Article citation information:
Szczucka-Lasota,
B., Węgrzyn, T., Cybulko, P. Application of Fe3Al
intermetallic phase filler metal in valve seating face hard facing. Scientific Journal of Silesian
University of Technology. Series Transport. 2023, 119, 257-265. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2023.119.15.
Bożena
SZCZUCKA-LASOTA[1],
Tomasz WĘGRZYN[2],
Piotr CYBULKO[3]
APPLICATION OF FE3AL
INTERMETALLIC PHASE FILLER METAL IN VALVE SEATING FACE HARD FACING
Summary. The exhaust
valve seat face is a surface that degrades slowly as a result of mechanical and
thermal stresses. There is extensive research on new materials for valves and
for the improvement of valve production technology. This paper discusses the
method of obtaining a filler metal made of Fe3Al intermetallic phase
and the method of TIG hard-facing on a valve seating face made of H9S2 steel,
evaluating the effects of essential hard-facing parameters. The resulting
hard-facing build up was assessed in terms of quality using industrial
radiography testing (RT).
Keywords: engine,
valve, intermetallic phase
1. INTRODUCTION
AND STATEMENT OF THE RESEACH PROBLEM FOR THE AUTOMOTIVE INDUSTUSTRY
The exhaust valve seat face is a surface that undergoes a slow process of degradation. The valves are made of heat- and creep-resistant steels. The most common valve steel grades are H9S2 or H10S2M, both having a perlite-martensitic structure. Both grades are mainly used in the automotive industry. They are characterised by high hardness and significant resistance to abrasion. These grades of steel are commonly used for:
H9S2 steel has a similar composition
and properties to H10S2M steel, but the lower molybdenum content results in
decreased resistance to oxidation and high-temperature creep at temperatures
lower by approximately 50° C. Valves made of H9S2 steel were used in this
study. The most important advantages of this steel include:
The impact of engine operating
conditions such as number of cycles, combustion pressure and temperature on
valve face wear were investigated by researchers such as Lewis [1], who
demonstrated a relationship between valve abrasive wear and the increase in
combustion chamber pressure. At the same time, a decrease in valve seating face
wear was observed as the temperature increased. This effect is attributed to a
film of metal oxides formed as a result of the exhaust valves coming into
contact with high-temperature exhaust gases [1]. Another study reported that
the wear of the valve seating face was mainly related to the misalignment of
the valve and valve seat and to the resulting abrasive wear that occurred at
the contact point [2]. Other studies indicate that the wear process is a
complex combination of oxidation, adhesion and abrasive wear processes [3-5].
Today, cobalt alloys, mainly Stellite, are used to strengthen the top layer of
the engine valve seating face. Apart from cobalt, these alloys contain up to
approx. 32% Cr, up to 14% W and 4% C. Stellites are exceptionally high hard, up
to 64 HRC (immediately after casting), but also brittle, show a high abrasion
and heat resistance (up to 950°C), and exhibit good corrosion resistance.
They are also used in friction couples, e.g. as the top layer of valve seats of
internal combustion engines, in gas turbines, aircraft turbines, in the
petrochemical industry and in the energy sector [6,7]. Stellite is used to
strengthen the surface of the valve seating face, which forms a friction couple
with the valve seat. The location of the surface hardening of the valve with
Stellite is presented in Figure 1.
Stellite reinforcement of valve
seating face surfaces is the most common method of preventing valve face
degradation in diesel and petrol engines. In recent years, various industries
have shown a growing interest in metal alloys with a base of intermetallic
phases (intermetallics) such as those found in the Fe-Al equilibrium phase
diagram. This is dictated by demands of designers for construction materials to
be able to operate in increasingly variable and aggressive conditions. The Intermetallics are phases of alloys
of two or more metals with well-established proportions [9-11]. The Fe3Al
intermetallic phase has a lower density (approx. 6 g/cm³) compared to the
Stellite currently used for valve seating face surfacing (approx. 9
g/cm³). The difference in specific gravity is due to the high percentage
of aluminium (low density) in the Fe3Al phase [7]. During the
operation of the valves, this component is exposed to combustion products of
fuel, but also of substances which, under high temperatures, have a degrading
effect on the exhaust valve seating face [13-14].
