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.

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: ?0.02 Cu: ?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

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? 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? 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? C. and 950? C., preferably 925? 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? 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? 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? C., 1h holding time; 880? C., 1 h holding time; 925? C., 20 min holding time. After quenching in oil, 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.

[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? 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? 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? 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? 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.

[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?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? C. and then held at that temperature for 435 min:

TABLE-US-00005 TABLE 6 Carburizing program 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

[0111] Thereafter, the hardened samples were subjected to tempering at different temperatures. Prior to tempering, the samples were painted with NoCarb? 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, ? 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? 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? 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.

[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? 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? 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? 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? C. Above 800? 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? C. The diagram further shows that the vanadium carbonitrdes are entirely dissolved at approx. 1100? 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? 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? 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? 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? 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? 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.

[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? 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? 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? C. and that all vanadium carbonitrides have dissolved at a temperature of 1120? 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? 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? C. and that the vanadium carbonitrides have dissolved at a temperature below 1200? C.

[0141] FIG. 10 shows a diagram produced in a fifth ThermoCalc? 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? 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? 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.