CA1103066A - High corrosion resistant and high strength medium cr and low ni stainless cast steel - Google Patents
High corrosion resistant and high strength medium cr and low ni stainless cast steelInfo
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- CA1103066A CA1103066A CA354,796A CA354796A CA1103066A CA 1103066 A CA1103066 A CA 1103066A CA 354796 A CA354796 A CA 354796A CA 1103066 A CA1103066 A CA 1103066A
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Abstract
HIGH CORROSION RESISTANT AND HIGH STRENGTH
MEDIUM Cr AND LOW Ni STAINLESS CAST STEEL
Abstract of the Disclosure The specification discloses a high corrosion resistant and high strength medium Cr and low Ni stainless cast steel and a method of preparing the same. The steel material comprises C: 0.1% or below, Si: 0.5% or below, Mn: 2.0% or below, P: 0.04% or below, S: 0.04% or below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% or below and the balance substantially of Fe, with said Mo and Cu contents in weight percentage being set to be in range of Mo+Cu: 5.0% to 7.0%; said percentages being by weight.
Further improvements in the steel are obtained by heat treatments. The resulting steel is stable and reliable in structure and performance and which is resistant to strong acids.
MEDIUM Cr AND LOW Ni STAINLESS CAST STEEL
Abstract of the Disclosure The specification discloses a high corrosion resistant and high strength medium Cr and low Ni stainless cast steel and a method of preparing the same. The steel material comprises C: 0.1% or below, Si: 0.5% or below, Mn: 2.0% or below, P: 0.04% or below, S: 0.04% or below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% or below and the balance substantially of Fe, with said Mo and Cu contents in weight percentage being set to be in range of Mo+Cu: 5.0% to 7.0%; said percentages being by weight.
Further improvements in the steel are obtained by heat treatments. The resulting steel is stable and reliable in structure and performance and which is resistant to strong acids.
Description
3~;6 The present invention relates to cast steel and to a method of producing the same. More particularly, the invention relates to medium chromium (Cr) low nickel (Ni) stainless cast steel having good corrosion resistance against strong acids, and to a method of producing the cast steel.
This application is a division of co-pending patent application Serial No. 294,192 filed on December 30, 1977.
With the recent remarkable developments in the chemi-cal industry and paper-manufacturing industry, etc., the requirement for materials having good corrosion resistance has been rapidly increasing. Although bronze has conven~
tionally been employed extensively as a reliable material having good corrosion resistance against strong acids, it has problems when required for large size facilities, due to its low allowable stress, elastic modulus and yield strength. Accordingly, martensite stainless steel of the 13 Cr group is generally employed for such purposes, while stainless steels of the 18-8 and 18-8-Mo groups are gen-erally used when subjected to strong acids. Meanwhile,in the field of stainless steel, high Cr low Ni two phase stainless steels having higher strength and corrosion re-sistance than the conventional stainless steels have been developed, and recently have been used for tubes for sea water heat exchangers, rolls for paper manufacturing, etc.
The two phase stainless steel as described above, however, has not yet been put into wide actual use, with various characteristics thereof still being left to be found. Accordingly, at the present stage, stainless steel of the 18-8 group or the 18-8-Mo group mentioned earlier is mainly used, but since stainless steels of the above ~ 3~6i~i;
kinds have an allowable stress lower than 13 Cr steel, they have not yet been brought into actual use with full confidence.
Accordingly, an essential object of the present invention is to provide a stainless cast steel of high corrosion resistance and high strength of the medium Cr low Ni group which is superior in yield strength to conventional stainless steels of the 18-8 group or lB-8-Mo group and which can be used in environments requiring contact with strong acids.
According to one aspect of the invention there is provided a high corrosion resistant and high strength medium Cr and low Ni stainless cast steel which consists ! essentially of, in weight percentage, C: 0.1% and below, Si: 1.5% and below, Mn: 2.0~ and below, P: 0.04% and below, S: 0.04% and below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% and below and the remaining portion substantially of Fe to form the material of said stainless cast steel, with said Mo and Cu contents in the weight percentage being set to be in range of Mo~Cu: 5.0% to 7.0%.
According to another aspect of the invention there is provided a method of producing a high corrosion resistant and high strength medium Cr and low Ni stainless cast steel which consists of the steps of preparing a molten material substantially of Fe, adding to said Fe material the following components: C: 0.1~ and below, Si: 1.5% and below, Mn: 2.0% and below, P: 0.04% and below, S: 0.04%
and below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5%
to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% and below in weight percentage to form the base material, setting the said Mo and Cu contents in the weight 3~
percentage to be in the range Mo + Cu: 5.0~ to 7.0% and casting the base material to form a stainless cast steel.
An advantage of the present invention, at least in preferred forms, is that it can provide a stainless cast steel of the above described type which is stable and reliable in structure and performance, and can be readily manufactured through simple processings at low cost.
