U.S. patent application number 15/839588 was filed with the patent office on 2018-04-19 for bainitic steel for rock drilling component.
This patent application is currently assigned to SANDVIK INTELLECTUAL PROPERTY AB. The applicant listed for this patent is SANDVIK INTELLECTUAL PROPERTY AB. Invention is credited to Tomas ANTONSSON, Johan LINDEN.
Application Number | 20180105905 15/839588 |
Document ID | / |
Family ID | 47559174 |
Filed Date | 2018-04-19 |
United States Patent
Application |
20180105905 |
Kind Code |
A1 |
LINDEN; Johan ; et
al. |
April 19, 2018 |
BAINITIC STEEL FOR ROCK DRILLING COMPONENT
Abstract
A bainitic steel comprising, in weight % (wt %) C: 0.16-0.23,
Si: 0.8-1.0, Mo: 0.67-0.9, Cr: 1.10-1.30, V: 0.18-0.4, Ni:
1.60-2.0, Mn: 0.65-0.9, P: 50.020, S: 50.02, Cu: <0.20, N:
0.005-0.012, balance Fe and unavoidable impurities.
Inventors: |
LINDEN; Johan; (GAVLE,
SE) ; ANTONSSON; Tomas; (Sandviken, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SANDVIK INTELLECTUAL PROPERTY AB |
Sandviken |
|
SE |
|
|
Assignee: |
SANDVIK INTELLECTUAL PROPERTY
AB
Sandviken
SE
|
Family ID: |
47559174 |
Appl. No.: |
15/839588 |
Filed: |
December 12, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14653486 |
Jun 18, 2015 |
|
|
|
PCT/EP2013/076740 |
Dec 16, 2013 |
|
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15839588 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/44 20130101;
C21D 2211/002 20130101; C21D 9/0075 20130101; C21D 9/22 20130101;
C22C 38/02 20130101; C22C 38/04 20130101; C21D 1/20 20130101; C21D
6/004 20130101; C21D 6/005 20130101; C22C 38/42 20130101; C21D
6/008 20130101; C22C 38/001 20130101; E21B 17/22 20130101; C22C
38/46 20130101 |
International
Class: |
C22C 38/46 20060101
C22C038/46; C21D 9/00 20060101 C21D009/00; C22C 38/42 20060101
C22C038/42; C21D 9/22 20060101 C21D009/22; C22C 38/02 20060101
C22C038/02; C21D 6/00 20060101 C21D006/00; E21B 17/22 20060101
E21B017/22; C21D 1/20 20060101 C21D001/20; C22C 38/00 20060101
C22C038/00; C22C 38/04 20060101 C22C038/04; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2012 |
EP |
12198569.1 |
Claims
1. A top hammer drill rod, comprising: a central rod portion
extending longitudinally from a first end to a second end; a case
hardened, threaded male connector at the first end; and a case
hardened, threaded female connector at the second end, wherein the
drill rod is formed from a steel comprising, in weight % (wt %): C:
0.16-0.23 Si: 0.85-0.95 Mo: 0.67-0.9 Cr: 1.10-1.30 V: 0.18-0.4 Ni:
1.60-2.0 Mn: 0.65-0.9 P: 0.020 S: .ltoreq.0.02 Cu: .ltoreq.0.20 N:
0.005-0.012 balance Fe and unavoidable impurities, wherein at least
one of the male connector and the female connector includes a core
region and a surface zone, wherein a microstructure of the surface
zone includes martensite, and wherein a microstructure of the core
region includes bainite.
2. The top hammer drill rod according to claim 1, wherein the
microstructure of the core region consists of martensite and
bainite.
3. The top hammer drill rod according to claim 2, wherein the
amount of Si in the steel is 0.85-0.95 wt %.
4. The top hammer drill rod according to claim 3, wherein the
amount of Si in the steel is 0.87-0.89 wt %.
5. The top hammer drill rod according to claim 2, wherein the
amount of Mo in the steel is 0.70-0.80 wt %.
6. The top hammer drill rod according to claim 2, wherein the
amount of Cr in the steel is 1.20-1.25 wt %.
7. The top hammer drill rod according to claim 2, wherein the
amount of V in the steel is 0.20-0.30 wt %.
8. The top hammer drill rod according to claim 2, wherein the
amount of N in the steel is 0.008-0.012 wt %.
9. The top hammer drill rod according to claim 2, wherein the top
hammer drill rod is used during air-cold top hammer drilling above
ground.
Description
RELATED APPLICATION DATA
[0001] This application is a continuation application of U.S.
application Ser. No. 14/653,486 filed Jun. 18, 2015, which is a
.sctn. 371 National Stage Application of PCT International
Application No. PCT/EP2013/076740 filed Dec. 16, 2013 claiming
priority of EP Application No. 12198569.1, filed Dec. 20, 2012, the
entire contents of each are incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to a bainitic steel according
to the preamble of claim 1. The present invention further relates
to a drill rod component according to the preamble of claim 7. The
present invention further relates to method for manufacture a drill
rod component according to the preamble of claim 10. The present
invention also relates to the use of the inventive bainitic steel
according to the preamble of claim 15.
BACKGROUND ART
[0003] Drilling rods for mining and construction work typically
comprises a central rod portion, a threaded male end and a threaded
female end. In operation, a drilling head or drilling bit is
screwed onto the male end of the rod and the drilling head is
driven into the rock or ground by a drill rig. One type of drilling
is the so called "top hammer drilling" in which the drilling rig is
arranged to provide high rotational movement and percussion to the
drill rod. As the length of the drill hole proceeds, the drill rod
may be extended by screwing further drill rods onto the end of the
precedent one.
