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Investigation of the joining area in the bodies of high-pressure stop valves produced by electroslag casting with melting-on.

The high-pressure stop valves are the basic parts of equipment used for extraction of oil and natural gas from large depths. In service at high pressure, the bodies of the valves are subjected to considerable loading. Therefore, to ensure reliable operation, the body of the valve is produced from medium-carbon higher strength alloyed steels which should also have a high level of plasticity for preventing brittle fracture [1].

The simplest and most economical method of producing the bodies of stop valves (Fig. 1) is electroslag casting with melting-on (ECMO). This method, developed at the E.O. Paton Electric Welding Institute, Kiev, provides for electroslag casting of only the central part of the body of the stop valve with simultaneous melting-on of previously produced branch pipes with flanges [2, 3].

At the start of investigations of the ECMO of the bodies of the stop valves in industry, this method was used for series production of components produced only from low carbon steels 20, 20G and 22K [4-6]. At the same time, in the industry where the pressure of the extracted gas and oil reaches 70 MPa and higher, and the content of the CO2 impurity does not exceed 6%, equipment uses the stop valves whose bodies are produced mainly from steels 34CrMo4 or 38KhM. These steels are difficult to weld and, consequently, during the development of the technology of ECMO of the blanks of the bodies of the stop valves, special attention was given to producing high-quality joints between the cast central part of the body and the flange branch pipe, produced from the rolled stock or forgings.

The method of ECMO combines electroslag casting and electroslag welding (ESW) in a single technological process. Since the rate of ESW is considerably lower than that of arc welding, the thermal cycle of ESW is extended. This creates suitable conditions for producing joints in low-weldability medium-carbon steels without cracks in the weld zone [7].

Experience with the ESW of this type of steels, including grades 30KhGSA, 30KhA and others, shows that the welded joint without cracks in the weld and with satisfactory properties can be obtained only using low carbon complexly alloyed welding wires [8, 9].

[FIGURE 1 OMITTED]

At the same time, in ESW of large components it is possible to produce almost full strength joints in 25KhN3MFA steel using a filler material of the same grade [10, 11], as a result of the considerably lower welding speed of these components and subsequent heat treatment.

[FIGURE 2 OMITTED]

To verify the possibilities of producing full strength joints in ECMO of the blanks of medium-carbon alloyed steels, experiments were carried out consisting of melting of the bodies of stop valves with a conventional passage section of 50 (Du50) and 80 mm (Du80), designed for operation at a pressure of up to 70 MPa.

In the first stage, to make the conditions of formation of the high-quality welded joints more difficult, the consumable electrode and the melted-on branch pipes were made of 40Kh, characterised by higher cracking susceptibility in welding in comparison with the steels of the same grade with molybdenum or nickel.

The quality of the welded joints was investigated on the vertical templates, taken from the central part of the melted blanks. Typical macrosections of the weld zone are shown in Fig. 2.

In melting of the blanks of the Du50 valve bodies, the depth of penetration of the branches decreases continuously, and in the Du80 blanks it is more stable. This variation of the nature of penetration is associated with differences in the mass and dimensions of the melted-on branches. Visual examination of the macrosections at a magnifications of 10 showed no defects in the form of lack of fusion cracks and slag inclusions. The macrosections show the zone of the cast metal, the fusion line and the heat affected zone.

The mechanical properties of the metal in the weld zone were evaluated after annealing of the cast blank at a temperature of 860[degrees]C followed by quenching in oil of templates from a temperature of 860[degrees]C and tempering at 650[degrees]C. The tensile and impact bend tests were conducted on the specimens taken from the cast electroslag metal, the metal of the branch and across the fusion line. The test results are presented in Table 1.

As indicated by the table, the mechanical properties of the metal of old and zones of the welded joint are higher than the requirements on the metal of the bodies, working at a pressure of 70 MPa [1]. Attention should be given to the unusually high plasticity and toughness of the metal in the heat affected zone. For example, the impact toughness in the zone is 1.5 times higher than in the cast electroslag and rolled metal of the branch pipe.

Similar results also obtained in ECMO of the blanks of a boss of 38KhS steel [12]. In this case, the impact toughness of the metal in the heat affected zone was more than 10% higher than the value in our the zones. At the same time, this was not detected in electroslag welding. The impact toughness after heat treatment was usually on the level of the properties of the parent metal and the weld metal [9].

The technological conditions of ESW differ from those in ECMO mainly by the speed of movement of the slag pool. The minimum speed (approximately 0.35...0.4 m/h) was used in electroslag welding of large blanks with the area of the welded section of several square metres [11, 13]. At the same time, in ECMO of relatively small blanks of the bodies of the stop valves, the melting rate of the central part of the body did not exceed 0.25 m/h i.e., it was almost 1.5 times lower. At this low rate, the thermal cycle of melting-on the branch pipes is longer than in ESW.