These and other objects and features of the present invention will become apparent from the following descrip-tion of preferred embodiments thereof with reference tothe accompanying drawings, in which;
Fig. 1 is a graph showing the results of comparative tests between conventional steels and steels according to embodiments of the present invention in which the weight reduction due to the corrosion of the sample stainless steels maintained for six hours in boiling 5% sulfuric acid are shown; and Fig. 2 is a graph also showing the results of com-parative tests between conventional steels and steels according to the present invention in which the hydro-chloric acid density and speed of corrosion ~g/cm2/24 ~;
hours) of the stainless steel samples maintained for twenty-four hours in 3% NaCQ + MoQ HC~ solution are shown.
Referring now to the drawings, preferred embodiments of the present invention are described in detail hereinbelow.
In order to overcome the strength-wise disadvan-tages inherent in the 18-8 and 18-8-Mo group stainless steels mentioned earlier, the present inventors have carried out various studies on the characteristics of the stainless steels in question, and as a result, have developed novel stainless cast steels which are superior in yield strength to conventional ~3~ :
18-8 and 18-8-Mo group stainless steels and which are resistant to corrosion by strong acids in actual use.
Before the detailed description, it is to be noted that the invention is particularly characterized by the following points.
- In a first embodiment, the stainless steel is composed in weight % of C: 0.1% or below, Si: 1.5% or below, Mn: 2.0% or below, P: 0.04% or below, S: 0.04% or below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%, Cu: 5.0% to 7.0%, N: 0.1% or below and the remainder substantially Fe.
In a second embodiment, the material of the first embodlment i6 subjected to a solution heat treatment at temperatures at least in the region of from 900 to 1,150C.
In a third embodiment, the resultant material of the second embodiment thus subjected to the solution heat treatment is further heated to 600 to 700C with subsequent cooling.
In a fourth embodiment, the resultant material of the third embodiment is further sub;ected to a precipitation hardening treatment at temperatures of 450 to 600C.
In a fifth embodiment, the stainless steel is composed in weight % of C: 0.1% or below, Si: 1.5% or below, Mn: 2.0% or below, P: 0.04% or below, S: 0.04% or below, Cr:
17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%, Cu: 2.5%
to 5.0%, W: 0.2% to 2.0%, N: 0.1% or below and the remainder substantially Fe, while said Mo and Cu are present in a range of Mo+Cu: 5.0 to 7.0 weight %.
In a sixth embodiment, the material of the fifth embodiment is subjected to a solution heat treatment at temperatures at least in the region from 900 to 1,150C.
~3~36~ :
In a seventh embodiment, the resultant material of the sixth embodiment is further heated to temperatures of 600 to 700c with subsequent cooling, In an eighth embodiment, the resultant material of the seventh embodiment is further subjected to a precipitation - hardening treatment at temperatures of 450 to 600C.
The reasons for limiting the range of the elements as described above will be described in detail hereinbelow.
It is preferable that the amount of the element C
is as small as possible, and more than 0.1% reduces the corrosion resistance. Although the element Si improves resistance against oxidation, more than 1.5% tends to reduce the tenacity. ~n is necessary for desulfurization, but inclusion of more than 2.0% reduces the corrosion resistance.
Inclusion of more than 0.04% of the element P obstructs the welding performance, while the amount of S should preferably be as small as possible from the viewpoint of resistance a~ainst pitting and is not more than 0.04%.
While Cr, which is the important element for forming stainless steel, remarkably improves the corrosion resistance, inclusion thereof up to 17.0% is not very effective, and if !~,~
there is more than 20.0%, the tenacity is reduced.
For improving the mechanical properties and general corrosion resistance of the steel to form martensite and ferrite structures, inclusion of Ni should preferably be in the region of 3.0 to 7.0%.
The amount of Cu, known as the element for improving the corrosion resistance of stainless steel against non- -oxidizing acids, is conventionally from 0.2 to 1.3% (the solid solubility phase in the ferrite phase i5 1.25% at 840C), and if the amount exceeds the above level, the Curich phase 3~6~
(~ phase) is precipitated for precipitation hardening, thus the strength of the material is remarkably improved, although excessive precipitation expedites the development of local corrosion and is not desirable from the viewpoint of tenacity.
Accordingly, the proper amount of Cu is between 2.5 and 7.0%
and is set to be in the region from 5.0 to 7.0% in the first to fourth embodiments, taking into account the composite addition effect with respect to Mo mentioned later, and in the region from 2.5 to 5.0% in the fifth to eighth embodiments from the viewpoint of the composite addition effect with respect to Mo and the addition of W referred to later.
The element Mo, which remarkably imp~roves the resistance against local corrosion, is required to be included in at least 1.5 to 2.5%, but more than 2.5% is not preferable from the viewpoint of strength, since martensite transformation is then started at normal temperatures or at temperatures less than the normal temperature, and thus the improvement of the corrosion resistance by composite addition together with Cu becomes important, with the proper amount of Cu for optimum result being in the region of 5.0 to 7.0% as described earlier.
