US3615368A

US3615368A - Nickel-chromium steel having increased resistance to corrosion

Info

Publication number: US3615368A
Application number: US736988A
Authority: US
Inventors: Anton Baumel
Original assignee: Gebrueder Boehler and Co AG
Current assignee: Gebrueder Boehler and Co AG
Priority date: 1967-06-19
Filing date: 1968-06-14
Publication date: 1971-10-26
Anticipated expiration: 1988-10-26
Also published as: AT289170B; DE1758424A1; DE1758424B2; FR1572641A; GB1223634A; SE342654B

Abstract

A nitric acid resistant nickel-chromium steel which, when welded, effectively prevents the formation of intermetallic deposits at the grain boundaries in the weld area. The composition of this steel consists essentially of up to 0.15% carbon, 2-6% silicon, 0-6% manganese, 16-25% chromium, 10-25% nickel, 0.08-2% nitrogen, and the remainder being iron and unavoidable impurities.

Description

ilnite @inies Pnieni Inventor Anton Baurnel lliapienberg, Austria Appl. No. 736,988 Filed June 14, 1960 Patented Oct. 26, 1971 Assignee Oebr. lBohler & (10., Alttiengeseilschait Vienna, Austria Priority June 19, 1967 Austria A5670/67 NlClKElL-CllllltOMlUM STlElElL i-lAVlNG iNClltlEASlED RESiSTANClE TO CORROSHON 3 Oiniins, No Drawings [56] References Cited UNlTED STATES PATENTS 2,984,563 5/1961 Tanczyn 2,820,708 1/1958 Waxweiler.

2,747,989 5/1956 Kirkby 3,337,331 8/1967 Ljunberg...

Primary Eraminer- Hyland Bizot Attorney1-iolrnan & Stern 75/128C 75/128C 75/128C 75/128C remainder being iron and unavoidable impurities.

ABSTRAQ'E: A nitric acid resistant nickel-chromium steel NlClliEL-CHMOMRUM STEEL HAVING llNCRlEASlElD RESISTANCE T CORROSHON This invention relates to a nickel-chromium steel which has in its unwelded and welded states an increased resistance to corrosion by reducing agents or agents which contain chlorine ions and particularly by oxidizing agents, such as nitric acid.

it is known that an austenitic nickel-chromium steel which contains chlorine 15-25% chromium, 7-20% nickel, 0.005-0'.2% carbon, 0-4% molybdenum, 0.l-% manganese and 2.5-6% silicon has an increased resistance to stress crack corrosion and that steels of substantially the same type exhibit an increased resistance to solutions of nitric acid. The practical use of these steels is limited, however, by the fact that they have a considerable disadvantage in the as-welded state. In the area which adjoins the seam weld, the welding heat induces the formation of a zone which is highly susceptible to intercrystalline corrosion because the silicon-containing steel is removed at an increased rate even in the absence of additional oxidizing agents, such as hexavalent chromium or trivalent iron. in these areas, a while silica layer is formed and the corrosion proceeds under this layer. This susceptible zone must not be confused with the known zone which is susceptible to intercrystalline corrosion in welded, nonstabilized, 8:18 nickel-chromium steels having an increased carbon content. The formation of this latter zone is induced by a deposition of chromium carbide at the grain boundaries. In the silicon-containing steels which are under consideration in the present case, the carbon content is so low that this deposition does not occur. Metallographic investigations have shown, however, that the welding heat induces in these high-silicon steels, at a temperature of about 850, the formation of an intermetallic deposit having highly increased chromium and silicon contents at the grain boundaries, so that these elements are depleted from the regions adjacent these grain boundaries. As these two elements are essential for the materials resistance to attack by nitric acid corrosion, the depleted regions ad joining the grain boundaries are more susceptible to nitric acid and a zone which is susceptible to intergranular corrosion is formed at a certain distance from the seam weld on both sides thereof.

As welding of the steel is generally inevitable in the manufacture of chemical equipment, the austenitic nickel-chromium steels which contain silicon and have an increased resistance to corrosion cannot be used in practice unless the above problem involving in the welding of such steels is solved.

My investigations have shown that the formation of intermetallic deposits at the grain boundaries in steels of this type at temperatures of about 850 C. can be retarded and the increased corrosion adjacent to the seam weld can be reduced, or avoided, if the steels contain nitrogen in an amount of 0.04-0.3711, preferably 0.08-0.2%.

The nickel-chromium steel according to the invention which has an increased resistance to corrosion in its unwelded and welded states is thus broadly characterized in that it contains up to 0.25% carbon, -10%, preferably 2-8%, silicon, 0-10% manganese, 13-30% chromium, 4-3070 nickel, 0-10% molybdenum and 0.04-0.3% nitrogen, balance iron and in evitable impurities.

According to the preferred embodiment of the invention, a steel which in its unwelded and welded states has a particularly high resistance to reducing agents or agents which contain chlorine ions contains up to 0.15% carbon, 2-6 percent silicon, 0-6 percent manganese, 16-25% chromium, 10-25% nickel, 2-6% molybdenum and 0.080.2% nitrogen, balance iron and inevitable impurities.

Without nitrogen, a steel of this type would also have a strong tendency to deposit intermetallic phases adjacent to the seam welds so that its susceptibility to an attack by agents such as sulfuric acids would be increased.

Best results as to the resistance to oxidizing agents, particularly nitric acid, in the unwelded and welded states of the steel, will be exhibited by a steel which contains up to 0.15% carbon, 2-6% silicon, 0-6% manganese, 16-25% chromium, 10-25% nickel and 0.08-0.2% nitrogen, balance iron and inevitable impurities.

A number of steel specimens having different nitrogen contents were subjected to welding tests and to comparative corrosion tests in boiling concentrated nitric acid, and dilute hydrochloric and sulfuric acids. These tests revealed a clear superiority of the nickel-chromium steels according to the invention. The same result was obtained when steel specimens were quenched from 1,l00 C. in water and subsequently tempered for 5 minutes to 10 hours. These specimens were tested for their notch impact strength relative to the tempering time and for their weight loss when submerged in boiling concentrated nitric acid for three times each time for a period of 48 hours. Best results were obtained with steels which contain 0.1 1% or 0.16% nitrogen. 1f the nitrogen content exceeds 0.22%, it appears necessary to avoid a formation of chromium nitrides, which particularly adversely affects the mechanical behavior.

Composition of Steels:

The filler material used when welding is the same as the composition of the nickel-chromium steel being welded, and moreover, the iron in the tiller material contains up to 20% delta ferrite as based on the weight of the filler material.

What is claimed is:

l. A nitric acid resistant nickel-chromium steel consisting essentially of up to 0.15% carbon, 2-6% silicon, up to 6% manganese, 16-25% chromium, 10-25% nickel, ODS-0.2% nitrogen, the remainder being iron and unavoidable impurities wherein, during welding, the formation of intermetallic deposits at the grain boundaries is effectively minimized.

2. A nickel-chromium steel as set forth in claim 8, which contains up to 20% delta-ferrite and constitutes a filler metal for welding.

3. A nickel-chromium steel as set forth in claim 1 wherein said filler metal is in the welded state.

Claims

2. A nickel-chromium steel as set forth in claim 8, which contains up to 20% delta-ferrite and constitutes a filler metal for welding.
3. A nickel-chromium steel as set forth in claim 1 wherein said filler metal is in the welded state.