Expression of the Self-Sharpening Mechanism of a Roller Cone Bit during Wear Due to the Influence of the Erosion Protection Carbide Coating

Round 1

Reviewer 1 Report

Conclusions: give them as bullets, one per each highlight.

Figure 4 needs some scale.

Eliminate some references to Coatings, there was a report considering this a bad practice, much better other journals.

The paper is nice, the application of this kind of tool on rocks and stones is amazing.

Sandstone. How did you deal with the deviations between one type and another? People working on Stone or Wood are always complaining about that. I like how some did with concepts more in the line of metal alloys. Some references are very interested but you missed some recent relative to work on other materials different than stone, one in wood in joinery industry is a good example, because the wear patterns and mechanism are not exactly the same, see https://doi.org/10.1177/0954405414534431 in Proc Mech engineers part B manufacture

Good work, please correct above points.

Author Response

Thank you for your review.

Figure 4 needs some scale.

Response: We agree with your statement. A scale has been added to the image, and a bit diameter of 215.9 mm has been added to the image description. The description under Figure 4: Worn roller cone drill bit (Ø 215.9 mm) after drilled interval.

Figure 4: Worn roller cone drill bit (Ø 215.9 mm) after drilled interval

Eliminate some references to Coatings, there was a report considering this a bad practice, much better other journals.

Response: We agree with your statement. We will remove one reference and keep two others. However, in this case, the references we cite are important from the standpoint of attaching various materials that increase the abrasion resistance of steel. These studies on the effectiveness of applying erosion control materials to steel for work in very demanding conditions are quite rare and have therefore been included in the references.

The paper is nice, the application of this kind of tool on rocks and stones is amazing.

Response: Thank you very much for your opinion. It is very encouraging to us.

Sandstone. How did you deal with the deviations between one type and another? People working on Stone or Wood are always complaining about that. I like how some did with concepts more in the line of metal alloys. Some references are very interested but you missed some recent relative to work on other materials different than stone, one in wood in joinery industry is a good example, because the wear patterns and mechanism are not exactly the same, see https://doi.org/10.1177/0954405414534431 in Proc Mech engineers part B manufacture

Response: The final depth of the borehole made with the bit that was the subject of our study was 1482.5 m. The drilling interval made with the bit under study was 610.7 m long. The rock material through which the drilling was carried out is not isotropic. It was basically sandstone, and in some places there were layers of carbonates and other material. As a rule, prior to the construction of deep wells, a geological survey of the rock material is carried out, during which the geomechanical parameters of the rock material are estimated. Based on the preliminary analysis, suitable bits are then selected for drilling through these materials. During drilling, the progress, the load on the bit and the analysis of the rock material coming out of the borehole are continuously measured. Based on this, the torque and load on the bit are adjusted on the fly. When transitioning to a rock material that differs significantly in its geomechanical parameters from the previous one, the bit is removed from the borehole and replaced with a more suitable one for drilling in this new material. The drilling regime (bit load, speed, torque, and flushing parameters) is also adjusted accordingly. We have not really monitored the wear mechanism during drilling in other materials such as wood and metal, because in this case, as you have already found out, it is a different operation of the bit - drill on the material and consequently a different type of wear. When drilling deep holes in rock, the method of operation is based mainly based on the crushing in the primary phase, and only then after "plowing" and removing material from the area of each tooth.

In the use of drill bits, for example, in wood or steel, the shape of the cutter of which differs markedly from the bits used in making deep holes, we think it is mainly a question of turning the material. However, studies of material wear and the development of new, more durable materials are very important. We have to admit that in the area of developing more durable materials for cutting tools, disproportionately more work has been put into the development of drills than into the development of roller cone bits, where, due to high product prices and a small number of quality manufacturers, we have to rely on their good will and their own development.

We have read the article. The topic is very interesting. We will cite it for reference.

Good work, please correct above points.

Response: Thank you. We will correct the points mentioned in connection with the changes in the article.

Reviewer 2 Report

1) Remove Fig. 3. It duplicates Fig. 2 and only misleads the reader.

2) Move Section 5.2 to Section 4 "Materials and Methods". Also, in Section 4, it is necessary to provide data on the composition of steel and wear-resistant coating.

3) Combine Tables 3-7 into one table.

4) In conclusion, add about the future prospects of your research. For example, that it is planned to search for the optimal composition of a wear-resistant coating, the results of which you will certainly present in the future. 

Author Response

Thank you for your review.

1) Remove Fig. 3. It duplicates Fig. 2 and only misleads the reader.

Response 1: Figure 3 shows the flow of the drilling around the bit rollers, which contains abrasive particles that cause wear of the protective coating. The purpose of this sketch is to show the movement of the drilling fluid, and thus the formation of micro-erosion channels that combine over time to form larger erosion channels. These can be seen in the image. We believe the figure is important to the presentation of the subject and we want to keep it in article.

2) Move Section 5.2 to Section 4 "Materials and Methods". Also, in Section 4, it is necessary to provide data on the composition of steel and wear-resistant coating.