The heating and cooling rate of the metal in the vicinity of the fusion zone was low and the holding time and high temperature considerably longer. Slow cooling of the blank is also supported by the large amount of heat in the cast section of the component in comparison with the metal of the welded joint in ESW. Evidently, all these factors influence the formation of the appropriate structures of the metal in the region of the joint which in turn determines the mechanical properties of the welded joint.

[FIGURE 3 OMITTED]

To determine the structure of the metal in the area of joining of the branch pipe with the cast body of the stop valve, the components were subjected to metallographic examination. Special attention was given to the structure in the blank after casting without subsequent heating, and also after annealing and quenching with tempering in the temperature conditions mentioned previously.

In the weld zone in ECMO, as in ESW, there are areas with different grain sizes (Fig. 3). The largest grains were found in the cast electroslag metal (Fig. 3a), adjacent to the heat affected zone. At the boundary with the cast metal, the heat affected zone also showed large grains (Fig. 3b). This was followed by the sections of the metal with a fine grains (Fig. 3c) and incomplete recrystallisation (Fig. 3d).

The cast electroslag metal consists of relatively large crystals, elongated in the direction of heat removal. At the fusion line, the width of the crystals is up to 1.2 and the length up to 5 mm. These crystals are characterised by a troostite structure, with individual areas of plate-shaped pearlite. Fine ferrite precipitates were found at the grain boundaries. The cast crystals grow on the partially melted large grains of the metal of the branch pipe. No microscopic cracks or other defects were found in the fusion zone.

The section with the large grains in the heat affected zone, adjacent to the fusion line, is approximately 4 mm wide. The section consists of equiaxed grains with the size of 0.55-1.2 mm, which corresponds to the grain size numbers -3 and -2 (GOST 5639-82). Fine ferrite precipitates are recorded at the grain boundaries, and the grain itself has a troostite structure.

The main difference between the structure of the metal in the weld zone in ECMO of 40Kh steel in comparison with that in electroslag welding of the alloyed medium carbon steels is the considerably larger grain size in the metal of the high-temperature section of the heat affected zone. For example, in ECMO the grain size of the metal in this area was -3 and -1 in comparison with - 1 and 1 ESW [9, 14].

This takes place as a result of the longer holding time of the metal at high temperature. The long-term holding of the metal in the vicinity of the solidus temperature results in the homogenising of the metal and maybe the reason for the improvement of its toughness after refining of the grain by quenching with tempering.

To verify this assumption, the method of local x-ray spectrum microanalysis was used to evaluate the chemical heterogeneity of the crystals on both sides of the fusion line, i.e., in the cast electroslag metal and in the adjacent section of the heat affected zone with large grains. The results of this evaluation are presented in Table 2.

As indicated by the results, the metal of the heat affected zone with a large grains shows almost no chemical heterogeneity with respect to the individual alloying elements, including carbon. At the same time, the crystals of the cast electroslag metal show a heterogeneity in this element.

The structure of the metal in the weld zone after this type of heat treatment became considerably finer in comparison with the structure after casting (Fig. 4). The metal in all sections of the weld zone consisted of the fine-dispersion homogeneous structure of tempered sorbite with the grain size corresponding to grain size number 11-13. In the cast and the parent metal, metallographic examination show the chemical heterogeneity, and at the same time the heat affected zone showed a lighter chemically homogeneous structure. The structure was found in the area which contained a large grains prior to heat treatment with the uniform distribution of carbon as a result of homogenising.

The zone is characterised by reduced hardness in comparison with the cast and parent metal. After heat treatment, the hardness of the metal of the light sections was HV1 1900-2180, and that of the parent metal HV1 1203-1224 MPa.

[FIGURE 4 OMITTED]

Analysing the results of metallographic investigations, we can determine the reasons for the increase of the plasticity and toughness of the weld metal in the high-temperature section of the heat affected zone, adjacent to the fusion line.

The large grains of the zone are characterised by the uniform distribution of carbon (Table 2) and, therefore, quenching is accompanied by the precipitation of carbides in the entire volume of the metal and by the formation of the grains with the size of 11-12 grain size points.

Subsequent high-temperature tempering results in the coalescence of the uniformly distributed fine dispersion carbides. This is accompanied by the dissolution of the finer and growth of larger carbide particles, and the ferrite matrixes depleted in carbon. This reduces the hardness, increases plasticity and toughness of metal [15]. Evidently, this process is more extensive in the zone with the uniform initial carbon distribution.

As a result of the experiments it was concluded that ECMO with subsequent quenching and tempering produces high-quality components from the alloyed medium-carbon steels. It was established for the first time that as a result of the longer thermal cycle of the process in the high-temperature at the heat affected zone the last vicinity and toughness of the metal are considerably higher than those of the cast electroslag and melted-on deformed metal.

References

[1.] Specifications for well mouth and fountain equipment, 6A APR, 01.02.1996.

[2.] Poleshchuk M.A., et al., Sovremenn. Metallurgiya, 2009, No. 2, 13-17.

[3.] Poleshchuk M.A., et al., Armaturostroenie, 2009, No. 4, 49-54.

