Pre Heating in short will be required to prevent cracking when the material being welded has a high carbon or alloy content . Welding causes a
sudden introduction of high temperature to the parts which may be at room temperature causing a thermal shock .
Weld metal shrinks when it cools and this sets up contraction stresses when the shrinkage is restrained by the surrounding cold metal . Shrinkage stresses in the weld metal and HAZ is especially high in highly restrained joints. Preheating is also used to minimise Distortion .
Weld metal shrinks when it cools and this sets up contraction stresses when the shrinkage is restrained by the surrounding cold metal . Shrinkage stresses in the weld metal and HAZ is especially high in highly restrained joints. Preheating is also used to minimise Distortion .
The rate of heat flow is greater in parts having thick
sections and metals with high thermal conductivity. This is a another reason
for shrinkage stresses . Preheating reduces the rate of heat flow away from the
weld and thus there is sufficient time for the thermal stresses to be re
distributed preventing the cracking phenomena.
There is a close relationship between thermal conditions ,
grain structure and hardness in the arc weld . The extent of the change in the
grain structure depends on the maximum temperature to which the metal is
subjected , the length of time the temperature exists , the composition of the
steel and the rate of cooling .Quick cooling from a high temperature may form hard and
brittle constituents in the HAZ parallel
to the line of fusion causing excessive
hardening and lowering ductility of the weld and HAZ .
The preheat temperature and its effect on the cooling rate of the weld as a function of heat input .
Relation of carbon content and preheat
Three rules of thumb can help in determining the need for pre heating carbon steel for metal arc welding :
1. Less than 0.20 % carbon : preheating is not required on plate under 1 inch thick and only rarely on plate over 1 inch thick .
2. 0.20% to 0,40% carbon : preheating is never required on plate under 1/2 inch tghick and sometimes on plate over 1/2 inch thick .
3. Over 0.40 % carbon : preheating is generally required on all plate thickness .
For low carbon steels the relatively small differences found in practise make insignificant change in these values. However with high carbon steels with appreciable amounts of alloying material the effect may be serious . Pre Heating prevents this phenomena from taking place and so the risk of cracking comes down .
Relation of carbon content and preheat
Three rules of thumb can help in determining the need for pre heating carbon steel for metal arc welding :
1. Less than 0.20 % carbon : preheating is not required on plate under 1 inch thick and only rarely on plate over 1 inch thick .
2. 0.20% to 0,40% carbon : preheating is never required on plate under 1/2 inch tghick and sometimes on plate over 1/2 inch thick .
3. Over 0.40 % carbon : preheating is generally required on all plate thickness .
For low carbon steels the relatively small differences found in practise make insignificant change in these values. However with high carbon steels with appreciable amounts of alloying material the effect may be serious . Pre Heating prevents this phenomena from taking place and so the risk of cracking comes down .
Slowing down the cooling rate by pre heating allows Hydrogen
if present to diffuse away from the weld and HAZ to avoid cracking . The
presence of stresses and Hydrogen cause toe cracks , root cracks , underbead
cracks and transverse cracks.
The amount of pre heat required increases with the alloying
content of the base metal . Certain elements increase the hardenability of the
steel thereby needing a slow cooling
rate . Generally the higher the carbon content of the steel the lower the
critical cooling rate requiring the need for preheating and slowing down the
cooling rate . Carbon and other elements present in the steel thus determine
the total hardenability and loss of ductility of the steel after rapid cooling
. Each element influence is somewhat different . 0.15 % phosphorous for example
demands as much pre heat as does 1.25 % copper.
In general the alloying
elements affect the preheating required in the following order .: manganese ,
chromium , molybdenum , nickel , carbon
, copper , silicon , vanadium and phosporous . The effect of these elements is cumulative .The presence of sulphur is not considered in calculating the
pre heating required . Although the presence of more than 0.045% sulphur may cause “ Hot short “ cracks
preheating does nothing to prevent it .
