Method of producing corrosion-, heat- and wear-resistant member, and the member produced

This invention relates to a method of producing a corrosion-, heat- and wear-resistant member which comprises packing an atomized powder of a high-carbon high-chromium steel into a capsule, heating the capsule packed with the powder, then extruding the capsule packed with the powder to obtain a stock not containing carbide grains greater than 3 .mu.m, working the stock, polishing a surface of the stock, and evaporating a film of TiC and/or TiN onto the polished surface, and further a corrosion-, heat- and wear-resistant member produced by the method. The corrosion-, heat- and wear-resistant member according to this invention is suitable for being members for tools used under extremely severe conditions, such as various tools for can manufacturing, molding tools for reinforced plastics, etc.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to evaporation coating of a high-carbon 
high-chromium steel with carbide or nitride of titanium. 
2. Description of the Prior Art 
As means for enhancing the wear resistance, corrosion resistance and heat 
resistance of a metallic material, there is a method of evaporating 
carbide or nitride of titanium. A coat of titanium carbide or nitride has 
an extremely high hardness (TiC: Hv 3300-4000, TiN: Hv 1900-2000) as well 
as corrosion resistance and heat resistance, and is therefore used 
particularly for reinforced plastic extruder screws, corrosion-, heat- and 
wear-resistant tools (for instance, forming and guide rolls, powder 
compacting dies, plastic working tools), etc. 
As a parent material to be coated with the carbide or nitride of titanium, 
a material having a strength suitable for making the most of the hardness 
of the coating is selected. For corrosion- and wear-resistance use, 
particularly, a high-carbon high-chromium steel such as JIS SUS440C is 
used. High-carbon high-chromium steels have sufficient strength and 
hardness for supporting the above-mentioned coating. When the high-carbon 
high-chromium steel coated with titanium carbide or nitride is applied to 
corrosion-resistance use, the steel is capable of preventing the problem 
that the coated member may become unusable due to fracture of the coating 
as a result of rapid progrss of corrosion from a broken portion of the 
coating, because the steel is not so inferior to the coating in corrosion 
resistance. Conventional materials coated with TiC and/or TiN, however, 
have difficulties in that the coating will flake or be broken relatively 
early, and are therefore not satisfactory. 
SUMMARY OF THE INVENTION 
This invention supplies a steel member provided with a TiC and/or TiN 
coating thereon and having a long life even with a high-carbon 
high-chromium steel as a parent material, and means for manufacturing such 
a steel member. 
These and other objects, advantages, features and uses will become more 
apparent as the description proceeds, when considered with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present inventors assumed the reason for the short life of the 
conventional coating to be the grain size or segregation of carbides in 
the parent material. Namely, high-carbon high-chromium steels are 
generally produced by the process of melting.fwdarw.casting.fwdarw.rolling 
(or forging), like common steels, and the steels thus produced contain a 
large number of so-called huge eutectic carbide grains segregated in such 
large size as not to be found in other steels, the huge carbide grains 
being peculiar to the high-carbon high-chromium systems. It is considered 
that since the huge eutectic carbide grains are harder than the matrix, 
upon polishing of the steel member the carbide grains protrude above the 
polished surface or the protruding carbide grains come off to form 
recesses in the polished surface, and the coating evaporated thereon also 
has recessed and protruding portions, so that the protruding portions, 
particularly, receive impacts from the opponent member brought into 
contact with the steel member, resulting in fracture. 
FIG. 2 shows an actual example of the case. FIG. 2 is a .times.1000 
micrograph of a cross section of a coat portion of a comparative member 
having a TiC+TiN double coating on a 400C parent material produced through 
melting, in which the carbide appears white in the parent material. As 
seen from the figure, huge carbide grains (about 10 to 15 .mu.m in 
diameter) protrude from the surface, lifting up the coating. There is a 
strong possibility that the huge carbide grains will be destroyed by the 
impact on collision with the opponent member. It is also possible that a 
stress based on the difference in thermal expansion coefficient between 
the carbide and the coating at the protruding portion will act on the 
coating, to accelerate fracture. 
