Manufacturing of exhaust gas recirculation (EGR) heat exchanger (cooler) for automotive
use using an Oxynon® furnance
M. Aida, S. Takahashi, K. Kanda, K. Watanabe and T. Ariga, Hiratsuka/JThis paper presents to introduce a new methodology (brazing method) for manufacturing Exhaust Gas
Recirculation (EGR)-Coolers.
An EGR-cooler is a heat exchanger that cools the exhaust gas down to an appropriate temperature before
returning the partial exhaust gas emitted from the engine back into the engine again. An EGR-cooler consists of acomponent where the exhaust gas passes, as well as a water-cooling compartment where the exhaust gas iscooled down.
Because the component where the exhaust gas passes needs to be capable of efficient thermal diffusion, it has agroup of fins which are made of processed thin stainless steel plates in a complex shape in order to reduce theflow rate of gas. The water-cooling compartment also has a feature to reduce the flow rate of gas.
Both of the above compartments cool down the exhaust gas by contacting each other through the thin stainlessplate. In order to mold the structure of these compartments, many assembling parts are connected each other bybrazing. For the brazing, high-temperature brazing filler metal, which is generally nickel filler alloy, is used. For thebase metal, austenite-stainless steel is used in order to enable it to endure the exhaust gas circulation. In order tobraze these metals, vacuum furnaces are generally used.
However, the method we are going to introduce in this paper does not use a vacuum furnace, but a continuous gas
atmosphere furnace (Oxynon furnace), which has a unique structure (Oxynon atmosphere).
In this study, we would like to introduce our knowledge, gained through this research, of manufacturing technology
of EGR-coolers using Oxynon furnaces.
1Introduction

An EGR-cooler is structured by a combination ofstainless steel (austenite) and nickel brazing filler
metal (BNi-5, Ni-Cr-Si-P, etc.). The heating
temperature for brazing is generally over 1000°C.
It is possible to observe some basic researchpresentations which are related to the EGR Cooler
manufacturing. [1], [2], [3], [4], [5], [6]In these studies, austenite-stainless steel plates were
mostly used for the base metal and BNi-5, as well as
newly developed Ni and Fe were used for the brazing
filler metal. Particularly, the brazing filler metal madeof Ni-Cr-Si-P was used in all of the above mentioned
studies and it was highly evaluated.
Vacuum heating was the heating method for brazingin those studies.
Referring to those past studies, we decided to use thebrazing filler metal made of Ni-Cr-Si-P in this study.
For this research, ferrite-stainless steel for the basemetal was chosen, which has been recentlydeveloped for practical use.
In addition, as for the heating method, the Oxynon
furnace was chosen instead of the vacuum furnace,
which has been generally used. Using the newheating method, the brazing properties was evaluatedand aimed to obtain the basic knowledge on themanufacturing of EGR-coolers.
In this study, the stainless steel (ferrite) and nickel
brazing filler metal were heated at a high temperature.
Prevention and reductive reaction of the oxidation of
the base metal and the brazing filler metal were
essential factors while brazing.

DVS 293

Oxynon furnace has sufficient properties to meet theabove criteria. The followings are the primary
properties:(1) All of the furnace’s internal structures are made ofhigh-purity graphite. Neutral gas (nitrogen orargon) is used for the internal atmosphere, so theinternal oxygen partial pressure can be reduced to
below 10-15 Pa. Therefore, there would be nocontamination with the brazed products.
(2) This is a mesh belt-type continuous furnace.
Because C/C composite belt is used for the meshbelt, a continuous feed under a high-temperature
heating at up to 2600℃ is possible.
(3) The furnace is linearly structured by a preheatingzone （ evaporation of the binder ） , a heatingzone and a cooling zone. Each brazed product iscontinuously brazed by passing through thesezones.
Due to the above properties, it can be said that theOxynon furnace is sufficiently capable of manufacturing EGR-coolers.
Furthermore, in order to obtain the knowledge of thebrazeability required for the EGR manufacturing using
Oxynon furnace, we tested the following properties aswell:(1) Spreading factor (wettability) of brazing filler metal(2) Mechanical properties (tensile shear strength, butt
shear strength, joint’s hardness distribution) ofbrazed joints(3) Corrosion resistance of brazed joints(4) Impact of heating atmosphere gas (Ar gas, N2 gasatmosphere)87

Details of the Oxynon furnace were presented at theprevious conference LÖT 2010 [7].

gas and evaluated various properties that could beimpacted by the difference between those gases,
such as wettability, joint mechanical properties, andcorrosion resistance.
2.4

Figure 1 Special specification OXYNON® FURNACE.

