Examination of the as-cast containing 1.17% C and 13.02% Mn exhibited the formation of carbides along austenite grain boundaries and other interdendritic areas due to Mn and C segregation. Segregation and carbide precipitation were studied using scanning electron microscope and electron probe microanalyzer. In this work, observation has been performed to understand the effect of solution treatment on the carbide dissolution. This experiment is for casting weighing 0.4 kg and 12.5 mm in section size. A single austenite phase at a cross section specimens was obtained for rapid and step heating procedures with holding time 45 minutes - 6 hours at 1050oC. In contrast, no holding time is required for slow heating procedure due to lower rate of heating about 18o/s. Observation on the surface exhibited decarburization, it may be by furnace gas during heating. This surface has depth 0.2-3 mm below the casting surface and formed partly martensitic structure.
Carbide precipitation, electron probe microanalyzer, solution treatment, decarburization.
Hadfield’s Manganese Steel is a very useful engineering material due to its high toughness, ductility, high work-hardening capacity and good resistance to wear. It is particularly useful for severe service that combines abrasion and heavy impact as in power shovel loader, bucket teeth, railway frogs, rock crusher, etc.
As-cast Hadfield’s manganese steel is quite brittle for normal use due to carbide precipitates [1-4]. It does not have sufficient physical properties to withstand impact due to carbides behavior that tends to brittleness [5]. Such an as-cast structure should be solution treated to the austenitizing temperature, at a certain holding time, then water quenched in agitated water to preserve the full austenite structure [1]. The mechanical properties are greatly affected by changes in its microstructure. The perfectly dissolution of grain boundary carbides to austenite grains has marked effect on its susceptibility to withstand impact under service condition. The toughness of these alloys is dependent on precipitates formed at grain boundaries. It is thus desirable to investigate these phenomena in these alloys. The present paper describes experimental results an investigation of as-cast and solution treated Hadfield’s manganese steel alloy.
The flow of experimental sequences and solution treatment procedures are described on fig 1 and fig 2.
Fig. 2 Solution treatment processes
The alloys made were obtained in many of chemical compositions of austenitic Fe-Mn-C steels in the comprehensive investigation. In this work, however, one of these steels was chosen to exemplify the solution treatment procedures applied.
The alloys were prepared from 16 kg and 1 ton induction furnace capacity under atmospheric condition. Five series of experiments were performed. The chemical compositions were tested by several optical spectrometers compared by wet analysis. The compositions believed for the last samples are 1.17 wt% C and 13.02 wt% Mn. The composition examination by EPMA on the normal matrix (after solution treatment for 1 hour by rapid heating) was obtained about 1.03 wt% C and 12.98 wt% Mn for intensities error of 7.63 and 1.52, respectively.
Some impurities appeared, beside the main elements, are 0.36 wt% Cr, 1 wt % Si, 0.011 wt% S, and 0.086 wt% P, so the total content of impurities being about 1.46 wt%. Such chrome contents will not affect the properties of alloy and ASTM A128 recommend in between 1.5-2.5 wt%. One percent of silicon is a higher content in austenitic manganese steel. This level will increase fluidity of molten metal during casting. In such a manganese alloy, sulphur with manganese will form manganese-sulphide instead of ferro-sulphide. So, it will not affect the alloy properties and also in ASTM A128, sulphur element is not considered
Melting and Pouring StudiesHadfield manganese steel has a difficulty in its casting practice. The molten metal reacted with silicate refractory by Mn and MnO to form (Mn2SiO4 + MnSiO3 and SiO2) [6] which caused erosion of a furnace wall. This phenomenon has occured when casting practice employed for the second melting process, where silicate refractory was used. After changing the refractory from silicate to the alumina, the same accident was not occuring. The important problem is to control the pouring temperature [7]. The resistance to cracking is promoted by the low pouring temperature [6] for about 1420 - 1450oC. However, the molten metal was freezing when pouring from a ladle into the mold even pouring temperature was 1550oC for about 5 seconds. The melting process for the experiment mentioned was using 16 kg induction furnace capacity with 10,000 Hz frequency. For the bigger capacity, 1 ton and 850 Hz frequency, such freezing molten metal at that temperature did not occur. It might be caused by higher frequency provides poorer of stirring effect when melting process done.
Solution-treatment StudiesThe important parameters affecting the properties of alloy are temperature, rate of heating, holding time, and rapidity of quenching. Austenitizing temperature determined does not exceed the eutectic transformation temperature to prevent melting of micro-segregated compound such as may be Fe3P. Thus, austenitizing temperature of 1050oC was chosen to compensate the temperature drop during furnace discharging until water quenched even the temperature of 980oC was sufficiently worth. The other reason for such a higher austenitizing temperature is to maintain perfect diffusion of atoms during carbide dissolution so that the time metal held at that temperature can be decreased.
The time metal held in austenitizing temperature was influenced by three distinct parameters. These are time required to: reach complete homogeneity entire the specimen parts, dissolve the carbides into austenite matrix, and diffuse perfectly by the atoms.
The first criteria could be predicted by hand-calculation using heat-transfer concept [10-12] or more convenient by numerical simulation using Ansys Software [13], based on the experimental data. Both of calculation methods were presented in reference [14].
