EFFECT OF HEAT TREATMENT ON THE MECHANICAL PROPERTIES OF CAST PRECIPITATION-HARDENING ALLOYS

A. D. KLEMENT’EV

(Presented by the scientific seminar of the departments of metal science and technology of heat treatment of metals, technology of metals, equipment and technology of welding production)

In studies of cast precipitation-hardening iron–cobalt–tungsten alloys [1], alloys were investigated that harden during casting into a metal mold (melting and pouring were carried out in a vacuum furnace). Tempering then followed. For this technological variant, alloys were developed that possessed good self-hardening during casting and a high degree of precipitation strengthening during tempering. Nevertheless, the effect of special heat treatment on the properties of these alloys is of interest.

As was shown [2–4], heat treatment has a significant effect on the structure, precipitation strengthening, and certain properties of the alloys studied. Moreover, the technological variant of heat-treated alloys imposes its own requirements on their chemical composition.

An important characteristic of tool materials is bending strength. This article considers the effect of heat treatment on bending strength in combination with the hardness and structure of the alloys.

The table gives variants of heat treatment and their effect on the hardness and bending strength of alloys 1 and 2. Chemical composition of the alloys: alloy 1—20% Co, 20% W, 0.2% Ti, 1% Mn, 0.5% V; alloy 2—20% Co, 10% W, 10% Mo, 0.25% Ti, 0.5% V.

The alloys were selected with allowance for the characteristic change in their structure and properties after quenching [3, 4]. The structure of alloy 1 before and after quenching does not fundamentally change in character (the fineness of grain increases). In the initial cast state, in accordance with the chemical composition [3], the structure has grains with sinuous boundaries and point precipitates of excess phase in the boundary regions. Unequal etchability of the grains of the solid solution indicates the presence of intracrystalline segregation and local premature decomposition of the solid solution.

Alloy 2 in the initial state has a structure analogous to that of alloy 1, but after quenching it acquires a polyhedral structure. Responsible for such restructuring of the structure are molybdenum,

vanadium and titanium [4]. In addition, the shift of the polymorphic transformation to the region of higher temperatures due to the indicated elements promotes, as a result of phase work-hardening, premature decomposition of the solid solution, reducing the degree of hardenability of the alloy. In some areas of the structure, in addition to polyhedra, small remnants of the initial phase of dendritic structure are detected (with sinuous boundaries). In the quenched state they have lower hardness, and after tempering higher hardness, than the polyhedra, which is in agreement with the regularity established earlier [4]. A small amount of excess phase, present in the form of point inclusions and completely dissolving during heating for quenching, does not prevent the appearance of a polyhedral structure [3, 4].

The alloys were quenched after heating in an electric tube furnace with ceramic heaters from 1300° with a 5-min hold and with various cooling-rate variants. Tempering was carried out in a lead bath at a temperature of 650° C for 30 min, ensuring a high level of hardness of the alloys.

In selecting heat-treatment variants, special attention was paid to the relationship between cooling rate, in particular in the high- and low-temperature regions (step quenching), and bending strength and precipitation hardening. Along the way, certain additional details of precipitation hardening were identified (isothermal quenching).

In order of decreasing cooling rate during quenching, the following cooling variants were used: in water, in oil, in air, and with slow cooling, which was achieved by gradually withdrawing the specimen from the high-heating zone of the tube furnace (with visual monitoring of cooling by heat colors).

Table 1

Effect of various heat-treatment variants on hardness and bending strength of alloys 1 and 2

Quenching in water (exp. 3, see table), by creating excessively large internal stresses, leads to a very significant decrease in strength compared with the initial state of the alloys (exp. 1). The high degree of hardenability of the alloys results in low hardness; accordingly, after tempering the hardness increases substantially (exp. 4) in comparison with alloys tempered directly after casting (exp. 2). When determining the hardness of tempered specimens, in a number of cases penetration of the diamond indenter is accompanied by characteristic cracking, and the hardness readings are thereby distorted.

Obviously, sharp quenching leads not only to an increase but also to nonuniformity of internal stresses in microvolumes, which intensifies the nonuniform precipitation of dispersed particles of the strengthening phase, thereby causing an even sharper heterogeneity of the alloy state after tempering.

It is characteristic that quenching in water carried out after normalizing gives a higher level of precipitation hardening and strength (exps. 11 and 12). Evidently, with this double heating a certain role is played by a reduction of intracrystalline segregation and some equalization of internal stresses.

Quenching in oil creates a lower degree of retention of the supersaturated solid solution and gives lower hardness after tempering (exps. 5, 6) than in the preceding case, but provides a practically acceptable level of strength, lower than in the initial state of the alloys.