[0004] Drill rods may be manufactured by forging and threading the
ends of a steel rod into mating male and female connectors.
However, the most common practice today is to manufacture the male
and female connectors separately and then attach the connectors
with friction welding to a respective end of a steel rod.
[0005] One problem related to drill rods is their relative short
service life, since the rate by which the drill rods wear out and
have to be replaced, has a direct impact on the total cost for the
drilling operation. A further problem is the strength of the rod.
If a rod breaks, it may take considerable time to retrieve it from
the drill hole.
[0006] In the past some work has been done to improve drill rods.
For example, W097/27022 is directed to the problem of soft material
zones occurring in the interface between the connector and the
central rod after friction welding. When a connector and a central,
rod are friction welded together, heat evolves in the interface
between connector and central rod. The heated zone is referred to
as "the Heat Affected Zone", (HAZ). In the HAZ the steel material
is annealed and a zone of soft material occurs in the interface
between rod and connector. The soft zone becomes the weakest part
of the drill rod and is typically the position where the drill rod
breaks. To solve this problem, WO97/27022 proposes a steel in which
the chemical composition has been balanced such that the hardness
of the most tempered portion in the HAZ has a hardness equal to the
core hardness of the drilling rod.
[0007] The steel described in WO97/27022 has lead to improvements
in the service life of drill rods, in particular in view of failure
in the interface between connector and central rod. However, the
overall service life of drill rods is still not sufficient.
[0008] Field observation has shown that today failure in drill rods
rarely occurs in the interface between connector and central rod.
Instead, the life length of the drill rods seems to be limited by
failure in the threaded portion of the connectors.
[0009] is Consequently, it is an object of the present invention to
solve at least one of the above problems. In particular it is an
object of the present invention to achieve an improved steel
composition which allows for the manufacturing of drill rods with
long service life. A further object of the present invention is to
achieve a cost effect drill component which can be used over a long
period of time. It is also an object of the present invention to
achieve a method for producing wear resistant drill components. Yet
a further object of the present invention relates to the use of the
improved steel composition in rock drilling components.
SUMMARY OF THE INVENTION
[0010] According to the invention at least one of these objects is
met by a bainitic steel comprising (in weight %):
[0011] C: 0.16-0.23
[0012] Si: 0.8-1.0
[0013] Mo: 0.67-0.9
[0014] Cr: 1.10-1.30
[0015] V: 0.18-0.4
[0016] Ni: 1.60-2.0
[0017] Mn: 0.65-0.9
[0018] P: <0.020
[0019] S: <0.02
[0020] Cu: <0.20
[0021] N: 0.005-0.012 wt %
balance Fe and unavoidable impurities.
[0022] The inventive steel is primarily intended for producing case
hardened components that are subjected to repeated wear at elevated
temperatures, i.e. 300-500.degree. C., for example case hardened
threaded connectors in drill rods. These components have a
martensitic surface zone and a bainitic-martensitic core.
[0023] Results from field test performed during top hammer drilling
have shown that case hardened drill rods manufactured from the
inventive steel last surprisingly longer than drill rods
manufactured from conventional steel.
[0024] During top hammer rock- or soil drilling above ground, the
drill rod is subjected to intensive percussion from the drilling
rig. The percussion causes a shock wave which progresses through
the interconnected drill rods down to the drill bit in the bottom
of the hole. As the shock wave progresses through the
interconnected rods, approximately 5% of its energy is lost in the
form of heat that mainly evolves in the threads of the male and
female connectors of the interconnected drill rods. Consequently,
the working temperature in the connectors during top hammer
drilling is high, typically up to 300.degree. C. but it may reach
500.degree. C. In above-ground top hammer drilling, air is
typically used for cooling the drill rods and also for removing the
drill cuttings. However, air is not an effective cooling fluid and
does not cool the rods sufficiently to avoid that the evolved heat
causes the martensitic case in the threads of the connectors of the
drill rods to transform into the softer phases cementite and
ferrite. In conventional drill rods, the transformation of the
martensite may cause the surface of the threads to soften and
eventually cause the connectors to wear out. As adhesive wear
resistance is in direct relation to the hardness.
[0025] The reason for the surprisingly long service life of the
drill rods manufactured from the inventive steel is not entirely
understood. However, without being bound by theory, it is believed
that the balanced amounts of the alloy elements silicon,
molybdenum, chromium and vanadium in the steel causes the
martensitic surface of the drill rod connectors to retain the
hardness at the high working temperatures during top hammer
drilling.
[0026] Silicon stabilizes epsilon carbide and retards therefore the
transformation of the hard martensitic surface zone of the
connectors into softer cementite and ferrite up to temperatures of
approximately 300.degree. C. However, as the temperature rises in
the connectors during drilling, the martensitic phase in the
surface of the case hardened connectors will eventually start to
transform into cementite and ferrite. The amount of martensite in
the surface zone of the connectors therefore drops and consequently
also the hardness of the surface zone drops. During the
transformation of the martensite into cementite and ferrite, carbon
is released into the steel.
[0027] In the inventive steel the alloy elements molybdenum,
chromium and vanadium forms hard and stable carbides with the
excess carbon resulting from the transformed martensitic phase. The
hard carbides precipitate in the remaining martensitic phase of the
connectors and compensate thereby for the hardness, that is lost by
transformation of martensite into cementite.
[0028] The core of the connectors consists of martensite and
bainite. Bainite is a fine mixture of the phases cementite and
ferrite. Bainite is stable at high temperatures and remains
therefore sufficiently strong to support the hardened surface zone
of the connectors at high working temperatures.
[0029] According to an alternative, the amount of Si is 0.85-0.95
wt % in the inventive steel.
[0030] According to an alternative, the amount of Mo is 0.70-0.80
wt % in the inventive steel.
