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HARDFACING

Hardfacing is gaining more and more importance in the most varied fields of modern engineering. In the case of wear, corrosion and other types of surface attack, the application of hardfacing permits the economical reconditioning of many structural members. Reconditioning may comprise workpieces subjected to metal-to-metal wear. Such as rollers, crane wheels, shafts, rails or which are subjected to metal-to-mineral wear, like building machinery, conveyor screws, excavator bucket edges, etc.

The possibilities of hardfacing not only lie in the field of reconditioning, but more and also in the manufacture of new tools and components. In this manufacturing process, a non-alloyed or low- alloy steel is used as carrier material to which are imparted special properties by welding on a suitable high quality weld metal. decisive factor in hardfacing is the knowledge of the wear conditions, that is, whether it is a frictional wear caused by metal or minerals, corrosion, cavitation, erosion or temperature. The selection of the welding consumable to be used will depend on these conditions. Besides this, it is most important to know the composition of the base metal to be surfaced, because of the measures likely to be taken, such as preheating and/or post weld treatment.

Depending on the use of the workpiece to be hardfaced, the weld metal shall meet the most varied demands. These can be cited as follows:

1. Hardness in the as-welded condition
2. Red hardness under thermal loads
3. Resistance to metal-to-metal wear
4. Abrassion resistance to frictional and grinding wear
5. Impact strength
6. Corrosion resistance
7. Resistance to scaling.

In some cases, more than one of these modes of load may occur simultaneously, so that the weld metal must posses the respective properties. Hence, a conscientious selection of the proper weld metal will be a must. Of course, there doesn’t exist a weld metal possessing all these properties in full measure. Nevertheless, in most cases it will be possible to find a satisfactory compromise in practice.

Welding Consumables

Welding consumables for hardfacing are standardized in DIN 8555. In this standard, the consumables are classified in alloy groups and hardfacing levels. Besides this, the manufacturing mode (e.g. GO = cast, GF = filled) as well as the properties of the weld metal (e.g. C = corrosion resistant, K = capable of being strain hardened) are indicated. By this, every weld consumable meeting this standard, can be specified as to it’s most essential properties.

Hardfacing of Tools

An interesting field of application in hardfacing is reconditioning of hardened tools, such as shear blades, punching dies, cutting edges, deep-drawing tools, etc. For this purpose, the so-called “step hardening weld method” has proven a success. Welding is carried out in the delayed transformation range which is situated between the perlite and bainite zones of TTT-curves of the steel in question. Thereby, the hardenability of the steel is taken into account. Generally, the temperature range in which welding shall be carried out is from 400°C to 600°C. This means, that step welding is based on the fact that in such steels the transformation of austenitic is clearly delayed within a certain temperature range.

The sequence of operations is as follows:

a) Slow heating of the workpiece to its hardening temperature. At this temperature the structure becomes austenitic.
b) Cooling to step temperature; that is, to the range in which the delay in austenitic transformation (according to TTT-diagram) is most pronounced.
c) Welding the workpiece at this temperature.
d) Cooling the workpiece to room temperature in still air or better in oil (50°C to 150°C).
e) Tempering to the respective temperature which in the case of high-speed steels lies in the range of 550°C to 570°C.
f) Machining the weld area.

For the solution of the many hardfacing problems, MAGMAWELD offers a complete line of welding consumables, such as covered electrodes, flux cored wire electrodes, solid wire electrodes for gas metal arc welding and submerged arc welding fluxes. A comprehensive description of the welding consumables for hardfacing applications is given in the respective product datasheets.

In the case of special problems, Magmaweld engineers will be glad to assist you.