GB2253804A - Open arc welding method and flux cored consumable electrode wires for use in the above method - Google Patents

Abstract

A method of welding employs an electric arc established between a welding wire and the work piece that takes place in air without a protective gas shield or alternatively in a nitrogen rich shielding gas equivalent to air. The flux cored weld metal is designed to contain a deficiency of nitrogen and relies on recovering this element from the surrounding air. It has been found that Nitrogen can be introduced in this manner and up to the thermodynamically maximum level can be obtained in the deposit, without the conventional problems of porosity. In particular high chromium martensitic deposits have been found to benefit by this treatment; and other alloys with a deliberate nitrogen addition can equally be welded by the method described.

Description

Title: Open arc welding method and flux cored consumable electrode wires for use in the above method.

We, S. Atamert and J. J. K. Stekly, of The Hay, Fowlmere, Royston, Herts, SG8 7QP, England, do hereby declare the invention, for which we pray that a patent may be granted to us, on the method and the particular welding wires used therein, all of which are further described in and by the following statements: The present invention relates to an open arc welding method, with no externally added gas shielding, for welding and cladding of metals of any description and analysis.

Arc welding consists of creating an electric arc between the electrode and the work piece. Should the electrode be a consumable item it is either a wire or a metal rod, and the heat of the arc melts the electrode and the work piece surface. Most welding processes require some form of protection from the atmosphere and this is achieved by the addition of a flux or a gas shield; primarily to restrict the access of the atmosphere to the molten metal. Should there be no protective gas or covering flux during the welding process it is termed "open arc welding".

In open arc welding, the surrounding atmosphere of air has an effect on the arc characteristics and on the chemistry of the weld deposit. Welding wires designed to work without gas protection have suffered from a deterioration and, at the least, an undesirable alteration of properties upon exposure to atmosphere during welding.

At the high temperatures that occur during welding, the solubility of nitrogen gas in the weld metal is relatively high and it decreases with temperature. As nitrogen pressure rises due to the decrease in solubility and possibly phase change caused by the decreasing temperature, porosity may occur; hence materials such as aluminium are added to combine with the nitrogen to reduce or eliminate porosity on cooling.

Heretofore, it has been known that a flux cored wire can be used in the open arc welding mode. The flux cored wire comprises a sheath or outer envelope of meta and a core of metal and fluxing compound powders.

For example the flux cored steel welding wires generally described under the AWS category E70T4 have a very high aluminium content to allow welding with no external gas protection, since the aluminium contained in the wire core combines, during the welding process, with the nitrogen and oxygen absorbed from the atmosphere and dissolved in the weld metal. The combination of the atmosphere with aluminium produces nitrides and oxides, thereby nullifying the absorbed gases' detrimental contribution.

Such prior art flux cored wires, operating in the open arc mode, present problems in the manufacture and in obtaining desired mechanical properties. The drawbacks include the need to add a substantial quantity of fluxing agents to generate a protective shield around the arc during welding and further the need to include alloying elements to interact with the gases absorbed from the atmosphere.

It is the proposal of this invention to use one of the elements absorbed from the atmosphere, namely nitrogen, as a desired addition to the weld metal; since the weld deposit chemistry will be designed deliberately to have a deficiency of nitrogen, as will be seen from the example below.

The composition of the wire, which includes additions to the core, is made in such a manner as to benefit by the addition of nitrogen from the surrounding atmosphere of an open arc weld. By contrast, the prior art products would also have contained nitrogen from the atmosphere; however, this element did not contribute to any of the properties of the weld metal in a positive sense and was not deliberately used to improve deposit properties. Although the prior art products would have contained nitrogen absorbed from the atmosphere since this process in open arc welding is unavoidable; it is however clear that such additions were invariably undesirable and considerable efforts were made to minimise or annul the deterioration of properties of the weld metal caused by the nitrogen absorption.

Accordingly, the invention covers the system of open arc welding, in which no additional gas shielding is added, with the surrounding atmosphere eom.prising air only, and the cored wire used in the process, all of which combined allows the beneficial use of the absorbed nitrogen.

To illustrate the method of applying the invention ferritie > /martensiticb steels can be considered. To obtain nitrogen at the required levels and sound metal after melting, only high pressure electro-slag remelting techniques have been found satisfactory.

As the application of high pressure nitrogen is not feasible during welding, it has been found by this invention, that nitrogen can be obtained at the desired level by two routes, either singly or in combination. The two possible routes are nitrogen from the air (Na) alone alternatively nitrogen from the welding wire (Nw) in combination with nitrogen from the air.

According to this approach, and in this specific example: Nrequjred Nv + N. with Nw > or =0 A typical group of materials which can be produced, according to this invention, are listed in examples.

The open arc welding method of the invention is carried out typically as follows: 1. Single electrode wire open arc welding In this operation a single wire is fed through a welding gun and the arc is established between the wire and the work piece.

No gas shielding is used for the protection of the arc.

2. As a twin or multiple electrode method.

In this operation any or all of the wires can carry welding current and any of the wires can be directed into the welding arc without carrying current, as a cold wire addition. No covering flux is added to these operations and no gas shielding is used.

The flux cored wires used in this invention will comprise an external sheath and a powder filled interior. The outer sheath may be made from any of the conventional metals, comprising steel base, nickel base, cobalt base and copper base.

Should the sheath be made of a steel base metal, a typical composition will be: C= .01 to 0.30% Si= < 1. 0% Mn= < 3. 0% Mo= < 1.5% Ni < 31 and the balance, besides trace elements, is iron.

