MXPA99011349A - Ultra-high strength cryogenic weldments - Google Patents

Abstract

Welding methods are provided for use in joining ultra-high strength, low alloy steels to produce weldments having tensile strengths greater than about 900 MPa (130 ksi) with weld metals having fracture toughness suitable for cryogenic applications according to known principles of fracture mechanics.

Description

CRYOGENIC WELDING OF VERY HIGH RESISTANCE DESCRIPTION OF THE INVENTION This invention relates to methods for producing very high strength welds with welded metals that have excellent fracture toughness at cryogenic temperature. More particularly, this invention relates to methods for producing very high strength welds with welded metals having excellent fracture toughness at cryogenic temperature on very high strength low alloy steels. Several terms are defined in the following specification. For convenience a Glossary of terms is provided in the same as before the claims. Frequently, it is necessary to store and transport pressurized volatile fluids at cryogenic temperatures, that is, at temperatures below about -40 ° C (-40 ° F). For example, there is a need for containers for storing and transporting liquefied and pressurized natural gas (PLNG) at pressures on the wide scale from about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at temperatures above -123 ° C. (-190 ° F). There is also a need for containers to safely and economically store and transport other pressurized fluids, such as methane, ethane and propane at cryogenic temperatures. In order for such brazed steel vessels to be constructed, the steel and its welds (see Glossary) must have adequate strength to withstand fluid pressure and adequate tenacity to prevent the initiation of a fracture, i.e., a failure event, in the operating conditions. As will be familiar to those skilled in the art, the Charpy V-notch (CVN) test can be used for the purpose of determining fracture toughness and fracture control in the design of vessels for transporting fluids at cryogenic temperature. pressurized, such as PLNG, particularly through the use of the transition temperature from ductile to brittle (DBTT). The DBTT defines two fracture regimes in structural steel. At temperatures below the DBTT, the fault in the Charpy V notch test tends to occur due to the fracture (fragile) of the low energy crack, whereas at temperature above the DBTT, the fault tends to occur through ductile fractures of the DBTT. high energy. Transportation and storage containers that are constructed of welded steel for cryogenic temperature applications mentioned above and for cargo bearing cryogenic temperature services must have DBTTs, as determined by the Charpy V-notch test, below the service temperature of the structure in order to avoid failure due to fragility. Depending on the design, service conditions and / or requirements of the applicable classification society, the required DBTT temperature shift may be from 5 ° C to 30 ° C (9 ° F to 54 ° F) below the temperature of service. ~ Nickel-containing steels conventionally used for cryogenic temperature structural applications, for example steels with nickel contents of more than about 3% by weight, have low DBTTs, but also have low tensile strengths. Typically, commercially available steels with nickel contents of 3.5% by weight, 5.5% by weight, and 9% by weight have DBTTs of about -100 ° C (-150 ° F), -155 ° C (-250 ° F) ), and -175 ° C (-280 ° F), respectively, and tensile strengths of up to approximately 485 MPa (70 ksi), 620 MPa (90 ksi), and 830 MPa (120 ksi), respectively. In order to achieve these combinations of strength and tenacity, these steels generally undergo expensive processing, for example a double annealing treatment. In the case of cryogenic temperature applications, the industry currently uses these steels containing commercial nickel due to their good tenacity at low temperatures, although their relatively low tensile strengths must be designed. These designs generally require excessive steel spouts to support the load, in applications of cryogenic temperature. Therefore, the use of these nickel-containing steels in cryogenic load-bearing temperature applications tend to be expensive due to the high cost of steel combined with the thicknesses of the steel required. Current commercial storage vessels for the transportation of liquefied natural gas to -162 ° C (-260 ° F) and atmospheric pressure (LNG) are typically constructed from the aforementioned commercial nickel-containing steels, austenitic stainless steels or aluminum. In LNG applications, the strength and toughness requirements for such materials, and for the welds that bind such materials, are different from those of the PLNG case. For example, in the discussion of the welding of steels with 2.25% by weight up to 9% by weight of nickel for cryogenic purposes, GE Linnert, in "elding Metallurgy", American elding Society, 3rd Ed., Vol. 2, 1967, pp. 550-570, list the Charpy V notch tenacity requirements (see Glossary) for welding as they vary from approximately 20 J to 61 J as measured at service temperature. Likewise, the 1995 publication, Det Norske Veritas (DNV) Rules For Classification of Ships, specifies that the materials used in newly constructed liquefied gas transport vessels must meet certain minimum Charpy V notch tenacity requirements. Specifically, the DNV Rules state that the plates and welds used for pressure containers with design temperatures ranging from -60 ° C to -165 ° C must cover a minimum Charpy toughness of 27 J at test temperatures ranging from 5 ° C at 30 ° C (9 ° F to 54 ° F) below the design temperature. The requirements listed by Linnert and the DNV Rules can not be directly applied to the construction of containers for transportation of PLNG (or other pressurized cryogenic fluids) since the PLNG containment pressure, typically approximately 2760 kPa (400 psia), is significantly more high for conventional LNG transportation methods, which is generally at or near atmospheric pressure. For PLNG storage and transportation containers, there is a need for more stringent toughness requirement, and therefore, a need for welds with better tenacity properties than those currently used to build