To investigate the above-mentioned point, various tests were carried out on 
parent materials obtained by the following three processes using 
water-atomized powders and gas-atomized powders of high-carbon 
high-chromium steels. The atomized powders, solidified through rapid 
cooling, contained small carbide grains, the average diameter of the 
carbide grains being about 1 to 2 .mu.m and, even at maximum, rarely 
exceeding 3 .mu.m. In the materials produced through the process of 
melting.fwdarw.casting, on the other hand, a large number of huge carbide 
grains 10 to 30 .mu.m in diameter are present. 
The parent materials served to the tests were produced by the four 
processes: (1) melted and cast material.fwdarw.rolling, (2) 
powder.fwdarw.press compacting.fwdarw.sintering.fwdarw.HIP (hot isostatic 
pressing), (3) powder.fwdarw.capsule packing.fwdarw.HIP.fwdarw.drawing (a 
kind of forging), and (4) powder.fwdarw.capsule 
packing.fwdarw.glass-lubricated hot extrusion. The parent materials to be 
tested differing in carbide grain size were prepared by the respective 
methods, and subjected to coating with carbide or nitride to obtain 
members to be tested, which were tested. 
TABLE 1 shows the chemical compositions of the materials under test, all 
being JIS SUS440C. 
TABLE 1 
______________________________________ 
Chemical Composition of Materials under Test 
Process C Si Mn Cr Mo 
______________________________________ 
1: Method and cast 
1.03 0.30 0.35 16.43 0.41 
material 
2: Powder - 1.05 0.25 0.33 16.51 0.42 
sintering - HIP 
3: Powder - HIP - 
1.02 0.27 0.37 16.50 0.42 
rolling 
4: Powder-extrusion 
1.02 0.27 0.37 16.50 0.42 
______________________________________ 
The material of Process No. 1, namely, the parent material to be tested 
prepared from a melted material was obtained by rolling an ingot into a 
round bar steel of 35 mm diameter, finish working the bar steel to 30 mm 
diameter, and subjecting the finish-worked bar steel to a heat treatment 
(1050.degree. C..times.20 min.oil cooling.fwdarw.200.degree. C..times.1 
hr.air cooling) and abrasive finishing. 
The parent material under test of Process No. 2 was obtained by compacting 
a water-atomized powder into a shape of 35 mm in diameter by 300 mm in 
length by a rubber press, subjecting the compact to 1190.degree. 
C..times.1 hr sintering, subjecting the sintered product to an 
1150.degree. C..times.1500 atm.times.1 hr HIP treatment in Ar, and 
subjecting the thus treated material to a heat treatment and abrasive 
finishing in the same manner as in No. 1. 
The parent material under test of Process No. 3 was obtained by packing a 
nitrogen gas-atomized powder into a mild steel sheet-made capsule 150 mm 
in diameter by 500 mm in length, subjecting the packed powder to the same 
HIP treatment as in No. 2, and subjecting the thus treated product to 
drawing (a kind of forging) to obtain a round bar 35 mm in diameter and to 
the same subsequent treatment as in No. 1. 
The parent material of Process No. 4 was obtained by packing a nitrogen 
gas-atomized powder into a mild steel sheet-made capsule 150 mm in 
diameter by 600 mm in height, heating the packed powder to 1030.degree. 
C., then immediately extruding the capsule packed with the powder by a 
2000-t horizontal extrusion press to obtain a round rod 35 mm in diameter, 
and subjecting the extrudate to the same subsequent treatment as in No. 1. 
For each of the parent materials relevant to the above processes, a large 
number of parent materials to be tested were prepared, part of which were 
served to material tests on the parent materials, another part were 
provided with an about 3.5 .mu.m thick coating of TiC by the CVD (chemical 
vapor deposition) method, and the remainder were provided with a double 
coating of TiC+TiN(about 2.0+1.5 .mu.m thick) by the CVD method. The same 
finish heat treatment as that for the parent materials under test was then 
carried out to obtain members to be tested. 
On each of the members under test prepared as above, endurance life tests 
of the coating were carried out by a roller pitting method using at least 
10 specimens each. The test results are shown in TABLE 2. 
