Welding , Hardfacing , Cladding and Cutting of metals: Choosing the Hardfacing Process and consumables

Choosing the Hardfacing Process and consumables:

Extensive work in research has resulted in the development of a wide range of alloys and welding procedures. The optimum alloy selection is made considering the component service conditions and feedback of the service performance.
For each industrial application and wear phenomena, there is a suitable welding consumable in the form of electrode, wire, powder, rod and strips to provide wear resistance.
Hardfacing can be applied by a number of welding processes. In most cases other than the Thermal spray processes, the equipment is identical or similar to that used for structural welding. Selection of the most suitable welding process for a given job will depend on a number of factors including:

Base metal composition:

The base metal chemical composition i.e. carbon and alloy content is an important factor in determining the process for hardfacing. Different processes may have different heat inputs to the base metal which render them unsuitable for some applications, e.g. manganese steels require low heat input and its tough properties may be lost (Becomes Brittle) if temperature during welding exceeds 260° C and so gas hardfacing would not be suitable.

Wear Factor Identification:

There are many types of wear. The wear predicted during the working life of the component has to be taken into account before deciding the process and consumables to be used to resist / minimize wear. In case of reclamation or preventive maintenance the component has to be inspected thoroughly to determine cause of wear before welding consumables can be selected.

Abrasion is the single most important mechanism which is the cause of all wear in industry:

Abrasion:

Abrasion (or metal-to-mineral wear) is further subdivided into:

High stress abrasion:

This occurs when the load between the abrasive material and wear material crushes the abrasive particles into smaller sizes (i.e. Ball mills and open gears in mining equipment)

Low stress abrasion:

Also referred to as scratching abrasion or erosion .This occurs when abrasive materials act as miniature chipping tools when they are transported along the surface in both a sliding and rolling action, in such a way that a reduction in particle size does not normally take place (i.e. feed Chutes, sand blast nozzles, pumps of slurry pipelines etc.).

Metal-to-metal wear:

Also referred to as adhesive wear. This occurs when there is movement of one component relative to the other with poor lubrication which results in scuffing, seizing and galling. Typical examples of metal-to-metal wear are

High Temperature:

Temperature, when it becomes sufficiently high, will lower hardness and strength and increase oxidation and subsequent scaling. More detrimental, however, are fluctuating temperatures, which lead to thermal fatigue cracking as in steel mill roll applications.

Corrosion:

Passive oxide films formed to protect the metal (Chromium oxide in SS) can be continuously removed in an abrasive wear environment. In a corrosive atmosphere this allows corrosion to erode the component at a constant high rate. The degradation of metals due to chemical reaction, whether by an acidic medium flowing through a pipe is also a reason for corrosion.

Impact:

This is the degradation of metals due to the repeated point loading of the component that causes the surface to fatigue rapidly and disintegrate (i.e. impact crusher or gyratory crushers).

Size, Shape and Function of the component:

The size and shape of the component is an important factor in choosing the process for Hardfacing. It determines whether the process will be manual, semi-automatic or fully automatic. Facilities available at site / workshop have to be taken into consideration. Surfacing large areas with gas or manual arc may often prove uneconomical. Parts of irregular shape may not be suitable for automatic applications.
It may not always be possible to manipulate heavy automatic equipment into areas where work must be done. Also, out-of position welding will limit the choice of processes. The type of Hardfacing consumable alloy required to be coated / welded on the component will depend on its function. Some processes are limited to certain consumable alloys available in the market.

Accessibility of the area to be Hardfaced:

The accessibility of the area to be surfaced will decide the process subsequently the consumable whether in form of electrode, wire, powder, rod and strips. The Hardfacing process whether manual or automated also depends on this.

State of the component:

This will depend on the whether the job is new and it needs a preventive hardfacing for extending its working life or whether it’s a repair and reclamation job. Large badly worn components requiring heavy rebuilding would be best suited to processes with high deposition rates.

Number:

If a large number of the same or similar items are to be Hardfaced, an automatic process would be most suited. The availability of the welding / thermal spray equipment and necessary automation will be a deciding factor in choosing the process.

Disclaimer

Welding , Hardfacing , Cladding and Cutting of metals