LNG storage containers. Baseplate Material Storage containers for pressurized cryogenic temperature fluids such as PLNG are preferably constructed from discrete plates of a very high strength low alloy steel. Three co-pending US provisional patent applications identify a number of very high strength, low alloy steels that can be welded with excellent cryogenic temperature toughness for use in the construction of storage vessels for transporting PLNG and other pressurized cryogenic temperature fluids. Improved steels are described in a co-pending provisional American patent application entitled "ULTRA-HIGH STRENGTH STEELS WITH EXCELLENT CRYOGENIC TEMPERATURE TOUGHNESS", which has a priority date of December 19, 1997 and is identified by the Patent Office and US Marks ("USPTO") as Application Number 60/068194; in a co-pending provisional American patent application entitled "ULTRA-HIGH STRENGTH AUSAGED STEELS WITH EXCELLENT CRYOGEKJIC TEMPERATURE TOUGHNESS" which has a priority date of December 19, 1997 and is identified by the USPTO as Application Number 60/068252; and in a co-pending provisional North American patent application entitled "ULTRA-HIGH STRENGTH DUAL PHASE STEELS ITH EXCELLENT CRYOGENIC TEMPERATURE TOUGHNESS". which has a priority date of December 19, 1997 and is identified by the USPTO as Application Number 60/068816. These steels are specifically suitable for many applications at cryogenic temperatures, including the transportation of PLNG, since these steels have the following characteristics for steel plate thicknesses of approximately 2.5 cm (1 inch) and above: (i) DBTT less than about -73 ° C (-100 ° F), preferably less than about -107 ° C (-160 ° F), in the base steel and in the HAZ welding, (ii) the higher tensile strength of 830 MPa (120 ksi), preferably greater than about 860 MPa (125 ksi), and more preferably greater than about 900 MPa (130 ksi), (iii) superior welding capacity, (iv) microstructure of substantially uniform thickness and properties, and (v) improved toughness over the commercially available standard high-strength low alloy steels. The steels described in the aforesaid co-pending provisional US patent applications may have tensile strength of more than about 930 MPa (135 ksi), or greater than about 965 MPa (140 ksi), or greater than about 1000 MPa (145 ksi). Other suitable steels are described in the European Patent Application published on February 5, 1997 and having the International Application number: PCT / JP96 / 00157, and the publication number WO 96/23909 (08.08.1996 Gazette 1996 / 36) (such steels preferably having a copper content of 0.1% by weight up to 1.2% by weight) and in a co-pending provisional US patent application entitled "ULTRA-HIGH STRENGTH, WELDABLE STEELS WITH EXCELLENT ULTRA-LOW TEMPERATURE TOUGHNESS ", which has a priority date of July 28, 1997 and is identified by the USPTO as the Application Number 60/053915. Welding Such steels can be joined together to form storage containers for pressurized cryogenic temperature fluids, such as PLNG by a suitable welding method to produce a weld that provides adequate strength and fracture toughness for the intended application. Such a welding method preferably includes a suitable welding process for example, without limitation, gas metal arc welding, inert tungsten welding gas ("GMAW") or submerged arc welding ("TIG"); a suitable consumable welding wire ("SAW"); a suitable consumable welding gas (if required); an adequate welding flow (if required); and suitable welding procedures, for example without limitation, preheating temperatures and welding heat inputs. A weld is a welded joint, which includes: (i) the welded metal, (ii) the heat affected zone ("HAZ"), and (iii) the base metal in the vicinity of the HAZ. Welded metal is the consumable welding wire (and the flux, if used) as deposited and diluted by the base metal plate portion that melts during the execution of a welding process. The HAZ is the portion of the base metal that does not melt during welding, although the microstructure and mechanical properties of the same are altered by the heat of the welding process. The portion of the base metal that is considered "within the vicinity" of the HAZ, and therefore a part of the weld, varies depending on factors known to those skilled in the art, for example without limitation, the width of the welding, the dimensions of the base metal plate that is welded and the distances between the welds. Properties of the Welds Desired for PLNG Applications For the purpose of constructing storage tanks for PLNG and fluid filters at pressurized cryogenic temperature, it is desirable to have a welding method that includes a consumable welding wire, a consumable welding gas, a process of welding, and welding procedures that will provide welds with tensile strengths and fracture tenacity suitable for the intended cryogenic application, in accordance with known principles of fracture mechanics as described herein. More particularly, in order to build the storage containers for PLNG, it is desirable to have a welding method that provides welds with tensile strength greater than about 900 MPa (130 ksi) and fracture toughness suitable for the PLNG application in accordance with the known principles of fracture mechanics, as described herein. The tensile strength of such welds is preferably greater than about 930 MPa / 135 ksi) more preferably greater than about 965 MPa (140 ksi), and still more preferably of at least about 1000 MPa (145 ksi). Commercially available welding methods currently use commercially available consumable welding wires that are not suitable for welding the aforementioned very high-strength low alloy steels and which provide welds with the desired properties for commercial cryogenic pressurized applications. Accordingly, the primary objects of the present invention are to improve the welding technology of the current technique for the ability to apply to very high strength low alloy steels to provide a welding method that will produce welds having higher tensile strengths of approximately 900 MPa (130 ksi) and fracture tenacities suitable for the intended cryogenic application in accordance with the known principles of fracture mechanics as described herein.