Response 2: Chapter 5.2. (Rock material) and the composition of steel and protective coatings were the subject of research. The results of these materials were obtained through laboratory testing and were therefore included in Chapter 5. Results. These data were not obtained from the manufacturer of the roller cone bit or, in the case of sandstone, from the literature. In fact, a characteristic sandstone sample was taken from a depth of 1482 m below the surface and then analysed in a laboratory for mineral composition to determine the proportion of abrasive constituents.

3) Combine Tables 3-7 into one table.

Response 3: Tables 3-7 have been combined into a single table. Reviewer 4 requests that the error in the measurements from EDS must be presented in Tables 3 - 7. Per your comment and reviewer 4's comment, the combined table takes the following form.

4) In conclusion, add about the future prospects of your research. For example, that it is planned to search for the optimal composition of a wear-resistant coating, the results of which you will certainly present in the future.

Response 4: Thank you very much for your kind idea. We will include this in the Conclusion. Colleagues in the metallurgy department of our faculty are currently investigating alloys that would increase the abrasion resistance of cutting components of TBM machines. We will test their results on roller cone drill bits.

Reviewer 3 Report

Abrasion of roller cone bit is important because wear is extremely aggressive and replacement is time consuming process. In addition, the drills bits themselves are expensive. 

Roller cone drills bits are used for drilling large diameter wells. The efficient work of the roller cone drill is influenced by the materials from which the teeth are made and the rocks are drilled. The rock is crushed at the bottom of the well and removed from the well using water. In order to reduce wear, research has been carried out on abrasion-resistant materials. 

I think we need to go deeper and provide answers to the following questions:

Unclear meaning of the text (lines72-75) 

Fig. 1 The work of the drill bit (forces, loads, motion kinematics / vectors) must be described in detail. It is not clear who spins the rolls. Maybe the soil makes the rollers spin? 

Roller / tooth material not provided (steel standard EN / ASTM / DIN) 

Fig. 2 It is not enough to specify "Load and high temperature area". What temperature and contact load is formed in the contact? The term „High temperature“ is also incomprehensible in the context of a cold water term. 

Fig. 3 What element is shown in the center of the drill? 

What equipment was used to cover the teeth of the drill rolls with carbide coatings? 

What material to cover the teeth of drill rollers? Maybe pure tungsten? What is the logic of that? What is the grain size of the hardening phase? 

What is 610.70 m (line 161). Maybe it's the length of the well? How many times the experiment was repeated? 

How was the stability of the rock properties ensured during this borehole length? 

Table 2. The steel grade must be specified. 

Figure 6. Poorly prepared image. The notes do not match the proportions. Need to increase the size and clearly show it (especially No 3). It is necessary to set metallurgicall names of the transition zone (coating - base). What grain size was used in the welding (coating) material? 

Fig.6. Certificates 3, 4 and 5 - "carbide coating" (cannot be identical). Also, it is not indicated the carbon concentration. 

I categorically disagree that the compositions in Table 3-6 do not contain carbon. How it can be called a Carbide coating table? 

Fig.. 7 and 8. What is the point of providing a Heating curve up to 1500-1600oC, if this is not the case in reality, the well temperature due to active cooling does not exceed 50oC (assumption). Therefore, it is necessary to talk about the local temperature of the tooth-primer  at the contact during work. 

Fig. 6 and 11 must be linked. If FIG. The "mixing zone" is shown in Fig. 11 but in Fig. 6 not. 

The method of diffusion zone analysis is recommended to use the article https://www.scientific.net/KEM.799.3: SEM and EDS mapping of sintered WC-Co particles in Fe based matrix: Z1 - hardmetal particle, Z2 - FeCrMnSi matrix. I - dissolution-reprecipitation zone, II - interdiffusion zone, III - core zone. 

The difference between the abrasion-reducing tungsten carbide coating and the properties (hardness) of the tooth steel materials results in uneven tooth wear. Depending on the tooth coating methods used by the authors (not specified), the hardness of the teeth during coating may be significantly reduced. 

In order to show wear, new and worn roller teeth need to be presented in one figure.

Author Response

Abrasion of roller cone bit is important because wear is extremely aggressive and replacement is time consuming process. In addition, the drills bits themselves are expensive. 

Roller cone drills bits are used for drilling large diameter wells. The efficient work of the roller cone drill is influenced by the materials from which the teeth are made and the rocks are drilled. The rock is crushed at the bottom of the well and removed from the well using water. In order to reduce wear, research has been carried out on abrasion-resistant materials.

Thank you for your review.

I think we need to go deeper and provide answers to the following questions:

Unclear meaning of the text (lines72-75)

Response: In this part we explain that the wear of the cutting part of the tool and the changed geometry of the teeth affect the drilling efficiency. That is, wear changes the geometry of the teeth, which changes the way the tooth acts on the rock. This leads to a reduction in the progress of drilling in the rock. The text is rearranged.

Fig. 1 The work of the drill bit (forces, loads, motion kinematics / vectors) must be described in detail. It is not clear who spins the rolls. Maybe the soil makes the rollers spin?

