EP0225425B1 - Low alloy steel having good stress corrosion cracking resistance - Google Patents
Low alloy steel having good stress corrosion cracking resistance Download PDFInfo
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- EP0225425B1 EP0225425B1 EP86108534A EP86108534A EP0225425B1 EP 0225425 B1 EP0225425 B1 EP 0225425B1 EP 86108534 A EP86108534 A EP 86108534A EP 86108534 A EP86108534 A EP 86108534A EP 0225425 B1 EP0225425 B1 EP 0225425B1
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- stress corrosion
- corrosion cracking
- low alloy
- alloy steel
- crystal grain
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- 238000005336 cracking Methods 0.000 title claims description 59
- 230000007797 corrosion Effects 0.000 title claims description 58
- 238000005260 corrosion Methods 0.000 title claims description 58
- 229910000851 Alloy steel Inorganic materials 0.000 title claims description 18
- 229910000831 Steel Inorganic materials 0.000 claims description 32
- 239000013078 crystal Substances 0.000 claims description 32
- 239000010959 steel Substances 0.000 claims description 32
- 229910052710 silicon Inorganic materials 0.000 claims description 23
- 229910052698 phosphorus Inorganic materials 0.000 claims description 21
- 229910052748 manganese Inorganic materials 0.000 claims description 20
- 229910052750 molybdenum Inorganic materials 0.000 claims description 14
- 229910052758 niobium Inorganic materials 0.000 claims description 14
- 229910052796 boron Inorganic materials 0.000 claims description 12
- 239000012535 impurity Substances 0.000 claims description 12
- 229910001566 austenite Inorganic materials 0.000 claims description 11
- 229910052684 Cerium Inorganic materials 0.000 claims description 10
- 229910052804 chromium Inorganic materials 0.000 claims description 10
- 229910052719 titanium Inorganic materials 0.000 claims description 10
- 229910052721 tungsten Inorganic materials 0.000 claims description 10
- 229910052759 nickel Inorganic materials 0.000 claims description 8
- 229910052720 vanadium Inorganic materials 0.000 claims description 8
- 229910052782 aluminium Inorganic materials 0.000 claims description 6
- 229910052717 sulfur Inorganic materials 0.000 claims description 6
- 238000012360 testing method Methods 0.000 description 15
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 12
- 230000035945 sensitivity Effects 0.000 description 12
- 239000011733 molybdenum Substances 0.000 description 8
- 238000010438 heat treatment Methods 0.000 description 7
- 239000000203 mixture Substances 0.000 description 7
- 230000000694 effects Effects 0.000 description 5
- 230000002265 prevention Effects 0.000 description 5
- 229910052718 tin Inorganic materials 0.000 description 5
- 229910045601 alloy Inorganic materials 0.000 description 4
- 239000000956 alloy Substances 0.000 description 4
- 229910052789 astatine Inorganic materials 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 238000005242 forging Methods 0.000 description 3
- 230000001965 increasing effect Effects 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- 238000005496 tempering Methods 0.000 description 3
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 239000007864 aqueous solution Substances 0.000 description 2
- 238000005452 bending Methods 0.000 description 2
- 230000000052 comparative effect Effects 0.000 description 2
- 238000001816 cooling Methods 0.000 description 2
- 230000006698 induction Effects 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000008018 melting Effects 0.000 description 2
- 230000003287 optical effect Effects 0.000 description 2
- 238000013001 point bending Methods 0.000 description 2
- 238000010791 quenching Methods 0.000 description 2
- 230000000171 quenching effect Effects 0.000 description 2
- 238000007670 refining Methods 0.000 description 2
- 229920006395 saturated elastomer Polymers 0.000 description 2
- 229910000975 Carbon steel Inorganic materials 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 238000006477 desulfuration reaction Methods 0.000 description 1
- 230000023556 desulfurization Effects 0.000 description 1
- 230000002708 enhancing effect Effects 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 230000002093 peripheral effect Effects 0.000 description 1
- 238000004881 precipitation hardening Methods 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 230000009466 transformation Effects 0.000 description 1
Images
Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
Definitions
- the invention relates to low alloy steel and more specifically to nickel-chrome-molybdenum steel having good stress corrosion cracking resistance, which material is used for steam turbines or the like.
