MAGNETIC ANALYSIS OF HIGH-MANGANESE AUSTENITIC STEEL IN STRONG FIELDS

Yu. D. Novomeiskii, A. I. Kolchuzhkina, V. A. Teremetskii

(Presented by the scientific seminar of the Department of Applied Mechanics, TIRiET)

Magnetic analysis of austenitic alloys makes it possible to investigate even the smallest phase changes by volume ($\gamma \to \alpha$ transformation, carbide precipitation, etc.), which have a great influence on the properties of these alloys.

The first work on the investigation of high-manganese steel in strong fields dates to 1887 [1]. In later works, relatively weak magnetic fields were used.

It is known that, in the cast state, the magnetic susceptibility of manganese austenite is higher than that of nickel austenite; in order to reduce the magnetic susceptibility, heat treatment of manganese steels is introduced, especially when they are used for parts of nonmagnetic design.

Additional alloying of austenitic alloys also affects their magnetic properties.

We investigated the influence of niobium additions on the magnetic susceptibility of manganese austenite in the cast state and after heat treatment.

Figure 1 shows a diagram of the apparatus for determining the relative value of the magnetic susceptibility of steel by the ponderomotive method in strong fields. The apparatus consists of an analytical balance having, instead of one pan, a holder made of nonmagnetic material with the test specimen.

In the equilibrium state, the specimen adjoins the end face of the core of an electromagnet producing a field of about 10,000 oersteds.

The relative value of the magnetic susceptibility is estimated from the force required to detach the specimen from the electromagnet core. The electrical circuit of the apparatus makes it possible to regulate the current in the electromagnet winding in order to maintain a constant field. The experimental heats were produced in crucibles with a basic fluorite lining and, in chemical composition, corresponded to GOST 2176-57 with niobium additions from 0.1 to 0.7% (by charge calculation).

After casting into a chill mold, specimens were cut out and ground on all faces to dimensions of $10 \times 10 \times 10$ mm.

Figure 2 presents the dependence of the magnetic susceptibility of high-manganese steel on the niobium content. The decrease in magnetic susceptibility in alloys with a niobium content up to 0.3% can be explained by an increase in the concentration of niobium in austenite and by refinement of the structure. Steel of optimal composition

Fig. 1. Diagram of the apparatus for magnetic analysis of austenitic alloys in strong fields.

1 — specimen; 2 — Armco-iron core (diameter 30 mm);
3 — electromagnet winding (8000 turns, diameter 0.8 mm);
4 — holder made of nonmagnetic material; 5 — analytical balance;
6 — selenium rectifier (64 volts, 12 amperes);
7 — ammeter with alternating-current scale (10 amperes);
8 — resistance variator; 9 — step-down transformer.

Fig. 2. Graph “Niobium content—magnetic susceptibility.”

Fig. 3. Microstructure of cast specimens (×600):
a—0.1% niobium; в—0.4% niobium;
б—0.7% niobium.

(0.3–0.4% niobium) has a minimum magnetic susceptibility. A further increase in the concentration of niobium in the steel leads to an increase in the content of the carbide phase and raises the magnetic susceptibility; a coarsening of the structure is also observed. Photographs of microsections (Fig. 3) confirm the indicated structural changes.

The linear dependence of the magnitude of the magnetic susceptibility on the niobium content after quenching (Fig. 2) corresponds to the amount of carbides in the steel and to the concentration changes associated with them, which agrees with the data of [2].

Quenching was carried out from a temperature of 1080°C in water (the usual regime for steel G13L). Repeated quenching of the steel (Fig. 4) changes the magnitude of the magnetic susceptibility of the steel in the following way: in specimens with a niobium content below the optimum, a decrease in magnetic susceptibility is observed after the first quench compared with the as-cast condition, owing to deep dissolution of manganese and niobium carbides; this process continues after the second quench as well, but to a lesser degree.

A similar mechanism of transformation suppression may also be assumed in alloys with a niobium content above the optimum.

In steels of optimum composition (with 0.3–0.4% niobium), an increase in magnetic susceptibility is observed after the first quench.

Apparently, the adopted quenching temperature is insufficient for austenitizing steel with niobium, but it promotes grain growth, which is the prevailing factor in increasing the magnetic susceptibility.

It may further be assumed that after the first quench the concentration of niobium in the austenite increases and impedes grain growth during subsequent heatings; after secondary quenching of the steel, further austenitization of the steel takes place; the magnetic susceptibility decreases.

The third and subsequent quenches lead to a slight increase in magnetic susceptibility due to the \(\gamma \to \alpha\) transformation.

Steel G13L with niobium additions in an amount of 0.3–0.4% was used for manufacturing parts of instruments for nonmagnetic applications without heat treatment after casting. Long-term tests under production conditions showed good results; scarce nickel-containing steel was successfully replaced, the service life of the parts and the reliability of their operation increased significantly, and the cost of the parts decreased.

Fig. 4. Effect of niobium on the magnetic susceptibility of high-manganese steel after repeated quenches

Conclusions

1. An apparatus has been developed for magnetic analysis of austenitic alloys in strong fields.

2. It has been established that additional alloying of high-manganese austenitic steel with niobium in an amount of 0.3–0.4% (by charge composition)

provides the minimum value of the magnetic susceptibility without additional heat treatment.

1. The effect of repeated quenching on the magnitude of the magnetic susceptibility of high-manganese austenitic steel with niobium has been investigated.

REFERENCES

1. G. A. Ewing, W. Low. Report of the British for the Advancement of Science, 1887, Manchester.

2. Nakagava Ruiti. G. Iron and Steel Inst. Japan, 1962, 48, No. 11, 1504—1506.