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TECHNICAL ARTICLE Development of AISI A2 Tool Steel Beater Head for an Impact Crusher in a Sinter Plant Goutam Mukhopadhyay ? Piyas Palit ? Sandip Bhattacharyya Received: 15 December 2014 / Revised: 23 February 2015 / Accepted: 23 February 2015 / Published online: 7 March 2015 C211 Springer Science+Business Media New York and ASM International 2015 Abstract Failure analysis of a tool steel (AISI A2) beater head of an impact crusher and development of suitable heat treatment process to improve its performance have been presented. The beater heads were failing prematurely by brittle fracture from its pin-hole locations. The investigation consisted of visual inspection, fractography, chemical ana- lysis, characterization of microstructures using optical and scanning electron microscopes (SEM), EDS analysis, and determination of micro-hardness profile. Microstructural characterization using SEM and EDS analysis revealed significant amount of coarse continuous Cr-carbide networks in the martensite matrix. It increased hardness (64 HRC) as well as heterogeneity of the matrix as depicted by the micro- hardness profile, and decreased the toughness (3 J) since coarse carbide networks are very hard and brittle. The austenitizing temperature as well as tempering temperature of heat treatment was found lower at the manufacturer’s end. The new recommended heat treatment resulted in lower amount of discontinuous Cr-carbides along with significant amount of fine precipitates uniformly distributed throughout the matrix which led to an optimum combination of both hardness (59 HRC) and toughness (6.5 J) required for the application. The beater heads manufactured following the recommended heat treatment exhibited better performance (life increased by 4 times) compared to the earlier ones. Keywords Impact crusher C1 Hammers C1 AISI A2 C1 Tool steel C1 Carbide network C1 Brittle fracture C1 Toughness C1 Heat treatment Introduction Sinter is used as a raw material in blast furnace for iron making. Sintering is a process of agglomeration of base mix into a porous mass, by incipient fusion. The base-mix materials are prepared by proportionate mixing of iron ore fines, ground flux materials, ground coke breeze, and revert mix, followed by layer wise stacking of the same in the storage yard. The granulation of raw materials for base mix is an important quality factor for sinter. The correct granulation is achieved by crushing in an impact crusher, followed by sieving and mixing different granulations in appropriate ratios. Limestone and dunite, used as fluxes in sinter, are reclaimed by means of a wheel on boom re- claimer and sent to three 150 tph Hammer Mills, called primary crushers from where they are then sent to sec- ondary crushers (Hammer Mills), where they are ground to a size of 3.2 mm. After crushing, these materials are screened and stored in proportioning bins. The impact crusher is extensively used for crushing the raw materials like limestone, dolomite, and dunite in the bedding and blending plant for sinter preparation. It crushes the raw materials using multiple rotating beater heads (hammers) by impact against the grinding liners fixed on the impact walls of the machine [1]. The beater heads are connected to a rotor shaft with the help of beater arms arranged circumferentially in a zig-zag fashion (Fig. 1). The fineness of the granulated product of the crusher can be altered by adjusting the distance between the rotating hammer heads and the impact walls. The beater heads of the impact crusher were failing very frequently (within 15 days) under brittle fracture. Gradual wear of the beater head at the striking face is a normal mode of failure while its sudden brittle fracture is a con- cern. The hammer heads or beater heads in an impact G. Mukhopadhyay ( goutam2007@yahoo.co.in 123 Metallogr. Microstruct. Anal. (2015) 4:114–121 DOI 10.1007/s13632-015-0192-6 crusher are exposed not only to wear, but also to impact or shock loads. Therefore, toughness of the beater head material is an important factor for extended service life of the hammers apart from its hardness or wear resistance properties [2–6]. A huge number of beater heads had to be replaced every month because of their frequent breakage which led to high maintenance cost and loss in production. Several works on alloy cast irons and tool steels like AISI H11, H13, D2 and M2 are available in the existing lit- erature [2–10] which deals with the microstructural and mechanical characterizations. But work on air hardening tool steel AISI A2 and its application as a beater head material in impact crusher (hammer mill) is rare in the open literature. Further, no industrial or application data on the performance of AISI A2 tool steel as a crushing ham- mer is available in the existing literature. This work has presented the analysis of root cause for the premature failure of the AISI A2 beater heads and develop- ment of suitable