[4.] Alikin A.P. and Boiko G.A., Electroslag casting in chemical engineering, in: Electroslag technology, Naukova Dumka, Kiev, 1983.

[5.] Medovar B.I., et al., Probl. Spets. Elektrometall., 1979, No. 10, 37-41.

[6.] Paton B.E., et al., Energomashinostroenie, 1977, No. 1, 27-29.

[7.] Makara A.M. and Gotal'skii Yu.N., Avt. Svarka, 1955, No. 5, 3-9.

[8.] Slutskaya T.M. and Isktra A.S., ibid, 1954, No. 5, 58-65.

[9.] Paton B.E. (editor), Electroslag welding and surfacing, Mashinostroenie, Moscow, 1980.

[10.] Andreev V.P., et al., Rafiniruyushchie pereplavy, 1975, No. 2, 87-95.

[11.] Roshchin M.B., et al., Svar. Proiz., 1975, No. 7, 14-17.

[12.] Tsygurov L.G., et al., Probl. Spets. Elektrometall., 1991, No. 1, 11-15.

[13.] Medovar B.I., et al., Rafiniruyushchie pereplavy, 1974, No. 1, 109-118.

[14.] GRabin V.F. and Denisenko A.V., Physical metallurgy of welding low- and medium-alloy steels, Naukova Dumka, Kiev, 1978.

[15.] Lakhtin Yu. M. and Leont'eva Yu. P., Material science, Mashinostroenie, Moscow, 1990.

Submitted 13.10.2009

M.A. Poleshchuk, T.G. Solomiichuk, G.M. Grigorenko, V.L. Shevtsov and L.G.

Puzrin

E.O. Paton Electric Welding Institute, Kiev
Table 1. Mechanical properties of the metal of the bodies of the
Du50 and Du80 stop valves made of 40Kh steel

Valve type   Sampling area   [[sigma].sub.T],   [[sigma].sub.B],
                                   MPa                MPa

Du50         Cast metal      553.3 ... 587.1    774.4 ... 782.0
                                  570.2              778.2

             Fusion line     528.3 ... 540.0    732.5 ... 744.9
                                  534.4              738.7
             Branch pipe       564 ... 583        775 ... 777
             metal                 573                776

Du80         Cast metal      573.1 ... 581.4    743.1 ... 743.8
                                  577.2              743.4

             Fusion line     542.5 ... 544.7    712.0 ... 717.6
                                  543.6              714.8

             Branch pipe     550.1 ... 567.2    725.5 ... 738.2
             metal                558.6              731.8

Requirements for the         [greater than or   [greater than or
pressure of 70 MPa [1].       equal to] 517      equal to] 655

Valve type   Sampling area      [delta] %           [PSI], %

Du50         Cast metal       17.0 ... 19.0      58.2 ... 59.4
                                   18.0               58.8

             Fusion line      17.0 ... 19.0      64.0 ... 66.0
                                   18.0               65.0
             Branch pipe      17.0 ... 18.7      46.6 ...  48.8
             metal                 17.8               47.7

Du80         Cast metal          19.321.7        51.2 ... 57.3
                                   20.5               54.5

             Fusion line         18.322.7        68.2 ... 69.9
                                   20.5               69.0

             Branch pipe      20.0 ... 20.7      50.0 ... 52.6
             metal                 20.3               51.2

Requirements for the         [greater than or   [greater than or
pressure of 70 MPa [1].        equal to] 17      equal to]  35

Valve type   Sampling area   KCU, J/[cm.sup.2]

Du50         Cast metal        90.4 ... 97.0
                                   93.7

             Fusion line     128.4 ... 132.8 *
                                   130.6
             Branch pipe     89.9 ... 103.5 *
             metal                 96.7

Du80         Cast metal       101.7 ... 111.0
                                   106.7

             Fusion line     152.2 ... 158.0 *
                                   155.1

             Branch pipe      86.1 ... 107.7
             metal                 96.9

Requirements for the         [greater than or
pressure of 70 MPa [1].        equal to]  20

Comment: 1. The nominator gives the scatter of the values, the
denominator of the mean values; 2. The asterisk indicates the
specimens in which the notch was made in the fusion line.

Table 2. Mass fraction of the elements in the crystals of cast
metal and the heat affected zone, %

Analysed object            C              Fe             Cr

Cast grain           0.25 ... 0.75   97.5 ... 96.5   1.0 ... 1.1
Grains in the
heat affected zone   0.35 ... 0.40   97.1 ... 96.7   1.0 ... 1.2

Analysed object            Mn              Si

Cast grain           0.60 ... 0.67    0.30 ... 0.35
Grains in the
heat affected zone        0.6              0.4
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Article Details
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Title Annotation:ELECTROSLAG TECHNOLOGY
Author:Poleshchuk, M.A.; Solomiichuk, T.G.; Grigorenko, G.M.; Shevtsov, V.L.; Puzrin, L.G.
Publication:Advances in Electrometallurgy
Date:Oct 1, 2009
Words:2636
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