The element W (tungsten), which is important in the fifth to eighth embodiments, has a particular effect for improvement of the corrosion resistance against strong acids when present together with Cu, Mo, etc. When 2.5 to 5% of Cu is present, the above effect is particularly conspicuous at the weight percentages of Mo+Cu from 5.0 to 7.0% and W of 0.2 to 2.0% as is clear from Example mentioned later.
Although the element N is important for improving the resistance against pitting, the tenacity tends to be reduced if N is contained in more than 0.1%, due to the precipitation of nitrides, and therefore, the amount of N is 3~6 set at no more than 0.1%.
It should be noted here that in the foregoing description, although the reasons for limiting the range of composition are described with reference to the effects of individual elements, the present invention is not based on the mere addition of these elements, but is characterized by exhibiting effécts greater than the sum of the effects of the individual elements in corrosion resistance and yield strength through interaction of the various elemente, as is clear from the Example explained in detail hereinbelow.
Example Table 1 below shows the chemical compositions and conditions of heat treatment for samples of comparative steels and steels according to the present invention.
-1~3~
.
T
,~I V ~ ~ X '~ V
~ o ~ 8 ::C ,.~ .__ 3 _ '3 .
~ 1ol I I I ! I I I i I 1 1 1 1 1 1 1 1 1 ~.
_ _ 3 :~ l l l I ~ oooo ~
_ .~ -- t ~
. g I I I I CO 0~ 0 ~ r~ co o o a~
~ ~ i _ _ _ . .
~ ~ r ooo oooooooooooooooooo Z o o o o o o o o o o ~o o o o o o ~o o o o o _ . _ . _ _ . _ . _ . _ _ .~ ~ a~ ~:r Lr) rt ~ r l ~ o ~t rt r-l ~ o u7 u~ a~
.~ __ CO O~ r l ~ D ~ Il~ l'l ~ ~ ~ ~r ~) ~ I Ln O ~ ~ ~ 1 i~ ~ ~ ~ r l .~ ~ l ¦ ~ N rt rt rt r l r-l r-~ r I r I r-l rt ~ ~ ~ ~~l ~ ~ ~
~ . _ r~ rt ~ in r~l i~ O t~l ~ rt ~ t~ a~ O O ~ 1~ ~ CO
r~ r~ _l ~ r~ r~ r~ r~ ~ ~ ~ ~ r~ r~ ~ ~ ~
..
U~ i U~ ~ U~ ~ r7 ~
U~ O O O O O O O O O O O O O O O O O O O O O
_l O'OO OOO`OOOOOOOOOOOOOOO
__ ~n._ ___ ____ _ __ __ rt r-l ~r ~1 ~D r~ Ct~ D ~D ~ O _I r l 0~ O r~ 1 i~: .
~ P~ . ~ ~ ~ ~ O ~I O O ~ O O ~ ~0 0 0 0 .' O O O O O O O O O O O O O O O O O O O O O
~--- -- - --_ ~ a~ c~ ~ I~ o o ~ ~ ~ G~ r-l O O _I
O O O O O O ~1 _i 0 r~ O O O O O O O ~ r-i r;
._ I _ .__ __ rt ~ _ r l rt CO O O (~
O ~ -1 rt r~/ rt ~ ~ O r ~ rt _l O
O O O --I ~I r I r-i ~ r1 O O O r-l O O O r~ r~ ~i r;
'- I ---- .. _ _ __ ___ CO '~ ~D ~ 1~ CO ~ ~ ~D r~ D CO ~ ~
C~ OOO OOOOOOOOOOOOOOOOOO
l O O O O O O O O O O O O O O (~ O O O O 0 ~ l ~1 ~ ~ ~ H ~ Z
1~ ~ ~
/ .~ ~ u~ io .
I ~ ~ rt ~i / ~-1 X ~ ~ a / ~ ~ ~n , , U~
__ _ _ _.
~1~3~6 In order to assess the corrosion resistance of each of the steels in Table 1 against boiling 5% sulfuric acid, corrosion tests were carried out on a laboratory scale, in which test pieces each having the dimensions of lO~ x 30 mmQ were immersed for 6 hours in the boiling acid with subsequent - measurement of the weight reduction.
In Fig. 1, showing the amount of weight reduction in %, the corrosion resistance against acids may be said to be extremely superior at weight reductions of less than 0.06~.
From the test results for the steels AISI 304, 321 and 316 given for comparison in Fig. 1, which are generally thought to be superior in corrosion resistance, it is noticed that the samples for 304 and 321 have quite large amounts of corrosion, while the sample 316 is still subjected to an appreciable amount of corrosion, although the corrosion resistance thereof is improved to a considerable extent by the addition of Mo. On the other hand, each of the steels according to the present invention is found to be superior in corrosion resistance.