Response: The mechanism of the movement of the roller cone drill bit is briefly explained in lines 91 to 95 nad Figures 1 and 2. The body of the bit is driven by the rotation of the drill rod. By attaching to the material and overcoming the forces at the bottom of the well, the rollers rotate about their axes. It is not the aim of this article to describe in detail the mechanism of action of the drill bit. The authors of the article have assumed that the description given in point 3 is sufficient, since we did not want to summarize the extensive subject of the operation of the bit in rock. To clarify the principle of operation, we provide two links to articles that discuss this topic:

Performance, simulation and field application modeling of rollercone bits - ScienceDirect

Feasibility study on roller-cone bit wear detection from axial bit vibration - ScienceDirect

We have added these two articles to the references.

Roller / tooth material not provided (steel standard EN / ASTM / DIN)

Response: The manufacturers of the roller cone drill bits does not state in its specifications from which steel standard the bit is made. This information is basically unobtainable as it is one of the secrets of the bit manufacturers. Unfortunately, we are unable to obtain this information. If we had it, we would gladly publish it. At the same time, it would save us a lot of work and research that we had to invest in studying the steel base of the bit's teeth. Normally, manufacturers of bits do not use standardized alloys. The amount of material used represents a critical mass, so steel manufacturers produce a batch with a chemical composition that differs from standard alloys and is specifically suited to the harsh conditions during drilling deep wels. In terms of composition, the base steel 14NiCrMo13-4 (1.6657) EN 10084-2008 comes closest, but the values of certain elements differ.

Fig. 2 It is not enough to specify "Load and high temperature area". What temperature and contact load is formed in the contact? The term „High temperature“ is also incomprehensible in the context of a cold water term.

Response: Figure 2 shows the areas where the temperature increases significantly in the area of the tooth that is in contact with the rock material. In fact, in this case, we could not determine with certainty the temperature at the tip of the teeth in contact with the rock. This is also explained in lines 291 to 303. In investigations in another case where there were microstructural changes in the tooth material, we found that the temperatures at the tip of the teeth in contact with the rock were well above 100°C. In the case of a change in the microstructure of the material in this second study, we estimated that the excess temperature at the tip of the tooth was somewhere in the range of 500°C.

On examination in a dilatometer, in the case described in the article, we have undoubtedly found that there is a significant difference in the thermal expansion coefficient between the tooth steel and the carbide coating. At temperatures above 100° C this is quite obvious. It is this difference between the coefficients that signifies an increase in internal stresses, which manifest themselves in cracks in the contact area between two different materials. Such cracks were found during the inspection.

Fig. 3 What element is shown in the center of the drill?

Response: Each bit roller has a different arrangement of teeth in rows. This is because the teeth or tooth paths do not overlap during drilling, and because the bottom of the hole is drilled evenly over the entire surface. The part of the bit (roller) that is in the center is called the spearpoint.

What equipment was used to cover the teeth of the drill rolls with carbide coatings?

Response: The carbide coating is applied to the tooth surface by the welding technique. The method of welding and the equipment used by the manufacturer to weld the carbide coating onto the teeth of the bit are a trade secret of the manufacturer, giving it a technological advantage over its competitors. Unfortunately, in the years that the authors have been engaged in such work, we have not been able to learn of it, despite several visits to the manufacturing plants. Not because of their own ignorance, but because they did not want to share this knowledge with us.

What material to cover the teeth of drill rollers? Maybe pure tungsten? What is the logic of that? What is the grain size of the hardening phase?

Response: The material applied to the teeth of the bit is tungsten grains in a ferrite base or matrix. The elemental composition is given in Tables 4 to 7. We do not know the grain size in the hardening phase as we do not have access to this type of information, which is a trade secret of the manufacturer. Such details influence the service life of the bit and represent a technological advantage over the competition.

What is 610.70 m (line 161). Maybe it's the length of the well? How many times the experiment was repeated?

Response: 610.7 m is the distance or length of the interval drilled with this bit. The total length of the well was 1482 m. Depending on the rock material drilled through and the diameter of the borehole in a particular interval, different bits were used for different sections. One test was performed with the bit discussed in our article because the bit was in a worn condition at the end of the test. Other bits at this site were not available due to their high price (over $10,000). Since the authors who carried out the drilling part of the research have been involved in deep drilling for many years and worked as rig managers early in their careers, the mechanism studied has been observed many times but has never been studied in the same manner as in this case.

How was the stability of the rock properties ensured during this borehole length?

Response: The stability of a borehole is ensured in various ways. In the part where drilling is going on, the wall of the well is unprotected, and stability is ensured by the pressure produced by drilling fluid (mud) with which the well is filled. In the higher parts, where the well is usually drilled with a larger diameter, it is permaneted with a casing. That was in our case. However, there are other examples of maintaining the stability of the well wall, such as overburden drilling (drilling and casing at the same time). This method is usually used in the construction of shallower boreholes up to a depth of 250 - 300 m.

Table 2. The steel grade must be specified.

Response: As we explained in one of the previous points, we do not know the actual name of the steel. We can guess it from various manuals and tables, but that cite it as standard would not be right in our opinion.