[0031] According to an alternative, the amount of Cr is 1.20-1.25
wt % in the inventive steel.
[0032] According to an alternative, the amount of V is 0.20-0. 30
wt %, preferably 0.2-0.25 wt % in the inventive steel.
[0033] According to an alternative, the amount of N is 0.005-0.008
wt % more preferred 0.008-0.012 wt %, in the inventive steel.
[0034] The invention also relates to component for rock drilling
comprising the inventive steel.
[0035] The component may be a threaded male or female connector for
a drill rod.
[0036] For example, the component is a drill rod comprising a
threaded male and a threaded female connector.
[0037] The invention also relates to a method for manufacturing a
component for rock drilling comprising the steps of: [0038] a.
forming a component for rock drilling as described above from the
inventive steel. [0039] b. heating said component to austenitizing
temperature; [0040] c. holding said component at austenitizing
temperature in a carbon containing atmosphere for a predetermined
time; [0041] d. cooling said component.
[0042] Preferably, said component is heated to a temperature of
900-1000.degree. C.
[0043] Preferably, said component is heated in an atmosphere of CO
and H.sub.2.
[0044] Preferably, the component is heated for 3-6 hours.
[0045] Preferably, the component is cooled in air.
[0046] The invention also relates to the use of the inventive
bainitic steel in case hardened connectors for drill rods during
air cold top hammer drilling above ground.
DETAILED DESCRIPTION OF THE INVENTION
[0047] The inventive steel comprises the following elements in
weight% (wt %): Carbon (C). Carbon is included in the inventive
steel for strength and to govern the final structure of the steel,
which should be bainitic. Carbon is also added to the inventive
steel for ensuring the formation of carbides. The carbides provide
a precipitation hardening effect in the bainitic structure of the
steel. The carbides further prevent the grains in the steel from
growing by coalescence, and thereby ensures fine grains in the
steel and consequently high strength. The carbon content should
therefore be at least 0.16 wt % in the steel. Too high carbon
content reduces the impact strength of the steel. Carbon should
therefore be limited to 0.23 wt %. Preferably, carbon is 0.18-0.20
wt %.
[0048] Silicon (Si) is used as deoxidizer in the manufacturing of
the steel and some amounts of silicon is therefore always present
in the steel. Silicon has a positive effect on the inventive steel
since it increases the hardenablity, i.e. the rate by which the
austenitic phase is transformed into martensite during quenching.
In the inventive steel, silicon is an important alloy element since
it retards the transformation of martensite into cementite and
ferrite.
[0049] Martensite is an unstable phase and when heated it
transforms, via various carbides, into cementite and ferrite which
leads to decreased hardness of the steel. Silicon stabilizes
epsilon carbide, which is one of the carbides that precedes the
cementite phase during the transformation of martensite and thereby
retards the transformation of martensite. Furthermore, during the
dissolving of the martensitic phase, carbon must diffuse through
the steel to the carbides in order for the carbides to grow. The
presence of silicon in the steel increases the carbon activity in
the steel which in turn retards the growth of the already formed
carbides and also the nucleation of new carbides. Also this
mechanism substantially retards the transformation of the
martensite. Silicon has therefore a positive effect on retaining
the strength of the surface zone in case hardened components of the
inventive steel at high temperatures.
[0050] However, silicon stabilizes ferrite and therefore will too
high amounts of silicon lead to an increase of the A1-temperature.
This has a negative effect as the steel during hardening must be
heated to higher temperature which causes grain growth in the
austenitic phase and thereby reduces the strength. Consequently,
the amount of silicon is limited to 0.80-1.0 wt % in the inventive
steel. Preferably, the amount of silicon is 0.85-0.95 wt %.
[0051] Molybdenum, chromium and vanadium are key elements in the
inventive steel since they form hard carbides which compensate for
the hardness drop when the martensitic phase transforms into
cementite and ferrite. The different carbide formers molybdenum,
chromium and vanadium form stable carbides at various temperatures.
Hence, at low temperatures and therefore moderate transformation of
the martensite, mainly molybdenum rich carbides are precipitated.
With increasing temperatures the transformation of martensite
increases. However at higher temperatures, chromium rich carbides
are first precipitated and subsequently, at even higher
temperatures, also vanadium rich carbides. This provides the effect
that the hardness of the martensite in the surface of the connector
is kept substantially constant over a wide range of working
temperatures.
[0052] Molybdenum (Mo), forms stable molybdenum rich carbides at a
temperature from 300.degree. C. up to approximately 500.degree. C.
and compensates for the hardness drop when the martensitic phase is
transformed into cementite and ferrite. To ensure that a sufficient
amount of carbides is precipitated, the amount of molybdenum shall
be at least 0.67 wt %. However, molybdenum stabilizes austenite and
has therefore a very strong influence on hardenability. Too high
amounts of molybdenum could therefore lead to the formation of
martensite in the core of the connector, which make the connector
brittle. High amounts of molybdenum could also cause the formation
of secondary hardness maximum. The upper limit for molybdenum is
therefore 0.9 wt % in the inventive steel. Preferably, molybdenum
is 0.67 to 0.83 wt % in the steel.
[0053] Chromium (Cr) forms stable chromium rich carbides with
carbon. Some chromium rich carbides are precipitated even at low
temperatures, i.e. 300.degree. C. However, the majority of the
chromium rich carbides are precipitated at temperature between
400-500.degree. C. To ensure that a sufficient amount of chromium
rich carbides are formed, the inventive steel should contain at
least 1.10 wt % chromium. Very high amounts of chromium could lead
to the formation of a so called secondary hardness maximum in the
steel at high temperatures, typically above 600.degree. C. This
phenomenon is generally caused by the formation of a large amount
of chromium carbides, and also of vanadium- and molybdenum
carbides. However, if the temperature of the steel is increased
further, the hardness rapidly drops due to growth of the
precipitated carbides which in turn steal carbon from other
precipitations in the steel. Chromium should therefore be limited
to 1.30 wt %. Preferably, the content of chromium is 1.20-1.25 in
the inventive steel to ensure that sufficient amount of carbides
are formed and that the formation of a secondary hardness maximum
is avoided.
