GB1589371A - Method of effecting welded joints whilst improving the resistance to embrittlement due to hydrogen - Google Patents

Description

(54) A METHOD OF EFFECTING WELDED JOINTS WHILST IMPROVING THE RESISTANCE TO EMBRITTLEMENT DUE TO HYDROGEN (71) We, ITALSIDER, S.p.A. of Via Corsica 4, Genoa, Italy, an Italian company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of effecting fusion welded joints in non-alloyed or low alloy structural steels whilst improving the resistance to embrittlement by hydrogen. By "non-alloyed or low alloy steel" is mean a simple carbon steel to which no alloying elements have been deliberately added, the steel containing only incidental impurity levels of such alloy elements.

The increasingly stringent requirements of present-day techniques have led to the manufacture of structural steels having mechanical features which can be defined as "mediumhigh" features. Among said steels may be mentioned e.g. the types ASA 60, ASTM A517F, ASTM A517B, AISI 4130, ASTM A285 and ASTM A537B whose tensile strength varies, according to the kind of steel, from about 35 kgs/mm2 to about 90 kgs/mm2. These steels on the one hand lead to several advantages like lower weight, lower thickness and/or larger size structures made therefrom; but on the other hand have disadvantages, among which the one most relevant to the problem faced by the present invention is that said steels are sensitive to the surrounding conditions, particularly regarding the tendency to be subject to alterations which may result in embrittlement and consequent breakage. In this regard, one of the most dangerous agents is the class of sulphides present in most forms of pollution and further in most important products, like for instance petroleum. In fact, said sulphides favour strongly the development of hydrogen, and thus are to be considered as one of the main factors leading to breakage as a result of embrittlement in most of the materials with high mechanical characteristics.

The problem has led to research throughout the world, in consequence of which many compositions of steel have been obtained, both without alloy elements or with a small percentage of alloy elements, which are capable of efficiently withstanding the phenomenon of sulphidic stress corrosion.

Among the numerous relevant technical publications, may be mentioned U.S. patent No. 3 600 161 by Nippon Steee, the report by H. Kihara at the seventh "World Petroleum Congress", Proceeding, vol. 5 (Mexico) 1967 pages 235-260 and the publication by R.W.

Saehle et al "Stress Corrosion cracking and Hydrogen embrittlement of iron-base alloys" ST.

Etienne Preprint n.F1. 12-14 June, 1973.

However, said metallurgical solutions still do not offer sufficient strength guarantees when welded structural elements are in contact with a sulphidic environment, owing to their high tendency to hydrogen cracking in that area which has been thermally altered by the welding, or when cathodic protection is provided, which naturally increases the amount of hydrogen available for the embrittlement phenomena. These features in the use of high strength steels are e.g. pointed out in the publication "Studies on sulphide corrosion cracking of high strength steels" of 1963 by the Welding Association of Japan, and in an article by H.

Kushiwaji and K. Shimoki in Japan Soc. for "Safety Engineering", 5, 314 (1966). In an article by J.F. Bates in "Materials Protection", January 1969, pages 33-40 that phenomenon is studied and test results reported. In a sulphidic environment, most of the test elements without any protection have a very high percentage of cracking by hydrogen embrittlement in extremely short periods of time, which even in the best cases do not reach one year.

Therefore, the use of structural steels with high mechanical characteristics in places polluted by sulphides, or anywhere where hydrogen in the elementary phase is available, is quite difficult, particularly for the welded structures of transport and stocking tanks, pipes for crude oil, and so on. Notwithstanding the great interest of the industry in the improvement of the resistance to the hydrogen embrittlement of welded joints, only a few suggestions suitable to countcract said phenomenon have been advanced.

As far as we know, there are two main methods for the protection against hydrogen embrittlement (besides obviously the embodiment of new compositions of steel). The first consists of subjecting the welded joints to heat treatments; and the second of protecting the welded joints with a coating of e.g. varnish or resin layers.

As may be casily understood. both said methods, though efficient to a degree, have some noteworthy limitations. Thus, the former method cannot be used with very large structures, and the lattcr is obviously cfficicnt only for a limited period of time, after which a new coating must be applied.

The invention intends to give a simple and practical answer to the technical requirements concerned. mitigating the problem of the absorption of hydrogen by said welded joints and their consequent embrittlement, by a welding method which ensures discharge of hydrogen away from the heat affected zone which, having a larger grain structure, is by itself more susceptible to embrittlemcnt by hydrogen than is the weld metal.

The principle of the invention is to create a galvanic cell in the welded zone, such that the hydrogen discharges from the weld metal. The weld metal is protected against hydrogen absorption through the addition of suitable alloy elements.