In terms of composition, the base steel is closest to 14NiCrMo13-4 (1.6657) EN 10084-2008, but the values of certain elements differ.

Figure 6. Poorly prepared image. The notes do not match the proportions. Need to increase the size and clearly show it (especially No 3). It is necessary to set metallurgicall names of the transition zone (coating - base). What grain size was used in the welding (coating) material?

Response: Figure 6 shows the points where the EDS analysis was performed on a steel base as well as in the mixing and coating zone. The size of the surface area of the analysis point is matched to the size of each microstructural component. The size of the measurement area of the EDS analyzes cannot be changed afterwards, as they have already been performed. Figure 6 is supplemented by demarcation of areas, and the names of these areas will be added to the description: tooth body - base, mixing zone - transition zone and coating.

Figure 6. Tooth SEM image: 1 – tooth body, 2 – matrix of carbide coating, 3 – carbide coating, 4 – carbide coating, 5 – carbide coating A – coating, B – transition zone (mixing zone), C – substrat (tooth body)

As for the grain size of the welding material, we have already explained this in one of the previous responses.

Fig.6. Certificates 3, 4 and 5 - "carbide coating" (cannot be identical). Also, it is not indicated the carbon concentration.

Response: The microstructure and macrostructure analyzes were performed using SEM Joel JSM 5610, which does not allow measurement of carbon concentration. However, from the experience of colleagues in metallurgy who have worked on similar alloys, they are known to contain carbon. The same sample was also analyzed using the SEM Zeiss CrossBeam 550, which gives us the qualitative and quantitative carbon concentration in the studied sample in the substrate, in the mixed zone and in the catings. We have not mentioned the carbon from the analyzes in the article because the EDS analyzer does not detect it with sufficient sensitivity. EDS analysis is not the most appropriate method for determining carbon concentration. The figure shows the carbon concentration resulting from EDS mapping. The figure shows the elemental concentration of carbon (purple spectrum).

I categorically disagree that the compositions in Table 3-6 do not contain carbon. How it can be called a Carbide coating table?

Response: We understand your indignation and agree with it. Based on your experience and knowledge, you know that carbon is present in such alloys, but we do not list it. Since we do not have the manufacturer's information on the composition of the carbide coating and the EDS method is not the most appropriate method for determining carbon in alloys, we have not included this information. The carbon content for the tooth body is given in Table 2, but this information is not available for the WC. It would be extremely difficult to obtain a quality sample of the carbide coating welded to the tooth body without removing a significant portion of the steel of the tooth body. However, the results of such a sample examined with an optical emission spectrometer would give a similar "poor" result as an examination of EDS.

Fig.. 7 and 8. What is the point of providing a Heating curve up to 1500-1600oC, if this is not the case in reality, the well temperature due to active cooling does not exceed 50oC (assumption). Therefore, it is necessary to talk about the local temperature of the tooth-primer at the contact during work.

Response: The measurement was carried out to such an extent that we were able to identify the stages of steel transformation beyond doubt. On this basis we obtained a guide value signifying the occurrence of the temperature at the tip of the tooth in contact with the rock, if steel transformation takes place. Locally, the temperatures can exceed 500°C, which depends on the position of the cooling medium, the intensity of the flow, the load of the teeth in contact with the rock and the type of rock through which we drill... The temperature generated by the crushing of the rock during contact between the tooth and the rock can be converted very rapidly into the components of the bit (substrate - the body of the tooth) and the drilling fluid. STA analysis was performed to characterize the substrate (tooth steel). Due to problems with the drilling fluid at higher temperatures, it is necessary to intensively add it, as the temperature in the well increases not only due to the influence of drilling, but also due to other local factors (thermal water, geothermal level, local temperature anomalies). In the area of the bit, however, the temperatures are much higher. Even when the bit is removed from the well, it is necessary to handle it with protective equipment that will prevent burns. This problem is particularly acute in the production of shallower wells, when the bit can not cool on its way to the surface.

Fig. 6 and 11 must be linked. If FIG. The "mixing zone" is shown in Fig. 11 but in Fig. 6 not.

Response: Figure 6 shows the locations of the EDS analyzes, while Figure 11 shows the types of damages in coating. Figure 6 is corrected as per your instructions and is shown in response to one of the points above.

The method of diffusion zone analysis is recommended to use the article https://www.scientific.net/KEM.799.3: SEM and EDS mapping of sintered WC-Co particles in Fe based matrix: Z1 - hardmetal particle, Z2 - FeCrMnSi matrix. I - dissolution-reprecipitation zone, II - interdiffusion zone, III - core zone.

Response: Thank you for pointing this out. We have read the article and it is very useful as it deals with the area of application of WC materials by manual or plasma transfer welding on steel surfaces. The article is cited for reference.

The difference between the abrasion-reducing tungsten carbide coating and the properties (hardness) of the tooth steel materials results in uneven tooth wear. Depending on the tooth coating methods used by the authors (not specified), the hardness of the teeth during coating may be significantly reduced.

Response: By Vickers hardness measurements we found that the hardness of the steel did not change over the entire cross-sectional area of the teeth body. The difference in hardness occurs only in the area of attachment of the carbide coating and in the carbide coating where it is significantly higher, and at the base of the tooth it is fairly constant.