- a nickel-chrome-molybdenum steel containing vanadium i.e. a high strength steel
- a nickel-chrome-molybdenum steel containing vanadium i.e. a high strength steel
- Such a steel is obtained by adding molybdenum or vanadium which is a fine carbide deposited element to nickel-chrome high strength steel sensitive to temper embrittlement as is known whereby increasing a restraint of softening, that is, a tempering resistance at a high tempering temperature.
- This steel is well suitable for the above-described use.
- the GB-PS 1 009 924 describes steel alloys containing C, Cr, Ni, Mo, Nb, V, Mn, Si, Al, P, S, H, N and balance Fe. It is said that this material has good yield strength and tensile strength, ductility, toughness and impact resistance. However, it does not suffice with respect to the a.m. requirements with turbines. It is generally known that intergranular stress corrosion cracking occurs in carbon steels which are subjected to tensile stress and to an environment containing OH.
- the object of the invention is to provide a nickel-chrome-molybdenum steel which meets the requirements of high stress corrosion cracking resistance even under severe application condition.
- the low alloy steel of claim 1 i.e. a low alloy steel having good stress corrosion cracking resistance containing
- S is an element which greatly deteriorates hot processing characteristics, and in view of preventing cracking during hot forging, the upper limit is set to 0.030 % in claims.
- Ni and Cr are elements indispensable to an increase in strength, improvement of hardenability an enhancement in toughness. Both the elements have each to be added in the amount in excess of 0.50 %. Preferably, Ni and Cr should be added in the amount in excess of 3.25 % and 1.25 %, respectively, in order to win further improvement of hardenability and toughness.
- the contents of said elements exceed 4.00 % and 2.50 %, respectively, the transformation characteristics are greately varied, and it takes a long time for heat treatment to obtain an excellent toughness, which is therefore impractical.
- the Ni content and Cr content are limited to the range of 0.50 to 4.00 % and 0.50 to 2.50 %, respectively and 3.25 to 4.00 % and 1.25 to 2.00 %, respectively, in claims 6 and 7.
- Mo enhances the corrosion resistance of the prior y grain boundary to materially reduce the sensitivity of intergranular stress corrosion cracking, as deposited in grains as a fine carbide during the tempering and greatly contributes to prevention of temper embrittlement and increase in strength.
- more than 0.25 % of Mo must be added; but when the content thereof exceeds 4.00 %, the aforesaid effects are saturated and the toughness begins to deteriorate.
- higher addition of Mo as necessary is uneconomical.
- the Mo content is limited to the range of 0,25 % to 4.00 %.
- V is an effective element which increases the strength of steel by formation of fine crystals and precipitation hardening .
- V is added as necessary but when the content thereof exceeds 0.30 %, the effect thereof is saturated, and therefore, in the claims, the upper limit is set to 0.30 %.
- Si, P and Mn are greatly concerned in the sensitivity of intergranular stress corrosion cracking. They are important elements which should be complementarily limited in relation to the size of crystal grain and a small addition of Ti, At, Nb, W, B Ce and Sn.
- Si is an element necessary for deoxidation during refining.
- the content of Si exceeds 0.15 %, the corrosion resistance of the prior y grain boundary deteriorates and the sensitivity or intergranular stress corrosion cracking materially increases. Therefore, in the claims, the upper limit of Si is set to 0.15 %.
- P is an impurity element which is segregated in the prior y grain boundary to deteriorate the corrosion resistance and to increase the sensitivity of intergranular stress corrosion cracking and to promote temper embrittlement.
- chrome-molybdenum steel and nickel-chrome-molybdenum steel according to JIS Standards the content thereof is limited to 0.030 % or less in view of temper embrittlement .
- said content is necessary to be further limited, thus in the claims 1 to 5 the content of P is set to 0.010 % or less.
- Mn is added for deoxidation and desulfurization during refining.
- the content of Mn exceeds 0.20 %, the a.m. segregation of grain boundary is promoted and the sensitivity of stress corrosion cracking materially increased; furthermore Si and P compositely act on the stress corrosion cracking, and the range of application thereof is greatly concerned in the size of crystal grains and the small addition of Ti, At, Nb, W, B, Ce and Sn, as is demonstrated by the invention.
- the sensitivity of stress corrosion cracking also depends on the prior austenite crystal grain size, and sufficient reliability cannot be obtained even if the a.m. alloy composition should be satisfied when the ASTM crystal grain size number is smaller than 3. Accordingly in the present claim 1 the prior austenite crystal grain size number is limited to above 4 in addition to the limitation of the a.m. alloy elements.
