TITLE:
Low alloy steel with improved abrasion resistance and impact toughness through non-
isothermal Quenching and Partitioning process
Field of the Invention
The present invention relates to a low alloy steel grade with improved abrasion resistance
and impact toughness. The invention further relates to process for producing a low alloy
steel grade with improved abrasion resistance and impact toughness.
Background of the Invention
The process of wear involves gradual and progressive loss of material from the surface of a
component due to relative motion between the active and counter body species either by
mechanical action or chemical reaction. Depending upon the environment of application, the
material interacts with different kinds of abrasives and as a result wears out. Therefore,
abrasive wear resistant materials become highly desirable in the industrial applications such
as agriculture, earth moving, excavation, mining, mineral processing, transportation etc.
The increasing cost of replacing worn parts in mining and earth moving equipment confronts
a continual challenge to materials development. As per the report of a national survey in
1997, for UK industries who have wear problem, the cost of wear was typically about 0.25%
of their turnover. Out of this, abrasive wear alone contributes to around 63% [1]. This
undesirable loss, particularly due to the process of wear, can be reduced to at least half by
replacing the currently used materials with new ones and/or by selecting a better design of
the components. However, a new design of a component is often coupled with other
associated risk, and also does not always make an economically viable alternative. To the
contrary, the use of new material can be considered as a better option.
Components in wear resistant applications, particularly in mining and earthmoving sectors,
are essentially required to have adequate abrasion resistance along with the ability to resist
chemical attacks. As impact loading is mostly unavoidable in such applications, the

requirement for good impact toughness also becomes one of the major concerns. Typically,
the material with high abrasion resistance is hard and brittle. Hence, these materials are
limited by low impact toughness, which leads to the generation of cracks while experiencing
sudden impact loading [2], Thus, there is a need to develop new grade of material which is
highly resistant to abrasion together with good impact toughness.
At present, the materials in abrasion resistant applications are either used in full or
tempered martensitic condition [3], Fully martensitic structure can provide high hardness
without any additional heat treatment after hot strip mill. However, the carbon content in
these materials is generally high which subsequently pose problems during welding and also
results in reduced impact toughness. The tempering process is used to improve toughness
with a compromise on hardness. Also, this process requires an additional facility after hot
strip mill, which adds up to an extra cost. The heat treatment process for direct quenching
and tempering technique is schematically shown in Fig. 1.
Objects of the Invention
An object of the present invention to propose a low alloy steel grade with improved abrasion
resistance and impact toughness produced directly from the hot strip mill.
Another object of the present invention is to propose a non-isothermal Quenching and
Partitioning (Q&P) process for producing a low alloy grade steel with improved abrasion
resistance and impact toughness.
Summary of the Invention
The present invention focuses on stabilizing an optimum amount of austenite through non-
isothermal Q&P process. Two different alloys were designed and cast at 40 kg scale. After
quenching in salt bath atmosphere, they were used to see the carbon partitioning for
different combinations of martensite and austenite content. The result shows that
stabilization of retained austenite is possible in one of the alloys, which depicts higher
impact toughness without any significant decay in abrasion resistance. However,

the hardness is somewhat compromised due to the presence of retained austenite and a
little tempering of martensite. The other alloy does not show any presence of retained
austenite. The possible reason could be an excessive amount of tempering during slow
cooling, which is evident in the microstructure. The tempered structure of this alloy resulted
in superior abrasion resistance without much decay in hardness value.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1: shows schematic of (a) hot rolling and direct quenching; (b) tempering process.
Figure 2 shows a process with various steps of non-isothermal Q&P process.
Figure 3: shows typical layout of hot rolling for non-isothermal Q&P process along with hot
rolled coil cooling profile.
Figure 4: shows comparison of the hot rolled coil cooling and salt bath furnace atmosphere
cooling profiles
Detailed description of the Invention
The present inventions based on studies and experimentations noted that the requirement
of good impact toughness along with superior abrasion resistance can be fulfilled by
obtaining some amount of retained austenite along with martensite in the microstructure.
This can be achieved through Quenching and Partitioning (Q&P) process. Q&P involves
partial transformation of austenite to martensite, followed by carbon diffusion from
martensite into untransformed austenite, which further stabilizes the remaining austenite to
room temperature. The final microstructure obtained from aforementioned process,
constitutes finely distributed retained austenite between the martensitic laths.
The concept of conventional isothermal Q&P process was proposed by Speer et al. in 2003
[4]. The process requires isothermal holding of material, containing a combination of
martensite and unstable austenite, at relatively high temperature for carbon partitioning. Till
date, this process has been investigated by several research groups mainly targeting cold
rolled products for automotive applications. Also, the above process calls for an additional
facility after hot strip mill to perform heat treatment.

