■ Summary
HENGIN supplied three new 3.0” (80mm) diameter grinding balls for metallurgical examination. All samples had been forged to the final size and shape and had excellent surface quality and roundness. There were no potentially detrimental cracks, laps or seams.
◆ On a weight basis, the balls were an average of 2.7% oversize relative to a 3.0” nominal ball.
◆ With an average surface hardness of Rockwell 63 HRC the balls did meet the recommended minimum 60 HRC surface hardness.
◆ Interior hardness readings were appropriate for a through hardened low residual stress ball design.
◆ With an average alloy calculated martensite start temperature, Ms(N), value of 442° F potential wear resistance of the alloy was higher than optimal. Marked ball wear testing provided data that lower Ms(N) alloys, when optimally heat treated, can produce lower wear rates.
◆ The three samples had an average calculated hardenability Grossman Di of 2.7”, which was less than optimal for the ball diameter.
◆ The potentially detrimental elements phosphorous and sulfur were at acceptable levels in the material.
◆ The average Shepard method estimated grain size at the surface (ASTM #8) and at the center (ASTM #6) is appropriate and indicates the thermal cycle during heat treatment was appropriate.
Based on the metallurgical properties obtained from the HENGIN 3.0” balls investigated for this report, the balls would be expected to provide a higher than optimal wear rate in normal impact secondary mill applications. The high martensite start temperature limits the balls potential wear rate performance. The surface hardness, hardness profile and grain size were all very good.Toughness should be adequate for the application, but only controlled drop ball testing or charge observations can determine ball toughness requirements for a specific application.
Upon arrival, the HENGIN 3.0” ball samples were marked for identification, thoroughly examined, weighed and metallurgically sectioned for subsequent hardness and chemistry evaluations. Sample sectioning was done by Bearcreek Metallurgical, LLC. Due to the sensitivity of heat treated high carbon steels to sample preparation, the metallurgical cutting practices utilized were designed for eliminating microstructural alteration through a low rate of metal removal and high coolant flow. The plane of the wafer extracted from the balls was random relative to the original bar rolling and ball forging axis. Hardness testing was performed on a Wilson Model 3JR Rockwell Hardness Tester using a “C” Brale
penetrator with a 150-kg load. For testing control, 65.6±0.5 HRC and 56.2±1.0 HRC calibration blocks were utilized to assure accuracy of the readings. Chemistry data was obtained through optical emission spectrographic (OES) and combustion analysis (LECO) methods by Colorado Metallurgical Services, Aurora, Colorado.
BALL SAMPLE - SIZE and DESCRIPTION
Sample Nominal Sample Sample Weight Percent Calculated Diameter
No. Size (in) Description (g) (lbs.) Oversize (in.) (mm)
1 3.0 New Whole Ball 1,867 4.12 2.7 3.03 76.9
2 3.0 New Whole Ball 1,871 4.13 2.9 3.03 76.9
3 3.0 New Whole Ball 1,867 4.12 2.7 3.03 76.9
SAMPLE
No. C Mn P S Si Ni Cr Mo Cu Ti V Nb Sn Al
1 0.77 0.77 0.022 0.003 0.19 <0.001 0.53 <0.001 0.02 0.004 0.003 <0.001 <0.001 0.008
2 0.76 0.77 0.018 0.004 0.19 <0.001 0.54 <0.001 0.02 0.004 0.003 <0.001 <0.001 0.007
3 0.78 0.76 0.013 0.002 0.18 <0.001 0.54 <0.001 0.01 0.004 0.004 <0.001 <0.001 0.016
◆ Prior Austenitic Grain Size - Shepherd Method
Sample No. Ball Surface Ball Center
1 ASTM #8 ASTM #6
2 ASTM #7 ASTM #6
3 ASTM #8 ASTM #6
◆ The criteria for grain size in grinding media steels are as follows:
Fine grain ASTM #7 through #9
Intermediate grain ASTM #4 through #6
Coarse grain ASTM #1 through #3
HENGIN provided three samples of 3.0” diameter grinding balls for metallurgical characterization. Surface quality and roundness of each of the samples was excellent. There were no potentially harmful cracks, surface seams or laps. Relative to the weight of a nominal 3.0” diameter ball, the samples were 2.7% oversize. The balls had been forged, heat treated, quenched and tempered. For optimal wear resistance in normal impact secondary mill applications, a minimum surface hardness of Rockwell 60 HRC is recommended. If ball breakage or spalling is noted in the ball charge, surface hardness levels below Rockwell 60 HRC may be required. The HENGIN balls tested did meet the recommended minimum surface hardness with its average 63 HRC. The hardness profile from the surface to the center indicates the balls were correctly through hardened and would be expected to have low residual internal stresses. Low residual stress is advantageous as it minimizes the cumulative effect of normal application induced stress.The microstructure shown in the figure below shows the expected fine tempered martensite with very little retained austenite. Conventional alloy design experience relies on a high volume fraction of retained austenite to provide bulk fracture toughness. Lower martensite start temperatures lead to higher percentages of metastable retained austenite in the microstructure. Retained austenite, when in service, can transform to martensite in a thin layer at the ball surface from impact stresses. This martensite is of very high hardness and excellent wear resistance.