In order to show wear, new and worn roller teeth need to be presented in one figure.

Response: We agree with the above. We hope you understand our plight, which we will describe. We do not have photographs of the condition of the bit prior to use. During the negotiation discussions to monitor the drilling operations and subsequent takeover of the bit for research purposes, the drillers have already performed a maneuver to lower the bit into the well. Therefore, we were not able to capture the initial condition with the photohraphy. However, our desire to pull out the bit almost at the end of the lowering maneuver fell on deaf ears because of the high costs involved in such a "non-emergency" situation. During drilling deep wells, any delay in drilling and any time without circulation of the drilling fluid means a financial loss and an unnecessary threat to the stability of the well.

Reviewer 4 Report

The manuscript concerns the characteristics of the teeth of a commercial roller cone bit covered with a protective coating used in the sandstone excavation process, in a worn state. The aim of the work is to identify the causes and mechanisms of tool wear. The undertaken topic fits well with the scope of the subject presented in Coatings, however, the level of research, methodology selection, presentation of results, their analysis and discussion are far from scientific excellence and should not be published in this form. 

The layout of the work is atypical,

Introduction - is a kind of abstract, does not fulfill its role, does not present the state of knowledge and achievements in the selected topic.

 Results :

Table 2 no Fe content in steel composition (!)

Tables 3-7 no error for concentration of elements by EDS ,

No Carbon in carbides (!) probably as the effect of EDS analysis software setting in which Carbon content is automatically excluded in the in quantitative measurements.

Comment: to show differences in Carbon content, please use EDS spectra for marked areas or EDS elemental distribution linescans or maps.

Lines 203-204-, what about the phase composition ( XRD ) and microstructure (LM or SEM images) of steel ?

Fig 12, The magnified image inserted in the overview requires an appropriate description in the drawing caption. It should not be deformed unidirectionally (flattened), but reduced proportionally in both axes, then only the scale bar will fulfill its role.  

Figs.13 and 14- add EDS maps or spectra to identify microstructure constituents

What was the load used in Vickers hardness measurement?

The discussion of the results is based on the knowledge of the Authors. No literature sources were cited to interpret the obtained results.

 Conclusions - it is rather a summary of the results along with the Author's comment. This section contains phrases that are not supported by results, e.g. lines 351-352 and some generalities, e.g.  lines 363-366.

Author Response

Comments and Suggestions for Authors

The manuscript concerns the characteristics of the teeth of a commercial roller cone bit covered with a protective coating used in the sandstone excavation process, in a worn state. The aim of the work is to identify the causes and mechanisms of tool wear. The undertaken topic fits well with the scope of the subject presented in Coatings, however, the level of research, methodology selection, presentation of results, their analysis and discussion are far from scientific excellence and should not be published in this form.

Thank you for your comments and suggestions.

The layout of the work is atypical,

Response: The layout and structure of the article are based on the journal's template.

Introduction - is a kind of abstract, does not fulfill its role, does not present the state of knowledge and achievements in the selected topic.

Response: In our opinion, the introduction is intended to introduce the subject of our investigation. The state of knowledge and achievements in the chosen subject are presented in Chapters 2 and 3. Chapters 2 and 3 will be moved to Chapter 1 Introduction.

Results:

Table 2 no Fe content in steel composition (!)

Response: Table 2 is completed with a value of Fe - 96.5.

Tables 3-7 no error for concentration of elements by EDS ,

Response: Reviewer 2's comment is to combine Tables 3 - 7 into one table. According to your comment and reviewer 2's comment, the combined table has the following form.

No Carbon in carbides (!) probably as the effect of EDS analysis software setting in which Carbon content is automatically excluded in the in quantitative measurements. 

Comment: to show differences in Carbon content, please use EDS spectra for marked areas or EDS elemental distribution linescans or maps.

Response: Your conclusion is correct. Why there is no carbon content is explained in the text under Table 7. The microstructure and macrostructure analyzes were performed using SEM Joel JSM 5610, which does not allow measurement of carbon concentration. However, from the experience of colleagues in metallurgy who have worked on similar alloys, they are known to contain carbon. The same sample was also analyzed using the SEM Zeiss CrossBeam 550, which gives us the qualitative and quantitative carbon concentration in the studied sample in the substrate, in the mixed zone and in the catings. We have not mentioned the carbon from the analyzes in the article because the EDS analyzer does not detect it with sufficient sensitivity. EDS analysis is not the most appropriate method for determining carbon concentration. The figure shows the carbon concentration resulting from EDS mapping. The figure shows the elemental concentration of carbon (purple spectrum).

Lines 203-204-, what about the phase composition (XRD) and microstructure (LM or SEM images) of steel?

Response: SEM of the complete tooth structure and carbide coating is shown in Figure 6.

It is a composite product that has functionally distributed properties. It consists of several zones, namely coating, transition zone and substrate (tooth body). For XRD analysis, material should be obtained from each zone and then analyzed. In this case, it is practically very difficult because the layers are quite thin. Hence the choice of scanning electron microscope (SEM) examination.