A welding method (which includes a consumable welding wire, a type of welding process, and the selection of certain parameters and welding practices) are provided to be used to join very high strength low alloy steels with excellent fracture toughness at cryogenic temperature for cryogenic applications. The welding method of this invention is formulated to produce a microstructure that produces a set of mechanical properties suitable for the demanding demands of pressurized cryogenic temperature fluid applications, such as the PLNG application. The welding method produces a welded metal that is dominated by a centered cubic crystal structure of very fine grain body (BCC). The welding method also provides a welded metal that has a low impurity content, and therefore, a low non-metallic inclusion content, and additionally, creates individual inclusions that are small in size. The fundamental effects of fine grain size on the strength and tenacity of structural steels, as well as the fundamental effects of low inclusion content on toughness, are well known to those skilled in the art. However, the techniques to achieve such characteristics in welded metals suitable for the PLNG application are not as widely known. The resulting weld from the use of the welding method of this invention has a tensile strength greater than about 900 MPa (130 ksi) and adequate toughness for the PLNG application, according to the known principles of fracture mechanics. BRIEF DESCRIPTION OF THE DRAWINGS The advantages of the present invention will be better understood by reference to the following detailed description and the accompanying drawings in which: FIGURE 1A illustrates a graph of critical failure depth, for a given failure length, such as a function of CTOD fracture toughness and residual stress; and FIGURE IB illustrates the geometry (length and depth) of a fault. Although the invention will be described in relation to its preferred embodiments, it will be understood that the invention is not limited thereto. On the contrary, it is intended that the invention cover all alternatives, modifications and equivalents that may be included within the spirit and scope of the invention, as defined by the appended claims. The present invention relates to a welding method for use in joining low-alloy steels of very high strength, whereby the resulting weld has very high strengths and excellent toughness at cryogenic temperature. These desirable properties are achieved, mainly through two aspects of micro-engineering of welded metal. The first feature is a cubic centered crystal structure (BCC) of very fine grain body and the second feature is a low non-metallic inclusion content wherein the individual inclusions are small in size. The welding method includes a consumable welding wire, a type of welding process, and the selection of certain parameters and welding practices. Preferred welding processes for the welding method of this invention are any of the gas-shielded processes such as gas metal arc welding (GMAW), inert gas welding of tungsten, plasma arc welding (TIG), or other derivatives (PAW). Preferred welding parameters and practice, such as heat input and shield gas consumption, are described hereinafter. Chemical Composition of Welded Metals In one embodiment, a welded metal chemistry according to the present invention comprises iron and alloying elements in the approximate amounts indicated in Table I below. Table I Alloy Element Upper Limit Preferred Upper Preferred Limit (% P) (% P) carbon 0.06 0.10 manganese (Mn) 1.60 2.05 silicon (Si) 0.20 0.32 nickel (Ni) 1.87 6.00 chromium (Cr) 0.30 0.87 molybdenum (Mo) 0.40 0.56 copper (Cu) -0- 0.30 aluminum (Al) -0- 0.020 zirconium (Zr) -0- 0.015 titanium (Ti) -0- 0.010 More preferably, the upper limit of the nickel content is approximately 4.00 % in weigh. The Effect of Fine Grain Size The fine grain size in the microstructure of a welded metal made in accordance with this invention increases the strength of the weld through the dislocation block. The fine grain size increases the tenacity to the crack by shortening the length of the dislocation accumulations that decrease the maximum possible stress intensity at the head of any individual stack. This makes the initiation of microcracking less likely. The lower accumulation intensity also improves the tenacity of the ductile fracture by reducing local micro-stresses, thus making the initiation of micro-lagoons less likely. Additionally, the fine grain size increases the overall toughness by providing many "obstructions" to the advancement of the crack (See Glossary for dislocation block definitions, crack toughness, dislocation accumulation, microcracking, micro-stress and micro-lagoons). Obtaining the Microstructure and the Grain Size The fine grain BCC structure is preferably dominated by the self-reaming rod martensite, ie it contains at least about 50 volume percent, more preferably at least about 70 percent in volume and still more preferred at least about 90 volume percent, of self-reeling rod martensite. However, significant amounts of lower bainite may also be present, for example up to about 49 volume percent. Minor components such as acicular ferrite, polygonal ferrite, and upper bainite (or other bainite degenerative forms) may also be present in smaller amounts, although preferably they do not constitute the dominant morphology. The desired martensitic / bainitic microstructure is achieved through the proper use of welded metal chemistry and adequate control of the cooling rate of welded metal. Such examples discussing the chemistries are given below. Low heat input welding is used so that the welded metal cools more quickly than with the higher heat inputs typically used. The heat input is defined as the welding voltage multiplied by the welding current and divided by the welding displacement velocity, that is, the arc energy. The low heat input weld used in the welding method of this invention has arc energies preferably within the range of about 0.3 kJ / mm to about 2.5 kJ / mm (7.6 kJ / inches to 63.5 kJ / inches), although more preferably within the range of approximately 0.5 kJ / mm to approximately 1.5 kJ / mm (12.7 kJ / inches to 38 kJ / inches). Different levels of "fine grain" size can be described within the desired microstructure and the low heat input welding technique is intended to reduce the size of each unit. A low weld heat input aids in the formation of a small column grain size, a small anterior austenite grain size, a small martensite / small bainite size, and a narrow martensite and / or bainite rod width . As used herein in reference to the structure, "fine grain" means the columnar grain size (width) which is preferably less than about 150 microns and more preferably about less than 100 microns; that the above austenite grain size is preferably less than about 50 microns, more preferably less than about 35 microns and even more preferably less than about 20 microns; and that the martensite / bainite package size is preferably less than about 20 microns, more preferably less than about 15 microns, more preferably less than about 10 microns. As used herein, "grain size" means the grain size as determined by the line intersection method, as is common to those skilled in the art. The Effect of Low Inclusion Content The low inclusion content tends to increase the fissure tenacity by eliminating potential cracking crack initiation sites and / or by reducing the number of micro-stress concentration sites. The low inclusion content tends to increase