- At, Ti, Nb, Ce, W, B and Sn are addition elements indispensable for enhancement of corrosion resistance of the prior y grain boundary and for great contribution to reduce the sensitivity of stress corrosion cracking of the grain boundary type.
- these six elements i.e., At, Ti, Nb, Ce, W and B
- more than one kind of these elements need be added in the amount of 0.001 % or more in total.
- the Nb addition set to 0.005 % or more is the most effective to reduce the stress corrosion cracking, relating to the limitations of Si + Mn + 20 P 5 0.50 %.
- the toughness is materially deteriorated.
- the total amount of addition of these elements is limited to the range of 0.001 to 0.50 %.
- similar effect to the addition of the aforesaid six elements may be obtained by addition of more than 0.003 % of Sn but when the content thereof exceeds 0.015 %, the temper embrittlement is increased to materially deteriorate the toughness.
- the content of Sn is limited to the range of 0.003 to 0.015 %.
- limitation of Mn content and/or range of Si + Mn + 20 P or limitation of size of crystal grains are necessary.
- NiCrMo steel according to the present invention contains optimum alloy elements having the excellent stress corrosion cracking resistance in the range of an optimum composition ratio and or has an appropriate microstructure (crystal grain size); and therefore, even if said steel is used for members subjected to a high load stress under the corrosion environment such as NaOH, OH- or the like, there is less possibility in producing stress corrosion cracking.
- Table 1 gives chemical compositions of sample steel used for stress corrosion cracking test and the prior y crystal grain size. These steels were produced by adjusting compositions and melting them in a high frequency induction electric furnace, thereafter making ingots, hot forging them into 25 mm thickness, heating them to a temperature for forming austenite and water quenching them, thereafter heating them up to 620 C and holding them for one hour and then cooling them at a speed of 4°C/min. The crystal grain size was variously varied by adjusting the heating temperature and its holding time. The sample steel thus produced was machined to produce a strip of testpiece of 1.5 mm thickness x 15 mm width x 65 mm length.
- Table 3 gives chemical composition of sample steel used for stress corrosion cracking test and the prior ⁇ crystal grain size.
- these steels were produced by adjusting compositions and melting them in a high frequency induction electric furnace, thereafter making ingots, hot forging them into 25 mm thickness, heating them to a temperature for forming austenite and water quenching them, thereafter heating them up to 620° C and holding them for one hour and then cooling them at a speed of 4 0 C/min.
- the crystal grain size was variously varied by adjusting the heating temperature and its holding time.
- the sample steel thus produced was machined to produce a strip of testpiece of 1.5 mm thickness x 15 mm width x 65 mm length.
- the corresponding testpiece was attached to a four-point bending constant load testing apparatus, bending stress corresponding to 60 % or 100 % of 0.2 % proof stress of the steel was applied thereto, the testpiece was immersed in 30 % NaOH aqueous solution at 150°C for one week or three weeks, and thereafter the presence of cracking and the depth of cracking of the testpiece were measured by observation with an optical microscope.
- Test III which has the most severe testing conditions, only steels corresponding to Nos. 85 to 94, that is, those which are fulfilled with Si + Mn + 20 P ⁇ 0.50 and ASTM crystal grain size number in excess of 4 have no stress corrosion cracking. Thus it is evident that this condition is the most effective embodiment to limit the prevention of stress corrosion craking.
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- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Heat Treatment Of Steel (AREA)
- Turbine Rotor Nozzle Sealing (AREA)
Description
- The invention relates to low alloy steel and more specifically to nickel-chrome-molybdenum steel having good stress corrosion cracking resistance, which material is used for steam turbines or the like.
- Materials which are used for steam turbines (operating with water vapor under high temerature and high pressure of approximately 300 °C and 70 bar) need excellent strength and toughness over a wide temperature range. Thus a nickel-chrome-molybdenum steel containing vanadium, i.e. a high strength steel, is used as a material to meet these requirements. Such a steel is obtained by adding molybdenum or vanadium which is a fine carbide deposited element to nickel-chrome high strength steel sensitive to temper embrittlement as is known whereby increasing a restraint of softening, that is, a tempering resistance at a high tempering temperature. This steel is well suitable for the above-described use.
