EP2763813A1

EP2763813A1 - Solder material for stainless steel components, particularly for the furnace soldering of heat exchangers for gases

Info

Publication number: EP2763813A1
Application number: EP12786853.7A
Authority: EP
Inventors: Benjamin Gracia; Carmen LARROSA LACUEY
Original assignee: Valeo Termico SA
Current assignee: Valeo Termico SA
Priority date: 2011-10-05
Filing date: 2012-10-05
Publication date: 2014-08-13
Also published as: WO2013053654A1; ES2401625A1; KR20140072161A; ES2401625B1

Abstract

Solder material (8) for stainless steel components, particularly for the furnace soldering of heat exchangers (1) for gases, said solder material (8) being made of an alloy containing boron, and is characterized in that it comprises a percentage of niobium capable of reacting with the nitrogen contained in the stainless steel itself of the components that are to be soldered, being liable to form nitrides of niobium and thus preventing the nitrogen from reacting with the boron of the solder material (8).

Description

WELDING MATERIAL FOR STAINLESS STEEL COMPONENTS, ESPECIALLY FOR OVEN WELDING OF HEAT EXCHANGERS

FOR GAS

The present invention relates to a welding material for stainless steel components.

The welding material of the invention is particularly applicable to the furnace welding of exhaust gas recirculation exchangers (EGRC).

BACKGROUND OF THE INVENTION

In some heat exchangers for gas cooling, for example those used in exhaust gas recirculation systems to the inlet of an internal combustion engine, the two heat exchanging media are separated by a partition.

The heat exchanger itself may have different configurations: for example, it may consist of a carcass inside which are arranged a series of parallel tubes for the passage of gases, the coolant circulating in the carcass around the tubes; in another embodiment, the exchanger comprises a series of parallel plates which constitute the heat exchange surfaces, so that the exhaust gas and the cooling liquid circulate between two plates in alternating layers.

In the case of tube bundle heat exchangers, the junction between the tubes and the carcass can be of different types. Generally, the tubes are fixed at their ends between two support plates coupled to each end of the carcass, the two support plates having a plurality of orifices for the introduction of the respective tubes. Said support plates are in turn attached to connecting means to the recirculation line.

These connection means may consist of a V-shaped connection or a peripheral rim connection or flange, according to the design of the recirculation pipe where the exchanger is assembled. In some cases, the support plate is integrated in one piece with the connecting means, thus forming a single connecting flange. The connection means may also consist of a gas deposit disposed at one or both ends of the carcass. At present, EGR heat exchangers are made of metal, generally made of austenitic stainless steel, in most cases AISI 304.

All the components of the heat exchangers, both stacked tube and stacked plates, are metallic, so that the heat exchangers are all assembled by mechanical means, then welded in the oven to ensure a level of tightness required for this application.

For EGR exchangers, a very common welding material for welding is the Ni620 alloy, according to the new ISO 17672: 2010 standard, and called BNi2 (by AWS), which contains boron. This alloy is economical and is the most used for baking stainless steel heat exchangers, especially for vacuum furnace welding.

The components of the heat exchanger are increasingly being manufactured in countries where the cost of manufacture is lower. However, there are sometimes problems because the components are not always welded satisfactorily.

The problem that has been detected is that the Ni620 solder material does not wet the surface of the component to be welded and does not extend, so that the resulting soldering is unsatisfactory, hence a risk leaks.

The composition of the N1620 solder material comprises: 6-8% chromium, 4-5% silicon, 2.75-3.5% boron, 2.5-3.5% iron, and the balance nickel.

To carry out the analysis tests, a few drops of this welding material are placed on a part of the component to be welded, which is then put in the oven for its welding as a standard heat exchanger.

When the drops of the solder material extend over the surface of the component to be welded, the soldering is performed satisfactorily; otherwise, the weld is deficient with the leakage problems that follow.

In the case of joints between the gas tubes and the support plates, the baking is good enough for the majority of the tubes, but if some tubes or even a single tube are not welded partially or totally, the result final is a leak.

To analyze this problem, various tests with different types of components and different manufacturing technologies of said types of components have been carried out, for example corrugated gas tubes, cooling and flanges made of cast iron.

In some cases, it has been detected that this problem is related to the annealing process of the components during their manufacture. In these cases, the manufacturer of these components uses continuous furnaces with an atmosphere of a mixture of nitrogen and hydrogen from an ammonia cracking process.

If the cracking process is not well controlled, a minimal amount of ammonia can produce a nitriding effect on the surface of the component to be welded, by depositing nitrogen which interferes with the subsequent oven welding.