the tenacity to the ductile fracture by reducing the number of microlaguna initiation sites. Increasing the welds made in accordance with this invention preferably have a low inclusion content, although they are not free from inclusion. The inclusions can contribute in a significant way to the optimal achievement of the properties of the welded metal. First, they act as deoxidants in the molten welded metal deposit. The low oxygen content in the shielding gas is preferred for making welds according to this invention, thus reducing the need for deoxidation; however, some deoxidation potential in the welded molten metal deposit is preferred. Secondly, the inclusions can be useful to control the columnar and anterior grain size growth of the austenite through grain limit setting. Limiting grain growth at elevated temperatures promotes smaller grain size at room temperature. However, due to the low heat input to make the welds according to this invention, it is helped to limit the grain size, the inclusion content can be reduced to a level which improves the toughness, but which still provides the effects of fixing useful grain limit. The welds made in accordance with this invention will achieve high resistances as previously observed. In the case of lower strength welded metals, it is often a characteristic designed to create a significant volume fraction of Ti-based inclusions for the purpose of nuclear acicular ferrite. For such solders of lower strength, acicular ferrite is the preferred microstructure due to its good strength and toughness properties. For the current invention, however, when the greatest strengths are of interest, it is an intentional feature to avoid a large volume fraction of inclusions that nucleate the acicular ferrite. Instead, it is preferred to create a microstructure dominated by rod martensite. Achieving the Size / Desired Inclusion Content The preferred low inclusion content in the welds according to the present invention is achieved by the selection and supply of an appropriate shielding gas, by maintaining a good cleaning of the weld and by using a welding consumable wire with low amounts of sulfur, phosphorus, oxygen and silicon. The specific chemistry of the consumable welding wire is designed to give the desired welded metal chemistry, which in turn is selected based on the desired mechanical properties. The desired mechanical properties depend on the specific container design; and this invention covers a range of welded metal chemistry capable of accommodating a design range. Using the welding method of this invention, the welded metal in volume will be diluted minimally by the base metal, and, therefore, the chemistry of the welding consumable of the welding consumable wire will be close to the same chemistry of the metal soldier as described below. According to the welding technique of this invention, the dilution is expected to be less than about 15%, but often less than about 10%. For areas near the weld metal center, dilution is expected to be less than about 5%. Using any well-known inverse dilution calculation method, one skilled in the art is able to calculate the consumable welding wire chemistry for use in the method of the present invention to obtain the desired welded metal chemistry. The shielding gas is preferably low in C02 and / or 02 content. Preferably, the shielding gas comprises less than about 10% by volume, more preferably less than about 5% by volume and even more preferably less than about 2% by volume, of C02 and / or 02. The main component of the protective gas is preferably argon; and the shielding gas preferably comprises about 80% by volume or more of argon, and more preferably more than about 90% by volume. Helium can be added to the shielding gas in amounts of up to 12% by volume to improve the characteristics of arc operation or penetration and welded bed profile. If necessary, for a specific storage vessel design the shielding gas impurities which tend to lead to non-metallic inclusion formation in the welded metal, as is known to those skilled in the art, can be further reduced by the supply of gas through a nanochemical filter, a device known to those with experience in the technique of precision TIG welding. To assist in achieving low inclusion content of welded metal in the welded metal, the consumable welding wire and the base material are preferably low in phosphorus sulfur and oxygen. The above characteristics of the welding method of this invention produce a welded metal preferably containing less than about 150 ppm of P, although more preferably less than about 50 ppm of P, less than about 150 ppm of sulfur, although more preferably preferable less than about 30 ppm of sulfur, and less than about 300 ppm of oxygen, although more preferably less than about 250 ppm of oxygen. For certain cryogenic storage vessel designs, the oxygen content of the welded metal is preferably controlled to less than about 200 ppm. With respect to the inclusion size, the low welding heat input that is preferred for welding in accordance with this invention, is selected to produce limited overheating and a rapid cooling rate, thereby eliminating the growth time of the inclusions in the molten welded metal tank. Additionally, small amounts Al, Ti, and Zr (less than about 0.015% by weight each) can be added individually or in combination to form small oxides. These elements are selected because of their known high affinity for oxygen. With respect to Ti, the amount of this element should be kept low, preferably less than about 0.10% by weight to avoid too much circular ferrite nucleation. The inclusions created by the invention are on average less than about 700 nm in diameter, preferably in the range from about 200 nm to about 500 nm in diameter. The number of non-metallic inclusions per unit area, for example of the surface of a section of welded metal created by this invention, which are greater than about 1000 nm in diameter which is preferably low, ie preferably less than about 250 per mm. The Balance Between Preheating and Heat Input The PLNG application requires a very high strength steel that may need some level of preheating to avoid cracking by welding. Preheating can alter the welding cooling rate (higher preheating temperatures that promote slower cooling), and it is an object of this invention to balance preheating and welding heat input to (1) avoid weld cracking and ( 2) produce a fine grain microstructure. Preheating is preferably between room temperature and about 200 ° C (392 ° F), although as will be common to those skilled in the art, the preheating temperature is preferably selected in consideration of the material's weldability and the input of welding heat. The welding capacity of the material can be determined using one of several test methods known to those skilled in the art, such as the Controlled Thermal Severity Test, the Y-groove test, the Canada Welding Institute test. . The "Models" can also serve for this purpose so that the welds of the real base and the welding metals are joined using the option manufacturing procedures. The models are preferably of sufficient size to impose the level of restriction that would occur with the actual storage container. Pulse Energy Supply In general, a pulse energy supply can be used with any of the gas shielding processes that are preferred for use in the welding method of this invention. Losses in arc stability or penetration capacity due to wire / gas chemistry selections can, to an important degree, be achieved using a pulse energy supply.