- However, recently it has been revealed that stress corrosion cracking is frequently encountered in low pressure steam turbines and peripheral equipment which use nickel-chrome-molybdenum steel with vanadium added thereto; if this happens in nuclear power stations it is really a significant problem. This stress corrosion cracking occurs mainly in a key way where a disk and a shaft are secured together and in a joint between a blade and a disk. Probably Na in the form of impurities in the vapor is concentrated as NaOH in crevices of such portions as described above to form a crack along a grain boundary in the presence of a high load stress when the turbine is operated.
- The GB-
PS 1 009 924 describes steel alloys containing C, Cr, Ni, Mo, Nb, V, Mn, Si, Al, P, S, H, N and balance Fe. It is said that this material has good yield strength and tensile strength, ductility, toughness and impact resistance. However, it does not suffice with respect to the a.m. requirements with turbines. It is generally known that intergranular stress corrosion cracking occurs in carbon steels which are subjected to tensile stress and to an environment containing OH. - Thus the object of the invention is to provide a nickel-chrome-molybdenum steel which meets the requirements of high stress corrosion cracking resistance even under severe application condition.
- According to the invention this object is solved by the low alloy steel of
claim 1, i.e. a low alloy steel having good stress corrosion cracking resistance containing - C : ≦ 0.40 %,
- Si : ≦ 0.15%,
- Mn ≦ 0.20 %,
- P : ≦ 0.010 %,
- S : ≦ 0.030%,
- Ni : 0.50 to 4.00 %,
- Cr : 0.50 to 2.50 %,
- Mo 0.25 to 4.00 % and
- V : ≦ 0.30%,
- In order to further specify the conditions of the combination of stress corrosion cracking sensitivity with nickel-chrome-molybdenum steels and alloy-composition, micro-alloying elements and microstructures, certain sample steels modified with the aforesaid various factors have been subjected to testing the stress corrosion cracking; and the test results have been analyzed.
- Further embodiments of NiCrMo steels having excellent stress corrosion cracking resistance are as follows:
- A low alloy steel having good stress corossion cracking resistance containing
- C : ≦ 0.40%,
- Si 0.15
- Mn : ≦ 0.60%,
- P : ≦ 0.010 % ,
- S : ≦ 0.030%,
- Ni : 0.50 to 4.00 %,
- Cr : 0.50 to 2.50 %,
- Mo : 0.25 to 4.00 % and
- V : ≦ 0.30 %
and further containing at least one of the kind selected from the following groups (i) and (ii):- (i) at least one kind of Al, Ti, Nb, W, B and Ce: 0.001 to 0.50 % in total,
- (ii) Sn : 0.003 to 0.015 %,
said Si, Mn and P being fulfilled with the relationship of Si + Mn + 20 P ≦ 0,75 %, the remainder being Fe and unavoidable impurities. - A low alloy steel having good stress corrosion cracking resistance containing
- C : ≦ 0.40%,
- Si : ≦ 0.15%,
- Mn : ≦ 0.60 %,
- P : ≦ 0.010 %,
- S : ≦ 0.030
- Ni : 0.50 to 4.00 %,
- Cr : 0.50 to 2.50 %,
- Mo : 0.25 to 4.00 % and
- V : ≦ 0.30 %
and further containing at least one kind selected from the following groups (i) and (ii):- (i) at least one kind of Al, Ti, Nb, W, B and Ce: 0.001 to 0.50 % in total,
- (ii) Sn : 0.003 to 0.015 %,
the remainder being Fe and unavoidable impurities, the prior austenite crystal grain size being in excess of 4 of ASTM crystal grain size number. - A low alloy steel having good stress corrosion cracking resistance containing
- C : ≦ 0.40%,
- Si : ≦ 0.15 %,
- Mn : ≦ 0.60 %,
- P : ≦ 0.010 %,
- S : ≦ 0.030 %,
- Ni : 0.50 to 4.00 %,
- Cr : 0.50 to 2.50 %,
- Mo : 0.25 to 4.00 % and
- V : ≦ 0.30 %
and further containing at least one kind selected from the following groups (i) and (ii):- (i) at least one kind of Al, Ti, Nb, W, B and Ce: 0.001 to 0.50 % in total,
- (ii) Sn : 0.003 to 0.015 %,
