Hardfacing and Surfacing by the Numbers
Chapter 10 of 10 · 10 min read
Economics and the wear record
Compare new, rebuilt and hardfaced wear parts by cost per tonne, include the change-out that price comparisons forget, find the break-even life, and keep the wear records that make the sums honest.
Every chapter of this book has ended, one way or another, with a question of money. Is it cheaper to rebuild a hammer than to buy a new one? Does a hardfaced edge earn back the cost of its deposit? Is overlay plate a better buy than a week of welding? The answers cannot come from the price of the part alone, because a wear part is not bought to be owned; it is bought to be worn away. What the customer is really buying is tonnes crushed, hectares worked or hours pumped, and the only fair comparison between two parts is what each costs per unit of that work.
This chapter turns the running case’s hammers and bucket edges into costs per tonne, shows why the cost of changing a part matters as much as the cost of the part, finds the break-even life a rebuild or a hardfacing must reach to pay, and sets out the wear records that make those sums trustworthy. It closes by bringing the book together.
Cost per unit of work
The basic measure is simple: everything it costs to put a part into service, divided by the work it does before it must be changed.
- New set
- A$6,000, lasts about 120,000 t
- Rebuilt set
- A$2,800, lasts about 85% as long, about 102,000 t
- Change-out
- about A$6,000 each time, whichever set is fitted
- new: (6,000 + 6,000) ÷ 120,000 ≈ A$0.100 per tonne
- rebuilt: (2,800 + 6,000) ÷ 102,000 ≈ A$0.086 per tonne
- at 600,000 t a year, the rebuilt sets save about A$8,200 a year
The rebuild wins, but by much less than the prices suggest. A rebuilt set at a little under half the price of a new one looks like a saving of more than half; per tonne, it is a saving of about 14%. The difference is the change-out, which costs the same whichever set goes in, and which the rebuilt set needs more often because it wears out sooner.
The cost the price comparison forgets
Leave the change-out out of the sum and the comparison changes completely. A new set would cost A$6,000 ÷ 120,000 = A$0.050 per tonne, and a rebuilt one A$2,800 ÷ 102,000 ≈ A$0.027 per tonne, a saving of nearly half. That is the figure a customer reaches by comparing quotes, and it is wrong, because every set that wears out has to be taken out of the crusher and replaced, and that work and the lost production that goes with it are part of what the hammers cost.
The change-out cost varies enormously between parts. Changing the hammers in an impact crusher means stopping the crusher, isolating it, opening it, unbolting and lifting heavy parts, and restarting, and every hour of that is an hour without crushed rock. Changing a tillage point takes a farmer a few minutes with a hammer. Changing a chute liner in a terminal may mean stopping a ship loading. The higher the change-out cost, the more a longer-lasting part is worth, and the less a cheaper, shorter-lived one saves.
The break-even life
The arithmetic can be turned around to ask how long a rebuilt part must last to pay at all.
With the quarry’s change-out cost of about A$6,000, the new set costs A$0.100 per tonne, so a rebuilt set at A$2,800 breaks even when (2,800 + 6,000) ÷ 0.100 = 88,000 tonnes, about 73% of the new set’s life. Rebuilt hammers that last 85% pay; rebuilt hammers that last 70% cost more than new ones, even at less than half the price. If the change-out cost were twice as large, the rebuild would need to reach about 82% of new life to pay.
The break-even figure is the most useful single number the workshop gives a customer, because it says what the rebuild must achieve rather than claiming what it will. It also tells the workshop where its quality matters most. A rebuild procedure that lets hammers come back embrittled, unbalanced or a few kilograms light can easily cost more than the rebuild saves.
