AU729937B2

AU729937B2 - Process for manufacturing a flux-cored wire with recrystallization annealing

Info

Publication number: AU729937B2
Application number: AU78692/98A
Authority: AU
Inventors: Denis Astier; Christian Bonnet
Original assignee: La Soudure Autogene Francaise
Current assignee: Lincoln Electric Co France SA
Priority date: 1997-08-12
Filing date: 1998-08-04
Publication date: 2001-02-15
Anticipated expiration: 2018-08-04
Also published as: CA2245046A1; AR013405A1; TW402537B; NO983587L; BR9803142A; EP0899052A1; KR19990023483A; AU7869298A; FR2767338A1; NO983587D0; JPH11123591A

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: La Soudure Autogene Francaise Actual Inventor(s): Christian Bonnet Denis Astier Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: PROCESS FOR MANUFACTURING RECRYSTALLIZATION ANNEALING A FLUX-CORED WIRE WITH Our Ref 538229 POF Code: 1290/214865 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1- 0 4 Oro The present invention relates to the field of the manufacture of welded metal tubes filled with filling elements, such as flux-cored wires intended for welding, involving one or more high-frequency recrystallization annealing steps.

Conventionally, when it is desired to reduce the diameter of a metal tube which may or may not contain filling elements, such as a flux-cored welding wire, the said metal tube is subjected to one or more steps of rolling and/or drawing the latter.

However, it is also known that the reduction in diameter of a metal tube by rolling and/or drawing causes the metallurgical phenomenon of work hardening.

Now, when the degree of work hardening increases, i.e. when the overall reduction ratio of the tube increases, there is an increase in the hardness of the material and a decrease in its ability to undergo subsequent deformations without fracture.

In other words, there is a certain threshold or 20 critical reduction ratio above which problems of tube fracture occur.

Currently, in order to be able to continue to roll and/or draw the tube beyond this threshold or critical ratio, the structure of the material is 25 regenerated by recrystallization annealing the structure.

For this purpose, the material of the tube is heated to a temperature above the so-called recrystallization temperature.

This recrystallization temperature depends on the nature of the material of which the metal tube is composed.

Thus, it is about 600 0 C in the case of a mild steel and about 750 0 C in the case of an austenitic stainless steel, for example.

Normally, this recrystallization annealing operation is carried out in a batch furnace or in a continuous furnace.

The hold time above the recrystallization temperature goes from a few hours (in the case of a 2 batch furnace) to a few minutes (in the case of a continuous furnace) However, it has been observed that when a metal tube filled with filling elements, such as a flux-cored wire, is made to undergo such a recrystallization annealing operation, undesirable chemical reactions occur between at least some of the various constituents contained in the filling elements.

Thus, rutile-type flux-cored welding wires generally contain filling elements consisting of a mixture of iron and iron-alloy powders which, alloyed with the sheath of the flux-cored wire, gives the weld a chemical composition tailored to the desired mechanical strength, ionizing elements making it S 15 possible to improve the stability of the electric arc, .oxides, such as TiO 2 A1 2 0 3 or SiO0 2 which are intended to form a slag, the physical properties of which, especially the melting point, viscosity and surface tension, are the source of the operating characteristics of this type of flux-cored wire, and powerful deoxidizing agents, such as Mg, Al or AlMg, S" which make it possible to lower the oxygen content of the molten metal deposited, thus improving the *..toughness properties of the weld.

25 Now, if such a powder mixture is raised to a high temperature, for example to 700C, during a recrystallization annealing treatment of the outer sheath of the tube containing it, oxidation-reduction reactions, which go more or less to completion, occur between the various constituents of the filling powder, these reactions being likely to make the wire unsuitable for its intended application.

The object of the present invention is therefore to provide a process allowing effective recrystallization annealing of a metal tube containing filling elements, such as a flux-cored welding wire, which process does not adversely affect the filling elements contained in the said tube, or does so only very slightly.

3 The invention therefore relates to a process for manufacturing a metal tube containing filling elements, comprising at least one recrystallization annealing step carried out by high-frequency induction heating.

