RU2257984C2 - Method for welding articles of cast iron with spheroidal graphite at mounting them - Google Patents

Abstract

FIELD: welding, namely arc welding of articles of cast iron with spheroidal graphite.

SUBSTANCE: method comprises steps of surfacing intermediate layer of iron-nickel alloy containing carbon, 1.6 -2.0%; nickel, 48.0-52.0%; iron, the balance onto surface of welded edges before welding process; realizing surfacing at cooling rate of surfaced layer in range 15 - 20°C/s in water cooled copper crystallizer at controlling flow rate of cooling water; after surfacing performing mechanical working in order to provide thickness of surfaced layer in range 5.0 - 7.0 mm.

EFFECT: possibility for providing uniform strength welded joint.

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Description

The invention relates to welding, in particular to arc welding of cast iron products with spherical graphite, and can be used in the power industry, oil and gas industry, construction. On the edge of the product (for example, pipes), a layer of the iron-nickel alloy developed by us is installed, the experimentally established thickness is 5.0-7.0 mm after the preparation (turning) of the edges for welding. The main technical and economic criterion for the thickness of the deposited layer is the minimum layer of iron-nickel alloy, which provides high technological strength of the welded joint. Surfacing is carried out by special iron-nickel electrodes or wire by the argon-arc welding method with preliminary and concurrent heating (250-300 ° C) and subsequent heat treatment (annealing at a temperature of 750 ± 20 ° C). Next, the product is transferred to turning, where they perform cutting edges for welding. This cutting was established experimentally: bevel angle 15 ° (30 ° total), blunting - 1.5 mm, clearance - 1.5 mm - from the standpoint of ensuring a defect-free welded joint.

Assembly welding is performed by iron-nickel electrodes without heating and heat treatment. This ensures equal strength, equal plasticity and high corrosion resistance of the welded joint, low cost of welded joints.

There is a known [1] method for welding products from cast iron with spherical graphite, which includes heating the product to 250-500 ° C and welding in an argon medium with a non-consumable electrode using a high-nickel additive. After welding, complex isothermal annealing is carried out at a temperature of 950-970 ° C.

The disadvantages of this method:

- the difficulty of ensuring high-quality protection of the weld during argon-arc assembly welding of pipelines;

- the complexity of ensuring and quality control of heating and heat treatment of welded joints at installation;

- extremely low productivity, complexity and, as a consequence, the high cost of implementing this welding method.

There is also known [2] a method of welding a cast iron product, according to which, in order to increase the resistance to cracks of a welded joint, restrictions are imposed on the heating rate for welding and welding conditions.

The structural homogeneity of the welded joint is achieved by annealing at a temperature of 800-850 ° C.

The disadvantages of this method:

- the difficulty of providing high-quality protection of the weld during assembly welding of pipelines in protective gas;

- the complexity of ensuring and quality control of preheating and heat treatment of welded joints at installation;

- extremely low productivity, complexity and, as a consequence, the high cost of welds made using this technology;

- despite the claimed special technological methods, the high heterogeneity of the chemical composition and structure of the welded joint.

The method of electric arc welding of cast iron (SU 1181830 A, B 23 K 28/00), in order to improve the quality of joints by preventing crack formation, recommends preliminary surfacing of the edges with nickel-containing welding materials in a section with a width of at least two and not more than three thicknesses of the metal being welded.

The idea of electric arc welding of cast iron with the use of preliminary surfacing of edges with nickel-containing welding material is certainly productive.

But in order to turn ideas into technology and achieve operational reliability of a welded joint, it is necessary:

- in order to reduce the zone of variable composition and increase the ductility of the welded joint, experimentally determine the content of Ni and C in the weld metal,

- to ensure equal strength and equally plastic welded joints, optimize the number of deposited layers and the thickness of the deposited layer after machining,

- to reduce the area bleached cast iron to optimize the cooling rate of the weld metal,

- to eliminate cracks in the weld and the heat-affected zone, develop a mode of annealing of the weld edges.

This method (AS No. 1181830 A) was chosen by us as a prototype.