Fig.
1. Location of Stellite reinforcement of the valve seating face [8]
2.
OWN RESEARCH
The parent metal
was made of H9S2 steel (in accordance with EN: X45CrSi9-3 - 1.4718). The
chemical composition of the steel is shown in Table 1.
Tab. 1.
Chemical composition of the parent material
|
Chemical
composition of H9S2 steel, % |
||||||
|
C |
Mn |
Si |
P |
S |
Cr |
Ni |
|
0.43 |
0.5 |
2.5 |
0.021 |
0.023 |
9.2 |
0.5 |
The intermetallic
phase of Fe3Al served as the filler metal. The chemical composition
of the intermetallic filler metal is shown in Table 2.
Tab. 2.
Chemical
composition of the filler metal
|
Chemical composition of the
Fe3Al phase, % |
||
|
Fe |
Al |
Other elements (mainly Zr) |
|
82.2 |
15.4 |
2.4 |
The Fe3Al
intermetallic phase belongs to the group of materials that are difficult to
process, and therefore hard to use it as a construction material for components
to be used during the hard-facing process. Waterjet cutting technology was used
to make the form of the filler rods. Processing materials which need higher
kinetic energy of the water stream, an abrasive is added to the water to
increase the erosion of the processed material [12]. The cutting process was
carried out on a Kimla Streamcut 3030 unit using garnet as the abrasive, with
the machine set to parameters shown in Table 3.
Tab.
3
Water jet cutting parameters
|
Parameter |
Value |
Unit |
|
Water pressure |
345 |
MPa |
|
Abrasive particle
size |
0.35 |
mm |
|
Head movement speed |
31 |
mm/min |
|
Water stream
diameter |
1.2 |
mm |
High-pressure water
and abrasive jet cutting was used to create filler rods with cross-sections of
1.3 mm × 1.3 mm and 1.4 mm × 1.4 mm. Such dimensions of the filler
metal were chosen due to the availability of conventional steel filler rods of
identical dimensions that are commonly used in TIG hard-facing. The method of
creating 1.3 mm × 1.3 mm filler metal is shown in Figure 2.
Fig. 2. Filler metal (rods) prepared
for TIG hard-facing (bright areas)
In order to determine the
feasibility of using TIG hard-facing to reinforce the top layer of the valve
seating face, the filler metal was welded onto the surface of the valve seat
face. In the process of TIG welding and TIG hard-facing, there is an important
difference in the way the filler metal (rod) is fed. In welding, the filler
metal must be fed to the weld pool to obtain sufficient fusion, while in
hard-facing the filler metal must be added to the arc to avoid excessive
penetration. This paper examines the impart of the most important welding parameters:
·
current
(60-70 A),
·
arc
voltage (18-20 V),
·
welding
speed (70-80 mm/min).
Argon with a flow rate of 8.5 l/min
was used as the shielding gas. A size 6 welding nozzle was used to increase the
uniformity of the shielding gas outflow and to keep the welding area clean. TIG
hard-facing on valve steel was carried out as follows:
·
single hard-facing
using a tungsten electrode (2.4 mm);
·
use of Fe3Al
filler metal, 1.3 mm × 1.3 mm and 1.4 mm
× 1.4 mm;
·
no preheating of
the valve steel;
·
the intermetallic
filler was fed manually.
Prior to TIG hard-facing, the surface of the
seating face was chemically cleaned using acetone. The characteristics of the
hard-facing process as a function of the selection of welding parameters are
shown in Table 4.
