Background of the invention
1. Field of the invention
[0001] The present invention is concerned with a method of manufacturing carburised steel
products with an increased hardening depth.
2. Description of the prior arts
[0002] It is generally known that the power transmitting components such as gears are heat-treated
with carburization for surface hardening considering the strength and wear durability
required by the transmitted load, where the required hardening depths are properly
designed by the transmitted loads. Particularly the increased hardening depth is required
for the large scale gear on which high loading transmission torque is acted. However,
in the conventional steel alloy materials for carburized gears, the hardening depth
is limited during carburization heat-treatment due to the material properties characterized
by the alloy elements of the steel composition, and particularly this phenomenon becomes
more apparent in the large scale gears(Ref. Fig. 1).
[0003] Fig. 1 represents the change in hardening depth with the gear diameter and quenching
rate of heat treatment.
[0004] It is noted that the hardening depth increases with the quenching rate, and under
the given quenching rate, the hardening depth decreases as the given diameter increases.
Accordingly, unless the hardening depth is good enough in case of large scale high
loading gears, the case crushing on spalling as indicated by Fig. 2 occurs.
Summary of the invention
[0005] It is an object of the present invention to provide a method of manufacturing carburized
steel products which not only increases the carbon permeability during carburization
heat treatment, but increases the hardening depth and thereby improves the strength
and wear durability of the carburized steel products by using boron treated steel
in which the hardening depth can be made deeper than in the conventional steels for
gears due to the effect of boron(B) element.
[0006] According to one aspect of the present invention, there is provided a method of manufacturing
a carburized steel product, comprising a step of a carburization heat treatment using
a boron treated steel, thereby a hardening depth is increased.
[0007] It is preferrable that the said heat treatment is performed at temperature range
925∼1050°C for at least 15 hours.
[0008] It is also preferrable that the said boron treated steel comprising, 0.18∼0.35 wt%
of C, 0.06∼0.3 wt% of Si, 0.5∼1.5 wt% of Mn, <1 wt% of Cr, <0.02 wt% of S, <0.03 wt%
of P, 0.01∼0.08 wt% of Al, <0.05 wt% of Ti, <0.012 wt% of N₂, <0.003 wt% of O₂ and
0.0005∼0.003 wt% of B.
Description of the preferred embodiments
[0009] The present invention makes use of the following characteristics of boron(B) treated
steel; (i) since the boron(B) treated steel does not comprises the expensive transition
metal elements such as Ni, Cr, and Mo, or comprises these metal elements by the minimum
amount, carbon(C) can be permeated more readily into the steel during carburization
treatment, and on the other hand, (ii) the use of boron(B) and the minimum amount
of alloying elements make it possible to increase the hardenability and hardening
depth of the carburized products.
[0010] In order to increase the hardening depth of the said boron(B) treated steel, it should
be carburized at 925 ∼ 1050 °C for the necessary time to achieve the carburisation
depth required for the final heat treated product. THis indicates that the hardening
characteristics of boron(B) treated steel is associated with the fact that carbon(C)
permeability into the surface of boron(B) treated steel increases apparently above
on a certain temperature.
[0011] If the carburization temperature is below 920°C, the carburized depth does not increase.
The carburization rate increases above 920°C. However, if the carburization temperature
exceeds 1050°C, the grains grow and become coarsend resulting in the deterioration
of the durability properties such as impact strength of the heat-treated product.
Accordingly, the carburization heat treatment should be performed in the said temperature
range of 920°C∼1050°C.
[0012] During carburization treatment, the alloying elements such as Al, Ti added by small
amounts contribute to lowering the grain growth, and the repeat quench is performed
in order to prevent further the grains from coarsening in the said temperature range.
[0013] The said repeat quench makes the coarsend grains redistributed finely, allowing the
grain growth to be affected little by the maximum carburization temperature.
[0014] The following example is submitted to illustrate the processing method and the effects
thereof but not to limited this invention.
Example
[0015] The specimens(⌀25x40mm) are made using boron(B) treated steel with chemical composition
as shown in Table 1, carburized at 930°C for the times as shown in table 2 and 3 respectively,
quenched at 850°C, and then annealed at 180°C for 2 hrs. Comparing the hardening depth
of boron treated steel with that of the conventional carburized steels used for gears,
it is noted that the hardening depth of the carburized boron treated steel is 1.5
times deeper than that of the other alloy steels. However, if the same experiments
as above is performed at 920°C, it is noted that the increase in the hardening depth
does not occur although the carburization is performed for the times as long as those
in table 3. That is, unless the carburization temperature is above 920°C, the effect
of boron treatment does not become apparent on the increase in carbon permeability
and hardening depth.
[0016] In order to confirm the said effect, the conventional alloy steel and the boron treated
steel of this invention have been heat heated at the condition as shown in table 4
with the difference in hardening depth shown in Fig. 3 and 4 respectively. As shown
in Fig. 3, it can be seen that the hardening depth of conventional carburized product(t1)
is less than that of the boron treated steel of this invention(t2).
Table 2
Carburization Temp. |
Time (hr) |
Hardening Depth(HV 513) |
|
|
SCM 21 |
SNCM 21 |
Boron Treated Steel |
930°C |
1.5 |
0.7mm |
0.7mm |
1.0mm |
3 |
0.9mm |
0.9mm |
1.3mm |
4 |
1.1mm |
1.0mm |
1.4mm |
5 |
1.1mm |
1.2mm |
1.7mm |
Table 3
Carburization Temp. |
Time (hr) |
Hardening Depth(HV 513) |
|
|
SCM 21 |
SNCM 21 |
Boron Treated Steel |
930°C |
1.5 |
0.8mm |
0.9mm |
1.1mm |
5 |
1.1mm |
0.9mm |
1.2mm |
7 |
1.6mm |
1.5mm |
1.6mm |
9 |
1.9mm |
1.7mm |
1.9mm |
Table 4
Treatment |
Temperature |
Time |
CP |
Holding |
950°C |
1Hr |
0 |
Carburization |
950°C |
12Hr |
1.0 |
Diffusion |
950°C |
10Hr |
0.8 |
Quenching |
860°C |
1Hr |
- |
Annealing |
180°C |
3Hr |
- |
[0017] As described in the above, if the steel products made of boron treated steel are
carburized at the proper temperatures, the hardening depths can be deeper than in
the conventional alloy steels resulting in the improvement of the strength of high
loading carburized products such as large scale gears and screws, particularly the
fracture strength like case crushing.
1. A method of manufacturing a carburized steel product, comprising a step of a carburization
heat treatment using a boron treated steel, thereby a hardening depth is increased.
2. The method of claim 1, wherein said heat treatment is performed at temperature range
925∼1050°C.
3. The method of claim 1, where in said boron treated steel comprising, 0.18∼0.35 wt%
of C, 0.06∼0.3 wt% of Si, 0.5∼1.5 wt% of Mn, <1 wt% of Cr, <0.02 wt% of S, <0.03 wt%
of P, 0.01∼0.08 wt% of Al, <0.05 wt% of Ti, <0.012 wt% of N₂, <0.003 wt% of O₂ and
0.0005∼0.003 wt% of B.
4. The method of claim 1, wherein further comprising a step of a repeat quenching is
performed after the carburization heat treatment at the said temperature range.
5. The carburized steel product manufactured by claims 1 , 2, 3 and 4.