TABLE 2 
__________________________________________________________________________ 
Deflective 
Maximum strength 
carbide grain 
before coating Film 
size after 
(kgf/mm.sup.2) thickness 
Production process 
quenching and 
Radial 
Axial 
Kind of of Roller 
for tempering 
direc- 
direc- 
coating coating 
pitting life 
No. 
patent material 
(.mu.m) tion 
tion 
(CVD) method 
(.mu.m) 
B.sub.10 Life 
__________________________________________________________________________ 
1 Melting .fwdarw. casting .fwdarw. 
24 183 289 TiC 3.5 2.5 .times. 10.sup.6 
rolling cycles 
(prior art method) TiC + TiN 
2.0 + 1.5 
2.7 .times. 10.sup.6 
2 Powder (water) .fwdarw. press 
6 230 232 TiC 3.5 4.9 .times. 10.sup.6 
compacting .fwdarw. sintering 
.fwdarw. HIP TiC + TiN 
2.0 + 1.5 
5.4 .times. 10.sup.6 
(comparative method) 
3 Powder (gas) .fwdarw. capsule 
5 225 368 TiC 3.5 6.2 .times. 10.sup.6 
packing .fwdarw. HIP .fwdarw. rolling 
(comparative method) TiC + TiN 
2.0 + 1.5 
6.9 .times. 10.sup.6 
4 Powder (gas) .fwdarw. capsule 
2 298 391 TiC 3.5 1.6 .times. 10.sup.7 
packing .fwdarw. hot extrusion 
(method of this TiC + TiN 
2.0 + 1.5 
2.1 .times. 10.sup.7 
invention) 
__________________________________________________________________________ 
As shown in TABLE 2, the maximum carbide grain diameter in the parent 
material under test is 24 .mu.m for Process No. 1 with the parent material 
prepared through melting, whereas the maximum carbide grain diameters for 
Nos. 2 to 4 with the parent materials prepared from powder are extremely 
small, 6 to 2 .mu.m. Particularly, in the parent material of No. 4 
according to the method of this invention, there is observed no carbide 
grain greater than 3 .mu.m. In the parent materials of No. 2 and No. 3, 
there are observed some carbide grains 5 to 6 .mu.m in size formed through 
growth during the HIP treatment. 
As to the deflective strength of the parent materials under test, the 
parent material of No. 4 is the highest both in radial direction and in 
axial direction, the parent material of No. 3 is the second highest, and 
the parent materials of No. 1 and No. 2 are low. The low deflective 
strength (particularly in radial direction) of the parent material of No. 
1 is considered to be due to the extremely coarse carbides, as compared to 
the carbides in the other parent materials. The reason for the low 
deflective strength of the parent material of No. 2 is considered to be 
that, since Process No. 2 comprises only the sintering.fwdarw.HIP 
treatment and does not comprise rolling or forging, the density of the 
parent material is less than 100% due to the presence of pores (voids), 
which are generally observed in materials prepared by this process. In the 
case of No. 4, the absence of the pores, which are liable to be formed in 
powder products, is also a great feature. 
Referring now to the results of the roller pitting test on the coated 
members under test, thought most of in this invention, the member of 
Process No. 4 showed a long life, about 3 to 8 times those of the members 
of the other processes. The member of No. 3, with the maximum carbide 
diameter of 5 .mu.m and with the density of approximately 100% due to the 
rolling after the HIP treatment, showed a life equal to about one-third of 
the life of the member of No. 4. This shows that the critical value of the 
maximum carbide grain size associated with the generation of the 
difference in life lies in the range of 3 to 5 .mu.m. 
As has been described above, the members under test containing carbide 
grains exceeding 3 .mu.m in size, namely, the members of Process Nos. 1 to 
3 did not give satisfactory results, and only the member with a maximum 
carbide grain diameter of not more than 3 .mu.m according to Process No. 4 
gave satisfactory results. It has also been confirmed that only Process 
No. 4, comprising packing the powder into a capsule followed by hot 
extrusion, makes it possible to maintain the diameter of the carbide 
grains. Namely, when a powder is used and a method other than Process No. 
4 is used with the intension of maintaining a small carbide grain diameter 
by inhibiting aggregation or growth of the carbide grains, it is 
inevitable to reduce a thermal history applied. In that case, it is 
impossible to obtain a 100% density, and the strength of the material 
itself becomes low, resulting in that the material obtained is unsuitable 
for use as a parent material. 
FIG. 1 shows a .times.3000 micrograph of a cross section of the coating of 
the double-coated member obtained by Process No. 4 according to the method 
of this invention. In the parent material portion, there is not observed 
any carbide grain exceeding 3 .mu.m in size, and the coating is extremely 
smooth. The surface of the double-coated member is free of recessed or 
protruding portions due to huge carbide grains, which are seen in the 
coating on the parent material consisting of conventional materials of 
FIG. 2 above. 