Referring also to Fig. 2, showing the speed o~
corrosion of each of the steels of Table 1 in a three %
solution of NacQ + HcQ, the results of which were obtained by short-time accelerated evaluation of the resistance against pitting by CQ concentration, the comparative steels AISI
304 and 321 had such large corrosion speeds at 0.02HcQ to 0.1 NHCQ, as the passive state thereof was difficult to maintain, i.e. they were subjected to active dissolution, while the AISI 316 steel and steels according to the present invention were in the passive state up to 0.06 NHCQ, with a consequent very slow corrosion speed. It is particularly to be noticed that the steels according to the present invention 1~3~
have a passive state which is even nore stable than the AISI
316 steel.
Table 2 below shows the effect of the precipitation hardening of Cu by the heat treatment of each of the steels in Table 1.
It is to be noted that in Tables 1 and 2, the symbols (a), (b) and (c) represent the following conditions employed in the heat treatment.
(a) Cooling by water after maintaining at a temperature of 1,050 C for 4 hours.
(b) Cooling by water after maintaining at a temperature of 1,050C for 4 hours, followed by reheating~ up to 680C with subsequent cooling with air.
(c) Cooling by water after maintaining at a temperature of 1,050 C for 4 hours, followed by reheating up to 680C with subsequent cooling by air, and further followed by reheating up to 550C with subsequent cooling in a furnace.
Table 2 Effect of the precipitation hardening of Cu by the heat treatment Classlficatlon 8eat0 2% yield strengeh kg/mm2 Comparativë AISI 304 (a)24.4 steels AISI 321 (a) 25~8 AISI316 (a)28.6 __ __ (a)38.5 B ~ (b)42.6 C (c)50.6 _ _ (a)37.4 E _ ( )40.8 F (c)49.1 . ._ Steels ofG _ (a) 35.6 H (b) 39.4 the present _ invention_ I _ (c) K (b) 46.3 L (c) 52.6 M (b) 44.7 . .
, ; (C) .... _ _ :
From the above Table 2, it can be seen that in the
This application is a division of co-pending patent application Serial No. 294,192 filed on December 30, 1977.
With the recent remarkable developments in the chemi-cal industry and paper-manufacturing industry, etc., the requirement for materials having good corrosion resistance has been rapidly increasing. Although bronze has conven~
tionally been employed extensively as a reliable material having good corrosion resistance against strong acids, it has problems when required for large size facilities, due to its low allowable stress, elastic modulus and yield strength. Accordingly, martensite stainless steel of the 13 Cr group is generally employed for such purposes, while stainless steels of the 18-8 and 18-8-Mo groups are gen-erally used when subjected to strong acids. Meanwhile,in the field of stainless steel, high Cr low Ni two phase stainless steels having higher strength and corrosion re-sistance than the conventional stainless steels have been developed, and recently have been used for tubes for sea water heat exchangers, rolls for paper manufacturing, etc.
The two phase stainless steel as described above, however, has not yet been put into wide actual use, with various characteristics thereof still being left to be found. Accordingly, at the present stage, stainless steel of the 18-8 group or the 18-8-Mo group mentioned earlier is mainly used, but since stainless steels of the above ~ 3~6i~i;
kinds have an allowable stress lower than 13 Cr steel, they have not yet been brought into actual use with full confidence.
Accordingly, an essential object of the present invention is to provide a stainless cast steel of high corrosion resistance and high strength of the medium Cr low Ni group which is superior in yield strength to conventional stainless steels of the 18-8 group or lB-8-Mo group and which can be used in environments requiring contact with strong acids.
According to one aspect of the invention there is provided a high corrosion resistant and high strength medium Cr and low Ni stainless cast steel which consists ! essentially of, in weight percentage, C: 0.1% and below, Si: 1.5% and below, Mn: 2.0~ and below, P: 0.04% and below, S: 0.04% and below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% and below and the remaining portion substantially of Fe to form the material of said stainless cast steel, with said Mo and Cu contents in the weight percentage being set to be in range of Mo~Cu: 5.0% to 7.0%.
According to another aspect of the invention there is provided a method of producing a high corrosion resistant and high strength medium Cr and low Ni stainless cast steel which consists of the steps of preparing a molten material substantially of Fe, adding to said Fe material the following components: C: 0.1~ and below, Si: 1.5% and below, Mn: 2.0% and below, P: 0.04% and below, S: 0.04%
and below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5%
to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% and below in weight percentage to form the base material, setting the said Mo and Cu contents in the weight 3~
percentage to be in the range Mo + Cu: 5.0~ to 7.0% and casting the base material to form a stainless cast steel.
An advantage of the present invention, at least in preferred forms, is that it can provide a stainless cast steel of the above described type which is stable and reliable in structure and performance, and can be readily manufactured through simple processings at low cost.