Bucket edges
The same method compares the bucket edges of Chapter 2.
| Part cost | Change-out | Life | Cost per tonne | |
|---|---|---|---|---|
| Plain replacement edge | A$1,800 | A$900 | about 60,000 t | about A$0.045 |
| Hardfaced replacement edge | A$2,450 | A$900 | about 120,000 t | about A$0.028 |
The hardfaced edge costs about A$650 more and lasts about twice as long, and per tonne it is about 38% cheaper, saving the quarry about A$10,250 a year at 600,000 tonnes. Its break-even life is (2,450 + 900) ÷ 0.045 ≈ 74,000 tonnes, about 1.24 times the plain edge’s life, so the hardfacing pays as long as it adds a quarter or more to the edge’s life. Here the change-out cost works for the more expensive option: because the hardfaced edge needs changing half as often, it saves half the change-outs as well as half the edges.
Downtime and planned shutdowns
Not every change-out costs the same. A part changed at a planned shutdown, when the plant is stopped anyway for other work, costs only the fitters’ time; a part that fails between shutdowns stops production unexpectedly and may damage other parts. The quarry’s crusher hammers are changed at scheduled shutdowns, so the workshop’s aim is a hammer that reliably lasts from one shutdown to the next, not one that lasts as long as possible on average. A rebuilt set that lasts 85% of new life on average but sometimes fails early may be worth less than one that always lasts 80%.
This is where the service interval of Chapter 9 joins the economics. If shutdowns fall every 100,000 tonnes, a set that reliably lasts 102,000 tonnes fits the schedule, while a new set that would last 120,000 tonnes is changed at the same shutdown with 18,000 tonnes of life unused. Under that schedule, the new set’s real cost is (6,000 + 6,000) ÷ 100,000 = A$0.120 per tonne, and the rebuilt set’s advantage grows. The cheapest part is the one whose life matches the plant’s rhythm.
The value of a spare set
Rebuilding needs a spare set, because the worn set comes out at the shutdown and the rebuilt one goes in, and the worn set is then rebuilt in time for the next shutdown. The spare set is money tied up in stock, but it is also what makes careful rebuilding possible: with a spare on the shelf, no rebuild is ever rushed, so the rotation and cooling of Chapter 5 are always followed. The quarry’s failure with a hurried rebuild, described there, cost far more than a spare set. The workshop counts the spare set as part of the rebuild scheme when it presents the economics, not as an extra.
Wear records
All of these sums depend on knowing how long parts last, and that comes only from records. A wear record does not need to be elaborate, but it must be kept for every part, every time.
- Avoid: Part, position and identificationserial or set number; which machine and position
- Avoid: Treatmentnew, rebuilt or hardfaced; procedure, filler and pattern used
- Avoid: Mass or key dimensions when fittedweighed or measured at fixed points with a template
- Avoid: Work donetonnes, hours or hectares from the plant's own counters, not estimates
- Avoid: Mass or dimensions when removedmeasured the same way as when fitted
- Avoid: Reason for removalworn to limit, cracked, spalled, broken, or changed at a shutdown
- Do: ? Photographs of the worn partshowing where the wear concentrated
From the record, the wear rate follows directly. The quarry’s new hammers lose about 15 kg in about 120,000 tonnes, about 0.125 kg per thousand tonnes; the rebuilt hammers lose their 12 kg of added deposit in about 102,000 tonnes, about 0.118 kg per thousand tonnes. The rebuilt deposit wears at much the same rate as the casting, which is what a manganese build-up should do; it lasts less long because it adds less metal, not because it wears faster. That distinction matters, because it says the way to longer rebuilt life is to restore more of the worn profile, within the build-up limits, not to change the filler.
Measuring the same way
Wear records are only as good as the consistency of their measurements. A hammer weighed with mud on it in one shutdown and clean in the next shows a false wear rate; an edge measured at its corner one time and its middle the next gives nonsense. The workshop weighs parts clean, on the same scales, measures dimensions at marked points with a template, and takes the work done from the plant’s own counters rather than from memory. It also records parts that failed early and why, because averaging a cracked hammer into the wear rate of sound ones hides the failure instead of explaining it.