Fig 12, The magnified image inserted in the overview requires an appropriate description in the drawing caption. It should not be deformed unidirectionally (flattened), but reduced proportionally in both axes, then only the scale bar will fulfill its role.

Response: Figure 12 is corrected accordingly.

Figure 12. Micro cracks and erosion channels.

Figs.13 and 14- add EDS maps or spectra to identify microstructure constituents

Response: Figures 13 and 14 show characteristic damage and no EDS measurement was carried out as the composition of the carbide coating is known from Figure 6 and Table 3 - 7.

What was the load used in Vickers hardness measurement?

Response: The load was 100 g. The load is added in the Materials and Methods section.

The discussion of the results is based on the knowledge of the Authors. No literature sources were cited to interpret the obtained results.

Response: The discussion is based on the authors' knowledge and experience in the use of roller cone drill bits in deep hole drilling and metallurgical knowledge in the manufacture of complex composite products (roller cone bits) with functionally distributed properties. No other literature sources are cited, as such research on roller cone drill bits has been relatively scarce and is poorly represented in technological and scientific practice. Numerous studies have been conducted on cutting tools whose composition and functioning differ significantly from the composition and functioning of roller cone bits, so we cannot easily draw parallels. At this point, we must admit that our profession lags far behind others or that cooperation between professions (drillers, mechanical engineers, metallurgists...) is poor and knowledge and experience are inadequately shared. Therefore, we hope that articles like this will break the ice in this field.

Conclusions - it is rather a summary of the results along with the Author's comment. This section contains phrases that are not supported by results, e.g. lines 351-352 and some generalities, e.g.  lines 363-366.

Response: We believe that the statement in lines 351-352 is supported by the results. By determining the tensile properties of the materials and by knowing exactly the mechanism of action of the roller cone bit and the flow of the drilling fluid, we can say with certainty that the carbide coating disintegrates because of its properties, the stresses, and the formation of new surfaces due to steel wear. The carbide coating resists transverse loads very poorly. We could easily determine this by breaking through a thin layer of the carbide coating with our fingers. This is not exactly a scientific method, but it does provide information about the material. By examining the rock, we have undoubtedly determined the composition of the rock material, which has a silicate content of over 47%, and the grain size of the debris, which is up to 3 mm. These particles, mixed with a fast-flowing drilling fluid that flows around the components of the bit, undoubtedly cause erosion of the material, even in the case of such erosion-resistant materials as the carbide coating. The erosion channels were detected using an electron microscope and are shown in Figures 13 and 14. Similar effects can be observed in nature - erosion along riverbeds or in the transport of silicate-containing suspensions in pipes. The study of roller cone bit wear that has been carried out is much more extensive and contains over 150 pages. Unfortunately, the length of the article is limited, and we have had to concentrate on the essentials that will interest the readers of the journal. We agree with you that many more topics need to be included in the article to present the mechanism of wear in more detail and how the protective carbide coating helps to reduce wear, but as written before, the length of the article is somehow limited by the concentration of the reader. The statements you cite as general (lines 363 - 366) may indeed be somewhat general. However, based on research and knowledge of metallurgy in relation to the formation of alloys, they indicate the direction in which the development of materials for roller cone drill bits should go. Currently, the situation in the industry for the manufacture and use of roller cone drill bits is that wear-resistant materials are joined to the tools mechanically or by welding. We would like professionals to start thinking and researching about new materials that do not depend so much on the way they are joined to the steel base. We will start developing such composite products (drill bits). Initially on a laboratory basis and then try to pass the results on to manufacturers if there is interest.

Round 2

Reviewer 1 Report

Paper is Ok

Author Response

Thank you. Kind regards.

Reviewer 3 Report

It is necessary to explain:

What equipment was used to cover the teeth of the drill rolls with carbide coatings?

What material to cover the teeth of drill rollers? Maybe pure tungsten? What is the logic of that? What is the grain size of the hardening phase?

Author Response

Thank you for your comments and suggestions.

It is necessary to explain:

What equipment was used to cover the teeth of the drill rolls with carbide coatings?

What material to cover the teeth of drill rollers? Maybe pure tungsten? What is the logic of that? What is the grain size of the hardening phase?

Response: As previously reported, to protect its technical knowledge and development, the manufacturer of the roller cone drill has not disclosed what welding technique it used to apply the carbide coating. However, we assume, as you have indicated, that the application of the carbide coating was accomplished by the plasma welding process. Since we do not know this for sure, we did not include it in the article. The added carbide material has a complex composition, as can be seen from the figures. As you can see from the figures, the material used consisted of the following microstructures:

• Prefabricated spherical WC balls, sizes 100 to 700 µm, bonded with cobalt binder
• Polycrystalline WC sizes from 10 µm to 100 µm
• Binder bonding WC spheres and WC polycrystals in Co and Fe-based (matrix) and also containing nanoparticles of WC in sizes from 0.06 µm to 0.25 µm

We can add the grain size in article.

Reviewer 4 Report

The manuscript still contains many imperfections that need to be addressed. The work requires significant corrections both in the way of presenting and interpreting the results, as well as in their discussion and therefore the work is still not suitable for publication in Coatings.