[0054] Vanadium (V) form very small vanadium rich carbides at
temperatures of 550-600.degree. C. and compensate therefore for the
hardness drop when the martensitic phase transforms into cementite
and ferrite at high temperatures. The inventive steel should
contain at least 0.18 wt % vanadium to ensure that a sufficient
amount of vanadium carbides is precipitated in the steel at high
working temperatures.
[0055] Vanadium also forms vanadium carbonitrides at high
temperatures, i.e. 900.degree. C. and above. The vanadium
carbonitrides are important since they prevent grain growth of the
austenitic phase during carburization of the steel. Too high
amounts of vanadium could lead to problems during hot working of
the steel since the carbonitrides becomes so stable that they do
not dissolve in the annealing step that precedes hot working.
Therefore vanadium must be limited to 0.40 wt % in the inventive
steel. Preferably, vanadium is 0.18-0.30 wt %, more preferred
0.20-0.30 wt %, even more preferred 0.20-0.25 wt %.
[0056] Manganese (Mn) is included in the inventive steel for
forming MnS with sulphur, which may be present as an impurity in
the steel. Manganese has a positive effect on hardenabilty of the
steel, since it lowers the Ms-temperature, i.e. the temperature at
which martensite start to form after austenitizing. The low
Ms-temperature also causes a fine bainitic structure in the core of
a connector manufactured from the inventive steel. This is positive
for ensuring a high strength in the core of the connector.
Manganese should be included in an amount of at least 0.65 wt % in
order to ensure MnS-types of sulfides. High amounts of manganese
could result in the formation of retained austenite in the steel,
due to that manganese lowers the Ms-temperature. Manganese should
therefore be limited to 0.85 wt %. Preferably the amount of
manganese is 0.70-0.80 wt % in the steel since this amount of
manganese also ensures a fine bainitic structure in the inventive
steel.
[0057] Phosphorus (P) is present as an impurity in the raw material
for the inventive steel. Phosphorous segregate to the liquid phase
during solidification of the steel and causes phosphorous rich
streaks in the solidified steel. A high phosphorous content
therefore has a negative impact on the ductility and impact
toughness of the steel. Therefore, phosphor should be limited to a
maximum of 0.020 wt %, i.e. 0-0.020 wt %, in the inventive
steel.
[0058] Sulphur (S) is also present as an impurity in the raw
material for the inventive steel. Sulphur forms sulphide inclusions
in the steel which has a negative impact on the ductility and
impact strength of the steel. Sulphur should therefore be limited
to 0.02wt %, i.e. 0-0.020 wt %, in the inventive steel, more
preferred to max 0.015 wt %.
[0059] Nickel (Ni) increases the impact strength of the steel and
is consequently an important element in the inventive steel which
is intended for drilling rods. Nickel further reduces the
Ms-temperature of the steel and increases thereby the hardenablity.
In order to ensure sufficient impact strength in the steel, the
nickel content should be at least 1.60 wt %. Too high content of
nickel could reduce the Ms-temperature too much and lead to the
formation of retained austenite in the steel. Retained austenite
could cause tensile stress in the martensitic phase, and thereby
reduce the strength of the steel. The nickel content should
therefore be limited to 2.0 wt % in the inventive steel. Nickel is
further an expensive alloying element and should for that reason be
present in as low amounts as possible. Preferably, the content of
nickel is 1.70-1.90 wt % in the inventive steel since this amount
of nickel yields a cost effective steel with sufficient impact
strength.
[0060] Cupper (Cu) is typically included in the scrap metal that is
used as raw material. Cupper may be allowed in amounts up to 0.20
wt %, i.e. 0-0.20 wt %.
[0061] Nitrogen (N). The inventive steel preferably contains
nitrogen to ensure that the stable vanadium carbonitrides are
formed during carburization. Preferably, the amount of nitrogen is
0.005 wt %, more preferred 0.008 wt %. If the steel contains too
much nitrogen, the vanadium carbonitrides will become too stable
and may not dissolve during heating to the hot working temperature
of the steel. Therefore the maximum amount of nitrogen is 0.012 wt
%.
[0062] In hot rolled condition, the inventive steel has a
throughout bainitic structure, i.e. a structure of cementite
(Fe.sub.3C) and ferrite (.alpha.-iron). By "hot rolled" is meant
that the inventive steel has been produced by casting, thereafter
been heated to a temperature of appoximately 1200.degree. C. and
subjected to hot rolling followed by cooling in air.
[0063] In case hardened condition, the inventive steel has a
martensitic surface zone and a bainitic/martensitic core.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] FIG. 1: A schematic drawing of a rock drilling component
manufactured comprising the inventive steel.
[0065] FIG. 2: A graph showing the results from experiments
performed on the inventive steel.
[0066] FIG. 3: A table showing the results from tests performed on
the inventive steel.
[0067] FIGS. 4 and 5: Surface and core hardness of samples in a
test performed on an inventive steel and a comparative steel.
[0068] FIGS. 6 to 10: Diagrams produceds in ThermoCalc.TM.
simulations performed on an inventive and a comparative steel.
