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Fatigue Life Improving of Drill Rod by Inclusion Control

  • Linzhu Wang , Shufeng Yang EMAIL logo , Jingshe Li , Wei Liu and Yinghao Zhou
Published/Copyright: September 15, 2015
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High Temperature Materials and Processes
From the journal High Temperature Materials and Processes Volume 35 Issue 7

Abstract

Large and hard inclusions often deteriorate the service performance and reduce the fatigue lifetime of drill rods. In this paper, the main reasons of the rupture of drill rods were analyzed by the examination of their fracture and it is found that the large inclusions were the main reason of breakage of rod drill. The inclusions were high of Ca content or Al2O3 rich. Smaller and better deformability inclusions were obtained by the optimization of refining slag, calcium treatment process and the flow control devices of tundish. Results of industrial experiment after optimization show that total oxygen content of drill rods decreased by more than 50%, macro-inclusions weight fraction decreased from about 4 mg/10 kg to about 0.3 mg/10 kg and the micro-inclusions average size decreased from 6 to 3.6 μm. The average using times of drill rods after optimization were increased by about 60%.

Keywords: drill rod; inclusion control; fatigue life; fracture

Introduction

Drill rod is consumptive tool made from hollow steel. It has been extensively used in quarries, open pit mines and construction sites. The fatigue life of drill rods is as short as 10 min to dozens of hours [1] because of its bad service condition in which it has to bear high-frequency impact, severe abrasion of rock and ore, stress of tension, compression, bending and torsion. With the power consumption of rock drill increasing, the requirement of drill rod is higher [2, 3]. Improving the quality and extending the service life of drill rods are urgent and arduous tasks.

As we all know, the nonmetallic inclusion is one of the factors affecting the performance of steel [4]. The properties of inclusions including size, morphology, distribution, melting point, hardness, plasticity and expansion coefficient have a significant influence on the mechanical property and fatigue life of steel [5–7]. Lots of researches indicate that crack initiated and propagated in the part of steel where there were inclusions or defects under the cyclic stress [6, 8–10]. Kiessling et al. [11] proposed that there was a relationship between the size of inclusions and fatigue limit. Larsson et al. [12] had an investigation on the spring steel and proposed that the inclusions with 10 μm size or smaller would not cause fatigue fracture. Inclusions with sharp corner appearance deformate in rolling process such as Al2O3. Malkiewicz [13] found that tapered clearance and crack are easily caused by the inclusions without deformability at refining temperature. It is found that the most effective measures of controlling inclusion in industrial production include slag refining [14–16], calcium treatment [17–19] and tundish metallurgy [20]. However, most of the present researches for drill rods are focusing on the stress and strain state analysis, heat and hot rolling process and pumping brazing [21, 22]. Therefore, it has a great significance to improve the service lifetime of drill rod by the inclusion controlling technology.

In this particle, industrial experiment was conducted in Shougang Guiyang Special Steel Company that has a large production of drill rods and its drill rod output accounts for 65% of the whole production in China. The main factors causing the breakage of drill rods were analyzed and corresponding production processes including refining slag, calcium treatment and the flow control devices of tundish were optimized to improve the fatigue life of drill rods.

Experimental process

Table 1 lists chemical composition of 95CrMo steel used for producing rod drill. Industrial process route for 95CrMo is electric arc furnace (EAF) steelmaking–ladle refining (LF)–vacuum degassing (VD) refining–tundish metallurgy–continuous casting–hot rolling–piercing. The sampling steps are shown in Table 2. Alloys and slag were added during tap. Sample 2 was taken after the first time steel composition adjusted during LF refining. The LF lasts for 50 min and the time of VD refining was about 23 min. A CaSi wire of 300 m was added in traditional production after VD refining. Steel samples during production process were taken with barrel-type samplers. Sample 6 was taken when the soft blowing lasts for 5 min after calcium treatment. Billets and finished products were also collected.

Table 1:

Chemical composition of 95CrMo (mass %).

