Wear of Metal Components

Wear of metal has been considered to be the second most costly loss-maker globally,

Following corrosion as the leader. If you were told that corrosion costs to the world has been determined to be $1930 billion dollars, out of a total world GWP for 2004 of $50,600 billion, you might be surprised. That’s 3.8% of the world’s total production that has disappeared into the air or the ground each year.

The figures are not quoted for wear, but it is believed that the losses are not far behind.

Most people come into contact with metal wear every day. We all know when that knife will not cut the steak anymore, or the barber has to sharpen his scissors (hair and paper are very abrasive media) and each male who shaves has to change his blade every 4 or 5 days, and these are just three of everyday examples, of which there are many. Wear is really quite complicated in the industrial workplace, as there are so many products in motion against so many different contacting surfaces, as described in this definition.

“Wear involves damage to a solid surface due to relative motion between that surface and one or more contacting substances, generally producing a progressive loss of material from the wearing surface. It may include oxidation, corrosion, creep, fatigue frictional effects, impact, pseudo machining of rough surfaces or cutting and deformation by abrasive particles.”

Whilst we may have learnt a great deal about certain types of wear in well-defined conditions (an example would be a shot or grit-blasting machine used to clean metal surfaces such as castings straight out of a sand mould or rusty brake shoes from vehicles) the multiplicity of surfaces and conditions means that the best choice in low-activity or unusual combinations of service are still relatively unknown.

Let us try to simplify some examples for you which will cover the various choice options in common use.

Did you know that:-

The best quality modern knives use high carbon – high chromium (400 series stainless steel) where the amount and size of the particles of the compound involving carbon and chromium, is carefully controlled to provide the ultimate in edge hardness and toughness. This involves carefully controlled to provide the ultimate in edge hardness and toughness. This involves “clean steelmaking”, often in a vacuum and being remelted once or twice to refine the grain.

High edge hardness at 55 – 60 Rockwell and good toughness has to be achieved (emphasis on hardness). The higher the carbon and chromium the more complex the compound and it gets harder and more brittle, so accurate control is critical. Cheap knives are often soft stainless steel (300 series) which cannot be hardened.

In contrast, hand saws and band saw blades need to be very flexible and should not fracture in service and which involves only plain high carbon steels which are hardened and tempered to form “tempered martensite” in the steel and no “hard” particles such as carbon/chromium compounds. The hardness is not so good (sacrificed for flexibility and toughness) so these products have to be constantly re-sharpened or replaced. Hardness at the tips of the teeth is only 45 – 50 Rockwell C.

Products which will suffer constant impact (grizzly bars in crushers, hammers in hammer mills, the cone and mantle of cone crushers etc.) use a special steel called Hatfield Steel (after its inventor Sir Robert Hatfield in 1882) or 14% manganese steel. This steel is unique, as it is used in its non-magnetic austenite condition, which is really quite soft (25 – 30 Rockwell C) and the impact on the surface causes localised martensite to form ( 60 Rockwell C) which ultimately wears away and is replenished by more impact and so on. This means that the substrate is very tough to impact stress and surface is very tough to wear. The problem occurs when less than ideal impact conditions exist, when the wear rate can be exceptionally fast.

In contrast, ripper teeth and the tooth-holder on excavators and diggers need to be about halfway between extremes. There is a great deal of impact when the bucket is driven at high speed, but the overburden being moved can be quite soft (coal, soil, some scoria types, limestone) so scuffing and gouging wear is prevalent. Ripper teeth are cast in medium carbon, nickel-chrome-molybdenum steels (Esco, Caterpillar, Komatsu as examples) and hardened and tempered to 45 to 48 Rockwell C where some wear is tolerated but the teeth and the holder do no rupture in service. In addition, the tooth-holder must be able to be welded to the bucket.

Wear plates in many industrial plants can be from wear resistant steels (Bisalloy 500 at 50 Rockwell C), the range of Ni Hard cast irons (NiHard 4 can accept some impact) through a range of alloy cast irons (15 Cr – 3 Mo – 2 Cu) to 27% chromium cast iron which is extremely brittle. Some equipment can use as much as three or four different wear-resistant materials in one piece of plant.

In consequence, it can be seen that you need to obtain expert advice on your particular wear problems to reduce the wear rate as low as possible by choosing the correct option.

Clive Jennings

Metallurgist

How can we Measure the Hardness of Steel?

Hardness is most commonly measured by one of two tests: Rockwell or Brinell

Brinell Test
A hardened steel ball is forced into the test surface and the diameter of the indentation is measured. The diameter of the indentation is related to the depth of the penetration: the softer the steel, the wider the indentation. A number is assigned: the higher the number, the harder the part is.

Rockwell Test
The Rockwell test is more commonly used and does not harm the surface as much as the Brinell test. An indenter point is loaded against the piece and the hardness is measured directly on the instrument. The different scales correspond to the different indenter tip used.