In other cases, in the absence of an annealing process, for example in the case of flanges made of cast iron, it is suspected that the cause of the problem is the quality of the raw material. However, the material certificate is correct, guaranteeing a certain percentage of nitrogen below the established limit.

In accordance with international standards, 1.4301 (AISI 304) austenitic stainless steel alloys can include up to 0.11% nitrogen, but can not be completely avoided due to the refining process. 'steel.

As a result, the nitrogen reacts with the boron contained in the Ni620 solder material and results in the problem of forming boron nitrides, which have a negative effect on the oven welding. In addition, the nitrogen is difficult to analyze and is generally not indicated in the material certificate, and according to the analytical technique, whether volumetric, spectroscopic or similar, different results are obtained. In the tests carried out by SEM microscope, it was found, in the components analyzed, the presence of superficial nitrogen near the drop of solder material which has not been sufficiently extended. Sometimes, using a SEM microscope, in some cases local nitrogen was detected around 10, reacting with boron.

It is known that the presence of niobium in steel is advantageous for baking when nitrogen is present in the steel.

When carbon and nitrogen are present, niobium reacts first with carbon to form NbC, then niobium reacts with nitrogen to form NbN.

Therefore, the amount of niobium required must be consistent with the following relationship:

% Nb% C% N

> +

93 12 14 This relationship is cited in US Patent 4461811 which further includes titanium and tantalum as stabilizers.

In the case of a low carbon austenitic stainless steel with <0.03% carbon and around 0.05% nitrogen, it is necessary to have a quantity of at least 0.56% of niobium.

In addition, the Ni620 solder material can have as impurities up to 0.06% carbon. In addition, in the case of AISI 304 type steel with <0.08% carbon and a maximum of 0.11% nitrogen, it would be necessary to have a quantity of at least 1.81% of niobium.

Spanish patent application No. 2351281, from the same proprietor as the present application, discloses a gas heat exchanger made of stainless steel, which comprises a ferritic stainless steel flange united to the core of the oven-baked heat exchanger. a solder material made from an alloy that comprises boron. The ferritic stainless steel of the flange further comprises a percentage of niobium capable of reacting with the nitrogen contained in the own ferritic stainless steel, forming nitrides of niobium, so that the nitrogen is not allowed to react. with the boron of the solder material. DESCRIPTION OF THE INVENTION

The object of the stainless steel component welding material, particularly for the furnace welding of gas heat exchangers, of the present invention is to overcome the disadvantages of solder materials known in the art by providing a solder material having adequate wettability, and without the need to modify the stainless steel material of the components to be welded.

The welding material for stainless steel components, in particular for furnace welding of gas heat exchangers, object of the present invention, is of the type which is manufactured in an alloy which comprises boron, and is characterized by the it comprises a percentage of niobium capable of reacting with the nitrogen contained in the own stainless steel of the components to be welded, being capable of forming nitrides of niobium and thus avoiding that the nitrogen reacts with the boron of the material of welding.

The advantages of using the new material of the invention are described below:

- The standard oven welding process and the oven itself can be used without any type of modifications.

- No special considerations are necessary to define the parts to be welded, nor the adjustments or manufacturing tolerances of the known exchangers.

- The quality of the solder joints improves due to a better wettability of this new welding material.

According to one embodiment of the present invention, the chemical composition of the solder material comprises, by weight, from 6 to 8% of chromium, from 4 to 5% of silicon, from 2.75 to 3.5% of boron, 2.5 to 3.5% iron and 0.5 to 4% niobium.

In this composition, the percentage of agglutinant to prepare the dough was not taken into account.

Preferably, the weight percentage of niobium added is 2%. Optionally, the solder material may include up to 20% chromium by weight in order to improve the corrosion resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate the description of what has been explained above, drawings are enclosed in which is shown, in schematic form and only by way of non-limiting example, a practical case of producing the welding material for steel components. stainless, particularly for furnace welding of gas heat exchangers of the present invention. In these drawings:

Figure 1 is a schematic longitudinal section of a conventional heat exchanger assembled and welded in the oven; and

Figure 2 is a partial perspective view of a metal core welded with a connection flange.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1 and 2 illustrate a heat exchanger 1 EGR made of stainless steel which comprises a core consisting of a bundle of tubes 2 arranged inside a carcass 3, intended for the circulation of gases with heat exchange with a cooling fluid, a connection flange 4 to the recirculation line, and inlet connection lines 5 and outlet 6 of the cooling fluid. In this case, said heat exchanger also comprises a gas tank 7 for the return of gases.