Questions and recommendations for Authors:

Provide the name of the tool manufacturer and give the grade of steel  according to Safety Data Sheets - each manufacturer is obliged to define this data for his product.

XRF analysis is given for the rock, why it is not done for the tool? It is given in the research methodology of the tools, but there are no results from this analysis.  

What was the phase composition of the steel and the coating. ( XRD analysis is recommended).

I completely disagree with the opinion of the authors presented in lines 266-269. EDS detects the presence of carbon. The quantitative analysis of this element may be burdened with a large error due to contamination of the microscope column, as well as the use of carbon tapes and deposition of thin carbon layer on the sample’s surface. Hence, in EDS software, carbon can be excluded from quantitative analysis. To show the differences in the chemical composition of selected areas of materials containing carbon, EDS spectra or maps are used. So, do not report quantitative analysis based on EDS - remove tables 2-7 with the calculated content of elements, since they do not give the full composition anyway- it makes no sense - carbon is in steel and carbon is the main component of tungsten carbide coating.

Replace the tables by EDS spectra for selected areas / points of EDS analysis as was recommended in the previous review - both for steel and for the coating- - what you didn't do, yet.

What was the purpose of the DSC analysis  and the conclusions of this analysis? Transition temperatures in steels are well enough described by Fe-Fe3C diagram. Figure 7 is not DSC plot registered for the coating itself, but for coated steel. Interpretation given by Authors for this plot is incomplete and incorrect. No discussion was given for the temperature range 1172-1570C, except 1286C. What was the basis for interpretation of the temperatures 1286 and 1600C as WC coating melting start and end, respectively (lines 287-289), bearing in mind that the melting point of WC is 2870C?

lines 391-393, figures 11 -14, please indicate regions of siliceous microparticles penetrating into the microstructure of the tool, as You claim,  by  adding EDS maps to show that regions.

Images of microstructures are still poorly described, for example Fig5. What is presented in the figure first on the right- the resign? 

There is an error of the name of the manufacturer of the “Jeol” microscope

Author Response

The manuscript still contains many imperfections that need to be addressed. The work requires significant corrections both in the way of presenting and interpreting the results, as well as in their discussion and therefore the work is still not suitable for publication in Coatings.

Thank you for your comments and suggestions.

Questions and recommendations for Authors:

Provide the name of the tool manufacturer and give the grade of steel according to Safety Data Sheets - each manufacturer is obliged to define this data for his product.

Response: For your information, the bit manufacturer is Smith (Schlumberger). We do not want to add the name of the manufacturer because we do not want the article to become commercial. We hope that you will understand this. The number 117 is an IADC code that characterizes some properties of the bit (first number - bit type, second number - rock type and third number - bearing type). Manufacturers of roller cone drills do not specify in their specifications (Safety Data Sheet), from which steel standard the bit is made, because it is a tool (similar to the fact that in private life you do not get a material specification for drills to drill short, small holes with hand tools). This information is basically unobtainable as it is one of the secrets of the bit manufacturers. If we had it, we would gladly publish it. At the same time, it would save us a lot of work and research that we had to invest in studying the steel base of the bit's teeth. Normally, manufacturers of bits do not use standardized alloys. The amount of material used represents a critical mass, so steel manufacturers produce a batch with a chemical composition that differs from standard alloys and is specifically suited to the harsh conditions during drilling deep wells. In terms of composition, the base steel 14NiCrMo13-4 (1.6657) EN 10084-2008 comes closest, but the values of certain elements differ.

XRF analysis is given for the rock, why it is not done for the tool? It is given in the research methodology of the tools, but there are no results from this analysis.

Response: An XRF analysis was performed for the tool, namely for the steel of the tooth base and for the carbide coating. The tooth base steel sample was also examined with ICP, which enables better quality results than XRF. In determining the composition of the carbide coating, however, the sample obtained was not of sufficient quality because the carbide coating on the tooth was slightly more than 2 mm thick, and consequently we did not publish the results of XRF analyses for the carbide coating. The carbide coating is a composite product that has functionally distributed properties. Instead, we performed an EDS analysis that allows investigation of individual phases (microstructural components).

What was the phase composition of the steel and the coating. (XRD analysis is recommended).

Response: For XRD analysis, material should be obtained from each zone and then analysed. In this case, it is practically very difficult because the layers are quite thin. A sample of carbide coating, the layer of which is just over 2 mm thick and welded to the tooth base, would be very difficult to obtain without this sample being contaminated with a large portion from iron of base steel, which means that the results would not be satisfactory. Since we did not obtain a quality sample of carbide coating for performing XRD analysis, in our opinion, the performance of XRD on tooth steel was also irrelevant. Hence the choice of scanning electron microscope (SEM) examination.

The figure shows the microstructure of two-phase bainite. Vickers hardness was measured on the sample. The average hardness value was 328 HV, which corresponds to a macro hardness of 33 HRC.

