US3650709A

US3650709A - Ferritic, austenitic, martensitic stainless steel

Info

Publication number: US3650709A
Application number: US871396A
Authority: US
Inventors: Lars Johan Gustaf Morsing
Original assignee: Avesta Jernverks AB
Current assignee: Outokumpu Stainless AB
Priority date: 1965-06-22
Filing date: 1969-10-13
Publication date: 1972-03-21
Anticipated expiration: 1989-03-21
Also published as: GB1080886A; DE1533158B1

Abstract

Rollable and weldable stainless steel with a high degree of tension and toughness within a wide range of temperatures both in a hardened and a tough-hardened condition comprising 5-15 percent by weight of ferrite, 10-40 percent by weight of austenite and the remainder tempered martensite.

Description

United States Patent Morsing 1451 Mar. 21,1972

[54] FERRITIC,AUSTENITIC, [56] References Cited MARTENSITIC STAIN LESS STEEL UNITED STATES PATENTS [721 Invent f Avesm 2,602,737 7/1952 Bindel ....75/l28N e 2,851,384 9/1958 Waxweiler... ....75/I28N [73] Assignee: Avesta Jernverks Aktiebolag, Avesta, 3,102,025 8/1963 Wilcox "-75/l28N Sweden 3,253,966 5/1966 Malagari... ...75/12s N 3,276,864 10/1966 Loginow ..75/l28N 22 F1led: 061.13, 1969 3,306,736 2/1967 Rundell ..75/l28N 21 i 51196.; 871,396

Related US. Application Data Continuation-impart of Ser. No. 856,872, July 3, 1969, which is a continuation of Sen No. 558,998, June 20, 1966, abandoned.

Primary Examiner-Hyland Bizot Attorney-Brumbaugh, Graves. Donohue & Raymond ABSTRACT 3 Claims, 1 Drawing Figure atented arch 21, 1972 3,650,709

NTOR

HAN 6U5TAF NURSING 1 LA R i L his ATTORNEYS FERRHTHC, AUSTENITHC, MARTENSITIC STAINLESS STEEL This is a continuation-in-part of my copending application Ser. No. 856,872 filed July 3, 1969 which is in turn a continua- Uliimate strength kgJmm. Hardness Brincll The Swedish Standards Association However, the weldability of steels of this type is not as good, since they are sensitive to grain growth. In welding, the'heatmin 60 max 260 non of my apphcann. N 558998 filed June 1966 actuated zone becomes coarse-grained, a circumstance which now abandoned. This invention relates to rollable and weldarenders the steels brittle ble stainless steel with a high tension and toughness within a The Steel accordin the resent invention is charac wide range of temperature both in a hardened and a toughterizedin that com rgises p hardened condition and to products produced from the steel. p Among the products welded constructions can especially be o 020 0 040 Emmy 0 g o 040 Carbon mentioned, e.g., pressure vessels where it is desirable to gummy silicon reduce the thickness of the material by using a material with a 0.2 2.0 preferably 0.6 1.0 Manganese high strength. The present invention relates further to a stainpekrab'y 'f less sheet iron material with a high strength, good corrosion 3 I g g; gtzl resistance anda low coeff cient of thermal expansion for use 0'02 preferably Nmogcn as Press platens. m h proces,s of pres.smg wanboards and the balance being iron and the contents of the alloying materilamlnates (formica). Finally, wire and strip products may also als mutually adapted in Such a way that the chromium memiond for example as intended to be used as equivalent plus the nickel equivalent are between 24.5 and z mate.r.lalS P of i 2 26.2 and 1.4 times the chromium equivalent minus the nickel usteilltlc stainless steels are mainly used for welded constructions of steel resistant to corrosion. These steels are nor- 2221; 3 :255???2223;"; fs rs i g g fi3: 2:52: mall known as 18-8 18 ercent Cr, 8 ercent Ni 1 -10 (18 gercem Cr, 10 peremgny pacer) 10 equivalent percent Ni +0.5 percent Mn 30 X (percent C pace", Ni plus about percent Mo), 2542 (25 percent percent N); as a result after the hardening the structure con- Cr 12 percfim Ni) 25 20 (25 percent Cr 20 percent Ni) etc. talns martensite, ferrite and austenite. From the analytic and They are characterized by a very good toughness, go structural point of view the steel may be defined as a transfortimy and Very good weldabmty. In comparison to umallowd mation form between an austenltlc and a ferrlte-martensitlc and low-alloyed steel of a ferrite-carbide structure (such as Steel V ferrite plus perlite, bainite, tempered martensite) they have a The Phase FomPosmon P the alloys fi to the low strength. The yield limit (0.2-limit) is particularly low, prese m q 15 shown m graphlcal form m the moon" in general below 25 kgJmmz' Owing to this circumstance a iii t l i e ii izgi fii the chromium equivalent is given as the ab construction exposed to mechanical stresses, e. a ressure vessel of austenitic stainless steel, often has an u necozomical Sciss? and the nlckel q a nt as he Otdtnate. The alloys acthickness. In order to reduce the stainless steel portion it is cordmg the P mvemlon he the fiashed area therefore sometimes necessary to make use of a stainless lin- ABCD, bemg Wlthm the three-Phase areal austemte P ing Combined with certain highustrength un a1]oyed or 1 1 tensite plus ferrite. Owing to this condition the structure of the loyed steels or to so-called compound steel, a method which in alloys will after rolling and hardening at above C and both cases leads to certain technical difficulties, e.g., welding pering a! 5 0-630 C., contain 5-15 percent f r problemg In pite of their low strength, however, {he 0 1H0 percent austenite, and the remainder tempered maraustenitic steels are very widely utilized owing to their great tensltei toughness, good ductility and particularly because they are The Steel is heat'n'ealed y means of hardening at more easy to weld. than 900 C. and subsequent tempering within a temperature In cases when greater strength is required and the demands range of500630 C. At the hardening temperature the strucon corrosion resistance are moderate certain stainless steels ture consists of austenite plus 5 to 15 percent ferrite. In the have up to now been used, their high strength being obtained process of cooling to room temperature or below no transforby means of a high carbon content. This refers to so-called mation of the ferrite takes place, while the austenite is transmartensite steels which reach their high strength in connecformed into martensite. However, a few per cent of the tion with hardening and tempering. In these steels chromium austenite remain untransformed. is the main alloying element. As an example some stainless By heating the hardened steel up to 500-630 C. a tempersteels standardized in Sweden having 13-17 percent of ing of the martensite and an equalization of the hardening chromi m m y e m n ione stresses is obtained, which is normal for toughened steel, and