heat treatment process to improve their performance. Experimental Procedure and Results Site Visit and Visual Observation The sinter plant has two raw materials bedding and blending (RMBB) units, namely, RMBB#1 and RMBB#2, which have 6 and 7 impact crushers. The impact crushers crush the flux materials used in sintering like lime stone, dolomite, pyroxenite, etc. The primary crushers which ro- tate at an rpm of around 675 first crush the raw materials from a mesh size of -50 mm into a size of -15 mm. These are then again crushed with the help of secondary crushers having an rpm of around 830 into a size of -3.15 mm. The capacity of a crusher is approximately 125 t/h and it contains 40–52 beater heads. Figure 1(a) shows a rotor shaft-beater head assembly. Beater heads are attached to the beater arms with the help of pins inserted through the holes present in the beater heads as shown in Fig. 1(b). Figure 2(a) and (b) shows failed beater heads which fractured from their pin-hole locations. The fracture surfaces (Fig. 2c and d) of the failed beater head reveal bright granular appearance suggesting brittle fracture. The other end of the beater head was found to be worn out (Fig. 2a and b) due to abrasion or impact with input flux materials while crushing. While gradual wear of the beater head surface is a desired mode of failure, fracture from the pin-hole locations during service is a concern which must be addressed. Fractography A small sample containing fracture surface was cut near the location of pin-hole from where the fracture initiated. The sample was ultrasonically cleaned for examining the frac- ture surface. The fracture surface was examined using a scanning electron microscope (SEM) (model: JXA6400, JEOL, Japan) operated at an accelerating voltage of 15 kV. Micrographs at various locations of the fracture surface were recorded at various magnifications. Examination of the fracture surface near crack initiation region as shown in Fig. 3 revealed cleavages indicating brittle fracture. Brittle fracture of the beater head suggests its failure under impact load during service. Materials A small piece of sample was cut from the failed beater head and prepared for its chemical analysis. Chemical analysis of the sample was carried out using x-ray fluorescence spectroscopy (XRF); carbon (C) and sulfur (S) content of the sample were determined using combus- tion infrared technique. The chemical analysis of the beater head is compiled in Table 1. The chemical analysis is found to be closer to AISI A2 (ASTM A681) grade of air hardening medium-alloy cold-work tool steel with marginally higher amount of chromium (Cr) and lower (a) (b) Beater Head Beater Arm Rotor shag332 Beater Head Beater Arm Pin Fig. 1 (a) Impact crusher showing rotor shaft-beater head assembly, (b) closer view of a beater head Metallogr. Microstruct. Anal. (2015) 4:114–121 115 123 amount of vanadium (V). Manganese, chromium, and molybdenum are the principal alloying elements in this grade of steel, which impart high hardenability and the steel can be hardened in air. AISI A2 provides an optimum combination of wear resistance and toughness required for crushing operation of raw materials in service. The heat treatment cycle given to the beater head as received from the manufacturer is presented in Fig. 4. The as-cast mate- rial was heat treated at 850–870 C176C for 1 h/in. followed by air cooling. The air-cooled material was tempered in two stages in the temperature range of 175–200 C176C for 3 h/in. Microstructural Examination A sample was cut from the failed beater head for microstructural examinations at the cross-section. The sample was then mounted in resin, ground, and polished using standard metallographic technique. The cross-section was examined under optical microscope (Leica, model: DMRX, Germany) after etching using Villela’s reagent (1 g picric acid, 5 mL hydrochloric acid, and 100 mL ethanol). Typical micrographs at the cross-section of the broken beater head are shown in Fig. 5(a) and (b). Mi- crostructural examination at the cross-section of the beater head reveals martensite matrix with networks of chain-like massive primary carbides at the grain boundaries (Fig. 5a and b). The carbides appear as bright phases which are preferentially clustered along grain boundary. The area fractions of carbides were measured at various fields using image analysis software. The average area fraction of carbides was found to be 7.2 ± 0.45%. The most significant material properties required for the beater head component are hardness, toughness, and wear resistance. As the amount of carbides increases, the hardness and wear resistance also increase but care has to be taken in heat treatment to avoid loss of toughness [2, 3, 6, 7]. The clusters of coarse carbides detrimentally affect the toughness; the carbides being brittle are sus- ceptible to initiate cracks during an impact in service [2, 8, 9]. (a) (b) (c) (d) A A B B Wear Pin hole Pin hole Fig. 2 (a, b) Beater head samples failed from the pin-hole locations, (c, d) closer view of fracture surfaces near the pin- hole of the beater heads Fig. 3 Fractography (at 92000) of failed beater head shows cleav- ages indicating brittle fracture 116 Metallogr. Microstruct. Anal. (2015) 4:114–121 123 Scanning Electron Microscopy and EDS Analysis The etched cross-section of the sample was examined with the help of the scanning electron microscope (SEM) operated at an accelerating voltage of 15 kV for its microstructural as well as elemental characterizations. Micrograph showed coarse carbide network at the grain boundary along with some fine globular precipitates with the martensitic matrix (Fig. 6a and b). Energy dispersive spectroscopy (EDS) of the grain boundary network as well as fine precipitates (as marked in Fig. 6a and b) was carried out for their elemental charac- terization and the results of the analyses were compiled in Table 2. The results of EDS analysis indicate that the grain boundary network and the fine precipitates within the matrix are chromium carbides. The carbide stoichiometry could not be determined by EDS micro-analysis. But literature survey on similar materials [2, 10, 11] like AISI H11, H13, M2, D2 and so on, and electron probe micro analysis of similar car- bides discussed in earlier literature [5, 12–15] suggests the grain boundary carbide network to be of M 7 C 3 type (Primary Carbide) and the fine globular precipitates as M 23 C 6 (where M = Cr) type (Secondary Carbide). The skeleton-like mor- phology of massive carbide network at the grain boundary as shown in Fig. 6(b) is apparently indicative of its eutectic origin (primary carbide), i.e., origin at the solidification stage [2, 5, 8–10, 13]. On the other hand, fine globular precipitates within the matrix indicate secondary carbides which form during heat treatment of the casting [2, 5, 8, 11]. Measurement of Hardness Both macro- and micro-hardness values were measured at the cross-section of the sample prepared from the failed beater head following the standard ASTM E384. Macro-hardness values were measured in a Vickers hardness testing machine with a load of 30 kgf. Five measurements were taken at random locations of the cross-section to get the average macro-hardness value; the macro-hardness value was found to be 781 ± 12 HV30 (equivalent to the micro-hardness profile is shown in Fig. 7(a). The hardness values are found to be varied in the range of 600–1000 HV0.05. Apart from hardness profile, micro-hardness values on some grain boundary carbide network were also measured separately. The average micro-hardness of carbide network was measured to 1375 ± 15 HV0.05; the measured micro-hard- ness value of carbide network is found to be similar to the value (1400 HV) for M 7 C 3 type of primary Cr-carbide reported in other existing literature [12, 16]. Measurement of Impact Toughness The impact tests of the samples were carried out using V-notch Charpy specimens in accordance with IS 1757:1988 [17] with the help of a standard impact testing machine (Striking Energy: 300 ± 10 J) at ambient temperature. At least three tests were carried out to get the average impact energy values of the beater head sample. The average impact energy value of the samples was 3 ± 0.2 J. Heat Treatment of Beater Head in Laboratory for Improvement The heat treatment given by the manufacturer of the beater head yielded a microstructure with significant coarse grain Table 1 Chemical analysis (wt%) of beater head sample Sample C Mn Si S P Cr Mo V W Beater head 1.01 0.67 0.50 0.030 0.026 5.7 1.02 0.12 0.010 ASTM A681 type A2 0.95–1.05 0.4–1 0.1–0.5 0.03 max 0.03 max 4.75–5.5 0.9–1.4 0.15–0.5 … Temperature ( °C) Time 850-870°C (1h/inch) 175-200°C (3h/inch) 175-200°C (3h/inch) Hardening Tempering 780° - 790°C (soaking) Temperature ( °C) Time 950-960°C (1h/inch) 250°C (3h/inch) 200°C (3h/inch) Hardening Tempering Room Temperature (a) (b) Room Temperature Fig. 4 (a) Typical heat treatment schedule given by the manufacturer of beater head, and (b) recommended heat treatment cycle of beater head Metallogr. Microstruct. Anal. (2015) 4:114–121 117 123 boundary carbide networks. The coarse grain boundary carbide network adversely affects the toughness of the material making it prone to fracture under impact [2, 8]. During present investigation, a suitable heat treatment was given to some as-cast beater head material (supplied by the same manufacturer). The heat treatment was carried out in a controlled-atmosphere furnace. The material was pre- heated to 788 C176C and held at this temperature until thor- oughly soaked. Then, it was heated to 954 C176C and held for 1 h/in. of greatest cross-section. After austenitization, the material was removed from the furnace and cooled in air followed by immediate tempering in two stages at a tem- perature of 200–250 C176C. Microstructural Characterization After Heat Treatment in Laboratory A sample was prepared form the beater head block heat treated