Said components 2 to 7 of the exchanger 1 are assembled together and soldered by welding with a solder material 8 made of an alloy which comprises boron.

The solder material 8 used is preferably Ni620, also referred to as BNi2, which is widely used for the furnace-welding of EGR heat exchangers, as it is suitable for most vehicle manufacturers' specifications. , being more economical and very easy to use.

In accordance with international standards, materials such as 1.4301 (AISI 304) austenitic stainless steel alloys may include up to 0.11% nitrogen, but it is not possible to avoid it completely because of the process. refining of steel. The problem encountered in the state of the art during the oven welding process is that the nitrogen reacts with the boron contained in the Ni620 solder material, making it difficult to weld the parts to be assembled. In addition, the nitrogen is difficult to analyze and is generally not indicated in the material certificate, and according to the analytical technique, whether volumetric, spectroscopic or similar, different results are obtained. Using an SEM microscope, it is possible, in some cases, to detect local nitrogen at around 10%, reacting with boron.

To solve this problem, the solder material 8 of the present invention comprises a percentage of niobium capable of reacting with the nitrogen contained in the own stainless steel, forming niobium nitrides, and thus preventing the nitrogen from reacting with the nitrogen. boron of the solder material 8.

It is known that the presence of niobium in steel is advantageous for baking when nitrogen is present in the steel. However, it is more economical to add niobium in the solder material 8 than to have to change all the components 2 to 7 of the exchanger 1 made of stainless steel, for example austenitic stainless steel 1.4301 by a niobium-containing stainless steel, such as 1.4550 austenitic stainless steel (AISI 347).

According to a preferred embodiment of the invention, the solder material comprises the Ni620 standard alloy with an addition of 2% niobium. As a result, the baking is improved compared to the standard alloy N620.

The chemical composition of the solder material of the invention comprises, by weight:

6 to 8% chromium

4 to 5% silicon

- 2.75 to 3.5% boron

- 2.5 to 3.5% iron

0.5 to 4% niobium

Optionally, the solder material can contain up to 20% chromium to improve corrosion resistance.

Some examples of wettability tests carried out with various components of the exchanger to be welded are then described by means of a Ni620 solder material with 2% niobium according to the invention.

According to a first example, the test was carried out with tubes 2 of gas. Firstly, small drops of standard Ni620 solder material are placed on the surface of the tube 2. Said drops have a size of about 2 mm in diameter. The tube 2 is then placed and welded in the furnace by means of a standard cycle of furnace welding in production.

When the solder material wets satisfactorily, it extends widely over the surface. However, if the base material of the component is contaminated with nitrogen, the solder material does not wet satisfactorily because it is held in place like a sphere, so that the welding process does not take place .

Additional drops of solder material with niobium added according to the invention are then placed. After a new oven-welding cycle, it can be seen that the niobium contains drops of melted material and satisfactorily extended on the surface of the tube 2 of gas. In contrast, the initial niobium-free solder material has not changed.

According to a second example, the test was carried out with a connection flange 4 assembled to the carcass 3, because it was also found an unsatisfactory firing due to nitrogen problems.

In this case, the flange 4 was welded with standard Ni620 solder material, and as a result, areas appeared with a weld bead having discontinuities. On the contrary, after having subsequently used the Ni620 solder material with niobium added according to the invention, it has been found that the weld is satisfactory with a suitable weld bead 8 as shown in FIG.

It should be noted that other problems have been detected on components with poor wettability, for example on the junction of the connection lines 5, 6 of the coolant to the carcass 3, and that these problems have been repaired satisfactory using the Ni620 solder material with 2% added niobium.

Therefore, the composition of the solder material of the present invention can reduce quality problems with two possible effects. First, it improves the profitability of current productivity and, secondly, reduces the cost of the heat exchanger, and thus increases the competitiveness of the company.

Claims

1. Welding material (8) for stainless steel components, in particular for furnace welding of heat exchangers (1) for gas, said welding material (8) being made of an alloy comprising boron, characterized in that it comprises a percentage of niobium capable of reacting with the nitrogen contained in the own stainless steel of the components to be welded, being capable of forming nitrides of niobium and thus preventing the nitrogen from reacting with the boron of the material welding (8).

The solder material (8) according to claim 1, wherein its chemical composition comprises, by weight, 6 to 8% chromium, 4 to 5% silicon, 2.75 to 3.5% boron. , from 2.5 to 3.5% iron and from 0.5 to 4% niobium.

The solder material (8) of claim 1 or 2, wherein the weight percent of niobium added is 2%.

The solder material (8) of claim 1 or 2 which includes up to 20% chromium by weight to improve corrosion resistance.