DESCRIPTION OF EMBODIMENTS
[0069] FIG. 1 shows schematically a longitudinal cross-section of a
drilling component according to a first embodiment of the present
invention. The drilling component shown in FIG. 1 is a MF-drilling
rod 1, which comprises a central rod portion 10. The first end of
the central rod 10 comprises a male connector 20 and the second end
of the central rod comprises a female connector 30. The male
connector 20 is provided with an external thread 21 and the female
connector is provided with an internal thread 31. The dimensions of
the male and the female connectors and the threads 21, 31 are
dimensioned such that the male connector 20 of a first MF rod can
be received in the female connector 30 of a second MF-rod. The
MF-rod further comprises a central channel 60, i.e. a bore that
extends through the entire MF-rod. The channel has one opening 61
in the center of the male connector and one opening 61 in the
centre of the s female connector. In operation, cooling fluid, such
as air is lead through the channel 60.
[0070] In FIG. 1, the male and the female connectors 20, 30 are
attached to the central rod portion 10 by friction welding which is
indicated by the dashed lines 11. However, the MF-rod in FIG. 1
could also be manufactured in one piece, i.e the male and the
female connectors 20 and 30 could be formed by forging and
threading the ends of the rod.
[0071] The connectors 20 and 30 are manufactured from the bainitic
steel according to the invention. The central rod 10 may be
manufactured from another type of steel, for example a conventional
low-alloyed carbon steel. However, the central rod could also be
manufactures from the bainitic steel according to the
invention.
[0072] The connectors 20 and 30 are case hardened and have a
bainitic core 40 and a martensitic surface zone 50. The martensitic
surface zone is 1-3 mm thick and extends from the surface of the
connector towards its centre.
[0073] Although the inventive drilling component has been described
with regards to a MF-rod it is obvious that it also could be any
other type of component that is subjected to repeated wear under
high working temperatures, for example a drifter rod.
[0074] Preferably, the inventive drilling component is manufactured
by a method which comprises the following steps.
[0075] In a first step, a drilling component is formed in a
bainitic steel according to the invention. This is typically
achieved by forging and threading a precursor of the inventive
steel into male and female connectors 20, 30. The precursor is
typically a portion of a solid rod that has been manufactured from
the inventive steel.
[0076] In a second step, the connectors are subjected to case
hardening. This is achieved in that the connectors are heated in a
furnace to austenitizing temperature, which for the inventive steel
is above 900.degree. C. The furnace could be of any type, e.g a pit
furnace. In order to ensure complete austenitizing of the
connectors and to avoid negative effects, such as grain
enlargement, the connectors should be heated to temperature between
900.degree. C. and 950.degree. C., preferably 925.degree. C.
[0077] The step of austenitizing of the connectors is performed in
a carbon rich atmosphere to ensure that the content of carbon is
increase in the surface zone of the connectors, so called
carburization. Typically the atmosphere in the furnace is a mixture
of the gases H.sub.2 and CO, for example cracked methane.
[0078] The connectors are kept in the furnace for a time period of
3-6 hours. The time governs the case depth, i.e. the thickness of
the martensitic surface zone. Preferably the time period is 5 hours
to ensure a sufficient case depth.
[0079] When the heating time has expired, the connectors, which now
are austenitized, are taken out of the furnace and are cooled in
the ambient air. Forced air cooling may be employed by blowing air
onto the connectors.
[0080] During cooling the carburized surface of the austenitized
connectors transforms into martensite and the core of the
connectors into a mixture of bainite and martensite.
[0081] The connectors may thereafter be subjected to a tempering
step to optimized the hardness of the martenistic surface.
Tempering is thereby performed at 200-300.degree. C. for 1
hour.
[0082] Finally, the connectors are attached to a central rod
portion by friction welding.
EXAMPLES
[0083] The inventive steel material is following described by four
non-limitating examples.
Example 1
[0084] Example 1 describes the results from field tests performed
with case hardened drill rods manufactured from the inventive
bainitic steel.
[0085] In a first step a heat of the inventive steel was produced.
The heat was produced by melting scrap metal in an electric arc
furnace, refining of the molten steel in a CLU converter and
subsequently cast in 24'' moulds to ingots.
[0086] The obtained inventive steel had the following
composition:
TABLE-US-00001 TABLE 1 Chemical composition of inventive steel C Si
Mn P S Cr Ni Mo V Cu N 0.19 0.87 0.72 0.004 0.009 1.15 1.66 0.70
0.20 0.13 0.009
[0087] From the inventive steel rods were produced. Some of the
rods were forged into threaded female type connectors and some into
threaded male type connectors.
[0088] The male and female type connectors were subjected to case
hardening. In a first step the connectors were carburized in a pit
furnace at a temperature of 925.degree. C. for a time period of 5
hours, the furnace contained an atmosphere of CO and H.sub.2.
[0089] After five hours the connectors were removed from the
furnace and allowed to cool in air. The case hardening resulted in
a martensitic layer which extended from the surface of the
connector towards the core which had bainitic/martensitic
structure.
[0090] The connectors were thereafter attached to the end of a
steel rod which also was manufactured from the inventive steel
material. A male connector was attached to one end of the rod and a
female connector to the other end. The connectors were attached by
friction welding.
[0091] Field testing was thereafter performed with the drilling
rods from the inventive steel at two different locations, Site A
and Site B. Drilling was performed with a drill bit having a
diameter of 115 mm and a drilling rig of the type Sandvik DP1500
was used. The drilling speed was approximately 1 meter/minute.
[0092] As comparison were also conventional drill rods used. These
rods were made of the steel grade Sanbar 64.
[0093] Nine rods of each type (inventive and conventional) were
used at Site A and 4 rods of each type at site B. The drill rods
were used until failure and the total number of meters drilled with
each rod was recorded as "drilling meter (dm)". Table 2 shows the
result of the testing as the average number of drilling meters
drilled per rod at site A and at site B.