The time required to dissolve the carbides was rough estimated by Fick's second law [8] [14] and the results is presented in the same paper above [14]. The time required to complete diffuse by carbon atoms do not need longer time since they occupy interstitial sites rather than manganese atoms in substitutional sites. It took about four minutes by atomic carbon to diffuse at mean distance of 150 m of as-cast grain diameter. Whereas, atomic manganese was consuming at least 6 hours at 1050oC to reach about 37 % concentration gradient from 50%. It took about 34 hours for ten percent, from 20.32 at.% (24.7 wt%) Mn to 12.58 at% (12.98 wt.%) Mn at distance 20 m. Such a diffusion path was measured in between one and half closer carbides, all calculations described above is presented in reference [14].
Metallographic StudiesAs-cast SpecimenThe microstructures of the as-cast specimen are shown in fig. 3a and 3b. Primarily, the carbides exist at the grain boundaries and white contrast carbide within the austenite grains. Cellular carbides along with white contrast were observed more at a grain edge than in its grain boundary. EPMA results revealed segregation of Mn and C at the grain boundaries, grain edges and within austenite grains.
(a)
(b)
Figure 3: Microstructures of as-cast Hadfield’s Manganese Steel.Carbide presipitates at austenite grain boundaries (a) and Cellular structure of carbides (b).Etched in 3% Nital. (17)
Maximum concentrations of Mn and C in the area of segregation were found to be 24.68 wt% and 4.07 wt% respectively. Segregation of a such carbon concentration is estimated occurs at the eutectic transformation corresponding to 4.2 wt% C and temperature about 1120 oC (equilibrium diagram of the system Fe-C). In addition, concentration of Mn is predicted close to its maximum solid solubility of 27 wt% Mn at 400 oC (equilibrium diagram of the system Fe-Mn). In the mean while, examinations of the Mn and C composition by EPMA in austenite matrix respectively are 11.59 wt% and 1.48 wt%. The observed segregation can be explained on the basis of rejection of solute and instability caused by the undercooling results in altering the planar solid-liquid interface to cellular or dendritic interfaces [15-16].
Rapid Heating The microstructures of solution treated specimens for 45 minutes held in the furnace is not found any carbides either in grain boundaries or within austenite grain (fig. 4a). Observation at higher magnification was predicted existing discontinues precipitates at grain boundaries (fig. 4b).
(a)
(b)
Figure 4: Microstructures of solution treated specimen
Rapid heating to 1050oC, held for 45 minutes, and water quenched at room temperature (a). Predicted discontinues precipitates at grain boundaries. Triangle regular shapes are etch-pits (b).Etched in 2% Nital/20% Na2S2O5. (17)
SEM examination of such precipitates indicated row of beads along grain boundaries. EPMA examination on this precipitates showed concentration of 14.71 wt% Mn and 1.33 wt% C while austenite matrix contains 12.98 wt% Mn and 1.03 wt% C so that the rows are not precipitates. It is clear that there is no precipitate at all grain boundaries and in austenite matrixes.
SEM observation showed the existing of triangle regular shapes near the grain boundaries indicates the etch-pits (fig. 4b). These pits thickness are in the range 0.1-1.4 m and exist at the whole of grains that have similar shapes and orientations with each other. The range of thickness is due to the difference excess concentration of vacancies produced by quenching from high temperature. The triangle arrangement of dislocation shown in fig. 4b consists of a tetrahedron of stacking faults on {111} planes with <110> type stair-rod dislocation along the edges of the tetrahedron [18]. EPMA examination on the tetrahedral defect provides concentration of 12.38 wt% Mn and 1.18 %wt C, while austenite matrix are 12.98 wt% Mn and 1.03 wt% C which are almost the same with.
The microstructure of specimen held for 1 hour till 6 hours in the furnace have the similar structure as before except grain coarsening has occured. The row of beads at grain boundaries are not found after holding time 3 hours (fig. 5a), it sign that austenite structure is free from carbides. After holding time 4-6 hours is hardly found the etch-pits in austenite matrix. The black spot spreading out on the whole microstructures are microporosities.
(a)
(b)
Figure 5: Microstructures of solution treated specimen. Rapid heating to 1050oC, held for 3 hours (a), 6 hours (b), and water quenched to room temperature. Etched in 2% Nital/20% Na2S2O5. (17)
Slow HeatingThe rate of heating employed in this experiment is about 0.18o/s and 2.2 kW power of convection furnace. No holding time was required for this procedure since such rate of heating ensures complete dissolution of carbides. The heat transfer calculation is presented in reference [14]. All grain surfaces are more clear from etch pits than direct heated specimen, but the grain size is coarser. For holding time 30 minutes, all surfaces are clear from etch-pits. The microstructures of the solution treated specimen by slow heating procedure are shown in Fig. 6a and 6b.