Depending on the case, the process of the invention may comprise one or more of the following characteristics: the frequency is at least 10 kHz, preferably at least 100 kHz, and advantageously between 250 kHz and 500 kHz, approximately; at least one recrystallization annealing step is carried out after at least one step of reducing the diameter of the tube by rolling and/or drawing; 15 it comprises, after a recrystallization *1 annealing step, at least one step of cooling the tube; the recrystallization temperature is greater than or equal to 700C, preferably greater than or equal to 750 0

C;

the filling elements in the tube comprise at least one metal powder chosen from iron powder, ferrosilicon powder, silico-manganese powder, manganese powder, ferro-silico-titanium powder, magnesium powder, aluminium powder and mixtures thereof; 25 the metal tube is a flux-cored wire.

The invention also relates to a flux-cored wire which can be obtained by the abovementioned process, which flux-cored wire may comprise a metal sheath made of mild steel.

Such a flux-cored wire can be used for the implementation of a welding process, such as a TIG, MIG, MAG, submerged-arc and vertical electroslag welding process.

More specifically, the inventors of the present invention have demonstrated that the regeneration of the tubular sheath, i.e. the recrystallization annealing, may be carried out in an effective manner by heating the tube, which has undergone one or more drawing and/or rolling steps, by means of a high- 4 frequency inductor and that, by operating in this way, the filling elements or powder contained in the said tube are almost unaffected or affected only minimally.

This is because the depth of penetration of the high-frequency currents, also called the "skin" depth, is defined by the following formula: -8 ;P 500 F rP in which: p uopr is the magnetic permeability (in H.m 1 of the material (here, the sheath of the tube); o is the magnetic permeability of free space (47rx10-7 H.m- 1 Pr is the relative permeability of the material; P is the resistivity (in Q.m) of the material; and f is the frequency (in Hz) of the 20 induction current.

Approximately 87% of the Joule-effect heating energy is released in this skin depth by the induced current.

Thus, for a given material, the skin depth is therefore inversely proportional to the square root of the frequency of the current in the inductor.

In the case of steels, the skin depth is also a function of temperature, since the magnetic permeability depends thereon.

For example, the skin depth for high-frequency currents is 0.7 mm at ambient temperature (about and 17 mm at 1000 0 C in the case of a frequency of 1 kHz whereas, at the same temperatures, it is 0.22 mm and 5.4 mm, respectively, for a frequency of 10 kHz and 0.04 mm and 1.1 mm, respectively, for a frequency of 250 kHz.

In the context of the manufacture of flux-cored wires intended for welding, the recrystallization annealing is generally carried out when the diameter of the flux-cored wire is about 4 mm, i.e. for a tubular sheath thickness of about 1.1 mm on average.

Taking into account the foregoing, in order to minimize the heating of the filling elements contained in the flux-cored wire and to avoid or minimize the chemical reactions between the various constituents, it is only necessary to heat the outer part of the tubular sheath using a sufficiently high frequency, i.e. of at least 1 kHz, the rest of the tubular sheath being essentially heated by thermal conduction to above the recrystallization temperature.

In other words, the inventors of the present invention have demonstrated that by carrying out the 15 process in this way the rise in the temperature of the filling elements contained in the flux-cored wire is not enough to generate undesirable chemical reactions between the various constituents of the said filling elements.

The invention will now be described in greater detail with the aid of examples which are given by way of illustration but which are in no way limiting.

a. Examples The following tests were carried out on various types of flux-cored wires intended for welding structural steels, these wires comprising a sheath made of mild steel having a carbon content of about 0.04%.

The recrystallization annealing is carried out on wires drawn and/or rolled to diameters of 4 mm and 4.7 mm, depending on the case, and at frequencies of 250 and 500 kHz, the power in the inductors being adjusted so as to reach various temperatures on the surface of the flux-cored wires.

After the recrystallization annealing step, the flux-cored wires are subjected to natural cooling to ambient temperature, or cooled by spraying with water.

6 After these various treatments, specimens are removed so as to measure their hardness and to examine, using metallography, whether the recrystallization has actually taken place.

The results obtained are given in Table I below.

TABLE I *r TYPE OF FLUX- TEMPERATURE COOLING RECRYSTALLIZATION

VICKERS

CORED WIRE (in OC) __HARDNESS 600 natural No n.d.

water No n.d.