2

Experimental Procedures

2.1Brazing filler metal

BNi-5 (Ni-Cr-Si) has been generally used for thebrazing filler metal with EGR-coolers.
The reason for the choice is because BNi-5 does notcontain boron (B) unlike the brazing filler metal madeof nickel and it has better corrosion resistancecompared to the brazing filler metal which contains
boron in the case where corrosive gas needs to beprocessed such as EGR. Because boron is notcontained, however, the melting temperature is high,which consequently requires a high brazing
temperature, as well as brazing skills.
In the past few years, due to the nickel price hike, theconsumption of nickel has declined and several new
brazing filler metals have been developed, forexample, a brazing filler metal made of Fe-base FeCr-Ni-Si-P or Fe-Cr-Ni-Si-P-Mn-Cu, which has areduced content of nickel.
In this study, among those available brazing filler
metals, the one with Ni-Cr-Si-P was chosen, which
has been well recognized as a brazing filler metal for
EGR-coolers, and evaluated its brazeability. Thebrazing filler metal powders was mixed with a binderand made into a paste.
2.2Base metals

Austenite-stainless steel (SUS304, SUS316) has
been generally adopted for the base metal in theconventional EGR-coolers for diesel engines, toprevent the corrosion of the exhaust gas. However,ferrite-stainless steel has been recently used for EGR
for gasoline engines. In this study, considering thisfact, we determined to adopt ferrite-stainless steel(SUS430, SUS444) for the base metal.
2.3

Atmosphere gas

In the case where stainless steel is brazed in Oxynon
furnace, argon gas is generally used. So far, we havealso conducted brazing of stainless steel using argon
gas. In this study, considering various conditions, weexperimentally used nitrogen gas in addition to argon
88

Wetting (Spreading) test

Wettability of the brazing filler metal is very importantwhen it comes to the brazeability evaluation. For themeasurement of wettability, as seen in Figure 2, weplaced 0.1 gram of brazing filler metal (paste) on astainless steel plate (30 mm × 30 mm ×t3 mm) andcalculated the spreading factor (A) from the spreading
area obtained after heating.
The spreading factor (A) was set to 1/2 of the x, yintercept length of an spreaded circle, due to the factthat the spreaded brazing filler metal becomes almosta round shape after the wettability test.
As for the brazing conditions to evaluate the impact
on wettability, the heating temperature was set atthree different points by adding 50°C to each liquidus
temperature, which were 1080°C , 1130°C , 1180°C.
The brazing time was gradually increased from 5
minutes to 15 and 30 minutes.
Also, the brazing temperature was set at a constant of1130°C and wettability was evaluated when argon gas
and nitrogen gas were used as atmosphere gas after
5, 15, 30 minutes of brazing time.
In addition, we examined the difference between
Oxynon heating and vacuum heating by observing theT-shaped brazing joint’s interface structure after
heating for 15 minutes at 1130°C.

Figure 2 Wetting (Spreading) test piece and these
specimen dimensions. Filler metal (paste state): 0.1 grams.

2.5

Joint strength

2.5.1 Lap shear test
Figure 3 shows the appearance of the test specimenthat was used in the experiment (JIS Z 3192). In atensile shear test with a lap joint brazing, theoverlapped area would make a large impact on theshear strength. However, in this experiment, theoverlapped area was configured as the same length
as the plate thickness (t 1.0). After the experimentconcluded, little deformation was found on the plate at
the brazed joint. Thus, we considered that the value
we obtained barely impacted the shape. About 0.025
grams of brazing filler metal was applied to theoverlapped area. We chose both of the genericvacuum heating method and Oxynon heating method
and adopted argon gas and nitrogen gas for theheating atmosphere to form a test specimen. TheDVS 293

brazing temperature was set to 1130°C which isconsidered to be the appropriate temperature for thebrazing filler metal (Ni-Cr-Si-P).

