue resistance because a very large loading is applied to the 
chain and the frequency in use is high. And also, it is required to have 
higher strength and toughness because an impact load is applied to the 
chain. In the conventional chain, therefore, there has been used a 
surface-hardened chain link formed by subjecting a chain link to gas 
carburizing, quenching and tempering treatments. 
As sectionally shown in FIG. 3, such a surface-hardened chain link is 
composed of an outermost surface layer 10, a hardened layer 11 enclosed 
with the outermost surface layer 10 and having a high carbon tempered 
martensite structure, and a core portion 12 enclosed with the hardened 
layer 11 and having a low carbon tempered martensite structure. 
In the conventional surface-hardened chain, Mn-B steel (SAE15B24), Ni-Cr-Mo 
steel (JIS SNCM220, SAM8620), Ni-Mo steel (SAE4620), Ni-Cr-Nn-Mo-B steel 
(see JP-A-61-276956) and the like were generally used as a starting 
material. 
However, the surface-hardened chains made frrm these starting materials 
were insufficient in the wear resistance, fatigue resistance, strength and 
toughness as mentioned below. 
In the surface-hardened chain made from Mn-B steel, Ni-Cr-Mo steel, 
Ni-Cr-Mn-Mo-B steel or the like, oxidation at crystal grain boundary 
(intergranular oxidation) was caused in a surface layer of a chain link by 
gas carburizing, so that the wear resistance and fatigue resistance in the 
surface layer were considerably deteriorated to bring about the premature 
degradation of the surface layer and also the strength and toughness were 
poor. 
In the surface-hardened chain made from Ni-Mo steel, retained austenite was 
existent in the surface layer and hence the wear resistance and fatigue 
resistance in the surface layer were considerably deteriorated to bring 
about the premature degradation of the surface layer and also the 
toughness in the surface layer was low. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide a surface-hardened 
chain having no intergranular oxidation in its surface layer and being 
minute in austenite crystal grain size and excellent in the wear 
resistance, fatigue resistance, strength and toughness. 
According to the invention, there is the provision of a surface-hardened 
chain comprising a plurality of connected chain links each made from a 
killed steel having a chemical composition comprising C: 0.17-0.35 wt %, 
Si: 0.10-0.25 wt %, Mn: 0.40-0.80 wt %, P: not more than 0.020 wt %, S: 
not more than 0.020 wt %, Ni: 0.40-1.5 wt %, Mo: 0.15-0.60 wt %, B: 
0.0005-0.006 wt % and the balance of Fe, said chain link cmprising a 
surface-hardened layer of a high carbon tempered martensite structure and 
a core layer of a low carbon teeeered rartensite structure. 
In preferable embodiments of the invention, a surface layer portion of the 
surface-hardened layer has a metal structure having no intergranular 
oxidation, and an austenite crystal grain size of the chain link is fine, 
and a carbon content of the surface-hardened layer is a range of 0.6-0.8 
wt %, or a range of 1.0-1.3 wt %.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The reason why the chemical composition of the killed steel used in the 
chain link is limited to the above range is as follows. 
When C content is less than 0.17 wt %, the hardenability lowers and the 
strength is insufficient, while when it exceeds 0.35 wt %, the toughness 
of the tempered martensite lowers. 
As Si content becomes small, the toughness is improved, but when it is less 
than 0.10 wt %, the improving effect is not obtained, while when it 
exceeds 0.25 wt %, the toughness lowers. 
When Mn content is less than 0.40 wt %, the hardenability and strength 
lower, while when it exceeds 0.80 wt %, the oxidation at the crystal grain 
boundary (intergranular oxidation) is undesirably caused. 
As P content becomes smll, the toughness is improved, and also as S content 
becomes small, the toughness is improved, Therefore, upper limits of P 
content and S content are 0.020 wt %, respectively. Moreover, when B is 
existent at a low P content as mentioned below, the effect of preventing 
low-temperature temper embrittlement can be obtained by synergistic action 
of P and B. 