For example, in the case where this invention is practiced using low heat input TIG welding and a low sulfur consumable wire, the penetration of the weld bed can be improved by using a pulse power supply. As will be known to those skilled in the art, the operating conditions taken into consideration in the design of storage vessels constructed from welded steel for storing and transporting pressurized cryogenic fluids, such as PLNG, include, among other things, the pressure and operating temperature, as well as additional stresses that are probably imposed on steel and welding. Measurements of standard fracture mechanisms such as (i) a critical stress intensity (KIC), which is a measure of the fracture toughness of flat deformation, and (ii) crack end opening displacement (CTOD ), which can be used to measure the tenacity of the plastic-elastic fracture both of which are known to those skilled in the art, can be used to determine the fracture toughness of steel and welds. Generally acceptable industrial codes for steel structure design, for example, as presented in the BSI publication "Guidance on methods for assessing the acceptability of flaws in welded structures", often referred to as "PD 6493: 1991", may used to determine the maximum allowable failure sizes for the vessel based on the fracture toughness of the steel and the weld (including HAZ) and the stresses imposed on the vessel. A person skilled in the art can develop a fracture control program to mitigate the initiation of the fracture through (i) an appropriate design of the container to minimize the stresses imposed, (ii) proper manufacturing quality control to minimize defects, (iii) appropriate control of the life cycle loads and pressures applied to the container, and (iv) an adequate inspection program to reliably detect faults and defects in the container. A preferred design philosophy of welded storage containers according to the present invention is "failure before spill," as is known to those skilled in the art. These considerations are generally referred to herein as "known principles of fracture mechanics". The following is a non-limiting example of application of those principles of known fracture mechanisms in a procedure for calculating the critical failure depth for a given failure length for use in a fracture control plan to prevent initiation into a container of pressure or container.

FIGURE IB illustrates a failure length fault 315 and failure depth 310. PD6493 is used to calculate the values for the critical failure size chart 300 shown in FIGURE IA based on the following design conditions for a container or pressure vessel: Container diameter: 4.57 m (15 feet) Wall thickness of container: 25.4 mm (1.00 inch) Design pressure: 3445 kPa (500 psi) Permissible circumferential pressure: 333 MPa (48.3 ksi) For the purpose of this example, a surface failure length of 100 mm (4 inches), for example, an axial fault located in a seam weld is assumed. Referring now to FIGURE IA, graph 300 shows the value of the depth of the critical fault as a function of fracture toughness CTOD and residual stress, for residual stress levels of 15, 50 and 100 percent of elastic limit. The residual stresses can be generated due to manufacturing and welding; and PD6493 recommends the use of a residual stress value of 100 percent of the yield strength in welding (including HAZ welding) unless the welds are relieved of stress using techniques such as post weld heat treatment (PWHT). or the release of mechanical stress.

Based on the CTOD fracture toughness of steel in pressure vessel at the minimum service temperature, the manufacturing vessel can be adjusted to reduce residual stresses and an inspection program can be implemented (both for initial inspection and inspection). service) to detect and measure the faults for comparison against the critical failure size. In this example, if the steel has a CTOD tenacity of 0.025 mm at the minimum service temperature (as measured using laboratory samples) and the residual stresses are reduced up to 15 percent of the yield strength of the steel, then the value for the critical failure depth is approximately 4 mm (see point 320 in FIGURE A). Following similar calculation procedures, known to those skilled in the art, the depths of critical failure can be determined for various fault lengths as well as various fault geometries. Using this information, a quality control program and inspection program (techniques, detectable failure dimensions, frequency) can be developed to ensure that failures are detected and resolved before they reach the critical failure depth or before the application of Design charges. Based on published empirical corrections between CVN, K? C and CTOD fracture toughness, the CTOD tenacity of 0.025 mm is generally related to a CVN value of approximately 37 J. That example is not intended to limit this invention to any. EXAMPLES In the following examples, a welding method according to the present invention is used to weld a base steel of the type described in a co-pending provisional American patent application entitled "ULTRA-HIGH STRENGTH, WELDABLE STEELS WITH EXCELLENT ULTRA- LOW TEMPERATURE TOGHNESS "with a priority date of December 19, 1997 is identified by the USPTO as Application Number 60/068816. For the purpose of these examples, the base steel comprises: 0.05% by weight of carbon, 1.70% by weight of manganese, 0.075% by weight of silicon, 0.40% by weight of chromium, 0.2% by weight of molidebne, 2.0% by weight of nickel, and 0.05% by weight of Niobium and other alloying elements within the scales defined in Application Number 60/068816, which include at a minimum level, from about 0.008% by weight to about 0.03% by weight of titanium, from about 0.001% by weight to about 0.05% by weight of aluminum, and from about 0.002% by weight to about 0.005% by weight of nitrogen. Additionally, the residues are substantially minimized in the base steel, for example the phosphorus content (P) is preferably less than about 0.01% by weight; the sulfur content (S) is preferably less than about 0.004% by weight; and the oxygen content (O) preferably is less than about 0.002% by weight. A steelware having this chemical is prepared to produce a very high strength double-phase steel plate having a microstructure comprising about 10 volume% up to about 40 volume% of a first phase substantially 100 volume% ferrite ("essentially") and about 60% by volume to about 90% by volume of a second phase of martensite in predominantly fine grain rod, fine-grained lower bainite, or mixtures thereof. In some way in greater detail, the base steel of those Examples is prepared by forming a slab of the desired composition, as described above; heating the ware to a temperature of from about 955 ° C to about 1065 ° C (1750 ° F-1950 ° F); hot rolling the earthenware to form the steel plate in one or more steps that provide approximately 30% to about 70% reduction in a first temperature scale in which the austenite is recrystallized, ie about about the temperature Tnr additionally hot rolling the steel plate in one or more passages providing about 40 percent to about 80 percent reduction on a second temperature scale below about the temperature Tnr and above about the transformation temperature Ar3 and finishing the rolling of the steel plate in one or more steps to provide about 15 percent to about 50 percent reduction in the inter-critical temperature scale of about the transformation temperature Ar3 and about the transformation temperature Arj. The hot-rolled steel plate is then quenched at a cooling rate of about 10 ° C per second to about 40 ° C per second (18 ° F / sec. - 72 ° F / sec.) To a Stopping Temperature of Tempered (QST) preferably below about the transformation temperature Ms plus 200 ° C (360 ° F), at which time the tempering is completed. The steel plate is allowed to cool to air at room temperature after the tempering is finished. (See Glossary for temperature definitions Tnr, and transformation temperatures Ar3 Ari, and Ms) EXAMPLE 1 In a first example of the method of the present invention, the gas metal arc welding (GMAW) process is used to produce a welded metal chemistry comprising iron and about 0.07% by weight of carbon, approximately 2.05. % by weight of manganese, approximately 0.32% by weight of silicon, approximately 2.20% by weight of nickel, approximately 0.45% by weight of chromium, approximately 0.56% by weight of molybdenum, less than approximately 110 ppm of phosphorus and less than 50 ppm of sulfur. The weld is made on a steel, such as the base steel described above, using a shielding gas based on argon with less than about 1% by weight of oxygen. The weld heat input is in the range of approximately 0.3 kJ / mm to approximately 1.5 kJ / mm (7.6 kJ / inch to 38 kJ / inch). Welding by this method provides a weld having a tensile strength greater than about 900 MPa (130 ksi), preferably greater than about 930 MPa (135 ksi), and still more preferably of at least about 965 MPa (140 ksi), and more preferably of about 1000 MPa (145 ksi). In addition, welding by this method provides a welded metal with a DBTT below about -73 ° C (-100 ° F), preferably below about -96 ° C (-140 ° F), more preferably below about -106 ° C (-160 ° F), and even more preferably below about -115 ° C (-175 ° F). EXAMPLE 2 In another example of the method of the present invention, the GMAW process is used to produce a welded metal chemistry comprising iron and about 0.10% by weight of carbon (preferably less than about 0.10% by weight of carbon, more preferably). preferable from about 0.07 to about 0.08% by weight of carbon), about 1.60% by weight of manganese, about 0.25% by weight of silicon, about 1.87% by weight of nickel, about 0.87% by weight of chromium, about 0.51% by weight molybdenum weight, less than about 75 ppm phosphorus, and less than about 100 ppm sulfur. Welding heat input is at the -scale of approximately 0.3 kJ / mm to approximately 1.5 kJ / mm (7.6 kJ / inches to 3-8 kJ / inches) heating of approximately 100 ° C (212 ° F) is used . The weld is made on a steel, such as the base steel described above, using argon-based shielding gas with less than about 1% by weight oxygen. Welding by this method provides a weld having a tensile strength greater than about 900 MPa (130 ksi), preferably greater than about 930 MPa (135 ksi), more preferably greater than about 965 MPa (140 ksi) and even more preferably of at least about 1000 MPa (145 ksi). In addition, welding by this method provides a welded metal with a DBTT below about -73 ° C (-100 ° F), preferably below about -96 ° C (-140 ° F), more preferably below about - 106 ° C (-160 ° F), and even more preferably below about -115 ° C (-175 ° F). EXAMPLE 3 In another example of the method of the present invention, the inert gas welding process of tungsten (TIG) is used to produce a weld metal chemistry that contains iron and approximately 0.07% by weight of carbon (preferably less than about 0.07% by weight of carbon), about 1.80% by weight of manganese, about 0.20% by weight of silicon, about 4.00% by weight of nickel, about 0.5% by weight of chromium, about 0.40% by weight of molybdenum, about 0.02% by weight in copper, about 0.02% by weight of aluminum, about 0.010% by weight of titanium, about 0.015% by weight of Zr, less than about 50 ppm of phosphorus and less than about 30 ppm of sulfur. The weld heat input is in the range of approximately 0.3 kJ / mm to approximately 1.5 kJ / mm (7.6 kJ / inches to 38 kJ / inch) and a preheat of approximately 100 ° C (212 ° F) is used. The welding is done on a steel, such as the above-described base steel, using an argon-based shielding gas of less than about 1% by weight oxygen. Welding by this method provides a weld having a tensile strength greater than about 900 MPa ( 130 ksi), preferably greater than about 930 MPa (135 ksi), preferably greater than about 965 MPa (140 ksi) more preferably greater than 1000 MPa (145 ksi). In addition, welding by this method provides a welded metal with a DBT below about -73 ° C (-100 ° F), preferably below