US20150344997A1 - Bainitic steel for rock drilling component - Google Patents

Bainitic steel for rock drilling component Download PDF

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Publication number
US20150344997A1
US20150344997A1 US14/653,486 US201314653486A US2015344997A1 US 20150344997 A1 US20150344997 A1 US 20150344997A1 US 201314653486 A US201314653486 A US 201314653486A US 2015344997 A1 US2015344997 A1 US 2015344997A1
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steel
component
inventive
alloy
hardness
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Johan Linden
Tomas ANTONSSON
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Sandvik Intellectual Property AB
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Sandvik Intellectual Property AB
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Assigned to SANDVIK INTELLECTUAL PROPERTY AB reassignment SANDVIK INTELLECTUAL PROPERTY AB ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: Antonsson, Tomas, LINDEN, JOHAN
Publication of US20150344997A1 publication Critical patent/US20150344997A1/en
Priority to US15/839,588 priority Critical patent/US20180105905A1/en
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/14—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes wear-resistant or pressure-resistant pipes
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/22—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for drills; for milling cutters; for machine cutting tools
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B17/00—Drilling rods or pipes; Flexible drill strings; Kellies; Drill collars; Sucker rods; Cables; Casings; Tubings
    • E21B17/02—Couplings; joints
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B17/00—Drilling rods or pipes; Flexible drill strings; Kellies; Drill collars; Sucker rods; Cables; Casings; Tubings
    • E21B17/22—Rods or pipes with helical structure
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite

Definitions

  • 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 .
  • Drilling rods for mining and construction work typically comprises a central rod portion, a threaded male end and a threaded female end.
  • 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.
  • the drill rod may be extended by screwing further drill rods onto the end of the precedent one.
  • Drill rods may be manufactured by forging and threading the ends of a steel rod into mating male and female connectors.
  • 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.
  • drill rods 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.
  • WO97/27022 is directed to the problem of soft material zones occurring in the interface between the connector and the central rod after friction welding.
  • the heated zone is referred to as “the Heat Affected Zone”, (HAZ).
  • HAZ Heat Affected Zone
  • 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.
  • 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.
  • an object of the present invention to solve at least one of the above problems.
  • 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.
  • Yet a further object of the present invention relates to the use of the improved steel composition in rock drilling components.
  • a bainitic steel comprising (in weight %):
  • the inventive steel is primarily intended for producing case hardened components that are subjected to repeated wear at elevated temperatures, i.e. 300-500° C., for example case hardened threaded connectors in drill rods. These components have a martensitic surface zone and a bainitic-martensitic core.
  • 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.
  • 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° C. but it may reach 500° C.
  • air is typically used for cooling the drill rods and also for removing the drill cuttings.
  • 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° C.
  • 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.
  • carbon is released into the 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.
  • 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.
  • the amount of Si is 0.85-0.95 wt % in the inventive steel.
  • the amount of Mo is 0.70-0.80 wt % in the inventive steel.
  • the amount of Cr is 1.20-1.25 wt % in the inventive steel.
  • the amount of V is 0.20-0.30 wt %, preferably 0.2-0.25 wt % in the inventive steel.
  • the amount of N is 0.005-0.008 wt % more preferred 0.008-0.012 wt %, in the inventive steel.
  • the invention also relates to component for rock drilling comprising the inventive steel.
  • the component may be a threaded male or female connector for a drill rod.
  • the component is a drill rod comprising a threaded male and a threaded female connector.
  • the invention also relates to a method for manufacturing a component for rock drilling comprising the steps of:
  • said component is heated to a temperature of 900-1000° C.
  • said component is heated in an atmosphere of CO and H 2 .
  • the component is heated for 3-6 hours.
  • the component is cooled in air.
  • 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.
  • the inventive steel comprises the following elements in weight % (wt %):
  • 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 %.
  • Silicon 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.
  • 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.
  • 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.
  • the amount of silicon is limited to 0.80-1.0 wt % in the inventive steel.
  • the amount of silicon is 0.85-0.95 wt %.
  • 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.
  • Molybdenum forms stable molybdenum rich carbides at a temperature from 300° C. up to approximately 500° 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.
  • Chromium (Cr) forms stable chromium rich carbides with carbon. Some chromium rich carbides are precipitated even at low temperatures, i.e. 300° C. However, the majority of the chromium rich carbides are precipitated at temperature between 400-500° 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° C. This phenomenon is generally caused by the formation of a large amount of chromium carbides, and also of vanadium- and molybdenum carbides.
  • Chromium should therefore be limited to 1.30 wt %.
  • 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.
  • Vanadium (V) form very small vanadium rich carbides at temperatures of 550-600° 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.
  • Vanadium also forms vanadium carbonitrides at high temperatures, i.e. 900° 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.
  • vanadium is 0.18-0.30 wt %, more preferred 0.20-0.30 wt %, even more preferred 0.20-0.25 wt %.
  • 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.
  • Manganese should therefore be limited to 0.85 wt %.
  • 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.
  • 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.
  • 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 0.02 wt %, i.e. 0-0.020 wt %, in the inventive steel, more preferred to max 0.015 wt %.
  • 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 hardenability. 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.
  • Cupper 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 %.
  • the inventive steel preferably contains nitrogen to ensure that the stable vanadium carbonitrides are formed during carburization.
  • 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 %.
  • the inventive steel In hot rolled condition, the inventive steel has a throughout bainitic structure, i.e. a structure of cementite (Fe 3 C) and ferrite (a-iron).
  • hot rolled is meant that the inventive steel has been produced by casting, thereafter been heated to a temperature of approximately 1200° C. and subjected to hot rolling followed by cooling in air.
  • the inventive steel has a martensitic surface zone and a bainitic/martensitic core.
  • FIG. 1 A schematic drawing of a rock drilling component manufactured comprising the inventive steel.
  • FIG. 2 A graph showing the results from experiments performed on the inventive steel.
  • FIG. 3 A table showing the results from tests performed on the inventive steel.
  • FIGS. 4 and 5 Surface and core hardness of samples in a test performed on an inventive steel and a comparative steel.
  • FIGS. 6 to 10 Diagrams produces in ThermoCalcTM simulations performed on an inventive and a comparative steel.
  • 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 female connector. In operation, cooling fluid, such as air is lead through the channel 60
  • 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 .
  • 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.
  • 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.
  • 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.
  • 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.
  • the inventive drilling component is manufactured by a method which comprises the following steps.
  • 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.
  • the connectors are subjected to case hardening.
  • the furnace could be of any type, e.g a pit furnace.
  • the connectors should be heated to temperature between 900° C. and 950° C., preferably 925° C.
  • 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.
  • the atmosphere in the furnace is a mixture of the gases H 2 and CO, for example cracked methane.
  • 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.
  • the time period is 5 hours to ensure a sufficient case depth.
  • 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.
  • 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° C. for 1 hour.
  • the connectors are attached to a central rod portion by friction welding.
  • the inventive steel material is following described by four non-limitating examples.
  • Example 1 describes the results from field tests performed with case hardened drill rods manufactured from the inventive bainitic steel.
  • 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.
  • the obtained inventive steel had the following composition:
  • the male and female type connectors were subjected to case hardening.
  • the connectors were carburized in a pit furnace at a temperature of 925° C. for a time period of 5 hours, the furnace contained an atmosphere of CO and H 2 .
  • 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.
  • 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.
  • the drilling rods of the inventive steel had a considerable longer operational life length than the rods of the conventional material.
  • test samples from an inventive steel was determined under laboratory conditions at various reheating temperatures.
  • 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.
  • the obtained inventive steel had the following composition:
  • the ingots were rolled into bars and the bars were cut into 5 cm long cylinders, which were used as samples.
  • 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.
  • 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)
  • the austenitizing temperatures was: 860° C., 1 h holding time; 880° C., 1 h holding time; 925° C., 20 min holding time.
  • the samples were reheated at the following temperatures: Non Reheated, 200° C., 300° C., 400° C., 500° C., 550° C., 580° C., 600° C., 650° C., 675° C. and 700° C.
  • 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 .
  • the test simulates the tempering effect that occurs in case hardened drill rods due to the heat that evolves in the couplings during drilling.
  • 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.
  • 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° C. for approx. 30 minutes and then hot rolled at 1200° 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.
  • 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, continuous casting and hot rolling.
  • the obtained casting of the inventive material was hot rolled to a bar having a diameter of 40 mm.
  • the bars of the inventive material were cut into samples in dimensions 40 ⁇ 130 mm.
  • Step 1 the samples were first heated for a period of 150 minutes to the process temperature of 925° C. and then held at that temperature for 435 min:
  • Step 1 Step 2 Step 3 Temperature, ° C. 925 925 925 Carbon potential (Cp) 0.80 0.60 0.40 Time, min 150 0 0 Hold time, min 435 100 180
  • the hardened samples were subjected to tempering at different temperatures. Prior to tempering, the samples were painted with No-CarbTM 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.
  • 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 .
  • 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 decreases 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 surface hardness of the comparative alloy for all tempering temperatures up to 600° C. That is, the inventive alloy has a higher tempering resistance than the comparative alloy.
  • the surface hardness of the inventive alloy remains much more stable with increasing tempering temperature than the surface hardness of the comparative alloy.
  • the surface hardness of the inventive alloy is essentially constant at 57 HRC up to 200° C. where it drops to 55 HRC and then proceeds essentially constant up to 300° C.
  • the surface hardness of the comparative alloy on the other hand drops continuously over the whole temperature interval.
  • 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.
  • 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.
  • 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.
  • a greater difference between hardness of the surface and the core results in more residual compressive stresses, which increases fatigue life.
  • 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.
  • simulations were performed in the program ThermoCalcTM 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.
  • 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 significant effect on the metallorganic structure in the core a final component.
  • FIG. 6 shows a diagram produced in a first ThermoCalcTM 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 composition of the alloy in the simulation is:
  • FIG. 6 shows the amount of various precipitated 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. These phases will not be discussed further.
  • the components are carburized and hardened at 930° C. At this temperature the crystal grains in the steel strive to coalesce into few and large grains.
  • 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.
  • FIG. 7 shows a diagram produced in a second ThermoCalcTM 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.
  • this simulation shows that a sufficient amount of vanadium carbonitrides exist in the alloy in the temperature interval of 900-1000° 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° C.
  • FIG. 8 shows a diagram produced in a third ThermoCalcTM 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:
  • FIG. 9 shows a diagram produced in a fourth ThermoCalcTM 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:
  • FIG. 10 shows a diagram produced in a fifth ThermoCalcTM 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:

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CN112322981A (zh) * 2020-11-06 2021-02-05 首钢贵阳特殊钢有限责任公司 一种凿岩用h22及h25钎杆中空钢
US20230136145A1 (en) * 2020-03-16 2023-05-04 Ejot Se & Co. Kg Method for producing a screw, and screw

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BR112022022553A2 (pt) * 2020-05-06 2022-12-13 Alleima Rock Drill Steel Ab Um novo aço bainítico
CN112695245B (zh) * 2020-12-03 2022-06-03 兰州兰石集团有限公司铸锻分公司 极寒地带钻机用低温钢及其热处理工艺

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