Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-08-02
2003-05-20
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S320000, C148S529000, C219S1370WM, C228S262410
Reexamination Certificate
active
06565678
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to weld metals with superior low temperature toughness for joining high strength, low alloy steels. This invention also relates to welding consumable wires and welding methods for producing such weld metals. Weld metals produced by the welding consumable wires and welding methods of this invention have microstructures that provide superior strength, toughness, and hydrogen cracking resistance. The welding consumable wires and welding methods of this invention are particularly well suited for mechanized field girth welding of high strength steel linepipe using the gas metal arc welding process to construct pipelines.
BACKGROUND OF THE INVENTION
Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
For industries that utilize steel structures, e.g., oil & gas, chemical, ship building, power generation, etc., it has become apparent that the selected use of high strength, low alloy (HSLA) steels is desirable. As used herein, “high strength, low alloy (HSLA) steel” includes any steel containing iron and less than about 10 wt % total alloy additives and having a yield strength of at least about 550 MPa (80 ksi). Utilizing a HSLA steel can result in lower costs for a structure due to the lower weight of the structure compared to that of the same structure built of a lower strength steel. Also, the use of HSLA steels can enable a structure to be built that could not practicably be built using a lower strength steel because very thick material would be necessary to provide structural strength, resulting in unacceptably high weight.
However, utilizing HSLA steels in some of the aforementioned structures may have certain drawbacks. Many commercially available HSLA steels are limited in their use, as compared to lower strength steels, particularly in fracture critical applications, because of the limited toughness (thus, limited defect tolerance) of their weldments. (See Glossary for definition of fracture critical.) Toughness in steel weldments may be considered from the standpoint of the ductile-to-brittle transition temperature (DBTT) as measured by the Charpy V-notch test, from the magnitude of the absorbed Charpy V-notch energy at specific temperatures, or from the magnitude of the fracture toughness at specific temperatures as measured by a test like the crack tip opening displacement (CTOD) test or the J-integral test; all of these toughness testing techniques being familiar to those skilled in the art. (See Glossary for definition of DBTT.)
Another potential drawback associated with the use of HSLA steels is the susceptibility for hydrogen cracking in their weldments. As the strengths of weld metals increase, their alloy contents typically increase, which creates higher hardenability and a tendency for transformation to martensite. The increased presence of martensite in higher strength weld metals combined with the higher residual stresses in higher strength weldments, generally leads to greater sensitivity for hydrogen cracking, as compared to lower strength weldments. In order to decrease the likelihood of hydrogen cracking in the weldments of HSLA steels, the steels are often preheated prior to welding, which can increase fabrication costs.
In addition to commercially available HSLA steels, new HSLA steels with superior strengths, e.g., yield strength of at least about 690 MPa (100 ksi), preferably at least about 760 MPa (110 ksi), more preferably at least about 828 MPa (120 ksi), and even more preferably at least about 896 MPa (130 ksi), and most preferably at least about 931 MPa to 966 MPa (135 to 140 ksi) are currently under development. For example, see International Application nos. WO 99/05336, WO 99/05334, WO 99/05328, WO 99/05335, and WO 98/38345. These new HSLA steels are particularly well suited for manufacturing high strength linepipe for constructing pipelines. For pipeline applications, the girth welds used to join individual linepipe segments preferably provide a high level of toughness due to the fracture critical nature of their service. Additionally, in certain environments, e.g., in arctic applications, the required girth weld toughness may need to be achieved at ambient temperatures as low as about −40° C. (−40° F.), or even as low as −60° C. (−76° F.). Therefore, in order to utilize commercially available HSLA steels, or the new HSLA steels currently under development, at low ambient temperatures, welding consumable wires and welding methods that provide weld metals and weldments with adequate strength, hydrogen cracking resistance, and, most importantly, toughness, at such temperatures are required.
Broadly speaking, there are two classes of welding wires and, therefore, weld metals, that are currently available for the purpose of joining HSLA steels with yield strengths in the range of about 690 MPa (100 ksi) to about 931 MPa (135 ksi). The first class of weld metal is commonly referred to by an acronym related to its microstructure, “LCBF”, which stands for low carbon bainitic ferrite. This type of weld metal is described in U.S. Pat. No. 5,523,540. The second class of weld metal is the martensitic type, which is also described in U.S. Pat. No. 5,523,540.
The LCBF weld metals were invented as an improvement over the martensitic type for welding of naval hull materials. One goal was that the LCBF microstructure be producible over a wide range of welding heat inputs. Examples of welds made at heat inputs from about 1.2 kJ/mm (30 kJ/in.) to 5 kJ/mm (127 kJ/in.) are given in U.S. Pat. No. 5,523,540. It is noted in U.S. Pat. No. 5,523,540 that it is necessary to provide relatively fast cooling rates to ensure that the martensitic-type weld metals transform completely to martensite. However, when applying a wide range of cooling rates to the LCBF weld metals, the microstructure transforms entirely to bainite, and martensite is avoided. Another goal of the LCBF weld metals is avoidance of hydrogen cracking without the requirement for preheating to drive off hydrogen. This permits a cost savings during fabrication. Meeting these goals for welding naval hull materials places certain demands on the chemistry for the LCBF weld metals, particularly on the carbon content. The LCBF weld metals described in U.S. Pat. No. 5,523,540 are limited to a maximum of 0.05 wt percent carbon, primarily to avoid martensite formation. It is generally believed that the LCBF microstructure is more stable over broad heat input ranges and that it is more resistant to hydrogen cracking than martensite.
In contrast to naval hull welding where avoidance of preheating is desired, preheating is routinely used in pipeline girth welding even for nominally low alloy grades of steel like API 5L X-65. Because of the fracture critical nature of each girth weldment and the expense associated with pipeline repairs, avoidance of hydrogen cracking in pipeline girth weldments is desirable. The use of preheating in pipeline girth welding is often seen as necessary to avoid or minimize hydrogen cracking that may otherwise occur under rugged field conditions that can result in less than optimal cleanliness. Pipeline construction using mechanized equipment can proceed at a rate of 100 to 400 welds per day (depending on the equipment employed and whether construction is onshore or offshore). Because hydrogen cracking can occur more than one or two days after welding, costly remedial action can result from the occurrence of this type of cracking during pipelaying. Thus, in the pipeline industry, welding preheat is seen as relatively cheap insurance to avoid hydrogen cracking and associated field repairs. This is particularly the case for offshore pipelines where the welds become essentially inaccessible soon after welding, and it is more cost effective to apply moderate preheat than it is to “pick-up” the constructed offshore line and conduct a repair.
With respect to structural integrity, each girth weld in a gas pipeline is fracture critical.
Bangaru Narasimha-Rao V.
Beeson Danny Lee
Fairchild Douglas P.
Koo Jayoung
Macia Mario Luis
ExxonMobil Upstream Research Company
Hoefling Marcy
Yee Deborah
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