What are the main parameters for Welding Costing?
For preparing estimates & costing of arc-welded fabrications, one must have technical calculation data to determine direct welding costs based on the following direct cost factors:
- Consumption of electrodes/ Filler wire to find the cost of consumables,
- Arc time to calculate welders hours cost, and
- Current consumption to consider the cost of electricity.
Cost Calculation for Welding consumables/ Electrode
The first step is to determine the quantity of electrodes required to complete the unit length of a given type of joint.
Once this is known, the arc time and current consumption can be determined by a simple calculation. As an example, we can take a 10 mm thick single-V butt joint of length 1 m, as shown in the below figure.
Here, the cross-sectional area, divided into segments A, B, C, and D, can be calculated by simple arithmetic. This area multiplied by ‘m’ gives the volume per meter length, which can be expressed in cm3.
Density of steel is 7.8 g/cm3. Using this factor, the weight of weld-metal required to fill the joint can be calculated.
With the above calculated total weight, by multiplying the cost of welding consumables (rod/ filler wire), we can determine the cost of welding consumables.
Weld Volume calculation for fillet weld
Standard tables for the weight of weld-metal for common fillet weld joints are given below for reference:
There are several methods is to deposit weld-metal from, say, five electrodes on a plate, using the mean amperage of the current range and leaving a stub of 50 mm in length, and determining its weight.
The Fillet Weld volume calculation is fairly simple once you have all of the measurements. First, you need to calculate the area of the fillet weld based on throat size using the formula given below.
Then, multiply two measurements i.e. area & weld material density together to get the total volume or Weld Weight of weld material required as explained below.
Alternatively one can refer to the electrode manufacturer for the quantity of electrodes required to deposit 1 kg of weld-metal, or the weight of weld-metal obtained from each electrode.
Weld Volume calculation for Butt weld
Weld volume for typical butt weld segments are given in the below table for reference purposes.
When doing these calculations, even small departures from prescribed dimensions in edge preparation or fit-up lead to significant increases in the volume of weld-metal required for the joint.
For example, an increase of 3° in a 60° V joint included angle leads to an increase of 5% in cross-section. The same is the case for inaccurate fit-up.
What is Electrode Deposition Efficiency?
Rather than providing the above-mentioned information, electrode manufacturers usually mention the deposition efficiency (DE) of the electrode in the electrode manufacturers’ handbooks.
Deposition Efficiency or DE is defined as the ratio of the weld deposit to the weight of core wire melted, expressed as a percentage.
For example, if an electrode has 90% DE, 90 g of weld-metal will be obtained from every 100 g of the core wire melted in the arc.
Deposition efficiency depends significantly on the type of electrode used, and lies between 80 and 95% for common types which do not contain iron powder and do not have appreciable proportions of ferroalloys, because some loss of metal occurs through spatter and evaporation in the arc.
Values over 100% are obtained when sufficient iron powder is incorporated in the flux covering.
Example calculation for electrode weight
You can calculate the weight of say, 400 mm length of 4 mm diameter wire, which is the length of core wire that is melted from one stick welding electrode, 4 x450 mm long, after allowing for 50 mm stub length.
lf this weight is AA, and the DE of the electrode is 90%, the weight of weld deposit yielded by the electrode is AA x 0.90. If the DE is 120%, the weight would be AA x 1.20.
Knowing the weight of weld-metal required to fill a joint and the weight of weld-metal deposited by one electrode, the total number of electrodes required for the joint can be estimated by using simple arithmetic.
Welding Arc Time Calculation
Arc time is pure welding time required to melt the quantity of electrodes necessary for completing a certain joint. To find this, one multiplies the number of electrodes required for the joint by the melting time per electrode.
The melting time must of course be determined for every type of electrode used and for every diameter of electrode used, because each of these has a different melting time. The melting time per electrode depends on the welding current used.
Widely varying welding currents are used in a shop depending on the conditions. When one wants to work accurately, it is recommended that one carries out tests to determine welding times for each electrode type and diameter for current conditions used in one’s own shop.
The pure welding time obtained by multiplying the number of electrodes by the melting time per electrode is exclusive of the time required for slag removal, changing of electrodes, setting up of the job, and other incidental but unavoidable operations.
For arriving at the total welding time, which includes all other time-consuming operations, there are two possible methods:
a) The accurate method consists of conducting time studies to find out incidental time expenditure in welding.
b) A less accurate method but which fully satisfies the requirements of estimation work, takes into account the incidental time expenditure by multiplying the pure welding time by a so-called weld time factor.
Cost of welding current consumption
This refers to the cost of electrical power used for welding. The following equations help to calculate this cost:
- Arc kilowatts = (Welding amps x arc voltage)/ 1,000
- Input kilowatt = (Arc kilowatts)/(% efficiency at arc voltage)
- Power cost per hour = Input kilowatts x rate for kwh
- Power cost/meter = Power cost per hour/ meter length of weld deposited in one arc hour.
- Power cost per Electrode = Power cost per hour/No. of electrodes consumed in one hour arc time
Welding Labor Cost Calculation
Direct labor cost of welding usually covers the wages of the welder and the helper who helps setting up the job, precleaning, etc.
From the number of electrodes consumed by a welder during a shift, the total weight of weld-metal deposited can be calculated.
The combined daily wages of the welder and his helper divided by the weight of weld-metal gives the labor cost per kg of weld-metal.
Overhead Cost in Welding
Overheads cover all the costs involved in shop management. They may include the costs of supervision, maintenance services, equipment depreciation and repair, cost of cleaning and lighting, etc.
Overheads may in some cases even include the total electric bill including welding power costs. For easy calculation, the relation between labor cost and overhead cost is determined and the resultant factor is used for calculating overhead costs. I
n an average shop, the overhead cost may be three to five times the labor cost.
Thus the final cost of 1 kg of weld-metal of an electrode consists of the following individual costs:
- Cost of electrode per kg of weld-metal
- Labour and overhead costs per kg of weld-metal
- Power cost per kg of weld-metal
- Equipment cost per kg of weld-metal.
Items 3 and 4 together normally amount to a maximum of 10% of the total costs.
In most cases, therefore, it suffices to compare the cost of electrodes and the labor and overhead costs involved in its use to judge the economics of various types of electrodes.