Factors that increase Hardness
High carbon and /or high alloy content of parent metal , coarse grained base metal , thick base metal ,work cold when welding is started and no extra heat applied , weldment cooled quickly after welding ,small bead welds ( tack welds ) , low welding current ; fast travel , high temperature heat source , high rate of heat transfer ( SMAW , SAW ) , multiple layer deposit .
Factors that decrease Hardness
Low carbon and or low alloy content of the metal , fine grained base metal , this base metal ( no quench effect on weld metal ) work hot when weld is started and kept hot during welding ,weldment allowed to cool slowly after welding , heavy bead or weld layer at slow speed , high welding current ; slow travel , low temperature heat source , slow rate of heat transfer ( gas welding , forge welding ) , single layer deposit .
Total hardenability can be expressed in terms of a carbon
equivalent . The two methods outlined in Annex XI of AWS D1.1-96 are: (1) heat
affected zone (HAZ) hardness control and (2) hydrogen control. The HAZ hardness
control method, which is restricted to fillet welds, is based on the assumption
that cracking will not occur if the hardness of the HAZ is kept below some critical
value. This is achieved by controlling the cooling rate. The critical cooling
rate for a given hardness can be related to the carbon equivalent of the steel,
which is defined as:
CE = C + ((Mn + Si)/6) + ((Cr + Mo + V)/5) + ((Ni + Cu)/15)
.
From the critical cooling rate, a minimum preheat temperature can then be
calculated.
Pre Heat Temperature as a function of CE
0.45 upto 0.6 95 to 210
above 0.6 210 to 370
Factors that increase cooling rate
Cold parent metal , Heavy sections ,High speed welding ,High welding current , welds made without protective slag conering , parent metal with high thermal conductivity { Copper , aluminium }, Welds in a fillet or in a deep groove , Local preheating in a large member .
Factors that decrease the cooling rate
Hot parent metal , Light section , slow speed welding , Low welding current ( combined with slow travel ) welds with a heavy layer of insulating slag , parent metal with low thermal conductivity , welds on a flat plate or on the edge of a plate or bar , preheating the entire
Pre Heat Temperature as a function of CE
Carbon Equivalent Suggested PreHeat ( ˚C )upto 0,45 optional
0.45 upto 0.6 95 to 210
above 0.6 210 to 370
Cold parent metal , Heavy sections ,High speed welding ,High welding current , welds made without protective slag conering , parent metal with high thermal conductivity { Copper , aluminium }, Welds in a fillet or in a deep groove , Local preheating in a large member .
Factors that decrease the cooling rate
Hot parent metal , Light section , slow speed welding , Low welding current ( combined with slow travel ) welds with a heavy layer of insulating slag , parent metal with low thermal conductivity , welds on a flat plate or on the edge of a plate or bar , preheating the entire
member or a large area.
How to Control Cooling Rate
1. Preheating is the most effective way of slowing the cooling rate.
2. Heat input from welding slows cooling by raising the temperature of the part.
3. Insulating the hot part immediately after welding with dry sand, lime, glass fiber blanket, etc. slows cooling.
How to Apply Preheat
Heating is done with gas or oil torches, ovens, furnaces , electrical induction coils or electrical resistance heating . AWS D1.1 code requires that the minimum preheat temperature be established at a distance that is at least equal to the thickness of the thickest member, but not less than 3 in. (75 mm) in all directions from the point of welding . To ensure that the full material volume surrounding the joint is heated, it is recommended practice to heat the side opposite of that which is to be welded and to measure the surface temperature adjacent to the joint. Before initiating the arc for each pass the temperature should be checked to verify that the minimum preheat temperature has been established as per the code requirement.
Checking the temperature of the part during welding may be needed to be sure it has not cooled. Heating again may be needed if welding is stopped for some reason . Interpass temperature is the temperature of the surface when welding all layers except the first layer. It is just as important and should usually be as high as the preheat temperature.