When Mo content is within a range of 0.15-0.60 wt %, the improvement of 
toughness and wear resistance is obtained. However, when it is less than 
0.15 wt %, the hardenability is not improved, while when it exceeds 0.60 
wt %, poor weld is frequently created in the formation of the chain link. 
When Ni content is less than 0.4 wt %, the hardenability is not improved, 
while when it exceeds 1.50 wt %, retained austenite is created in the 
hardened layer to cause temper embrittlement. 
When B content is less than 0.0005 wt %, the hardenability and the above 
synergistic effect are not obtained, while when it exceeds 0.006 wt %, the 
hardenability and the synergistic effect are deteriorated. According to 
the invention, when B content is within a range of 0.0005-0.006 wt %, the 
strength at grain boundary is improved without causing the intergranular 
oxidation and hence the improvement of hardenability and toughness of 
hardened layer and the synergistic effect are obtained. 
In the invention, the chain link comprising the surface-hardened layer of 
the high carbon tempered martensite structure and the core layer of the 
low carbon tempered martensite structure is advantageously obtained by 
subjecting the chain link of the killed steel to a treatment of 
carburizing--quenching--tempering or a treatment of 
carburizing--nitriding--quenching--temering. 
The carbon content of the surface-hardened layer can properly be adjusted 
in these treatments. Therefore, it is ipportant to control the carbon 
content of the surface-hardened layer to a proper range in accordance with 
the desired properties or applications. For instance, the carbon content 
of the surface-hardened layer is favorable to be 0.6-0.8 wt % in 
applications requiring the toughness or 1.0-1.3 wt % in applications 
requiring the wear resistance. 
The following examples are given in illustration of the invention and are 
not intended as limitations thereof. 
As shown in FIG. 1, a chain 4 is formed by bending a round bar of 7.1 mm in 
diameter having a chemical composition as shown in Table 1 to form a chain 
link 1 and connecting these link chains to each other at a pitch p of 21 
mm and subjecting opposed ends of a parallel portion 2 in each of the 
chain links 1 to upset butt welding to automatically form a weld part 3. 
Therefore, these chain links 1 are successively engaged with each other at 
a shoulder portion 5. In FIG. 2 is shown one chain link 1 taken out from 
the chain 4. As shown in FIG. 2, a point A of the chain link 1 or a center 
of an inner face of the shoulder portion 5 in the chain link is a position 
of creating a maxamm wearing, and a point B in the vicinity of a 
borderline between the shoulder portion 5 and the parallel portion 2 is a 
position of creating maximum tensile stress, and a point C of the chain 
link 1 or a center of an outer surface of the shoulder portion 5 is a 
position of creating a second larger tensile stress. 
TABLE 1 
__________________________________________________________________________ 
Sample Chemical composition (wt %) 
No. Classification 
C Si Mn P S Ni Cr Mo B 
__________________________________________________________________________ 
1 Mn--B steel 15B24 
0.19 
0.32 
1.41 
0.032 
0.025 
-- -- -- 0.002 
2 SNCM220 0.26 0.21 
0.85 
0.028 
0.026 
0.51 
0.63 
0.15 
-- 
SAE8620 
3 JP-A-61-276956 
0.250.22 
1.56 
0.025 
0.075 
1.56 
0.65 
0.15 
0.0008 
(Ni--Cr--Mn--Mo--B steel) 
4 SAE 4620 0.32 0.18 
0.65 
0.027 
0.025 
1.81 
-- 
0.21 
5 Ni--Mn--Mo--B steel 
0.18 
0.16 
1.0 
0.031 
0.025 
1.30 
-- 
0.12 
0.0003 
6 Acceptable steel 
0.1518 
0.45 
0.015 
0.011 
0.45 
-- 
0.20 
0.0015 
7 Acceptable steel 
0.1518 
0.45 
0.015 
0.011 
0.45 
-- 
0.20 
0.0015 
8 Acceptable steel 
0.1635 
0.62 
0.008 
0.005 
0.87 
-- 
0.51 
0.003 
9 Acceptable steel 
0.1323 
0.75 
0.001 
0.002 
0.95 
-- 
0.15 
0.002 
__________________________________________________________________________ 
Each of the above chains of Sample Nos. 1-6 is subjected to a carburizing 
treatment in a gas carburizing furnace at a carburizing temperature of 
900.degree. C. using an endothermic converted gas (mixed gas of CO, 
H.sub.2 and N.sub.2) produced from methane (natural gas) and air as a 
carrier gas and methane (natural gas) as an enrich gas, and oil-quenched 
and then tempered at 200.degree. C. The chain of Sample No. 7 is subjected 
to carburizing and nitriding at 880.degree. C. by using an endothermic 
converted gas (mixed gas of CO, H.sub.2 and N.sub.2) as a carrier gas and 
methane (natural gas) and ammonia gas (NH.sub.3) as an enrich gas, and 
oily-quenched and then tempered at 200.degree. C. The chains of Sample 
Nos. 8-9 is subjected to a gas carburizing at 930.degree. C. by using CO 
rich endothermic converted gas as a carrier gas and butane as an enrich 
gas, and oil-quenched and then tempered at 200.degree. C. 