As has been described above, it was found out that in order to deposit a 
satisfactory titanium carbide or nitride film on a high-carbon 
high-chromium steel by evaporation, the maximum diameter of chromium 
carbide grains in the parent material should be not more than 3 .mu.m. It 
was also found out that in order to obtain such a parent material, it is 
most suitable to pack an atomized powder in a capsule and subject the 
powder-packed capsule to glass-lubricated hot extrusion. This invention 
has been attained based on the findings. 
WORKING EXAMPLES 
TABLE 3 shows examples of chemical composition. 
TABLE 3 
______________________________________ 
Examples of Chemical Composition 
(wt %) 
No. C Si Mn Cr Mo V W Nb Co 
______________________________________ 
1 1.05 0.35 0.38 16.20 -- -- -- -- -- 
2 1.95 0.58 0.23 24.05 -- -- -- -- -- 
3 1.02 0.27 0.37 16.50 0.42 -- -- -- -- 
4 1.80 0.31 0.45 20.20 3.04 -- -- -- -- 
5 0.85 0.33 0.68 15.30 -- 1.01 -- -- -- 
6 1.32 0.55 0.27 17.50 -- -- -- 1.24 -- 
7 1.55 0.62 0.31 18.20 -- -- 3.01 -- -- 
8 1.88 0.60 0.28 21.05 -- -- -- -- 3.54 
9 1.75 0.45 0.35 18.03 1.00 -- -- -- 2.02 
10 1.95 0.55 0.24 23.01 -- -- 2.05 0.80 1.00 
11 1.94 0.22 0.41 17.21 1.05 0.50 -- 0.07 2.01 
12 1.95 0.30 0.35 17.55 0.98 0.49 1.01 0.06 1.45 
______________________________________ 
EXAMPLE 1 
Of a gas-atomized powder of Composition No. 3 (corresponding to SUS440C) in 
TABLE 3, a minus 35 mesh portion (about 500 .mu.m or below) was packed in 
a mild steel sheet-made capsule to prepare a billet 205 mm in diameter by 
600 mm in length. The billet was heated to 1030.degree. C., and then 
extruded by a 2000-t extrusion press at an extrusion pressure of 150 
kgf/mm.sup.2 to produce a bar stock of 65 mm diameter. The bar stock was 
worked, quenched from 1050.degree. C. and subjected to 200.degree. C. 
tempering. The thus treated bar stock was polished, coated with TiC by CVD 
and subjected to a further similar heat treatment to produce a roll. When 
the roll thus obtained was used as a guide roll for wire rod, the life of 
the roll was about 2000 hr. On the other hand, a roll produced similarly 
from a conventional material prepared through melting showed flaking of 
the coat after about 1000 hr of use as a guide roll. Thus, the member 
obtained according to this invention showed a life improved to about 2 
times as compared to the life of the corresponding member according to the 
prior art. 
EXAMPLE 2 
A gas-atomized powder of Composition No. 4 in TABLE 3 was used to produce 
the same type of roll member as in Example 1 in the same manner as in 
Example 1. When the roll member was served to the practical operation 
test, the life of the roll member was 2500 hr, about 2.5 times the life of 
the conventional roll member prudced from a melted material. This 
improvement in life is considered to arise from the improved adhesion 
between the coating and the parent material due to the extremely high 
hardness, HRC 63, of the parent material (in the case of Example 1, HRC 
59) as a result of the higher-C, higher-Cr and higher-Mo composition as 
compared to SUS440C. 
EXAMPLE 3 
A gas-atomized powder of Composition No. 10 in TABLE 3 was used to produce 
a bar stock 80 mm in diameter in the same manner as in Example 1. The bar 
stock was worked to produce an extruder screw for a reinforced plastic 
having a high silica content. The screw was coated with TiC in a thickness 
of 3.5 to 4.0 .mu.m, quenched from 1070.degree. C., and subjected to 
250.degree. C. tempering and to final polishing, to obtain a product for a 
practical operation test. The life of the screw shown upon the test was 
1.8 times the life of a screw produced from a melted SUS440C material by a 
predetermined heat treatment and the above-mentioned TiC coating 
treatment, and was 1.5 times the life of a screw formed from Ferrotic, 
which is the material frequently used for such screws. For all of the 
three kinds of extruder screws, the end of the life was due to lowering in 
the extrusion pressure as a result of wear of the screw. The longer life 
of the screw of this example as compared to the life of the screw produced 
from the melted SUS440C material is considered to be due to the 
improvements in the adhesion of the coat and in the ruggedness of the 
surface. 