said Si, Mn and P being fulfilled with the relationship of Si + Mn + 20 P ≦ 0.75 %, the remainder being Fe and unavoidable impurities, the prior austenite crystal grain size being in excess of 4 of ASTM crystal grain size number. - A low alloy steel as particularly mentioned in
claim 5 in which Si, Mn and P are in the relationship of Si + Mn + 20 P ≦ 0.50 %. - A low alloy steel as particularly mentioned in
claim 6 wherein Ni : 3.25 to 4.00 % and Cr : 1.25 to 2.00 %. - A low alloy steel having good stress corrosion cracking resistance containing
- C : ≦ 0.40 %,
- Si : ≦ 0.15%,
- Mn : ≦ 0.60 %,
- P : ≦ 0.010 %,
- S : ≦ 0.030 %,
- Ni : 3.25 to 4.00 %,
- Cr : 1.25 to 2.00 %,
- Mo : 0.25 to 4.00 % and
- V : ≦ 0.30 % and
- Nb : 0.005 to 0.50 %,
said Si, Mn and P being fulfilled with the relationship of Si + Mn + 20 P ≦ 0.50 %, the remainder being Fe and unavoidable impurities. - The specific inventive use of these steels according to
claim 8 is for steam turbines or the like where these high mechanical properties are desired. - In the following, the influences of the chemical components and ASTM crystal grain size number will be described and this demonstrates the reasons for the %-data:
- C is an element for securing the strength. However, this element increases the sensitivity of stress corrosion cracking, and when its content exceeds 0.4 %, toughness is deteriorated in relation to other alloy elements. Therefore, in claims, the upper limit is set to 0.40 %.
- S is an element which greatly deteriorates hot processing characteristics, and in view of preventing cracking during hot forging, the upper limit is set to 0.030 % in claims.
- Ni and Cr are elements indispensable to an increase in strength, improvement of hardenability an enhancement in toughness. Both the elements have each to be added in the amount in excess of 0.50 %. Preferably, Ni and Cr should be added in the amount in excess of 3.25 % and 1.25 %, respectively, in order to win further improvement of hardenability and toughness. When the contents of said elements exceed 4.00 % and 2.50 %, respectively, the transformation characteristics are greately varied, and it takes a long time for heat treatment to obtain an excellent toughness, which is therefore impractical. Thus in the
claims 1 to 5 the Ni content and Cr content are limited to the range of 0.50 to 4.00 % and 0.50 to 2.50 %, respectively and 3.25 to 4.00 % and 1.25 to 2.00 %, respectively, inclaims 6 and 7. - Mo enhances the corrosion resistance of the prior y grain boundary to materially reduce the sensitivity of intergranular stress corrosion cracking, as deposited in grains as a fine carbide during the tempering and greatly contributes to prevention of temper embrittlement and increase in strength. In order to obtain such effects, more than 0.25 % of Mo must be added; but when the content thereof exceeds 4.00 %, the aforesaid effects are saturated and the toughness begins to deteriorate. Furthermore, higher addition of Mo as necessary is uneconomical. Thus in the claims, the Mo content is limited to the range of 0,25 % to 4.00 %.
- V is an effective element which increases the strength of steel by formation of fine crystals and precipitation hardening . V is added as necessary but when the content thereof exceeds 0.30 %, the effect thereof is saturated, and therefore, in the claims, the upper limit is set to 0.30 %.
- Si, P and Mn are greatly concerned in the sensitivity of intergranular stress corrosion cracking. They are important elements which should be complementarily limited in relation to the size of crystal grain and a small addition of Ti, At, Nb, W, B Ce and Sn.
- Si is an element necessary for deoxidation during refining. When the content of Si exceeds 0.15 %, the corrosion resistance of the prior y grain boundary deteriorates and the sensitivity or intergranular stress corrosion cracking materially increases. Therefore, in the claims, the upper limit of Si is set to 0.15 %.