Figure 3 Tensile shear strength test specimen (JIS Z 3192)
and these dimensions.

2.5.2 Butt shear test
Figure 4 explains the overview of the test specimen
we used. We conducted brazing on a specimen wherea stainless steel stick of Ø = 10 mm and h = 10 mm
was placed on a stainless steel plate (SUS444,SUS430) of 30 mm × 30 mm × t6 mm. Brazing filler
metal of 0.1 gram was applied between the stainlesssteel plate and the stainless steel stick. The sameconditions from the wettability test were applied to thebrazing conditions, in which the heating temperatures
were 1080°C, 1130°C, 1180°C and the heating times
were 5 minutes, 15 minutes, 30 minutes. In order tocalculate the maximum shear stress, the brazed test
specimen obtained was fixed to the jig indicated in
Figure 5 and a compressed load was added.

Figure 4 Butt shear strength test specimen and thesedimensions.

2.5.3 Corrosion test
Using the test specimen obtained from the wettability
test, a salt spray test (5% salt water) was conducted.
Salt water was sprayed for up to 40 hours and thespecimen’s surface was examined after the test.
As for the test conditions, argon gas and nitrogen gas
were adopted for the atmosphere, a stainless steel
plate (SUS430, SUS444) was used for the base metaland Ni-Cr-Si-P was used for the brazing filler metal.
3Results and discussions

3.1Spreading and wetting

The spreaded brazing filler metal became almost around shape as seen in Figure 6. The applied brazing
filler metal evenly spread out in x-axis and y-axis
directions. Therefore, it can be considered that thebrazing filler metal has good wettability.
In the case where Oxynon furnace and argon gas
atmosphere are adopted, the wettability is barely
impacted by the heating temperature changes even if
it was increased from 1080°C to 1180°C. However,
we found that the spreading area tends to get slightly
bigger as the brazing time at each temperature getslonger than 5 minutes. As for the base metal, thespreading area when SUS430 is used was about 2030% bigger than that when SUS444 was used.
Furthermore, as for the spreading area when argon
gas and nitrogen gas were used in an Oxynon furnace,
SUS444 did not generate a big difference but SUS430
had about 30-40% bigger spreading area whennitrogen gas was used for the atmosphere, compared
to argon gas. We will further examine thesedifferences on the spreading area in the future. These
results indicate the feasibility of the use of nitrogengas for the atmosphere gas in Oxynon furnace inorder to have better wettability, instead of limiting theuse to argon gas. Figure 7 explains the observation
results of T-shaped brazed joint’s structures in avacuum furnace and an Oxynon furnace. On thebrazed joint’s interface, we found that the base metal
was melted (erosion) by the fused brazing filler metal.
The level of erosion does not largely change between
Oxynon heating and vacuum heating. In both furnaces,
fillets were favorably formed.
3.2

Figure 5 The jig equipment for the butt shear strength test
specimen.

DVS 293

Joint strength

From the results of the tensile shear strength test, it isclear that there was almost no difference between
vacuum furnace and Oxynon furnace on the strength.
Although the difference between argon gas
atmosphere and nitrogen gas atmosphere was subtle,
it was found that nitrogen gas atmosphere would
increase the strength with both of Oxynon heating andvacuum heating. As for the butt shear strength, there
was almost no impact from the difference on theheating temperature and the heating time with
Oxynon furnace. Also, there were no big differences

89between argon gas atmosphere and nitrogen gas
atmosphere with Oxynon furnace.

Argon(Oxynon)
Vacuum
1130℃ – 15min SUS430

Argon(Oxynon)
Vacuum
1130℃ – 15min SUS444

Figure 7 Metallographic examination of T-shaped brazed
joints interface. The difference of heating atmosphere(Oxynon furnace, Vacuum furnace) and base metal.

Figure 8 Tensile shear strength test results. Difference ofheating atmosphere and brazing conditions（temperature,time.