The filling of the core of the wire, according to the invention, has a fill ratio of powder to sheath as defined by the formula below, in the range between 10 and 50% as fill percentage.

weight of fill powder x x100 % = fill percentage.

weight of sheath material.

The powder composition of the core may comprise any of the commonly used ferro-alloys, metal powders, oxides and fluxes generally used in the welding industry; however combined in such a manner as to benefit by tfle nitrogen addition from the eventual arc welding process.

Typical examples of welding wires that can be used according to the present 'invention are listed in the table below, as examples 01, 02 and 03. All compositions listed are considered as all weld metal analysis produced by the open arc welding process.

Hereafter and according to the invention, it is possible to obtain sound weld metal containing nitrogen; the process relying on the pressure of nitrogen from the atmosphere remaining at normal levels, or alternately a protective shielding gas around the welding arc, containing primarily nitrogen, and hence similar to air for all practical purposes.

The open arc welding process and the cored wire invention are industrially advantageous in that they allow welding to occur in a simple and manageable manner and furthermore the weld metal deposit has substantial advantages over materials and methods used to date.

Example: It has been found that weld metal deposits of high chromium content (4 - 16%)produced by open arc welding with flux-cored arc welding wires can be designed to produce porosity-free martensitic weld deposits with up to the maximum thermodynamically possible nitrogen content.

In these alloys the replacement of nitrogen for carbon is relevant and will be explained further. The use of this invention makes it possible to weld these materials in air and thereby absorb at least part of the required nitrogen from the atmosphere.

The technique involves the use of nitrogen from the air and from the wire. Welding wires can be used, by this technique, for joining and cladding purposes of any iron-based plate.

High chromium (4-16%) and high carbon (up to 0.5%) content alloys are used extensively in applications where high hardness, high temperature corrosion and wear resistance are important (e.g. continuous casting rolls). The as-welded microstructure in these steels consists of mainly martensite with some retained ferrite. The first solid phase to form in the liquid is delta ferrite which later transforms to either delta plus austenite or to a single phase austenite depending on the composition and cooling rate. As the temperature decreases, the austenite transforms to martensite but the ferrite remains unchanged. The presence of ferrite is believed to be detrimental to strength, but its presence is associated with improved machineability.

The ferritic solidification is preferred to austenitic solidification, because the segregation of impurities to grain boundaries is reduced and consequently solidification cracking is less likely to occur.

Due to the various welding techniques used, and particularly due to the multiple thermal cycling experienced by the weld beads, the structure will also contain M23 CE type carbides which preferentially nucleate at austenitic boundaries or martensitic lath boundaries. These carbides are chromium rich and tend to be thermodynamically stable below about 9000 C. In practice, the formation of these carbides is unavoidable and the volume of carbides increases with increasing carbon content.

These carbides, as is well documented, cause chromium depletion zones adjacent to grain boundaries and result in a detrimental effect known as weld decay. In corrosive environments chromiumdepleted regions are sensitive to pit nucleation and, subsequently, stress corrosion cracking.

In order to avoid these problems considerable work has been done to replace carbon; nitrogen additions have found preference and dramatic improvements have since been achieved. The levels of nitrogen content required to achieve the same tensile strength or hardness as is readily available from carbon additions are not easily obtained with simple nitrogen additions.

The two possible routes to obtain nitrogen additions are nitrogen from the air (Na) and nitrogen from the welding wire (Nw). In order to achieve the maximum hardness weld metal the maximum thermodynamic level of nitrogen will be required. This Nx will be consist of: Niax = Nv + Na with Nv > or =0 In this group typical materials which can be used according to this invention are listed below: Table: Weld metal chemical composition of possible materials benefitting by the use of this invention.

C Mn Si Cr Mo Ni W V Co Fe Ol .04 4. 1.0 12.7 0. 5 0. - - - balance 02 . 04 1. 2 0. 7 12. 5 0. 4 4. 0 - - - balance 03 . 04 1. 2 0. 5 12. 8 0. 6 5. 0 0. 8 0. 5 2. 0 balance For cladding purposes particularly, deposits with the analysis as indicated in the above table have to reach hardnesses above Rc 40; and this is only possible by the use of the invention as described.

Claims (6)

CLAIMS:

1. An open arc welding method, achieved by establishing an electric arc which takes place in air, requiring no additional gas shield, to produce a weld metal in which nitrogen as recovered from the surrounding natural atmosphere is a deliberate addition to the weld metal.

2. A flux cored wire produced such that it can be used in an open arc welding method, and the consequent weld metal designed to contain nitrogen which is either totally or in part absorbed from the atmosphere during a welding process of any description.

3. A weld deposit achieved by any arc process, which uses a gas as a shield, which for all practical purposes is substantially the same as air and encompasses methods of the above claims 1.

and/or 2.

4. A weld deposit obtained by any method which depends on nitrogen as a part substitute for carbon and at least a portion of the nitrogen having been recovered from air or from a gas shield similar to air for all practical purposes.

5. An open arc welding method, or a method with a gas shield equivalent to air, achieved by designing the wire to produce a weld metal containing alloys in the 4-16% chromium range, in which nitrogen will be a deliberate substitute for at least some of the carbon and encompassing any of the above claims.

6. A welding method in which a deliberately large proportion of the thermodynamic maximum nitrogen solubility level is achieved by nitrogen recovered from the surrounding air or from a gas which is similar to air for practical purposes.