ElementCSiMnCrMoNiPSCu
Content0.90–1.000.15–0.400.15–0.400.80–1.200.15–0.30–≤0.025≤0.025≤0.25
Table 2:

Sampling steps of 95CrMo drill rod.

No.TimeType
Sample 1After EAF steelmaking tapping (suppressed as 0 min)Steel
Sample 2Middle of LF refining (at about 15 min)Steel and slag
Sample 3End of LF refining (at about 35 min)Steel and slag
Sample 4Before VD refining (at about 40 min)Steel and slag
Sample 5At the end of VD refining (at about 63 min)Steel
Sample 6After soft blowing (at about 70 min)Steel

The content of total oxygen was measured by the infrared absorption method. The inclusions in metallographic samples were observed and analyzed by a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS) and automated particle explorer (ASPEX). The cylinder (Ф80 × 100 mm) was electrolyzed for the analysis of macro-inclusions.

Results and discussion

Examination of broken drill rod

Drill rods broken during drilling process were collected to be cut and grinded roughly by 600 # abrasive paper. The broken drill rods were selected to be observed in SEM-EDS in which inner cracks were obvious and Figure 1 shows the surface morphology of fracture. About 40 exposed inclusions in cracks were observed as shown in Figure 2 and it indicates that these inclusions were the main reason causing the breakage of drill rod. It is found that the inclusions in the fracture were very large which were almost more than 10 μm (the inclusions larger than 10 μm called “large inclusions” in this paper) and the largest inclusions were about 100 μm. Most failures were caused by these large inclusions, which was also proposed in the study of spring steel [12]. The composition results of inclusions show that high Ca content (CaO% + CaS% ≥ 70%) inclusions in the fracture account for 42%. Al2O3-rich inclusions of which the sizes are about or more than 10 μm account for 23%. These hard inclusions would be stress raiser and cause brittle fracture. It indicates that the composition control and the elimination of inclusions were poor during production process of drill rods. It has a great significance to study the effect of refining slag, calcium treatment and structure of flow control device in the tundish on the control of inclusions.

Figure 1: Fracture morphology.
Figure 1:

Fracture morphology.

Figure 2: Inclusions in fracture of drill rods.
Figure 2:

Inclusions in fracture of drill rods.

Influence of refining slag on steel cleanliness

The chemical compositions variation of conventional refining slag and experimental refining slag during refining process are given in Table 3. There are mainly two noticeable differences between conventional and experimental slags, namely basicity and the change of Al2O3 content. As shown in Figure 3, deoxidization capacity of experimental refining slag was stronger and the total oxygen decreased by 25–29% after optimization at the end of LF. Twenty inclusions were observed and the size of inclusions in the steels with experimental slag decreased from about 4 to 2 μm. It indicates that high basicity slag is more effective in improving the cleanliness of 95CrMo steel.

Table 3:

Chemical composition of refining slag.

Heat no.Sample no.SiO2, %Al2O3, %CaO, %MgO, %Others, %RC/A
Heat 1Sample 223.4511.1153.675.566.212.294.83
Heat 1Sample 323.4811.0550.498.806.182.154.57
Heat 1Sample 423.607.0857.636.095.602.448.14
Heat 2Sample 217.837.8166.114.064.193.718.46
Heat 2Sample 317.748.3064.764.584.623.657.80
Heat 2Sample 417.6110.7961.125.165.323.475.66
Figure 3: Influence of refining slag on cleanliness variation during refining process: (a) total oxygen and (b) inclusion size.
Figure 3:

Influence of refining slag on cleanliness variation during refining process: (a) total oxygen and (b) inclusion size.

In order to analyze the effect of refining slag on the inclusion composition, the liquid area of CaO-Al2O3-MgO-SiO2 slag system and the melting point of composite inclusions are calculated by FactSage 6.4 which are shown in Figure 4 with the experimental compositions and their morphologies. It is shown in Figure 4 that the liquidus temperature of conventional refining slag is in the range of 1,700–1,900℃ which is lower than experimental refining slag. It indicates that rapid slag melting caused an increase of CaO in inclusions of which size was large. Al2O3 content decreased in the inclusions by using experimental refining slag and Al2O3 content kept increasing in the experimental slag which demonstrates that it improved the absorption ability of this slag for Al2O3. The increase of MgO in the inclusions as shown in Figure 4(b) indicates that high basicity slag has a positive effect on the formation of MgO in the steel. It is found that the size of the inclusions significantly decreases with increasing ratio of MgO to Al2O3 as shown in Figure 5, which was also proposed by Park [23]. It indicates that the harm of inclusions to destroy the performance of drill rod was reduced by optimizing the refining slag to modify the composition of inclusions.