Yield Ultimate Condi- Carbon Chromium Nickel limit strength Hardness tion (percent) (percent) (percent) kg/ntnf kglmrri Brinell 4 0.ls0.25... min 12.0.. min 70 90-105....... 270-320 -3 0.30-0.40... min 13.0.. min 150...... *455-625 -3 max 0.25.... min l6........2.5-3.0..... min 65...... 85l00....... 250-300 *Vickers. The figures refer to the condition according to the standards. ***The Swedish Standards Association.

The drawback re ardin these steels is the hi h carb c ntent which means %hat th e steels grow sensitife to cr i ks in toughiless of the Stee} 'f However what dlsEmguishes this type of steel is that in the process of tempering welding slmie the whlch 15 Created m the heatac' within this range of temperature austenite is also created tuated zone is hard and brittle. In order to avoid the formation wh.ch stable also at low tem eramres This creation of of cracks in these steels it is necessary to pre-heat them in the bl I it incr Ses with anpincreasea tem in {em ff of weidling p Fi f the g tui e ii; :2 aTaout 230 C If heated up to high r IBEIPG'JZUICS ere are aso errlte austenitic stain ess stees aving a pomparativglyiigh yield limit such as, for example, the folowmg Stan at :21 2324 ing the temperature and the length of time of the tempering Carbon m x .1 different levels of strength having yield limits varying between 2:1 50 and 80 kg./mm. are obtained. A longer tempering time Molybdenum 1% LL13 75 reacts in the same direction as an increase in the tempering Yield limit kgJmm. min 42 temperature.

In spite of the low carbon content the steel has a high strength. It differs from the martensite steels having a high carbon content by being easy to weld. The martensite which is created in the heat-actuated zone is tough owing to the low carbon content of the steel. Consequently, this zone is not embrittled which it is as regards the chromium steels with a high carbon content. The austenite which is contained in the steel also contributes to the toughness of the heat-actuated zone. Consequently, the steel is not sensitive to cracks in the process of welding and can be welded without pre-heating. In order to obtain maximum toughness characteristics it is suitable to anneal after the welding within a temperature range of 500-630 C. This produces an equalization of the welding stresses and a tempering of the martensite in the hardening zone. The annealing temperature that should be chosen depends on the temperature used in the tempering in connection with the tough hardening of the steel.