The idea of carbon partitioning during non-isothermal slow cooling of hot rolled coil was
proposed by Thomas et al. in 2008 [5]. This process does not require any additional facility
to carry out heat treatment. It mainly utilizes the coil heat for carbon partitioning. Hence, it
is an integrated non-isothermal Q&P process.
A process (200) comprising various steps shows the non-isothermal Q&P process in Fig. 2.
At step (204) steel is casted with composition (all in wt.%) 0.10 - 0.25 carbon, 1.0 - 1.8
Manganese, 0.40 - 0.6 Silicon, 0.15 - 0.25 Chromium, 0.10 - 0.15 Molybdenum, 0.4 - 0.6
Nickel, 0.34 - 0.69 Carbon equivalent, 0 - 0.1 Vanadium, 0 -0.08 Titanium, 0 - 0.01 Boron,
balance being Iron and residual impurities (in wt.%).
At step (208) the steel alloy is homogenized by austenitizing at 1150-1200°C for 30-45
minutes.
At step (212) the steel alloy is quenched in salt bath at 200°C to 250°C for 30 to 45
seconds; and at step (216) steel alloy is cooled to room temperature by extremely slow rate
of 0.25-0.50 °C/min.
The carbon partitioning from martensite to austenite happens during slow non-isothermal
cooling of the hot rolled coil after quenching at the run out table. The austenite has
sufficiently higher carbon content as compared to the initial alloy carbon concentration (i.e.
Cy > Ci) and remains stable. It does not transform to martensite on cooling to room
temperature. Here it is worth mentioning that the quench temperature (QT) is same as the
coiling temperature (CT) and the partitioning start temperature (PTi). Thus, a single
temperature (i.e. QT = CT = PTi) controls both the amount of untransformed initial
austenite as well as the driving force for carbon partitioning that is in other words the
partitioning kinetics. Hence, the choice of optimum quench temperature is fairly more critical
in the non-isothermal Q&P process as compared to the isothermal Q&P process, where QT
and PTi are different. The thermal profile (T vs t) of a hot rolled coil is also shown in Fig. 3,
which was calculated by B. Nelson of ArcelorMittal [5]. Here it can be noted that

temperature drop with time is extremely slow. Hence, depending upon the quench
temperature and alloy composition, it may provide sufficient driving force for carbon
partitioning.
Calculation of the cooling atmosphere which could closely resemble the actual coil cooling
profile is done. After recording the cooling profile of different furnaces, it was found that a
well-controlled salt bath furnace atmosphere could be effectively used for the cooling profile
akin to hot rolled coil cooling. Figure 4 shows the salt bath furnace atmosphere cooling
along with coil cooling profile taken from Fig. 3.
The next step was to design an alloy composition considering the following factors:
1. Higher Ms temperature to provide more driving force for carbon partitioning at the
same quench temperature.
2. Low carbon equivalent to eliminate the problem of weldability and low carbon
content to improve impact toughness.
3. Higher hardenability to eliminate bainite or pearlite formation during quenching
below Ms temperature.
4. Addition of less micro-alloying elements to have more solute carbon for partitioning.
5. The optimum amount of Si addition to prevent carbide formation during carbon
partitioning.
The chemical composition of the proposed alloys, their carbon equivalent (CE) values
and Ms temperature calculated empirically as well as through dilatometry experiment are
shown in Table 1. In the Alloy-1, Si was added to prevent carbide formation. Earlier
research shows that 1.5 wt % Si addition is required to completely avoid carbide formation
[6]. However, the present investigation does not aim at stabilizing very high level of
retained austenite. Hence, to avoid harmful effect of higher Si addition, such as brittleness
and poor surface properties [7], 0.5 wt % Si was added to check the possibility of retained
austenite stabilization. Vanadium was included in order to eliminate the detrimental effect of
nitrogen. The Alloy-2 contains boron to increase the hardenability, particularly in thicker
plates. For this to happen boron must remain in solid solution. As boron is a strong nitride
former stoichiometric amount of Ti was added. Ni was added to improve impact toughness
and also to act as an austenite stabilizer.