Sample 1, 1000x Nital etch
With an average alloy calculated Ms(N) value of 442º F, the 3.0ʺ HENGIN balls would be anticipated to develop a higher than optimal wear rate in normal impact secondary mill grinding applications. Higher alloy carbon content would be the best way to lower the martensite start temperature for enhanced wear resistance. Alloy hardenability was slightly low, but acceptable, for the ball size with a calculated Grossman Di of 2.7ʺ. Hardenability elements utilized were manganese and chromium. Phosphorus and sulfur, at elevated levels, can develop grain boundary films or non-metallic inclusions, respectively, which can reduce impact toughness. These potentially harmful elements, however, were at acceptable levels in the material. The average Shepard method estimated grain size of ASTM #8 at the surface and ASTM #6 at the center in the extracted wafers was categorized as a mix of “fine” and“intermediate”. The grain sizes were very good. Fine and intermediate grain microstructures have greater fracture toughness than coarse grain microstructures. Aluminum was used as the grain refining element in sample #3, the other samples did not have measurable grain refining elements. No defects were noted in the centerline portion of the samples and there were no indications of detrimental hydrogen. Hydrogen-assisted cracking can result in ball breakage and increased wear rate.Laboratory tests are available for measuring grinding media material toughness, but these tests only measure a small material segment and cannot be scaled to the impact conditions that occur in application. Structural integrity and spalling resistance of a grinding ball are more appropriate characteristics for evaluating toughness. Only controlled drop ball tests or conducting tests in the actual application can be considered viable techniques.
Below is a tabular summary of the metallurgical characteristics of the nominal 3.0ʺ (80mm) ball samples. Included are calculated values of Ms(N) (Martensite start temperature), Di (Grossman hardenability of composition) and the weighted volumetric hardness. The calculated Ms(N) value
can be used to estimate heat treatment quenching characteristics as well as the relative wear rates for optimally heat treated materials in a specific application. An alloy with a lower Ms(N) will develop lower wear rates. The Di calculated value can be used to determine the adequacy of the total alloy content for the specific ball size. Prior austenitic grain size is an indication of the compatibility of the heat treatment cycle with the alloy composition and an important characteristic of material toughness.
Sample Ms(N) Ideal Weighted Acceptable Acceptable Optimal Optimal Hydrogen
Temp Diameter Volumetric Composition Heat Surface Prior Indications
(Inches) Hardness Treatment Hardness Austenitic
(HRC) Grain Size
1 440°F 2.7 63.0 No* Yes Yes Yes None noted
2 446°F 2.7 62.0 No* Yes Yes Yes None noted
3 440°F 2.7 62.7 No* Yes Yes Yes None noted
Notes: Ms(N) is the martensite start temperature (degrees F) calculated via the Nehrenberg method.Di is the Grossman method ideal critical diameter (in.) calculated from the alloy composition.Weighted volumetric hardness is a measure of the section average Rockwell hardness.Prior austenitic grain size estimate by the Shepherd Method.
*Too high Ms(N) temperature for optimal wear resistance.Based on the metallurgical properties obtained from the HENGIN 3.0” balls investigated for this
report, the balls would be expected to provide a higher than optimal wear rate in normal impact secondary mill applications. The high martensite start temperature limits the balls potential wear rate performance. The surface hardness, hardness profile and grain size were all very good.Toughness should be adequate for the application, but only controlled drop ball testing or charge observations can determine ball toughness requirements for a specific application.
■ In January 2021, the filling rate of SAG mill ball is 10%, the grading is Φ 100 mm: Φ 80 mm = 1:5, the ball consumption is 0.87 kg / T, the roundness rate is less than 1%, and the crushing rate is less than 1%. The life cycle of SAG mill shell liner is 3 months.
■ In February 2021, the filling rate of SAG mill ball is 12%, the grading is Φ 100 mm: Φ 80 mm = 1:5, the ball consumption is 0.79 kg / T, the roundness rate is less than 1%, and the crushing rate is less than 1%. The SAG mill shell liner has been used for 3 months.
■ The use effect of grinding ball has achieved the expected goal. The next step is to adjust the filling rate and grading of grinding ball, so as to achieve lower wear and better crushing and grinding effect.