A characteristic feature of this steel is also that it has a certain content of ferrite. As regards hardable steels having a high content of carbon the problem has been how to adapt the analysis in such a way that ferrite is avoided. It is a well-known fact that ferrite in the martensitic steels with a high content of carbon lowers the toughness, which is the reason why the content of ferrite-forming substances must be maintained at a low rate when high toughness is required. If, on the other hand, the content of carbon is low, it has been proved that a good toughness of martensitic steels is obtained in spite of the existence of ferrite in the structure.

Thanks to this it has been possible to keep the content of chromium high at about 16 percent, and it has also been possible to add about 1 percent molybdenum. This means a considerably higher resistance to corrosion as compared with the results obtained with the previously known ferrite-free steels, whose content of chromium must be limited to about 14 percent if there is no molybdenum, and to about 13 percent if about 1 percent molybdenum is desired. Since the steel according to the present invention is hardenable it differs from the ferrite containing steels according to the French Patent specification No. 803.361 having 0.020-about 0.300 percent C, 1.3-3.0 percent Mn, 1.5-6.5 percent Ni, 16.0-23.0 percent Cr and possibly up to about 2.5 percent Cu or 3 percent M or 3 percent W. In these steels with their composition so adapted that two phases, ferrite plus austenite are obtained, the object of using ferrite is to reduce the sensitivity to inter-granular corrosion characteristic for austenitic steels having a high content of carbon.

The high toughness of the steel is also made obvious by the circumstance that the curve for impact work at various temperatures is rectilinear without any marked transition from a high to a low value and with a transition temperature below l96 C. As the transition temperature at impact tests it is usual to indicate the temperature at which the curve for impact work versus the temperature for an impact test specimen with an area of 10 X 10 mm?, a V-indication 2 mm. deep and a support distance of 45 mm. passes the value 2.8 kg. For welded constructions, especially for pressure vessels, it is desirable to use material with a low transition temperature; it is often prescribed that the transition temperature shall lie below 0 C. or 20 C. At 196 C. the present steel shows an impact work of about kg., which is considerably above 2.8 kg., a value which is regarded as a criterion of the transition temperature.

The steel is also characterized in that the high temperature yield limit up to 300-400 C. is only slightly less than the yield limit at room temperature.

The steel has a good resistance to irradiation with neutrons in a nuclear reactor, and it has even after being irradiated to neutron doses of 3X10 neutrons/cm. 1 mev.) a transition temperature for impact tests below 80 C.

The steel has a low coefficient of expansion. At 20100 C. it is 12 X it is thus of the same magnitude as unalloyed steel and considerably lower than the figures for austenitic steel.

EXAMPLE 1 5 Sheet iron havin a thickness of 14 mm. was roduced from g a steel having the following composition:

Percent 10 P 0.011 5 0.005 Cr 16.0 Ni 4.7 Mo 1.02 N 0.03s

the remainder consisted of iron and unintentional impurities.

The sheet iron was heated up to 900 C., which temperature was maintained for 1 hour; it was then hardened in air and tempered at 600 C., the latter temperature being maintained for 6 hours. After testing the sheet iron heat-treated in this manner the following strength values were obtained:

Yield limit, 0.2-point" 60 kgJmm. Ultimate Strength 91 kgJmm. Elongation, S X d 20% Impact work, 20 kv. 7.2 kg.

- Hardness, Brinell 278 At a higher temperature the following strength values were obtained:

100C. 200c. 300C. 400C. 500 c.

Yield limit,

kgJmm. 60 so 60 s9 51 Ultimate strength,

kgJmm. 84 so 79 13 63 Elongation,

s x d 2o l8 l6 14 14 Impact work at different temperatures:

Testing temperatures l96C. 74C. 20 c. 0C. +20" c.

Impact work,

kv., kg. 5.3 6.3 6.7 7.1 1.2

The sheet iron was welded by means of coated electrodes having a similar composition. No cracks were noticed either in the weldings or in the zone actuated by the welding heat.