about -96 ° C (-140 ° F), more preferably below about -106 ° C (-160 ° F), and even more preferably below about -115 ° C (-175 ° F). Welded metal chemistries similar to those mentioned in the examples can be made using either GMAW or TIG welding processes. However, TIG welds are anticipated to have lower impurity content and a more highly refined microstructure than GMAW welds, and therefore tenacity at the improved low temperature. While the present invention is described in terms of one or more preferred embodiments, it should be understood that other modifications may be made without departing from the scope of the invention, which is set forth in the appended claims. The welding method of this invention can be used with many different steels of the very high strength low alloy steels, described herein, which are provided for the purposes of example only. Glossary of terms: Ari transformation temperature: the temperature at which the transformation of austenite to ferrite or for ferrite plus cementite is completed during cooling; Ar3 transformation temperature: the temperature at which the austenite begins to transform to ferrite during cooling; BCC: cubic centered body; Tenacity (Charpy-V Notch) Energy in Feet-pounds or Joules Charpy measured on the rupture in a V-Charpy notch sample; Fissure toughness The strength of a steel to crack fracture, whose property (eg, without limitation) can be measured using the CTOD test or can be established using the DBTT from a notch-V Charpy test group; cooling speed cooling speed in the center, or substantially in the center, of the thickness of the plate; cryogenic temperature any lower temperature of about -40 ° C (-40 ° F); CTOD open crack of displacement CVN: notch Charpy in V DBTT (Ductile to Temperature delineates the two regimes of Transition of Fragility): fracture in structural steels; At temperatures below the DBTT, the fault tends to occur by low energy crack fracture (fragility), while at temperatures above the DBTT, the fault tends to occur by high energy ductile fracture; dislocation: a linear imperfection in a crystalline arrangement of atoms; dislocation block: a phenomenon whereby an obstacle (such as the grain boundary or a precipitate) prevents or hides the movement of dislocations in a metal; accumulation of dislocation: occurs when a plurality of dislocations that move in it, or almost in the same, inclined plane, move within an obstacle and stack one next to another; essentially substantially 100% fine grain structure means that the columnar grain size (width) is preferably less than about 150 microns, and more preferably less than about 100 microns; that the above austenite grain size is preferably less than about 50 microns, and even more preferably less than about 35 microns, and even more preferably less than about 20 microns; and that of the martensite / bainite package size is preferably smaller, about 20 microns, more preferably less than about 15 microns, and even more preferably less than about 10 micras GMAW gas metal arc welding; grain size grain size as determined by the line interception method HAZ: area affected by heat; temperature range from approximately intercritical: transformation temperature Ar3 to approximately the transformation temperature Ari on heating; K-IC- critical stress intensity factor; kJ: kilojoule; kPa: thousands of Paséales; ksi: thousands of pounds per square inch; low alloy steel: a steel containing iron and less than about 10% by weight of total alloy additives; low-solder inlet welding with heat-arc energies preferably within the range of about 0.3 kJ / mm to about 2.5 kJ / mm (7.6 kJ / inch to 63.5 kJ / inch), though more preferably within the scale from about 0.5 kJ / mm to about 1.5 kJ / mm (12.7 kJ / inch to 38 kJ / inch) non-metallic inclusion content the unit area number per lower: non-metallic inclusion; for example, of the surface of a welded metal cut created by this invention, which are greater than about 1000 nm in diameter is preferably less than about 250 per mm2. Maximum permissible failure size: length and depth of critical failure; microranging the first example of the separation of material at the beginning of the initiation of slit fracture microtension tension occurs at a sub-granular scale around a simple (or group of) discontinuity (or discontinuities), which may include , for example, an inclusion, a precipitate, or an area of a second phase; microvacio a cavity occurs near a discontinuity in a steel matrix such as an inclusion, a precipitate, or a small area of a second phase; MPa: millions of Paséales; Transformation temperature Ms the temperature at which the transformation of austenite or martensite begins during cooling; ppm: parts per million; Tempered as used in the description of the present invention, accelerated cooling by any means by which a fluid selected by its tendency is used to increase the cooling rate of the steel, as opposed to cooling to air Temperature of stopping the highest temperature, or tempered (QST) substantially the highest reached on the surface of the plate, after the quenching is stopped, due to the heat transmitted from the average thickness of the plate; Slab a piece of steel having any dimensions tensile strength: in tension test, the maximum load ratio to the original cross-sectional area; TIG welding: welding of inert gas of tungsten; Tra temperature: the temperature below which the austenite does not recrystallize; USPTO: United States Patent and Trademark Office; and welding: a welded joint, which includes: (i) the welded metal, (ii) the heat affected zone (HAZ), and (iii) the base metal in the "close proximity" of the HAZ. The portion of the base metal that is considered within the "close proximity" of the HAZ and therefore, a part of the weld, varies depending on the factors known to those skilled in the art, for example, without limitation, the width of the weld, the size of the article that was welded, the number of welding required to manufacture the article, and the distances between the welds.