These and other objects and features of the present invention will become apparent from the following descrip-tion of preferred embodiments thereof with reference tothe accompanying drawings, in which;
Fig. 1 is a graph showing the results of comparative tests between conventional steels and steels according to embodiments of the present invention in which the weight reduction due to the corrosion of the sample stainless steels maintained for six hours in boiling 5% sulfuric acid are shown; and Fig. 2 is a graph also showing the results of com-parative tests between conventional steels and steels according to the present invention in which the hydro-chloric acid density and speed of corrosion ~g/cm2/24 ~;
hours) of the stainless steel samples maintained for twenty-four hours in 3% NaCQ + MoQ HC~ solution are shown.
Referring now to the drawings, preferred embodiments of the present invention are described in detail hereinbelow.
In order to overcome the strength-wise disadvan-tages inherent in the 18-8 and 18-8-Mo group stainless steels mentioned earlier, the present inventors have carried out various studies on the characteristics of the stainless steels in question, and as a result, have developed novel stainless cast steels which are superior in yield strength to conventional ~3~ :
18-8 and 18-8-Mo group stainless steels and which are resistant to corrosion by strong acids in actual use.
Before the detailed description, it is to be noted that the invention is particularly characterized by the following points.
- In a first embodiment, the stainless steel is composed in weight % of C: 0.1% or below, Si: 1.5% or below, Mn: 2.0% or below, P: 0.04% or below, S: 0.04% or below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%, Cu: 5.0% to 7.0%, N: 0.1% or below and the remainder substantially Fe.
In a second embodiment, the material of the first embodlment i6 subjected to a solution heat treatment at temperatures at least in the region of from 900 to 1,150C.
In a third embodiment, the resultant material of the second embodiment thus subjected to the solution heat treatment is further heated to 600 to 700C with subsequent cooling.
In a fourth embodiment, the resultant material of the third embodiment is further sub;ected to a precipitation hardening treatment at temperatures of 450 to 600C.
In a fifth embodiment, the stainless steel is composed in weight % of C: 0.1% or below, Si: 1.5% or below, Mn: 2.0% or below, P: 0.04% or below, S: 0.04% or below, Cr:
17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5% to 2.5%, Cu: 2.5%
to 5.0%, W: 0.2% to 2.0%, N: 0.1% or below and the remainder substantially Fe, while said Mo and Cu are present in a range of Mo+Cu: 5.0 to 7.0 weight %.
In a sixth embodiment, the material of the fifth embodiment is subjected to a solution heat treatment at temperatures at least in the region from 900 to 1,150C.
~3~36~ :
In a seventh embodiment, the resultant material of the sixth embodiment is further heated to temperatures of 600 to 700c with subsequent cooling, In an eighth embodiment, the resultant material of the seventh embodiment is further subjected to a precipitation - hardening treatment at temperatures of 450 to 600C.
The reasons for limiting the range of the elements as described above will be described in detail hereinbelow.
It is preferable that the amount of the element C
is as small as possible, and more than 0.1% reduces the corrosion resistance. Although the element Si improves resistance against oxidation, more than 1.5% tends to reduce the tenacity. ~n is necessary for desulfurization, but inclusion of more than 2.0% reduces the corrosion resistance.
Inclusion of more than 0.04% of the element P obstructs the welding performance, while the amount of S should preferably be as small as possible from the viewpoint of resistance a~ainst pitting and is not more than 0.04%.
While Cr, which is the important element for forming stainless steel, remarkably improves the corrosion resistance, inclusion thereof up to 17.0% is not very effective, and if !~,~
there is more than 20.0%, the tenacity is reduced.
For improving the mechanical properties and general corrosion resistance of the steel to form martensite and ferrite structures, inclusion of Ni should preferably be in the region of 3.0 to 7.0%.
The amount of Cu, known as the element for improving the corrosion resistance of stainless steel against non- -oxidizing acids, is conventionally from 0.2 to 1.3% (the solid solubility phase in the ferrite phase i5 1.25% at 840C), and if the amount exceeds the above level, the Curich phase 3~6~
(~ phase) is precipitated for precipitation hardening, thus the strength of the material is remarkably improved, although excessive precipitation expedites the development of local corrosion and is not desirable from the viewpoint of tenacity.
Accordingly, the proper amount of Cu is between 2.5 and 7.0%
and is set to be in the region from 5.0 to 7.0% in the first to fourth embodiments, taking into account the composite addition effect with respect to Mo mentioned later, and in the region from 2.5 to 5.0% in the fifth to eighth embodiments from the viewpoint of the composite addition effect with respect to Mo and the addition of W referred to later.
The element Mo, which remarkably imp~roves the resistance against local corrosion, is required to be included in at least 1.5 to 2.5%, but more than 2.5% is not preferable from the viewpoint of strength, since martensite transformation is then started at normal temperatures or at temperatures less than the normal temperature, and thus the improvement of the corrosion resistance by composite addition together with Cu becomes important, with the proper amount of Cu for optimum result being in the region of 5.0 to 7.0% as described earlier.
