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gordon stephenson
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I would think the reason will be two fold, Maybe some of the cooling air from the grinder is helping to cool the weld and secondly you are removing some of the mass of hot metal by grinding, so less hot metal to retain the heat.
chunkolini
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What about friction producing heat? and replacing any heat lost. Grinding does produce a jolly lot of heat.
Is this one of those 'I think I have noticed it so it must be happening things' (like the greenhouse effect?); I see them all the time, cant think of one right now but they crop all over the place. Just recalled one, I was sure for ages that stainless steel has a distinctive smell when being ground or welded. Does it?
Chunko'.
Is this one of those 'I think I have noticed it so it must be happening things' (like the greenhouse effect?); I see them all the time, cant think of one right now but they crop all over the place. Just recalled one, I was sure for ages that stainless steel has a distinctive smell when being ground or welded. Does it?
Chunko'.
Iain Westland
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But it doesnt generate the heat of a weld does it....
Paul.
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That'll be the hexavalent chromium rotting your nasal passages away then.
And when you reach the stage of not noticing wind passing, yer fooked
Gordon, you're most probably right. Even more peculiar, when I thought about it last night, I observed the same effect welding sheet metal and softly hammering the bead to prevent it from warping too much.
Seems like nobody has observed a similar effect - more evidence that I'm going senile.
But thats irrelevant. Its not a comparison between welding and grinding, its a comparison between grinding and not. The welding has finished, now, does leaving it or grinding it cause it to cool faster. Grinding puts heat into the material, leaving it does not.
Al.
Al.
Iain Westland
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Read NTwoO post again, the grinders heat is less than the heat in the welded piece so there is heat transfer from the weld to the grinder.
similar transfer to air is slower. Although the welding has finished the weld is still much hotter than an initial attack with a grinder.
Iain
similar transfer to air is slower. Although the welding has finished the weld is still much hotter than an initial attack with a grinder.
Iain
chunkolini
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Sorry guys not convinced on this one.As soon as you start looking for an effect you will find one, there were several experiments done on this by psychologists in the 60's/70's.
How are you identifying how fast the material cools?
Technic Al, where are you?
Chunko'.
How are you identifying how fast the material cools?
Technic Al, where are you?
Chunko'.
Iain Westland
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from the wise old wiki
Conduction is the transfer of heat by direct contact of particles of matter. The transfer of energy could be primarily by elastic impact as in fluids or by free electron diffusion as predominant in metals or phonon vibration as predominant in insulators. In other words, heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from atom to atom. Conduction is greater in solids, where atoms are in constant contact. In liquids (except liquid metals) and gases, the molecules are usually further apart, giving a lower chance of molecules colliding and passing on thermal energy.
Heat conduction is directly analogous to diffusion of particles into a fluid, in the situation where there are no fluid currents. This type of heat diffusion differs from mass diffusion in behaviour, only in as much as it can occur in solids, whereas mass diffusion is mostly limited to fluids.
Metals (eg. copper, platinum, gold, iron, etc.) are usually the best conductors of thermal energy. This is due to the way that metals are chemically bonded: metallic bonds (as opposed to covalent or ionic bonds) have free-moving electrons which are able to transfer thermal energy rapidly through the metal.
As density decreases so does conduction. Therefore, fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas: fewer collisions between atoms means less conduction. Conductivity of gases increases with temperature. Conductivity increases with increasing pressure from vacuum up to a critical point that the density of the gas is such that that molecules of the gas may be expected to collide with each other before they transfer heat from one surface to another. After this point in density, conductivity increases only slightly with increasing pressure and density.
To quantify the ease with which a particular medium conducts, engineers employ the thermal conductivity, also known as the conductivity constant or conduction coefficient, k. In thermal conductivity k is defined as "the quantity of heat, Q, transmitted in time (t) through a thickness (L), in a direction normal to a surface of area (A), due to a temperature difference (ΔT) [...]." Thermal conductivity is a material property that is primarily dependent on the medium's phase, temperature, density, and molecular bonding.
A heat pipe is a passive device that is constructed in such a way that it acts as though it has extremely high thermal conductivity.
Transient Conduction vs. steady state conduction. Steady state conduction is the form of conduction which happens when the temperature difference is constant, so that an equilibration time, the spatial distribution of temperatures in an object does not change (for example, a bar may be cold at one end and hot at the other, but the gradient of temperatures along the bar do not change with time). There also exist situations wherein the temperature drop or raise occurs more drastically, such as when a hot copper ball is dropped into oil at a low temperature, and the interest is in analysing the spatial change of temperature in the object over time. This mode of heat conduction can be referred to as unsteady mode of conduction or transient conduction. Analysis of these systems is more complex and (except for simple shapes) calls in for the application of approximation theories.