Field trials
The surest way to compare two treatments is to run them side by side in the same service. The workshop’s bucket-bottom trial in Chapter 9 put three patterns on buckets working in the same pit over the same season, and the comparison was fair because everything else was the same. Trials on one bucket each are better than nothing but can mislead, because one bucket may have worked harder material or a rougher operator; trials on several parts of each kind, or with treatments alternated between positions on the same machine, are better. The workshop writes a short plan for each trial, saying what is being compared, how it will be measured and how long it will run, before the parts go into service, so that the result cannot be argued away afterwards.
Pricing the work honestly
The economics also guide how the workshop prices its own work. Quoting a rebuild at a little under half the price of a new part invites the customer to compare prices; presenting the cost per tonne, with the change-out included and the break-even life stated, invites the customer to compare value, and it is a comparison the workshop can stand behind because it is built from the customer’s own records. Where the records show that a rebuild will not pay, because the part’s change-out cost is low and the rebuild’s life is short, the workshop says so. A customer told honestly that a new part is the better buy is more likely to trust the workshop’s next recommendation.
Bringing it together
This book began with how surfaces wear: abrasion, impact, erosion, adhesion, corrosion and heat, and the recognition that the right deposit depends on which of them dominates. It moved through building up and hardfacing, the dilution that changes what a deposit really is, the families of hardfacing alloys and their trade between hardness and toughness, the special care of manganese steel, the carbide deposits that resist abrasion and crack in a harmless pattern, the processes that lay deposits, the preheat and buffers that keep them from cracking, and the patterns and thicknesses that put the alloy where it does most good.
Each of those choices shows up in the end as a cost per unit of work. A deposit chosen for the wrong mechanism wears out early; one diluted too much is softer than its data sheet; one laid too thick spalls; one laid by too slow a process costs too much to lay; one placed in the wrong pattern protects the wrong places. The wear record is how a workshop finds out which of these has happened, and the cost per tonne is how it shows a customer what the right choice is worth. Machine Part Repairs by the Numbers covers the wider repair decisions that hardfacing sits within, and Welding Health and Safety by the Numbers covers the fume and heat that hours of surfacing bring.
A failure: the cheap rebuild
This is an illustration. A plant elsewhere had changed to a cheaper rebuilder for its crusher hammers, whose rebuilt sets cost about a third of a new set rather than a little under half. Nobody kept wear records, and the change was judged a success on price.
A year later, the plant’s maintenance planner counted the shutdowns. The cheap rebuilt sets had lasted, on average, about 60% of a new set’s life, and two had cracked between shutdowns, stopping the crusher. At a change-out cost similar to the quarry’s, the cheap sets cost about (2,000 + 6,000) ÷ 72,000 ≈ A$0.111 per tonne, more than new hammers, before counting the two unplanned stoppages. The plant went back to a rebuilder whose procedure followed the filler maker’s guidance, and started keeping the records that would have shown the problem within months.
Exercise: tillage points per hectare
A farm runs a cultivator with 30 points over about 2,000 hectares a year. A plain point costs A$40 and lasts about 80 hectares; a point hardfaced in the workshop costs A$65 and lasts about 160 hectares. Changing a point costs about A$15 in labour each time. Prices exclude GST. Compare the cost per hectare of each, and the yearly cost for the cultivator.
Per point, the plain point costs (40 + 15) ÷ 80 ≈ A$0.69 per hectare, and the hardfaced point (65 + 15) ÷ 160 = A$0.50 per hectare.
Each of the 30 points works the whole 2,000 hectares, so the yearly cost is about 30 × 2,000 × 0.69 ≈ A$41,250 with plain points and 30 × 2,000 × 0.50 = A$30,000 with hardfaced ones, a saving of about A$11,250 a year. The hardfaced point breaks even at (65 + 15) ÷ 0.69 ≈ 116 hectares, so it pays as long as it lasts about 45% longer than a plain point. The farm’s own records, kept for a season, would show whether it does.
