FIELD OF THE INVENTION
The present invention generally relates to hydraulic excavators. More particularly,
the invention relates to a method for surface treatment of medium carbon steel
to produce tooth points with high impact toughness and enhanced wear
resistance.
BACKGROUND OF THE INVENTION
Different expensive grades of ferrous materials are generally used for
manufacturing items that require high strength as well as abrasion resistance. A
typical application is that of tooth points of hydraulic excavators. For this and
similar other applications, high alloy Cr-Ni-Mo steels, austempered ductile iron,
Hadfield steel, etc. are used. Such materials provide a unique combination of
high impact toughness and wear resistance so as to withstand the severe
conditions that the components are subject to in the field. Their cost varies in
the range of Rs. 90 to 120 per kg. But it is seen in several applications that in
spite of using such expensive materials, the life of tooth points remains a cause
of concern for the owner of the excavators. Tooth points are replaced,
depending on the abrasive nature of the material being handled, anywhere
between 5 to 25 times during the life of the bucket. Hence there is a drive for
reducing the cost of manufacturing.
The life of ground engaging tools like tooth-points, contribute significantly to the
operating cost of hydraulic excavators. The load encountered by such
components consists of the impact during the digging and the subsequent
abrasion from the surface. Materials should posses both high abrasive wear
resistance and impact toughness to withstand this loading condition.
The traditional approach of material design is to choose expensive grades of
ferrous materials for achieving a unique combination of high hardness and
superior impact toughness.
Predominantly excavator manufacturers use F-Steel for manufacturing tooth
points.
The chemical composition of various materials used for manufacturing tooth
points (F steel, 35C8S, ADI, Mn (HadfieW) Steel and Alloy forged steel) includes
the inventive materials are presented in Table - 1.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for surface
treatment of medium carbon steel to produce tooth points with high impact
points enhanced wear resistance properties for hydraulic excavators, which
eliminates the disadvantages of prior art.
Another object of the invention is to propose a method for surface treatment of
medium carbon steel to produce tooth points with high impact points enhanced
wear resistance properties for hydraulic excavators, which adapts a known grade
of steel and surface hardened to develop a new grade of steel.
A further object of the invention is to propose a surface hardened medium plain
carbon steel with high impact and high wear resistance properties, which allows
producing of tooth-points of hydraulic excavator having higher impact and wear-
resistance properties.
SUMMARY OF THE INVENTION
According to the invention, an inexpensive grade of medium carbon steel (35C8)
is selected and surface hardened to achieve improved impact and wear
resistance properties for producing tooth points of hydraulic excavators. The
surface treated steel thus developed is named 35C8S. Tooth points
manufactured with this new grade of steel have been validated by testing in
laboratories and through field applications.
The invention thus deals with casting and heat treatment of tooth points using a
known medium carbon steel and surface hardened to 53HRC by a heat treatment
process.
In this invention, suitably surface-hardened plain carbon steel (35C8S chemical
composition given in Table - 1) is proposed as an alternative material for specific
applications. The rational selection of steel grade 35C8S, apart from being
inexpensive, are that the base metal should have sufficient ductility to withstand
the bending load during impact, and that the surface hardened layer provides
the necessary abrasive wear resistance. Dry sand abrasion test and impact test
have been conducted for a comparative evaluation of the newly developed
material vis-a-vis the conventional materials used for manufacturing tooth point
(see Figure 1 and Figure 2). During actual operation when the excavator digs the
ground, the factors like corrosion, erosion, humidity, particle shape of the ground
material etc. significantly contribute to the life of the tooth point. Hence
validations of experimental results were further re-established with field trials.
Dry sand abrasion resistance (Figure 1) of the newly developed 35C8S, F Steel,
ADM, ADI-II, Alloy forged steel and Hadfied Steel. (Chemical compositions -
Table -1) is shown in Figure 2 35C8S is found to have higher wear resistance
than the rest of the prior art grades. Thus, two types of tooth points on type
produced from prior art grade F-Steel, and the other typed produced from newly
developed grade of 35C8S, were manufactured and put to field study for
determination of actual life.
As shown in figure-2, the wear resistance of 35C8S is 1.7 times higher than F
steel. Optical microsturctural analysis shows that micro-segregation of alloying
elements results in brittle regions in F steel. However in 35C8S this phenomenon
is absent and consequently the microstructure exhibits predominantly uniform
fine martensititc structure (Figure 5). Scanning electron microscopy analyses
Figure 6(a,b) and Figure 7 (a,b) established that the micromechanics of the wear
causes the generation of micro-cracks originating between the scratch marks
caused by the sand particles. Because of the predominantly uniform fine
martensitic structure of 35C8S, stress induced transformation has not resulted in
wkJerspread micro cracks as observed in F steel. Furthermore 35C8Sis cheaper
than F steel (approximate cost of F-Steel (approximate cost of F-Steel is '95/kg,
and 35C8S '83/kg).
Figure 1 - Shows a dry sand rubber wheel abrasion test setup, the sample
placed inside the holder can be pressed against the rubber wheel
by a load of 135N during the test.
Figure 2 - Graphically shows Volume loss (mm3) vs Hardness (BHN) of
different grades of materials used for tooth points.
Figure 3 - Schematically shows a tooth-point configuration
Figure 4a - Shows a hydraulic excavator in operation mode.
Figure 4b - Shows tooth points position in a bucket of the excavator including
tooth points prepared of prior art steel and the newly developed
35C8S steel.
