THE ECONOMICS OF HARDFACING
1.1 General
The importance of hardfacing is widely recognized. There are many documented instances of
savings that have been made in many industries by the use of hardfacing.
However, this does not imply that hardfacing is best in all
instances and an analysis of the costs involved in hardfacing and other
alternatives should be made. The
accurate calculation of costs involved may be quite complex, as many factors
must be considered to determine whether hardfacing is cost effective.
This section explains in detail the factors involved in
costing. It is necessary that these
factors be understood since their importance will differ from job to job and
from workshop to workshop. Labor and
overhead will always be major costs and will vary depending on company, size,
complexity, technology level and type of output.
It is essential that each organization keeps accurate cost
data concerning their own particular operation since reliable costing will be
required for quotations, comparison between maintenance methods, evaluation of
design and evaluation of welding procedures.
While labor and overhead, and material costs are the easiest
costs to identify in computing the cost of any hardfacing application, the
following factors should be considered, since they also will impact the total
cost of the job.
(a) Design
The design or geometry of the original
component will determine the amount of weld deposit required and ease of
applying it.
(b) Position
The highest deposition rates are
achieved in the flat and horizontal positions.
(c) Process
The welding process and process
parameters (procedure) chosen will govern the time required as determined by
deposition rate, deposition efficiency and operator factor.
(d) Costs
Additional costs associated with the
work such as transportation, machining, heat treatment and inspection.
A comparison of deposition rates, expressed as pounds per
hour, at different current setting is shown in Table 1.1.A.
Table 1.1.A.
|
Current/Amps Stick Electrode Semi-Automatic Submerged Arc |
|
110 1.8 - - |
|
125 2.0 - 3 |
|
150 2.6 - 4 |
|
175 3.0 6 4 |
|
200 3.7 8 6 |
|
225 4.2 10 8 |
|
275 7.0 12 10 |
|
325 9.0 14 12 |
|
375 - 16 14 |
|
425 - 18 17 |
|
475 - - 20 |
|
525 - - 24 |
Deposition efficiency is expressed as a percentage and is determined
by dividing the weight of metal deposited by the weight of the consumable
expressed as a percentage. Efficiencies
will vary with process as shown in Table 1.1.B.
Table 1.1.B. Deposition
Efficiencies
|
Process Deposition Efficiency % |
|
Manual (flux-coated electrode) 60-70 |
|
GTAW (TIG) 95 |
|
GMAW (solid and metal-cored) 95 |
|
FCAW (open-arc or gas-shielded) 85-90 |
|
SAW (submerged arc) 90-95
(wire only) |
|
Gas Welding (bare rod) 95 |
Operator factor or efficiency is expressed as a
percentage and is the effective deposition hours divided by total hours
expressed as a percentage. Time study
methods or arc monitors are used to determine the operator factor and some
typical factors are shown in Table 1.1.C.
|
Process Operating Factor % |
|
Manual 30 |
|
GTAW 20 |
|
GMAW 60 |
|
FCAW 60 |
|
SAW 65 |
*Semi automatic processes
Activities
that affect the operator factor include:
preparation, setting up, idle time, instruction, crane time, preheating,
deslagging, operator skill and degree of automation.
1.2. Hardfacing
Cost Calculation
1. Flux
Cost (submerged arc welding only) per lb of weld metal deposited
Unit
Price ($/lb) x Consumption Rate (lb/hr) ÷ Deposition Rate of Wire (lb/hr)
(generally
1 lb of flux is used for each pound of wire deposited)
2. Shielding
Gas Cost (gas shielded processed only) per lb of weld metal deposited
Unit
Price ($/ft3) x Flow Rate (ft3/hr) ÷ Deposition Rate of
Wire (lb/hr)
Example: Argon/CO2 @ $.15/ft3
Flow
Rate of 30 ft3 per hour $0.15 x 30 = $.64/lb
Deposition
Rate of Wire @ 7 lbs/hr 7
3. Power
Cost per lb of weld metal deposited
[Unit
price ($/kw hour) x kw per hour]÷ Deposition Rate (lb/hr)
Example Power Cost @ $0.126/kw hour
Kilowatts
= (welding volts x welding amps) ÷ 1000
Voltage
@ 21
Current
@ 200 amps $0.126 x 4.2 = $.08/lb
Deposition
Rate of Wire @ 7 lbs/lb 7
4. Welding
Material Cost per lb of weld metal deposited
Wire/Electrode
Price ($/lb) ÷ Deposition efficiency (%)
Example Welding Wire @ $5/lb $5 = $5.26/lb
Deposit
Efficiency @ 95% .95
5. Labor
Cost per lb of weld metal deposited
Labor
Charge ($/hr) ÷ [Deposition Rate (lb/hr) x Operating factor (%)]
Example Labor Cost @ $25/lb $25 = $5.95/lb
Deposition
Rate @ 7 lbs/hr 7 x .60
Operating
Factor @ 60%
6. Overhead
Cost per lb of weld metal deposited
Overhead
price ($/hr) ÷ [(Deposition Rate (lb/hr) x Operating factor (%)]
Example Labor Cost @ $10/lb $10 = $2.38/lb
Deposition
Rate @ 7 lbs/hr 7 x .60
Operating
Factor @ 60%
7. Total
Cost per lb of weld metal deposited
8. TOTAL COST OF HARDFACED COMPONENT
Width of hardfacing (in) x
Thickness of hardfacing (in) x Length of
hardfacing (in) x Total
cost/lb (7 above) x 0.3 = Total hardfacing cost
· Metric
Equivalent - Width of hardfacing (cm) x Thickness of hardfacing (cm)
x Length of
hardfacing (cm) x Total
cost/kg (7 above) x .008 = Total hardfacing cost
This calculation provides the means to determine the total
cost of depositing a hardfacing layer of a given thickness based on a given
width and length, with the deposited cost per pound previously determined.
