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Heat input is the total amount of electrical energy supplied to a weld during the process of welding. It is a parameter for arc welding procedure and must be controlled to ensure the quality of the sound weld.
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Keys Terms: Arc welding, Heat input, Voltage, Current, Heat Affected Zone (HAZ), Thermal efficiency factor, Factors influencing heat input.
What Is Heat Input?
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Heat input is defined as the measure of the amount of electrical energy transferred during welding. It is a kind of tool used in the evaluation of welding procedures within a given process. Some of the factors which are influenced by the heat input are weld size, cooling rate, the material used, and its properties.
So far most of the fabricators don’t bother about the heat input. For some reason, this is completely fine. But when we are welding the materials whose microstructure is delicate, at that point, it is important to realize the heat input.
It is because of a huge bearing on the cooling rate. Higher cooling rates and heat transfer can cause embrittlement in the heat-affected zones. This probably going to happen when welding material is vulnerable to hydrogen-induced cracking.
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| Concepts Related To Heat Input | ||
|---|---|---|
| Heat Load Formula | Temperature | Heat Rate Formula |
| Thermal energy | Joule’s Law | Heating Effect |
Formula For Heat Input
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Formula for heat input is
\(\frac{Voltage * Current*Time}{DistanceTravelled * 1000}\)It can also be written as:
\(\frac{60*Current*Volts}{1000*(distancetravelledin/min)}\)The accurate measurement of arc voltage is difficult here, yet we generally measure load voltage at the output terminals of the current source.
| Unit - Energy Per Unit Length |
In Europe, this tends to be in kJ/mm, though in America, it tends to be in kJ/in. Subsequently, the heat input is the arc energy under the European system. In this way, the formula that holds true with regards to this system is
| Heat Input = Thermal efficiency factor Arc energy |
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Things To Remember
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- Heat input is the amount of electrical energy transferred to the welds during the welding process.
- Heat input is a critical parameter for the Arc welding process.
- Increased heat input brings about cracking in the weld metal and heat-affected zone (HAZ).
- The importance of heat input in welding is that when heat input increases, the cooling rate of the weld is reduced.
- In the European system, units are kJ/mm, whereas, in the American system, units are kJ/in.
Important PYQs Based On Heat Input Formula
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- One mole of a perfect gas expands isothermally to ten times its original volume. The change in entropy is… [JKCET 2004]
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- Enthalpy of a compound is equal to its… [VITEEE 2010]
- For Which One Of The Following Reactions Will There Be… [VITEEE 2010]
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- Three moles of an ideal gas expanded spontaneously into the vacuum. The work done will be… [NEET 2010]
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- If Enthalpies Of Formation For C _2 H _4g CO _2 G [NEET 1995]
- A gas is allowed to expand in a well-insulated container against a constant external pressure of… [NEET 2017]
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- A reaction occurs spontaneously if… [NEET 1995]
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Sample Questions
Ques: Your heat input at 0.3619 kJ/mm is too high, but you also noticed that your weld is oversized. Instead of lowering your amperage or voltage, you decide to travel faster. You now complete the same weld in 90 seconds at the exact same parameters. What is your heat input now? (4 Marks)
Ans: Travel Speed =
\(\frac{Lengthofweld}{Timetoweld}\)Travel Speed =
\(\frac{2500mm}{1.5min}\)= 1666 mm/min
Arc energy
=
\(\frac{(60sec/min)*(325amp)*(29volts)}{(1000j/kJ)*(1666mm/min)}\)= 0.3394 kJ/mm
Heat input = Thermal efficiency factor Arc energy
= 0.3394kJ/mm 0.8
= 0.2715 kJ/mm
Ques: Your weld at 400 inches and 39 volts per minute. The weld is 35 inches long and welding takes 4 minutes. While welding you find that the system shows an amperage of 425. What is your heat input? (4 Marks)
Ans: Travel Speed =
\(\frac{Lengthofweld}{Timetoweld}\)Travel Speed =
\(\frac{35inch}{4min}\)= 8.75 inches per min
Heat Input =
\(\frac{60*Current*Volts}{1000*(distancetravelledin/min)}\)Heat Input =
\(\frac{60*425*39}{1000*8.75}\)= 113.65 kJ/in
Ques: A heat engine operates at a voltage of 1500 V. Flowing current is 3400 A. Find out the heat input if the distance is 24 meters with time 80 sec. (3 Marks)
Ans: Given-
Voltage = 1500 V,
Current = 3400 A,
Distance = 24 m,
Time = 80 sec
Heat Input =
\(\frac{Voltage*Current*Time}{Distancetravelled*1000}\)=
\(\frac{1500*3400*80}{24*1000}\)= 17000 Joules/meter
Ques: Calculate the heat input for a procedure qualification test coupon welded with a current range of 140A to 190 Amp, 16-18 Volts and travel speed of 80 mm/min to 110 mm/min. (3 Marks)
Ans: After reading the question, we understand that heat input shall be having two values as:
- Maximum heat input
- Minimum heat input
For minimum heat input, the values of current and voltage will be on its lower side as a multiplication factor and travel speed on its higher side as it’s a dividend factor.
Minimum Heat Input (J/min) =
\(\frac{140*16*60}{110} = 1221.8 J/min\)= 1.22 kJ/mm
Maximum Heat Input (J/mm) =
\(\frac{190*18*60}{80}\)= 2565 J/min = 2.57 kJ/mm
Ques: What is the maximum heat input in the welding? (2 Marks)
Ans: There is a heat input restriction applied to some steels that will experience joint strength reduction upon welding, a maximum heat input of 2.5 kJ/mm for 15 mm thickness. This value is dependent on steel grade.
Ques: Why is heat input important in welding? (1 Mark)
Ans: The importance of heat input in welding is that as it increases, the rate of cooling of the weld gets reduced. When the cooling rate reduces, most of the material will exhibit increased grain size in the weld.
Ques: If a welder takes 2 minutes to complete an 18 inches long weld. He keeps the voltage at 24 volts and the current at 120 amps. What is your heat input? (2 Marks)
Ans: Travel Speed =
\(\frac{Lengthofweld}{Timetoweld}\)=
\(\frac{8 inches}{2min}\)= 9 in/minHeat Input =
\(\frac{24*120*60}{9}\)= 19200J/in = 19.2 kJ/in
Ques: Define Arc Energy. (2 Marks)
Ans: Arc energy is defined as the energy supplied by the power source to the workpiece
AE =
\(\frac{V*I*60}{T*S}\)Units of Arc energy is J/mm.
Ques: You are using a conventional power source welding machine and using the GTAW arc welding process. The parameters used for welding shown on the machine are current 125A and voltage 13V. 250mm of length is welded in 4 minutes of time. What will be heat input? (3 Marks)
Ans: Travel Speed =
\(\frac{Lengthofweld}{Timetoweld}\)=
\(\frac{250 mm}{4min}\)= 62.5 mm/min
Heat Input =
\(\frac{12*125*60}{62.5}\)= 1560 J/mm
Ques: You are using a special waveform on a power source with GTAW process. The reading on the display for energy used for the length of 500mm is 560J. What will be heat input? (2 Marks)
Ans: Heat Input
=
\(\frac{TotalEnergykJ}{WeldBeadLength}\)=
\(\frac{560KJ}{500mm}\)= 1.12 kJ/mm
= 1120 J/mm
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