- P is an impurity element which is segregated in the prior y grain boundary to deteriorate the corrosion resistance and to increase the sensitivity of intergranular stress corrosion cracking and to promote temper embrittlement. In chrome-molybdenum steel and nickel-chrome-molybdenum steel according to JIS Standards, the content thereof is limited to 0.030 % or less in view of temper embrittlement . However, in order to reduce the stress corrosion cracking, said content is necessary to be further limited, thus in the
claims 1 to 5 the content of P is set to 0.010 % or less. - Mn is added for deoxidation and desulfurization during refining. When the content of Mn exceeds 0.20 %, the a.m. segregation of grain boundary is promoted and the sensitivity of stress corrosion cracking materially increased; furthermore Si and P compositely act on the stress corrosion cracking, and the range of application thereof is greatly concerned in the size of crystal grains and the small addition of Ti, At, Nb, W, B, Ce and Sn, as is demonstrated by the invention. Thus it is necessary, in terms of prevention of stress corrosion cracking in
claim 1 wherein the small addition of Ti, Ai, Nb, W, B, Ce and Sn ist not effected, to limit the content of Mn to the range of 0.20 % or less, and to strictly limit the contents so that the contents of Si, P and Mn fulfill the relationship of (Mn + Si + 20 P) < 0.30 %, i.e. the total of the weight % of contained Mn, the weight % of contained Si and 20 times of weight % of contained P is less than 0.30 %. On the other hand, it has been found from the detailed study of the influence of the microstructure in an attempt of further enhancing the reliability of sensitivity of stress corrosion cracking that the sensitivity of stress corrosion cracking also depends on the prior austenite crystal grain size, and sufficient reliability cannot be obtained even if the a.m. alloy composition should be satisfied when the ASTM crystal grain size number is smaller than 3. Accordingly in thepresent claim 1 the prior austenite crystal grain size number is limited to above 4 in addition to the limitation of the a.m. alloy elements. - The proper range of Mn content and/or Si + Mn + 20 P in
claims 2 to 5 is different from the case ofclaim 1. This results from the fact that the sensitivity of stress corrosion cracking is greatly related to the size of crystal grains and containment of small addition elements of Ti, Ai, Nb, W, B, Ce and Sn. It is possible to further enhance the reliability of the sensitivity of stress corrosion cracking by a proper combination of these various conditions. - At, Ti, Nb, Ce, W, B and Sn are addition elements indispensable for enhancement of corrosion resistance of the prior y grain boundary and for great contribution to reduce the sensitivity of stress corrosion cracking of the grain boundary type. In order to obtain such effects, among these six elements, i.e., At, Ti, Nb, Ce, W and B, more than one kind of these elements need be added in the amount of 0.001 % or more in total. Among them the Nb addition set to 0.005 % or more, is the most effective to reduce the stress corrosion cracking, relating to the limitations of Si + Mn + 20
P 5 0.50 %. However, when the total of addition exceeds 0.50 %, the toughness is materially deteriorated. Thereby, the total amount of addition of these elements is limited to the range of 0.001 to 0.50 %. On the other hand, in case of Sn, similar effect to the addition of the aforesaid six elements may be obtained by addition of more than 0.003 % of Sn but when the content thereof exceeds 0.015 %, the temper embrittlement is increased to materially deteriorate the toughness. Thus the content of Sn is limited to the range of 0.003 to 0.015 %. In order to reduce the stress corrosion cracking, however, the microalloying element addition, limitation of Mn content and/or range of Si + Mn + 20 P or limitation of size of crystal grains are necessary. Thus it is necessary to fulfill either condition Mn ≦ 0.60 % and Si + Mn + 20 P ≦ 0.75 %, and the size of the prior austenite crystal grain is above 4 of ASTM crystal grain number. The former condition and the latter condition correspond to required conditions ofclaims 2 and 3, respectively. It has been found that if both conditions are simultaneously fulfilled, the excellent stress corrosion cracking properties as ofclaims 2 and 3 may be further improved. This requirement is defined inclaim 4. And it has been found that by the provision of Si + Mn + 20 P 0.50 %, further improvement with respect to the reduction of stress corrosion cracking according toclaims claim 5. - NiCrMo steel according to the present invention contains optimum alloy elements having the excellent stress corrosion cracking resistance in the range of an optimum composition ratio and or has an appropriate microstructure (crystal grain size); and therefore, even if said steel is used for members subjected to a high load stress under the corrosion environment such as NaOH, OH- or the like, there is less possibility in producing stress corrosion cracking.
- The embodiment of
claim 1 is described. - Table 1 gives chemical compositions of sample steel used for stress corrosion cracking test and the prior y crystal grain size. These steels were produced by adjusting compositions and melting them in a high frequency induction electric furnace, thereafter making ingots, hot forging them into 25 mm thickness, heating them to a temperature for forming austenite and water quenching them, thereafter heating them up to 620 C and holding them for one hour and then cooling them at a speed of 4°C/min. The crystal grain size was variously varied by adjusting the heating temperature and its holding time. The sample steel thus produced was machined to produce a strip of testpiece of 1.5 mm thickness x 15 mm width x 65 mm length.