Normalizing gives unequal results for the selected alloys (exps. 7 and 8). For alloy 1, the strength level after high-temperature treatment and after tempering is higher than in the initial state (exps. 1 and 2); however, the degree of precipitation hardening (61 HRC) is insufficient.

In alloy 2, owing to a higher overall level of hardening in comparison with alloy 1, the hardness after tempering (exp. 8) is sufficiently high (63 HRC), but the strength proves to be lower than in the initial state of the alloy. As for the relatively low precipitation strengthening of both alloys, this is explained by the insufficient hardenability of the alloys during cooling in air.

The difference in strength levels compared with the initial states in the alloys is explained as follows. The high strength of alloy 1 is due to the lower stressed state of the alloy during quenching and to partial equalization of the alloy composition within individual grains. The nature of the microstructure does not change in this case. In alloy 2, the appearance of a polyhedral structure is accompanied by grain growth and, as noted above, premature decomposition of the solid solution is stimulated. It should be noted that during rapid cooling in the course of quenching in water, signs of naphthalene-like fracture are found in this alloy; they are barely noticeable in the fractures of specimens cooled in oil. Apparently, to a certain extent these shortcomings are also inherent in normalizing.

Slow cooling, as should be expected, reduces the hardenability of the alloys and the level of their precipitation hardening during tempering (exps. 9 and 10). The somewhat lower strength in comparison with the normalized state is associated with increased decomposition of the solid solution during cooling from high temperatures.

The positive effect of normalizing on the strength of alloy 1 indicates the expediency of combining it with variants of heat treatment that lead to a high degree of precipitation hardening, that is, with quenching in water and oil. The corresponding experiments sho-

showed that, if normalizing is not able to prevent the unfavorable consequences of rapid cooling in water (experiments 11 and 12), then its combination with cooling in oil gives very good results (experiments 13 and 14), substantially higher in comparison with the initial state of the alloy. Apparently decisive here is the leveling of the solid solution and of the inhomogeneity associated with the conditions of nonequilibrium primary crystallization. Repeated stresses and periodic processes of precipitation and dissolution of the excess phase may also play a certain role.

The effect of the cooling rate in the region of high and low temperatures during step quenching is more conveniently observed on alloy 2, in which, at the quenching temperature, the excess phase dissolves completely and a homogeneous polyhedral structure is formed.

In one case, cooling was carried out from the quenching temperatures to 600–700° (visually) in oil, followed by cooling in air (experiment 15); in another—to the same intermediate temperatures in air, with final cooling in oil (experiment 17).

The first variant of step quenching gives higher strength (experiments 15 and 16) than single-stage treatment in oil (experiments 5 and 6) or normalizing (experiments 7 and 8). After tempering, a somewhat lower hardness is observed (by 0.5 HRC), but a higher strength than with single-stage cooling in oil, and still more so than with normalizing. The explanation for this lies in the degree of fixation of the supersaturated solid solution and the stressed state of the alloy.

The second variant of step quenching (experiments 17 and 18) is inferior in all respects to single-stage cooling in oil (experiments 5 and 6), normalizing (experiments 7 and 8), and the preceding variant of step quenching (experiments 15 and 16).

Isothermal quenching (experiment 19) was carried out in a lead bath at 650° for 30 min. The hardness and strength obtained in this case are entirely insufficient, which is apparently explained by the excessively high temperature of the isothermal holding.

The considered variants of heat treatment of precipitation-hardening alloys make it possible to conclude that it is advisable to combine normalizing with subsequent quenching in oil. For alloys that are sensitive to increased stresses in the low-temperature region, but that possess good self-hardening capacity, it is advisable to use step quenching with initial cooling in oil and final cooling in air.

REFERENCES

1. A. D. Klement’ev. Investigation of cast precipitation-hardening cutting alloys. Izvestiya vuzov MVO SSSR, “Ferrous Metallurgy,” No. 3, publ. by the Siberian Metallurgical Institute, 1959.

2. A. D. Klement’ev. Microstructure and properties of iron–cobalt–tungsten alloys that harden during casting. Izvestiya TPI, vol. 209, in press.

3. A. D. Klement’ev. Effect of quenching conditions on the microstructure and properties of cast iron–cobalt–tungsten alloys. Izvestiya TPI, vol. 209, in press.

4. A. D. Klement’ev. Effect of additions and quenching conditions on the microstructure and properties of cast iron–cobalt–tungsten alloys. Izvestiya TPI, vol. 209, in press.