ON THE WELDABILITY OF HIGH-MANGANESE AUSTENITIC STEELS

Yu. A. Evtyushkin

(Presented by the scientific seminar of the Department of Equipment and Technology of Welding Production)

At present, in developing new alloys it is necessary to take into account a number of requirements not only of a technological nature, such as: casting properties, forgeability, machinability by cutting, but also weldability conditions. It should be noted that in a number of cases weldability is one of the main requirements imposed on an alloy. In this case, assessment and investigation of weldability are necessary for developing the technology and materials for surfacing. In addition, at present a number of enterprises have raised the question of welding parts of mining equipment, dredge buckets, made of steel Г13. However, there are no data on this question in the literature. Not a single welding handbook, including the most complete catalog of electrodes of the latest edition, gives even one grade of electrodes for welding steel Г13 [1].

The weldability of high-manganese austenitic steels may be assessed by considering the following factors:

1. Obtaining alloyed austenite with special properties in the weld metal.
2. Preventing the decomposition of austenite and spalling in the heat-affected zone.
3. The tendency of the weld to form hot cracks.

The possibility of obtaining austenite with special properties, i.e., an analysis of physical weldability, can be made on the basis of the phase diagram (Fig. 1, [2]). In the present case, the portion of the diagram adjoining the iron side is of interest. Manganese belongs to the alloying elements that expand the gamma region. When manganese and carbon are jointly present in steel, the effect of expansion in the gamma region is intensified. If the carbon and manganese contents are increased simultaneously, then even upon reaching 3% carbon and 30% manganese no significant change in properties is observed in comparison with the classical composition. The presence of lines of secondary transformations on the ternary phase diagram of the iron–manganese–carbon system is very conditional (Fig. 2). This is confirmed by the phenomenon of transcrystallinity of welded seams. Thus, under appropriate conditions it is possible to obtain austenite capable of strengthening under cold deformation and ensuring high wear resistance.

The thermal weldability of manganese steels is assessed by their relation to the thermal cycle of welding. The austenitic structure of the weld, stable at room temperature, is very sensitive to the rate of—

cooling rate (Fig. 3, [3]). Under real conditions of cooling of a part during welding or surfacing, precipitation of carbides in the weld metal can be observed; hence there is a decrease in the stability of austenite and its decomposition with the formation of a ferrite–carbide mixture of high hardness. To increase the stability of austenite, it is necessary to reduce the carbon content and introduce stabilizing elements [4, 5].

Fig. 1. Phase diagram of iron—manganese

Fig. 2. Section of the ternary phase diagram of the iron—manganese—carbon system

Tests showed that manganese steel with 0.8–1.2% niobium is less sensitive to the thermal cycle during welding and surfacing. Decomposition of austenite in steel with niobium begins at higher temperatures than in steel G13. When comparing the structure of weld metal of type G13 and G13 with niobium after holding at 600–650°, it is seen that the degree of decomposition of austenite in the steel with niobium is considerably smaller.

Fig. 3. Diagram of isothermal decomposition of austenite in high-manganese steel G13

With normal heat input, during welding of steel with niobium no precipitation of carbides is observed in the heat-affected zone, even without the use of special technological measures. The reserve of austenitic structure in the near-weld zone makes it possible to prevent spalling. However, manganese welding wire is in short supply, and wire with a low carbon content is generally not produced. Experiments were carried out to study electrodes based on common austenitic steels of the 18-8 type with a fluorite–calcium coating. These electrodes with additions of niobium may be recommended for welding manganese steels. The thermal weldability of these steels has much in common.

One of the reasons for the poor weldability of high-manganese austenitic steels is the high content of phosphorus, silicon, and oxygen [6]. These impurities form low-melting eutectics, which are located along grain boundaries. Under the action of the thermal cycle

during welding and surfacing, because of the eutectic the strength of the grain boundaries decreases, and conditions arise for the formation of hot cracks. In the presence of shrinkage stresses, cracks can lead to the destruction of the product. In G13 steel this phenomenon is aggravated by the precipitation of carbides along the grain boundaries; therefore, the cleanliness of the grain boundaries when welding manganese steels is of great importance.

Fig. 4. Microstructure of the weld metal without niobium ×500

Fig. 5. Microstructure of the weld metal with niobium ×500

By introducing niobium into the weld metal in optimal amounts, it is possible to clean the grain boundaries to some extent. Figures 4 and 5 show, respectively, the microstructure of G13 steel and G13 steel with niobium. From the comparison it is evident that niobium plays a useful role. Figure 6 presents the change in the microstructure of the heat-affected zone and of the weld when welding austenitic manganese steels. Thus, technological weldability can be increased by introducing niobium into the electrodes and by using components that are low in sulfur and phosphorus.

Fig. 6. Microstructure of the welded joint

REFERENCES

1. Catalog Electrodes for Arc Welding and Surfacing. Naukova Dumka Publishing House, 1967.

2. E. Houdremont. Special Steels, translated from German, Vol. I, Metallurgizdat, 1959.

3. I. I. Frumin. Automatic Electroslag Surfacing. Metallurgizdat, 1961.

4. Yu. D. Novomeiskii, V. I. Livshits. Properties and Application of High-Manganese Austenitic Steel. TTU Publishing House, 1964.

5. V. N. Kashcheev, Yu. A. Evtyushkin, Yu. D. Novomeiskii. Wear-resistant surfacing of impact-action crusher teeth. Scientific Communications of KrasTSNII, 1963.

6. E. N. Morozovskaya. Structure of the near-weld zone during surfacing on G13L steel. Automatic Welding, No. 7, 1967.