- In the stress corrosion cracking test the corresponding testpiece was attached to a four-point bending constant load testing apparatus; bending stress corresponding to 60% of 0.2 % proof stress of the steel was applied thereto; the testpiece was immersed in 30 % NaOH aqueous solution at 150°C for one week, and thereafter the presence of cracking and the depth of cracking of the testpiece were measured by observation with an optical microscope. The results of the stress corrosion cracking tests are shown in Table 1, and in Fig. 1 and 2. As can be seen from the Table and the figures, and steels (
Nos 1 to 24) according to the present invention have no stress corrosion cracking therein. On the other hand, comparative steels (Nos. 25 to 50) outside the claims have stress corrosion cracking of the grain boundary type. Particularly, as will be seen from Figs. 1 and 2, it is evident that no stress corrosion cracking is produced by the provision of Mn ≦ 0.20 %, Si + Mn + 20 P ≦ 0.30 %, and ASTM grain size number in excess of 4. It is found from these facts that the limitation of Mn, Si and P, the limitation of (Mn + Si + 20 P) and the limitation of crystal grain size are very effective in prevention of stress corrosion cracking. - The embodiment of
claims 2 to 5 is described. - Table 3 gives chemical composition of sample steel used for stress corrosion cracking test and the prior γ crystal grain size. Similarly to Example 1, these steels were produced by adjusting compositions and melting them in a high frequency induction electric furnace, thereafter making ingots, hot forging them into 25 mm thickness, heating them to a temperature for forming austenite and water quenching them, thereafter heating them up to 620° C and holding them for one hour and then cooling them at a speed of 40 C/min. The crystal grain size was variously varied by adjusting the heating temperature and its holding time. The sample steel thus produced was machined to produce a strip of testpiece of 1.5 mm thickness x 15 mm width x 65 mm length.
- In the stress corrosion cracking test, the corresponding testpiece was attached to a four-point bending constant load testing apparatus, bending stress corresponding to 60 % or 100 % of 0.2 % proof stress of the steel was applied thereto, the testpiece was immersed in 30 % NaOH aqueous solution at 150°C for one week or three weeks, and thereafter the presence of cracking and the depth of cracking of the testpiece were measured by observation with an optical microscope.
- The results of these tests are shown in Table 3.
- As will be apparent from the test results, the steels (Nos. 51 to 94) according to the present invention corresponding to
claims 2 and 3 includingclaims - On the other hand, even in Test II more severe conditions in terms of stress than in Test I steels (Nos. 66 to 87) corresponding to
claims - Moreover, in Test III which has the most severe testing conditions, only steels corresponding to Nos. 85 to 94, that is, those which are fulfilled with Si + Mn + 20 P ≦ 0.50 and ASTM crystal grain size number in excess of 4 have no stress corrosion cracking. Thus it is evident that this condition is the most effective embodiment to limit the prevention of stress corrosion craking.
said Si, Mn and P being fulfilled with relationship of Si + Mn + 20 P < 0.30 %, the remainder being Fe and unavoidable impurities, the prior austenite crystal grain size being in excess of 4 of ASTM crystal grain size number.
Claims (8)
said Si, Mn and P being fulfilled with relationship of Si + Mn + 20 P ≦ 0.30 %, the remainder being Fe and unavoidable impurities, the prior austenite crystal grain size being in excess of 4 of ASTM crystal grain size number.
and further containing at least one of the kind selected from the following groups (i) and (ii):
said Si, Mn and P being fulfilled with the relationship of Si + Mn + 20 P ≦ 0,75 %, the remainder being Fe and unavoidable impurities.
and further containing at least one kind selected from the following groups (i) and (ii):
the remainder being Fe and unavoidable impurities, the prior austenite crystal grain size being in excess of 4 of ASTM crystal grain size number.
and further containing at least one kind selected from the following groups (i) and (ii):
said Si, Mn and P being fulfilled with the relationship of Si + Mn + 20 P ≦ 0.75 %, the remainder being Fe and unavoidable impurities, the prior austenite crystal grain size being in excess of 4 of ASTM crystal grain size number.