The thus surface-hardened chains have properties as shown in Table 2, 
respectively. Moreover, the chains of Sample Nos. 1-5 (using the 
conventional steel material) have a depth of a total carburized-hardened 
layer of 0.3 mm and a surface carbon content C.sub.s in a surface layer 
portion of 0.8 wt %, respectively. 
TABLE 2 
__________________________________________________________________________ 
Stress Carbon 
Austenite 
creating 
Intergrannular 
Stress at 
Total Fatigue 
Wear 
content in 
crystal 
cracks 
oxidation in 
breakage 
elongation 
limit 
resisting 
surface layer 
Sample 
.sigma..sub.Crain size 
surface layer 
.sigma..sub.B 
at breakage 
.sigma..sub.F 
ratio 
portion 
No. Classification 
number 
(MPa) 
portion 
(MPa) 
E (%) 
(MPa) 
AW C.sub.s 
__________________________________________________________________________ 
(%) 
1 Mn--B steel 15B24 
5.2 520 presence 
805 4.3 242 0.34 
0.8 
2 SNCM220 457 4.8 
presence 
786 
4.0 
0.8 
SAE8620 
3 JP-A-61-276956 
567 5.5 
presence 
805 
4.5 
0.8 
(Ni--Cr--Mn--Mo--B 
steel) 
4 SAE 4620 
574 5.3 
absence 
4.8 
0.8 
5 Ni--Mn--Mo--B steel 
7.1 
583 
presence 
835 
8.6 
0.8 
6 Acceptable steel 
720 7.5 
absence 
12 
0.7 
(carburizing) 
7 Acceptable steel 
absence 
14 
0.6 
(carburizing- 
nitriding) 
8 Acceptable steel 
746 7.8 
absence 
14.5 
1.2 
(curburizing) 
9 Acceptable steel 
737 8.5 
absence 
14 
1.2 
(curburizing) 
__________________________________________________________________________ 
In the chains of Sample Nos. 1-3, the oxidation at crystal grain boundary 
is created in an outermost surface layer as shown in FIG. 4 and the 
quenching is insufficient and the austenite crystal grain size number is 
4.8-5.5 (the smaller the numerical value, the larger the grain size) and 
the strength and toughness are low. Furthermore, the level of the wear 
resistance AW is as low as 0.34-0.39 (the larger the numerical value, the 
better the wear resistance). And also, the fatigue limit .sigma..sub.F 
indicating the fatigue resistance is as low as 230-242 MPa (the smaller 
the numerical value, the lower the fatigue limit). Moreover, stress at 
breakage .sigma..sub.B indicating the strength 786-805 MPa and total 
elongation at breakage E indicating the toughness is 4.0-4.5%. 
In the chain of Sample No. 4, the intergranular oxidation is not existent 
in the surface layer portion, but stress creating cracks .sigma..sub.C is 
as low as 574 MPa and the total elongation at breakage E indicating the 
toughness of the hardened layer is as low as 4.8%. Furthermore, retained 
austenite is existent in the hardened layer and the wear resisting ratio 
AW indicating the wear resistance is as low as 0.381. 
