MC, M7C3 carbides in ROLTEC,TOUGHTEC, WEARTEC, AISI A2, D2 and D6 steels

Figure 1: Microstructure of 12% Cr-steel, AISI D2. Both variants have been forged to a medium sized dimension. Scale bar: 50 µm.

Figure 2: Microstructure of a) an ingot cast steel with 0.7% C and 5% Cr and b) a spray formed steel alloyed with 4% V and low in Cr. Both variants have been forged to a medium sized dimension and heat treated to 60 HRC. Scale bar: 20 µm.

Table 1: Nominal chemical composition and dimension of investigated steel grades.

Figure 3: Microstructure of a 10% V steel after hot working to a medium sized dimension and heat treated to 62 HRC. Scale bars: 10 µm.

Table 2: Heat treatment procedure for the investigated steels.

Table 3: Volume fraction (%) of carbide and carbide type after heat treatment to 5563 HRC.

Figure 4: Microstructure of AISI A2, D2, D6, ROLTEC, TOUGHTEC and WEARTEC in heat treated condition. Scale bar: 10 µm.

Carbide name: MC, M7C3
Record No.: 732
Carbide formula: MC, M7C3
Carbide type: MC, M7C3
Carbide composition in weight %: No data
Image type: LM, SEM
Steel name: ROLTEC,TOUGHTEC, WEARTEC, AISI A2, D2 and D6 steels
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Cold work tool steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: No data
Note: Spray forming of tool steels give unique opportunities to develop steels with specific properties as this process allows a high degree of freedom as regards alloying elements. Furthermore, the rapid solidification rate develops a microstructure suitable for many tool steel applications. The manufacturing of high alloyed tool steels with a good yield has before only been possible via powder metallurgy, but is now also possible with use of spray forming. Spray forming of tool steels on an industrial scale have been introduced on a number of specially designed steel grades making use of the benefits of the spray forming technique. The properties of the new developed spray formed steels are unique in several aspects. Steels with high to excellent abrasive wear resistance in combination with good ductility and a working hardness ranging from 4566 HRC are available. This paper presents the properties of the spray formed tool steels produced on an industrial scale and these are compared with conventional manufactured grades. Also, industrial application test are presented.

Spray forming adds newopportunities to design cold work tool steels with properties not found in conventional produced cold work tool steels. During casting of conventional ingots the segregation of carbon and carbide forming elements are known to give material properties in the final hot worked bar that are strongly directional oriented and on a low level in the transverse direction. This is caused by the formation of strong carbide in segregatied areas. Preferentially chromium carbides are formed as cold work steels are based on high amount of carbon and chromium to obtain a properties profile consisting of high hardness after heat treatment, adequate hardenability and wear resistance. Ductility is due to this alloying concept and manufacturing process a factor, which is not able to be greatly optimised. The more rapid solidification occurring at spray forming gives less time for segregation of carbon and alloying elements and results in lower segregation and a finer solidification microstructure. Thus, the resulting microstructure of the hot worked bar is more homogeneous, but nevertheless much improvement in wear resistance and ductility can not be gained when spray forming steels of type D2. In Fig. 1 a comparison is made of the microstructure appearance for a 12%Cr-steel, AISI D2, manufactured via ingot casting and forging and via spray forming followed by forging.

The conventionally manufactured Cr-alloyed steels are based entirely on the soft chromium carbide type (M7C3; ~1700 HV), which is less advantageous from a wear resistant point of view. The hard and wear resistant vanadium carbide (MC; 2800 HV) is a more interesting alternative in designing a wear resistant and ductile tool material. This concept has been used in the development of alloys suitable for spray forming. In addition to an excellent wear resistance the ductility is significantly increased compared to what can be achieved via conventionally produced tool steels due to a homogeneous distribution of relatively small carbides. Figure 2 illustrates the uneven microstructure of chromium carbides in a martensitic matrix of a conventional ingot cast steel with 0.7% C and 5% Cr versus the more homogeneous microstructure of rounded vanadium and chromium carbides of a spray formed steel with 4% V and a lower chromium content, which promotes both wear resistance and ductility.