I completely disagree with the opinion of the authors presented in lines 266-269. EDS detects the presence of carbon. The quantitative analysis of this element may be burdened with a large error due to contamination of the microscope column, as well as the use of carbon tapes and deposition of thin carbon layer on the sample’s surface. Hence, in EDS software, carbon can be excluded from quantitative analysis. To show the differences in the chemical composition of selected areas of materials containing carbon, EDS spectra or maps are used. So, do not report quantitative analysis based on EDS - remove tables 2-7 with the calculated content of elements, since they do not give the full composition anyway- it makes no sense - carbon is in steel and carbon is the main component of tungsten carbide coating.

Response: We agree with the statement. EDS gives us a comparative value for carbon. However, as you yourself have noted, the measured value is subject to a large error. We did not cite it in the article because it is a known fact in science. However, in scientific and technological practice, where carbon concentrations are quoted, but interpreted qualitatively (a comparable difference in carbon concentration between different microstructural components). In the article, we list under Table 3: Carbon concentration for each microstructural component is not listed in Table 3 because quantitative values for carbon obtained by analysis of EDS are not quantitatively accurate.

Replace the tables by EDS spectra for selected areas / points of EDS analysis as was recommended in the previous review - both for steel and for the coating- - what you didn't do, yet.

Response: The spectrum has no quantitative definition for a single element, because it shows the intensity as a function of wavelength for each element. For your information, we provide images and tables of the spectrum in response.

What was the purpose of the DSC analysis  and the conclusions of this analysis? Transition temperatures in steels are well enough described by Fe-Fe3C diagram. Figure 7 is not DSC plot registered for the coating itself, but for coated steel. Interpretation given by Authors for this plot is incomplete and incorrect. No discussion was given for the temperature range 1172-1570C, except 1286C. What was the basis for interpretation of the temperatures 1286 and 1600C as WC coating melting start and end, respectively (lines 287-289), bearing in mind that the melting point of WC is 2870C?

Response: Fe-Fe3C diagram: In practice, the temperatures and equilibrium phases are also influenced by additional alloying elements and impurities that are not included in the phase diagram above. If you want to consider the influence of all elements, you should do a calculation e.g. with the software package Thermo Calc. Therefore, the measured reference temperatures from the melting range of the investigated samples are important, because they contain the influences of other elements and do not coincide with those in the Fe-Fe3C diagram.

In Figure 8, it can be seen that the high temperature peaks exceeding 1289.6 °C are related to the melting of the binder components in the composite carbide coating. Basically, the particles of WC remained solid after melting above 1600 °C. We will add this information in text.

In the text we have added additional explanations to the commentary on figure 7. To wit: At this temperature the matrix of the steel has the crystal structure of austenite. And: The low temperature eutectics begins to melt at 1352.1 °C and up to 1483.5 °C. Then the primary crystals of austenite begin to melt, and the melting is completed at 1524.5 °C.

lines 391-393, figures 11 -14, please indicate regions of siliceous microparticles penetrating into the microstructure of the tool, as You claim, by adding EDS maps to show that regions.

Response: The above pictures clearly show the damage caused by the penetration of silicate particles in the drilling fluid into shallow cracks and channels, causing an erosional effect. These silicate particles are now no longer present in the samples as they were washed out during sample preparation for analysis. We assume that you know how to prepare samples for examination with an electron microscope, because you described this in part in one of the comments. The specimen is embedded in resin and then polished and chemically treated. As a result, it is not possible to observe free silicate particles in cracks and erosion channels with a depth of a few µm, because they are simply no longer present on the observed surfaces. It is possible that there is another particle somewhere below the surface that was not treated during sample preparation. However, this is below the surface of the investigation and we should be really lucky to detect it with EDS because we would have to locate the exact erosion channel that is below the surface that we do not see, that is within the range of the wave and that contains the silicate particle. You probably agree that this would be very fortunate.

In any case, the silicate particles in the drilling fluid treated the surface of the bit and teeth while the drilling fluid washed around them, as has undoubtedly been established. The authors of the article, who have been involved in the technology of deep well drilling with roller cone or other types of bits for many years, know the influence of the flow of drilling fluid and the erosion of the rock particles on the components of the bits. Technical logic and the experience of engineers lead to predictions and findings in cases where theory does not exactly match practice. For this reason, the principle is that theory is translated into practice and then practice into theory. Only in this way can we make progress. As a top engineer and expert, which can be seen from your comments, we think you understand that it is not possible to describe everything in a short article.

Images of microstructures are still poorly described, for example Fig5. What is presented in the figure first on the right- the resign? 

Response: Figure 5 has been corrected and a comment has been added to explain what is in Figure 5.

Figure 5. Cross section of a worn tooth with a protective carbide coating.

Description: Figure 5 shows a cross-section through one of the teeth and the position of the carbide coating. Figure 5 combines three images of the tooth sample examined, namely a and b macroscopically and c microscopically. Images a and b show a change in wear-related geometry, and image c shows that the larger complex WC particles are more wear resistant than the smaller nanoparticles in the coating matrix.

There is an error of the name of the manufacturer of the “Jeol” microscope

Response: Thank you for notice that. It is typing mistake. We apologise for that. We will correct this.