in the Laboratory for examination of the microstructure fol- lowing the procedure as described in ‘‘Microstructural Examination’’ section. The optical microstructures are shown in Fig. 8(a) and (b). The microstructures reveal predominantly Fig. 5 Microstructures of failed beater head: (a) Microstructure (at 950) shows martensite matrix along with carbide networks at the grain boundaries, and (b) microstructure at magnified view (at 9200) shows continuous carbide network at the prior austenite grain boundary Fig. 6 SEM micrographs at the cross-section of beater head sample: (a) micrograph (at 91000) shows coarse carbide network at the grain boundary along with fine precipitates within martensite matrix, and (b) micrograph (at 91500) shows locations of EDS analysis on the coarse carbide network as well as fine precipitates within the matrix Table 2 Results of EDS analysis (wt%) at different locations as shown in Fig. 6 Locations C Si Cr Mn Fe Mo Remarks 1 11.71 0.40 16.68 0.68 64.5 6.03 Secondary carbide 2 19.35 … 40.92 … 30.63 8.71 Primary carbide 3 … 1.00 4.83 0.50 92.71 0.96 Matrix 4 9.64 0.48 12.40 … 73.72 3.76 Secondary carbide 5 17.21 … 31.67 … 45.08 6.03 Primary carbide 6 … 1.05 5.33 … 92.23 1.39 Matrix 0 200 400 600 800 1000 1200 1 2 3 4 5 6 7 8 9 10111213141516171819 Micro-vickers hardness (Hv) Micro-Hardness Profile Failed Beater Head (heat treated at Manufacturer's end) Ag332er recommended heat treatment (a) (b) Fig. 7 Micro-hardness profiles measured at the cross-section of the beater heads: (a) Failed beater head, and (b) after recommended heat treatment 118 Metallogr. Microstruct. Anal. (2015) 4:114–121 123 martensitic matrix with a small amount of primary eutectic carbides at the grain boundary. But these carbides are not in the form of continuous network rather they are discontinuous and present at some discrete locations on the grain boundaries un- like that observed in Fig. 5(a) and (b). Scanning electron mi- croscopy (SEM) as well as EDS analysis of the carbides was carried out to study their morphology and type. SEM and EDS analyses show discontinuous primary Cr-carbides (M 7 C 3 type) at the grain boundary and numerous globular fine carbide par- ticles (secondary carbides-M 23 C 6 type) uniformly distributed throughout the matrix as illustrated in Fig. 9 and Table 3. Mechanical Characterization After Heat Treatment in Laboratory After carrying out the heat treatment of the as-cast beater head block in the Laboratory, the sample was tested for hardness and impact toughness values following the pro- cedure described in ‘‘Measurement of Hardness’’ and ‘‘Measurement of Impact Toughness’’ sections, respec- tively. The macro-hardness values were measured to be 670 ± 10 Hv (equivalent to during heat treatment fine globular precipitates of secondary carbides which were uniformly distributed throughout the matrix were formed from the coarse continuous carbide network Fig. 8 (a) Microstructure (at 950) of beater head after suggested heat treatment, (b) Microstructure (at 9200) shows martensite matrix with discontinuous carbides at discrete locations on the prior austenite grain boundary Fig. 9 Discontinuous carbide network (at 92500) at prior austenite grain boundary along with fine globular Cr-carbide precipitates within the matrix Metallogr. Microstruct. Anal. (2015) 4:114–121 119 123 making them discontinuous or isolated at certain locations apart from reducing their amount [5]. The heat treatment schedule (Fig. 4) given by the manufacturer showed a lower austenitizing temperature during hardening as well as temperature during tempering [2, 18]. Because of lower austenitizing temperature during heat treatment, the mas- sive primary carbides which generated fine secondary carbides afterwards were not fully dissolved in the matrix; this yielded coarse carbide network with a little amount of fine secondary carbide precipitates imparting brittleness or poor toughness to the material [2, 5, 19]. A proper heat treatment schedule as described in ‘‘Heat treatment of Beater Head in Laboratory for improvement’’ section was recommended to the manufacturer to improve the microstructure of beater head; austenitizing tem- perature was increased to 954 C176C after preheating at 788 C176C and tempering temperature was increased to 200–250 C176C. The new heat treatment ensured significant dissolution of coarse primary carbide network in the matrix followed by precipitation of fine globular secondary car- bide particles uniformly distributed throughout the matrix (Figs. 8 and 9). The recommended heat treatment schedule resulted in improved microstructure having an optimum combination of both hardness and toughness. The impact toughness of the material was found to increase from 3 to 6.5 J, i.e., by 117% with a hardness value desired as per the specification of the grade AISI A2. The hardness profile (Fig. 7b) also exhibited a relatively smooth trend compared to the earlier one ensuring a more uniform matrix. The new beater he