The element W (tungsten), which is important in the fifth to eighth embodiments, has a particular effect for improvement of the corrosion resistance against strong acids when present together with Cu, Mo, etc. When 2.5 to 5% of Cu is present, the above effect is particularly conspicuous at the weight percentages of Mo+Cu from 5.0 to 7.0% and W of 0.2 to 2.0% as is clear from Example mentioned later.
Although the element N is important for improving the resistance against pitting, the tenacity tends to be reduced if N is contained in more than 0.1%, due to the precipitation of nitrides, and therefore, the amount of N is 3~6 set at no more than 0.1%.
It should be noted here that in the foregoing description, although the reasons for limiting the range of composition are described with reference to the effects of individual elements, the present invention is not based on the mere addition of these elements, but is characterized by exhibiting effécts greater than the sum of the effects of the individual elements in corrosion resistance and yield strength through interaction of the various elemente, as is clear from the Example explained in detail hereinbelow.
Example Table 1 below shows the chemical compositions and conditions of heat treatment for samples of comparative steels and steels according to the present invention.
-1~3~
.
T
,~I V ~ ~ X '~ V
~ o ~ 8 ::C ,.~ .__ 3 _ '3 .
~ 1ol I I I ! I I I i I 1 1 1 1 1 1 1 1 1 ~.
_ _ 3 :~ l l l I ~ oooo ~
_ .~ -- t ~
. g I I I I CO 0~ 0 ~ r~ co o o a~
~ ~ i _ _ _ . .
~ ~ r ooo oooooooooooooooooo Z o o o o o o o o o o ~o o o o o o ~o o o o o _ . _ . _ _ . _ . _ . _ _ .~ ~ a~ ~:r Lr) rt ~ r l ~ o ~t rt r-l ~ o u7 u~ a~
.~ __ CO O~ r l ~ D ~ Il~ l'l ~ ~ ~ ~r ~) ~ I Ln O ~ ~ ~ 1 i~ ~ ~ ~ r l .~ ~ l ¦ ~ N rt rt rt r l r-l r-~ r I r I r-l rt ~ ~ ~ ~~l ~ ~ ~
~ . _ r~ rt ~ in r~l i~ O t~l ~ rt ~ t~ a~ O O ~ 1~ ~ CO
r~ r~ _l ~ r~ r~ r~ r~ ~ ~ ~ ~ r~ r~ ~ ~ ~
..
U~ i U~ ~ U~ ~ r7 ~
U~ O O O O O O O O O O O O O O O O O O O O O
_l O'OO OOO`OOOOOOOOOOOOOOO
__ ~n._ ___ ____ _ __ __ rt r-l ~r ~1 ~D r~ Ct~ D ~D ~ O _I r l 0~ O r~ 1 i~: .
~ P~ . ~ ~ ~ ~ O ~I O O ~ O O ~ ~0 0 0 0 .' O O O O O O O O O O O O O O O O O O O O O
~--- -- - --_ ~ a~ c~ ~ I~ o o ~ ~ ~ G~ r-l O O _I
O O O O O O ~1 _i 0 r~ O O O O O O O ~ r-i r;
._ I _ .__ __ rt ~ _ r l rt CO O O (~
O ~ -1 rt r~/ rt ~ ~ O r ~ rt _l O
O O O --I ~I r I r-i ~ r1 O O O r-l O O O r~ r~ ~i r;
'- I ---- .. _ _ __ ___ CO '~ ~D ~ 1~ CO ~ ~ ~D r~ D CO ~ ~
C~ OOO OOOOOOOOOOOOOOOOOO
l O O O O O O O O O O O O O O (~ O O O O 0 ~ l ~1 ~ ~ ~ H ~ Z
1~ ~ ~
/ .~ ~ u~ io .
I ~ ~ rt ~i / ~-1 X ~ ~ a / ~ ~ ~n , , U~
__ _ _ _.
~1~3~6 In order to assess the corrosion resistance of each of the steels in Table 1 against boiling 5% sulfuric acid, corrosion tests were carried out on a laboratory scale, in which test pieces each having the dimensions of lO~ x 30 mmQ were immersed for 6 hours in the boiling acid with subsequent - measurement of the weight reduction.
In Fig. 1, showing the amount of weight reduction in %, the corrosion resistance against acids may be said to be extremely superior at weight reductions of less than 0.06~.
From the test results for the steels AISI 304, 321 and 316 given for comparison in Fig. 1, which are generally thought to be superior in corrosion resistance, it is noticed that the samples for 304 and 321 have quite large amounts of corrosion, while the sample 316 is still subjected to an appreciable amount of corrosion, although the corrosion resistance thereof is improved to a considerable extent by the addition of Mo. On the other hand, each of the steels according to the present invention is found to be superior in corrosion resistance.