The welding process used will affect the pre heat requirements
How to Control Cooling Rate
1. Preheating is the most effective way of slowing the cooling rate.
2. Heat input from welding slows cooling by raising the temperature of the part.
3. Insulating the hot part immediately after welding with dry sand, lime, glass fiber blanket, etc. slows cooling.
How to Apply Preheat
Heating is done with gas or oil torches, ovens, furnaces , electrical induction coils or electrical resistance heating . AWS D1.1 code requires that the minimum preheat temperature be established at a distance that is at least equal to the thickness of the thickest member, but not less than 3 in. (75 mm) in all directions from the point of welding . To ensure that the full material volume surrounding the joint is heated, it is recommended practice to heat the side opposite of that which is to be welded and to measure the surface temperature adjacent to the joint. Before initiating the arc for each pass the temperature should be checked to verify that the minimum preheat temperature has been established as per the code requirement.
Checking the pre heat Temperature.
Temperature Sticks ( crayons ) , Contact Thermometers , Hand held electrical device ( Thermocouple ) with digital readout , ultrasonic gun ,
Checking the temperature of the part during welding may be needed to be sure it has not cooled. Heating again may be needed if welding is stopped for some reason . Interpass temperature is the temperature of the surface when welding all layers except the first layer. It is just as important and should usually be as high as the preheat temperature.
The welding process used will affect the pre heat requirements
SAW combines fast travel with slower cooling rates under a
blanket of flux and better protection against contamination that would
participate in crack formation. This advantage permits preheat temperatures to
be brought down by 200 to 300 degrees.
Low hydrogen electrodes also substantially reduce the pre
heat required . They are effective in preventing contamination of base and weld
metal safeguarding against underbead cracks and reducing the possibility of
root bead cracks.
TIG welding also offers fast deposition and protection
against underbead and weld cracking . In
some cases the use of Low hydrogen electrodes , TIG welding or SAW entirely eliminate the need for
preheating .
SS electrodes of 18-8 or higher alloy content offer excellent
ductility and will make crack free welds at low pre heat temperatures . However their use may not be possible if the
weld metal is required to match the parent metal in strength .
Preheating of austenitic manganese steel , martensitic
stainless steels and tool and die steels require separate considerations. Low or minimum
preheat, low heat input, and low interpass temperature
are used on Manganese steels. Manganese steel becomes brittle if
overheated. While a 100°F to 200°F preheat may be
required, do not allow interpass temperatures to
exceed 500°F.
Some alloy steel components require a specific heat treatment to perform properly in service. This must be considered when preheating and welding. Contact the part maker for information. Preheating is always required when welding hardened alloy steel , but the preheat temperature must never exceed that at which the metal was previously tempered . The minimum pre heat temperature at which any hardened metal is welded is 70 F . On plate ¾ in thick , the minimum is 150 F . On plate 1 ½ in thick it is 300 F.
Some alloy steel components require a specific heat treatment to perform properly in service. This must be considered when preheating and welding. Contact the part maker for information. Preheating is always required when welding hardened alloy steel , but the preheat temperature must never exceed that at which the metal was previously tempered . The minimum pre heat temperature at which any hardened metal is welded is 70 F . On plate ¾ in thick , the minimum is 150 F . On plate 1 ½ in thick it is 300 F.
Heavier plate thicknesses always require more preheat than
thin plates . Restrained welds and confined welds such as fillets , always
require more preheat than butt welds where plates are free to move .
It is not enough to heat the steel to the proper preheating
temperature . The steel must be kept in this temperature till the weld is
completed .The interpass temperature must be held at the same level as the pre
heat temperature .
In Summary preheating of the weldment area achieves better weld penetration and slows the cooling process, thus allowing added relief of stresses and reduced hardening of the materials.
In Summary preheating of the weldment area achieves better weld penetration and slows the cooling process, thus allowing added relief of stresses and reduced hardening of the materials.
The ASME Code sections take cognizance of the foregoing, in
some cases allowing exemption from postweld stress relieving provided preheating of a specified temperature is used.
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