The purpose of the invention is the elimination of the disadvantages of this welding method by optimizing the technological modes of surfacing.

The invention relates to welding, in particular to electric arc welding with piece electrodes, and may find application in the construction of pipelines in the oil and gas industry, in the power system.

As our long-term studies have shown, the required level of ductility of welded joints of VChShG is achieved only when the nickel content in the first layer of deposited metal is> 35 wt.%.

In world practice, and in particular in GOST 2246-70 "Welding steel wire", only one brand of high-nickel wire, unalloyed with chromium is used - Sv-08N50 and its functional analogues in foreign standards.

The use of chromium-containing welding materials for surfacing on high-strength cast iron (as, for example, in the prototype patent - OZL-7 (08X20N9G2B) leads to a sharp decrease in the ductility of the welded joint due to the formation of brittle chromium carbides of the Cr ₇ C ₃ and Cr ₂₃ type in the fusion zone C ₆ .

Therefore, the development of electrodes and a nickel-iron alloy for argon-arc surfacing was based on the Sv-08N50 welding wire (GOST 2246-70).

The proposed method of mounting welding is implemented as follows.

1. Fuse RDS or ARDS ends of the welded pipes.

1.1. Heated to 250-300 ° C. Check the heating temperature with a thermal pencil or pyrometer.

1.2. Surfaced in three layers with a thickness of 7.0-9.0 mm Electrodes based on 08N50 wire based on calculations and experiments increased the C content in the deposited metal.

Welding current 180 ± 20 A. Layer-by-layer cooling. Edges do not overheat above 300-350 ° C. Surfacing is carried out in a water-cooled copper mold.

1.3. Slowly cool.

1.4. Annealed in a continuous furnace according to the developed regime.

1.5. Edges for welding are machined to ensure a weld layer thickness of 5.0-7.0 mm.

2. Assembly welding with electrodes based on wire type 08N50 with the provision of the developed composition of the weld due to the calculated composition of the electrode coating. Welding current 190 ± 20 A.

Welding is performed without heating and heat treatment.

The mechanical properties of welded joints (pilot testing) are given in table No. 1.

Table number 1
Mechanical properties of welded joints,
made by the claimed technology σ _VR , kgf / mm ² δ,% α, ° a _n , kgcm / cm ² 44.0 7.0 60 2,8 43.8 5.7 53 2.6 43.7 6.7 48 3,1

It should be noted that the ductility of welded joints made by electrodes on the 08N50 welding wire is significantly higher than the performance of the PANCH-11 high-nickel wire [1-2]. In particular, the bend angle of the welded joint during PANCH-11 welding does not exceed 20 °.

The area of surfacing and machining of pipe ends is organically inscribed in the technological scheme of the pipe shop between centrifugal pipe molding operations and an annealing furnace, where pipes with weld-welded and welded-through edges undergo heat treatment.

The composition of the alloy for surfacing is determined experimentally on the basis of ensuring equal strength and high ductility of the welded joint.

Based on the great personal experience in developing materials for cast iron welding [4-6], 50Ni - 50Fe (for example, welding wire 08N50, GOST 2246-70) was taken as the base alloy.

The optimum carbon content in the alloy was established experimentally on the basis of the dependence: carbon content in the alloy - the length of the zone bleached welded joints of pipes from VChShG.

The carbon content in the deposited metal was varied in the range of 0.5-2.5% due to changes in the content of graphite in the electrode coating of the main type.

We proceeded from the concept that the carbon content in the welding wire and cast iron should be of the same order.

The minimum bleaching zone was observed when the carbon content in the deposited metal of the electrode was 1.6-2.0 wt.% (Average value - 1.8 wt.%) - table No. 2.

Table number 2.
The influence of carbon content on the length of the zone bleached welded joints of pipes from ductile iron. The content of C in the weld metal, wt.% 0.8 1,2 1,6 2.0 2,4 The length of the zone bleached, microns 500 300 fifty fifty 100

Reducing the bleaching zone increases the ductility of the welded joint (table No. 3).