The properties of three commercial spray formed alloys, ROLTEC,TOUGHTEC and WEARTEC are presented in this paper. ROLTEC and WEARTEC are aimed for cold work applications, but also for engineering applications where a high wear resistance is required. TOUGHTEC is regarded as a steel aimed for plastic and hot work applications in that a higher priority is given to ductility compared to the other two grades, but still with an excellent wear resistance. As reference steels three conventional produced grades are used, AISI A2, D2 and D6. Table 1 shows the nominal chemical composition and dimension of the steels investigated. For the conventional grades the main alloying element is 5 to 12% Cr balanced with carbon to give a sufficient hardness after heat treatment and a combination of abrasive wear resistance and ductility. The spray formed grades are alloyed with high amounts of vanadium, 3.78.9% and balanced contents of chromium, 4.27.0% and molybdenum, 2.33.2% in order to achieve a very good abrasive wear resistance in combination with a good ductility and superior hardenabilty. This involves that hardness can be maintained also in big section sizes after heat treatment and/or can a lower cooling speed be used at hardening resulting in lower dimensional changes. All conventional grades are produced in full scale via ingot casting. The spray formed grades are manufactured as billets with a diameter of 500 mm and a length of approximately 2 meter. The spray forming process is described in more detail in literature.

Also higher alloyed vanadium steel grades are possible to produce via spray forming for further hot working to suitable dimensions for various applications. An example is given in Fig. 3, where the microstructure of a 10% V steel manufactured via spray forming and powder metallurgy is shown. Both production methods result in a homogeneous distribution of mostly hard vanadium carbides in a martensitic matrix, but there is a significant difference in size of the carbides with the spray formed carbides having an average size of around 610 m compared to about 2 µm for the PM method. This microstructure difference increases abrasive wear resistance substantionally for the spray formed steel grade, without reducing ductility too much. The effect of carbide size on abrasivewear resistance is illustrated inFig. 4, where three alloys are manufactured in full scale either via conventional ingot casting (Conv.), spray forming (SF) or powder metallurgy (PM). The alloys have all been heat treated to 6061 HRC. The lowest wear resistance is shown by D2 in the whole carbide size range due to the fact that the carbides are entirely of the softer chromium carbide type (M7C3; ~1700HV). With an increasing amount of hard vanadium carbides (MC; 2800 HV) and an increasing carbide size there is a significant improvement in wear resistance.

All steels have been tested as regards mechanical properties for ductility with unnotched specimens, 71055mm, and for abrasive wear resistance with an internal standard pin-on-disc method against SiO2. Fracture toughness testing has been performed for the spray formed grades and compared to reference data for cold work tool steels. The steels have been heat treated according to standard procedures to achieve a hardness between 4864 HRC, albeit mostly to 6063 HRC, as shown in Table 2.

The microstructure of the spray formed grades display significant differences compared to the conventional ingot cast steels. The spray formed grades are characterized by an even carbide distribution with mostly hard vanadium carbides embedded in a martensitic matrix. Depending on the alloy content the volume fraction of carbides varies between 920%. The size of the carbides are typically 115 10 µm, whereas for D2 and D6 the carbide size can be up to 100 µm in the longitudinal and 20 µm in transverse direction, i.e. a significant aspect ratio of the carbide shape is present in conventionally produced grades. Also, due to the longer solidification time at ingot casting a heavily banded carbide structure appears in high alloyed conventional produced steels. In Fig. 4 the microstructures of the investigated alloys are shown after hardening and tempering. The volume fraction of carbides in the investigated steel grades after heat treatment generating a hardness of 5863 HRC is presented in Table 3. Data has been determined by using the point counting technique based on SEM pictures.
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