670 natural No 202 water No 207 "slag-free" 750 natural Yes 110 water Yes 160 800 natural Yes 110 water Yes 155 900 natural Yes 115 water Yes 177 600 natural No n.d.

water No n.d.

670 natural No 217 water No 220 rutile 750 natural Yes 116 water Yes 165 800 natural Yes 114 water Yes 188 900 natural Yes 117 _water Yes 180 not determined In the tests, similar results were obtained whatever the diameter of the flux-cored wire, the nature of the filling powders and the high frequency used.

-7- However, as may be seen from the above table, the temperature reached proves to have a major effect on the recrystallization of the tubular sheath.

This is, because it is apparent that, under these very rapid heat treatment conditions, it is necessary to reach a higher temperature (at least 7500C) than during a conventional treatment in a continuous furnace or a batch furnace, during which the temperature is generally only 6000C.

I0 Furthermore, the hardness measurements show that when the temperature reached has allowed the tubular sheath to be recrystallized the resulting softening, i.e. the difference in hardness between the non-recrystallized state and the recrystallized state, 15 is very different depending on the mode of cooling chosen, despite the very low carbon content of the tubular sheath.

More specifically, the recrystallized fluxcored wires cooled by water spraying retain a relatively high hardness, despite the recrystallization, and were unable subsequently to be drawn down to a diameter of 1.2 mm without fracture, whereas the flux-cored wires cooled naturally, i.e. by heat exchange with the ambient atmosphere, have a lower hardness and have been able to be drawn without fracture down to the said diameter of 1.2 mm.

Such wires, drawn to a diameter of 1.2 mm, were compared with flux-cored wires which had undergone a conventional recrystallization treatment at 6000C for 1 hour in a batch furnace and also with wires containing the same filling powders and manufactured according to the "Chemetron" process, which process does not require an intermediate heat treatment given that, by means of this process, the wires are formed and filled with powder at a diameter of 5 mm.

It follows from these comparative tests that: the wires according to the invention which are annealed by high frequency and cooled naturally behave, from an operating standpoint, in a manner 8 similar to the unannealed wires obtained using the "Chemetron" process; the wires treated in a batch furnace result in much less stable electric arcs and in a much greater degree of spatter, therefore in operating characteristics which are markedly inferior to those of the wires according to the invention, thereby confirming the existence of the problem of reactions between the various elements constituting the filling elements of the flux-cored wire when these are subjected to recrystallization annealing using a conventional process.

In other words, high-frequency recrystallization annealing followed by sufficiently i 15 slow cooling allows effective recrystallization of the tubular sheath, i.e. allows the latter to be regenerated, making it possible for the said tube subsequently to undergo drawing and/or rolling steps without causing fracture and at the same time guaranteeing operating quality of the flux-cored wire when it is used in a welding process, given that the properties of the filling elements are not affected by the high-frequency recrystallization annealing heat treatment.

According to the invention, frequencies greater than 100 kHz are preferred, for example frequencies of between 250 and 500 kHz.

However, it is possible to use lower frequencies, but the minimum frequency which can be used should not be less than 10 kHz.

This is because, by operating in this manner, most of the energy is released by the Joule effect in the outer quarter of the tube thickness, when the tube is at low temperature, in order to reach slightly more than one third of the thickness at 7000 (because of the variation in magnetic permeability) and then the entire thickness when the temperature reaches and exceeds the Curie point (769°C in the case of steel).

-9 Furthermore, the differences observed with regard to the hardness of the material after recrystallization annealing, depending on the mode of cooling chosen, may be explained by the difference in cooling rate involved in the two modes of cooling chosen (natural cooling or water spraying) This is because, when the cooling is "natural" it takes approximately 30 seconds for the flux-cored wire to go from the annealing temperature (greater than 700 0 C) to a temperature of about 400 0 C, whereas cooling by spraying water results in a time to cool down to ambient temperature of about one second.

Additional tests (Tests 1 to 4) have shown that the final hardness (final Vickers hardness) after recrystallization and cooling down to ambient temperature (To) is in fact mainly determined by the Stime to cool between the recrystallization temperature (TR) and a temperature of approximately 400 0

C.