Thus, the present invention provides a method of effecting fusion welded joints in nonalloyed or low-alloy structural steels, whilst improving the resistance to hydrogen embrittle mcnt, which compriscs providing the weld zone a galvanic cell such that any hydrogen discharge occurs from the weld metal, the lattcr being protected from the action of hydrogen by incorporating in the weld metal chromium and/or molybdenum, wherein said galvanic cell is provided by the incorporation in said weld metal of an enrichment of nickel, compared to the content of the structural steel, the joint being welded using a metal wire containing nickel, and chromium and/or molybdenum; the nickel content of the wire being related to the nickel content of the structural steel to be welded by the expression: (O/oNi in the steel + 0.40 < %Ni in the wire < (O/oNi in the Stccl + 1.0%) and the chromium content of the wire being from 1.5 to 3 times the wire nickel content; and the molybdenum content of the wire being from 0. l S to 0.5 times the wire nickel content, and wherein the welding is carried out with a thermal value of from 0.5 to lkWh/m. In the method according to the invention, in the weld metal there is provided, with respect to the structural steel which is to be welded, an enrichment in nickel. The higher content of nickel in the weld metal, with rcspect to the adjacent heat affected zone, creates between the weld metal and the heat affected zone a cell such that the hydrogen discharges from the weld metal; the chromium and/or molybdenum oppose the absorption of hydrogen by the weld metal favouring the hydrogen re-combination and its removal in the form of bubbles.

Further, the welding takes place at a low thermal value, i.e. between 0.5 and IkWh/m.

Tcsts were made under severe conditions designed to emphasise the effects of sulphidic stress corrosion. To this end, common steel shects with a 25mm thickness were welded with a high thermal value, e.g. 1.5 kWh/m, with common welding wires known in the trade as ARCOS T/OMo and flow type 2 M. and with test wires according to the method of the invention. From said welded sheets were obtained test pieces perpendicularly to the welding scam. Said pieces were then stressed axially with a static load up to 60%, 70% and 80% of the value of the steel tensile strength.

Similar pieces were obtained from non-welded shects and subjected to similar loads (60%, 70% and 80%). Then, the test pieces were immersed in a buffer solution consisting of CH3COOH 0.5 M and Na2S 0.0lem. The solution pH was about 4. An automatic recorder recorded the test times until brcaking.

The test results, taken in isolation, were difficult to intcrpret, and thus it was necessary to relate all the data to the behaviour of the steel in the non-welded phase. For this reason there was dcfincd a first index of stability S of the test piece which, considering the exponential behaviour of the cracking times as a function of the applied load, was calculated for the various test pieces by the expression: S = T6() + 2 T70'; + 4 T80 where T60% T7()% and T8()% define the cracking times at the various percentages of load applied, respectively. From the valucs of the stability index S were obtained an embrittlement index, defined as the sensitivity percentage referrcd to the sensitivity of the non-welded metal. The embrittlement index NI was thus calculated by the expression: SB - SI NI = x 100 SB wherein SI is the stability index of the welded piece and Sb the one of the corresponding non-welded base steel. For exemplification purposes are herebelow reported the data obtained for two quite commonly used classes of steels, known as ASA 60 and ASA 64 respectively, whose weight percent compositions are respectively: ASA 60: C 0.19, Mn 1.46, Si 0.38, P 0.022, S 0.008, Cr 0.029, V 0.06; ASA 64: C 0.17, Mn 1.25, Si 0.31, P 0.012, S 0.017, Cr 0.30 Ni 0.28, V 0.05, Mo 0.09. The compositions of the welding wires used, besides the known ARCOS T30Mo, are reported in Table I.

TABLE I Wire No. C Si Mn Ni Cr Mo S P 258 0.050 0.018 0.068 3.02 1.52 - 0.011 0.019 259 0.044 0.016 1.02 3.02 - - 0.008 0.015 260 0.048 0.020 0.095 2.00 1.97 - 0.008 0.015 262 0.053 0.028 0.10 0.99 2.50 - 0.010 0.016 263 0.051 0.051 1.96 1.00 - 0.30 0.012 0.016 265 0.052 0.037 1.98 0.52 - 0.59 0.011 0.016 Table 2 shows the data of index of embrittlement NI %: In evaluating the data appearing in these two Tables, it is to be considered that both the welding conditions and the sulphidic stress corrosion conditions were exaggerated so as to create maximum sensitiveness of the welded joints to hydrogen embrittlement. With the environment and load applied unchanged if the welding is performed by a multipass method with a thermal value of 0.6 kWh/m, the values of NI for the joints welded by the method according to the invention decrease for instance for the ASA 60 steel and the wire 262 from 43% to 31 %, and for the commercial wire T30Mo from 73% to 30%.

TABLE 2 Wire 25 mm thick plates welded with multipass seams and a thermal value up to 1.5 kWh/m ASA 60 ASA64 S NI% S NI% Comm.T30Mo 500 73 440 63 258 637 65 445 63 259 462 75 339 71 260 680 63 409 65 262 1044 43 725 38 263 1039 43 702 40 265 886 52 526 55 As shown in Tables l and 2, for the steels considered the test wires 258 and 260 contain an excess of nickel in comparison to that utilized in the method of the invention, and thus they are still too sensitive to the hydrogen effect. Wire 259, without chromium, is even more sensitive. Wires 262 and 263, with the necessary contents of nickel and chromium or molybdenum show the best results. For the ASA 60 steel, wire 265 has the right content of nickel, but molybdenum is in excess and thus the joint is still more sensitive.