TABLE-US-00002 TABLE 2 Results from drilling Site Conventional rod
Inventive rod Site A 2400 dm (average) 3200 dm (average) Site B
2100 dm (average) 3100 dm (average)
[0094] As can be seen in table 1, the drilling rods of the
inventive steel had a considerable longer operational life length
than the rods of the conventional material.
Example 2
[0095] In a second example, the hardness reduction of test samples
from an inventive steel was determined under laboratory conditions
at various reheating temperatures.
[0096] In a first step, a heat of the inventive steel was produced.
The heat was produced by melting scrap metal in an electric arc
furnace, refining of the molten steel in a CLU converter and
subsequently casting in 24'' moulds to ingots.
[0097] The obtained inventive steel had the following
composition:
TABLE-US-00003 TABLE 3 Chemical composition of inventive steel C Si
Mn P S Cr Ni Mo V Cu N 0.20 0.89 0.79 0.011 0.013 1.27 1.75 0.77
0.21 <0.01 0.008
[0098] The ingots were rolled into bars and the bars were cut into
5 cm long cylinders, which were used as samples.
[0099] The samples were thereafter subjected to a simulated
hardening treatment. This treatment included heating to
austenitizing temperature, holding at austenitizing temperature for
a pre-determined temperature and subsequently cooling in oil which
was heated to room temperature. Thereafter the hardened samples
were subjected to reheating in order to simulate heating during
drilling operation. After reheating, the samples were cooled in
air. After cooling of the reheated samples, the hardness was
measured in the surface, on the middle of the radius and in the
center of each sample. The hardness was measured in Vickers
(HV1)
[0100] As reference, one sample of each series was left as hardened
but in non reheated condition.
[0101] Twelve samples were used for each austenitizing temperature.
The austenitizing temperatures was: 860.degree. C., 1h holding
time; 880.degree. C., 1 h holding time; 925.degree. C., 20 min
holding time. After quenching in oil, the samples were reheated at
the following temperatures: Non Reheated, 200.degree. C.,
300.degree. C., 400.degree. C., 500.degree. C. 550.degree. C.,
580.degree. C., 600.degree. C., 650.degree. C., 675.degree. C. and
700.degree. C.
[0102] The result of the measurement graphically demonstrated in
FIG. 2. FIG. 2 shows a graph in which the result for each
austenitizing temperature is shown as a mean value for the measured
hardness at each reheating temperature The specific measurement
values are shown in table 4, see FIG. 3.
[0103] It should be noted that the experiment is performed on
non-carburized samples. However, from graph in FIG. 2, it is clear
that the hardness of the three different samples series is almost
constant from the non-reheated samples up to 650.degree. C. It is
believed that the constant hardness is due to the stabilizing
effect of silicon on the martensitic phase at low temperatures and
by the precipitation of hard and stable carbides of chromium,
molybdenum and vanadium at higher temperatures which compensates
for the transformation of martensite into cementite and ferrite. At
700.degree. C., a secondary hardness maximum is formed and
thereafter the hardness sharply drops due to that the Cr-, Mo- and
V-carbides coalescence into fewer and coarser precipitations. The
growth of the Cr-, Mo- and V-carbides further causes the remaining
martensite to dissolve into cementite and ferrite and thereby the
hardness decreases even further.
[0104] It is evident that a carburized sample of the inventive
steel material, at all reheating temperatures, would be harder than
the non-carburized samples. However, it is believed that the
hardness of a carburized sample would also exhibit an essentially
constant hardness up to approximately 650.degree. C.
Example 3
[0105] In a third example a comparison was made on the surface- and
core hardness of hardened and tempered samples of an alloy
according to the invention and a comparative alloy. The test
simulates the tempering effect that occurs in case hardened drill
rods due to the heat that evolves in the couplings during drilling.
For comparision, an alloy similar to the alloy disclosed in
document WO97/27022 was selected. WO97/27022, discloses an alloy
which is optimized for friction welding and is briefly discussed
under the section "Background of the invention" of the present
application.
[0106] The chemical composition of the inventive and comparative
alloys are shown in table 5 below. Comp 0.09 denominates the
comparative alloy and Inv 0.22 denominates the inventive alloy.
TABLE-US-00004 TABLE 5 Chemical composiition of test alloys % C %
Si % Mn % P % S % Cr % Ni % Mo % V % Cu % N Comp 0.19 0.89 0.30
0.005 0.002 1.25 1.79 0.75 0.09 0.020 0.002 0.09V Inv 0.22V 0.20
0.89 0.70 0.060 0.027 1.20 1.84 0.70 0.22 0.13 0.009
[0107] A 1 kg heat of the comparative alloy was produced by
conventional methods including: melting of scrap metal in a
induction furnace, refining and casting. The casting was preheated
in a furnace in 700.degree. C. for approx. 30 minutes and then hot
rolled at 1200.degree. C. into a square bar having the dimensions
13 mm. The bar was then slowly cooled in air and cut into 13'13 mm
samples.
[0108] A 75 ton heat of the inventive alloy was produced by
conventional methods used in production, including: melting in an
EA-furnace, AoD treatment, ladle refining, continious casting and
hot rolling. The obtained casting of the inventive material was hot
rolled to a bar having a diameter of 40 mm.
[0109] The bars of the inventive material were cut into samples in
dimensions 40.times.130 mm.