Lumped System Analysis. A common approximation in transient conduction, which may be used whenever heat conduction within an object is much faster than heat conduction across the boundary of the object, is Lumped system analysis. This is a method of approximation that suitably reduces one aspect of the transient conduction system (that within the object) to an equivalent steady state system (that is, it is assumed that the temperature within the object is completely uniform, although its value may be changing in time). In this method, a term known as the Biot number is calculated, which is defined as the ratio of resistance to heat transfer across the object's boundary with a uniform bath of different temperature, to the conductive heat resistance within the object. When the thermal resistance to heat transferred into the object is less than the resistance to heat being diffused completely within the object, the Biot number is small, and the approximation of spatially uniform temperature within the object can be used. As this is a mode of approximation, the Biot number must be less than 0.1 for accurate approximation and heat transfer analysis. The mathematical solution to the lumped system approximation gives Newton's law of cooling, discussed below.
Even if the Biot number is not less than 0.1, analysis can be continued, but the accuracy of the result reduces. This mode of analysis has been applied to forensic sciences to analyse the time of death of humans. Also it can be applied to HVAC (heating, ventilating and air-conditioning, or building climate control), to ensure more nearly instantaneous effects of a change in comfort level setting.[1]
Conduction is the transfer of heat by direct contact of particles of matter. The transfer of energy could be primarily by elastic impact as in fluids or by free electron diffusion as predominant in metals or phonon vibration as predominant in insulators. In other words, heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from atom to atom. Conduction is greater in solids, where atoms are in constant contact. In liquids (except liquid metals) and gases, the molecules are usually further apart, giving a lower chance of molecules colliding and passing on thermal energy.
Heat conduction is directly analogous to diffusion of particles into a fluid, in the situation where there are no fluid currents. This type of heat diffusion differs from mass diffusion in behaviour, only in as much as it can occur in solids, whereas mass diffusion is mostly limited to fluids.
Metals (eg. copper, platinum, gold, iron, etc.) are usually the best conductors of thermal energy. This is due to the way that metals are chemically bonded: metallic bonds (as opposed to covalent or ionic bonds) have free-moving electrons which are able to transfer thermal energy rapidly through the metal.
As density decreases so does conduction. Therefore, fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas: fewer collisions between atoms means less conduction. Conductivity of gases increases with temperature. Conductivity increases with increasing pressure from vacuum up to a critical point that the density of the gas is such that that molecules of the gas may be expected to collide with each other before they transfer heat from one surface to another. After this point in density, conductivity increases only slightly with increasing pressure and density.
To quantify the ease with which a particular medium conducts, engineers employ the thermal conductivity, also known as the conductivity constant or conduction coefficient, k. In thermal conductivity k is defined as "the quantity of heat, Q, transmitted in time (t) through a thickness (L), in a direction normal to a surface of area (A), due to a temperature difference (ΔT) [...]." Thermal conductivity is a material property that is primarily dependent on the medium's phase, temperature, density, and molecular bonding.
A heat pipe is a passive device that is constructed in such a way that it acts as though it has extremely high thermal conductivity.
Transient Conduction vs. steady state conduction. Steady state conduction is the form of conduction which happens when the temperature difference is constant, so that an equilibration time, the spatial distribution of temperatures in an object does not change (for example, a bar may be cold at one end and hot at the other, but the gradient of temperatures along the bar do not change with time). There also exist situations wherein the temperature drop or raise occurs more drastically, such as when a hot copper ball is dropped into oil at a low temperature, and the interest is in analysing the spatial change of temperature in the object over time. This mode of heat conduction can be referred to as unsteady mode of conduction or transient conduction. Analysis of these systems is more complex and (except for simple shapes) calls in for the application of approximation theories.
Lumped System Analysis. A common approximation in transient conduction, which may be used whenever heat conduction within an object is much faster than heat conduction across the boundary of the object, is Lumped system analysis. This is a method of approximation that suitably reduces one aspect of the transient conduction system (that within the object) to an equivalent steady state system (that is, it is assumed that the temperature within the object is completely uniform, although its value may be changing in time). In this method, a term known as the Biot number is calculated, which is defined as the ratio of resistance to heat transfer across the object's boundary with a uniform bath of different temperature, to the conductive heat resistance within the object. When the thermal resistance to heat transferred into the object is less than the resistance to heat being diffused completely within the object, the Biot number is small, and the approximation of spatially uniform temperature within the object can be used. As this is a mode of approximation, the Biot number must be less than 0.1 for accurate approximation and heat transfer analysis. The mathematical solution to the lumped system approximation gives Newton's law of cooling, discussed below.
Even if the Biot number is not less than 0.1, analysis can be continued, but the accuracy of the result reduces. This mode of analysis has been applied to forensic sciences to analyse the time of death of humans. Also it can be applied to HVAC (heating, ventilating and air-conditioning, or building climate control), to ensure more nearly instantaneous effects of a change in comfort level setting.[1]