Examination of the cracking areas confirms what has been previously said in this specification: while, for the commercial wire, as well as for wires 258, 259 and 260, the embrittlement cracking always occurred in the heat affected zone; for wires 263 and 262 cracks occurrred in the weld metal and the cracked surfaces showed cracking by corrosion rather than cracking by hydrogen embrittlement. For wire 265 with the ASA 60 steel, the cracking occurrred by hydrogen embrittlement in the weld metal while with the ASA 64 steel there was a brittle crack in the heat affected zone. As already said, the test conditions were exaggerated; however, the results of these exaggerated tests already show the remarkable improvement obtainable in comparison to the joints welded with conventional methods.

Expressed in terms of cracking times, this improvement may be represented, with reference to the ASA 60 steel with an applied load of 80% Rm, by about 10 hours for the conventionally welded steel. about 120-130 hours for the steel welded according to the invention, and about 180-200 hours for the non-welded steel.

Therefore, it is to be appreciated that according to the invention it is possible. by a method which is extremely simple and no more expensive than the ones already known, to obtain considerable improvements in the resistance to hydrogen embrittlement of welded joints and thus a higher quality of welded structure, by simply modifying the welding operation and thereby avoiding the need for any further heat treatment of the joints or any periodic surface coating.

WHAT WE CLAIM IS: 1. A method of effecting fusion welded joints in non-alloyed or low-alloy structural steels whilst improving the resistance to hydrogen embrittlement which comprises providing in the weld zone a galvanic cell such that any hydrogen discharge occurs from the weld metal, the lattcr being protected from the action of hydrogen by incoporating in the weld metal chromium and/or molybdenum, wherein said galvanic cell is provided by the incorporation in said weld metal of an enrichment of nickel, compared to the content of the structural steel, the joint being welded using a metal wire containing nickel, and chromium and/or molybdenum; the nickel content of the wire being related to the nickel content of the structural steel to be welded by the expression: Ni in the Steel + 0.40%) < %Ni in the wire < (O/oNi in the steel + 1.0%) and the chromium content of the wire being from 1.5 to 3 times the wire nickel content; and the molybdenum content of the wire being from 0. l S to 0.5 times the wire nickel content, and wherein the welding is carried out with a thermal value of from 0.5 to 1kWh/m.

2. A method according to Claim I , wherein the metal wire has substantially the composition of wire 262 or 263 as herein exemplified.

3. A structural steel having a weld of improved resistance to hydrogen embrittlement produced by the method of either of the preceding claims.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. the weld metal and the cracked surfaces showed cracking by corrosion rather than cracking by hydrogen embrittlement. For wire 265 with the ASA 60 steel, the cracking occurrred by hydrogen embrittlement in the weld metal while with the ASA 64 steel there was a brittle crack in the heat affected zone. As already said, the test conditions were exaggerated; however, the results of these exaggerated tests already show the remarkable improvement obtainable in comparison to the joints welded with conventional methods. Expressed in terms of cracking times, this improvement may be represented, with reference to the ASA 60 steel with an applied load of 80% Rm, by about 10 hours for the conventionally welded steel. about 120-130 hours for the steel welded according to the invention, and about 180-200 hours for the non-welded steel. Therefore, it is to be appreciated that according to the invention it is possible. by a method which is extremely simple and no more expensive than the ones already known, to obtain considerable improvements in the resistance to hydrogen embrittlement of welded joints and thus a higher quality of welded structure, by simply modifying the welding operation and thereby avoiding the need for any further heat treatment of the joints or any periodic surface coating. WHAT WE CLAIM IS:

1. A method of effecting fusion welded joints in non-alloyed or low-alloy structural steels whilst improving the resistance to hydrogen embrittlement which comprises providing in the weld zone a galvanic cell such that any hydrogen discharge occurs from the weld metal, the lattcr being protected from the action of hydrogen by incoporating in the weld metal chromium and/or molybdenum, wherein said galvanic cell is provided by the incorporation in said weld metal of an enrichment of nickel, compared to the content of the structural steel, the joint being welded using a metal wire containing nickel, and chromium and/or molybdenum; the nickel content of the wire being related to the nickel content of the structural steel to be welded by the expression: Ni in the Steel + 0.40%) < %Ni in the wire < (O/oNi in the steel + 1.0%) and the chromium content of the wire being from 1.5 to 3 times the wire nickel content; and the molybdenum content of the wire being from 0. l S to 0.5 times the wire nickel content, and wherein the welding is carried out with a thermal value of from 0.5 to 1kWh/m.

2. A method according to Claim I , wherein the metal wire has substantially the composition of wire 262 or 263 as herein exemplified.

3. A structural steel having a weld of improved resistance to hydrogen embrittlement produced by the method of either of the preceding claims.