In the chain of Sample No. 5, the austenite crystal grain size number is 
improved as compared with those of Sample Nos. 1-4, but the intergranular 
oxidation is existent in the surface layer portion and the other 
properties are substantially the same level as in Sample Nos. 1-4. 
The chains of Sample Nos. 6-9 correspond to examples according to the 
invention. In these examples, it is common to provide the following 
results: 
(1) There is no oxidation at crystal grain boundary (intergranular 
oxidation) in the surface layer portion. 
(2) The austenite crystal grain size number of 7.5-8.5 is obtained by the 
grain size control in the steel-making, so that the resulting crystal 
grains are fine. 
(3) The retained austenite is not existent in the surface layer portion. 
Therefore, each of the chain links according to the invention comprises a 
surface-hardened layer of high carbon tempered martensite structure and a 
core layer of low carbon tempered martensite structure because the 
intergranular oxidation as shown in FIG. 4 is not caused and hence the 
outermost surface layer 10 as shown in FIG. 3 is not existent in the 
surface layer portion. 
The properties of each of the chains of Sample Nos. 6-9 are mentioned as 
follows: 
Chain of Sample No. 6 
(1) The toughness of the surface-hardened layer is improved by the addition 
effect of B because stress creating cracks .sigma..sub.C is 720 MPa and 
total elongation at breakage E is 12%. 
When Samle No. 6 is compared with Sample No. 4, the stress creating cracks 
.sigma..sub.C is increased by 25% because .sigma..sub.C (No. 
6)/.sigma..sub.C (No. 4)=720/574=1.25. Further, the total elongation at 
breakage E in Sample No. 6 is higher by 2.5 times or more than those of 
Sample Nos. 1-4. 
(2) The wear resistance is considerably improved. 
That is, the wear resisting ratio AW indicating the wear resistance is 4.66 
times and 4.28 times of Sample Nos. 2 and 4, respectively, because AW(No. 
6)/AW(No. 2)=1.63/0.35=4.66 and AW(No. 6)/AW(No. 4)=1.63/0.381=4.28. 
(3) The fatigue resistance is improved. 
The fatigue limit .sigma..sub.B indicating the fatigue resistance in Sample 
No. 6 is higher by 1.44 times that of Sample No. 4 because .sigma..sub.F 
(No. 6)/.sigma..sub.F (No. 4)=360/250=1.44. 
(4) The stress at breakage (strength) is improved. 
The stress at breakage .sigma..sub.B in Sample No. 6 is higher by 1.12 
times than that of Sample No. 4 because .sigma..sub.B (No. 
6)/.sigma..sub.B (No. 4)=910/811=1.12. 
Chain of Sample No. 7 
The surface carbon content C.sub.S is 0.6 wt %, which is lower than the 
surface carbon content of 0.8 wt % in the conventional chains of Sample 
Nos. 1-4. This shows that the toughness becomes higher. Furthermore, the 
stress creating cracks .sigma..sub.C relating to the toughness and the 
total elongation at breakage E are improved with those of Sample Nos. 1-4. 
And also, the stress .sigma..sub.C and total elongation E are higher than 
those of Sample No. 6. 
Chains of Sample Nos. 8 and 9 
The surface carbon content C.sub.S is 1.2 wt % in Sample No. 8 and 1.0% in 
Sample No. 9, which are higher than that (0.7 wt %) of Sample No. 6. This 
shows that the wear resistance becomes higher. That is, Sample Nos. 8 and 
9 tend to be used in applications requiring higher wear resistance rather 
than the toughness by increasing the carbon content in the surface layer 
portion as compared with those of Sample Nos. 6 and 7. 
Particularly, the wear resisting ratio AW of Sample No. 8 is highest among 
those of Sample Nos. 1-9 and is higher by 4.69 times than that of Sample 
No. 5 indicating the highest wear resisting ratio among the conventional 
samples because AW(NO. 8)/AW(No. 5)=1.84/0.392=4.69. 
In FIG. 5 is sectionally shown a structure of a surface-hardened layer of 
an embodiment of the chain link according to the invention at the same 
scale as in FIG. 4 showing the structure of the conventional chain link. 