[0110] The samples were subsequently carburized and hardened by
forced air cooling. Carburizing of the samples was performed
according to the following program in an athmosphere of
Propane/Nitrogen/Methanol. In Step 1 the samples were first heated
for a period of 150 minutes to the process temperature of
925.degree. C. and then held at that temperature for 435 min:
TABLE-US-00005 TABLE 6 Carburizing program Step 1 Step 2 Step 3
Temperature, .degree. C. 925 925 925 Carbon potential (Cp) 0.80
0.60 0.40 Time, min 150 0 0 Hold time, min 435 100 180
[0111] Thereafter, the hardened samples were subjected to tempering
at different temperatures. Prior to tempering, the samples were
painted with NoCarb.TM. inorder to prevent decarburization. Table 7
below shows the tempering temperature for each sample. one sample
of each alloy was left untempered. Each of the remaining samples
was tempered for 30 minutes.
TABLE-US-00006 TABLE 7 Tempering temperatures Sample 1 2 3 4 5 6 7
8 9 10 Temperature, .degree. C. Untempered 150 180 200 250 300 400
500 600 700
[0112] After tempering, the core and surface hardness of each
sample were measured. The surface hardness was measured in HRC and
the core hardness by Vickers measurement (HV30). The surface
hardness of the various samples is shown in FIG. 4. The core
hardness of the samples is shown in FIG. 5.
[0113] From FIG. 4 it can be concluded that the untempered samples
of the inventive and the comparative alloy have similar surface
hardness. This is due to that the structure in the surface of the
respective untempered samples essentially consists of martensite.
The hardness of the tempered samples descreases with increasing
tempering temperature. However, from the graphs in FIG. 4 it is
clearly visible that the surface hardness of the inventive alloy is
higher than the the surface hardness of the comparative alloy for
all tempering temperatures up to 600.degree. C. That is, the
inventive alloy has a higher tempering resistance than the
comparative alloy.
[0114] Surprisingly, the surface hardness of the inventive alloy
remains much more stable with increasing tempering temperature than
the surface hardness of the comparative alloy. As can be seen in
FIG. 4, the surface hardness of the inventive alloy is essentially
constant at 57 HRC up to 200.degree. C. where it drops to 55 HRC
and then proceeds essentially constant up to 300.degree. C. The
surface hardness of the comparative alloy on the other hand drops
continuously over the whole temperature interval.
[0115] At higher temperatures the dissolving rate of the martensite
increases and the vanadium carbides coaleces to coarser particles
which results in decreasing surface hardness. At 700.degree. C. the
vanadium carbides become unstable and the surface hardness of both
the inventive and the comparative samples drops rapidly.
[0116] From FIG. 5 it can be concluded that the core hardness in
the inventive samples is slightly lower than in the comparative
samples. The main reason for the relative low core hardness of the
inventive alloy is that the high amount of vanadium in combination
with the selected nitrogen content produces stable vanadium
carbonitrides during the carburizing step of the samples. The small
vanadium carbonitrides prevents grain growth during the carburizing
step and increases the impact toughness of the core. The small
grains also lowers the hardenability of the alloy and ensures
thereby that the core, after hardening, substantially consists of
bainit which is less hard but more tough than martensite.
[0117] Conclusion
[0118] The results from the third example show a better tempering
resistance in the inventive alloy than in the comparative alloy.
The surface hardness of the inventive alloy is more stable compared
with the comparative material.
[0119] In rock drilling, the ability to have a stable surface
hardness is crucial for the wear resistance. A material that will
keep the surface hardness even though the temperature increases
during drilling will withstand wear better, as adhesive wear
resistance is in direct relation with the hardness. The relation
between surface hardness and core hardness is also an important
factor for threads used in drilling rods. The desired relation is a
hard surface for better wear resistance together with a tough core
for better impact resistance. Also a greater difference between
hardness of the surface and the core results in more residual
compressive stresses, which increases fatigue life. With this in
mind the inventive alloy with high vanadium content is advantageous
compared with the comparative material having a low vanadium
content, it provides a higher surface hardness together with a
tougher core, while it is the opposite for the comparative
material.
Example 4
[0120] In a fourth example, simulations were performed in the
program ThermoCalc.TM. 3.0 and database TCFE7. The purpose of the
simulations was to confirm the results from the measurements of the
core hardness on the inventive and the comparative samples in the
third example. A further purpose was to confirm that the good
result of core hardness of the inventive sample exist over a
preferred range of nitrogen and vanadium of the inventive
alloy.
[0121] The simulations shows the stability of vanadium
carbonitrides at various temperatures in inventive and comparative
alloys. As will be described further below, the presence of
vanadium carbonitrides at the carburizing temperature or the
hotworking temperature will have a signinficant effect on the
metallografic structure in the core a final component.
[0122] FIG. 6 shows a diagram produced in a first ThermoCalc.TM.
simulation of the stability of vanadium carbonitrides that are
formed in an inventive alloy having a vanadium content of 0.2 wt %
and a nitrogen content of 0.005 wt %. The overall compostion of the
alloy in the simulation is:
[0123] 0.019 C; 0.9 Si; 0.75 Mo; 1.2 Cr; 0.20 V; 1.8 Ni; 0.78 Mn;
0.005 N
[0124] FIG. 6 shows the amount of various percipitated phases in
moles that exist in the alloy system at different temperatures. The
y-axis shows the amount of precipitated phases and the x-axis shows
the temperature. Line 1 shows the amount (in moles) of vanadium
carbonitrides that exists in the alloy system at various
temperatures. The other lines shows in the diagram shows other
phases that are present in the inventive alloy system.
[0125] These phases will not be discussed further.
[0126] When line 1 is followed in FIG. 6, it can be seen that the
precipitation of vanadium carbonitrides increases with increasing
temperature in the temperature range of 700-800.degree. C. Above
800.degree. C. the precipitation of vanadium carbonitrides ceases
and the precipitated vanadium carbonitrides start to dissolve due
to equilibria in the alloy system. Consequently, less vanadium
carbonitrides may exist in the alloy system at high temperatures.