EXAMPLE 4 
A gas-atomized powder of Composition No. 7 in TABLE 7 was used to produce a 
gate part (die) of a molding machine for a reinforced plastic having a 
high silica content. The production conditions were the same as in Example 
3. Upon a practical operation test, the gate part showed a life of about 
2.1 times the life of a gate part produced from a melted SUS440C material. 
While the gate part produced from the melted material came to the end of 
life through local uneven wear, the gate part obtained according to this 
invention came to the end of life through uniform wear. The difference in 
life between the two kinds of gate parts is considered to arise from the 
lowered adhesion of the coating on the conventional gate part due to the 
ununiformity of the structure of the parent material prepared through 
melting, and from a reduction in film thickness at protruding portions of 
the coating, and the resultant flaking, during the use of the conventional 
gate part. 
EXAMPLE 5 
A gas-atomized powder of Composition No. 5 in TABLE 3 was used to produce a 
bar stock under the same conditions as in Example 1. The bar stock was 
worked, quenched from 1050.degree. C., subjected to 200.degree. C. 
tempering, then polished, coated with TiC+TiN, and again subjected to heat 
treatment and polishing under the same conditions as above to produce a 
throat for a beer bottle capping machine. When served to a practical 
operation test, the throat showed a longer life of about 7 months, as 
compared to the life of about 3 months of a throat produced from a melted 
SUS440C material. The throat produced from the melted material showed 
flaking at a large number of portions of the coat, accompanying 
longitudinal flaws in the introducing portion (approach) of the throat. On 
the other hand, the throat produced according to this invention showed 
uniform wear, probably because of the absence of recessed or protruding 
portions which, if present, would give rise to stress concentration. 
EXAMPLE 6 
A gas-atomized powder of Composition No. 2 in TABLE 3 was used to produce a 
bar stock under the same conditions as in Example 1. The bar stock was 
worked, quenched from 1130.degree. C., subjected to 200.degree. C. 
tempering, then polished, coated with TiC+TiN, and again subjected to heat 
treatment and polishing under the same conditions as above to produce a 
seamer roll for can manufacturing. In consideration of the severe use 
conditions of the seamer roll and in order to make clear the difference 
between the member produced according to this invention and a member 
produced from a melted material, 12 rolls each were produced from the two 
kinds of materials, and were used as cap seamer rolls for beverage cans to 
perform a practical operation test. The rolls produced according to this 
invention were found able to seam an average of 1,500,000 cans, whereas 
the rolls produced from the melted material were found able to seam an 
average of 800,000 cans. Of the 12 rolls produced from the melted 
material, three were broken after seaming 350,000 cans, 480,000 cans and 
690,000 cans, respectively. On the other hand, none of the rolls produced 
according to this invention were broken, and each of the rolls has a life 
of at least 1,200,000 cans. An investigation of the fracture surfaces of 
the broken rolls produced from the melted material revealed that the 
origin of the fracture was coarse carbide grains 20 to 30 .mu.m in size, 
for all the three broken rolls. Besides, eight of the 12 rolls produced 
from the melted material showed partial flaking of the coating, and, as a 
result of corrosion in part of the parent material, came to the end of 
life. On the other hand, eleven of the 12 rolls produced according to the 
invention showed uniform wear, and only one of the rolls showed local 
pit-like flawing. These results indicate the excellent strength of the 
parent material in the rolls produced according to this invention as well 
as superior adhesion between the parent material and the coating. 
As has been described above, according to this invention it is possible to 
deposit a satisfactory film of TiC and/or TiN on a high-carbon 
high-chromium steel by evaporation, which has not hitherto been 
achievable. The invention was applied, by way of example, to members used 
under extremely severe conditions, such as various tools for can 
manufacturing, molding tools for reinforced plastics, etc., whereby it was 
possible to enhance the reliability and useful life of the equipment 
employing the members.