Hard Facing
Welding Filler Metals
Wear is one of the main causes of expensive material losses and cost intensive break downs of machines and equipment. Frequently, the actual wear, i.e. the unwanted erosion due to the mechanical action of all sorts of substances and agents on the workpiece surface is also accompanied by corrosive effects.
Consequently, the most important measure at the disposal of the practising engineer is the use of hardfacing either as a method of repair, e.g. the restoration of the original condition or as preventive protection in the form of hardfaces against wear or cladding against corrosion. Wear is usually the result of several factors, which means that the choice of the right welding filler material is especially important for wear resistant hardfacing. The purpose of the present publication is to help the user make this choice.
First of all, some important rules need to be followed:
Pre-heating
Before welding is started, it should be ascertained whether pre-heating is required. There are usually two reasons for this:
- The base metal is susceptible to cracking, e.g. highly carburised steels
- The weld metal is susceptible to cracking
Allowance must be made for the fact that a weld, depending on its hardness and wear resistance, is sensitive to some degree to shrinkage stresses and tends therefore to crack.
If properly used, no metal will be deposited from the electrode. To cut a section (e.g. a circle) from a plate, a hole is first pierced in it by concentrating the arc at one spot and pushing the electrode into the molten pool until it melts through to the other side. The hole can be enlarged or extended into a cut as required.
This is particularly noticeable when welding hypereutectic Cr hard alloys with 2% to 5% C and up to 33% Cr, the weld metal of which almost always contains cracks. Because the PACWELD filler materials are normally used only where the wear is by abrasive action, shrinkage cracks are not important here. In all other cases where a crack-free weld is required, it is necessary to carry out pre-heating. Normally to temperatures of about 200°C to 400°C.
High alloy steels need to be pre-heated correspondingly higher, i.e. to 400°C to 600°C; in some cases, a post-heating treatment appropriate to the base metal has to be carried out.
Buffer Layers
There is often a considerable difference between the thermal expansion of the base and that of the weld. To compensate for this difference, buffer layers are deposited, preferably with elastic, austenitic welding filler materials, e.g. PACWELD 101, PACWELD 103, PACWELD 105, PACWELD 535.
It must be decided on the basis of the particular application whether and when it makes sense to deposit buffer layers. If the base metal consists of work-hardened high manganese steel, the surface must be grounded or a buffer layer must be deposited with the types of filler material already mentioned, otherwise cracking off of the wear resistant weld metal must be expected.
Filling
In cases where wear and erosion are considerable it is not sensible, either economically or technically, to hardface the entire area of lost metal with wear resistant weld metal. It is recommended that the level is filled up roughly to the original contour. this is to be done for example:
- For low alloy steels, cast steel with PACWELD 103 / PACWELD 105
- For high alloy tool steels with PACWELD 101 (maximum 2 to 3 layers)
- For high manganese steels with PACWELD 535 (works hardenable)
Figure 1
Hard Facing Techniques
Hardfacing may be applied as a continuous surface, as stringer beads or dot patterns. The type of deposit employed will depend on the nature of the environment and the area to be covered.
Where a continuous surface is required, care should be taken to ensure that weld runs overlap (see Figure 1) and for fine abrasion or erosion the weld runs should be deposited at right angles to the direction of travel of the abrasive material.
Stringer Beads
Applications where large irregular pieces of abrasive material are present, for example in quarries, hardfacing of items such as bucket teeth requires continuous stringer beads which run parallel to the direction for the abrasive material. The 'rock' etc, will then ride on top of the weld bead leaving the base material unaffected.
Conversely, fine abrasive material like sand and glass requires a stringer pattern running across the direction of flow to produce pockets of captured material which form a 'material on material' barrier to wear.
Some environments like earthmoving will involve a combination of the two conditions and the best hardfacing patter is therefore a criss-cross or chequered patter.
Dot Patterns
Secondary wear areas requiring less protection are quickly treated using a regular dot pattern which allows material capture between the dots while providing high spots during impact and sliding contact.
Dual Electrode Technique
Where very high deposition rates are required, a second tubular electrode can be introduced into the weld pool in a similar way to a gas welding filler rod. Currents of up to 300 amps may be used with this technique and with a little practice the welder will be able to combine the right degree of arc movement between workpiece and filler rod to ensure thorough fusion to the parent metal.
Cast Welding
The dual electrode technique may also be combined with a graphite or copper mould into which very thick deposits of hardfacing can be cast welded. This technique is particularly useful for rebuilding/protecting hammers, bolt heads etc.
Where severely worn components are being rebuilt alternate layers of Grade 33/35 and 100/108 should be deposited to minimise stress. These should be applied after restoring the component to its approximate shape with a conventional electrode giving a deposit compatible with the original base metal. Typically, a low grade 316 type stainless steel welding rod may be used.