The amount of of carbonitrides in the alloy system therefore
decreases with increasing temperature. In the alloy system of FIG.
6 it can be seen that a relatively high amount of vanadium
carbonitrides exists in the alloy system in the temperature
interval of 900-1000.degree. C. The diagram further shows that the
vanadium carbonitrdes are entirely dissolved at approx.
1100.degree. C.
[0127] The above distribution of vanadium carbonitrides would
ensure good core properties in a component manufactured from the
inventive alloy for the following reasons:
[0128] Firstly, in production of components for rock drilling, the
components are carburized and hardened at 930.degree. C. At this
temperature the crystal grains in the steel strive to coalesce into
few and large grains.
[0129] Generally, the grain size of a steel influences the
hardenability of the steel in the sense that the hardenability of
the steel increases with increasing grain size. After hardening, a
steel with a small grain size will therefore, have a predominant
bainitic structure whereas a steel with large grains will have a
martensitic structure.
[0130] The presence of the relatively large amount of vanadium
carbonitrides at 930.degree. C. in FIG. 6 would effectively prevent
grain growth in the inventive steel by blocking the crystal grains
of the alloy from coalescing. This would in turn result in small
grains in the inventive alloy and a predominatly bainitic structure
in the core of a hardened component manufactured thereof. This is
important for the strength and impact toughness of the core as well
as its structural stability at high temperature.
[0131] Secondly, from FIG. 6 it may be concluded that all vanadium
carbonitrides are dissolved at approx. 1100.degree. C. This is of
course important for the hotworkability of the steel. However, more
important is the absence of the negative effect that vanadium
carbonitrides remaining after hotworking would have on the grain
size during hardening of the alloy. In the hardening step remaining
vanadium carbonitrides would coalesce into few and very large
particles. These particles would have little effect on preventing
grain is growth during carburization/hardening and the result would
be a component with a core of mainly martensitic structure having
low toughness and therefore poor impact strength.
[0132] FIG. 7 shows a diagram produced in a second ThermoCalc.TM.
simulation of the stability of vanadium carbonitrides that are
formed in an inventive alloy with a vanadium content of 0.2 and a
nitrogen content of 0.012. This simulation confirms the conclusions
of the first simulation. Hence, also this simulation shows that a
sufficient amount of vanadium carbonitrides exist in the alloy in
the temperature interval of 900-1000.degree. C. to ensure a
bainitic structure in in the core of the alloy after hardening. It
may further be concluded from the diagram that the vanadium
carbonitrides are completely dissolved at approx. 1130.degree.
C.
[0133] It can be noted that the higher nitrogen content in the
alloy of the second simulation results in the precipitation of more
vanadium carbonitrides at 930.degree. C. in comparision with the
first simulation. This is of course positive for ensuring the
bainitic structure of the core.
[0134] FIG. 8 shows a diagram produced in a third ThermoCalc.TM.
simulation of the stability of vanadium carbonitrides that are
formed in an inventive alloy with a vanadium content of 0.3 wt %
and a nitrogen content of 0.005 wt % The simulated alloy had the
following composition:
[0135] 0.019 C; 0.9 Si; 0.75 Mo; 1,2 Cr; 0.1 V; 1.8 Ni; 0.78 Mn;
0.005 N
[0136] Also this simulation shows that a sufficient amount of
vanadium carbonitrides are precipitated at 900-1000.degree. C. and
that all vanadium carbonitrides have dissolved at a temperature of
1120.degree. C.
[0137] In comparision to the first and second simulations more
vanadium carbonitrides are precipitated in the third simulation.
The reason for this is the higher vanadium content in this
alloy.
[0138] FIG. 9 shows a diagram produced in a fourth ThermoCalc.TM.
simulation of the stability of vanadium carbonitrides that are
formed in an inventive alloy with a vanadium content of 0.3 wt %
and a nitrogen content of 0.012 wt %. The simulated alloy had the
following composition:
[0139] 0.019 C; 0.9 Si; 0.75 Mo; 1,2 Cr; 0.1 V; 1.8 Ni; 0.78 Mn;
0.005 N
[0140] Also this simulation shows that a sufficient amount of
vanadium carbonitrides exists at the temperature range of
900-1000.degree. C. and that the vanadium carbonitrides have
dissolved at a temperature below 1200.degree. C.
[0141] FIG. 10 shows a diagram produced in a fifth ThermoCalc.TM.
simulation of the stability of vanadium carbonitrides that are
formed in a comparative alloy with low vanadium content (0.1 wt %)
and a nitrogen content of 0.005 wt %. The simulated alloy is
similar to the alloy used in Example 3 and has the following
composition:
[0142] 0.019 C; 0.9 Si; 0.75 Mo; 1,2 Cr; 0.1 V; 1.8 Ni; 0.78 Mn;
0.005 N
[0143] From line 1 in FIG. 10 it can be concluded that that a very
small amount of vandium carbonitrides are exist in this alloy at
the temperature intervall of 900-1000.degree. C. In this alloy the
amount of of vandium carbonitrides is too small to prevent grain
growth during carburization which in turn would result in increased
hardenability and martensite formation in the core of a hardened
component manufactured this alloy. The simulation therefore
confirms the measurements that was made on the core hardness of the
comparative alloy of Example 3.
[0144] To summarize, from the five ThermoCalc.TM. simulations and
the results from the physical experiment 3 it may be concluded that
an optimal balance of surface hardness and core hardness in achived
in the inventive alloy. The optimal balance of surface- and core
hardness makes the inventive alloy very suitable for use in
rockdrilling components.
* * * * *