Thus, the test results given in Table II below show that, from an industrial standpoint, it is advantageous to use only natural cooling (still air) or *else slightly accelerated cooling (by spraying water, for example) from the annealing temperature (TA) down to about '4000C and then to supplement this cooling by additional cooling by spraying a coolant, such as water, so as to obtain a temperature of the material which is approximately equal to ambient temperature and to do so by minimizing the length of "buffer" wire in the cooling loops.

10 TABLE II Test No.

1 2 3 4 Transition from TR to 400 0

C

Transition from 400"C to To

CM

water still air blown air still air Hardness 1 sec.

30 sec.

10 to 15 sec.

30 sec.

CM

Water Still air Blown air water Hardness 1 sec.

several minutes 1 min Final Vickers hardness 165 115 140 1 sec. 125 CM: Cooling mode It follows that the cooling of the tube preferably comprises at least one slow cooling step from approximately the recrystallization temperature (TR) to an intermediate temperature and, optionally, at least one rapid cooling step from approximately the intermediate temperature to ambient temperature (To).

Advantageously, the slow cooling step takes at least 10 seconds, preferably at least 20 seconds, and/or the rapid cooling step is carried out by means of a coolant and preferably by spraying water.

Normally, the intermediate temperature is between 300°C and 5500C, preferably about 400°C.

Advantageously, at least part of the induction heating step and/or of the slow cooling step is carried out in an inert protective atmosphere so as to avoid or minimize any surface oxidation of the tube.

The inert protective gas used is nitrogen, argon or a gas mixture containing at least one of these two gases, or any other gas or gas mixture known to those skilled in the art.

11 Throughout the description and claims of this specification the word "comprise" and variations of that word, such as "comprises" and "comprising", are not intended to exclude other additives or components or integers.

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Claims

1. A process for manufacturing a metal tube containing filling elements, comprising at least one recrystallization annealing step carried out by high-frequency induction heating, the said frequency being at least 10 kHz.

2. A process according to claim 1, wherein the frequency is at least 100 kHz.

3. A process according to either of claims 1 and 2, wherein the frequency is between 250 kHz and 500 kHz, approximately.

4. A process according to any one of claims 1 to 3, wherein at least one recrystallization annealing step is carried out after at least one step of reducing the diameter of the tube by rolling and/or drawing.

A process according to any one of claims 1 to 4, wherein it comprises, after a recrystallization annealing step, at least one step of cooling the tube.

6. A process according to claim 5, wherein the cooling of the tube comprises at least one slow cooling step from approximately the recrystallization temperature to an S 15 intermediate temperature and, optionally at least one rapid cooling step from approximately the intermediate temperature to ambient temperature.

7. A process according to claim 6, wherein the slow cooling step lasts at least seconds, preferably at least 20 seconds, and/or in that the rapid cooling step is carried out by means of a coolant and, preferably, by spraying water.

8. A process according to claim 6, wherein the intermediate temperature is Sbetween 3000C and 5500C, preferably about 4000C.

9. A process according to any one of claims 1 to 8, wherein the recrystallization temperature is greater than or equal to 7000C, preferably greater than or equal to 7500C.

10. A process according to any one of claims 1 to 9, wherein the filling elements in the tube comprise at least one powder chosen from iron powder, ferro-silicon powder, silico-manganese powder, manganese powder, ferro-silico-titanium powder, magnesium powder, aluminium powder and mixtures thereof.

11. A process according to any one of claims 1 to 10, wherein the metal tube is a flux-cored wire. C:\WINWORD\GAYNODELETE\58229.DOC

12. A process according to any one of claims 1 to 11, wherein at least part of the induction heating step and/or of the slow cooling step is carried out in an inert protective atmosphere.

13. A flux-cored wire which can be obtained by the process according to any one of claims 1 to 12.

14. A flux-cored wire according to claim 13, wherein it comprises a metal sheath made of mild steel.

Use of a flux-cored wire according to either of claims 13 and 14 for the implementation of an arc welding process.

16. TIG, MIG, MAG, submerged-arc or vertical electroslag welding process which can employ a flux-cored wire according to either of claims 13 and 14.

17. A process according to claim 1 substantially as hereinbefore described with reference to any of the examples. DATED: 3rd August, 1998 15 PHILLIPS ORMONDE FITZPATRICK Attorneys for: LA SOUDURE AUTOGENE FRANCAISE C:\WINWORD\GAY\NODELETE\538229.DOC