STRUCTURE AND PROPERTIES OF DIE STEELS WITH MOLYBDENUM IN THE AS-CAST STATE

I. O. Khazanov, V. F. Zhidovin

(Presented by Professor A. N. Dobrovidov)

The need to study the properties of tool steels in the as-cast state is due to the steadily increasing extent to which cast tools are being used in industry. From the techno-economic point of view, a cast tool, as compared with a tool made from forged blanks, has a number of advantages:

1) lower material cost and the possibility of using tool-steel scrap within the enterprise;

2) the possibility of modifying the steel in order to change its chemical composition and, consequently, its properties in the desired direction;

3) the possibility of manufacturing parts of the most complex configuration, which is difficult to achieve by machining;

4) increased service life in a number of cases.

Steels for hot dies must possess sufficient dimensional stability, high wear resistance at elevated temperatures, and heat resistance. Increasing wear resistance under heating is possible by increasing resistance to tempering and strength characteristics under heating, and also as a result of increasing the amount of excess carbides [1]. However, since these steels must possess sufficient toughness, the route indicated above can be used only in exceptional cases. In this connection, the question arises as to what possibilities are afforded by alloying practice (without increasing the carbon content) for increasing dimensional stability and, accordingly, the wear resistance of the tool under heating. It is known that steel with 0.3–0.4% carbon and 2% chromium, with a tungsten content of 8–10%, is very sensitive to thermal shocks and cracks easily; therefore tools made from such steel fail more quickly because of the formation of deep cracks than because of loss of dimensions due to wear [2]. The resistance of steel to thermal shocks can be increased by alloying it with molybdenum [3].

In the present work, the influence of molybdenum on the properties of steels was studied during additional alloying of die steels 5KhNV and 4Kh8V2, as well as when tungsten was replaced by molybdenum in steel 4Kh2V8 (in a ratio of 2:1) up to complete displacement of tungsten. The steels were melted in an induction furnace with an acid lining. The charge consisted of tool-steel scrap and 60% ferromolybdenum. The steels were cast into metal molds. The chemical composition of the steels is given in Table 1.

The hardness of steel 5KhNV in the as-cast state (after tempering at 300°C with a holding time of 6 hours to relieve stresses) varies, depending on the molybdenum content, from 31 HRC to 41 HRC. The minimum hardness is found in steel 5KhNV without an addition of molybdenum. The highest hardness is found in the steel with 1.90 and 2.72% molybdenum. Steel 4Kh8V2 after casting and the same tempering has a hardness of 56–57 HRC, regardless of its molybdenum content. Since, when cast in a chill mold, steel 5KhNV does not acquire the hardness required for small-sized die tooling, its structure in the state immediately after casting was not investigated.

Table 1*

* The content of manganese, silicon, sulfur, and phosphorus in all heats, and the content of vanadium in heats 1–8, correspond to GOST 5950-63.

The structure of steel 4Kh8V2 after casting is fine-acicular martensite; the martensite crystals are surrounded by an uneven network of a “light” constituent, which has thickenings at grain junctions and the appearance of interlayers along grain boundaries. The structure has a clearly expressed dendritic morphology. On the basis of data from measurements of the microhardness of the main solid solution (fine-acicular martensite) and of the “light” constituent, it has been suggested [4] that the “light” constituent is a more highly alloyed cryptocrystalline martensite.

With an increase in molybdenum content from 0.98 to 4.69% (heats 5–8), the grain of the steel is considerably refined. The network of the “light” constituent becomes somewhat thicker. The dendritic morphology of the structure is retained.

The structure of heats 9–13 after casting also consists of martensite and the “light” constituent. In the structure of heats 9 and 13 the amount of the “light” constituent is smaller; regions of increased etchability appear. As a rule, they are located at grain junctions and are surrounded by the “light” constituent. In the structure of annealed and hardened steel

these regions are absent. This gives grounds to suppose that they are the result of nonequilibrium crystallization of the steel.

As a preliminary heat treatment, isothermal annealing was selected, with heating to 860°C and isothermal holding at 750°C for steels of heats 5–16 and at 700°C for steels of heats 1–4. The hardness of the steels in the annealed condition (heats 1–8) increases with increasing molybdenum content from 215 HB in 5KhNV steel without molybdenum addition to 241 HB at a molybdenum content of 2.72%, and from 192 HB to 217 HB at molybdenum contents in 4Kh8V2 steel of 0.98 and 4.69%. The hardness of the steels of heats 9–13 decreases as tungsten is replaced by molybdenum, from 207 HB in the steel with 8% tungsten (0% molybdenum) to 141 HB in the steel with 4% molybdenum (0% tungsten).

The microstructure of the steels of heats 1–4 in the annealed condition consists of granular pearlite and regions of excess ferrite. As the molybdenum content increases, the amount of excess ferrite increases; the dendritic structure becomes more distinctly expressed. The regions of excess ferrite form the axes of dendrites. The structure of heats 5–16 in the annealed condition is sorbite-like pearlite and carbides.