Referring also to Fig. 2, showing the speed o~
corrosion of each of the steels of Table 1 in a three %
solution of NacQ + HcQ, the results of which were obtained by short-time accelerated evaluation of the resistance against pitting by CQ concentration, the comparative steels AISI
304 and 321 had such large corrosion speeds at 0.02HcQ to 0.1 NHCQ, as the passive state thereof was difficult to maintain, i.e. they were subjected to active dissolution, while the AISI 316 steel and steels according to the present invention were in the passive state up to 0.06 NHCQ, with a consequent very slow corrosion speed. It is particularly to be noticed that the steels according to the present invention 1~3~
have a passive state which is even nore stable than the AISI
316 steel.
Table 2 below shows the effect of the precipitation hardening of Cu by the heat treatment of each of the steels in Table 1.
It is to be noted that in Tables 1 and 2, the symbols (a), (b) and (c) represent the following conditions employed in the heat treatment.
(a) Cooling by water after maintaining at a temperature of 1,050 C for 4 hours.
(b) Cooling by water after maintaining at a temperature of 1,050C for 4 hours, followed by reheating~ up to 680C with subsequent cooling with air.
(c) Cooling by water after maintaining at a temperature of 1,050 C for 4 hours, followed by reheating up to 680C with subsequent cooling by air, and further followed by reheating up to 550C with subsequent cooling in a furnace.
Table 2 Effect of the precipitation hardening of Cu by the heat treatment Classlficatlon 8eat0 2% yield strengeh kg/mm2 Comparativë AISI 304 (a)24.4 steels AISI 321 (a) 25~8 AISI316 (a)28.6 __ __ (a)38.5 B ~ (b)42.6 C (c)50.6 _ _ (a)37.4 E _ ( )40.8 F (c)49.1 . ._ Steels ofG _ (a) 35.6 H (b) 39.4 the present _ invention_ I _ (c) K (b) 46.3 L (c) 52.6 M (b) 44.7 . .
, ; (C) .... _ _ :
From the above Table 2, it can be seen that in the
2% yield strength, although the comparative AISI 304, 321 and 316 steels have extremely low values in the region of 24 to 27 kg/mm J the steels of the present invention each have high values. More particularly, the heat treatment conditions (b) show higher effects than those of (a), while the heat treatment conditions (c) show also higher effects than those of (b).
Table 3 below shows the 0.2% yield strength in kg/mm2 of the stainless steels of the present invention without any heat treatment (i.e., in the state as they are cast), and it can be seen from Table 3 that the steels of the present invention are superior to the comparative steels in this respect also.
Table 3 Yield strengths of steels of the present invention without heat treatment .. .. .
Classification 0.2% yield strength Kg/mm _ ..... ._ .. , .
D' 36.8 , . ... ___ Steels of the G' 35.0 present J' 38.7 invention _ M' 39.1 ~;
__ _ _ -As ls clear from the foregolng description, although the strength increase in the steels according to the present invention is mainly attributable to the inclusion of Cu in the amounts of the predetermined range and also to the specific heat treatment, the effect is particularly conspicuous in the steels subjected to the heat treatment under the conditions (c) mentioned earlier, i.e., a solution heat treatment at a temperature of 900 to 1,150C, heating to a temperature from 600 to 700C with subsequent cooling, and further a precipitation hardening treatment at temperatures of 450 to 600C. In the above case, the reasons for limiting the temperatures for the .
~1~3~6 ' second treatment to 600 to 700C are that the martensite transformation rate of the steels according to the present invention (temperatures for starting counter-transformation are in the region of 700 to 750C) in the first solid solution heat treatment is 80 to 85~, and that the rate of martensite formation is much improved by heating to the temperature immediately below the above counter-transformation temperature with subsequent cooling after the first heat treatment mentioned above. The temperatures are specified to be from 600 to 700C, because such temperature range is best suited for the purpose. ~'-Steels including elements in the composition range of the stainless cast steel according to the present invention and those further sub~ected to the heat treatment are much superior in yield strength than the conventional stainless steels, and thus practical stainless steels having stable corrosion resistance against strong acids, especially in the chemical industry, paper manufacturing industry, etc., are advantageously;produced. The steels of the present invention which are particularly suitable for use in suction roll shells for paper manufacturing can also be used for any industrlal components and parts which require the varlous characteristics t'~j as described in the foregoing.
Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the ar~. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention as defined by the appendant claims, they should be construed as included therein.
Table 3 below shows the 0.2% yield strength in kg/mm2 of the stainless steels of the present invention without any heat treatment (i.e., in the state as they are cast), and it can be seen from Table 3 that the steels of the present invention are superior to the comparative steels in this respect also.
Table 3 Yield strengths of steels of the present invention without heat treatment .. .. .
Classification 0.2% yield strength Kg/mm _ ..... ._ .. , .
D' 36.8 , . ... ___ Steels of the G' 35.0 present J' 38.7 invention _ M' 39.1 ~;
__ _ _ -As ls clear from the foregolng description, although the strength increase in the steels according to the present invention is mainly attributable to the inclusion of Cu in the amounts of the predetermined range and also to the specific heat treatment, the effect is particularly conspicuous in the steels subjected to the heat treatment under the conditions (c) mentioned earlier, i.e., a solution heat treatment at a temperature of 900 to 1,150C, heating to a temperature from 600 to 700C with subsequent cooling, and further a precipitation hardening treatment at temperatures of 450 to 600C. In the above case, the reasons for limiting the temperatures for the .