Tab. 4
Selection of TIG hard-facing
parameters using Fe3Al intermetallic phase filler
|
Used process
parameters |
Characteristics of the hard-facing
process |
|
Current: 60 A Arc voltage: 18 V Welding speed 80 mm/min, filler metal: 1.3
mm × 1.3 mm |
Incorrect
process parameters. Problems were observed with the formation of a correct
weld pool. This was caused by too low welding current during hard-facing. |
|
Current: 64 A Arc voltage: 19 V Welding speed 80 mm/min, filler metal: 1.3
mm × 1.3 mm |
Correct process parameters. In the initial
phase of the hard-facing, a slight local spill of the filler metal could be
observed, related to the formation of the weld pool. No problems were observed with the
formation and control of the shape and guidance of the weld pool |
|
Current: 64 A Arc voltage: 18 V Welding speed 90 mm/min, filler metal: 1.3
× 1.3 mm |
Correct process parameters. In the initial
phase of the hard-facing, a slight local spill of the filler metal could be
observed, related to the formation of the weld pool. No other excessive spill
on the remaining circumference of the valve seating face was identified. The application of the intermetallic
phase was fully controlled. |
|
Current: 68 A Arc voltage: 19 V Welding speed 90 mm/min, Filler metal: 1.3
mm × 1.3 mm |
Problems were
observed with the formation of a correct weld pool. Issues with the uniform
application of the filler metal into the weld pool. This was caused by too
high welding current during hard-facing. |
|
Current: 62 A Arc voltage: 18 V Welding speed 80 mm/min, filler metal: 1.4
mm × 1.4 mm |
Incorrect
process parameters. Problems were observed with the formation of a correct
weld pool. This was caused by too low welding current during hard-facing. |
|
Current: 66 A Arc voltage: 18 V Welding speed 80 mm/min, filler metal: 1.4
mm × 1.4 mm |
Correct process parameters. In the initial
phase of the hard-facing, a slight local spill of the filler metal could be
observed, related to the formation of the weld pool. No problems were
observed with the formation and control of the shape and guidance of the weld
pool |
|
Current: 66 A Arc voltage: 18 V Welding speed 80 mm/min, filler metal: 1.4
mm × 1.4 mm |
Correct process parameters. In the initial
phase of the hard-facing, a slight local spill of the filler metal could be
observed, related to the formation of the weld pool. No other excessive spill
on the remaining circumference of the valve seating face was identified. The application of the intermetallic
phase was fully controlled. |
|
Current: 70 A Arc voltage: 18 V Welding speed 80 mm/min, filler metal: 1.4
mm × 1.4 mm |
Problems were
observed with the formation of a correct weld pool. Problems were observed
with the uniform application of the filler metal into the weld pool. This was
caused by too high welding current during hard-facing. |
The information presented in the
table indicates that current is the most important welding parameter, having a
significant effect on the form of the build-up weld. In the case of the 1.3 mm
× 1.3 mm filler metal, it was noted that the current should be set at 64
A, while for a weld with a cross-section of 1.4 mm × 1.4 mm, the current
should be set at 66 A. The arc voltage and welding speed in the tested range
had no significant effect on the appearance of the weld.
Fig. 3 shows an
example of the valve seat face after hard-facing using the most favourable
welding parameters (I=64 A).
Fig. 3.
Valve seating face after TIG hard-facing, filler metal cross-section:
1.3 mm × 1.3 mm, I= 64 A
It was observed that the surface
seating face has the correct form only with the appropriate welding current of
I=64 A (filler metal cross-section 1.3 mm × 1.3 mm). It was observed that
the surface of the seating face has the correct form only with the appropriate
current of I=66 A (filler metal cross-section 1.4 mm × 1.4 mm). In order
to analyse the quality of the process, the two obtained hard-facings were
subjected to industrial radiography test (RT). These studies make it possible
(Table 5) to detect internal, subsurface and surface discontinuities in test
objects. RT examinations involve exposing an object to X-rays. The image of the
scanned object is recorded on radiographic film. This method is often used to
inspect welded joints, among other things. Testing and evaluation of welds was
performed in accordance with PN-EN ISO 17635 for non-destructive testing of
welds, and in compliance with the general rules for metals according to PN-EN
ISO 17636-2 for non-destructive testing of welds. Radiography was conducted in
accordance with PN - EN ISO 6520-1 Welding and allied processes.