said Si, Mn and P being fulfilled with the relationship of Si + Mn + 20 P ≦ 0.50 %, the remainder being Fe and unavoidable impurities.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP249707/85 | 1985-11-06 | ||
JP60249707A JPS62109949A (en) | 1985-11-06 | 1985-11-06 | Nicrmo steel having excellent stress corrosion cracking resistance |
Publications (3)
Publication Number | Publication Date |
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EP0225425A2 EP0225425A2 (en) | 1987-06-16 |
EP0225425A3 EP0225425A3 (en) | 1988-10-05 |
EP0225425B1 true EP0225425B1 (en) | 1991-08-21 |
Family
ID=17197003
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP86108534A Expired - Lifetime EP0225425B1 (en) | 1985-11-06 | 1986-06-23 | Low alloy steel having good stress corrosion cracking resistance |
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EP (1) | EP0225425B1 (en) |
JP (1) | JPS62109949A (en) |
DE (1) | DE3680995D1 (en) |
Cited By (1)
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DE102014016073A1 (en) * | 2014-10-23 | 2016-04-28 | Vladimir Volchkov | stole |
Families Citing this family (11)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPS63255344A (en) * | 1987-04-13 | 1988-10-21 | Japan Steel Works Ltd:The | Ultra-high purity turbine rotor shaft material |
US5383768A (en) | 1989-02-03 | 1995-01-24 | Hitachi, Ltd. | Steam turbine, rotor shaft thereof, and heat resisting steel |
CA2009120C (en) * | 1989-02-03 | 1998-05-12 | Masao Siga | Steam turbine, rotor shaft thereof, and heat resisting steel |
DE69217508T3 (en) * | 1991-03-20 | 2004-05-06 | Hitachi, Ltd. | Steel for armature shafts of electrical machines |
JPH04362155A (en) * | 1991-06-10 | 1992-12-15 | Japan Steel Works Ltd:The | High purity steel for high and low pressure integrated turbine rotor |
FR2748036B1 (en) * | 1996-04-29 | 1998-05-22 | Creusot Loire | LOW ALLOYED STEEL FOR THE MANUFACTURE OF MOLDS FOR PLASTIC MATERIALS |
ES2576453T3 (en) * | 2007-04-13 | 2016-07-07 | Sidenor Investigación Y Desarrollo, S.A. | Hardened and tempered steel and procedure for obtaining parts of said steel |
US20130323075A1 (en) * | 2012-06-04 | 2013-12-05 | General Electric Company | Nickel-chromium-molybdenum-vanadium alloy and turbine component |
DE102016005532A1 (en) | 2016-05-02 | 2017-11-02 | Vladimir Volchkov | stole |
JP7534102B2 (en) * | 2020-02-27 | 2024-08-14 | 日本製鋼所M&E株式会社 | NiCrMo steel and its manufacturing method |
EP4008801A1 (en) * | 2020-12-01 | 2022-06-08 | CRS Holdings, LLC | High strength, high impact toughness steel alloy, an article of manufacture made therefrom, and a method for making the same |
Family Cites Families (4)
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US3254991A (en) * | 1962-06-29 | 1966-06-07 | Republic Steel Corp | Steel alloy and method of making same |
US3438822A (en) * | 1966-10-31 | 1969-04-15 | United States Steel Corp | Method of making fine-grained steel |
DE2754524B2 (en) * | 1977-12-07 | 1980-07-03 | Jurij Fedorovitsch Leningrad Balandin | stole |
SE442024B (en) * | 1980-08-05 | 1985-11-25 | N Proizv Ob T Mas Proizv Ob Iz | STOLEN |
-
1985
- 1985-11-06 JP JP60249707A patent/JPS62109949A/en active Pending
-
1986
- 1986-06-23 EP EP86108534A patent/EP0225425B1/en not_active Expired - Lifetime
- 1986-06-23 DE DE8686108534T patent/DE3680995D1/en not_active Revoked
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Publication number | Priority date | Publication date | Assignee | Title |
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DE102014016073A1 (en) * | 2014-10-23 | 2016-04-28 | Vladimir Volchkov | stole |
Also Published As
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DE3680995D1 (en) | 1991-09-26 |
JPS62109949A (en) | 1987-05-21 |
EP0225425A2 (en) | 1987-06-16 |
EP0225425A3 (en) | 1988-10-05 |
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