~1~3~6 ' second treatment to 600 to 700C are that the martensite transformation rate of the steels according to the present invention (temperatures for starting counter-transformation are in the region of 700 to 750C) in the first solid solution heat treatment is 80 to 85~, and that the rate of martensite formation is much improved by heating to the temperature immediately below the above counter-transformation temperature with subsequent cooling after the first heat treatment mentioned above. The temperatures are specified to be from 600 to 700C, because such temperature range is best suited for the purpose. ~'-Steels including elements in the composition range of the stainless cast steel according to the present invention and those further sub~ected to the heat treatment are much superior in yield strength than the conventional stainless steels, and thus practical stainless steels having stable corrosion resistance against strong acids, especially in the chemical industry, paper manufacturing industry, etc., are advantageously;produced. The steels of the present invention which are particularly suitable for use in suction roll shells for paper manufacturing can also be used for any industrlal components and parts which require the varlous characteristics t'~j as described in the foregoing.
Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the ar~. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention as defined by the appendant claims, they should be construed as included therein.
Claims (8)
1. A high corrosion resistant and high strength medium Cr and low Ni stainless cast steel which consists essentially of, in weight percentage, C: 0.1% and below, Si: 1.5% and below, Mn: 2.0% and below, P: 0.04% and below, S: 0.04%
and below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5%
to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% and below and the remaining portion substantially of Fe to form the material of said stainless cast steel, with said Mo and Cu contents in the weight percentage being set to be in range of Mo+Cu: 5.0% to 7.0%.
and below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5%
to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% and below and the remaining portion substantially of Fe to form the material of said stainless cast steel, with said Mo and Cu contents in the weight percentage being set to be in range of Mo+Cu: 5.0% to 7.0%.
2. A steel as claimed in claim 1, wherein said material of said stainless cast steel has been subjected to solu-tion heat treatment at temperatures in range from 900 to 1,150°C.
3. A steel as claimed in claim 2, wherein said material has been further subjected to heating to temperatures from 600 to 700°C with subsequent cooling.
4. A steel as claimed in claim 3, wherein said material has been further subjected to a precipitation hardening treatment at temperatures in range from 450 to 600°C.
5. A method of producing a high corrosion resistant and high strength medium Cr and low Ni stainless cast steel which consists of the steps of preparing a molten material substantially of Fe, adding to said Fe material the following components: C: 0.1% and below, Si: 1.5% and below, Mn: 2.0% and below, P: 0.04% and below, S: 0.04%
and below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5%
to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% and below in weight percentage to form the base material, setting the said Mo and Cu contents in the weight percentage to be in the range Mo + Cu: 5.0% to 7.0% and casting the base material to form a stainless cast steel.
and below, Cr: 17.0% to 20.0%, Ni: 3.0% to 7.0%, Mo: 1.5%
to 2.5%, Cu: 2.5% to 5.0%, W: 0.2% to 2.0%, N: 0.1% and below in weight percentage to form the base material, setting the said Mo and Cu contents in the weight percentage to be in the range Mo + Cu: 5.0% to 7.0% and casting the base material to form a stainless cast steel.
6. A method as claimed in claim 5, further including the step of subjecting said base material to a solution heat treatment at temperature in range from 900 to 1,150°C.
7. A method as claimed in claim 6, further including the step of heating said base material having been subjected to said solution heat treatment, to temperatures from 600 to 700°C, with subsequent cooling.
8. A method as claimed in claim 7, further including the step of subjecting said base material having been subjected to said heating with the subsequent cooling, to a precipitation hardening treatment at temperatures in range from 450 to 600°C.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA354,796A CA1103066A (en) | 1977-06-30 | 1980-06-25 | High corrosion resistant and high strength medium cr and low ni stainless cast steel |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP79195/1977 | 1977-06-30 | ||
| JP7919577A JPS5413414A (en) | 1977-06-30 | 1977-06-30 | Medium cr low ni stainless cast steel of high corrosion resistance and high strength |
| CA294,192A CA1097949A (en) | 1977-06-30 | 1977-12-30 | High corrosion resistant and high strength medium cr and low ni stainless cast steel |
| CA354,796A CA1103066A (en) | 1977-06-30 | 1980-06-25 | High corrosion resistant and high strength medium cr and low ni stainless cast steel |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1103066A true CA1103066A (en) | 1981-06-16 |
Family
ID=27165444
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA354,796A Expired CA1103066A (en) | 1977-06-30 | 1980-06-25 | High corrosion resistant and high strength medium cr and low ni stainless cast steel |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1103066A (en) |
-
1980
- 1980-06-25 CA CA354,796A patent/CA1103066A/en not_active Expired
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