Figure 4: Phase diagram of MgO–CaO–Al2O3–SiO2 slag with experimental compositions: (a) conventional slag and (b) experimental slag.
Figure 4:

Phase diagram of MgO–CaO–Al2O3–SiO2 slag with experimental compositions: (a) conventional slag and (b) experimental slag.

Figure 5: Relationship of inclusion size and its composition.
Figure 5:

Relationship of inclusion size and its composition.

Influence of calcium treatment on inclusions

CaSi wires of 0, 180, 300 m were fed, respectively, after VD refining. The composition and size change of inclusions in sample 5 (before calcium treatment) and sample 6 (after soft bellowing for 6 min) are shown in Figures 6 and 7.

As shown in Figure 6, the effect of calcium wire length on the composition of inclusions shows that the content of calcium compounds increased obviously after CaSi wire feeding and it decreased in the steel without Ca feeding. CaS content was high in the inclusions with 300 m wire feeding. The size of inclusions decreased with 0 and 180 m CaSi wire as shown in Figure 7. The growth of inclusions in the steel with 300 m CaSi wire feeding was the highest.

Figure 6: Effect of CaSi wire length on composition of inclusions.
Figure 6:

Effect of CaSi wire length on composition of inclusions.

Figure 7: Effect of CaSi wire length on the size of inclusions.
Figure 7:

Effect of CaSi wire length on the size of inclusions.

The research works on inclusions composition, and size distribution in billets was done to analyze the effect of calcium treatment, as shown in Figures 8 and 9. Traditional Al2O3–CaO–CaS ternary plot at 1,873 K was drawn by FactSage 6.4, and the size and composition of inclusions are shown in Figure 9. The size of inclusions left in billets increased with longer CaSi wire, and average size of inclusions in the billet with 300 m CaSi wire feeding was largest. The average diameter of inclusions in the billets without calcium treatment was only 3.3 μm.

Figure 8: Inclusions size distribution in 95CrMo billets.
Figure 8:

Inclusions size distribution in 95CrMo billets.

Figure 9: Inclusions distribution in billets and Al2O3–CaO–CaS ternary plot at 1,873 K.
Figure 9:

Inclusions distribution in billets and Al2O3–CaO–CaS ternary plot at 1,873 K.

There were mainly Al2O3–CaO–CaS inclusions in the billets and the content of CaS in inclusions increased with longer CaSi wire feeding as shown in Figure 9. It is found that there were large solid inclusions which were more than 10 μm and some inclusions covered with CaS in the billets with 300 m CaSi wire feeding. The inclusions in the billet without CaSi wire feeding were found to have good deformation capacity or to be small solid inclusions. Large inclusions were liquid inclusions, or the inclusions with liquid core at 1,873 K such as calcium aluminates covered with CaS and CaO · 2Al2O3 have good deformation capacity. Others were not more than 5 μm in spite of high melting inclusions which are harmless to the performance of drill rod. It indicates that calcium treatment is not suitable for the production of 95CrMo drill rods.

Fluid flow in the tundish

The water model system of four-strand caster tundish was built in the laboratory to study the effect of impact zone structure on the tundish metallurgy. A 1:4 model was established and it was necessary to satisfy the dynamic similarity and geometrical similarity. The residence time distribution (RTD) curve that reflects the similarity of flow characteristics of each strand in multi-strand was obtained by stimulus–response technique [24]. The original porous baffle and the new designed porous baffle in tundish are shown in Figures 10 and 11. Flat porous baffle was applied in original tundish and the angle between the porous baffle and bottom of tundish was 82°. Higher rectangular hole and lower trapezoidal hole were used as deflector hole. The optimized porous baffle was designed as shown in Figure 11. There was, respectively, a hole on the porous baffle and the holes inclined upward.