Geometric weld inconsistencies in metals were classified using PN- EN ISO 5817
Welding – Fusion-welded joints of steel, nickel, titanium and their
alloys.
Tab. 5
Analysis of RT images of a valve
hard-faced with Fe3Al intermetallic phase
|
Valve appearance / filler metal
dimensions [mm] |
RT
image |
Radiographic
image interpretation |
|
Before hard-facing (valve only) |
|
No
defects found internally |
|
After hard-facing with 1.4 mm filler metal |
|
No
defects found internally |
|
After hard-facing with 1.3 mm filler metal |
|
No
defects found internally |
The images in Table
5 confirm that the two tested hard-facings made using the Fe3Al
intermetallic phase filler metal do not show any welding imperfections or
defects. For quality level B according to PN-EN ISO 5817, the maximum
permissible dimension of a single gas bubble is d≤0.2·a, but no
more than 3 mm. Based on these guidelines, the acceptable bubble size for the
test object is 1 mm. The dimensions of the identified discrepancies are below 1
mm. On this basis, it can be concluded that the created hard-facing falls into
quality level B.
The final point of
the study was to analyse the quality of hard-facing in terms of microstructure.
Figure 4 illustrates the structure of the hard-facing welds and the correct
fusion at the interface between the steel surface of the valve and the
intermetallic build-up.
Analysis of the image indicates fusion
is correct, and the weld is free of defects and welding inconsistencies, which
is confirmed by the result of the radiographic tests.
Fig. 4.
Microstructure after TIG hard-facing of the valve,
1.3 mm × 1.3 mm filler metal, I= 64 A
3. CONCLUSION
This article presents the
possibility of hard-facing steel used for the valve seating face of an internal
combustion engine using an intermetallic filler metal. The Stellite
reinforcement used to date does not meet the demands presented by developers of
modern means of transport. The study has shown that the use of TIG hard-facing
at specific process parameters allows the correct fusion of the Fe3Al
intermetallic phase with the valve seating face material made of H9S2 steel.
The quality of the weld build-up depends on the cross-sectional dimensions of
the filler metal and the welding parameters. This paper examines the combined
effects of welding current, arc voltage and welding speed used in hard-facing
on the quality and form of the weld build-up. It was found that satisfying
results were obtained using a Fe3Al filler metal in two sizes, 1.3
mm × 1.3 mm and 1.4 mm × 1.4 mm. Proper execution of
the hard-facing weld was confirmed by photographic evidence, the results of
radiographic tests and microstructure analysis. Based on the conducted
research, it can be concluded that the analysed technology valve steel
hard-facing with Fe3Al phase filler metal can be successfully
applied in the construction of internal combustion engine valves.
Acknowledgement
The paper is a part of the COST project, CA
18223 and the part of presented research was funded by Silesian University of
Technology grant number BK-277/RT1/2021.
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Received 20.11.2022;
accepted in revised form 21.02.2023
Scientific Journal of Silesian University of Technology. Series Transport is licensed under a Creative Commons
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[1] Faculty of Transport and Aviation Engineering,
The Silesian University of Technology, Krasińskiego 8 Street, 40-019
Katowice, Poland. Email: bozena.szczucka-lasota@polsl.pl. ORCID:
https://orcid.org/0000-0003-3312-1864
[2] Faculty of Transport and Aviation Engineering, The Silesian University of Technology, Krasińskiego 8 Street, 40-019 Katowice, Poland. Email: tomasz_wegrzyn@polsl.pl. ORCID: https://orcid.org/0000-0003-2296-1032
[3] Medgal Sp. z o. o.,
Niewodnicka 26A Street, 16-001 Księżyno, Poland. Email: piotr.cybulko@gmail.com. ORCID:
https://orcid.org/0000-0003-1146-1892