Figure 5a - Mkxostructure details alloy composition (35C8S) according to the
invention.
Figure 6(a,b) SEM micrographs of abraded surface for F-Steel used in prior art.
Figure 7(a,b) SEM micrographs of abraded surface for 35C8S used in the present
invention.
DETAIL DESCRIPTION OF THE INVENTION
According to the invention, for dry sand wear resistance test, firstly samples of
prior art F steel were hardened and tempered by heating at 880°C for two hours
and then quenching in polymer grade-P15, tempering at 350°C for 4.5 hours to
achieve a hardness of 44-47HRC. Samples of 35C8S were hardened and
tempered to 350-400 BHN and then surface hardened to 53 HRC minimum (case
depth of 2-4 mm) by surface hardening in a salt bath. Heating at temperature
880°C for 45 minutes and then quenching in water, tempering at 250°C for 2
hours.
Specimens (S) were used for dry sand wear test. The sand used for these tests
conformed to AFS 50/70 sand, the grain fineness number was 50 and the
hardness was 7 in Mohs scale. The sand flow rate was regulated by a hopper
designed for maintaining flow between rubber wheel (RW) and the specimen (S)
at 250 grams per minute. The load applied was 130 N and number of revolutions
was 6000 in 30 minutes (Figure 1). Weights of the specimens before and after
the test were measured by an electronic weighting machine with least count
0.001 g. The weight loss because of wear was subsequently converted to volume
loss. Seven samples for each grade were tested to obtain the mean weight loss.
The temperature of the specimen surface was 70°C maximum at the end of the
test.
To test materials in field applications, tooth points (Figure 3) of a hydraulic
excavator (HE) were prepared with innovative 35C8S through CO2 molding
technique. They were subsequently hardened and tempered to 350-400 BHN and
then surface hardened to 53 HRC minimum.
Figure 2 shows results of the dry sand abrasive wear resistance of the.different
materials generally used for manufacturing tooth point as obtained from the
testing. The wear resistance of the innovative 35C8S is superior to all the grades
and is 1.76 times greater than the prior art F steel - - the most frequently used
for manufacturing tooth points in Ex-70 hydraulic excavators.
As shown in figure 4(b), two numbers of each F steel of prior art and 35C8S
tooth points were fitted in the same bucket (B) of hydraulic excavator (HE and
tested in field as shown in Figure 4. To observe the wear pattern, the reduction
in length of each tooth point (1,2,3,4) was observed at the end of the test
(Completed in 85.1 hours).
Table 2 illustrates the rate of decrease in tooth point's length in field testing. The
innovative 35C8S had 20% better wear resistance than F steel.
The mircosturcure of the prior art F steel consists of martensite, retained
austenite and untempered martensite (white segregates), as shown in Figure 5a.
In the innovative 35C8S, fine martensitic structure at the surface of the specimen
was observed with no untempered martensitic region Figure 5b.
Figure 6a and 6b illustrates the SEM micrographs of the wear surface of prior art
F-steel. The width of the continuous and uniform scratch mark produced by the
sand particles is 2.27nm. Numerous fissures, which eventually lead to the micro
cracking and subsequent removal of material, were observed between the
scratch marks. The high strains resulting from the abrasion are including stress
induced transformation of retained austenite to martensite which eventually
leads to micro-cracks.
Figure 7a and 7b illustrates the abraded surfaces of the innovative 35C8S. The
width of the scratch mark is 0.69 urn (69% less than of prior art F-steel) and the
micro fissures present between the cracks are relatively less as compared to that
of prior art F steel. Because of the low alloy content of the steel, the amount of
retained austenite is negligible and hence the formation of microcracks due to
stress induced transformation of retained austenite and consequent wear loss is
less than F-Steel.
From results of field trials it was evident that the innovative 35C8S tooth point's
posses 20% better life than that of prior art F steel. It is also cheaper by 12%.
In real application, no fracture or damage was observed in 35C8S tooth points so
it can be used as a preferable material for manufacturing tooth points.
WE CLAIM
1. A method for surface treatment of medium carbon steel to produce tooth
points with high impact points enhanced wear resistance properties for
hydraulic excavators, the method comprising the steps of:
- Providing a medium steel sample heaving a chemical composition
of carbon 0.371, manganese 0.768, silicon 0.121, sulphur 0.029,
phosphorus 0.055 and traces of chromium, nickel and
molybdenum;
- Hardening and tempering to 265~280 BHN
- Surface hardening of the sample to 53 HRC by dipping at 2-4mm
(case depth) in a salt bath followed by heating at temperature
880°C for 45-minutes, quenching in water, and tempering at 250°C
for 2-hours.
2. A method for surface treatment of medium carbon steel to produce tooth
points with high impact points enhanced wear resistance properties for
hydraulic excavators, as substantially described and illustrated herein with
reference to the accompanying drawings.

The invention relates to a method for surface treatment of medium carbon steel
to produce tooth points with high impact points enhanced wear resistance
properties for hydraulic excavators, the method comprising the steps of:
providing a medium steel sample having a chemical composition of carbon 0.371,
manganese 0.768, silicon 0.121, sulphur 0.029, phosphorus 0.055 and traces of
chromium , nickel and molybdenum; and hardening and tempering to 265~280
BHN; Surface hardening of the sample to 53 HRC by dipping at 2-4mm (case
depth) in a salt bath followed by heating at temperature 880°C for 45-minutes,
quenching in water, and tempering at 250°C for 2-hours.