1.3 Cost
Advantage
The
determination of the cost advantage of hardfacing over replacement enables
management to reach an informed decision on whether to hardface a particular
component.
The cost
advantage (CA) of hardfacing compared with a new component, in $ per unit
output, is given by:
Equation
1: CA = PCN PCR
OPN OPR
PCN = Prime cost of new
component (S)
= Cost of new component + downtime cost (new)
PCR = Prime cost of hardfacing ($)
= Cost of hardfacing + downtime cost (hardfacing)
OPN = Work output during life of new component
OPR = Work output during life of hardfaced
component
Where
there is a positive cost advantage, hardfacing is likely to be the best
solution, and an indication of the annual cost savings can be determined.
This
simple equation for cost advantage does not take into account factors such as
plant depreciation and replacement, cost of capitalization, taxation benefits,
and other factors that may affect the decision.
Most of
the data needed for the above calculation is relatively easy to obtain and will
be available as predetermined costs in many shops. The cost of hardfacing can be calculated using the method shown
in Table 1.3.
Other
factors may require consideration, such as the value of work output for the
hardfaced component which may not be known and has to be estimated initially.
1.4. Annual
Cost Savings and Extra Expenditure
Where significant extra expenditure is required to
implement a new hardfacing repair method which has a positive cost advantage
from equation 1, a supporting case will normally be required by
management. Data required will include
the annual cost savings and extra expenditure.
The annual cost savings (CS) expressed in
$/year by using hardfacing instead of a new component is given by equation 2:
Equation 2: CS
= CA x PR
CA = Cost advantage from equation 1 ($/unit
output).
PR = Production rate (Averaged over the year
including downtime, and here
assumed equal for both new and repaired components)
(unit output/year).
Example 1 – Shear blade for Cold
Punching ¾” (18 mm) Mild Steel
Blade cost =$100
Installation cost =$20/hr
x 0.5hr = $10
Downtime cost =$210.00/hr
x 0.5hr = $105
Prime cost - new =215
Work output - new =2000 cuts
Cost per cut =0.1075
Hardfaced
Component
Cost of hardfacing (see
1.2) =$60
Installation cost and downtime cost =$115 (same as new component since a second blade had
already been hardfaced.
Prime cost - hardfaced =175
Work output - hardfaced =11,500 cuts
Cost per cut =0.0152
Cost Advantage
(CA) = 215 minus 175 =
0.1075 - 0.0152
2000 11500
Note: The figures shown are hypothetical.
This shows a positive cost
advantage, meaning hardfacing is beneficial.
A reduced prime cost for the hardfacing method contributes to this
advantage, but the increased work output is the major contribution.
If the usage of the shear
blade increases to 46000 cuts per year the savings increases as follows:
= 0.0923 x 46000
= $4246 (i.e. annual saving by using hardfacing instead of
new component).
At the exit from the
furnace is the clinker crusher, on which the hammers had to be changed every
month. The shutdown of this crusher to
change the set of 36 hammers caused a production shutdown lasting 2 days.
·
Cost of Lost Production:
2 days at 1000
tons/day and $17 per ton $34,000
·
Cost of Set of 36 hammers:
Cost of 36
hammers $450.
·
Labor and overhead $300.
·
Total cost of machine downtime $34,750.
Annual loss through 12 shutdowns $417,000.
After a careful study, a hard overlay using a continuous
wire was applied and enabled the service life of the hammers to be prolonged by
100% - i.e. from one to two months.
Downtime and production costs were halved and productivity increased.
·
Cost of lost production: $34,000
x 6 = $204,000
·
Cost of hammers: ($450
+ $300) x 6 = $4,500
·
Cost of hardfacing overlay: $550
x 6 = $3,300
·
Total annual cost: $211,800
Total annual saving: ($417,000 - $211,800) $205,200