Figure 10: Original tundish: (a) tundish structure and (b) porous baffle structure.
Figure 10:

Original tundish: (a) tundish structure and (b) porous baffle structure.

Figure 11: Optimized tundish: (a) tundish structure and (b) porous baffle structure.
Figure 11:

Optimized tundish: (a) tundish structure and (b) porous baffle structure.

As shown in Figure 12, the peak concentration for the tundish with original tundish was high. There was a short circuit flow and the response time was short as shown in Figure 12 and it was not beneficial to the floatation of inclusions. The simulating study showed that a large difference in flow characteristics exited in original tundish which was same with the result of macro-inclusions above. The changing trend of concentration for four strands in the tundish with optimized porous baffle was the same, which indicates that each flow was in good consistency. The RTD curves in Figure 13 were smooth and the blending effect was improved.

Besides, when compared with the original tundish structure, the optimized impact zone was larger which suppressed the surface fluctuation and slag entrapment. The upward angle of deflector hole was beneficial to extend the residence time of steel and it has better effect on floatation and removal of inclusions.

Figure 12: RTD curves in tundish with original porous baffle.
Figure 12:

RTD curves in tundish with original porous baffle.

Figure 13: RTD curves in tundish with new designed porous baffle.
Figure 13:

RTD curves in tundish with new designed porous baffle.

Optimized industrial experimental results

The optimized industrial experiment was conducted with experimental refining slag, no calcium treatment and new designed tundish. The cleanliness of 95CrMo drill rod before and after optimization is compared and the results are shown in Table 4. About 300 kN high-frequency fatigue testing machine was adopted for the simulation of drill rod fatigue life. Rock drill, drill rod and rock were fixed on the test stand and rock drill was pushed by the moving cylinder. The powder was removed by high pressure water. The average stress was 10 kN and alternating stress was 94 kN.

Table 4:

Cleanliness of rod drill before and after optimization.

ProductionT.O./ppmMicro-inclusion Average size/μmMacro-inclusion Weight (mg/10 kg steel)w(MgO)/w(Al2O3) ratio in inclusionArea fraction of micro-inclusion (ppm)
High CaHigh CaS
Tradition12–246.14–170.192.772.47
Optimization6–93.60.12–0.50.251.320.02

The cleanliness of 95CrMo was improved obviously after optimization. Total oxygen content decreased by 50–62.5% and macro-inclusions weight decreased from about 4 mg/10 kg to about 0.3 mg/10 kg. Fine inclusions were obtained by controlling their composition. It is found when the ratio of MgO to Al2O3 and CaO content in inclusions increased, the micro-inclusions average size decreased from 6.1 to 3.6 μm and their deformability was improved.

A few drill rods were chosen randomly for the simulation of their fatigue life and the results were shown in Figure 14. As it is seen, the drill rods produced by original process were easy to break up and their fatigue life was unstable. The average using times of drill rods after optimization increased by about 60% which reached about 85–88 × 104 times and their fatigue life was more stable than traditional rod drills. It indicates that the optimization of refining slag, CaSi wire feeding and tundish structure has a good effect on the fatigue life of drill rods.

Figure 14: Using times of drill rods produced before and after optimization.
Figure 14:

Using times of drill rods produced before and after optimization.

Conclusion

  1. The examination of drill rod fracture shows that the large inclusions of which size is almost more than 10 μm were the main reason causing breakage of drill rod and these inclusions were of high Ca content or Al2O3 rich.

  2. Industrial experimental results indicate that increasing the basicity of refining slag has a positive effect on improving the cleanliness of 95CrMo drill rod, calcium treatment is not suitable for obtaining good deformation capacity and small inclusions of 95CrMo drill rod, and optimized tundish has better effect on floatation and removal of inclusions.