Claims (12)

1. CLAIMS 1. A method of welding a base metal, the method is characterized in that it comprises the step of: (i) welding using a gas shield welding process, a shielding gas based on argon, and a consumable wire of welding to produce: (a) a welded metal with a transition temperature from ductile to brittle of less than about -73 ° C (-100 ° F) and having a centered cubic crystal structure of fine-grained body of less about 50% by volume of self-reaming rod martensite and less than about 250 non-metallic inclusions greater than about 10000 nm in diameter per mm2, as measured on a surface of a piece of weld metal, and comprising iron and following alloying elements: about 0.06% by weight to about 0.10% by weight of carbon; about 1.60% by weight to about 2.05% by weight of manganese; about 0.20% by weight "up to about 0.32% by weight of silicon, about 1.87% by weight to about 6.00% by weight of nickel; about 0.30% by weight to about 0.87% by weight of chromium; about 0.40% by weight to about 0.56% by weight of molybdenum; and (b) a weld having a tensile strength greater than about 900 MPa (130 ksi). The method according to claim 1, characterized in that the welded metal further comprises at least one additive selected from the group consisting of from 0% by weight to approximately 0.30% by weight of copper, 0% by weight up to about 0.020% by weight of aluminum; 0 wt% to about 0.015 wt% zirconium, and 0 wt% to about 0.010 wt% titanium. 3. The method of compliance with the claim 1, characterized in that the gas-armored welding process is done with a heat input in the range of 0.5 kJ / mm to about 1.5 kJ / mm (12.7 kJ / inches to 38 kJ / inches). 4. The method of compliance with the claim 1, characterized in that the gas-armored welding process is metal arc welding with gas, and the welded metal comprises iron and approximately 0.07% by weight of carbon, approximately 2.05% by weight of manganese, approximately 0.32% by weight of silicon, about 2.20% by weight of nickel, about 0.45% by weight of chromium, about 0.56% by weight of molybdenum, less than about 110 ppm of phosphorus and less than about 50 ppm of sulfur. 5. The method of compliance with the claim 4, characterized in that the gas-shielded welding process is done with a heat input in the range from about 0.3 kJ / mm to about 1.5 kJ / mm (7.6 kJ / inches at 38 kJ / inch). The method according to claim 1, characterized in that the gas-armored welding process is gas-metal arc welding and the welded metal comprises iron and approximately 1.60% by weight of manganese, approximately 0.25% by weight of silicon, about 1.87% by weight of nickel, about 0.87% by weight of chromium, about 0.51% by weight of molybdenum, less than about 75 ppm of phosphorus and less than about 100 ppm of sulfur, and less than about 0.10% by weight of carbon. 7. The method of compliance with the claim 6, characterized in that the armored welding process -with gas is made with a shielding gas based on argon with less than about 1% by weight of oxygen. The method according to claim 6, characterized in that the gas-armored welding process is done with a heat input in the range of about 0.3 kJ / mm to about 1.5 kJ / mm (7.6 kJ / inches to 38 kJ / inches). The method according to claim 1, characterized in that the gas-armored welding process is inert gas welding of tungsten, and the welded metal comprises iron and about 1.80% by weight of manganese, about 0.20% by weight of silicon, about 4.00% by weight of nickel, about 0.5% by weight of chromium, about 0.40% by weight of molybdenum, about 0.30. % by weight of copper, approximately 0.02% by weight of aluminum, approximately 0.010% by weight of titanium, approximately 0.015% by weight of zirconium, less than approximately 50 ppm of phosphorus and less than approximately 30 ppm of sulfur, and less than approximately 0.07% by weight of coal 10. The method according to claim 9, characterized in that the gas-armored welding process is done with a heat input in the range of approximately 0.3 kJ / mm to approximately 1.5 kJ / mm ( 7.6 kJ / inches to 38 kJ / inches) and a preheat of approximately 100 ° C (212 ° F). 11. A welding of at least 2 edges of a base metal using a gas-armored welding process, a welding gas based on argon, and a consumable welding wire, where the weld has a tensile strength of at least about 900 MPa (139 ksi), and characterized in that it comprises: (i) a welded metal with a transition temperature from ductile to brittle of less than about -73 ° C (-100 ° F) and having a centered cubic crystal structure of fine-grained body of at least about 50 volume percent of martensite in self-reeling rod and less than about 250 non-metallic inclusions greater than about 1000 nm in diameter per mm2, as measured on a surface of a portion of welded metal, and then comprises iron and the following alloying elements: about 0.06% by weight to about 0.10% by weight of carbon; about 1.60% by weight to about 2.05% by weight of manganese; about 0.20% by weight to about 0.32% by weight of silicon; about 1.87% by weight to about 4.00% by weight of nickel; about 0.30% by weight to about 0.87% by weight of chromium; and about 0.40% by weight to about 0.56% by weight of molybdenum; (ii) a zone affected by heat; and (iii) portions of base metal in the vicinity of the HAZ. 12. The welding according to claim 11, characterized in that the welded metal also comprises at least one additive selected from the group consisting of 0% by weight of about 0.30% by weight of copper, 0% by weight up to about 0.020% by weight. aluminum weight; 0 wt.% To about 0.015 wt. Zirconium, and 0 wt.% To about 0.10 wt.% Titanium.