When selecting the optimal regime for final heat treatment, as many as six temperature quenching regimes were tested for each steel. The quenching temperatures that provided maximum hardness without signs of overheating in the structure were taken as optimal. For steels of heats 1–4 the quenching temperature interval is 880–970°C; for steels of heats 5–8, 1050–1150°C; and for steels of heats 9–16, 1100–1200°C. Analysis of fractures and of the microstructure of the quenched specimens showed that, when heated to the indicated temperatures, the steels studied are not prone to overheating.

Fig. 1. Microstructure of steel of heat 15 in the annealed condition, ×440. Etching with a 3% solution of nitric acid in alcohol

Fig. 2. Microstructure of steel of heat 15 after quenching from 1200°C in oil, ×440. Etching with a 3% solution of nitric acid in alcohol

The microstructure of the steels of heats 1–4 in the quenched condition consists of martensite and carbides. As the molybdenum content increases, the amount of carbides increases, and the acicularity of the martensite becomes less distinctly expressed.

The main structure of the steels of heats 5–16 in the quenched condition is martensite, the grains of which are surrounded by a network of a “light” constituent. There is no fundamental difference between the microstructures of the steels

in the as-cast and hardened states there is none (Figs. 1 and 2). The only difference is that the “light” constituent in the hardened state is etched more strongly. This indicates some equalization of the structure during annealing and heating for hardening.

Fig. 3. Heat resistance of 5KhNV steel as a function of molybdenum content. The numbers on the curves are heat numbers.

Successive tempering of specimens hardened from the optimum temperatures showed that steels of heats 5–16 are prone to secondary hardening. In steels of heats 5–8, the peak of secondary hardness lies in the temperature range 500–550°C. With increasing alloying by molybdenum, the peak of secondary hardness shifts toward the upper value of the indicated temperature interval. Steels of heats 9–16 have maximum secondary hardness in the temperature range 550–600°C. An increase in the molybdenum content in 5KhNV steel to 2.72% (heat 4) causes a slowing of the softening of the steel during tempering in the temperature range 500–550°C. Evidently, the processes of dispersion hardening also begin to make themselves felt here.

Fig. 4. Effect of carbon and vanadium content on the heat resistance of steels. The numbers on the curves are heat numbers.

The heat resistance of steels of heats 1–4 increases with increasing molybdenum content from 425°C (heat 1) to 585°C (heat 4) (Fig. 3). The effect of molybdenum on the heat resistance of steels of heats 5–8 is manifested to a lesser degree. Increasing the molybdenum content from 0.98% (heat 5) to 4.69% (heat 8) raises the heat resistance by 25°C (560 and 585°C, respectively).

Steels of heats 9–12 have a heat resistance of 630–640°C. Steel of heat 13 has reduced heat resistance (590°C). This was significantly promoted by the low carbon content in this steel. The greatest heat resistance was exhibited by steels of heats 15 and 16 (Fig. 4). These steels are close in composition to the steels of heats 11 and 12 and differ from them in carbon content (heat 15) and in carbon and vanadium content (heat 16).

The heat resistance was taken to be the temperature to which, when heated for 6 hours, the steel retains a hardness of 45 HRC.

For mechanical tests, specimens of steels of all heats were tempered to a hardness of 50–51 HRC. Bend tests and impact-toughness tests were carried out on standard specimens. The average values of the ultimate strength in bending and impact toughness (in both cases 5–6 specimens were tested) are given in Table 2.

Table 2

The addition of 1% molybdenum to 5KhNV steel increases impact toughness from 1.0 to 1.4 kgm/cm² and the ultimate strength in bending from 143 to 250 kg/mm². Increasing the molybdenum content to 2.72% practically does not lead to any further increase in the mechanical properties of this steel.

Additional alloying of 4Kh8V2 steel with molybdenum increases impact toughness and the ultimate strength in bending, but the effect of molybdenum is manifested in this case less than in 5KhNV steel.

Conclusion

An increase in the mechanical properties and heat resistance of cast die steels 5KhNV and 4Kh8V2 can be achieved by additional alloying of these steels with molybdenum. The optimum molybdenum content should be considered: in 5KhNV steel—1%, in 4Kh8V2 steel—2.3–2.5%.

LITERATURE

1. K. Bungardt, O. Mölders. Chernye metally, 3, 27, 1966.
2. I. Elfmark. Metal Treatment and Drop Forging, vol. 33, No. 250, 1966.
3. A. P. Gulyaev, S. L. Rustem, G. N. Orekhov, G. P. Alekseeva. Metallovedenie i termicheskaya obrabotka metallov, 7, 2, 1958.
4. I. O. Khazanov, Yu. M. Lozinskii. Izvestiya TPI, 162, 270, 1967.