Theoretical calculation of retained austenite variation with quench temperature was
performed using the model developed by Speer et al. [4], Based on the results of these
calculations a quench temperature was selected which predicted the maximum amount of
retained austenite. In the present case the temperature was 250°C. For both the alloys,
mechanical properties were evaluated for Q&P treated at 250°C.
Mechanical properties (on ASTM standard samples) and the retained austenite volume
fraction (through XRD) for above mentioned experiments are shown in Table 2.
In case of Alloy-1, Q&P treated sample shows the presence of retained austenite. The
retained austenite lead to high impact toughness without any significant decay in abrasion
resistance. However, the hardness value is somewhat compromised.
In case of Alloy-2, the retained austenite is not present in the Q&P treated sample. The
tempered structure of this alloy resulted in superior abrasion resistance without much decay
in hardness value.


Having 8-10% (by volume) austenite and rest martensite.
Having abrasive wear volume loss for low alloy steel is 245-364 mm3 on a dry sand
rubber wheel test machine.
Having Charpy impact toughness of low alloy steel at room temperature is 33-102
J/cm2.
Having Charpy impact toughness of low alloy steel at -40 deg. C is 10-33 J/cm2.
Having hardness (at 1 kg load) of low alloy steel is 255 Hv-348 Hv.
Having Yield Strength of low alloy steel is 461-703 MPa.
Having ultimate Tensile Strength of low alloy steel is 640-952 MPa.
Having Total elongation of low alloy steel is 16-27 %.

WE CLAIM:
1. A low alloy steel with abrasion resistance and impact toughness comprising
0.10 - 0.25 - carbon
1.0-1.8 - Manganese
0.40 - 0.6 - Silicon
0.15-0.25 -Chromium
0.10-0.15 -Molybdenum
0.4 - 0.6 - Nickel
0.34 - 0.69 - Carbon equivalent
0-0.1 -Vanadium
0 -0.08 - Titanium
0 - 0.01 - Boron
balance being Iron and residual impurities.
2. A process for producing low alloy steel comprising steps of:
casting a steel alloy with composition
0.10-0.25 -carbon,
1.0-1.8 Manganese,
0.40 - 0.6 Silicon,
0.15-0.25 Chromium,

0.10 - 0.15 Molybdenum,
0.4 - 0.6 Nickel,
0.34 - 0.69 Carbon equivalent,
0-0.1 Vanadium,
0 -0.08 Titanium,
0 - 0.01 Boron,
balance being iron and residual impurities (in wt.%);
homogenizing the steel alloy by austenitizing at 1150-1200°C for 30-45 minutes;
quenching in salt bath at 200°C to 250°C for 30 to 45 seconds; and
cooling the steel alloy to room temperature by extremely slow rate of 0.25-0.50
°C/min.
3. The process as claimed in claim 1, wherein the low alloy steel comprises 8-10% (by
volume) austenite and rest martensite.
4. The process as claimed in claim 1, wherein abrasive wear volume loss for low alloy
steel is 245-364 mm3 on a dry sand rubber wheel test machine.
5. The process as claimed in claim 1, wherein Charpy impact toughness of low alloy
steel at room temperature is 33-102 J/cm2.
6. The process as claimed in claim 1, wherein Charpy impact toughness of low alloy
steel at -40 deg. C is 10-33 J/cm2.

7. The process as claimed in claim 1, wherein Hardness (at 1 kg load) of low alloy steel
is 255 Hv-348 Hv.
8. The process as claimed in claim 1, wherein Yield Strength of low alloy steel is 461-
703 MPa.
9. The process as claimed in claim 1, wherein Ultimate Tensile Strength of low alloy
steel is 640-952 MPa.
10. The process as claimed in claim 1, wherein Total elongation of low alloy steel is 16-
27 %.