Table number 3.
The effect of carbon content in the deposited metal of the electrode on the bend angle of the welded joint. The content of C in the weld metal, wt.% 0.8 1,2 1,6 2.0 2,4 Bending angle during static bending, ° twenty thirty 90 90 60

Thus, the optimum carbon content in the deposited metal of the 50Ni - 50Fe type was experimentally established. On this basis, the composition of the electrode coating of the electrodes and the alloy obtained by the metallurgical redistribution in the form of a welding wire were developed.

Then experimentally determined the minimum number of deposited layers and the thickness of the deposited layer after machining required from the conditions for ensuring the operational strength of welded joints.

The edges of the pipes were surfaced using a water-cooled copper crystallizer. The number of deposited layers varied from 2 to 4, while the total thickness of the deposited layers was 7.0–9.0 mm.

The weld edges were machined under a V-shaped groove (thickness 5.0-7.0 mm) and welded with the developed electrodes.

The minimum number of layers - 3 and the thickness of the deposited layer - 5.0-7.0 mm after machining provide high operational strength of welded joints of pipes from VChShG - table No. 4.

Table No. 4.
The influence of the number of layers and the thickness of the deposited layer on the operational strength of welded joints. Number of deposited layers 2 3 4 Thickness of the deposited layer after preparation (turning) of the edges 3 5 7 Tensile strength, kgf / mm ² 17,2 45.6 47.2 Relative extension, % 6.7 12.1 13,2 Bending angle during static bending, degrees nine 57 60

The standard technology for the production of centrifugally cast pipes involves annealing in a continuous furnace at a temperature of 930 ± 20 ° C.

We have developed a mode of annealing centrifugally cast pipes from VChShG with deposited edges at a temperature of 750 ± 20 ° C for 30 minutes. This mode provides equal strength and high ductility of pipes and welded joints, significantly reduces their thermal deformation and cost - table No. 5.

Table No. 5.
Effect of annealing temperature on the mechanical characteristics and thermal deformations of welded joints Annealing temperature, ° С 650 700 750 800 930 Tensile strength, kgf / mm ² 41.2 44,4 47.6 41.2 43.5 Relative extension, % 4.8 6.7 7.7 4.6 3.2 Bending angle, degrees 31 38 59 48 41 Thermal deformation of welded joints, mm from the longitudinal axis 4 4 5 eleven eighteen

In order to optimize the cooling rate of the deposited metal and ensure its directed crystallization during cooling, a surfacing method using a water-cooled copper crystallizer is proposed and implemented.

A distinctive feature of this method is the provision of calculated cooling rates of the weld joint and, as a consequence, the operational strength of welded joints from high-frequency alloys.

The cooling rate of the iron-nickel deposited metal of the welded joint is controlled by the flow rate of the cooling mold water.

The mold provides a more favorable from the standpoint of the absence of defects and high operational strength structure and properties of the weld metal and welded joint - table No. 6.

Table number 6 The cooling rate of the weld metal, ° C / s 10 fifteen twenty 25 The angle of bend of the weld, degrees 34 57 60 28 Tensile strength, kgf / mm ² 41.3 48,2 49.1 36.6

References

1. RF patent No. 2064383.

2. RF patent No. 2177861.

3. USSR copyright certificate No. 1181830 - prototype.

4. Copyright certificate of the USSR No. 833407.

5. Copyright certificate of the USSR No. 1050837.

6. USSR author's certificate No. 1074691.

Claims (1)

A method of assembly welding of nodular cast iron products, in which an intermediate layer of iron-nickel alloy is deposited onto the edge surface before welding, characterized in that the deposition is carried out at a cooling rate of the deposited layer of 15-20 ° C / s in a water-cooled copper crystallizer with an adjustable flow rate of cooling water, an iron-nickel alloy is used an alloy containing C 1.6-2.0%, Ni 48.0-52.0%, Fe - the rest, and after surfacing, mechanical processing is performed to ensure the thickness of the deposited layer is 5.0-7 , 0 mm.