Spreading factor = [A]xyAx y2

Figure 6 Wetting (Spreading) test results. Spreading ratio
comparison of the defferent temperature, time andatmospher gas.
Figure 9 Butt shear strength test results. The effect ofbrazing condition on the temperature,time and atmosphere(argon, nitrogen gas).

90DVS 293

3.3

Corrosion test

5Figure 10 shows the transition of corrosion with thewettability test specimen before/after for 40 hours salt
spray test.
Even though a slight discoloration was observed on
the surfaces of the brazing filler metal and the basemetal, this type of discoloration can be easily removed
through a simple lapping treatment. No corrosion
which could incur a weight loss was found.

Before
SUS430

BeforeSUS444After(40hrs)
Argon (Oxynon) 1130℃ –15minAfter(40 hrs)
Argon (Oxynon)
1130℃ –

15min

Conclusion

Not using vacuum furnaces that has been generally
adopted for the manufacturing of EGR-coolers,
Oxynon continuous heating furnace that wedeveloped by ourselves was used and examinedvarious properties required for the manufacture, suchas spreading factor (wettability), mechanicalproperties (tensile shear test, butt shear test,
hardness distribution), corrosiveness, etc. throughbasic tests, from which we successfully obtainednecessary results. After considering these test results,
we tested the manufacturing of EGR-coolers using
Oxynon furnace in a combination of ferrite-stainlesssteel and a brazing filler metal made of Ni-Cr-Si-P.
As a result, we are confident that there would be noproblems in manufacturing EGR-coolers using
Oxynon furnace due to its preferable properties.
In addition, we conducted various tests to examine
brazeability (wettability) test, tensile shear test, Buttshear test, hardness distribution, corrosion test) witheach material of brazing filler metal (BNi-5, Ni-Cr-Si-P,Fe-base) required to manufacture EGR-coolers using
austenite-stainless steel. We obtained preferable
results from all of these tests as well. From all of theabove results, we are pleased to say that there would
be also no problems for the manufacture of EGRcoolers using Oxynon furnace when austenitestainless steel is adopted for the base metal.

DVS 293[1] U. Persson：Iron-based brazing filler metals forhigh temperature brazing of stainless steel,
Proceedings of the 8th International Conferenceon Brazing, High Temperature Brazing andDiffusion Bonding, LÖT 2010 Aachen, DVS
Berichte 263, p. 38-41[2] T. Hartmann and D. Nützel： Nickel-chromiumbased amorphous brazing foils for continuous
furnace brazing of stainless steel, Proceedings of
the 8th International Conference on Brazing,High Temperature Brazing and Diffusion Bonding,
LÖT 2010, Aachen, DVS Berichte 263, p. 42-47[3] T. Hartmann and D. Nützel: Chromium containing
amorphous brazing foils and their resistance toautomotive
exhaustgas
condensate,
Proceedings of the 5th International Brazing andSoldering Conference, IBSC 2012, LasVegas,USA, p. 394-401[4] Y. Miyazawa and T. Ariga Other ： Brazing offerritic stainless steel and microstructure at thebrazing joint, Proceedings of the 5th International
Brazing and Soldering Conference, IBSC 2012,Las Vegas, USA, p. 236-239

Figure 10 Spreading test piece surface after corrosion
testing. Salt water spray corrosion test. (40 hours).

4References[5] O. Mars and M. Stroiczek and U.Persson:
Erosion control of Stainless Steel brazing alloys
Proceedings of the 5th International Brazing andSoldering Conference, IBSC 2012, Las Vegas,USA, p. 169-173[6] M. Weinstein and L Lee and C.J.Skinner: Further
developments in Boron free Nickel-ChromiumPhosphorous-Silicon brazing filler metals,Technical Article 2012 Wall Colmonoy[7] S. Takahashi and K.Kanda Other: Development
Low-Oxygen Atmosphere Controlled Furnace
and its Application to Brazing Technology ,Proceedings of the 8th International Conferenceon Brazing, High Temperature Brazing andDiffusion Bonding, LÖT 2010 Aachen, DVS
Berichte 263, p. 289-295

91