Example 2 70 mm sheet iron of the same charge as that used in Example l was hardened in air after being heated up to 900 C. for 2 hours and tempered at 600 C. for 6 hours. The following strength values were obtained:

Yield limit 64 kgJmm. Ultimate strength 92 kgJmm. Elongation, 5 X d 18% Impact work, kv. 7.3 kg.

EXAMPLE 3 A forging round with a diameter of 170 mm. and 250 mm. in

length was heat-treated by heating up to 900 C. for 4 hours subsequent cooling in air and tempering at 575 C. for 6 hours.

The composition of the steel in percent by weight was:

C Si Mn P S Cr Ni Mo N 0.035 0.30 1.00 0.035 0.025 16.5 4.7 1.08 0.030

V 7 'llhe f ollowing strength values were obtained:

Contraction,

Impact work at different temperatures:

Testing temperature -l96 C. -74 C. -20 C. C. +20 C. Impact work,

kv., kg. 6.2 11 l2 13 14 EXAMPLE 4 From 70-mm. sheet iron of the same charge as that in Example 1 tensile and and impact test specimen were taken which were emitted in a nuclear reactor at 240260 C.

Strength tests prior to and after the irradiation resulted in the following values:

Quantity of Irradiation Yield Ultimate Elonganeutrons/cm. Temperature limit strength tion 7:

1 mev.) kgJmmFkg/mm. Unirradiated 66 88 18 5-10 265 C. 90 102 17 Transition Temperature for Impact Tests Impact test specimen 3 X 3 mm. T 0.17 is the temperature at which impact work 0.17 kg. is obtained and corresponds approximately to the temperature T 2.8, at which impact work 2.8 kg. is obtained for a normal test specimen.

Quantity of Irradiation neutrons/cm. Temperature T0,l7 HV 1 mev.)

Unirradiated l92 293 5.000 X 10" 265C. l37 332 1 claim:

1. Welded constructions of stainless steel characterized in that both the basic metal and the weld metal comprise steel of a tough hardened type, containing:

Percent 0020-0040 Carbon 0.2-1.0 Silicon 0.22.0 Manganese 15-18 Chromium 4-6 Nickel 0.2 Molybdenum 0020-0080 Nitrogen and the balance iron with the usual impurities, the contents of the alloying materials being mutually adapted such that the chromium equivalent plus the nickel equivalent are between 24.5 and 26.2 and 1.4 times the chromium equivalent and the nickel equivalent are between 16.5 and 19.0, where the chromium equivalent is percent Cr percent Si percent Mo and the nickel equivalent percent Ni 0.5 percent Mn 30 (percent C percent N), said stainless steel having been hardened by heating up to at least 900C. to obt'ain'a structure of austenite and ferrite only and then rapidly cooled to a low temperature so that most of the austenite was transformed into martensite, and thereafter tempered by heating up to 500-630 C. to obtain a structure comprising 5-15 percent by weight of ferrite, 1040 percent by weight of austenite and the remainder tempered martensite.

2. Steel according to claim 1 wherein the steel contains:

Percent 0030-0040 Carbon 0.3-0.5 Silicon 0.6-1.0 Manganese 16-17 Chromium 4.5-5.5 Nickel 0.8-1.3 Molybdenum 0.0200.050 Nitrogen 3. Welded constructions produced from the steel according to claim 1, intended to be exposed to neutron irradiation.

zg gg UNITED STATES PATENT OFFICE CERTIFlC-ATE 0F CORRECTION Patent: No. 3,650,709 Dated March 21, 1972 lnve Lars Johan Gustaf Morsing It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 32, "5,000" should be -5 ,o0--; Column 6, line 13, "equivalent and the" should be -equivalent minus the--. 1

Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR., ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,650,709 Dated March 21, 1972 InVentOr() Lars Johan Gustaf Morsing It is certified that errot appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 32, "5,000" should be 5,o0--; Column 6, line 13, "equivalent and the" should be ---equivalent minus the--.

Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,J'R. ROERI GOTTSCHALK Attesting Officer Commissioner of Patents

Claims

2. Steel according to claim 1 wherein the steel contains: Percent 0.030-0.040Carbon0.3-0.5Silicon0.6-1.0Manganese16-17Chromium4.5-5.5Nickel0.8-1.3Molybdenum0.020-0.050Nitrogen
3. Welded constructions produced from the steel according to claim 1, intended to be exposed to neutron irradiation.