  3. The cleanliness of 95CrMo was improved obviously after optimization of whole production processes. Total oxygen content decreased by more than 50%, macro-inclusions weight decreased from about 4 mg/10 kg to about 0.3 mg/10 kg, the micro-inclusions average size decreased from 6 to 3.6 μm and their deformability was improved. The average using times of drill rods after optimization increased by about 60% and their fatigue life was more stable than traditional drill rods.

Funding statement: Funding: The authors would like to acknowledge the funding of the National Natural Science Foundation of China (grant nos 51474076 and 51474085) and the Fundamental Research Funds for the Central Universities (grant no. FRF-TP-14-114A2).

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Received: 2015-2-23
Accepted: 2015-7-4
Published Online: 2015-9-15
Published in Print: 2016-8-1

©2017 by De Gruyter

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Articles in the same Issue

  1. Frontmatter
  2. Research Articles
  3. A New Method to Produce Ni–Cr Ferroalloy Used for Stainless Steel Production
  4. Mechanical and Electrochemical Characterization of Super-Solidus Sintered Austenitic Stainless Steel (316L)
  5. Effect of γ→α Phase Transformation on Refining Austenite Grains of Microalloyed Steel in Continuous Casting by Simulation
  6. Fatigue Life Improving of Drill Rod by Inclusion Control
  7. Influence of Basicity and MgO on Fluidity and Desulfurization Ability of High Aluminum Slag
  8. Effect of Sputtered AlY Coating on High-Temperature Oxidation Behavior of Stainless Steel
  9. Optimal Design of Nozzle for Supersonic Atmosphere Plasma Spraying
  10. Oxidation Behaviors of Inconel 740H in Air and Dynamic Steam
  11. Line-Profile Analysis Combined with Texture Analysis for Characterizing Dislocation Distribution in Texture Components of Cold-Rolled Copper Sheets
  12. Microstructure Analysis of HPb59-1 Brass Induced by High Current Pulsed Electron Beam
  13. Thermal Treatment Method for Synthesis and Characterization of the Octahedral Magnetic Nanostructures of Co3O4 from a New Precursor
  14. Phases Transition and Consolidation Mechanism of High Chromium Vanadium-Titanium Magnetite Pellet by Oxidation Process
  15. Effect of Fiber Laser Treating on Magnetic Domains in the Grain-Oriented Silicon Steel: Imaging Domains by Bitter, MFM and Kerr Microscopy
https://doi.org/10.1515/htmp-2015-0044
Keywords for this article
drill rod; inclusion control; fatigue life; fracture
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Frontmatter
  2. Research Articles
  3. A New Method to Produce Ni–Cr Ferroalloy Used for Stainless Steel Production
  4. Mechanical and Electrochemical Characterization of Super-Solidus Sintered Austenitic Stainless Steel (316L)
  5. Effect of γ→α Phase Transformation on Refining Austenite Grains of Microalloyed Steel in Continuous Casting by Simulation
  6. Fatigue Life Improving of Drill Rod by Inclusion Control
  7. Influence of Basicity and MgO on Fluidity and Desulfurization Ability of High Aluminum Slag
  8. Effect of Sputtered AlY Coating on High-Temperature Oxidation Behavior of Stainless Steel
  9. Optimal Design of Nozzle for Supersonic Atmosphere Plasma Spraying
  10. Oxidation Behaviors of Inconel 740H in Air and Dynamic Steam
  11. Line-Profile Analysis Combined with Texture Analysis for Characterizing Dislocation Distribution in Texture Components of Cold-Rolled Copper Sheets
  12. Microstructure Analysis of HPb59-1 Brass Induced by High Current Pulsed Electron Beam
  13. Thermal Treatment Method for Synthesis and Characterization of the Octahedral Magnetic Nanostructures of Co3O4 from a New Precursor
  14. Phases Transition and Consolidation Mechanism of High Chromium Vanadium-Titanium Magnetite Pellet by Oxidation Process
  15. Effect of Fiber Laser Treating on Magnetic Domains in the Grain-Oriented Silicon Steel: Imaging Domains by Bitter, MFM and Kerr Microscopy
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