(a)
(b)
Figure 6: Microstructures of solution treated specimen. Slow heating from room temperature to 1050oC, no held (a), held for 30 minutes (b), and water quenched to room temperature. Etched in 2% Nital/20% Na2S2O5. (17)
Step HeatingThe microstructures of the solution treatment are shown in Fig. 7a and 7b. In fact, this heating procedure offered more advantage than heating processes as mentioned earlier. All microstructures observed are clear from etch-pits, grain boundary carbides, carbides in grain, and having finer grain size started from 1 hour until 3 hours holding time in a furnace.
(a)
(b)
Figure 7: Microstructures of solution treated specimen. Variation continuously and isothermally heating from room temperature to 575oC (held for 3 hours) then 980oC (held for 2 hours), and 1050oC, held for 1 hour (a), held for 2 hours (b), and water quenched to room temperature. Etched in 2% Nital/20% Na2S2O5. (17)
During the metal held in a furnace at 575oC for 3 hours, phases transformation of grain boundary carbide and pearlite was occuring. When temperature was increased above 700oC, say 850oC, the carbide at grain boundaries dissolved to form thin film but does not perfectly dissolve until 925oC. Increasing temperature to 980oC and the metal held for 2 hours, the grain boundary carbides became completely dissolve. During the completion of dissolution, there were not significant grain growth due to solute drag effect [16] employed by the carbides so that grain growth was stagnant. Increasing temperature to 1050oC takes an advantage to dissolve remaining carbide within grain to enter into austenite matrix. So, it can be noted that the appropriate holding time for dissolving carbides are at austenitizing temperature 980oC and the temperature 1050oC is to complete dissolution. This statement is reasonable because the metal held for longer time at just 1050oC, the grain boundaries will move and the grains will grow so fast.
Surface Decarburization, Magnetic Testing, and Hardness Testing StudiesThere was a surface decarburization during heating may be due to gas reactions in the furnace. The layer in which the decarburization occurs, in fact, tend to form a martensitic structure (fig. 8). The distance of this layer is just 0.2 - 3 mm and the thicker layer is produced by step heating with 6 hours holding time.
(a)
(b)
Figure 8: Surface decarburization. 1 mm thickness of decarburization was measured from the specimen held for 3 hours at 1050oC with slow heating procedure (a) and Martensitic structure on the area of carburization (b). Etched in 2% Nital/20% Na2S2O5. (17
The results of the magnetic testing proved that the specimens were nonmagnetic after all solution treatment procedures done. This finding indicates that all microstructures (nonmagnetic) are free form carbides. All surfaces of decarburization were attracted by magnet.
Brinell hardness for as-cast of Hadfield’s manganese steel was observed in between 223-229 for ten times examination. Whereas for solution treated specimen which rapid heating procedure was obtained in between 163-166 respectively for 45 minutes - 6 hours holding time in the furnace. Brinell hardness number for slow heating is 118-145 at which the holding time is in between 0 - 6 hours. While Brinell hardness for step heating is 136-158 and 1 - 3 hours holding time .
V-Notch Charpy Impact Testing StudiesThe results of impact value for all procedures is illustrated in fig. 9. From this curve the highest impact value is obtained for step heated specimen which held in the furnace for 1 hour and the lowest is for slow heated specimen held 2 hours in the furnace. From this curve is also indicated that longer time the specimen held in the furnace has a tendency decrease of impact value up to two hours. Finally, it can be concluded that for six hours the specimen held in the furnace will decrease the impact values.
Figure 9: Variation of V-notch Charpy impact value with isothermal holding time in 17 wt% C and 13.02 wt% Mn cast steel. (Testing condition: at room temperature)
ACKNOWLEDGMENTSAppreciation is expressed to those who have made contributions to the research. I am indebted to the following individuals: Ir. Hatman Bahar, Manager of Surface and Heat Treatment, Ir. H. Dani Ramdani, Foundry Manager, PT. Pindad (Indonesia); Satyawan Ari Nugroho, Head of PPC and Engineering Department, PT. Baramulti Metalika (Indonesia); and Eric, Director of PT. Hanco (Indonesia).
CONCLUSIONSSeveral main points which can be concluded from this work are:
- The alloy composition resulted from casting is suitable for the Hadfield manganese steel that is 1.17 wt% C and 13.02 wt% Mn.
- Heat transfer simulation and diffusion time calculation for section size specimen of 12.5 X 12.5 X 12.5 mm indicated that 45 minutes is appropriate time to hold specimen at 1050oC for rapid heating procedure.
- Heat transfer calculation and metallographic observation (fig. 6a) proved that for rate of heating 0.18oC/s with slow heating procedure, no holding time is required.
- For the same holding time, solution treatment employed by step heating procedure provides smaller austenite grain size.
- The highest impact value is obtained for the specimen which is solution treated at 1050oC with step heating procedure. This impact value, however, has no great difference between step and slow heating also the time consumed for slow heating procedure was lower.
- Brinell hardness number for as-cast of Hadfield manganese steel was observed in between 223-229.
- As solution-treated specimens have hardness in between 163-166, 118-145, 136-158 respectively for rapid heating, slow heating and step heating.
- Recommendation for heating of Hadfield manganese steel is slow heating procedure due to the time consumed by this process is lower and the impact value has no great difference between slow and step heating procedure about 17 J/cm2.
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