As seen from FIG. 5, in the chain link according to the invention, the 
oxidation is not caused at the crystal grain boundary, and the retained 
austenite is not existent in the surface layer portion, and the austenite 
grain size becomes fine. 
In FIG. 6 is shown a graph showing a relation between tensile stress 
.sigma. and total elongation at breakage E applied to the chain link, in 
which .sigma..sub.C is a stress creating cracks and .sigma..sub.B is a 
stress at breakage and E is represented by the following equation: 
EQU E=(1-1.sub.0)/1.sub.0 
wherein 1.sub.0 is an initial length before the application of tensile 
stress and 1 is a length after the application of tensile stress. 
In FIG. 7 is shown a graph showing results based on a fatigue test of a 
surface-hardened chain. This graph shows a relation between stress of 
loading chain .sigma. (i.e. tensile stress of fatigue limit of chain 
.sigma..sub.F) and repeat number n when tensile stress .sigma. applied to 
the chain link is varied between upper limit tensile stress .sigma..sub.U 
and lower limit tensile stress .sigma..sub.L as shown in FIG. 8. 
A curve A (.sigma..sub.F-A) shows the result of the surface-hardened chain 
according to the invention, and a curve B (.sigma..sub.F-B) shows the 
result of the conventional surface-hardened chain. 
For example, when .sigma..sub.L =50 MPa, the fatigue limit .sigma..sub.F-A 
of the surface-hardened chain according to the invention is 360 MPa, while 
the fatigue limit .sigma..sub.F-B of the conventional surface-hardened 
chain is 250 MPa. 
In FIG. 9 is shown a graph showing a relation between chain rotating number 
N and pitch wearing ratio .increment.p in a test for the wear resistance 
of a surface-hardened chain, in which a curve A shows the result of the 
surface-hardened chain according to the invention, and a curve B shows the 
result of the conventional surface-hardened chain, and N is a rotating 
number between chain links in the test (N=2m when lifting-up and 
lifting-down number of electric chain block is m), and N.sub.0 is a 
rotating number defined in the test, and .increment.p is represented by 
the following equation: 
EQU .increment.p=(p-p.sub.0)/p.sub.0 .times.100(%) 
wherein p.sub.0 is an initial pitch of the chain link and p is a pitch of 
the chain link after the test. Further, the wear resisting ratio AW is 
defined by AW=1/.increment.p, in which the larger the numerical value, the 
better the wear resistances 
When N.sub.0 is 1.times.10.sup.4 in the chain link having a diameter of 7.1 
mm and a pitch of 21 mm, the pitch after the test is 21.08 in the chain 
link according to the invention (p.sub.1) and 21.5 in the conventional 
chain link (p.sub.2). Therefore, in the curve A, .increment.p.sub.1 
=(21.08-21)/21.times.100=0.381 and A=2.63, while in the curve B, 
.increment.p.sub.2 =(21.5-21)/21.times.100=2.3 and AW=0.42. 
In FIG. 10 is shown a distribution of carbon content in a section of a 
chain link obtained by subjecting a chain link having a carbon content of 
0.23 wt %, a diameter of 7.1 n and a pitch of 21 mm to a carburizing so as 
to provide a surface carbon content C.sub.S of 0.6 wt %, for the 
application requiring the toughness or 1.1 wt % for the application 
requiring the wear resistance as an example. 
As seen fran the above, the steel of Sample No. 3 corresponding to 
JP-A-61-276956 has a drawback that the intergranular oxidation is caused 
in the surface layer portion of the chain link because relatively large 
amounts of Cr and Mn are existent in addition to B. On the contrary, the 
steel according to the invention does not cause the intergranular 
oxidation in the surface portion of the chain link because Cr is not 
existent and the Mn content is controlled to a level lower than that of 
Sample No. 3. 
As mentioned above, according to the invention, the occurrence of the 
intergaanular oxidation in the surface layer portion of the chain link 
during the carburizing, which has been observed in the conventional 
technique, can effectively be prevented and also the austenite crystal 
grain size can be made fine, so that there can stably be provided 
surface-hardened chains having excellent wear resistance, fatigue 
resistance, toughness and strength.